Developments in the Formulation and Reinforcement of Concrete (Woodhead Publishing in Materials)

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Developments in the formulation and reinforcement of concrete

Related titles: Durability of concrete and cement composites (ISBN 978-1-85573-940-6) Concrete and other cement-based composites are by far the most widely used manmade construction materials in the world. However, major problems of infrastructure deterioration have been caused by unanticipated premature degradation of these materials. This book provides an up-to-date review of several of the main forms of degradation, examining what is known about their causes and control. Trends in modelling and prediction of service lives are also examined. Advanced civil infrastructure materials (ISBN 978-1-85573-943-7) In recent decades, developments in materials in response to the call for more durable infrastructures have led to many exciting advancements. The purpose of this book is to provide an up-to-date review of several emerging infrastructure materials that may have a significant impact on repair and/or new construction. An area that is given high priority in this book is the materials design concept that has led to the creation of advanced composites; such design methodology has been made possible by several key advancements in materials science and mechanics. Also, each chapter concludes with selective examples of real world applications using such advanced materials. This book will be an invaluable reference for undergraduate and graduate students of civil engineering, materials, architecture and building engineering, and will serve as a guide for researchers and professional engineers in the construction industry. Durability of composites for civil structural applications (ISBN 978-1-84569-035-9) This comprehensive book on durability of FRP composites will make it easier for the practising civil engineer and designer to use these materials on a routine basis. It addresses the current lack, or inaccessibility, of data related to the durability of these materials, which is proving to be one of the major challenges to the widespread acceptance and implementation of FRP composites in civil infrastructure. The book should help further the acceptance of composites for civil structural applications, by providing this source for practising engineers, decision makers, and students involved in architectural engineering, construction and materials, disaster reduction, environmental engineering, maritime structural technology, transportation engineering and urban planning. Details of these and other Woodhead Publishing materials books, as well as materials books from Maney Publishing, can be obtained by: • •

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Developments in the formulation and reinforcement of concrete Edited by Sidney Mindess

Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals & Mining CRC Press Boca Raton Boston New York Washington, DC

Woodhead publishing limited Cambridge, England

Woodhead Publishing Limited and Maney Publishing Limited on behalf of The Institute of Materials, Minerals & Mining Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2008, Woodhead Publishing Limited and CRC Press LLC © 2008, Woodhead Publishing Limited The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the author and the publishers cannot assume responsibility for the validity of all materials. Neither the author nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 978-1-84569-263-6 (book) Woodhead Publishing ISBN 978-1-84569-468-5 (e-book) CRC Press ISBN 978-1-4200-7609-7 CRC Press order number WP7609 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Project managed by Macfarlane Production Services, Dunstable, Bedfordshire, England (e-mail: [email protected]) Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International Limited, Padstow, Cornwall, England

Contents

Contributor contact details Introduction S Mindess, University of British Columbia, Canada

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6

Autoclaved aerated concrete R Klingner, University of Texas at Austin, USA Introduction to autoclaved aerated concrete Applications of autoclaved aerated concrete Structural design of autoclaved aerated concrete elements Seismic design of autoclaved aerated concrete structures Design example: three-story autoclaved aerated concrete shear-wall hotel Further background on earthquake performance of autoclaved aerated concrete shear-wall structures Development of seismic design factors (R and Cd) for ductile autoclaved aerated concrete shear-wall structures Acknowledgements References High-density and radiation-shielding concrete P Lessing, Idaho National Laboratory, USA Introduction Applications/case studies The case of DUAGG® and DUCRETE® Future trends Sources of further information and advice References

ix

xiii

1 1 4 6 11 14 34 41 42 42 44 44 47 47 66 73 76 v

vi 3

3.1 3.2 3.3 3.4 3.5 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 5 5.1 5.2 5.3 5.4 5.5 6

6.1 6.2 6.3 6.4 6.5 7 7.1 7.2 7.3 7.4

Contents High-strength concrete O E Gjørv, Norwegian University of Science and Technology, Norway Introduction Applications Future trends Sources of further information and advice References

79

Sprayed concrete (shotcrete) N Banthia, University of British Columbia, Canada Introduction Mix proportioning and process implications Strength and stiffness Kinematics and rebound Toughness, impact resistance and fiber reinforcement Concluding remarks Acknowledgements References

98

79 80 90 93 95

98 99 101 102 108 111 111 111

Hot weather concreting C Ishee, Florida Department of Transportation, USA Introduction Applications/case studies Future trends Sources of further information and advice References

114

Underwater concrete A K Al-Tamimi, College of Engineering, The American University of Sharjah, United Arab Emirates Introduction Development of underwater concrete Quality control of underwater concrete Application/case study References

136

Fibrous concrete reinforcement S Mindess, University of British Columbia, Canada Introduction How do fibres work? Types of fibres Mix proportioning, fabrication and placement

154

114 114 131 133 134

136 137 138 144 151

154 155 156 157

Contents

vii

7.5 7.6 7.7 7.8 7.9 7.10

What do fibres do? High performance fibre reinforced concrete Hybrid fibre systems Applications of fibre reinforced concrete Concluding remarks References

158 161 163 164 165 166

8

Lightweight concrete T W Bremner, University of New Brunswick, Canada Introduction Applications/case studies Production of lightweight concrete Future trends Sources of further information and advice References

167

Self-compacting concrete (SCC) M Geiker, Technical University of Denmark, Denmark Significance of self-compacting concrete Selected properties of self-compacting concrete Applications/case studies Future trends Sources of further information and advice References

187

8.1 8.2 8.3 8.4 8.5 8.6 9 9.1 9.2 9.3 9.4 9.5 9.6 10

167 176 178 181 183 184

187 188 195 200 202 204

Recycled materials in concrete C Meyer, Columbia University, USA 10.1 Introduction 10.2 Fly ash 10.3 Ground granulated blast furnace slag (GGBFS) 10.4 Recycled concrete 10.5 Recycled waste glass 10.6 Recycled tires 10.7 Recycled plastics 10.8 Other recycled materials 10.9 Future trends 10.10 References

208

11

231

Foamed concrete V Bindiganavile and M Hoseini, University of Alberta, Canada 11.1 Introduction 11.2 Definitions and classifications

208 210 211 213 215 220 222 223 225 227

231 232

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Contents

11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11

Materials Mix design Production of foamed concrete Properties of foamed concrete Fiber reinforced foamed concrete Applications Research needs Acknowledgements References

232 235 236 237 248 250 251 252 252

12

Polymer concrete Y Ohama, Nihon University, Japan Introduction Production techniques for polymer concrete Practical applications, recycling and quality standards Future trends Sources of further information and advice References

256

Future developments in concrete L Czarnecki, Warsaw University of Technology, Poland, W Kurdowski, Institute of Mineral Building Materials, Poland and S Mindess, University of British Columbia, Canada Introduction Does concrete have a future? General factors influencing the development of concrete Functional concrete Nanocement and nanoconcrete Concluding remarks References

270

12.1 12.2 12.3 12.4 12.5 12.6 13

13.1 13.2 13.3 13.4 13.5 13.6 13.7

Index

256 257 259 263 268 268

270 271 272 276 279 282 282

285

Contributor contact details

(* = main contact)

Chapter 3

Introduction

Professor Odd E. Gjørv Norwegian University of Science and Technology, NTNU Department of Structural Engineering Rich. Birkelandsv. 1a NO-7491 Trondheim Norway E-mail: [email protected]

Dr Sidney Mindess Department of Civil Engineering University of British Columbia Vancouver BC Canada E-mail: [email protected]

Chapter 1 Professor Richard E. Klingner The University of Texas at Austin Engineering 1 University Station C2100 Austin, TX 78712-0284 USA E-mail: [email protected]

Chapter 2 Dr Paul A. Lessing Idaho National Laboratory Box 1625 Idaho Falls, ID 83415-2218 USA E-mail: [email protected]

Chapter 4 Dr Nemy Banthia Department of Civil Engineering University of British Columbia Vancouver BC Canada E-mail: [email protected]

Chapter 5 Dr Charles Ishee Florida Department of Transportation 5007 Northeast 39th Avenue Gainesville, FL 32609 USA E-mail: [email protected]

ix

x

Contributor contact details

Chapter 6

Chapter 9

Dr Adil K. Al-Tamimi Department of Civil Engineering College of Engineering The American University of Sharjah P.O. Box 26666 Sharjah United Arab Emirates E-mail: [email protected]

Associate Professor Mette R. Geiker Department of Civil Engineering Technical University of Denmark Brovej, Building 118 DK-2800 Kgs. Lyngby Denmark E-mail: [email protected]

Chapter 10 Chapter 7 Dr Sidney Mindess Department of Civil Engineering University of British Columbia Vancouver BC Canada E-mail: [email protected]

Dr Christian Meyer 622 SW MUDD Mail Code 4709 500 West 120th Street New York, NY 10027-6699 USA E-mail: [email protected]

Chapter 8

Chapter 11

Professor Emeritus Theodore W. Bremner Honorary Research Professor Department of Civil Engineering 17 Dineen Drive University of New Brunswick Fredericton NB Canada E3B 5A3 E-mail: [email protected]

Dr Vivek Bindiganavile* and M. Hoseini Structural Engineering University of Alberta 3-020 Markin/CNRL Natural Resources Engineering Facility Edmonton Alberta Canada T6G 2W2 E-mail: [email protected]

Contributor contact details

xi

Chapter 12

Chapter 13

Professor Yoshihiko Ohama 14-10-402 Hiyoshi 2-chome Kohoku-ku Yokohama 223-0061 Japan E-mail: [email protected]

Dr Lech Czarnecki* Warsaw University of Technology Warsaw Poland Email: [email protected] Dr Wieslaw Kurdowski Institute of Mineral Building Materials Cracow Poland Dr Sidney Mindess University of British Columbia Vancouver BC Canada E-mail: [email protected]

Introduction

S MINDESS, University of British Columbia, Canada

Concrete is the most widely used man-made material in the world; indeed, of all materials, only water is used in greater quantities (Table I.1). In many ways, concrete literally forms the basis of our modern society. Almost every aspect of our daily lives depends, directly or indirectly, on concrete. We need only consider the obvious examples: roads, bridges, runways, dams, water conduits, buildings of all types, and so on, to realize its importance. However, because it is so ubiquitous, we tend to take it for granted. The general view of concrete can be expressed by the often-quoted remark, “You mix together cement, gravel and water, and it gets hard. What else is there to know about concrete?” Concrete production is now, of course, a mature technology. “Modern” concretes have been in use since the middle of the 19th century, and the ancient Greeks and Romans both produced mortars which may be considered to be forerunners of today’s concretes. It is thus not surprising that concrete is often considered to be a rather “boring” topic, compared to the more dramatic modern discoveries in such areas as information technology, communications, or medical technology. However, in this case, appearances are deceiving; in fact, modern concretes constitute a sophisticated family of materials. Portland cement itself is a complex material, manufactured by first burning an intimate mixture of limestone and clay or shale in a kiln at temperatures in the range of 1400°C to 1500°C, and then intergrinding the resulting clinker with gypsum. In producing concrete, this cement is now commonly combined with one or more supplementary cementing materials, such as fly ash, silica fume, blast furnace slag, or limestone powder. In addition, modern concretes usually contain one or more chemical admixtures to modify the properties of either the fresh or the hardened concrete. Concrete is thus a very complex, though not completely understood, system. Today, exciting developments in cement and concrete are underway. There are several forces driving this resurgence in cement and concrete research. Perhaps the most significant is the drive to make the concrete industry much more sustainable. The production of Portland cement is highly energy intensive. As well, the cement industry is a significant conxiii

xiv

Introduction Table I.1 Annual production of selected materials Concrete Steel Salt Sugar Oil

∼3.8 billion m3 ∼8.7 billion tonnes ∼1 billion tonnes ∼200 million tonnes ∼135 million tonnes ∼5.2 billion tonnes

tributor of greenhouse gases: the production of one tonne of cement leads to almost one tonne of CO2 being released into the atmosphere. This is estimated to account for about 6% of the worldwide production of greenhouse gases. Thus, driven by environmental (and hence governmental) pressures, there is a real incentive for the concrete industry to become much “smarter” about concrete production. This involves, amongst other things, an increasing use of supplementary cementing materials to replace Portland cement, and an increased emphasis on durability rather than strength. There is a movement in the industry to performance specifications rather than the current prescriptive specifications for concrete; this should permit modern concrete producers to make much more efficient use of their materials. Related to this is the development of a large number of special concretes that are optimized for particular applications. These concretes are now available only because of the advances in our understanding of the chemistry and physics of concrete, and the development of the admixtures mentioned above. Indeed, most of the chapters in the present volume deal with these new types of concretes. The authors have described both the underlying science and the production applications of these new materials. A careful reading of these chapters should lead to a better understanding of the possibilities inherent in concrete technology – we are limited only by our imaginations in the types of concretes that will be available in the future.

1 Autoclaved aerated concrete R KLINGNER, University of Texas at Austin, USA

1.1

Introduction to autoclaved aerated concrete

Autoclaved aerated concrete (AAC) is a concrete-like material with very light weight, obtained by uniformly distributed, closed air bubbles. Material specifications for this product are prescribed in ASTM C1386. Because AAC typically has one-sixth to one-third the density of conventional concrete, and about the same ratio of compressive strength, it is useful for cladding and infills, and for bearing-wall components of lowto medium-rise structures. Because its thermal conductivity is one-sixth or less that of concrete, it is energy-efficient. Because its fire rating is slightly longer than that of conventional concrete of the same thickness, it is very fire-resistant. It is not susceptible to mold. Because of its internal porosity, it has very low sound transmission, and is acoustically very effective.

1.1.1 Historical background of AAC AAC was first produced commercially in Sweden, in 1923. Since that time, its production and use have spread to more than 40 countries on all continents, including North America, Central and South America, Europe, the Middle East, the Far East, and Australia. This wide experience has produced many case studies of use in different climates, and under different building codes. Background material on experience with AAC in Europe is given in RILEM (1993). In the US, modern uses of AAC began in 1990, for residential and commercial projects in the southeastern states. US production of plain and reinforced AAC started in 1995 in the southeast, and has since spread to other parts of the country. A nationwide group of AAC manufacturers was formed in 1998 as the Autoclaved Aerated Concrete Products Association (http://www.aacpa.org/). Design provisions for AAC are provided in the 1

2

Developments in the formulation and reinforcement of concrete

Code and Specification of the Masonry Standards Joint Committee (MSJC), and in the technical manuals available on the web site of the AACPA. The AACPA includes one manufacturer in Monterrey, Mexico, and many technical materials are available in Spanish as well as English.

1.1.2 AAC elements AAC can be used to make unreinforced, masonry-type units, and also factory-reinforced floor panels, roof panels, wall panels, lintels, beams, and other special shapes. These elements can be used in a variety of applications including residential, commercial, and industrial construction. Reinforced wall panels can be used as cladding systems as well as load-bearing and non load-bearing exterior and interior wall systems. Reinforced floor and roof panels can be efficiently used to provide the horizontal diaphragm system while supporting the necessary gravity loads.

1.1.3 Materials used in AAC Materials for AAC vary with manufacture and location, and are specified in ASTM C1386. They include some or all of the following: fine silica sand; Class F fly ash; hydraulic cements; calcined lime; gypsum; expansive agents such as finely ground aluminum powder or paste; and mixing water. Details of the mixture designs used by each producer depend on the available materials and the precise manufacturing process, and are not publicly available. The finely ground aluminum powder or paste produces expansion by combining with the alkaline slurry to produce hydrogen gas. AAC can be reinforced internally in the manufacturing process with welded wire cages, and also at the job site with conventional reinforcement.

1.1.4 How AAC is made Overall steps in the manufacture of AAC are shown in Fig. 1.1, and described below. Sand is ground to the required fineness in a ball mill, if necessary, and is stored along with other raw materials. The raw materials are then batched by weight and delivered to the mixer. Measured amounts of water and expansive agent are added to the mixer, and the cementitious slurry is mixed. Steel molds are prepared to receive the fresh AAC. If reinforced AAC panels are to be produced, steel reinforcing cages are secured within the molds. After mixing, the slurry is poured into the molds. The expansive agent creates small, finely dispersed voids in the fresh mixture, which increases the volume by approximately fifty percent in the molds

Autoclaved aerated concrete

3

Cement Lime Water Ball mill

Sand/fly ash

Expanding agent

Mix

Reinforcement wires Cast

Longitudinal cut Expansion Autoclave

Anticorrosive treatment

Assembling

To job site

AAC products

1.1 Overall steps in manufacture of AAC.

within three hours. Within a few hours after casting, the initial hydration of cementitious compounds in the AAC gives it sufficient strength to hold its shape and support its own weight. After cutting, the aerated concrete product is transported to a large autoclave, where the curing process is completed. Autoclaving is required to achieve the desired structural properties and dimensional stability. The process takes about 8–12 hours under a pressure of about 174 psi (12 bars) and a temperature of about 360°F (180°C) depending on the grade of material produced. During autoclaving, the wire-cut units remain in their original positions in the AAC block. After autoclaving, they are separated for packaging. AAC units are normally placed on pallets for shipping. Unreinforced units are typically shrink-wrapped, while reinforced elements are banded only, using corner guards to minimize potential localized damage that might be caused by the banding.

1.1.5 AAC strength classes AAC is produced in different densities and corresponding compressive strengths, in accordance with ASTM C1386 (Precast Autoclaved Aerated Concrete Wall Construction Units). Densities and corresponding strengths are described in terms of “strength classes” (Table 1.1).

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Developments in the formulation and reinforcement of concrete

Table 1.1 Typical material characteristics of AAC in different strength classes Strength class

Specified compressive strength lb/in2 (MPa)

Nominal dry bulk density lb/ft3 (kg/m3)

Density limits lb/ft3 (kg/m3)

AAC 2.0

290 (2.0)

AAC 4.0

580 (4.0)

AAC 6.0

870 (6.0)

25 31 31 37 44 50 44 50

22 28 28 34 41 47 41 47

(400) (500) (500) (600) (700) (800) (700) (800)

(350)–28 (450)–34 (450)–34 (550)–41 (650)–47 (750)–53 (650)–47 (750)–53

(450) (550) (550) (650) (750) (850) (750) (850)

Other strength classes within these ranges and densities may be produced depending on specific design requirements.

Table 1.2 Dimensions of plan AAC wall units AAC unit type

Width, in. (mm)

Height, in. (mm)

Length, in. (mm)

Standard block Jumbo block

2–15 (50–375) 4–15 (100–375)

8 (200) 16–24 (400–610)

24 (610) 24–40 (610–1050)

Table 1.3 Dimensions of reinforced AAC wall units Product type

Thickness, in. (mm)

Height or width, in. (mm)

Typical length, ft (mm)

Wall panel Floor panel Lintel/beam

2–15 (50–375) 4–15 (100–375) 4–15 (100–375)

24 (610) 24 (610) 8–24 (200–610)

20 (6090) 20 (6090) 20 (6090)

1.1.6 Typical dimensions of AAC units Typical dimensions for plain AAC wall units (masonry-type units) are shown in Table 1.2. Typical dimensions for reinforced AAC wall units (panels) are shown in Table 1.3.

1.2

Applications of autoclaved aerated concrete

Autoclaved aerated concrete (AAC) can be used in a wide variety of structural and non-structural applications (Barnett et al. 2005), examples of

Autoclaved aerated concrete

5

which are shown in the following figures. Figure 1.2 shows an AAC residence in Monterrey, Mexico, in which the AAC is used as structure and envelope. Figure 1.3 shows an AAC hotel in Tampico, Mexico, in which the AAC is again used as structure and envelope. Figure 1.4 shows an AAC cladding application on a high-rise building in Monterrey, Mexico. In each of the above applications, the thermal and acoustical efficiency of the AAC makes it an attractive choice for building envelope.

1.2 AAC residence in Monterrey, Mexico (courtesy Xella Mexicana).

1.3 AAC hotel in Tampico, Mexico (courtesy Xella Mexicana).

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Developments in the formulation and reinforcement of concrete

1.4 AAC cladding in Monterrey, Mexico (courtesy Xella Mexicana).

1.3

Structural design of autoclaved aerated concrete elements

1.3.1 Integrated US design context for AAC elements and structures Prior to October 2003, proposed AAC masonry buildings in the US had to be approved on a case-by-case basis. Since that date, project approvals can be obtained under the general evaluation-service reports ICC ES 215 (2003) and ICC ESR-1371 (2004). Since early 2005, project approvals for AAC masonry structures can be obtained through the inclusion of design provisions for AAC masonry in the mandatory-language Appendix A of the 2005 MSJC Code and Specification. It is also expected that reinforced AAC panels will be analogously addressed through ACI 318. This design context is shown schematically in Fig. 1.5 and is discussed in more detail in the rest of this chapter. Loads for structural design of AAC should be taken from appropriate load codes, such as ASCE 7. AAC masonry elements are designed using the provisions of Appendix A of the 2005 MSJC Code and Specification. Reinforced AAC panels are designed using manufacturers’ recommendations.

1.3.2 US design and construction provisions for elements and structures of AAC masonry In the US, development of masonry design provisions by an ANSI consensus process is the responsibility of the Masonry Standards Joint Committee (MSJC), sponsored by the American Concrete Institute (ACI), the American

Autoclaved aerated concrete

7

Model codes

AAC masonry design appendix in MSJC Code and Specification

ASTM specifications unique to AAC masonry

R, Cd

ASTM specifications for AAC material

Reinforced AAC panel appendix in ACI 318

ASTM specifications unique to reinforced AAC panels

1.5 Integrated US design background for AAC elements and structures.

Society of Civil Engineers (ASCE), and The Masonry Society (TMS). The MSJC Code and Specification is essentially referenced directly by US model codes (International Building Code and NFPA Code). The MSJC design provisions cover a wide variety of design approaches (strength, allowable-stress, empirical) and materials (clay, concrete, glass block). Based on the combination of test results from the University of Texas at Austin, the University of Alabama at Birmingham, and elsewhere, a strength design approach was developed for AAC masonry, with provisions that are generally similar to current strength-design provisions for other types of masonry, and for reinforced concrete. The proposed design provisions, commentary, and “super-commentary” were introduced, refined by, and approved by MSJC in 2004, in the form of a mandatory-language Appendix to the 2005 MSJC Code and Specification. They produce final designs similar to those produced by the proposed ACI provisions for reinforced AAC elements, described below. Flexural resistance of AAC masonry elements is computed assuming yielded flexural reinforcement and an appropriate equivalent rectangular stress block. Maximum reinforcement is limited to ensure tensioncontrolled behavior. Deformed reinforcement must be used, and must be surrounded by grout. Development and splice requirements are the same as for conventional masonry; only the grout is considered, and bond failure and splitting are addressed. In-plane shear resistance of AAC masonry elements is computed as the sum of resistance from masonry plus deformed reinforcement in intermediate bond beams only. In-plane shear resistance from AAC masonry is checked with respect to web shear, crushing of the diagonal strut, and

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Developments in the formulation and reinforcement of concrete

sliding shear. Out-of-plane resistance of AAC masonry elements is computed using beam shear equations similar to those used for conventional masonry. Capacity design for shear is required. These design requirements are accompanied by corresponding construction requirements in the MSJC Specification, which is mandated by the MSJC Code. Construction requirements address quality assurance, materials and execution.

1.3.3 US design provisions for reinforced AAC panels In the US, development of design provisions for reinforced concrete under the ANSI consensus process is the responsibility of ACI Committee 318. The latest version of that committee’s document, ACI 318-05, is essentially referenced directly by US model codes. The design provisions of ACI 318-05 address the strength design of a wide variety of conventional reinforced concrete elements similar to AAC applications, including prefabricated wall panels. Based on the combination of test results from the University of Texas at Austin, the University of Alabama at Birmingham, and elsewhere, a strength design approach has been developed for reinforced AAC elements, that is consistent with ACI 318-05, whose provisions are generally similar to ACI 318-05 strengthdesign provisions for reinforced concrete elements, and that produce final designs similar to those produced by the proposed MSJC provisions for AAC masonry. The first set of proposed design provisions, commentary, and “supercommentary” was introduced to ACI Subcommittee 523A (Autoclaved Aerated Cellular Concrete) in the Fall of 2002. Because ACI 523A is a relatively new subcommittee, the design provisions, commentary, and “supercommentary” were introduced as appendices to a non-mandatory design guide on AAC. After the guide has been approved by Committee 523, it will be offered to ACI 318 as a basis for a mandatory-language appendix to that document. Because these provisions must be discussed and refined within ACI Committee 318 as well as ACI 523, their timetable for approval will probably be extended longer than for their counterpart provisions for AAC masonry. It is hoped that this will be approved for the 2014 edition of the ACI 318 document.

1.3.4 Handling, erection and construction with AAC elements AAC masonry units are laid with a polymer-modified, thin-bed mortar. AAC panels are lifted and placed using specially designed clamps, and are aligned using alignment bars.

Autoclaved aerated concrete

9

When AAC elements are used as a load-bearing wall system, the floor and roof systems are usually designed and detailed as horizontal diaphragms to transfer lateral loads to shear walls. The tops of the panels are connected to the floor or roof diaphragms using a cast-in-place reinforced concrete ring beam. AAC floor and floor panels can be erected on concrete, steel or masonry construction. All bearing surfaces should be level and minimum required bearing areas (to prevent local crushing) should be maintained. Most floor and roof panels are connected by keyed joints that are reinforced and filled with grout to lock the panels together and provide diaphragm action to resist lateral loads. A cast-in-place reinforced concrete ring beam is normally placed along the perimeter of the diaphragm, completing the system.

1.3.5 Electrical and plumbing installations in AAC Electrical and plumbing installations in AAC are placed in routed chases. Care should be taken when laying out chases to ensure that the structural integrity of the AAC elements is maintained. Do not cut reinforcing steel or reduce the structural thickness of the AAC elements in critical areas. When analyzing the vertically spanning AAC element, horizontal routing should be permitted only in areas with low flexural and compressive stresses. In contrast, when the AAC element is intended to span horizontally, vertical routing should be minimized. When possible, it may be advantageous to provide designated chases for large quantities of conduit or plumbing.

1.3.6 Exterior finishes for AAC Unprotected exterior AAC deteriorates when exposed to cycles of freezing and thawing while saturated. To prevent such freeze-thaw deterioration, and to enhance the aesthetics and abrasion resistance of AAC, exterior finishes should be used. They should be compatible with the underlying AAC in terms of thermal expansion and modulus of elasticity, and should be vapor permeable. Many different types of exterior finishes are available, and the most common are discussed here. Polymer-modified stuccos, paints or finish systems are the most common exterior finish for AAC. They increase the AAC’s water-penetration resistance while allowing the passage of water vapor. Heavy acrylic-based paints containing aggregates are also used to increase abrasion resistance. There is generally no need to level the surface, and horizontal and vertical joints may be chamfered as an architectural feature, or may be filled.

10

Developments in the formulation and reinforcement of concrete

Masonry veneer may be used over AAC panels in much the same way that it is used over other materials. The veneer is attached to the AAC wall using masonry ties. The space between the AAC and the masonry can be left open (forming a drainage wall), or can be filled with mortar. When AAC panels are used in contact with moist or saturated soil (for example, in basement walls) the surface in contact with the soil should be coated with a waterproof material or membrane. The interior surface should either remain uncoated, or be coated with a vapor-permeable interior finish.

1.3.7 Interior finishes for AAC Interior finishes are used to enhance the aesthetics and durability of AAC. They should be compatible with the underlying AAC in terms of thermal expansion and modulus of elasticity, and should be vapor permeable. Many different types of interior finishes are available, and the most common are discussed here. Interior AAC wall panels may have a thin coat of a mineral-based plaster to achieve a smooth finished surface. Lightweight interior gypsum-based plaster may provide a thicker coating to level and straighten walls, and to provide a base for decorative interior paints or wall finishes. Interior plasters have bonding agents to enhance their adhesion and flexibility, and are commonly installed by either spraying or troweling. When applied to the interior surface of exterior AAC walls, gypsum board should be attached using pressure-treated furring strips. When applied to interior walls, moisture-resistant gypsum board can be applied directly to the AAC surface. For commercial applications requiring high durability and low maintenance, acrylic-based coatings are often used. Some contain aggregates to enhance abrasion resistance. When ceramic wall tile is to be applied over AAC, surface preparation is normally necessary only when the AAC surface requires leveling. In such cases, a Portland cement- or gypsum-based parge coat is applied to the AAC surface before setting the ceramic tile. The ceramic tile should then be adhered to the parged wall using either a cement-based thin-set mortar or an organic adhesive. In moist areas such as showers, only a Portland cement-based parge coat should be used, and the ceramic tile should be set with cement-based thin-set mortar only.

1.3.8 Typical construction details for AAC elements A wide range of construction details for AAC elements is available on the web sites of individual manufacturers, accessible through the web site of the AACPA. An example is given in Fig. 1.6.

Autoclaved aerated concrete

11

Load-bearing vertical wall system

Interior bearing wall

1/2″

Exterior bearing wall

1/2″

1.6 Construction details for load-bearing wall panels (courtesy Aercon Florida).

1.4

Seismic design of autoclaved aerated concrete structures

Because it has been used extensively in Europe for more than 70 years, AAC has been extensively researched there (RILEM 1993). Outside of the US, seismic qualification of AAC components and structures is based on experience in the Middle East and Japan. In the US, it is based indirectly

12

Developments in the formulation and reinforcement of concrete

on that experience, and directly on an extensive experimental and analytical research program conducted at the University of Texas at Austin, and described further here and in Tanner et al. (2005a,b), Varela et al. (2006) and Klingner et al. (2005a,b). That research program developed design models, draft design provisions, and seismic design factors (R and Cd). In the rest of this chapter, the US approach to seismic design of AAC structures is summarized; a design example is presented; and the research background for the design procedure is reviewed.

1.4.1 Basic earthquake resistance mechanism of AAC structures Structures whose basic earthquake resistance depends on AAC elements are generally shear-wall structures. Lateral earthquake loads are carried by horizontal diaphragms to AAC shear walls, which transfer those loads to the ground. General response of shear-wall structures to lateral loads is discussed in the Masonry Designers’ Guide (MDG 2006), and is not repeated here. Earthquake design of AAC shear-wall structures is similar to earthquake design of conventional masonry shear-wall structures. A complete design example is given later in this document. The technical justification for the design steps is given in the Commentary to Appendix A of the 2005 MSJC Code and Specification, and is also discussed at the end of this chapter.

1.4.2 Seismic design factors (R and Cd) for ductile AAC shear-wall structures in the US Because AAC structures (whether of masonry units or reinforced panels) in practically all parts of the US must be designed for earthquake loads, it is necessary to develop seismic design factors (R and Cd) for use with ASCE 7, the seismic load document referenced by model codes such as the 2003 IBC. The seismic force-reduction factor (R) is intended to account for ductility, and for structural overstrength. It is based on observation of the performance of different structural systems in previous strong earthquakes, on technical justification, and on tradition. Because AAC is a new material in the US, its seismic design factors (R and Cd) must be based on laboratory test results and numerical simulation of the response of AAC structures to earthquake ground motions. The proposed factors must then be verified against the observed response of AAC structures in strong earthquakes. Values of R and Cd for ductile AAC shear-wall structures have been proposed in two code-development forums.

Autoclaved aerated concrete •

•

13

In October 2002, seismic design factors were proposed to and approved by ICC ES (a model-code evaluation service), as part of a proposed IC ES listing for AAC structural components and systems produced by members of the Autoclaved Aerated Concrete Products Association (AACPA). That listing is intended to make it easier to use such systems throughout the US, until the consensus design provisions proposed above are incorporated in MSJC and ACI documents, and are referenced by model codes. In 2005 and 2006, the same seismic design factors were considered by the Building Seismic Safety Council and by the International Code Council. In September 2006, the ICC Structural Committee approved the R and Cd values, shown in Table 1.4, for reinforced AAC masonry, for inclusion in the 2007 IBC Supplement. The values will become final if they are sustained in ICC public comment hearings in May 2007.

1.4.3 ASTM specifications for AAC construction ASTM traditionally deals with specifications for materials and methods of test. For the past several years, standards-development work regarding AAC has been going on in two ASTM committees: •

In 1998, ASTM Subcommittee C-27.60 (Precast Concrete Elements of AAC) developed a material standard for AAC: C1386-98 (Standard Specification for Precast Autoclaved Aerated Concrete Wall Units). Subcommittee C27-60 has also developed a standard for reinforced AAC panels: C1452-00 (Standard Specification for Reinforced Autoclaved Aerated Concrete Units). That subcommittee has also developed a standard method of test for determining the modulus of AAC: C1591: (2004) (Standard Test Method for Determining the Modular of Elasticity of AAC).

Table 1.4 R and Cd values for reinforced AAC masonry Response modification coefficient, R

2

System overstrength factor, Ω0

2.5

Deflection amplification factor, Cd

2

System limitations and building height limitations (feet) by seismic design category as determined in Section 1616.3 A or B

C

D

E

F

NL

35

NP

NP

NP

14 •

Developments in the formulation and reinforcement of concrete In 2003, ASTM Subcommittee C-15.10 (Autoclaved Aerated Concrete Masonry) developed a standard for AAC masonry: C1555-03a (Standard Practice for Autoclaved Aerated Concrete Masonry). That standard references the AAC material provisions of ASTM C1386-98, and also contains construction provisions. It has been incorporated into the 2005 MSJC Specification.

1.5

Design example: three-story autoclaved aerated concrete shear-wall hotel

This example illustrates the preliminary design of a three-story AAC shearwall hotel in Asheville, North Carolina, a zone of moderate seismic risk, using the loading provisions of the 2003 IBC. The principal lateral forceresisting elements of the structure are transverse shear walls. This example problem is carried out using the AAC masonry design and detailing provisions of the 2005 MSJC Code and Specification.

1.5.1 Design steps 1)

Choose design criteria: • propose plan, elevation, materials, fAAC′ • calculate D, L, W, E loads • propose structural systems for gravity and lateral load 2) Design transverse shear walls for gravity and earthquake loads 3) Design exterior walls for gravity and wind loads • earthquake loads will be carried by longitudinal walls in-plane • out-of-plane wind loads will be carried by longitudinal walls out-ofplane using vertical and horizontal strips

1.5.2 Step 1: Choose design criteria The plan and elevation of the building are shown in Figs 1.7 and 1.8. Architectural constraints Water-penetration resistance:

Movement joints:

A single-width AAC masonry wall will be used. Exterior protection will be provided by low-modulus acrylic stucco. To control crack widths from shrinkage of AAC walls, use vertical control joints every bay.

Autoclaved aerated concrete

15

North

Elevator

8-in. AAC planks, untopped

20 ft

Stairs

10 ft 20 ft

20 ft typ 20 ft typ

7 @ 20 ft = 140 ft

1.7 Design criteria: plan of the building. Roof, parapet R 3 3 @ 11 ft 2 1 Typical facade

1.8 Design criteria: elevation of the building.

Design for fire Use and occupancy: Group B Use Type I or Type II construction (noncombustible material) No area or height restrictions 2- or 3-hour rating required Must meet separation requirements of Table 602 of the 2003 IBC Bearing walls: 4-hr rating (8-in. nominal AAC masonry OK) Shafts: 2-hr rating (8-in. nominal AAC masonry OK) Floors: 2-hr rating (planks and topping OK) Specify materials 12-in. AAC masonry units (ASTM C1555), fully mortared Thin-bed mortar (ASTM C1555)

16

Developments in the formulation and reinforcement of concrete

Class 6 AAC (fAAC′ = 6 MPa or 870 psi), assumed unit weight 45 pcf Deformed reinforcement meeting ASTM A615, Gr. 60 Floors and roof of untopped AAC planks with diaphragm reinforcement Structural systems Gravity load:

Lateral load:

Gravity load on roof and floors will be transferred to transverse walls. Gravity load on corridor will be transferred to spine walls. Lateral load (earthquake will govern) will be transferred by floor and roof diaphragms to the transverse shear walls, which will act as statically determinate cantilevers. (See Tables 1.5 and 1.6.)

1.5.3 Summary of design procedure Calculate design lateral load from earthquake (All section and table references are to the 2003 International Building Code) 1)

Determine the structure’s seismic use group (related to nature of occupancy), and select the corresponding seismic importance factor, IE, in accordance with Section 1616.2 and Table 1604.5. 2) Determine the site class (A through F) in accordance with Section 1615.1.1 and Table 1615.1.1. Table 1.5 Calculate design roof load due to gravity Dead load

Planks EPDM membrane, gravel HVAC, roofing

Live load

Ignore reduction of live load based on tributary area

30 lb/ft2 20 lb/ft2 30 lb/ft2 80 lb/ft2 total 20 lb/ft2

Table 1.6 Calculate design floor load due to gravity Dead load

Planks HVAC, floor finish, partitions

Live load

Use weighted average of corridor and guest rooms. Ignore reduction of live load based on tributary area

30 lb/ft2 20 lb/ft2 50 lb/ft2 total 60 lb/ft2

Autoclaved aerated concrete 3)

4)

5) 6)

7)

8)

17

Determine the ordinates of the maximum considered response acceleration for short periods, SMS, and for a 1-second period, SM1, depending on the geographical location of the structure (Figure 1615), and adjusted for site class effects, in accordance with Section 1615.1.2 (Equations 16-38 and 16-39). Determine the key ordinates of the design response spectrum for short periods, SDS, and at 1 second, SD1, as two-thirds the values determined in Step 3 above (Section 1615.1.3, Equations 16-40 and 16-41). Determine the design response spectrum using the key ordinates from Step 4 and Section 1615.1.4 (Equations 16-42 and 16-43). Determine the structure’s seismic design category (A through F) based on its seismic use group (Step 1), the key ordinate SDS, and the key ordinate SD1, using Section 1616.3, Table 1616.3(1), and Table 1616.3(2). Determine the required design approach for each seismic design category in accordance with Section 1616.6, including, for higher seismic design categories, the effects of plan structural irregularities (Table 1616.5.1.1) and vertical structural irregularities (Table 1616.5.1.2). Determine the seismic load effect, E and Em, for use in the load combinations of Section 1605, including the effects of redundancy (r), system overstrength (Ω0), in accordance with Section 1617.

Now we will discuss each step in more detail, with our example for Asheville, North Carolina. Step 1: Determine the structure’s seismic use group (related to nature of occupancy), and select the corresponding seismic importance factor, IE, in accordance with Section 1616.2 and Table 1604.5 In accordance with Section 1616.2, • •

•

Seismic Use Group II structures are those whose failure would result in substantial public hazard, or so designated by the building official; Seismic Use Group III structures are those containing essential facilities required for post-earthquake recovery, or so designated by the building official; and Seismic Use Group I structures are those not assigned to Seismic Use Group II or III.

Step 2: Determine the site class (A through F) in accordance with Section 1615.1.1 and Table 1615.1.1 In accordance with Table 1615.1.1, site classes are assigned as shown in Table 1.7.

18

Developments in the formulation and reinforcement of concrete

Table 1.7 Site class definitions Site class

Soil profile name

A B C D E E

Hard rock Rock Very dense soil and soft rock Stiff soil profile Soft soil profile –

F

–

Average properties in top 100 ft

其

Described in terms of soil shear wave velocity, standard penetration resistance, and undrained shear strength

Described in terms of plasticity index, moisture content, and undrained shear strength Described in terms of vulnerability to liquefaction or collapse, high organic content, very high plasticity, or very high flexibility

Step 3: Determine the ordinates of the maximum considered response acceleration for short periods, SMS, and for a 1-second period, SM1, depending on the geographical location of the structure (Figure 1615), and adjusted for site class effects, in accordance with Section 1615.1.2 (Equations 16-38 and 16-39) Determine the maximum considered earthquake acceleration response in % g for short periods (Figure 1615(1)) and for 1-second periods (Figure 1615(2)) as a function of geographical location. For Asheville, North Carolina, for example, SS = 0.40 g, and S1 = 0.13 g. For illustration, assume Site Class D (stiff soil profile). Then the acceleration-dependent site coefficient, Fa, is 1.48 (Table 1615.1.2(1)), and the velocity-dependent site coefficient, Fv, is 2.28 (interpolating in Table 1615.1.2(2)). Then the maximum considered short-period response acceleration is: SMS = Fa ⋅ SS = 1.48 ⋅ 0.40 g = 0.59 g And the maximum considered 1-second response acceleration is: SM 1 = Fv ⋅ S1 = 2.28 ⋅ 0.13 g = 0.30 g Step 4: Determine the key ordinates of the design response spectrum for short periods, SDS, and at 1 second, SD1, as two-thirds the values determined in Step 3 above (Section 1615.1.3, Equations 16-40 and 16-41) These design spectral ordinates correspond to values with a 10% probability of exceedance within a 50-year period.

Autoclaved aerated concrete

19

Continuing with our example for Asheville, North Carolina, the design response acceleration for short periods is: SDS =

2 2 ⋅ SMS = ⋅ 0.59 g = 0.39 g 3 3

and the design response acceleration for a 1-second period is: SD1 =

2 2 ⋅ SM 1 = ⋅ 0.30 g = 0.20 g 3 3

Step 5: Determine the design response spectrum using the key ordinates from Step 4 and Section 1615.1.4 (Equations 16-42 and 16-43) Define T0 ≡ •

SD1 S and TS = D1 . Then: SDS SDS

For periods less than or equal to T0, the design spectral response acceleration, Sa, is given by Equation 16-20: Sa = 0.6

• •

SDS T + 0.4SDS T0

(Equation 16-20)

For periods greater than T0 and less than or equal to TS, the design spectral response acceleration, Sa, is equal to SDS. For periods greater than TS, the design spectral response acceleration, Sa, is given by Equation 16-21: S=

SD1 T

(Equation 16-21)

The resulting design acceleration response spectrum is given in Fig. 1.9.

Response acceleration, g

0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

0.2

0.4

0.6

1 0.8 Period, sec

1.2

1.4

1.6

1.8

1.9 Design response spectrum for Asheville, NC site class D (stiff soil).

20

Developments in the formulation and reinforcement of concrete

Step 6: Determine the structure’s seismic design category (A through F) based on its seismic use group (Step 1), the key ordinate SDS, and the key ordinate SD1, using Section 1616.3, Table 1616.3(1), and Table 1616.3(2) According to Table 1616.3(1), a structure in Seismic Use Group I (the default case), and with a short-period design spectral ordinate, SDS, of 0.39 g, is assigned to seismic design category C (Table 1.8). The asterisk refers to footnotes that do not apply in this case. According to Table 1616.3(2), a structure in Seismic Use Group I (the default case), and with a 1-sec period design spectral ordinate, SD1, of 0.48 g, is assigned to seismic design category C (Table 1.9). The asterisk refers to footnotes that do not apply in this case. The two tables assign the structure to the same seismic design category, C. Had they assigned the structure to different categories, the more severe classification would have governed. Because the structure is less than or equal to 35 ft in height, and is assigned to SDC C, it can be designed as ordinary reinforced AAC masonry according to the draft provisions of the 2007 Supplement to the 2006 IBC. Continue with the design from Fig 1.9:

Table 1.8 Seismic design category based on short-period response accelerations Value of SDS

SDS < 0.167 g 0.167 g ≤ SDS < 0.33 g 0.33 g ≤ SDS < 0.50 g 0.50 g ≤ SDS

Seismic use group I

II

III

A B C D*

A B C D*

A C D D*

Table 1.9 Seismic design category based on 1-second period response accelerations Value of SD1

SDS < 0.167 g 0.067 g ≤ SDS < 0.133 g 0.133 g ≤ SDS < 0.20 g 0.20 g ≤ SDS

Seismic use group I

II

III

A B C D*

A B C D*

A C D D*

Autoclaved aerated concrete

21

Step 7: Determine the required design approach for each seismic design category in accordance with Section 1616.6, including, for higher seismic design categories, the effects of plan structural irregularities (Table 1616.5.1.1) and vertical structural irregularities (Table 1616.5.1.2) Plan structural irregularities include: • • • •

plan eccentricities between the center of mass and the center of stiffness; re-entrant corners; out-of-plane offsets; non-parallel systems.

These can increase seismic response. Vertical structural irregularities include: • • • • •

stiffness irregularity; mass irregularity; vertical geometric irregularity; in-plane discontinuity in vertical lateral-force-resisting elements; discontinuity in capacity – weak story.

These can also increase seismic response. The building under consideration here has no plan or vertical structural irregularities. Step 8: Determine the seismic load effect, E and Em, for use in the load combinations of Section 1605, including the effects of redundancy (ρ) and system overstrength (Ω0), in accordance with Section 1617 Overstrength need not be addressed, because the lateral-force-resisting system is statically determinate. Redundancy must be addressed. In our case, redundancy is the same on every floor:

ρ = 2−

20 rmax A

(Equation 16-54)

For shear walls, r is the maximum value of the product of the shear in the wall or wall pier and 10/lw, divided by the story shear. Because there are 16 uncoupled transverse shear walls, the shear per wall is 1/16 times the total story shear, and r=

( 161 ) ⋅ ( 1020 ) = 0.0313

22

Developments in the formulation and reinforcement of concrete

The area A refers to the diaphragm level above the story in question:

ρ = 2− r = 2−

20 rmax A

20

0.0313 3 × 50 × 140

r = 2 − 4.41 r = −2.41 However, ρ must not be less than 1.0, and need not exceed 1.5 (2003 IBC, Section 1617.2.1). The former value governs in this case. Finally, in accordance with the 2003 IBC, Section 1617.1.1, the design seismic load effect E is E = rQE Now compute the seismic base shear. In accordance with the 2003 IBC, Section 1617.4, seismic base shear is to be computed using the procedures of ASCE 7-02, Section 9.5.5.: V = CsW where Cs is the design seismic coefficient (defined below), and W is the building weight. In accordance with ASCE 7-02, Section 9.5.5.2.1: Cs = In our case:

SDS R I

( )

(Equations 16-36, 16-37)

SDS = 0.39 g R =2 (ordinary AAC masonry shear wall systems) I = 1.00 (ASCE 7-02, Table 9.1.4) Therefore SDS 0.39 = = 0.20 R 2 I 1 This is multiplied by the redundancy factor of 1.0, giving a product of 0.40. In other words, the building must be designed for 40% of its weight, applied as a lateral force. This force is distributed triangularly over the height of the building. The weight of a typical floor is its area, times the dead load per square foot, plus the interior transverse wall weight, plus the spine wall weight, plus the weight of the exterior walls. For simplicity, assume that the roof weighs the same as a typical floor, and ignore the parapet. Cs =

( ) ()

Floor weight: 50 lb/ft2 × 50 × 140 ft2 = 350 kips Transverse wall weight: 7 × 20 × 11 ft2 × 45 lb/ft2 = 69.3 kips

Autoclaved aerated concrete

23

Spine wall weight: 2 × 130 × 11 ft2 × 45 lb/ft2 = 128.7 kips Perimeter wall weight: 2 × (140 + 50) × 12 ft2 × 45 lb/ft2 = 188.1 kips Total weight of a typical floor is 736.1 kips. The design base shear (Table 1.10) is calculated assuming a linear distribution of forces over the height of the structure. Total design base shear is 2208 kips × 0.20 = 441.7 kips. At the roof level, the factored design lateral force is the factored design base shear (441.7 kips), multiplied by 0.50 (the quotient of WH/SUM) for the triangular distribution, or 220.9 kips. At the next level down, the factored design lateral force is 441.7 kips, multiplied by 0.333, and so forth. At each level, the factored design moment is the summation of the products of the factored design lateral forces above that level, each multiplied by its respective height above that level. The load factor for seismic loads is 1.0. Factored design shear and moment diagrams for the building are shown in Table 1.11 and Fig. 1.10. Table 1.10 Design base shear Level

Weight

Height

W×H

WH/SUM

R 3 2

736.1 736.1 736.1 2208.3

33 22 11

24,291 16,194 8,097 48,582

0.50 0.333 0.167

Table 1.11 Factored design shear Level

Fu, k

H, ft

Vu, k

Mu, k-ft

R 3 2

220.9 147.2 73.6

33 22 11

220.9 368.1 441.7

0 5923 29,616 44,424

0

220.9 k 147.2 k 73.6 k

2,430

220.9 368.1

6,479

441.7

Vu, kips

1.10 Moment diagrams for the building.

11,338 Mu, kip-ft

24

Developments in the formulation and reinforcement of concrete

20 ft

4 ft

1.11 Transverse wall I beam.

1.5.4 Step 2: Design building for gravity plus earthquake loads (All references are to the 2005 MSJC Code and Specification) The transverse direction is critical for this building. The 16 transverse walls are conservatively assumed to be uncoupled, so that each functions as an independent cantilever. Design each transverse wall as I beam, assuming flange widths of 4 ft (Fig. 1.11). Shear design of a typical transverse wall for earthquake loads From the 2005 MSJC Code, Appendix 3.4.1.2.1: fVAAC = f 0.95 w t f AAC 1+ ′

Pu 2.4 f AAC ′ wt

Include the effects of axial load, assuming that a typical transverse wall carries its self-weight plus the distributed floor weight on a tributary width of 20 ft: Self-weight of wall: 9.9 kips/floor Floor weight: 60 lb/ft2 × 20 × 20 = 24 kips/floor Total unfactored axial load at base, P, is 3 × (9.9 + 24) = 101.7 kips. Assume that the critical load case is Pu = 0.9D. Then fVAAC = 0.8 × 0.95 × 240 × 11.9 in.2 870 1 +

0.9 × 101, 700 2.4 870 × 240 × 11.9

fVAAC = 0.8 × 0.95 × 240 × 11.9 in.2 870 1 + 0.45 fVAAC = 77, 165 lb

Autoclaved aerated concrete

25

This exceeds (1/16 walls) times the factored design base shear (1/16 × 441.7 kips = 27.6 kips), and the transverse walls are satisfactory for shear thus far. While floor-level bond beams are required, no shear reinforcement is required. Check sliding at the base of the wall. By A.1.8.5 and A.3.4.1.2.3: VAAC = m ACC Pu Conservatively assume that Pu comes from 0.9D only: VAAC = 1.0 × 0.9 × 101.7 kips = 91.5 kips fVAAC = 0.8 × 91.5 = 73.2 kips Sliding shear resistance governs. In-plane flexural design of transverse shear walls for earthquake loads Each transverse shear wall has a plan length of 20 ft. The factored base moment per wall is (1/16) × 11,338 ft-kips, or 708.63 ft-kips. The critical load case is 0.9D + 1.0E. The factored axial load (see above) is 0.9 × 101.7 kips, or 91.5 kips. Using a spreadsheet, the interaction diagram for the wall is shown in Fig. 1.12. Flexural reinforcement consisting of 1 #4 bar at each end is required. The bars should be placed in grouted cores at least 12-in. square (at intersections of web and flanges). Check splice requirements and percent area requirements. Assume a 2000-psi grout strength by the proportion specification of ASTM C476. By A.3.3.3.1: 1600 1400

φ Pn, kips

1200 1000 800 600 400 200 0 0

500

1000

1500

2000 2500 φ Mn, ft-kips

3000

3500

4000

1.12 Strength interaction diagram by spreadsheet AAC transverse shear wall f ′AAC = 870 psi, 20 ft long, 12 in. thick, #4 bars at ends.

4500

26

Developments in the formulation and reinforcement of concrete ld =

0.13db2 f yg K AAC fg′

=

0.13 × 0.52 × 60, 000 × 1.0 = 7.58 in. 12 − 0.5 2, 000 2

(

)

12 inches governs. By Code A.3.3.1, the maximum percent area in a plastic hinge zone is 3%. For a 12-in. square core, a #4 bar easily satisfies this requirement. Because the wall is symmetrically reinforced, maximum reinforcement limitations (Code A.3.3.5) are satisfied. Now check capacity design for shear (Code Section A.1.3). First try to meet the capacity design provisions of that section. At an axial load of 91.5 kips, the nominal flexural capacity of this wall is 750 ft-kips, divided by the strength reduction factor of 0.9, or 833 ft-kips. The ratio of this nominal flexural capacity to the factored design moment is 833 divided by 708.6, or 1.18. Including the additional factor of 1.25, that gives a ratio of 1.47:

φVn ≥ 1.47Vu Vn ≥

1.47 1.47 Vu = Vu = 1.84Vu = 1.84 × 27.6 = 50.7 kips φ 0.8

Vn (governed by sliding shear) is 91.5 kips, considerably greater than this. The wall is satisfactory without shear reinforcement. A nominal 8-in. wall could probably be used instead of 12 in. Comments • The most laborious part of this design is calculation of the design lateral force for earthquake loads. Once that calculation is done, design of the lateral-force-resisting system is straightforward, even for a region of moderate seismic risk such as Knoxville. • This structural system could be designed for increased capacity. Increased flexural capacity would be quite easy to achieve, but increased shear capacity (to meet capacity design requirements) would probably require intermediate bond beams.

1.5.5 Step 3: Design exterior walls for gravity plus out-of-plane wind (All references are to the 2005 MSJC Code and Specification) The critical panel will be at the top of the building, where the wind load is highest. The panel must be designed for out-of-plane wind. Load effects in vertical jamb strips will be increased by the ratio of the plan length of openings to the total plan length.

Autoclaved aerated concrete

27

Use factored wind load (components and cladding) on wall ρu = 50 lbs/ft2 Reinforcement in a 3-in. grouted cell at 4 ft. on center (Fig. 1.13): h = 11ft t = 12 in. The wall is considered simply supported at top and bottom (Fig. 1.14). Flexural capacity a)

Determine design moment wu = p ⋅ width = 50 lb/ft 2 ⋅ 4 ft = 200 lb/ft Mu =

(Section A.3.2)

wl 2 200 ⋅ (11)2 = = 3, 025lb / ft = 36, 300 lb / in. 8 8

48 in.

1.13 Plan view of 4 ft section of wall.

1.14 Wall supported at top and bottom.

28 b)

Developments in the formulation and reinforcement of concrete Try #4 bar T = As f y = 0.20 ⋅ 60, 000 = 12, 000 lb a=

T 12, 000 = = 0.34 in. 0.85 f ACC ′ b 0.85 ⋅ 870 ⋅ 48

( a2 ) = 12.0 ⋅ (6 − 0.234 ) = 70, 000 lb/in.

Mn = As ⋅ f y ⋅ d −

ΦMn = 0.9 ∗ 70, 000 lb/in. = 63, 000 lb/in. > Mu OK Use a #4 bar. Outside of a plastic hinge zone, Code A.3.3.1 imposes a maximum bar area of 4.5% of the cell. Using a 3-in. grouted core, the area ratio is (0.5/3)2, or 0.028, easily satisfying the requirement. This bar size will easily satisfy the maximum reinforcement limitations of A.3.3.5 for out-ofplane flexure, and the design is satisfactory for flexure.

Shear capacity (a)

Determine factored loads and maximum shear force for a single panel. wu = 50 psf, and the panel is 4 ft wide: Vu =

(b)

200 ⋅ 11 = 1, 100 lb 2

Determine shear capacity of panel: VAAC = 0.9 f AAC ′ An + 0.05Pu = 0.9 870 ⋅ 48 ⋅ 6 = 7645 lb ΦVAAC = 0.8 ∗ (7645) = 6, 116 lb > Vu = 1, 100 lb

OK

1.5.6 Step 4: Design floor diaphragms for in-plane actions Design requirements for AAC floor diaphragms are not given in the 2005 MSJC Code and Specification, because that can be applied to many different types of floor systems. The design procedure given here is based on the requirements of the ICC ES 215 evaluation report, which was developed based on research at the University of Texas at Austin. The procedure is also given at the AACPA website (www.aacpa.org). f AAC ′ = 870 psi fgrout ′ = 2000 psi

Autoclaved aerated concrete

29

f y = 60, 000 psi Ring beam reinforcement 2 #5 Grouted key reinforcement 1 #5 Factored transverse lateral load in each bay, Fu = 220.9 kips/16 bays = 13.81 kips (a)

Design diaphragm for flexure, assuming that load is uniformly distributed along span (Fig. 1.15)

Plan view of diaphragm

Fu

Grouted keys

Ring beam

240 in.

(a) Elevation

b = 240 in.

(b)

1.15 (a) Design diaphragm for flexure, assuming that load is uniformly distributed along span, plan view; (b) Design diaphragm for flexure, assuming that load is uniformly distributed along span, elevation view.

30

Developments in the formulation and reinforcement of concrete M=

wu l 2 Fu × l 13, 810 ⋅ 240 = = = 414, 188 lb/in. 8 8 8

T = As f y = 2 ⋅ 0.31 ⋅ 60, 000 = 27, 200 lb a=

C 37, 200 = = 0.09 in. 0.85 fgrout ′ b 0.85(2000)(240)

d = length of key − ring beam/2 − 2 ∗ U-block thickness = 240 − 4 − 4 = 2387 in.

( a2 ) = 0.9 ⋅ 37, 200 ⋅ (238 − 0.09)

ΦMn = ΦAs f y ⋅ d −

= −7, 970, 000 lb/in. ≥ Mu (b)

OK

Design diaphragm for shear based on adhesion (i) Panel-to-panel joint (Fig. 1.16(a))

The total resistance is the adhesion of the grouted area plus the adhesion of the thin-bed mortar area. bgrout = 5in. bthin−bed = 3in. Vgrout = t grout ⋅ bgrout ⋅ l = 36 ⋅ 5 ⋅ 240 = 43, 200 lb Vthin − bed = t thin − bed ⋅ bthin − bed ⋅ l = 18 ⋅ 3 ⋅ 240 = 13, 000 lb Vtotal = Vgrout + Vthin − bed = 55, 200 lb

φVtotal = 0.67 ⋅ 55, 200 lb = 36, 980 lb > Vu = (ii)

Fu = 6, 900 lb OK 2

OK

Panel bond beam joint (Fig. 1.16(b))

bgrout = 8 in. Vgrout = t grout ⋅ bgrout ⋅ l = 36 ⋅ 8 ⋅ 240 = 69, 100 lb Fu = 6, 900 lb 2 Design diaphragm for shear based on truss model fVtotal = 0.67 ⋅ 69, 100 = 46, 300 lb > Vn

(c)

OK

One #5 bar in each grouted key. Each plank is 2 ft wide, so there are 10 planks. The load applied to each node is 1/10 of the total load, or 1.38 kips.

Autoclaved aerated concrete

31

Section D - D AAC floor panel

Grouted key AAC joint

Thin bed mortar at AAC joint (a)

Bond beam

Bond beam AAC joint

AAC floor panel

(b)

1.16 (a) Design diaphragm for shear based on adhesion: panel-topanel joint; (b) Design diaphragm for shear based on adhesion: panel bond beam joint.

In this model the compression chords act as diagonal compression members. There are two types of nodes: loaded nodes (on the upper side of Fig. 1.17) and unloaded nodes (on the lower side) (Fig. 1.18). The critical diagonal compression occurs in the panels next to the support. The component of that compression parallel to the transverse walls is onehalf the total factored load on the panel, or one-half of 13.81 kips, or 6.91 kips. The total compressive force in the diagonal, and also the tension force

32

Developments in the formulation and reinforcement of concrete Fiu Node 1

Node 3

Tension reinforcement

Compression strut

Node 2 Node 4

1.17 Design diaphragm for shear based on truss model.

Node 1

Node 2

Cpanel T grouted

Fiu T ring3

T ring3

Cpanel Cpanel Node 3

T ring1

Node 4

Cpanel Tring beam

T ring3 Cpanel T grouted

T ring2

T ring2

1.18 Loaded and unloaded nodes.

Autoclaved aerated concrete

33

in the associated tension tie, is essentially that shear, because of the aspect ratio of the panels: C panel =

6.91 kips 6.91 kips × 20 2 + 2 2 = = 6.97 kips = Tgrouted 2 20 cos tan −1 20

(

)

Check capacity of compression strut: Wstrut = 6 in. Tpanel = 8 in. Fstrut = 6.94 kips/48 = 145 psi < 0.75 (0.85 f AAC ′ ) = 0.75(0.85)(870) = 555 psi OK Check capacity of tension tie in grouted key: Tgrouted key = 6.94 kips < ΦAs f y = 0.75 ∗ 0.31 ∗ 60, 000 = 14, 000 lb

OK

Tension ties in ring beams have already been checked, and are satisfactory.

1.5.7 Overall comments •

•

• •

Although it is located in a region of moderate seismic risk, this building needs comparatively little reinforcement, because of the large plan area of its bearing walls. Considerable simplicity in design and analysis was achieved by letting transverse shear walls resist lateral loads as statically determinate cantilevers. Design of AAC bearing walls is inexpensive and straightforward for this type of building. Many types of floor and roof system are possible with AAC. To adapt this design to other types of floor or roof elements, the unit weight would have to be changed appropriately; the connection details would have to be changed appropriately; and the diaphragm actions would have to be checked appropriately. For example, if hollow-core prestressed concrete planks were used, the unit weight would increase, and so would the seismic base shear and overturning moment. Shear design of the transverse shear walls would still govern. Details of the connections between walls and floor or roof would be similar to those used with the AAC planks. Shears in the horizontal diaphragms would be transferred in topping only.

34

Developments in the formulation and reinforcement of concrete

1.6

Further background on earthquake performance of autoclaved aerated concrete shear-wall structures

1.6.1 Introduction to earthquake performance of AAC shear-wall structures Because the performance of AAC shear walls is fundamental to the earthquake resistance of AAC structures, the University of Texas studies first examined the performance of AAC shear walls under reversed cyclic loads like those imposed by strong earthquakes.

1.6.2 Brief description of AAC shear-wall tests at the University of Texas at Austin The Texas tests involved a group of 17 AAC shear-wall specimens, with aspect ratios (height divided by plan length) from 0.6 to 3. Some of the specimens were of AAC masonry; others were of reinforced panels, oriented horizontally; and still others were of reinforced panels, oriented vertically. To develop design equations for shear capacity, some of the specimens were designed to fail in shear, with low aspect ratios and heavy flexural reinforcement. Others were designed to fail in flexure, with high aspect ratios and light flexural reinforcement. AAC material strengths ranged from the lowest to the highest commercially available. The test setup is shown in Fig. 1.19.

Lateral load

External reinforcement Tie down boxes for rods

Axial load system

External reinforcement Tie down boxes for rods

1.19 Test setup for ACC shear-wall specimens (University of Texas at Austin).

Autoclaved aerated concrete

35

1.6.3 Overall performance of AAC shear walls tested at UT Austin AAC shear walls tested at UT Austin behaved reliably. The sheardominated walls failed in a variety of mechanisms, discussed further below. The flexure-dominated walls failed, as expected, by yielding of the longitudinal reinforcement. As shown in Fig. 1.20, a typical flexure-dominated AAC shear wall reaches displacement ductilities of about 4, and story drifts of about 1% with little degradation in strength or stiffness. Shear-dominated walls failed by web-shear cracking, flexure-shear cracking, or sliding shear. Based on observed performance of AAC shear walls, design formulas were developed for each of these behaviors, for walls laid with unreinforced AAC masonry units, and also for walls made of reinforced AAC panels. The design formulas for AAC masonry have been incorporated as described above into the 2005 MSJC Code and Specification; the design formulas for reinforced AAC panels are being incorporated into the draft ACI 523A Design Guide.

1.6.4 Two-story AAC assemblage tested at UT Austin

30

133

20

88

10

44

0 –1.2

–0.8

–0.4

0.0

0.4

0.8

1.2

0

–10

–45

–20

–89

–30

–134 Drift ratio (%)

1.20 Typical load-displacement curves for flexure-dominated AAC shear wall.

Base shear (kN)

Base shear (kips)

To confirm the validity of the design procedures that had been developed based on tests of AAC shear walls, a complete two-story AAC assemblage, consisting of AAC shear walls and AAC floor planks, was built at UT Austin and tested under reversed cyclic loading.

36

Developments in the formulation and reinforcement of concrete

Description of assemblage The two-story assemblage specimen consisted of two flanged walls connected by floor slabs (Fig. 1.21). The walls were constructed of vertically oriented AAC panels with internal reinforcement and additional fieldplaced longitudinal reinforcement, and the floor slabs were constructed of internally reinforced AAC panels. Vertically oriented panels were selected instead of AAC masonry units. On the upper level, the floor panels were oriented longitudinally; on the lower level, transversely. A complete description of the specimen is provided in Tanner (2005a). Vertically oriented, reinforced panels were selected because those had shown the most critical behavior in the shear wall tests described above. The reinforcement in the shear walls of the two-story assemblage Specimen consisted of flexural (longitudinal) reinforcement and foundation dowels (Fig. 1.22). The flexural reinforcement continued up the height of the specimen with a splice just above the first-story slab (first elevated slab). The dowels extended 24 in. (61 mm) above the foundation, and were included to increase the sliding shear capacity of the specimen. They were also placed at the level of the first elevated slab to prevent sliding at the bed of leveling mortar placed between the vertical panels and the slab.

North

1.21 Isometric view of two-story AAC assemblage specimen.

Autoclaved aerated concrete

37

#4 Vertical reinforcement #5 Dowels

North

50

222

40

178

30

133

20

89

10

44

0

0

4

8

12

16

20

24

28

32

36

40

0

–10

–45

–20

–89

–30

–134

–40

–178

Force per story (kN)

Force per story (kips)

1.22 Plan view of horizontal section at base, showing flexural reinforcement and dowels in two-story AAC assemblage specimen.

–222

–50 Time

1.23 Loading history for two-story AAC assemblage specimen.

Instrumentation was used to measure global and local behavior of the two-story assemblage specimen. Measurements included applied loads, horizontal displacements, and various local deformations. Before the assemblage was tested, the AAC material was verified to have a compressive strength of 495 psi (3.4 MPa) and a splitting tensile strength of 45 psi (0.31 MPa). The assemblage was subjected to the reversed cyclic loading history shown in Fig. 1.23. After the maximum load was reached, the specimen was loaded under displacement control to tip displacements of 0.5 in. (13 mm), 0.8 in. (20 mm) and 1.5 in. (38 mm). A constant axial load was applied through the self-weight of the specimen and the loading equipment. The total axial load on the specimen is 60 kips (267 kN).

38

Developments in the formulation and reinforcement of concrete

1.6.5 Summary of assemblage behavior Overall assemblage behavior is summarized in Fig. 1.24. Total base shear is the summation of the equal shears applied to each floor level. Positive displacements are to the south; negative, to the north. The displacements shown in the plot include base sliding. The following behaviors were observed in the two-story AAC assemblage: • • • • • •

flexural cracking at the base; minor vertical cracking on the north end of the east wall; web-shear cracking in the webs of both walls in the lower story; yielding of the flexural reinforcement at all four corners of the assemblage; separation of the flanges from the webs in the lower story; and separation of the vertical joint at the location of reinforcement at the northeast corner.

160

712

120

534

80

356

40

178

0 –1.0 –0.8 –0.6 –0.4 –0.2 –40

0.0

0.2

0.4

0.6

0.8

0 1.0 –178

–80

–356

–120

–534

–160

–712 Average drift ratio in % (south positive)

1.24 Overall hysteretic behavior of two-story AAC assemblage specimen.

Total base shear (kN)

Total base shear (kips)

The loads at which these behaviors were observed were consistent with the predictions based on previous tests of AAC shear walls, and also consistent with the design procedures and equations. After the flexural reinforcement yielded, both walls exhibited flexural behavior, consisting largely of rigid-body rocking. Vertical displacements were observed at the wall bases on the tension side, due to yielding and bond deterioration of the tensile reinforcement. Crushing of the compression toe was avoided, due to lateral support by the flanges. The tested flexural capacity of each wall fell within predicted limits corresponding to

Autoclaved aerated concrete

39

neglecting the contribution of flexural dowels and including the contribution. After flexural yielding, distributed web-shear cracks continued to form in the walls at the lower level. Diagonal cracks formed around the dowels, separating those dowels from the webs of the AAC shear walls. This reduced the effectiveness of the dowel action, which in turn reduced the sliding-shear capacity of each wall. Degradation of dowel action is also identified by spalling of AAC around the diagonal cracks. Maximum slip between the AAC shear walls and the foundation exceeded 0.5 in. (13 mm). The wall displacement, corrected for this slip, is shown in Fig. 1.25. After three cycles of flexural rocking, to displacement drift ratios of 0.32% (loading south) and 0.24% (loading north), vertical cracks began to form at the interface between the web and the flanges. As the displacements increased, the flange panel did not slide with the web in the direction of loading, resulting in local damage to the flange and finally instability of the flange at both the north and south ends of the specimen. At the north end, the flange damage was accompanied by a large vertical crack in the east web. Testing of the two-story assemblage specimen was halted due to this damage. Final cracking patterns for each exterior face of the specimen are shown in Figs 1.26–1.28. The cracks shown in grey formed at the time of yielding of the flexural reinforcement; subsequent cracks are shown in black.

1.6.6 Summary of response of two-story AAC assemblage

80

356

60

267

40

178

20

89

0 –0.6 –20

–0.4

–0.2

0.0

0.2

0.4

0 0.6 –89 –178

–40 E Top W Top

–60

Force per story (kN)

Force per story (kips)

After removing the measured base slip, the two-story assemblage specimen reached drift ratios between 0.24% and 0.42%, and final displacement ductilities ranged from 2.8 to 5.8 (Table 1.12).

–267 –356

–80 Drift ratio in % (south positive)

1.25 Hysteretic behavior of two-story AAC assemblage specimen with slip removed.

40

Developments in the formulation and reinforcement of concrete

1.26 Cracks in the east wall at the end of the test.

1.27 Cracks in the west wall at the end of the test.

1.28 Cracks in the south and north wall at the end of the test.

Autoclaved aerated concrete

41

Table 1.12 Drift ratios and displacement ductilities for each wall, with base slip removal Shear wall/Direction of loading

Displacement ductility

Drift ratio (%)

East wall/south East wall/north West wall/south West wall/north

5.8 2.8 4.4 2.8

0.42 0.24 0.37 0.24

For design purposes, these results justify an assumption of an available flexural ductility of at least 3.0, reasonably consistent with that observed in previous tests of flexure-dominated AAC shear walls at UT Austin.

1.7

Development of seismic design factors (R and Cd) for ductile autoclaved aerated concrete shear-wall structures

The seismic force-reduction factor R specified in seismic design codes is intended to account for energy dissipation through inelastic deformation (ductility), and for structural overstrength. The factor R is based on observation of the performance of different structural systems in previous strong earthquakes, on technical justification, and on tradition. For innovative structures such as AAC structures, the force-reduction factor R and the corresponding displacement-amplification factor Cd must be based on laboratory test results and numerical simulation of the response of those structures subjected to earthquake ground motions. The proposed factors must then be verified against the observed response of those structures in strong earthquakes. Using the observed experimental response of the ductile (flexuredominated) AAC shear walls and the two-story assemblage tested at UT Austin, inelastic analytical models were developed and subjected to ground motions representing design earthquakes for highly seismic regions of the US. For flexure-dominated AAC structures, values of R and Cd were established that would result in a suitably low probability of failure under those design earthquakes, and under maximum considered earthquakes. For use with the 2000 IBC, the R value is 3, and the Cd value is also 3. Those values are consistent with the values for moderately reinforced shear walls of conventional masonry. ICC hearings have approved the lower values of R and Cd used in the example problem.

42

Developments in the formulation and reinforcement of concrete

1.8

Acknowledgements

Some of the material in this chapter is adapted from a draft guide to AAC construction, under preparation by Committee 523A of the American Concrete Institute. The author acknowledges the contributions of colleagues to that material. The University of Texas research whose results are described here was supported financially and technically by the Autoclaved Aerated Concrete Products Association.

1.9

References

ACI 318-05 (2005): ACI 318-05 Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute, Farmington Hills, Michigan, 2005. ACI 523A (2002) Autoclaved Aerated Cellular Concrete, American Concrete Institute, Farmington Hills, Michigan, 2002. ASCE 7-05 (2005): Minimum Design Loads for Buildings and Other Structures (ASCE 7-02), American Society of Civil Engineers, Reston, Virginia, 2005 (with Supplement). ASTM C1386 (1998): ASTM C1385 (Standard Specification for Precast Autoclaved Aerated Concrete (PAAC) Wall Construction Units), ASTM International, West Conshohocken, Pennsylvania. ASTM C1452 (2000): ASTM C1452 (Standard Specification for Reinforced Autoclaved Aerated Concrete Units), ASTM International, West Conshohocken, Pennsylvania. ASTM C476 (2002): Standard Specification for Grout for Masonry, ASTM International, West Conshohocken, Pennsylvania, 2002. ASTM C1555 (2003): ASTM C 1555 (2003a) (Standard Practice for Autoclaved Aerated Concrete Masonry), ASTM International, West Conshohocken, Pennsylvania. ASTM C1591 (2004): ASTM C 1591 (Standard Test Method for Determining the Modulus of Elasticity of AAC), ASTM International, West Conshohocken, Pennsylvania. ASTM A615 (2006): Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete, ASTM International, West Conshohocken, Pennsylvania, 2006. Barnett et al. (2005): Barnett, R. E., Tanner, J. E., Klingner, R. E. and Fouad, F. H. “Guide for Using Autoclaved Aerated Concrete Panels: I – Structural Design,” ACI Special Publication SP 226, Caijun Shi and Fouad H. Fouad, Editors, American Concrete Institute, Farmington Hills, Michigan, April 2005, pp. 17–28. IBC 2000: International Building Code, 2000 Edition, International Code Council, Washington, DC, 2000. IBC 2003: International Building Code, 2003 Edition, International Code Council, Falls Church, Virginia, 2003. IBC 2006: International Building Code, 2006 Edition, International Code Council, Washington, DC. ICC ES 215 (2003): “Acceptance Criteria for Seismic Design Factors and Coefficients for Seismic-Force-Resisting Systems of Autoclaved Aerated Concrete (AAC),”

Autoclaved aerated concrete

43

Evaluation Report AC215, ICC Evaluation Service, Inc., Whittier, California, November 1, 2003. ICC ESR-1371 (2004): “Autoclaved Aerated Concrete (AAC) Block Masonry Units,” Evaluation Report ESR-1371, ICC Evaluation Service, Inc., Whittier, California, October 1, 2004. Klingner et al. (2005a): Klingner, R. E., Tanner, J. E., Varela, J. L., Brightman, M., Argudo, J. and Cancino, U., “Technical Justification for Proposed Design Provisions for AAC Structures: Introduction and Shear Wall Tests,” ACI Special Publication SP 226, Caijun Shi and Fouad H. Fouad, Editors, American Concrete Institute, Farmington Hills, Michigan, April 2005, pp. 45–66. Klingner et al. (2005b): Klingner, R. E., Tanner, J. E. and Varela, J. L., “Technical Justification for Proposed Design Provisions for AAC Structures: Assemblage Test and Development of R and Cd Factors,” ACI Special Publication SP 226, Caijun Shi and Fouad H. Fouad, Editors, American Concrete Institute, Farmington Hills, Michigan, April 2005, pp. 67–90. MDG (2006): Masonry Designers’ Guide, 5th edition, Phillip J. Samblanet, ed., The Masonry Society, Boulder Colorado, 2006. MSJC Code and Specification (2005): ACI 530-05 / ASCE 5-05 / TMS 402-05 (Building Code Requirements for Masonry Structures) and ACI 530.1-05 / ASCE 6-05 / TMS 602-05 (Specifications for Masonry Structures), American Concrete Institute, Farmington Hills, Michigan; American Society of Civil Engineers, Reston, Virginia; and The Masonry Society, Boulder, Colorado. RILEM (1993): Autoclaved Aerated Concrete: Properties, Testing and Design, RILEM Recommended Practice, RILEM Technical Committees 78-MCA and 51-ALC, E & FN Spon, London. Tanner et al. (2005a): Tanner, J. E., Varela, J. L., Klingner, R. E., “Design and Seismic Testing of a Two-story Full-scale Autoclaved Aerated Concrete (AAC) Assemblage Specimen,” Structures Journal, American Concrete Institute, Farmington Hills, Michigan, vol. 102, no. 1, January–February 2005, pp. 114–119. Tanner et al. (2005b): Tanner, J. E., Varela, J. L., Klingner, R. E., Brightman M. J. and Cancino, U., “Seismic Testing of Autoclaved Aerated Concrete (AAC) Shear Walls: A Comprehensive Review,” Structures Journal, American Concrete Institute, Farmington Hills, Michigan, vol. 102, no. 3, May–June 2005, pp. 374– 382. Varela et al. (2006): Varela, J. L., Tanner, J. E. and Klingner, R. E., “Development of Seismic Force-Reduction and Displacement Amplification Factors for AAC Structures,” EERI Spectra, vol. 22, no. 1, February 2006, pp. 267–286.

2 High-density and radiation-shielding concrete P LESSING, Idaho National Laboratory, USA

2.1

Introduction

2.1.1 Definition of high-density concrete High-density concrete is produced using special heavy aggregates and can have a density of up to 400 lb/ft3. Why would anyone be interested in a heavyweight concrete when low-density concrete has so many beneficial construction and insulation applications? The answer is that heavyweight concrete is primarily useful for nuclear radiation shielding but can also be useful for counterweights and blast shielding (applications where physical space is limited). Gamma-rays and X-rays can be shielded by a mass of material containing heavy atoms. To shield against neutrons it is necessary to have a mass of material that contains atoms that can both “thermalize” and capture neutrons. The hydrogen atom in water that is chemically bonded in concrete “thermalizes” the fast neutrons which may then be captured by other atoms such as boron which have high neutron-capture cross-sections. Reference books are available that cover aggregate characteristics, mix proportioning, and standards for conventional high-density concrete.1,2

2.1.2 Characteristics of various heavy aggregates Historically inexpensive high-density mineral aggregates such as barite, ferro-phosphorus, goethite, hematite, ilemnite, limonite, magnetite, and steel punching and steel shot have been used to produce high-density concrete. Table 2.1 shows typical densities of heavy aggregates and the concrete manufactured using these aggregates. Boron-containing additions such as colemanite, boron frits, and borocalcite have been used to improve the neutron-shielding properties of heavy concrete. However, they may negatively affect the setting and strength of the concrete. 44

High-density and radiation-shielding concrete

45

Table 2.1 Physical properties of traditional heavyweight aggregates and concrete Aggregate

Water % (retained or chemically bound)

Aggregate specific gravity

Aggregate bulk density lb/ft3

Concrete density lb/ft3

Geothite (hydrous iron ore) Barite (barium sulfate) Hematite (iron ores) Magnetite (iron mineral) Ferro-phosphorus (slag) Steel punchings or shot

10–11

3.4–3.7

130–140

180–200

0

4.0–4.6

145–160

210–230

–

4.9–45.3

180–200

240–260

–

4.2–5.2

150–190

210–260

0

5.8–6.8

200–260

255–330

0

6.2–7.8

230–290

290–380

2.1.3 Requirements for radiation-shielding concrete High level functional requirements for spent nuclear fuel storage applications were gathered into a report by Haelsig.3 Haelsig tabulates the requirements that he determined to be applicable to the conceptual design of a multi-purpose cask (MPC) and allocates them to MPC subsystems. In several appendices, Haelsig lists the design criteria of several vendor supplied concrete casks that were then licensed by the Nuclear Regulatory Commission (NRC). The design criteria/requirements vary by manufacturer, but are in the general categories of: service life, criticality safety limit, surface contact dose, dose for storage, storage facility dose limits, ambient environment, fuel cladding temperature limits, maximum decay heat power, concrete temperature limits, canister internal pressure limits and leak tightness, seismic ground accelerations, tornado loads, flooding loads basis, snow and ice loads, cask drops and fuel impact acceleration, and overall shipping package width on transport vehicle. Obviously, the mechanical and physical properties of a heavy concrete (e.g., attenuation of radiation, density, strength, thermal conductivity, etc.) have an impact on how a cask design meets most of these functional requirements. For instance, the Haelsig report calculates how the wall thickness will be greatly reduced using a heavy versus ordinary concrete while still meeting the radiation criteria.4 However, no physical properties are specified for the concrete (other than in-service temperature requirements). The physical properties of a specific grade of concrete need to be

46

Developments in the formulation and reinforcement of concrete

Table 2.2 Maximum temperatures of concrete in storage cask Source

References cited

Normal “bulk” temp.

“Local” longterm temp.

Short-term (accident) temp.

Depleted Uranium Concrete Container Feasibility Study5

ACI-349 Appendix A and NRC Guidance ACI-349 Appendix A

66°C (150°F)

93–149°C (200–300°F)

177–343°C (350–650°F)

≤66°C (≤150°F)

≤93°C (≤200°F)

177(surface)– 343°C (local) (350–650°F)

66°C (150°F)

149°C (local) (300°F)

Pacific Nuclear “Nuhoms” System Sierra Nuclear “VSC” System6

Babcock & Wilcox Fuel Company7

ACI-349 Appendix A and NRC Guidance

121°C (250°F)

coupled with a specific cask design in order to meet the overall functional requirements. Table 2.2 shows the important aspect of temperature requirements for concrete spent fuel storage casks listed by various manufacturers. Pacific Nuclear and Sierra Nuclear appear to interpret NRC guidance somewhat differently. Pacific Nuclear’s criteria distinguish between duration and location (short versus long term, surface versus bulk, and local). Examining Table 2.2, it appears that the long-term maximum exposure (local) temperature for concrete in spent nuclear fuel storage casks should be less than 149°C (300°F). This is fairly consistent with a performance test sponsored by the Electric Power Research Institute (EPRI).8 A relevant standard from the American Cement Institute (ACI) standard 349 Appendix A.4 states: A.4.1- The following temperature limitations are for normal operation or any other long-term period. The temperatures shall not exceed 150°F except for local areas, such as around penetrations, which are allowed to have increased temperatures not to exceed 200°F . . .”

ACI-349 Appendix A.4 was obviously written for conventional concrete using rock aggregate such as quartzite and not for synthetic aggregate. Therefore, to qualify under ACI-349, long-term exposures should be kept under 200°F (93.3°C). Using this standard, concrete fabricated with synthetic aggregate would qualify if the strength did not deteriorate at temperatures from 90°C to 125°C.

High-density and radiation-shielding concrete

2.2

47

Applications/case studies

In the United States, depleted uranium (uranium having 235U content less than natural uranium’s 0.711 wt%) has been generated as tails from uranium enrichment and spent fuel reprocessing. This comprises approximately 500,000 metric tons (uranium content). About 470,000 metric tons are stored as pure DUF6 in steel cylinders at US gaseous diffusion enrichment sites. There is approximately 225,000 metric tons of elemental fluorine associated with this stored DUF6. Intact cylinders normally contain DUF6 with purity exceeding 99.9%. In addition, the UDOE-EM currently owns about 19,500 metric tons of elemental uranium (MTU) in the form of high purity DUO3 resulting from historical weapons production programs at US defense complexes. In 2002, the DOE awarded Uranium Disposition Services, LLC (UDS) a contract to design, build and operate two DUF6 conversion facilities at Paducah, Kentucky and Portsmouth, Ohio. The facilities were designed to convert DUF6 into uranium oxide for disposal (or an alternate use) and aqueous hydrogen fluoride which is to be sold. The facilities were expected to be in full operation by June 2008.9 When the conversion facilities are operational, large quantities of high purity depleted uranium oxide powder will be available. This material could be used as a feedstock to manufacture aggregate suitable for inclusion into radiation shielding heavy concrete.

2.3

The case of DUAGG® and DUCRETE®

In the mid-1990s the Idaho National Laboratory (INL) developed new methods to produce high-density aggregate (synthetic rock) primarily consisting of depleted uranium oxide.10 The objective was to develop a low-cost method whereby depleted uranium oxide powder (UO2, U3O8, or UO3) could be processed to produce high-density aggregate pieces (DUAGG) having physical properties suitable for disposal in low-level radioactive disposal facilities or for use as a component of high-density concrete used as shielding for radioactive materials.11

2.3.1 Fabrication of DUAGG High purity, dense, sintered UO2 and U3O8 pellets were fractured to form aggregate. This aggregate was used with Portland cement and sand to fabricate a heavy concrete. In addition, INL conducted experiments where UO2 and U3O8 powder was sintered using liquid phase sintering techniques

48

Developments in the formulation and reinforcement of concrete

to produce DUAGGTM. The DUAGG was also used as a heavy aggregate in concrete. Figure 2.1 shows a process flow diagram for the production of synthetic DUAGGTM heavy aggregate. The DUAGG consisted of approximately 80% uranium oxide powder pressed and then densified via liquid phase sintering where the liquid was provided by about 20% synthetic basalt. Physical properties of materials: UO3: crystal density – 7.3 g/cm3; bulk density – 3 g/cm3 UO2: crystal density – 10.9 g/cm3; bulk density – 4 g/cm3 Additives: average bulk density – 2.5 g/cm3 DUAGG: approx. green density – 5.4 g/cm3; approx. sintered density – 8.6 g/cm3 Crushed DUAGG: approx. bulk density – 5.2 g/cm3 DUAGG compositions The compositions of several synthetic basalts are listed in Table 2.3. Historically, Composition Number 5 of Table 2.3 has been the formula most frequently used in various waste encapsulation studies at INL. The composi-

UO3 + dirt (weigh)

Ducrete process Clay & additives (weigh)

Waste heat to dryer

Heat

Storage bin Combine?

1.) Rotary calciner gas fired

2.) Grind - attrition mill (wet)

3.) Mixer / Blender

Waste heat to dryer

Size: (comparable to jellybeans)

5.) Agglomerator

Exhaust

Exhaust

Briquettes

Storage bin

Pre heat

Storage bin

6.) Briquettor

8.) Mix concrete intensive shear? pump from bottom?

4.) Dryer

Storage bin

7.) Furnace - 1300 °C dry H2 atmosphere gas or electric? continuous or batch? 9.) Cast concrete forms, etc.

Cement + Sand + Water

2.1 Process flow diagram for production of DUAGGTM heavy aggregate (illustrative).

High-density and radiation-shielding concrete

49

Table 2.3 Chemical analyses of five synthetic basalt melt compositions (wt %) Composition SiO2 No.

Al2O3 FeO

Fe2O3 CaO

MgO Na2O K2O

TiO2

ZrO2

1 2 3 4 5

8.34 8.09 7.87 6.93 5.20

6.62 5.30 7.09 2.76 0.79

3.85 3.76 3.31 3.38 2.77

2.15 2.00 1.34 1.74 1.46

0.59 2.94 6.62 11.4 21.4

0.14 1.24 3.06 4.96 9.91

48.7 46.8 45.9 41.6 32.6

17.1 17.3 15.5 17.0 16.4

9.21 9.74 7.08 7.61 6.70

3.29 2.79 2.20 2.64 2.86

Table 2.4 Average analysis of 224 North American basalts Melt

SiO2

Al2O3

FeO

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

ZrO2

Avg.

49.9

16.0

8.4

2.7

9.7

6.5

2.7

0.8

1.6

1.5

tions can be compared to the average of 224 analyses of natural North American basalts, as shown in Table 2.4. Comparison of Table 2.3 and Table 2.4 shows that some of the INL synthetic basalts are enriched in iron oxides at the expense of the alumina and silica content. In addition, Compositions 4 and 5 have been highly enriched in TiO2 and ZrO2 to promote the formation of the crystalline phases (e.g. zirconolite (CaZrTi2O7), which can incorporate fairly large amounts of uranium oxide into its crystalline structure). Compositions 4 and 5 form liquid phases that can result in a higher degree of crystallinity upon cooling to room temperature than some of the other basalt compositions. Synthetic basalt is made using several precursor chemicals. For initial experiments, precursor chemicals were mixed in given ratios, melted, and cooled. The resultant mass was ground into a fine powder. Later experiments demonstrated lower cost methods to replace the mixing, melting, and grinding steps of producing synthetic basalt. It was discovered that uranium oxide powder could be mixed directly with calcined soil, additive chemicals, and clay and then the DUAGG could be green-pressed and sintered at quite moderate temperatures to good densities. The local INL soil used for the production process is designated Subsurface Disposal Area (SDA) Lake Bed Soil and has the following chemical analysis (in weight percentages): 69.9 SiO2, 13.2 Al2O3, 4.7 FeO, 4.1 CaO, 1.9 MgO, 1.5 Na2O, 3.3 K2O, 0.8 TiO2, 0.11 MnO2, 0.10 BaO, 0.06 ZrO2, 0.06 B2O3, 0.04 NiO, 0.02 SrO, and 0.02 Cr2O3. A typical batch formulation using soil and additives for melt # 5 is given in Table 2.5. Batch formulas can be adjusted for soil of different compositions. Soils available at the INL make excellent starting precursor material because they have a

50

Developments in the formulation and reinforcement of concrete

Table 2.5 Batch formula of “Melt No. 5” basalt using “SDA” soil Chemical component

Number of moles

Batch wt%

SDA soil Al2O3 Fe2O3 CaO MgO NaHCO3 K2CO3·1.5 H2O TiO2 ZrO2

0.5917 0.0003 0.0541 0.0274 0.0192 0.0316 0.0022 0.1733 0.0570

51.36 0.88 11.84 2.11 1.06 3.64 0.50 18.98 9.63

Table 2.6 Favored mill forms for Melt No. 5 basalt using “SDA” soil and “Allen” clay Chemical component

Number of moles

Batch wt%

SDA soil Allen Kaolin clay Fe2O3 CaO MgO NaHCO3 K2CO3·1.5 H2O TiO2 ZrO2 Al2O3

0.4061 0.1889 0.0808 0.0588 0.0353 0.0181 0.0230 0.2440 0.0812 –

36.183 16.901 11.548 2.952 1.274 1.357 3.402 17.434 8.950 –

very fine particle size, are of volcanic origin (a composition similar to basalt), and contain very little organic matter. It was also discovered during DUAGG production that part of the soil can be replaced with clay. The benefits are that clay acts as (1) a lubricant that sticks less to metal pressing dies and (2) a powder binder. Clay greatly aids in the green forming of the aggregate when using mass production methods such as briquetting. A variety of different generic types of clays were investigated for use in the DUAGG production process, including Kaolin, Ball, and Bentonite clays. An example was H-56 clay which was supplied by K-T Clay Company. A similar batch formulation for Melt No. 5 basalt uses “Allen” type Kaolin clay from K-T Clay Company as shown in Table 2.6. The “Allen” clay exhibits a great deal of plasticity and was determined to be a satisfactory choice for inclusion in the mill batches. DUAGG that was pressed and sintered (reducing atmosphere), using the batch formula of Table 2.6, appeared to have good strength and showed

High-density and radiation-shielding concrete

51

nondetectable amounts of uranium during leaching studies. However, during the leaching tests, trace amounts of iron were found to be leaching from the DUAGG. The leaching has no effect on the DUAGG shielding performance. However, if the DUAGG is buried in a landfill, such leaching may eventually lower DUAGG strength. Therefore, a new composition was devised in which extra CaO was substituted for the iron oxide, and boron oxide was added for increased neutron attenuation. This formulation, designated as SRP-5, is shown in Table 2.7. The molar formula is shown in Table 2.8. Sufficient moles of Ti were provided so that it would be theoretically possible to form a stoichiometric amount of CaZrTi2O7 (zirconolite) if the Ti atoms reacted with all of the Zr and a portion of the Ca with sufficient Ti remaining to react with the remaining Ca to form CaTiO3. Boron was added in an amount equivalent to about 1 wt%. Table 2.7 Boron-added DUAGGTM compositiona Sample no.

UO2 content vol. %

Type of clay additive

Description of mill additions

Anticipated bulk density (g/cm3)

Anticipated apparent density (g/cm3)

Anticipated open porosity %

SRP-5

80

Allen

Boron-no iron

>8.07

>8.46

<1.00

Note: SRP indicates use of UO3 starting powder from the Savannah River Site. a Many of the compounds found in the soil are also found in basalt compositions. Therefore, batches can be formulated that combine soil with the proper amount of makeup compounds – sometimes called mill additions, additives, etc. – to form a basalt upon heating. The dirt and compounds melt and form in situ the liquid phase that is responsible for liquid phase sintering of the uranium oxide powder.

Table 2.8 Molar formula for boron-added synthetic basalt Constituent

Moles

SiO2 Al2O3 Fe2O3 CaO B2O3 MgO K2O Na2O TiO2 ZrO2

0.5942 0.0996 0.0190 0.1668 0.0669 0.0430 0.0265 0.0127 0.2423 0.0812

52

Developments in the formulation and reinforcement of concrete

DUAGG fabrication process

1

20

0

0 DTA

–1

–20

–2

–40

–3

–60 TGA

–4

SR UO3 Yellow cake

–5 0

100

200

–80

300

400 500 600 700 Temperature (deg.C)

2.2 Thermal analysis of as-received UO3 powder.

800

–100 900 1000

Delta-T (microvolts)

Delta-W (percent)

Depleted uranium oxide feedstock (UO3) typically was received in 55-gal drums that weighed about 1300 lb. The powder had been stored for a considerable amount of time and therefore was heavily hydrated. It was assumed that the as-received (hydrated) UO3 powder would be highly agglomerated with the existence of both “soft” and “hard” agglomerates. The particle sizes were measured from a submicron size to more than 100 μ. Particles of larger sizes are assumed to be agglomerates of the small single crystallites. A plot for thermogravimetric analysis (TGA) and differential thermal analysis (DTA) for heating as-received powder is shown in Fig. 2.2. The loss of water is continual upon heating from room temperature to more than 700°C. Both physically adsorbed water (that which is desorbed at a temperature less than 200°C) and chemically bonded (hydrated) water combine for a total weight loss of 5 wt%. After high temperature calcination of as-received UO3 powder, a combined TGA/DTA plot during heating (more specifically, re-calcining) shows that the weight loss is almost completely eliminated (Fig. 2.3). However, a small DTA peak occurs at 890°C, possibly associated with conversion of UO3 to U3O8. After calcination, at more than 900°C, the UO3 changes in color from yellow to dark gray. The overall composition calls for 80 vol% UO3 (converted to approximately 95 vol% UO2 during sintering in a reducing atmosphere) and 20 vol% artificial basalt. Because the UO3 has a density of 8.01 g/cm3 and the basalt has density of about 3.0 g/cm3, the overall composition can be

High-density and radiation-shielding concrete 0.5

53 5

TGA

Delta-W (percent)

DTA –5

–0.5

–1

–1.5

–2 0

–10

–15

SR UO3 Calcined 100

200

Delta-T (microvolts)

0

0

300

400 500 600 700 Temperature (deg.C)

800

–20 900 1000

2.3 Thermal analysis of UO3 powder after calcination. Table 2.9 Mill formula for boron-modified DUAGGTM Chemical component

Batch wt%

SDA soil Allen Kaolin clay Fe2O3 CaO MgO Na2CO3 K2CO3 TiO2 ZrO2 B2O3

36.00 16.82 – 6.97 1.27 0.85 2.84 17.34 8.91 9.00

calculated to be approximately 91.4 wt% UO3 and 8.6 wt% artificial basalt. The batch formula for the boron-modified composition is shown in Table 2.9. Using the formula to fabricate aggregate, the sintering temperature could be slightly lowered and the product proved to be leach resistant for uranium and iron ions. The resultant microstructure showed increased interaction of the liquid phase with the uranium-oxide grains, causing rounding of the grains. The DUAGG appeared to be quite strong, but its fracture behavior tended to be more glasslike than that of DUAGG fabricated from the original batch formula of Table 2.5 (Basalt No. 5). Wet attrition milling was originally chosen as a very efficient method for grinding the UOx powder and mixing in the added powders (i.e., calcined

54

Developments in the formulation and reinforcement of concrete

and ground soil, clay, and calcined mill additions). When compared to vibratory and conventional ball milling, attrition milling is capable of producing the finest product for a fixed specific energy input. However, dry milling was also utilized in later productions with good effect. The powder sticking problem during dry milling was overcome through the use of a lubricant. The “dry” lubricant coats the surface of the powder particles and helps to avoid severe agglomeration, but generally must be diluted and added through the use of an appropriate liquid solvent. Small levels (less than 0.5 wt%) of the organic milling aid proved to be effective. Because UO3 based mixes have a high density, the recommended amount of organic milling aid was in the range 0.1 to 0.3 wt%. The processing advantages of dry milling for process simplification (i.e., the reduction of subsequent drying and low temperature calcining steps) and dispersion of milling aids (e.g., dispersing agent) should not be overlooked, and may prove sufficiently attractive for large production to outweigh the advantages of wet milling while alleviating some of the problems caused by wet milling (e.g., hydration). After drying, but before the calcination step, the cake was broken up as much as possible to help prevent the formation of hard lumps. If the cake was broken into a fine powder, a subsequent de-lumping step (after low-temperature calcination) was not necessary. The powder was then transferred into the calcining oven (air atmosphere) and the temperature was increased to 400°C. The temperature is maintained at 400°C for onehalf hour. This temperature was sufficient to remove about 80% of the chemically absorbed water on the UOx without removing the hydrated waters from the kaolin clay. The binder selected for our initial tests was “Flamsperse,”a primarily on the basis of low cost and has been known to be used in briquetting. “Flamsperse” is calcium lignosulfonate (a byproduct of the wood pulp industry). Later, other binders were also utilized, including clays of the montmorillonite group (composition of (Mg, Ca)O. Al2O3.5SiO2.H2O; e.g. bentonite). The use of bentonite is classified as a “matrix” type agglomeration binder and, therefore, can be used in conjunction with “film” type organic binders. Excellent binders are acrylic emulsion binders like Duramax B-1020 available from the Rohm and Haas Company.b Although much more expensive than Flamsperse, these binders are water soluble, have greater binding power, and (most importantly) decompose by depolymerization (“unzipping”) without requiring oxygen during binder burnout. Therefore, the binders burn out completely with no residue when using reducing atmospheres like nitrogen or hydrogen in argon. a

Flambeau Paper Corporation, Phone (715) 762-5235, Park Falls, Wisconsin 54552. Rohm and Haas Company, Independence Mall West, Philadelphia, Pennsylvania 19105.

b

High-density and radiation-shielding concrete

55

For developmental work, samples of mixed powders were dry-pressed in a single-acting metal die, by hand using a uni-axial hydraulic press, to make disks (0.875 in. diameter and approximately 0.375 in. high). Pressures of about 17,000 to 18,000 psi were used. Later automatic briquetting was utilized. For developmental tests, the disks were fired (sintered) in a tube furnace using reducing (4% hydrogen in argon) atmosphere to maintain UO2 as the primary phase. The batch firing rate was 10°/minute with a 2-hour hold at 700°C and 3-hour hold at 900°C and a 1-hour hold at 1100°C. The rate was increased to a rate of 15°/minute until 1300°C, where the temperature was maintained for one-half hour and then the furnace cooled at its natural rate. About 10 kg of DUAGG was produced by hand pressing at INL.12 The process was then transferred to industry13 for improvements and development of a pilot process14 at Starmet CMI, Barnwell, S.C. that included a continuous firing “pusher” kiln. DUAGG properties DUAGG pellets were produced at Starmet CMI using either UO2 or U3O8 powder as the starting feedstock. All pellets were hand pressed at 10,000 psi, and sintered in forming gas (H2 + N2) for 30 minutes at 1250°C. The DUAGG densities of three pellets are described in Table 2.10. Mounted and polished specimens were examined in the SEM as shown in Figs 2.4–2.6. Their microstructure features and chemistry are discussed below. There were four major phases present in varying proportions in the three pellets examined. These phases are described below: (1)

Uranium oxide phase The uranium oxide phase was determined by X-ray diffraction to be predominantly UO2 for all samples. The UO2 appears in all specimens as rounded particles (evidence of liquid phase sintering) and measured approximately 1 to 5 μ. In pellets that started with UO2 there is a small amount of spherical porosity internal to the UO2 particles. The UO2 particles appear to form a semi-continuous phase, with some necking between adjacent particles indicating coalescence.

Table 2.10 DUAGGTM densities ID

Description

% Theoretical density

% Apparent porosity

#2 #3 #4

Batch 2 – UO2 Batch 3 – U3O8 Batch 1 – UO2

98.0 93.9 93.3

0.09 2.33 2.22

56

Developments in the formulation and reinforcement of concrete

2.4 Backscattered electron image of fracture surface of pellet #3. The arrow highlights the U-Ti-oxide phase. The light rounded particles are UO2, while the black areas surrounding the UO2 is the silicate phase, which was molten during sintering.

2.5 Backscattered electron image of polished cross-section of pellet #2. The area depicted by the marker 1 is the Ti-oxide phase. The black appearing silicate phase can be seen surrounding both the UO2 and the Ti-oxide particles.

(2)

Silicate phase The silicate phase appears to wet the U oxide phase and fills space between all other phases, again indicating liquid phase sintering. The silicate phase contains a mixture of Si, Al, Ca, Ti, K, Na, Zr, Fe, and Mg. The approximate atomic ratio of these elements (from quantita-

High-density and radiation-shielding concrete

57

2.6 Backscattered electron image of the original surface of pellet #3. Note the needle-like Ti-oxide phase interspersed with the rounded UO2 particles.

tive EDS – not counting oxygen) was: 54Si, 15Al, 15Ca, 5Ti, 5K, 2Na, 2Zr, 1Mg, 0.5Fe. Analyses suggested a fairly homogeneous, crystalline silicate phase. (3) U-Ti-oxide phase The mixed uranium titanium phase is present in greatest quantity in pellet #3 (where starting material was U3O8) and essentially absent in pellet #2 (pre-reduced UO2). It is found in small quantities in pellet #4 (pre-reduced UO2 with higher percent of additives in the mix). This phase has a predominantly plate or needle-like morphology. The UTi-oxide phase is highlighted by an arrow in Fig. 2.4. This phase was also observed in other DUAGGTM samples made from U3O8 as the starting material. This phase did not contain Ca, and it was the only U-containing phase other than the uranium oxide. This result suggests that the U is more chemically active during sintering when it is reduced in-situ from U3O8 to UO2. (4) Ti-oxide phase This phase contains small amounts of Mg, Fe, and Zr, but no Ca or U. This phase is present as blocky or needle-like particles in large quantity in pellet #2, as shown in Fig. 2.5. Internally it is present in smaller amounts in pellet #3 and pellet #4. There is also a tendency for this phase to segregate in larger quantities to the DUAGGTM surface for all samples, as shown in Fig. 2.6. Later, Starmet produced a short run of DUAGGTM in their pilot plant. A photograph of the sintered DUAGG briquettes is shown in Fig. 2.7. A US penny (left) and

58

Developments in the formulation and reinforcement of concrete

2.7 Photograph of sintered DUAGGTM briquettes produced by Starmet.

pre-sintered briquette (right) are shown at the bottom right for comparison.

2.3.2 Fabrication of DUCRETE® Proportioning, mixing, and placing INL demonstrated that dense depleted-uranium oxide (UO2 and U3O8) pellets could be used as large aggregate to replace conventional gravel in Portland cement based concrete15 (DUCRETETM). The strength value for this DUCRETETM was in the range of typical construction grade concrete for an equivalent curing time (7 days). From a mixing and casting perspective, the 1 : 1.5 : 14 (cement : sand : UO2, by weight) mixture contained as high a UO2 aggregate loading as could be recommended for casting large structures. This mixture only showed a 7.3% reduction in compressive strength compared to the volume equivalent gravel aggregate concrete. Visual examinations showed no deleterious interactions between the UO2 pellet aggregate and the cement/sand/water matrix when cured for 7 days at room temperature. Analysis of data showed the possibility of a slight degradation in compressive strength for temperatures between 90°C (194°F) and 150°C (301°F) for times up to 28 days. The mean tensile strengths were increased by exposure to temperatures up to 150°C (301°F).

High-density and radiation-shielding concrete

59

Reference concrete fabricated with gravel aggregate showed a 1291 psi decrease in mean compressive strength with exposure to excessive temperatures of 250°C (482°F) while volume equivalent samples fabricated using depleted uranium oxide aggregate either cracked or completed disintegrated when exposed to 250°C (482°F). This temperature is 100°C (212°F) higher than expected for anticipated application of DUCRETETM for spent nuclear fuel storage or shipping. Detailed microscopic analysis showed that the surface of both the UO2 and U3O8 large aggregate, contained in Portland cement, severely cracked (spalled) when exposed to moist air at 250°C (482°F) for 14 days. Since one “bare” piece of U3O8 disintegrated after exposure to 250°C (457°F), there is evidence of physical/chemical changes within the piece severe enough to cause stresses large enough to cause fracture throughout the body. Subsequently, the root-cause of the cracking of the heavy concrete made with pure UO2 or U3O8 aggregate during exposure to high temperature, moist atmospheres was determined. Careful SEM analysis produced evidence to support the conclusion that oxygen diffuses from the cement through the surface into the volume of the dense uranium oxide aggregate.16 This increases the oxygen to uranium (O/U) ratio, particularly at the surface of the aggregate. The concurrent decrease in crystal density in the affected region causes a volume increase that leads to an outward expansion of the aggregate. This outward expansion results in a build-up of tensile stresses within the cement/sand cement matrix. Eventually these stresses lead to fracture of the Portland cement that is bonding the sand and large aggregate together. Therefore, a liquid-phase sintered DUAGGTM was developed (as described above). The idea was that during sintering, the liquid phase basalt would surround the uranium oxide grains and provide a barrier to oxidizing gases (oxygen or water vapor) from the atmosphere or pore-liquid from the Portland cement. This barrier would greatly reduce the amount of oxygen uptake at the surface of the aggregate and thus curtail expansion of the surface and subsequent cracking of Portland cement.17 Subsequent development work over a number of years has demonstrated that this technology succeeded far beyond initial expectations. Early INL results showed that sintered DUAGGTM had a bulk density somewhere between 8.50 g/cc and 8.75 g/cc. Other INL work indicated a cement : sand : coarse-aggregate ratio of 1 : 1.5 : 14, giving DUCRETETM good workability and casting characteristics when using dense UO2 aggregate. This batch ratio of DUCRETETM, using a 8.50 g/cc coarse DUAGG, yielded a final density of 6.65 g/cc (415 lb/ft3). This can be compared to an equivalent concrete made with quartzite coarse aggregate that had a final density of 2.47 g/cc (154 lb/ft3). If the DUAGGTM has a bulk density of

60

Developments in the formulation and reinforcement of concrete

8.75 g/cc, the final density will increase to 6.79 g/cc (424 lb/ft3). Using volume ratios, early work projected that the optimum cement : sand : coarseDUAGG ratio might be about 1 : 1.5 : 11.6. Larger DUCRETETM batches were further developed at Starmet where the heavyweight batches proved to overload ordinary mixing equipment. This brings up the possibility of using pre-placed aggregate where a water/ cement/sand slurry could be pumped through the aggregate from the bottom.18 The use of fine aggregate DUAGG (sand) to replace quartzite sand might be able to be used in order to boost the overall concrete density. However, substitution of a hydrated sand such as Colemanite (Ca2B6O11 . 5H2O) would provide additional neutron attenuation and probably result in better overall radiation shielding (a thinner wall thickness – see next section). Figure 2.8 shows a block of DUCRETETM that was cast at Starmet and then sawed down the middle. Figure 2.9 shows a 30 gallon “Over Pack” Shielding device that was fabricated at Starmet and the interior filled with DUCRETETM prior to closure welding. Physical properties Table 2.11 shows the density and compressive strength of some experimental samples of DUCRETETM briquettes that were fabricated (formed using roll compaction) at Starmet using fly ash, microsilica, and metal fiber

2.8 DUCRETETM blocks fabricated by Starmet.

High-density and radiation-shielding concrete

61

2.9 DUCRETETM 30 gallon “Over Pack” shielding fabricated at Starmet.

Table 2.11 Density and strength of DUCRETETM fabricated at Starmet Sample no.

Measured density (lb/ft3)

Strength (psi)

DUAGG densitya (g/cm3)

Small fines

6 7 8 13 14

5.66 5.72 5.86 4.81 4.73

5101 4310 4430 3880 4390

8.1b 8.1c 8.1c 7 7

Fly ash Fly ash None Micro silica Micro silica

a

DUAGG briquettes are crushed and screened to yield American Concrete Institute No. 8 size fraction. b DUAGG density of 8.6 gm/cm3 has been made in laboratory settings at INL. This density aggregate will produce a DUCRETE density of nearly 6.4 g/cm3. c Sample contained about 0.36 wt% metal fibers to increase DUCRETE flexural strength.

additives.19 It was expected that further adjustments in the pilot plant would have increased the density of the DUAGGTM. Radiation-shielding properties The high density of DUCRETETM (e.g., 5.87 g/cm3 versus 2.24 g/cm3 for normal concrete) results in a superior shielding material for spent fuel and high-level waste applications where the source radiation is composed of both gamma and neutron flux. Figure 2.10 shows the results of calculations20

62

Developments in the formulation and reinforcement of concrete Wall thickness (in.) 30 Neutrons

Gamma

25 20 15 10 5 0

DUCRETE

Uranium metal

Lead metal

Stainless steel

Concrete

Dr. J. Sterbentz, INEEL Comparison of wall thickness to attenuate neutron and gamma dose from 24 PWR spent fuel assemblies

2.10 Calculation result showing DUCRETETM can give equivalent shielding performance to other materials, but at a reduced wall thickness.

comparing the thickness of DUCRETETM compared to normal concrete and other materials to shield the combined neutron and gamma dose from a certain amount of spent fuel. If desired, the ratio of aggregate to the cement phase fraction in the DUCRETETM mixture can be varied to adjust the gamma and neutron attenuation characteristics to an overall optimum. At higher DUAGGTM volume fraction, the external dose is driven by the neutron leakage because there is too little water in the cement matrix. At lower DUAGGTM volume fraction, the neutron leakage is minimized but the external dose is dominated by gamma leakage. Another calculation21 provides comparative shielding results for a certain waste package using DUCRETETM, magnetite concrete, steel, or a steelpolyethylene composite as shown in Fig. 2.11. This figure again emphasizes the benefit of having both low Z and high Z material in the shielding. The 100% steel shield has no neutron moderator and thus the external radiation dose is dominated by neutrons. Figure 2.12 shows the results of actual measurements14 using a 60Co gamma source where the half value layer thickness (HVL) for three densities of DUCRETETM are compared to ordinary concrete and other materials. For steel cask systems, separate neutron shields containing hydrogenous

High-density and radiation-shielding concrete Magnetite C.

Composite

Carbon steel

63

DUCRETE

1.00E+03 1.00E+02

Dose rate (rem/h)

1.00E+01 1.00E+00 1.00E-01 1.00E-02 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07

10

0

20

30

40

50

60

70

80

90

100

110

Shield thickness (cm)

2.11 Contact dose rate vs. shield thickness for 21-PWR WP with bounding SNF.

Half value thickness (cm)

6 5 Concrete Magnetite

4

DUCRETE Steel Lead Uranium

3 2 1 0

0

5

10

Material density

15

20

(g/cm3)

2.12 Half value layer thickness for DUCRETETM shielding compared to other traditional shielding materials at 1.25 MeV.

material are added since the large thickness of steel needed to attenuate neutrons is impractical. A principal objective at INL was to produce DUAGGTM with acceptable physical, mechanical and chemical properties such that it could be used to produce DUCRETETM storage containers for spent nuclear fuel. In particular, depleted Uranium Aggregate’s (DUAGG’s) chemical composition was optimized with respect to several technical parameters:

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Developments in the formulation and reinforcement of concrete

(1) density, (2) microstructure (fine grained with minimum of large porosity), (3) chemical stability as the aggregate in concrete, (4) leach resistance (primarily for uranium but also any other elements that might cause degradation of mechanical properties or could be coupled to uranium leaching), (5) neutron and gamma ray attenuation when the DUAGG is incorporated into Portland cement to form a concrete (DUCRETETM), and (6) cost. Therefore, radiation shielding efficiency is only one factor in making a suitable shielding material. Really the best way to compare the total effectiveness of a shielding system would be to compare the final cost to the customer for cask systems having the same performance features. Such data are difficult to obtain because of proprietary business considerations and, for DUCRETETM, no large casks have yet been fabricated or licensed. Lacking such data, Quapp14 compared the relative unit material costs and general fabrication costs per pound of material. It was estimated (at that time) that installed concrete costs about $0.12 per kg, steel costs about $1.10 per kg, DUCRETETM considerably less than $2.00 per kg, metallic lead about $1.65 per kg, and depleted uranium metal about $22 per kg. While fabrication cost for concrete and DUCRETETM are low (forming and pouring), fabrication cost for the metals are considerably higher (forging, rolling, welding, machining, etc.). In addition, metal shielding systems require additional separate neutron shields adding to the overall cask cost. Consequently, the overall cost effectiveness of DUCRETETM was estimated to be similar to concrete on a performance adjusted basis and considerably less than steel for storage applications. Corrosion resistance results Table 2.12 shows the strength test results measured at INL on DUCRETE (1 : 1.5 : 14 weight ratio of cement : sand : DUAGG) versus ordinary concrete (1: 1.5 : 3.5 weight ratio of cement : sand : gravel). A water/cement ratio of slightly over 0.4 was used in conjunction with a “superplasticizer” (Mighty 150). The DUAGG was −1/2″ to +4 mesh and the gravel was −3/8″ to +4 mesh in size. The sand was clean and sieved to between −7 and +50 mesh. Ordinary Type I Portland cement was used. After casting and curing for seven days in water (ASTM standard C192 – Method of Making and Curing Concrete Test Specimens in the Laboratory), the compression test cylinders were exposed to moist flowing air in an environmental chamber. The air bubbled through water held at room temperature prior to introduction into the heated chamber. After exposure, sulfur end caps were cast onto the 2″ dia. × 4″ high compression specimens according to ASTM standards C192 and C617. The DUCRETETM and

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Table 2.12 Compressive strengths of concrete and DUCRETETM after high temperature exposures in unsaturated steam Sample ID

Aggregate type

Aging temp. (°C)

Aging time (days)

Compressive strength (psi)

OST-1 OST-2 ORT-1 ORT-2 OST-3 OST-4 ORT-3 ORT-4 OST-5 OST-6 OST-7 ORT-5 ORT-6 OST-9 OST-10 ORT-8 OST-11 OST-12 ORT-9

Gravel Gravel DUAGG DUAGG Gravel Gravel DUAGG DUAGG Gravel Gravel Gravel DUAGG DUAGG Gravel Gravel DUAGG Gravel Gravel DUAGG

Baseline Baseline Baseline Baseline 100 100 100 100 150 150 150 150 150 250 250 250 250 250 250

Baseline Baseline Baseline Baseline 28 28 28 28 14 28 28 28 28 14 14 14 28 28 28

3899 4535 3500 4790 5033 4239 6007 5399 3700 5193 6998 4659 3883 1655 3026 2911 2349 1545 2084

Average strength (psi) 4217 4145 4636 5703 3700 6096 4271 2341 2911 1947 2084

concrete cylinders were broken using an Instron testing machine according to ASTM C39-72 which utilizes a spherical bearing block to minimize shear stresses at the end caps. The results show that DUCRETETM is as strong as an otherwise equivalent concrete made with gravel aggregate. For exposure temperatures of 100°C and 150°C (for times up to 28 days), the average strengths for the gravel concrete and DUCRETETM did not decrease. In some cases the strengths actually increased. When compared to the baseline sample, the compressive strengths of the DUCRETETM samples did not decrease after exposure to 250°C for times up to 28 days (none of the DUCRETETM samples showed any visual evidence of cracking and they were integral prior to strength testing). Clearly the DUAGGTM was chemically stabilized toward oxidation at temperatures up to 250°C in moist air. Table 2.13 shows some leaching results using the US EPA 1311 test (Toxicity Characterization Leaching Procedure). Other leach tests22 could also have been utilized. Greatly reduced leaching was observed for DUAGGTM when compared to UO3, U3O8, or UO2. Even more strenuous leach testing of DUAGGTM, fabricated at Starmet, has been performed by the Oak Ridge National Laboratory (ORNL).23 These accelerated exposure studies used ASTM C289-94 Standard Test Method where at a consistent surface-to-liquid ratio of 1 : 10 the sintered

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Developments in the formulation and reinforcement of concrete Table 2.13 Leach results using TCLP test for different forms of uranium Uranium form

Concentration in leachate (mg-U/liter)

DUAGG UO2 U3O8 UF4 UO3

4 170 420 7367 6900

The UO3 is from the DOE Savannah River Site and was recovered from reprocessing. The U3O8 and DUAGG were manufactured at Starmet CMI from SRS UO3. The UF4 was converted from UF6.

DUAGGTM samples were exposed to (1) distilled water, (2) 1 N sodium hydroxide solution, and (3) saturated water extract of high alkali cement. There were three exposure temperatures and six exposure times: 25, 66, and 150°C at intervals of 30, 60, 90, 180, 360, and 730 days. The release rate for DUAGGTM was only 0.25 mg/(m2 day) after 1 month at 66°C in deionized water and about 0.40 mg/(m2 day) after 13 months in cement pore solution. By comparison, a reference UO2 had previously shown a release rate of 5 mg/(m2 day) after 8 days at 70°C in de-ionized water. After >24 months of exposure the release rate of uranium in a cement pore solution was low and no deleterious products from the alkaliaggregate reaction were seen. Evidence was later presented by ORNL24 that a protective layer of re-crystallization products from the basalt phase of the DUAGGTM covers the surface and this slows the release of uranium. These precipitated phases are due to the high silica content of the basalt, and are thought to include: Schoepite (UO3 . 2H2O), Soddyite (U5Si2O19 . 6H2O), and Uranophane (Ca[UO2]2[SiO3OH]2 . 5H2O).

2.4

Future trends

The largest applications for heavy concrete shielding are for spent nuclear fuel (SNF) and high level waste (HLW) containers. Most current shielding systems for SNF or vitrified HLW use either steel or concretec because of their relatively low cost, wide availability, known fabrication characteristics, and radiation-shielding effectiveness. For steel cask systems, separate neutron shields typically containing hydrogenous material are added since the thickness of steel required to attenuate neutrons to c

All concrete systems use some steel for structural and/or thermal purposes but the predominant material for shielding is concrete.

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acceptable external doses is impractical. While ordinary concrete is not a particularly good gamma shield (per unit thickness), it is cheap enough that the practical approach is to use copious volumes. In sufficient thickness, it shields both the neutron and gamma radiation source terms effectively. On the straight basis of aggregate cost it is impossible to compare DUAGGTM with gravel, since gravel is inexpensive having been mined and washed rather than fabricated at a high temperature like DUAGGTM. However, there are overall economic factors that need to be considered. These include some environmental advantages to using the huge stockpiles of depleted uranium oxide (converted from corrosive UF6) for some useful purpose rather than burial.25 More importantly, it would seem that the key to gaining the full economic benefit and acceptance of DUCRETETM is to devise a combination storage and transportation cask which uses DUCRETETM as the shielding.26 An empty DUCRETETM cask can be transported via rail-freight from the manufacturing facility to a storage site and ultimately (after filling) to a repository or disposal location. This feature also provides an economic advantage as most concrete storage systems are not transportable. New concrete shielding must be built at each location where they are used. This transportable feature assures that if the United States opens an interim spent fuel storage facility at Yucca Mountain or at private sites, that the DUCRETETM shield can be transported to that site and re-used. There is a precedent for such a system in the GNB CONSTOR Cask27 (see Fig. 2.13) which uses high density concrete (4.1 g/cm3) combined with steel to provide a low cost system for transportation and storage of spent nuclear fuel.

2.4.1 Casks for spent nuclear fuel Shipping casks Transportability is a necessary design feature for the use of DUCRETETM casks at storage sites and then for their reuse as part of the repository waste package or at LLW disposal sites. The high density of DUCRETE allows the cask to be considerably smaller in diameter and remarkably lighter than conventional concrete casks. This is illustrated in Fig. 2.14. The DUCRETETM cask is 35 tons lighter and 100 cm smaller in diameter than the Sierra Nuclear Cask (SNC Model VSC) ventilated storage cask manufactured using ordinary concrete.28

2.4.2 Repository overpacks Figure 2.15 shows one particular concept developed by Duke Power for a DUCRETETM high level waste (HLW) storage container or “overpack”.

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Secondary lid (welded) Sealing plate (welded) Primary lid bolted Trunnion Fuel assembly Outer liner Inner liner Reinforcement Basket Heavy concrete

2.13 GNB CONSTOR combination storage and shipping cask (uses a heavy-concrete liner).

X-section of concrete cask (92 ft2)

Air outlet

SNF fuel assemblies

X-section of DUCRETE cask (44 ft2)

Cask lid

Multipurpose sealed basket (shell wall) Concrete cask liner Concrete

Air inlet duct Air entrance

2.14 Sierra nuclear ventilated spent-fuel dry storage cask made with concrete compared to one made with DUCRETETM.

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DUCRETETM Overpack System MPC ( 3 8 in.)

24 in.

13 in.

61 in. MPC ( 3 8 in.) canister

147 in.

120 in.

½ in. Steel shell

DUCRETE TM concrete (12 in.)

90 in.

DUCRETETM thickness (in.)

Surface dose rate (mrem/h)

Dose rate @ 2 (mrem/h)

10 11 12 13 14 15 16

41 27 19 15 12 9 8

10 6 4 3 2 2 1

2.15 Drawing of a high level waste (HLW) container made from DUCRETETM – also, shielding effectiveness vs. wall thickness.

This container holds four each of sealed HLW canisters. High level waste is much different than spent nuclear fuel and must be packaged differently. The surface dose rate (mrem/hr) drops off rapidly with DUCRETE wall thickness. Figure 2.16 shows a schematic of waste container stored in an underground repository. Provisions are made for access roads and gantry cranes for moving and positioning the waste containers.

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Developments in the formulation and reinforcement of concrete

Gantry crane Concrete liner Access road

Rock

SG98 0051a

2.16 Schematic of waste containers stored in an underground repository.

2.4.3 Russian RBMK heavy concrete spent fuel casks Russia has approximately the same amount of stored depleted uranium as does the United States. Russia also has the need to transport and store spent nuclear fuel from various reactors. They have already adapted a metal/ heavy-concrete dry storage and transportation cask (GNB CONSTOR type – see Fig. 2.13) for spent fuel from their RBMK-type reactors. This cask contains heavy concrete utilizing steel shot and barium sulfate. Several years ago Oak Ridge National Laboratory (ORNL), Teton Technologies, General Nuclear Services, Inc. (GNSI) and VNIIEF of Russia joined together to collaborate on DUCRETETM cask design and fabrication.29 Both ORNL (Y-12) and VNIIEF were to produce 100–200 kg of DUAGGTM for testing. GNSI was to perform chemical testing and optimization of DUAGGTM and GNSI was to handle cask design and modeling tasks. Some additional work was to be performed by a Russian team30 on micro-reinforcement (metal and/or polymeric fibers) of concrete to increase fracture toughness and therefore the ability to absorb energy (from a rocket attack or airplane crash). The lead institution was the Russian Federal Nuclear Center – All Russia Science and Research Institute of Experimental Physics (RFNC-VNIIEF). The RFNC-VNIIEF is located in Sarov, Nizhniy Novgorod Oblast, Russia. This city was originally established in 1946 as Arzamas-16 as the location of the Soviet Union’s nuclear weapons research and development facilities. In 1994, Arzamas-16 was renamed Kremlev and in 1995 President Yeltsin officially changed the city’s name to Sarov (its original historical name). Sarov is sometimes called the “Russian Los Alamos”, but the RFNC-

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VNIIEF also performs research in many other areas including materials science and mechanical engineering. Future production of DUAGGTM A report31 as of July 2005 indicated that RFNC-VNIIEF had made significant progress in improving and simplifying a DUAGGTM formulation and its production and had made DU concrete with high compressive strength and high density (6.4 g/cm3). Work was also ongoing on UO2 – stainless steel cermets with large (1 mm dia.) UO2 particulates mixed with stainless steel in about a 50 vol% ratio. For the last couple of years ORNL has not been funded for participation in the Russian collaboration. It is thought that the RFNC-VNIIEF is going forward with plans to produce casks. However, it is not clear if they will produce the UO2 – stainless steel cermet or large quantities of DUAGGTM to produce their version of DUCRETETM. Also, it is not clear what might be the source of funding for such developments.

2.4.4 Shields The use of heavy concrete (probably “toughened”) for shielding may have a future due to the proliferation of terrorist attacks on various structures. Toughened DUCRETETM A toughened version of DUCRETETM is thought to be necessary for applications where enhanced mechanical performance is desired like thin-walled (2 inch thick) waste containers, the corners/edges of large casks, for casks with thin (or no) metal walls, casks “hardened” against terrorist attacks, or for blast shields. Spent nuclear fuel sitting in certain dry-storage casks in the yards of nuclear power plants are thought to be vulnerable to terrorist attacks. The US Army Engineer Waterways Experiment Station (WES) supports the US Army Corps of Engineers mission in Survivability and Protective Structure using the Geodynamics Research Facility and the Projectile Penetration Research Facility. They have worked on high density and toughened concrete,32 sometimes in conjunction with Sandia National Labs on terminal effects of projectiles.33,34 Many additives and fabrication techniques are known to increase the compressive strength of concrete; however, for spent fuel and high level waste containers the critical design criteria are related to impact and dropping. In these cases, the relevant property is not compressive strength,

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but: (1) flexural strength (maximum stress at failure under bending), (2) impact resistance, (3) energy absorption, and (4) fracture toughness. These properties of concrete can be increased through the use of fibrous reinforcement. For shielding applications, steel fibers have the advantages (over plastic fibers) of holding stresses while under load and greatly increasing the tensile/bend (flexural) fracture strength of the concrete and moderately increasing the compression strength (likely due to shear effects). The level of property improvement gained in fibrous concrete is dependent on a number of factors including: (a) concrete mix, (b) fiber aspect ratio (length/diameter), (c) fiber strength, (d) fiber volume fraction, (e) concrete age, (f) fiber/matrix bond, (g) fiber modulus of elasticity (stiffness). Steel fibers include the categories of spun fibers and wires. The stainless steel versions of the wires are very corrosion resistant. Larger diameter steel fibers (wires) can be added to concrete with large aggregate (e.g., no larger than 3/8″ diameter) while smaller aggregate is necessary when using chopped fibers. The larger aggregate sizes will usually accommodate less fiber per cubic yard. Metal mats (loosely woven long continuous fibers) can only be filled with cement grout (small aggregate/sand or cement only) since small particles are necessary to penetrate between the continuous fibers in the mat. An example of the use of metal mats is the SIMCON (slurry infiltrated mat concrete) fabricated using MmatTEC promoted by Ribbon Technology Corp. In tests comparing the mat with normal fiber reinforcement, the mat materials offer a substantial improvement in flexural strength (increasing from 300 psi for ordinary concrete to 3000–6000 psi for SIMCON) and also large increases in fracture toughness (measured as area under load vs. deflection curve). Ribbon Technology Corp. has accumulated a large amount of fracture strength and toughening data using ordinary Portland cement mixes with various metal fibers and metal mats.35 The advantage of using metal mats is that a lower volume fraction of metal is needed when compared to chopped metal fibers in order to achieve the same level of toughening, plus the mat can be placed in specific volumes of a cast shape (e.g., edges or outside) in order to give the desired effect at a lower cost. Of course, polymeric fibers can also be added to concrete or DUCRETETM to provide toughening.36 All types of polymers have been utilized in concrete: polypropylene, polyethylene, polyester, acrylic, aramid, etc. Since the polymers have hydrogen atoms as part of their long chains, they also could help attenuate neutrons in DUCRETETM used as shielding. However, some concern has been expressed over polymeric fibers’ longterm stability in spent nuclear fuel casks due to temperature and radiation damage.

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73

Sources of further information and advice

Below is a list of laboratories or companies that have been involved with research and development of DUAGGTM and DUCRETETM and companies that manufacture dry storage casks. Some cask manufacturing companies utilize concrete-liners and others do not. An example of a dry cask “system” manufactured by Holtec International is shown in Fig. 2.17. Figures 2.18 and 2.19 show an Energy Solutions Model VSC-24 (concrete lined cask) being loaded, in-service, and being shipped via rail inside a special Model TS125 transportation cask.

2.17 A “Holtec” dry storage case being set in place on a “pad”.

2.18 Energy Solutions VSC-24 cask being loaded (left) and in-service on pad at a nuclear plant (right).

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Developments in the formulation and reinforcement of concrete

2.19 Energy Solutions VSC-24 cask will be shipped inside an Excellox6 transportation cask Model TS125 via rail car.

2.5.1 US national laboratories Idaho National Laboratory Paul A. Lessing, PhD Idaho National Laboratory Materials Department MS-2218 Box 1625 Idaho Falls, Idaho 83415-2218, USA Tel: 208-26-8776 E-mail: [email protected] Oak Ridge National Laboratory Leslie R. Dole, PhD Oak Ridge National Laboratory PO Box 2008 MS-6166 Oak Ridge, TN 37831-6166, USA Tel: 865-576-0382 E-mail: [email protected] Web page: http:/www.dole.nu.lesdole/

2.5.2 US technology development and worldwide cask fabrication companies The Idaho National Laboratory licensed their DUAGGTM and DUCRETETM technology to Teton Technologies who sublicensed to Starmet Corporation of Barnwell, SC. Starmet assembled and briefly ran a DUAGGTM fabrication pilot plant, but Starmet is no longer in business.

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Dry cask technology advice Teton Technologies William Quapp, MS, PE 860 W. Riverview Dr. Idaho Falls, Idaho 83401, USA Tel: 208-535-9001 Email: [email protected] Pacific Development Services Inc. John C. Ritchie, BSc, MBA 1802 N Carson St., Suite 212 Carson City, NV, 89701, USA Tel: 702-940-7832 Email: [email protected] Dry cask fabrication companies There have been many changes in licenses and company ownership within the dry cask fabrication community within recent years. Below is a partial list of companies that currently manufacture dry spent fuel casks. AREVA (France) 2 Rue Paul Dautier, F-78141 Velizy-Villacoublay Cedex, France, Tel: +33 1 39 26 30 00 Fax: +33 1 39 26 27 00 Vectra Technologies sold the NUHOMS® license to AREVA, then Framatome, in the early 1990s. AREVA currently fabricates several different types of storage casks. Their Transnuclear TN series of metal casks have self-contained steel and borated resin shielding; no concrete is used. Energy Solutions, LLC [USA] 423 West 300 South, Suite 200 Salt Lake City, Utah 84101 Tel: 801-649-2000 E-mail: [email protected] Energy Solutions sells the VSC-24 cask system (see Fig. 2.18). Equipos Nucleares SA (ENSA) [Spain] C/ Jose Ortega y Gasset 20, 5 Floor, E-20006 Madrid, Spain, Tel: +34 91 5 553 617 Fax: +34 91 5 563 149 ENSA’s main focus is on spent fuel transport as opposed to dry cask storage; however, they do manufacture canisters for dry storage systems. Holtec International [USA] Holtec Centre, 555 Lincoln Drive West, Marlton, NJ 08053, USA, Tel: +1 856 797 0900

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Holtec is a major manufacturer of casks at the present time. They have complete system solutions (wet storage, dry storage, and shipping). (See Fig. 2.17.) Siempelkampstraße 45, 47803 Krefeld, Germany, Tel: +49 2151 894 481 Fax: +49 2151 894 488 Holtec Manufacturing Division (HMD) Keystone Commons, 200 Braddock Avenue, Pittsburgh, PA 15145, USA, Tel: +1 412 823 3773 Fax: +1 412 823 6669 [See Above comments] GNS Gesellschaft für Nuklear-Service mbH [Germany] Hollestraße 7A, D-45127 Essen, Germany, Tel: +49 201 109 1828 Fax: +49 201 109 1125 GNS is the major European supplier for dry spent fuel cask storage, their technology is cast iron. NAC International [USA] Atlanta Corporate Headquarters 3930 East Jones Bridge Road, Norcross, GA 30092, USA, Tel: +1 678 382 1229 Fax: +1 678 382 1429 USEC purchased NAC International in late 2004.

2.5.3 Russian institutes Serge G. Ermichev (e-mail: [email protected]) V.I. Shapovalov N.V. Svirodov RFNC-VNIIEF 37 Mir Avenue, Sarov, 607190 Nizhniy Novgorod region Russia Tel: (83130) 4-24-14 or 4-57-92 V.K. Orlov VNIINM, Moscow, Russia

2.6

References

1 Waddell J.J. and Dobrowolski J.A., Concrete Construction Handbook, Third Edition, McGraw-Hill, 1993. 2 Kosmatka S.H. and Panarese W.C., Design and Control of Concrete Mixtures, Thirteenth Edition, Portland Cement Association, 5420 Old Orchard Road, Skokie, Ill 60077-1083.

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3 Haelsig R.T., “Depleted Uranium Concrete Container Feasibility Study”, EGG Report MS-11400, September 1994. 4 MPC transport 10 mrem/h at 2 m and 200 mrem/h on contact; MPC storage 20 mrem/h contact; DHLW storage 20 mrem/h on contact. 5 Haelsig R.T., “Depleted Uranium Concrete Container Feasibility Study”, Prepared under INEL contract to Scientech, Inc., 1585 N. Skyline Drive, Idaho Falls, ID 83402 by Packaging Technology, Inc., 4401 A Industry Drive East, Tacoma WA 98424-1800, May 16, 1994. 6 “Safety Analysis Report for the Ventilated Storage Cask System” PSN-91001 Rev.0 Pacific Sierra Nuclear Associates, Scotts Valley, CA, October 1991. 7 “BR-100 Shipping Cask Preliminary Design Report” 51-1177082-01, page II-311, Babcock & Wilcox Company, Lynchburg, VA. 8 ”Performance Testing and Analyses of the VSC-17 Ventilated Concrete Cask”, EPRI Report TR-100305, Project 3073-1, PNL-7831, UC-85, Final Report 1992, Prepared by Pacific Northwest Laboratories and Idaho National Engineering Laboratory (EG&G). 9 Chillicothe Gazette, Report, June 28, 2006. 10 Lessing P.A., “Development of DU-AGG (Depleted Uranium Aggregate)”, INEL-95/0315, Idaho National Laboratory, Idaho Falls, Idaho, September 1995. 11 Quapp W.J. and Lessing P.A., “Radiation Shielding Composition”, U.S. Patent No. 5,786,611; Granted July 28, 1998. 12 Lessing P.A., “Process Description for the Production of Depleted Uranium Aggregate”, INEL/INT-97-00662, Idaho National Engineering Laboratory, June 1997. 13 Lessing P.A. and Gillman H., “DU-AGG Pilot Plant Design Study”, INEL96/0266, Idaho National Engineering Laboratory, July 1996. 14 Quapp W.J., Miller W.H., Taylor J., Hundley C., and Levoy N., “DUCRETE: A Cost Effective Radiation Shielding Material”, Spectrum 2000, Sept. 24–18, Chattanooga, TN. 15 Lessing P.A., “Development of ‘DUCRETE’ ”, INEL-94/0029, Idaho National Engineering Laboratory, Box 1625, Idaho Falls, ID 83415, October 1994. 16 Lessing P.A., “Development of DU-AGG (Depleted Uranium Aggregate)”, INEL-95/0315, Appendix B, Idaho National Engineering Laboratory, Idaho Falls, Idaho, September 1995. 17 Quapp W.J. and Lessing P.A., “Radiation Shielding Composition”, U.S. Patent 6,166,390; Granted December 26, 2000. 18 “Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing”, ACI 304.3R-89, American Concrete Institute, Detroit, MI. 19 Quapp W.J., “An Advanced Solution for the Storage, Transportation and Disposal of Spent Fuel and Vitrified High Level Waste”, Global ’99: International conference on future nuclear systems, summary article in Nuclear News; 42 (12), Nov. 1999. 20 Sterbentz J.W., “Shielding Evaluation of a Depleted Uranium Heavy Aggregate Concrete for Spent Fuel Storage Casks”, INEL-94/0092, Idaho National Engineering Laboratory, October 1994. 21 Tang J., “Shielding Characteristics of Various materials on PWR Waste Packages”, BBAC00000-001717-0210-00008 Rev 00, DOE Yucca Mountain Project Office, M&O Contractor, February 1998.

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22 “American National Standard Measurement of the Leachability of Solidified Low-Level Radioactive Wastes by a Short-Term Test Procedure”, ANSI/ANS16.1–1986 or “Static Leach Test Method” in the Nuclear Waste Materials Handbook of the Materials Characterization Center, MCC-1P, Pacific Northwest National Laboratory, Richland, WA. 23 Dole L.R., Ferrada J.J., and Mattus C.H., “Radiation Shielding for Storage and Transportation Cask Using Depleted Uranium Oxide in Cementitious Matrices”, U.S./Russian Depleted Uranium Workshop: Review of ISTC Projects, Oak Ridge National Laboratory, May 17–21, 2004. 24 Dole L.R., Ferrada J.J., and Mattus C.H., “Cask Size and Weight Reductions Through the Use of DUCRETE”, Russian-American Workshop on Use of Depleted Uranium and Review of International Science and Technology Center (ISTC) Projects, Moscow/Serov, Russia, June 19–23, 2005. 25 Quapp W.J. and Cooley C.R., “New Options for Managing Depleted Uranium Materials”, International Conference on Nuclear and Hazardous Waste Management (SPECTRUM ’98), Denver, CO, Sept. 13–17, 1998, American Nuclear Society – ISBN: 0-89448-635-7. 26 Hopf J.E., “Conceptual Design Report for a Transportable DUCRETE Spent Fuel Storage Cask System”, INEL-95/0167, Idaho National Engineering Laboratory, April 1995. 27 Diersch R. and Jack A., “GNB, The CONSTOR Steel-Concrete Cask for Transport and Storage of Spent Nuclear Fuel from RBMK”, INMM, Washington, DC, January 1998. 28 Powell F.P., “Comparative Economics for DUCRETE Spent Fuel Storage Cask Handling, Transportation, and Capital Requirements” INEL-95/0166, Idaho National Engineering Laboratory, April 1995. 29 Haire M.J., Doe L.R., Arrowsmith H.W., Denton M.S., Shapovalov Vi.I., and Matveev V.Z., “A Collaboration to Develop the Next-Generation SNF/HLW Cask”, 2003 International High-Level Radioactive Waste Management Conference, March 30–April 2, 2003, Las Vegas, Nevada. 30 Dole L.R., Ferrada J.J., and Mattus C.H., “Radiation Shielding for Storage and Transportation Cask Using Depleted Uranium Oxide in Cementitious Matrices”, J.S./Russian Depleted Uranium Workshop: Review of ISTC Projects, May 17–21, 2004, Oak Ridge National Laboratory, Oak Ridge, TN. 31 “ORNL Foreign Trip Report #226719, Report of Foreign Travel to Russia for M.J. Haire, C.W. Forsberg, L.R. Dole, and R.G. Wymer”, Oak Ridge National Laboratory, July 13, 2005. 32 Moxley R.E., Adley M.D., and Rohani R., “Impact of Thin-Walled Projectiles with Concrete Targets”, Shock and Vibration 2, 1995 5 355–364. 33 Forrestal M.J., et al., “An Empirical-Equation for Penetration Depth of OgiveNose Projectiles into Concrete Targets”, International Journal of Impact Engineering, 1994 15 (4) 395–405. 34 Frew D.J., et al., “The Effect of Concrete Target Diameter on Projectile Deceleration and Penetration Depth”, International Journal of Impact Engineering, 2006 32 (10) 1584–1594. 35 Hackman L.E., et al., “Slurry Infiltrate Mat Concrete (SIMCON)”, Concrete International, 1992 (12) 53–56. 36 Fiber Reinforced Concrete, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077-1083, ISBN 0-89312-091-X, 1991.

3 High-strength concrete O E GJØRV, Norwegian University of Science and Technology, Norway

3.1

Introduction

The theoretical basis for producing high-strength concrete was originally developed in the field of ceramic materials in the late 1950s and early 1960s. Based on single-phase polycrystalline ceramic materials, it was shown that reduced particle dimension increased the strength. The dependence of the particle size on the strength was explained on the basis of Griffith’s theory for the rupture of brittle materials with internal cracks. In accordance with this theory, the strength of the material should increase with decreasing pore and particle size by a square root law. Later on it was shown that there was a similar relationship between microstructure and strength for cement pastes with densely packed cement particles at a very low water/cement ratio. However, it was not until the early 1970s that new and very effective agents for dispersing the fine cement particles in water became available, and then, a tremendous advance in the production of high-quality concrete was achieved. At the same time, large quantities of ultra-fine condensed silica fume particles also became available. Therefore, a commercial basis for production of concrete with very high density and strength was established, and a rapid development of high-strength concrete took place. Since a low porosity concrete with high density also will enhance the overall performance of the material, the term “high-performance concrete” was also soon introduced, which is inclusive of the term “high-strength concrete”. More and more, however, the term “high-performance concrete” was mostly used and specified for concrete durability rather than for concrete strength. In the literature, there are a number of definitions of both “high-strength concrete” (HSC) and “high-performance concrete” (HPC), but as properly discussed by Aïtcin in his book on high-performance concrete (Aïtcin 1998), there is no clear consensus about the meaning of either of these terms. In the literature, some people try to define high-strength concrete as different from “normal strength concrete”, “ordinary concrete” or “usual 79

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Developments in the formulation and reinforcement of concrete

concrete”, but what is “normal”, “ordinary” or “usual” is rapidly changing. Also, some definitions are based on a maximum water/cement ratio, but nor is the term “water/cement ratio” easy to define any longer. For many years, when concrete was mostly based on pure Portland cements and simple procedures for concrete production, the concept of water/cement ratio was the fundamental basis for characterizing concrete quality. Since a number of different cementitious materials and reactive fillers are now being used for concrete production, the concrete properties are more and more being controlled by various combinations of such materials. In addition, the concrete properties are also increasingly being controlled by the use of various types of processed concrete aggregate, new concrete admixtures and sophisticated production equipment. Therefore, the old and very simple terms “water/cement” or “water/binder ratio” for characterizing concrete quality successively lost their meaning. As a consequence, performance definition and specification for concrete quality based on performance criteria are more commonly being applied. In order to stimulate the use of high-performance concrete for highway applications in the USA, the Federal Highway Administration in the early 1990s defined high-performance concrete (HPC) by four durability and four strength parameters, which included (Goodspeed et al. 1996): Durability • freeze/thaw durability • scaling resistance • abrasion resistance • chloride permeability. Strength • compressive strength • elasticity • shrinkage • creep. Based on requirements for each parameter, four different performance grades were defined, and details of test methods for determining the performance grades given. Then, applications of the various HPC grades for various exposure conditions were recommended.

3.2

Applications

3.2.1 General At an early stage, high-strength or high-performance concrete was mostly applied to high-rise buildings, bridges and offshore structures, but it was successively applied to a variety of other applications such as:

High-strength concrete • • • • • • •

81

harbor and coastal structures hydraulic structures underground construction pavements and industrial floors water treatment plants storage facilities for aggressive waste and chemicals concrete products.

For some of the applications, the mechanical properties were decisive, while for others, the durability properties were the most important; but very often, it was the combined enhanced mechanical and durability properties which were the basis for the various applications. For the various partners in new projects, the importance and benefits of applying high-strength or high-performance concrete were somewhat different. For the owner, increased service life, reduced concrete volume and costs, reduced construction time and increased space and comfort in highrise buildings by reduced swaying were the most important. For the designer, improved properties such as compressive strength, E-modulus, durability and rapid attainment of final creep as well as reduced dead load were important. For the contractor, fast track construction and cheaper alternatives were important. For the concrete producer, the use of high-tech production increased the profit and the market, as well as giving beneficial effects on conventional production. Also, from an environmental point of view, savings in cement and aggregate and increased service life contributed to a more sustainable development. Over the years, a number of technical committees and working groups have summarized the extensive literature and experience from the various areas of application of high-strength or high-performance concrete, a brief outline of which is given in the following.

3.2.2 High-rise buildings For many years, the columns of high-rise buildings were the largest application of high-strength concrete in buildings. In 1965 and in the early 1970s, a number of buildings with high-strength concrete were constructed in the Chicago area with columns typically having a design compressive strength of 62 MPa, but successively, much higher design strengths were achieved (Table 3.1 and Fig. 3.1). Very often, the high E-modulus for improved stiffness of the columns was the motivation for the selection of high-strength concrete.

3.2.3 Bridges For long-span bridges, both high strength and low weight are important. For the bridges listed in Table 3.2, design strengths of up to 79 MPa based on

82

Developments in the formulation and reinforcement of concrete

Table 3.1 Buildings with high-strength concrete (FIP/CEB 1990) Building

Location

Year+

Total stories

Lake Point Tower Midcontinental Plaza Frontier Towers Water Tower Plaza Royal Bank Plaza River Plaza Helmsley Palace Hotel Richmond – Adelaide Toronto Larimer Place Condominiums Texas Commerce Tower City Center Project Trump Tower 499 Park Avenue Petrocanada Building S.E. Financial Center Chicago Mercantile Exchange 1130 A.Michigan Ave. Pacific Park Plaza

Chicago Chicago Chicago Chicago Toronto Chicago New York Centre

1965 1972 1973 1975 1975 1976 1978 1978

70 50 55 79 43 56 53 33

52 62 62 62 61 62++ 55 61

Denver

1980

31

55

Houston Minneapolis New York New York Calgory Miami Chicago

1981 1981 – – 1982 1982 1982

75 52 68 27 34 53 40

52 55 55 59 50 48 62+++

Chicago Emeryville, CA Melbourne Seattle Dallas Chicago Paris

– 1983

– 30

52 45

– 1983 1983 1986 1988

44 76 72 15 –

55 66 69 97 65

Chicago Seattle Seattle Seattle

1989 1989 1989 1989

79 58 44 62

83 115 115 94

Collins Place Columbia Center Interfirst Plaza 900 N. Mich. Annex Grande Arche de la Défence South Wacker Tower Two Union Square Pacific First Center Gateway Tower

Max design strength (MPa)

+ Year in which high strength concrete was cast. ++ Two experimental columns of 76 MPa strength were included. +++ Two experimental columns of 97 MPa strength were included.

normal weight and up to 69 MPa based on lightweight concrete were applied. For cantilever bridges, the use of normal weight or lightweight concrete was sometimes varied from one span to another. In Norway, two floating bridges for strait crossings were also built, where a 65 MPa type of lightweight concrete was typically used for the floating pontoons (Meaas et al. 1994; Hasselø 2001). Many bridges are also exposed to very severe environments (Fig. 3.2), and in particular, chloride penetration and steel corrosion have proved to

High-strength concrete Building height, ft 12,000 12,000 psi 1000 (82.7 MPa) (82.7) 12,000 (82.7)

900 800

Building height, m 300 12,000 (82.7)

19,000 (131.0)

17,000 (117.2) 19,000 (131.0)

12,500 (82.2)

700

83

14,000 (96.5)

600 500

14,000 (96.5)**

400

250 200 150 100

300 200

50

100

So ut h

Pr ud en t

31 1

Tw o

0

ia l C Pla hi z c a 1 ag * W 98 o ac 9 ke r C Dr hi iv c e So 1 ag * ci 98 o et 9 y C Ce O le n ne ve te Pe la r† ac 19 nd ht 91 re e Ce At nte la r* n Tr 199 ta u N mp 1 ew Yo Pal Da rk ac in C e* Bo 19 ity sw 91 or M th 22 in T 5 ne ow W ap er es t W 19 olis * ac 91 ke r C Dr hi iv c e G 1 ag * at 9 o ew 88 ay To Se we a r† Pa 19 ttle ci 90 fic Fi rs tC e Se ntr a e† Tw 1 ttle o U 989 ni on Sq Se uar a e† 19 ttle 88

0

* Reinforced concrete frame † Composite concrete/steel frame ** Also includes one experimental column of 17,000 psi

3.1 High-strength concrete in buildings (FIP/CEB 1990).

represent a special challenge for their durability and long-term performance (Gjørv 2002). For many bridges, experience has shown that specification of a concrete with high strength is not necessarily enough to ensure proper durability, and in some cases, deep chloride penetration has been observed even during concrete construction, before the concrete has gained sufficient maturity and density (Fig. 3.3).

3.2.4 Offshore structures When the first concept for fixed offshore concrete structures in the North Sea was introduced in the late 1960s, the offshore technical community showed much skepticism. At the same time, however, the results of a comprehensive field investigation of more than 200 conventional concrete sea structures along the Norwegian coastline were published, demonstrating that the general condition of these structures was quite good, even after service periods of up to 50–60 years (Gjørv 1968, 1994). These results contributed, therefore, to convincing the most skeptical operators that concrete could also be a possible and reliable construction material for offshore installations in the North Sea. However, the appearance of corrosion on embedded steel that typically took place in all of these conventional concrete structures after a service period of only 5–10 years was not acceptable

Table 3.2 Bridges with high-strength concrete (FIP/CEB 1990) Bridge

Location

Year

Max span Max design (m) strength (MPa)

Willows Bridge Nitta Highway Bridge San Diego to Coronado Kaminoshima Highway Bridge Ootanabe Railway Bridge Fukamitsu Highway Bridge Akkagawa Railway Bridge Kylesku Bridge Selbjørn Bridge Deutzer Bridge Pasco-Kennewick Intercity Coweman River Bridge Linn Cove Viaduct N Parrot Ferry Bridge Ottmarsheim Pont de Tricastin Tower Road Bridge Pont du Pertuiset Pont de Joigny Arc sur la Rance Giske Sandhornøya Boknasundet Helgelandsbrua

Toronto Japan California Japan

1967 1968 1969 1970

48 30 43 86

41 59 41+ 59

Japan Japan Japan Scotland Norway Germany Washington Washington Carolina California France France Washington France France France Norway Norway Norway Norway

1973 1974 1976 – 1977 1978 1978 – 1979 1979 1979 – 1981 1988 1988 1988 1989 1989 1990 1990

24 26 46 79 212 185 299 45 54 195 172 142 49 110 – – 52 154 190 425

79 69 79 53 40 69+ 41 48 41 43+ 30+ 30+ 62 65 60 60 55 55+ 60+ 65

+ Lightweight concrete.

3.2 Storseisund Bridge (1989), which is one of the many concrete bridges severely exposed along the Norwegian coastline (courtesy of J Brun).

High-strength concrete

85

3.3 Helgelandsbrua (1990) in Norway, in which a high chloride penetration already took place during concrete construction.

3.4 Development of offshore concrete structures in the North Sea (courtesy of Aker-Kvæner).

to the offshore technical community. Therefore, in order to gain acceptance for the first offshore concrete platform, both increased concrete quality and concrete cover beyond that required by current concrete codes and much stricter programs for quality assurance and quality control had to be introduced. During the construction of the Ekofisk Tank (1973), the first edition of Recommendations for Design and Construction of Concrete Sea Structures was published by the international organization for prestressed concrete (FIP 1973). Shortly after, Det Norske Veritas (DNV 1974) in their Rules, the Norwegian Petroleum Directorate in their Regulations (OD 1976) and ACI in their Recommendations (ACI 1978) adopted the new and stricter durability requirements for fixed offshore concrete structures. After the first breakthrough for use of concrete for offshore installations in the North Sea in the early 1970s, rapid development took place (Fig. 3.4).

86

Developments in the formulation and reinforcement of concrete

During the period from 1973 to 1995, altogether 28 major concrete platforms containing more than 2.5 million cubic meters of high-performance concrete were installed, and by 2007, there were 34 concrete structures in the North Sea, most of which were produced in Norway (Table 3.3). In other parts of the world, a number of offshore concrete structures have also been produced, and so far, a total of 50 various types of offshore concrete structures have been installed (Moksnes 2007). For the first North Sea concrete structures in the early 1970s, it was not so easy to produce a high-strength concrete which also contained entrained air for ensuring proper frost resistance. Extensive research programs were carried out, however, and the quality of concrete and the specified design strength increased from project to project. Thus, from the Ekofisk Tank which was installed in 1973, to the Troll A Platform installed in 1995, the design strength successively increased from 45 to 80 MPa. Also, the water depths for the various installations successively increased; in 1995, the Troll A Platform was installed at a water depth of 303 m. From the tip of the skirts to the top of the shafts of this gravity base structure, the total height was 472 m, which is taller than the Empire State Building. After production in one of the deep Norwegian fjords, the Troll A Platform containing 245,000 m3 of high-strength concrete, 100,000 t of reinforcing steel and 11,000 t of prestressing steel, was moved out to its final offshore destination, and this operation was the biggest movement of a man-made structure ever (Fig. 3.5). In 1995, the Heidrun platform was also installed in deep water of 350 m, but this structure was a tension leg floating platform consisting of lightweight concrete with a design strength of 65 MPa.

3.5 The Troll A Platform (1995) on its way out to the final destination in the North Sea (courtesy of Aker-Kværner).

High-strength concrete

87

Table 3.3 Offshore concrete structures with high-strength concrete in the North Sea (Moksnes 2007) Year

Field

Operator

Platform type

1973

Ekofisk

Phillips

1975 1975 1975

Beryl A Brent B Frigg CDP1

Mobil Shell Elf

1976 1976 1976

Brent D Frigg TP1 Frigg MCP-01

Shell Elf Elf

1977 1977 1977 1978 1978

Dunlin A Frigg TCP2 Statfjord A Cormorant A Ninian Central

Shell Elf Mobil Shell Chevron

1978 1981 1982

Brent C Statfjord B Maureen ALC

Shell Mobil Phillips

1981

Schwedeneck A

Texaco

1981

Schwedeneck B

Texaco

1984 1986 1987 1988

Statfjord C Gullfaks A Gullfaks B Oseberg A

1989 1989

Gullfaks C North Ravenspurn

1989 1992 1992

Ekofisk Barriere NAM F3 Snorre CFT

Mobil Statoil Statoil Norsk Hydro Statoil Hamilton Bros. Phillips Shell Saga

Caison, Jarlan Wall GBS 3 shafts GBS 3 shafts GBS 1 shaft, Jarlan Wall GBS 3 shafts GBS 2 shafts GBS 1 shaft, Jarlan Wall GBS 4 shafts GBS 3 shafts GBS 3 shafts GBS 4 shafts GBS 1 shaft, Jarlan wall GBS 4 shafts GBS 4 shafts GBS Art. column GBS Monotower GBS Monotower GBS 4 shafts GBS 4 shafts GBS 3 shafts GBS 4 shafts

1993 1993

Sleipner A Draugen

Statoil Shell

1994

Heidrun Found

Conoco

1995 1995 1995 1995

Harding Troll A Heidrun TLP Troll B

1999

South Arne

BP Shell Conoco Norsk Hydro Amerada Hess

Depth (m)

Concrete volume (m3)

70

80,000

118 140 98

52,000 64,000 60,000

140 104 94

68,000 49,999 60,000

153 104 145 149 136

90,000 50,000 87,000 120,000 140,000

141 145 92

105,000 140,000 –

25

3,620

16

3,060

145 135 141 109

130,000 125,000 101,000 116,000

GBS 4 shafts GBS 3 shafts

216 42

244,000 9,800

Protection Ring GBS Suction anchors, 3 cells GBS 4 shafts GBS Monotower Suction anchors, 19 cells GBS foundation GBS 4 shafts Concrete TLP Concrete Semi

75 43 310

105,000 23,300 7,800

82 251

77,000 85,000

350

28,000

106 303 350 340

37,000 245,000 63,000 41,000

60

35,000

GBS 1 shaft

88

Developments in the formulation and reinforcement of concrete

3.2.5 Special applications Along with the extensive programs for production of offshore concrete structures, extensive research for further development of high-strength and high-performance concrete took place. Thus, in the early 1990s, concretes based on high-quality natural rock aggregate and lightweight aggregate with compressive strengths of up to 198.6 MPa (Gjørv and Rønning 1992) and 102.4 MPa (Zhang and Gjørv 1991), respectively, were obtained, the latter having a fresh concrete density of 1865 kg/m3. As a spin-off of this research, a number of other special applications such as high-strength concretes for highway pavements, industrial floors and hydraulic structures were also developed. In the Scandinavian countries, extensive maintenance and rehabilitation of highway pavements due to heavy traffic from studded tires had been a big problem for many years. In 1985, therefore, an accelerated load facility for full-scale testing of the abrasion resistance of highway concrete pavements exposed to heavy traffic from studded tires was built in Norway (Gjørv et al. 1990). By increasing the concrete strength from 50 to 100 MPa, the abrasion of the concrete was reduced by approximately 50%, and at 150 MPa, the abrasion was comparable to that of high-quality massive granite blocks (Fig. 3.6). Compared to a high-quality asphalt highway pavement, this represented an increased service life of the highway pavement by a factor of approximately ten. Later, various types of concrete with high abrasion resistance for a variety of other applications were developed.

Partial abrasion (mm/104 rev.)

3.0

Wet

Dry Wet Type of aggregate

Syeniteporphyr Hornfels Quartzdiorite Jasper

2.5 2.0 Dry 1.5 1.0 Massive granite (wet) 0.5

Massive granite (dry) 50

150 100 28 days compressive strength (MPa)

3.6 Relationship between compressive strength and abrasion resistance of concrete (Gjørv et al. 1990).

High-strength concrete

89

Compressive strength (MPa)

500

100

50

10 0.3

0.4

0.5

0.6

0.7

Porosity

3.7 Relationship between compressive strength and porosity of cement paste (Bache 1981).

Gradually, a new generation of cementitious materials with extremely low porosities and ultra high-strength properties were also developed (Fig. 3.7). Different techniques such as DSP, MDF and RPC were applied in order to reduce the porosity, but all these materials had one thing in common by optimizing the packing of all particles involved. As a result, water/binder ratios of 0.10 to 0.20 were achieved, which were much lower than those of traditional high-performance concrete. The DSP materials (densified with small particles) took advantage of the combined action of condensed silica fume and superplastcizers. By also replacing the rockbased aggregate by small particles of high-quality ceramic aggregate such as calcined bauxite, new materials with compressive strengths of 140 to 500 MPa, flexural strengths of 20 to 70 MPa and E-moduli of 50 to 100 GPa became available for a variety of new industrial applications and new concrete products. The MDF materials (macro defect free) also took advantage by additional use of a polyvinyl alcohol polymer (PVA), which first acts as a powerful dispersant of the particle system and then acts as reactive binder. As a result, impressive flexural strengths of up to 200 MPa were achieved. The RPC materials (reactive powdered concrete), which were the latest type of ultra high-strength materials developed, went one step further than the DSP materials in down-scaling the maximum size of the coarse aggregate to 300 μm. By such a small size aggregate in combination with pressure on the material during hardening, impressive compressive strengths of up to 800 MPa were achieved.

90

Developments in the formulation and reinforcement of concrete

3.3

Future trends

3.3.1 General In many countries there has been a rapidly increasing deterioration of many important concrete infrastructures (Gjørv 2002). This is not only a technical and economic problem, but also has a great impact on available resources, environment and human safety (Gjørv and Sakai 2000). For new concrete infrastructures, therefore, there is a great need for proper application of high-performance concrete. In many countries, there is also a rapid development of high-rise buildings, and increasing population and shortage of land area also tend to move more activities and constantly new types of activities into more severe marine environments. All of these developments will require more high-performance concrete in the years to come. For many of the bridges and offshore installations which have been exposed to a severe environment for some time (Figs 3.2 and 3.8), experience has shown that the use of high-strength concrete alone does not necessarily ensure proper durability and long-term performance. Although the overall condition of the concrete structures in the North Sea appears to be quite good (Gjørv 1994, Moksnes and Sandvik 1996, FIP 1996), several of these structures already have some extent of steel corrosion, and for some of them, very costly repairs have been carried out and costly protective measures applied. Many of the observed corrosion problems can be related to lack of proper quality control or special problems during concrete construction, but experience also clearly demonstrates that a high compressive

3.8 The offshore concrete structures in the North Sea are exposed to a very harsh and hostile environment.

High-strength concrete

91

Chloride concentration (% Cl as weight of concrete)

0.35

elevation: + 14.4 m elevation: + 7.8 m elevation: – 11.5 m

0.3 0.25 0.2 0.15 0.1 0.05 0 0

10

20

30

40

50

60

70

Distance from the exposed surface (mm)

3.9 Chloride penetration into a 20-year-old North Sea concrete platform (Sengul and Gjørv 2007).

strength alone does not necessarily give a high resistance against chloride penetration. For one of the concrete platforms in the North Sea which was subjected to a very thorough investigation after a service period of approximately 20 years, Fig. 3.9 reveals that a deep chloride penetration both above and below water had taken place (Sengul and Gjørv 2007). Although the scatter of test results was relatively high, the chloride penetration was deepest in the upper part of the splash zone (+14.4 m) and lowest in the constantly submerged part of the shaft (−11.5 m). In the upper part of the shaft, a chloride front of approximately 0.07% by weight of concrete at a depth of approximately 60 mm was observed. For the nominal concrete cover of 75 mm specified, this indicated that an early stage of steel corrosion had already been reached. A specified water/cement ratio of less than 0.40 and a minimum cement content of 400 kg/m3 in combination with a nominal concrete cover of 75 mm, had not been sufficient to prevent chloride penetration from reaching embedded steel within a service period of approximately 20 years. While it is relatively easy to control the resistance of concrete, both against freezing and thawing and expansive alkali reactions, by following established precautions and procedures, extensive experience demonstrates that electrochemical corrosion of embedded steel represents the most critical and greatest threat to the durability and long-term performance of concrete structures in chloride exposed environments. Therefore, in order to gain a more controlled durability and service life of new important concrete structures in severe environments, a rapid international development on both probability-based durability design and performancebased concrete quality control has taken place (Gjørv 1993, 2002). A

92

Developments in the formulation and reinforcement of concrete

probability-based durability design has already been applied to several new major projects (Gehlen and Schiessl 1999, McGee 1999). In order to meet the very strict requirements based on such a durability design, a proper type of cement or binder system has proved to be very important (Årskog et al. 2007). Thus as a result of a proper durability design, the Rion-Antirion Bridge (2001) in Greece was produced with a high-performance concrete based on blast furnace slag cement (Fig. 3.10). A probability-based durability design has already been shown to provide a very good basis for a better utilization of high-performance concrete for new concrete structures in severe environments.

3.3.2 Probability-based durability design A probabilistic approach to the durability design was developed in the European research project “DuraCrete” in the late 1990s (DuraCrete 1999). Further development and simplification of the results from this project have provided the basis for new specifications and guidelines both for durability design and performance-based concrete quality control of new important concrete structures in Norwegian harbors (Norwegian Association for Harbor Engineers 2007a, b). Over the last few years, the above Norwegian specifications and guidelines have been successfully applied to a number of projects with important concrete structures in severe environments. One of these projects includes a new large city development with a service life requirement of 300 years (Fig. 3.11), which is currently under construction in Oslo harbor (Årskog and Gjørv 2007). Although a probability-based durability design does not guarantee a given service life of a new concrete structure, such a design provides the basis for engineering judgment of all of the factors which are considered the most relevant for the durability, including the scatter and variability of all these factors. Hence, a good engineering basis for comparing and select-

3.10 The Rion-Antirion Bridge (2001) in Greece was produced with a high-performance concrete based on blast furnace slag cement.

High-strength concrete

93

3.11 Utilization of high-performance concrete in a new city development project in Oslo harbor, where all the concrete substructures have a service life requirement of 300 years (courtesy of Tjuvholmen KS).

ing one of several technical solutions for a given environment is obtained, and durability requirements, which are possible to verify and control during concrete construction, are specified. Extensive experience has shown that much of the durability problems, which occur after some time of service, can be related to lack of proper quality control and special problems during concrete construction. Upon completion of a new structure, therefore, it is very important to provide documentation of the construction quality and durability achieved before the structure is handed over to the owner. As part of the durability design, the owner must also be provided with a service manual for a regular condition assessment and preventive maintenance of the structure. For concrete structures in chloride-containing environments, it is the regular monitoring of the real chloride penetration and assessment of the future corrosion probability in combination with protective measures which provide the ultimate basis for achieving a more controlled durability and service life.

3.4

Sources of further information and advice

International conferences “Very High Strength Cement-Based Materials” (1984), Boston, USA, Proc. of an Int. Symp., ed. by J F Young, Matrs. Res. Soc., Pennsylvania, ISBN 0-931837-07-3. “Utilization of High Strength Concrete” (1987), Stavanger, Norway, Proc. of 1st Int. Symp., ed. by I Holand, D Holland, B Jakobsen and R Lenchow, Tapir, NTNU, Trondheim, ISBN 82-519-0797-7.

94

Developments in the formulation and reinforcement of concrete

“High Strength Concrete” (1990), Proc. of 2nd Int. Symp., Berkeley, USA, ed. by W T Hester, ACI SP-121. “High Strength Concrete” (1993), Lillehammer, Norway, Proc. of 3rd Int. Symp., ed. by I Holand and E Sellevold, Norwegian Concrete Association, Oslo, ISBN 82-91341-00-1. “Utilization of High Strength/High Performance Concrete” (1996), Paris, France, Proc. of 4th Int. Symp., Paris, ed. by F de Larrard and R Lacroix, École Nationale des Ponts et Chaussées, Paris, ISBN 2-85978-257-(1-3-5). “Utilization of High Strength/High Performance Concrete” (1999), Sandefjord, Norway, Proc. of 5th Int. Symp., ed. by I Holand and E. Sellevold, Norwegian Concrete Association, Oslo, ISBN 82-91341-25-7. “Utilization of High Strength/High Performance Concrete” (2002), Leipzig, Germany, Proc. of 6th Int. Symp., ed. by G Kønig, F Dehn and T Faust, Leipzig University, Leipzig, ISBN 3-934178-18-9. “Utilization of High Strength/High Performance Concrete” (2005), Washington, USA, Proc. of 7th Int. Symp., ed. by H G Russel, ACI SP-228. “Concrete Under Severe Conditions – Environment and Loading” (1995), Sapporo, Japan, Proc. of 1st Int. Symp., ed. by K Sakai, N Banthia and O E Gjørv, E & FN Spon, London and New York, ISBN 0-419-19870-9. “Concrete Under Severe Conditions – Environment and Loading” (1998), Tromsø, Norway, Proc. of 2nd Int. Symp., ed. by O E Gjørv, K Sakai and N Banthia, E & FN Spon, London and New York, ISBN 0-419-23850-6. “Concrete Under Severe Conditions – Environment and Loading” (2001), Vancouver, Canada, Proc. of 3rd Int. Symp., ed. by N Banthia, K Sakai and O E Gjørv, Univ. of British Columbia, Vancouver, ISBN 0-88865-782. “Concrete Under Severe Conditions – Environment and Loading” (2004), Seoul, Korea, Proc. of 4th Int. Symp., ed. by B H Oh, K Sakai, O E Gjørv and N Banthia, Seoul National University and Korea Concrete Institute, Seoul, ISBN 89-8949902-X 93530. “Concrete Under Severe Conditions – Environment and Loading” (2007), Tours, France, Proc. of 5th Int. Symp., ed. by F Toutlemonde, K Sakai, O E Gjørv and N Banthia, Laboratoire Central des Ponts et Chaussées, Paris, ISSN 2-7208-2495-X.

State-Of-The-Art Reports “State-Of-The-Art-Report on High Strength Concrete” (1984), Report by ACI Committee 363–84, ACI Journal. “Guide for the Design and Construction of Fixed Offshore Concrete Structures” (1984), Report by ACI Committee 357R-84, ACI. “High Strength Concrete” (1987), ACI Report SCM-15, ACI. “Literature Review of High Strength Concrete Properties” (1988), by L J Parrott. Review carried out by C and CA Services, January. “High Strength Concrete” (1990), State-Of-The-Art-Report, FIP SR 90/1/CEB Bulletin d’Information No. 197, London. “High Performance Concrete” (1991), by P Zia, M L Leming and S M Ahmed, A State-Of-The-Art-Report by the Strategic Highway Research Programme, National Research Council, Washington, DC, USA.

High-strength concrete

95

“State-Of-The-Art-Report on Offshore Concrete Structures for the Arctic” (1991), Report by ACI Committee 357, 1R-91, ACI. “High Strength Concrete” (1992), ACI Compilation 17, ACI. “State-Of-The-Art-Report on High Strength Concrete” (1992), Report by ACI Committee 363R-92, ACI. “Application of High Performance Concrete” (1994), FIP/CEB Bulletin d’Information No. 222, Lausanne, ISBN 2-88394-025-8. “High-Strength Concrete” (1994), by J A Farny and C Panarese, Portland Cement Association, Engineering Bulletin, Skokie, USA. “High Performance Concrete” (1995), Recommended Extensions to the Model Code 90. Research Needs, CEB Bulletin d’Information No. 228, Lausanne, ISBN 2-88394-031-2. “Durability of Concrete Structures in the North Sea” (1996), State-Of-The-ArtReport, FIP, London, ISBN 1-874266-30-1. “Curing of High-Performance Concrete: Report of the State-of-the-Art” (1999), by K W Meeks and N J Carino, NISTR 6295, Gaithersburg, USA.

Books “High Strength Concrete” (1985), ed. by H Russel, ACI SP-87. “Production of High Strength Concrete” (1986), by M B Peterman and R L Carrasquillo, Noyes Publications, ISBN 0-8155-1057-8. “High Performance Concrete: From Material to Structure” (1992), ed. by Y Malier, E & FN Spon, London, ISBN 0-419-17600-4. “High Performance Concrete in Severe Environments” (1993), ed. by P Zia, ACI SP-140. “High Performance Concrete: Properties and Applications” (1994) ed. by S P Shah and S H Ahmad, Edward Arnold, London, ISBN 0-340-059922-1. “High Performance Concrete” (1998), by P-C Aïtcin, E & FN Spon, London and New York, ISBN 0-419-19270-0.

3.5

References

ACI Committee 357 (1978), “Design and Construction of Fixed Offshore Concrete Structures”, ACI. ACI Committee 363R-84 (1984), “State-Of-The-Art-Report on High Strength Concrete”, ACI Journal. Aïtcin P-C (1998), “High Performance Concrete”, E & FN Spon, London, ISBN 0-419-19270-0. Årskog V, Ferreira, M, Liu, G and Gjørv O E (2007), “Effect of Cement Type on the Resistance against Chloride Penetration”, Proc. V. 1 of 5th Int. Conf. on Concrete Under Severe Conditions: Environment and Loading, ed. by F Toutlemonde, K Sakai, O E Gjørv and N Banthia, Laboratoire Central des Ponts et Chaussées, Paris, ISSN 2-7208-2495-X, 367–374. Årskog V and Gjørv O E (2007), “A New City Development Project in Oslo Harbor With 300 Years Service Life Requirement”, Proc. V. 1 of 5th Int. Conf. on Concrete Under Severe Conditions: Environment and Loading, ed. by F Toutlemonde, K Sakai, O E Gjørv and N Banthia, Laboratoire Central des Ponts et Chaussées, Paris, ISSN 2-7208-2495-X, 851–862.

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Developments in the formulation and reinforcement of concrete

Bache H H (1981), “Densified Cement/Ultra-Fine Particle-Based Materials”, Aalborg Portland, Aalborg, Denmark. DNV – Det Norske Veritas (1974), “Rules for Design, Construction and Inspection of Fixed Offshore Structures”, Oslo, Norway. DuraCrete (1999), “General guidelines for durability design and redesign, The European Union – Brite EuRam III, Project No. BE95–1347: Probabilistic performance based durability design of concrete structures”, Report No. T7-01-1. FIP (1973), “Recommendations for the Design and Construction of Concrete Sea Structures”, London. FIP (1996), “Durability of Concrete Structures in the North Sea”, State-Of-The-ArtReport, FIP, London, ISBN 1-874266-30-1. FIP/CEB (1990), “High Strength Concrete”, State-Of-The-Art-Report, FIP SR 90/1/ CEB Bulletin d’Information No. 197, London, UDC 624-012-46. Gehlen C and Schiessl P (1999), “Probability-Based Durability Design for the Western Scheldt Tunnel”, Structural Concrete (2) 1–7. Gjørv O E (1968), “Durability of Reinforced Concrete Wharves in Norwegian Harbours”, Ingeniørforlaget, Oslo, 208 p. Gjørv O E (1993), “Durability of Concrete Structures and Performance-Based Quality Control”, Proc. Int. Conf. on Performance of Construction Materials in the New Millenium, ed. by A S El-Dieb, M M R Taha and S L Lissel, Shams University, Cairo, ISBN 977-237-191, 10 pp. Gjørv O E (1994), “Steel Corrosion in Concrete Structures Exposed to Norwegian Marine Environment”, Concrete International, 16, (4) 35–39. Gjørv O E (2002), “Durability and Service Life of Concrete Structures”, Proc. of 1st FIB Congress 2002, Session 8, 6, Japan Prestressed Concrete Engineering Association, Tokyo, 1–16. Gjørv O E and Rønning H R (1992), 1st Prize Award in a Compressive Strength of High Strength Concrete Contest, Norwegian Ready Mix Concrete Association, Oslo, Norway. Gjørv O E and Sakai K (2000), “Concrete Technology for a Sustainable Development in the 21st Century”, Proc. of Int. Workshop in Lofoten, Norway, ed. by O E Gjørv and K Sakai, E & FN Spon, London and New York, ISBN 0-419-25060-3, 386 p. Gjørv O E, Bærland T and Rønning H R (1990), “Abrasion Resistance of HighStrength Concrete Pavements”, Concrete International, 12, (1) 45–48. Goodspeed C H, Vanikar S and Cook R A (1996), “High Performance Concrete Defined for Highway Structures”, Concrete International, 18, (2) 62–67. Hasselø J A (2001), “Experiences with Floating Bridges”, Proc. of 4th Int. Symp. on Strait Crossings, Bergen Norway, ed. by J Krokeborg, Swets & Zeitlinger Publ., Lisse, ISBN 90-2651-845-5, 333–337. McGee R (1999), “Modelling of Durability Performance of Tasmanian Bridges”, Proc. of 8th International Conference on the Application of Statistics and Probability, Sydney, Australia. Meaas P, Landet E and Vindøy V (1994), “Design of Sahlhus Floating Bridge (Nordhordland Bridge)”, Proc. of 3rd Int. Symp. on Strait Crossings, Ålesund, Norway, ed. by J Krokeborg, Balkema Publ., Rotterdam, ISBN 90-5410-388-4, 729–734. Moksnes J (2007), Private communication.

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Moksnes J and Sandvik M (1996), “Offshore Concrete in the North Sea – A Review of 25 Years Continuous Development and Practice in Concrete Technology”, Proc. of Odd E Gjørv Symp. on Concrete for Marine Structures, ed. by P K Mehta, CANMET/ACI Ottawa, Canada, 1–22. Norwegian Association for Harbor Engineers (2007a), “Durable Concrete Harbour Structures-Recommended Specifications for New Concrete Harbor Structures”, TEKNA, Oslo, 2. edit., (In Norwegian). Norwegian Association for Harbor Engineers (2007b), “Durable Concrete Harbour Structures Practical Guidelines for Design and Durability, Quality Control”, TEKNA, Oslo, 2. edit., 48 p. (In Norwegian). OD – Norwegian Petroleum Directorate (1976), “Regulations for the Structural Design of Fixed Offshore Structures”, Stavanger, Norway. Sengul O and Gjørv O E (2007), “Chloride Penetration into a 20-year-old North Sea Concrete Platform”, Proc. V. 1 of 5th Int. Conf. on Concrete Under Severe Conditions: Environment and Loading, ed. by F Toutlemonde, K Sakai, O E Gjørv and N Banthia, Laboratoire Central des Ponts et Chaussées, Paris, ISSN 2-7208-2495-X, 107–116. Zhang M H and Gjørv O E (1991), “Mechanical Properties of High-Strength Lightweight Concrete”, ACI Materials Journal, 88, 240–247.

4 Sprayed concrete (shotcrete) N BANTHIA, University of British Columbia, Canada

4.1

Introduction

Shotcrete is an all-encompassing term used to describe pneumatically projected concrete or mortar using either the dry-mix process or the wet-mix process. “Gunite”, an old term, refers only to the dry-mix process. In the dry-process (Fig. 4.1), a bone-dry cementitious mixture is blown to the nozzle through a hose, where water is added by the nozzleman. Since an intimate mixing of the water and dry materials does not occur at the nozzle, dry-process shotcrete relies heavily on the skills of the nozzleman who manipulates the nozzle in order to produce an effective mixing on the application surface. The amount of water added at the nozzle is critical, as insufficient water will increase both the material rebound and dust, while excessive water will cause the mix to slough off. Only small aggregates are used in dry-process shotcrete, since large aggregates tend to rebound from the application surface. Cement contents are generally high as effective cohesiveness is required to reduce rebound and produce a certain shotcrete “build-up”. In the wet-process, a concrete mixture (typically ready-mix concrete) including all of the mix water is fed to the hopper of the machine, and the mixture is then pumped to the nozzle where compressed air is added to accelerate the mix onto the application surface (Fig. 4.1). Due to the reduced possibility of rebound, large aggregates (12.5 mm or larger) are often used in the mix. Since the nozzleman has no control over the mixture proportioning of the final product, the quality of wet-mix shotcrete is far less dependent upon the skills of the nozzleman. With less dust and less rebound, wet-mix shotcrete is becoming the preferred process of producing shotcrete. The choice of the process – dry or wet – however, depends upon the prevailing conditions at the site and the engineering culture of the region. One way to understand the differences between the two types of shotcrete and concrete is to compare their penetration resistances in the fresh state (Fig. 4.2; Refs. 4 and 5). Such curves are obtained by inserting an 98

Sprayed concrete (shotcrete)

99

A B

Dry-process A: Bone-dry cementitious materials B: Water Wet-process A: Premixed materials with water B: Air

Penetration resistance (MPa)

4.1 Dry-process and wet-process shotcrete (adapted from Ref. 1). 2.5 Dry-mix 2 1.5 1 Cast or wet-mix 0.5 0 0

2

4 6 Deflection (mm)

8

10

4.2 Consistency of shotcrete as measured by instrumented penetrometer.

instrumented penetrometer in the fresh state, and are often used in the case of dry-process shotcrete to determine its water content and ascertain its acceptability. As seen in Fig. 4.2, right after its placement, the in-situ penetration resistance of dry-process shotcrete is significantly higher than both cast concrete and wet-process shotcrete. It is generally believed that, in the case of shotcrete, the early products of cement hydration never get the time to rearrange themselves within the mix, resulting in a much greater penetration resistance.

4.2

Mix proportioning and process implications

The matrix in shotcrete is different from that in cast concrete, and has its own distinct features. Typical mix designs are given in Table 4.1. Placement using pneumatic compaction and lack of forms in shotcrete requires that the material be more cohesive, adhere well to the surface and resist sloughing off. Aggregates preferred in shotcrete are therefore generally rounded. Highly angular aggregates are known to cause problems in pumping, and

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Developments in the formulation and reinforcement of concrete

Table 4.1 Typical mix designs for dry- and wet-process fiber reinforced shotcrete Components

Dry-process 3

kg/m Cement Silica fume additives Blended aggregate Steel fibers Accelerators Superplasticizer Water reducer Air entraining agent Water Total

420 50 1670 60 13 – – – 170 2383

Wet-process % dry materials 19.0 2.2 75.5 2.7 0.6 – – – – 100

kg/m3

% wet materials

420 40 1600 60 13 6L 2L If required 180 2321

18.1 1.7 68.9 2.6 0.6 0.3 0.1 If required 7.7 100

also increase the possibility of rebound. Further, aggregates used in shotcrete tend to be smaller than in cast concrete. High aggregate rebound and placement difficulties further dictate that shotcrete mixes contain far less total aggregate content (fine and coarse) than cast concrete (2,3). While cast concrete has anywhere from 50–60% coarse aggregate by mass, shotcrete mixes only have 30% coarse aggregate content, which is further reduced during shooting due to rebound. Supplementary cementing materials (such as fly ash, silica fume and metakaolin) are used more commonly in shotcrete than in conventional cast concrete in order to achieve improved pumpability, better adhesion and cohesion, greater build-up thickness, reduction in the required quantity of accelerator, and a reduction in rebound. Shotcrete mixes already have higher than normal cement contents (400–450 kg/m3), and a further increase in the in-situ cement content occurs due to the higher than proportional rebound of aggregates during shooting; in the dry-process, it is not uncommon to have in-situ cement contents approaching 600–700 kg/m3 (4,5). Once in place, shotcrete also has a different spatial distribution of its various components. Water in dry-mix shotcrete, given that it is introduced only at or near the nozzle, is far less uniformly distributed through the placement than in cast concrete. In the wet-process, although the water is uniformly distributed, the pneumatic compaction results in internal voids that are far different in size ranges and spatial distribution than in cast concrete. Furthermore, the lack of bleed channels in shotcrete produces a material with different internal structure and transport properties than traditional cast concrete. Shotcrete, therefore, owing to its distinct mix design and placement procedures, develops a number of properties different from cast concrete.

Sprayed concrete (shotcrete)

101

4.3 X-ray image showing a preferential fiber orientation in shotcrete (6).

Unlike ordinary fiber reinforced concrete, in which fiber orientation is known to be three-dimensional random, in shotcrete, fibers are distributed in a more or less two-dimensional random fashion (6) (Fig. 4.3), due to the pneumatic placement. This introduces anisotropy, and has a clear influence on the reinforcing efficiency of the fibers.

4.3

Strength and stiffness

Shotcrete displays a different rheology, strength gain mechanism, compressive/tensile strength ratio, and creep characteristics. A commonly used indicator of matrix quality in cast fiber reinforced concrete is the water/cement ratio, which is expected to have a unique relationship with strength. The application of this relationship, however, requires, among other things, that the basic condition of complete consolidation be met – a condition rarely met in shotcrete. Another difficulty in applying a water/cement ratio vs. strength relationship to dry-process shotcrete is that an exact determination of the in-situ water/cement ratio in the mix is never possible (5). These factors when combined result in a poor correlation between the water/ cement ratio and strength of dry-process shotcrete, as illustrated in Fig. 4.4 (7). In the case of shotcrete, therefore, a direct measurement of porosity is necessary for assessing its strength and quality. When the volume of permeable voids (ASTM C642-97) is plotted as a function of strength, a much better correlation is seen to exist (5,7).

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Developments in the formulation and reinforcement of concrete

f¢c (MPa)

90

30 0

Exponential curve

Armelin; 1997 Bindiganavile; 1998

60

fc = 132.6e–1.93w/c R = 0.32 0

0.1

0.2

0.3

0.4

0.5

0.6

4.4 Water/cement ratio vs. dry-mix shotcrete compressive strength (7). Note a poor correlation.

For wet-mix shotcrete, while the water/cement ratio is the same as the one in the initial mix, strengths can sometimes be as much as 30% lower than its cast counterpart (8). Shotcrete has richer mixes to begin with, and during the process of shooting the rebound of aggregate renders the mixes even richer. It is well known that for concrete of a given water/cement ratio, leaner mixes with a higher aggregate/cement ratio develop higher strengths (9) and wet-mix shotcrete appears to follow a similar trend. However, when the flexural strength is considered, comparisons indicate that richer shotcrete mixes acquire greater flexural strengths than their cast counterpart for an identical water/cement ratio (8). Another property of interest is the deformability of the matrix itself as defined by its elastic modulus. Dry-mix shotcrete with a greater loss of aggregate through rebound exhibits a consistently lower elastic modulus than both wet-mix shotcrete and cast concrete (10). Wet-mix shotcrete, on the other hand, develops a different internal structure and creep characteristics from those of cast concrete (10).

4.4

Kinematics and rebound

One primary concern with the dry-process shotcrete, as discussed previously, is the high rebound; nearly 20–40% of material and up to 75% of fibers may be lost through rebound (4). The use of various mineral admixtures in shotcrete has therefore increased dramatically in order to control rebound (3). The rebound performance of four mineral admixtures – silica fume (SF), carbon black (CB), high reactivity metakaolin (HRM) and fly ash (FA) – in dry-process shotcrete is compared in Fig. 4.5 at 10% cement replacement rate (5). Notice that steel fiber rebound is always greater than the material rebound and increases proportionately with it. Note also a definite increase in rebound with an increase in the mean particle size of the admixture. The shape of the particle also appears to be important; while fly ash, silica fume and carbon black are all spherical, HRM has an irregular “platelet” structure.

Sprayed concrete (shotcrete)

Rebound (%)

40 30

CB - 0.05 μm SF - 0.1 μm HRM - 1.0 μm Fly ash - 10 μm Cement - 50 μm

Fiber y = 0.1853x + 35.598 R2 = 0.0412

Material

20

103

y = 2.3022x + 22.343 R2 = 0.3371

10 0 –1.5

–1

–0.5

0

0.5

1

1.5

2

Log (a); a = Mean size (μm)

4.5 Material and fiber rebound as a function of admixture particle size. Table 4.2 Particle velocities in dry-mix shotcrete Study

Reported particle velocity (m/s)

Stewart (1933)11 Ryan (1973)12 Valencia (1974)13 Ward and Hill (1977)14 Parker (1976)15 Blume et al. (1978)16

90–50 90–120 135 35–56 30–60 27–35

An additional concern, as discussed previously, is that the rebound material consists primarily of aggregates. Lack of sufficient aggregate volume for volumetric stabilization and insufficient curing render shotcrete highly susceptible to cracking due to plastic shrinkage (17). Pneumatic compaction in shotcrete necessarily requires its placement at a high velocity. In Table 4.2 some typical measurements of particle velocity are given (1). Notice the wide scatter in measured velocity values. Further, due to differences in particle shape, size and specific gravities, various particles will acquire different velocities in the shotcrete stream – a fact not reflected in Table 4.2. Using high-speed photography, an extensive investigation was carried out at the University of British Columbia on understanding the kinematics of particle motion in shotcrete. Figure 4.6 shows some high-speed images of a particle as captured by a high-speed camera running at 1000 frames/sec. These images were used to develop a generalized kinematic model of particle motion in shotcrete and combined with plasticity theory to predict rebound (18–21). Essentials of this model as applied to aggregates particles are presented below. Based on high-speed imaging, aggregate velocity (V) was found to be inversely proportional to its size (f) at a given air volume and would be described by a general expression (A and B are constants): 1 V = A ⎡⎢ ⎤⎥ ⎣f ⎦

B

4.1

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Developments in the formulation and reinforcement of concrete

(1) t = 0 ms

(2) t = 16 ms

(3) t = 33 ms

(4) t = 53 ms

Rebound

4.6 High-speed images of a fiber in dry-process shotcrete stream.

In modeling rebound in shotcrete, the usual assumption of fresh concrete being a Bingham fluid is of limited usefulness since it fails to account for the elastically stored energy in the fresh substrate, which when transferred back to the impinging particles may cause it to “pop” out of the bed. In addition, it is necessary to quantify the resistance a particle would experience to its penetration into the substrate at a high speed (which brings into relevance the strain-rate sensitivity of the substrate) and the adhesion developed between the substrate and the particle which would then resist debonding of the latter. For an experimental assessment of this process, a small portable penetrometer with a 25.4 mm diameter hemispherical ball was developed which is inserted into the bed of fresh shotcrete and then pulled out. In actual modeling of rebound, the process could then be divided into two phases, the penetration phase and the reaction phase. During the penetration phase, one could make use of the theory of plasticity (22) to develop the parameters of the hydrostatic stress-field that developed around the impinging aggregate. The final depth

105

b

Sprayed concrete (shotcrete)

p

dh

da

p

R

c

p a

p

b

du(a)

dh

p a p p p

4.7 Contact stress field developed during shotcreting.

of penetration of the particles would be determined simply by balancing the work necessary to create the indentation (pd · Va) and the kinetic energy available in the fast moving particle (W1). In other words (Fig. 4.7): W1 = pd ⋅ Va

4.2

In the reaction phase, the rebound energy (W2) is given by (22) W2 =

3p 2 2 3 ⎡ 1 − n c2 1 − n c2 ⎤ + p a* ⎢ 10 Ei ⎥⎦ ⎣ Ec

4.3

The coefficient of restitution, e, which is the percentage of energy returned to the particle for rebound is then given by e2 =

V′ V ′2 W2 = = 2 V V W1

4.4

From Eqns 4.2, 4.3 and 4.4, e can be further written as e2 =

( ) ( )( )

3p 2 4 R 10 p

3/4

1 m E* 2

−1 / 4

⋅ p ⋅ ( pd )−3 / 8 ⋅ V −1 / 4

4.5

where m is the mass of the impacting particle, p is the static penetration resistance (peak contact stress as determined using the penetrometer), pd is the dynamic penetration resistance and V is the particle velocity. The values of pd are obtained experimentally as the slope of the particle kinetic

106

Developments in the formulation and reinforcement of concrete

energy (W1) vs. volume displaced (Va) curve (Eqn 4.2). The values of p generally varied between 0.3 MPa (wet consistency) and 1.0 MPa (dry consistency) with a typical values of 0.5 MPa (Fig. 4.2). The dynamic contact stress, pd, on the other hand was typically greater than 3 MPa and depended strongly upon the mix design parameters. Equation 4.5 can be further simplified by using the expressions, Ec = Y/ecp, where Y is the yield strength of the substrate given by Y = p/3. e2 =

( )

3p 2 4 R 10 p

3/4

(2.25e cp )

( ) m 2

−1 / 4

⋅ p1 / 2 ⋅ pd −3 / 8 ⋅ V −1 / 4

4.6

If one can assume that ecp is a constant, Eqn (4.6) can be rewritten in terms of a constant K and an impact factor, Y: e = K .y where K =

4.7

( )

3p 2 4 R 10 p

3/4

(2.25e cp )

( ) m 2

−1 / 4

and y = p1 / 2 ⋅ pd −3 / 8 ⋅ V −1 / 4

Notice that in the final form (Eqn 4.7) the rebound energy of a particle can be entirely represented by the impact factor Y. With a known value of the coefficient of restitution e, the available energy for rebound W2 can be calculated. This can then be equated to the energy necessary for debonding (WD) given by WD = [s 0 (pa *2 )]d *

4.8

And finally, the condition of rebound is given by W2 ≥ 1 ⇒ REBOUND WD

4.9

During the computations, the process of rebound was modeled as a stochastic process with particle velocity, static and dynamic contact stresses and the ultimate concrete strain taken as the stochastic variables. The model thus determined the probability of rebound for a given particle size and when all particle sizes in a given mix were considered, the overall rebound could be predicted. Some model predictions are compared with the experimental findings in Figs 4.8–4.11. In Fig. 4.8, aggregate rebound is plotted as a function of aggregate size. Notice that the model predicts a linear variation in rebound with the logarithm of aggregate size, which relates very well to the experimental observations. In Fig. 4.9, the influence of silica fume content is shown. Once again, the model accurately predicts a decrease in rebound with an increase in the silica fume content as observed in reality. In Fig. 4.10, rebound dependence on cement content is shown. Notice a drop in rebound with an increase in the cement content, and a good prediction by

Aggregate rebound (% by mass)

Sprayed concrete (shotcrete)

107

80 70 60 50

Experimental, R2 = 0.89

40

2

Analytical, R = 0.99

30 20

Experimental Analytical

10 0 0.01

0.1

1 10 Aggregate size (mm)

100

4.8 Rebound as predicted by model for various aggregate sizes.

Overall rebound (% by mass)

50 Experimental Analytical

45 40

Analytical, R2 = 1

35 30

Experimental, R2 = 0.56

25 20 –2

0

2

4

6

8

10

12

14

16

18

Silica fume content (% mass subst to cement)

Overall rate of rebound (% by mass)

4.9 Rebound as predicted by model for various silica fume contents. 55 Experimental Analytical

50 45 40

Experimental, R2 = 0.65

35 30

2 Analytical, R = 0.99

25 20 300

350 400 450 500 550 Cement content of the mix (kg/m3)

600

4.10 Rebound as predicted by model for various cement contents.

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Developments in the formulation and reinforcement of concrete

Overall rebound (% by mass)

70 60

Analytical, R2 = 0.94

50 Experimental, R2 = 0.84

40 Experimental Analytical

30 20

0

1

2

3

4

5

6

7

8

9

Shooting consistency to 9 mm cylindrical needle (MPa)

4.11 Rebound as predicted by model for various shooting consistencies.

the model. Finally, the influence of shooting consistency on rebound is plotted in Fig. 4.11. Notice an increase in the rebound when shotcrete with a higher shooting consistency (i.e., a greater resistance to penetration needle) is shot. Note also a good match between the experimental findings and the model predictions. In the case of fibers, the fiber rebound is shown to be proportional to a specific fiber parameter called the modified aspect ratio (l/√d) (23). High fiber rebound in shotcrete remains a critical issue and significant further research is needed.

4.5

Toughness, impact resistance and fiber reinforcement

Due to the nature of the applications, requirements of material deformability, toughness and energy absorption are often greater in shotcrete than in cast concrete. In repair applications, a greater toughness and cracking resistance is required for durability. When used as ground covering or support, two distinct situations may arise: in the first, slow quasi-static ground movements may occur and in the second, rapid or dynamic ground deformations may occur. The latter, often called “rock-bursts”, impose very severe toughness requirements on shotcrete as discussed below. For an enhancement of quasi-static toughness, energy absorption and impact resistance fibers are generally used in shotcrete. Fibers control cracking, and act as stress-transfer bridges thus enhancing resistance to crack growth and fracture (24). Some typical load deflection responses for fiber reinforced shotcrete beams are shown in Fig. 4.12. While plain shot-

Sprayed concrete (shotcrete)

Load (kN)

20 18 16 14

109

DD fiber (0.5%) hybridized with 0.5% of secondary micro fiber

DD fiber at 0.5%

12 10 8 6 4 2

DD fiber at 0.3%

DD fiber at 0.1%

0 0

0.2

0.4

0.6

0.8 1 1.2 Displacement (mm)

1.4

1.6

1.8

2

4.12 Toughness of shotcrete (ASTM C1018) for various fiber systems. 35

Material Fiber

Rebound (%)

30 25 20 15 10 5 0 Plain-Dry Plain-Wet 30 kg/m3

60

30

60

kg/m3

kg/m3

kg/m3

Dry-mix

Wet-mix

4.13 Comparison of two processes for fiber reinforced shotcrete: rebound.

crete beams would fail in a brittle manner, fiber reinforced shotcrete beams would carry loads far beyond the peak load resulting in a curve with a long descending branch. The area under this curve is a measure of the absorbed energy and it is often called “toughness”. Note in Fig. 4.12 that the improvements in “toughness” are proportional to fiber volume fraction, and that fiber hybridization appears to be highly effective. The process used for shotcrete, dry or wet, is expected to influence not only the rebound values but also the strength, toughness, and long-term durability of fiber reinforced shotcrete. In Fig. 4.13, the two processes are compared on the basis of rebound (25). Notice that the rebound in the dryprocess – for both material and fiber – far exceeds the rebound in the wetprocess. Note also that the percentage rebound is inversely proportional to the initial fiber content in the design mix.

110

Developments in the formulation and reinforcement of concrete

Load (kN)

20 16 Batched: 60 kg/m3 In-situ: 50 kg/m3 Batched: 30 kg/m3 In-situ: 24 kg/m3

12 8 4 0

Plain dry-mix shotcrete

0

1

0.5

1.5

2

2.5

3

3.5

4

Deflection (mm) (a)

Load (kN)

20 16

Batched: 60 kg/m3 In-situ: 57 kg/m3

12 8 4 0

Batched: 30 kg/m3 In-situ: 28 kg/m3

Plain wet-mix shotcrete

0

0.5

1

1.5

2

2.5

3

3.5

4

Deflection (mm) (b)

4.14 Comparison of flexural toughness for the two processes: (a) dryprocess and (b) wet-process.

In Fig. 4.14, “flexural toughness” curves of fiber reinforced shotcrete produced by the two processes are compared. These curves were obtained as per the ASTM C1018 procedure.* Notice that the “toughness” of wetprocess shotcrete for a given initial design mix fiber content is greater than that of the dry-process shotcrete. This is expected, as the fiber rebound in the dry-process is far greater than that in the wet-process. However, a closer observation of Fig. 4.14 reveals that even for a given effective in-place fiber content, the wet-process produces a greater toughness. The reasons for this are not clear, and in fact the observation is a bit puzzling as the fibers in the dry-process shotcrete are expected to be more preferentially aligned (see Fig. 4.3) than in the wet-process shotcrete and this preferential orientation would be expected to produce a better toughness. This, however, does not appear to be the case. Rock-bursts occur in tunnels in hard-rock mines due to high in-situ, mining and seismically induced stresses. Three main mechanisms (26) are * ASTM C1018 has recently been withdrawn, and replaced by ASTM C1609. However, the procedure for obtaining the load vs. deflection curves is essentially the same in both standards.

Sprayed concrete (shotcrete)

111

recognized: seismically induced rock falls where a seismic wave accelerates a volume of rock, rock mass fracturing and bulking due to strain build-up at an opening, and finally, rock ejection due to momentum transfer from a remote disturbance. When ejection of a rock occurs, it is not uncommon to have rocks almost 1 m size ejected with typical ejection velocities of about 6 m/s (27) and as high as 50 m/s (28). Typically, support systems are expected to withstand a dynamic energy release of about 100 kJ/m2 (29,30), and although some full-scale impact tests have been reported (31), there has hardly been any systematic study of the influence of such dynamic impact events on shotcrete linings of different thickness, with different types of reinforcement including fibers, and rock bolting patterns. Particularly lacking is a thorough understanding of the constitutive response of shotcrete materials under variable strain-rates, and the interaction between the retaining elements and the containment elements and its influence on the structural response of the entire assembly.

4.6

Concluding remarks

This chapter presents a brief state-of-the-art on shotcrete. It demonstrates that due to its unique mix designs, placement techniques, compaction dynamics, strength gain mechanisms, and internal structure, shotcrete is distinctly different from cast concrete, and hence our conventional understanding of cast concrete should be applied to shotcrete only with caution. Significant further efforts are necessary for a fundamental understanding of the nature of shotcrete and to develop materials suitable for various applications. This chapter places particular emphasis on fiber reinforced shotcrete and highlights the various benefits of fiber inclusions in shotcrete.

4.7

Acknowledgements

The author would like to thankfully acknowledge the continued financial support of the Natural Sciences and Engineering Research Council of Canada.

4.8

References

1 Austin, S.A. and Robins, P.J. (Eds), Sprayed Concrete: Properties, Design and Applications, Whittles Publishing, 1995. 2 Gilkey, H.J., “Water-Cement Ratio vs. Strength: Another Look”, J. of American Concrete Institute, 57(10), 1961, 1287–1312. 3 Morgan, D.R., McAskill, N., Neill, J. and Duke, N. F., “Evaluation of silica fume shotcretes”, Proceedings of CANMET/ACI workshop on condensed silica fume in concretes, Montreal, 1987.

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4 Armelin, H.S., “Rebound and Toughening Mechanisms in Steel Fiber Reinforced Dry-mix Shotcrete”, Ph.D. Thesis, University of British Columbia, 1997. 5 Bindiganavile, V. and Banthia, N., “Rebound in Dry-Mix Shotcrete: Influence of Type of Mineral Admixture”, ACI Materials Journal, 97(2), 2000, 1–5. 6 Ramakrishnan, V., Coyle, W.V., Dahl, L.F. and Schrader, E.K., “A Comparative Evaluation of Fiber Shotcretes”, Concrete International, Jan. 1981. 7 Banthia, N. and Bindiganavile, V., Proc. International Conference on Infrastructure Regeneration and Rehabilitation, Sheffield, 1999. 8 Banthia, N., Trottier, J.-F. and Beaupré, D., “Steel Fiber Reinforced Shotcrete: Comparisons with Cast Concrete”, ASCE, J. of Materials in Civil Eng., 6(3), 1994, 430–437. 9 Neville, A., Properties of Concrete, Fourth edn. John Wiley and Sons, Inc., 1996. 10 Chan, C., Banthia, N., and Sakai, K., Proc. Int. Workshop on Concrete Technology for a Sustainable Development in the 21st Century, Svolvaer, Norway, June 1998. 11 Stewart, E.P., New Test Data Aid Quality Control of Gunite. Engineering NewsRecord, Nov. 9, 1933, 4. 12 Ryan, T.F., Gunite: A Handbook for Engineers. Cement and Concrete Association, Wexham Springs, 1973, 63. 13 Valencia, F.E., Practical Aspects of Shotcrete Application. Use of Shotcrete for Underground Structural Support, SP-45, ACI/ASCE, Detroit, 1974, 114–129. 14 Ward, W.H. and Hills, D.L., Sprayed Concrete – Tunnel Support Requirements and the Dry-mix Process, Shotcrete for Ground Support, SP-54, ACI, Detroit, 1977, 475–532. 15 Parker, H.W., Field Oriented Investigation of Conventional and Experimental Shotcrete for Tunnels, PhD. Thesis, University of Illinois at Urbana-Champaign, USA, 1976, 630. 16 Blumel, O.W., Lutsch, H. and Stehno, G., State-of-the-Art Shotcrete Technology. Shotcrete for Underground Support III, Engineering Foundation, New York, 1978, 15–26. 17 Banthia, N. and Campbell, K. “Restrained Shrinkage Cracking in Bonded Fiber Reinforced Shotcrete”, RILEM – Proc. 35, The Interfacial Transition Zone in Cementitious Composites, Eds. Katz, Bentur, Alexander and Arligui, E and F N. Spon, 1998, 216–223. 18 Armelin, H. S. and Banthia, N., ‘Mechanics of Aggregate Rebound in Shotcrete – (Part 1), RILEM, Materials and Structures, 31, March 1998, 91–98. 19 Armelin, H.S. and Banthia, N., “Development of a General Model of Aggregate Rebound in Dry-mix Shotcrete – (Part 2)”, RILEM Materials and Structures, 31, April, 195–202. 20 Armelin, H.S., Banthia, N., Morgan, D.R. and Steeves, C., “Rebound in Dry-Mix Shotcrete”, Concrete International, 19(9), 1997, 54–60. 21 Armelin, H.S., Banthia, N. and Mindess, S., “Kinematics of Dry-Mix Shotcrete”, ACI Materials J., 96(3), 1999, 283–290. 22 Hill R., The Mathematical Theory of Plasticity, Oxford University Press, London, 1950. 23 Armelin, H.S. and Banthia, N., “Steel Fiber Rebound in Dry Mix Shotcrete: Influence of Fiber Geometry”, ACI Concrete International, 20(9), 1998, 74–79.

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24 Banthia, N., Trottier, J.-F., Wood, D. and Beaupré, D., “Steel Fiber Dry-Mix Shotcrete: Influence of Fiber Geometry”, ACI Concrete International, 14(5), 1992, 24–28, and “Influence of Fiber Geometry in Wet-Mix Steel Fiber Reinforced Shotcrete”, ACI Concrete International, 16(6), 1994, 27–32. 25 Bindiganavile, N. and Banthia, N., “Process Dependence of Shotcrete for Repair”, International Journal of Materials & Product Technology, UK, 2005, 23(3), 240–256. 26 Kaiser, P.K., Proc. 3rd Int. Symp. on Rock Burst and Seismicity in Mines, Balkema, Rotterdam, 1993, 13–27. 27 Jager, A.J., et al., Proc. Int. Deep Mining Conference: Technical Challenges in Deep Level Mining, South African Institute of Mining and Metallurgy, 1990, 1155–1177. 28 Ortlepp, W.D., 3rd Int. Symp. on Rock Burst and Seismicity in Mines, Balkema, Rotterdam, 1993, 101–106. 29 Stacey T.R., Ortlepp, W.D. and Kirsten H.A.D., J of the South African Institute of Mines and Metallurgy, May–June 1995, 137–140. 30 Kirsten, H.A.D., “Fiber Reinforced Shotcrete”, World Tunneling, Nov. 1997, 411–414. 31 Tannant, D.D., Kaiser, P.K. and McCreath, D.R., Large Scale Impact Tests on Shotcrete, Laurentian University, March 1995, 45.

5 Hot weather concreting C ISHEE, Florida Department of Transportation, USA

5.1

Introduction

Hot weather concreting is generally referred to as the placement of concrete in weather conditions which require attention to avoid excessive heating or drying of the placed concrete. Building codes and specifications generally limit the maximum placing temperature of concrete to 30°C [85°F] to ensure that the concrete does not set early or show signs of plastic shrinkage cracking upon hardening. When the concrete is placed at temperatures above this, it is commonly termed hot weather concreting. Hot weather concrete does not only refer to placing at higher temperature, but can include other situations that would cause similar effects such as high air temperature, low relative humidity, wind velocity and intensity of solar radiation. Any of these situations or combination thereof can adversely affect the quality of fresh and hardened concrete properties (ACI 305R-99). Most of the time these situations cannot be avoided and a high quality concrete is desirable to produce a structure that will have a long service life. Certain precautionary measures are required to ensure the desired service life is obtained.

5.2

Applications/case studies

Difficulties in placing concrete in high temperatures include the likelihood of problems with transporting, finishing and consolidation, which typically results in a hardened concrete product with lower strength and subject to cracking. These effects can be overcome to produce a high performing concrete if material selection and evaluation is performed carefully, concrete placing temperatures are controlled, plastic properties are taken into consideration, and anticipated hardened properties are used in the design of the structure. 114

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5.2.1 Material selection and mix design evaluation One of the best practices to ensure adequate performance from a concrete mixture is to select a concrete mix design that is going to be appropriate for the placement and environment that the structure will be exposed to. Many of the typical mix design calculations can be used in choosing the proportion of coarse aggregates, fine aggregates, cement, mineral admixtures, chemical admixtures and water. One of the more difficult issues with hot weather concreting is to address the selection of cement type, mineral admixture addition and chemical admixtures used for the concrete mix. Another aspect of concrete mix design that is often overlooked is creating a trial batch and exposing it to conditions that simulate hot weather concrete conditions to ensure a successful placement.

Selection of cement Selection of the most appropriate cement for an application can be difficult. ASTM C150 specifies the use of five different types of Portland cements. The two types of cements applicable to hot weather concrete are a slowersetting Type II or Type IV. The Type II cement is generally selected because of the lower heat of hydration and lower content of tricalcium aluminate (C3A). The Type IV cement is best for massive and hot weather concrete applications but is not readily available in the United States or other parts of the world. Concretes produced with the slower-setting cements will be less likely to exhibit plastic-shrinkage cracking. Selecting a Portland cement with a slower rate of heat development and simultaneously producing a lower dissipation of heat from the concrete will result in lower peak temperatures. This will result in less thermal expansion and a lower risk of thermal cracking (ACI 305R-99). Most recently, the specification for cement (ASTM C150 or AASHTO M 85) has included a maximum heat of hydration to reduce the amount of heat generated by the typical Type II cement. The Florida Department of Transportation allows a maximum on 335 kJ/kg [80 cal/g] for concrete placed in severe environmental conditions. If the Type II cement exceeds this limit, but is less than 370 kJ/kg [88 cal/g], then the concrete mix design must have a mineral admixture to replace a portion of the cement to reduce the total amount of heat generated (FDOT 2007). If available, blended hydraulic cement (ASTM C1157 or ENV197-1) is better for most hot weather applications. Blended hydraulic cements are obtained by creating a composite of reactive materials such as: Portland cement clinker, granulated blast furnace slag, pozzolanic materials, fly ash, burnt shale or silica fume. Selection of an appropriate hydraulic cement would be based on four requirements:

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(1)

If the hydraulic cement contains lime as calcium as CaO and magnesia as MgO, then their presence must be in the form of free lime and not periclase. This can be tested by measuring the autoclave expansion and contraction (ASTM C595). (2) Investigating the demand for water of the hydraulic cement. As the amount of the secondary material increases the lowering of water demand is a considerable attribute. (3) Have hydraulic cement that has a heat of hydration between 200 kJ/kg [48 cal/g] and 250 kJ/kg [60 cal/g] at 72 hours. This type of hydraulic cement should produce enough heat to set the concrete in a workable time, but not so much heat as to induce cracking. (4) Determine if there is potential for any delayed expansion problems in the concrete. Duggan and Scott (1989) proposed a test which states that if more than a 0.05% expansion occurs within 20 days of casting then there is a high risk of deleterious expansion and potential of delayed expansion problems (Owens 1992). One of the trends in current construction practices is to obtain access to in-place concrete elements as quickly as possible. As such, there is an increased demand for early strength requirements. Therefore, there has been a general trend to increase the Portland cement content in a typical mix design. However, for hot weather concrete applications, such practice is contradictory to getting a better performing element. Hot weather concreting results in an increase in the hydration rate and thereby increases the early-age properties. When additional Portland cement is added to the mix, additional production of heat will be generated within the element at early ages. This excessive heat production will result in an increase in the thermal expansion of the concrete and increase the potential for thermal cracking. To reduce the risk of excessive early age thermal expansion, the reduction of Portland cement within the mixture and the replacement of portions of the Portland cement with mineral admixtures are beneficial. Additionally, lower water content in the concrete below 150 liters/m3 [252 lbs/yd3] is desirable to reduce the plastic shrinkage cracking (Owens 1992).

Use of mineral admixtures Mineral admixtures have been used in concrete for as long as concrete has been made. However, some of the benefits of mineral admixtures are just recently being understood. Some typical mineral admixtures used in concrete are fly ash, ground granulated blast furnace slag and silica fume. Each of these mineral admixtures affects the properties of the concrete differently in hot weather conditions. Fly ash is the byproduct of coal burning power plants and is used to replace portions of the Portland cement in mixture design. ASTM C618

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outlines the requirements of fly ash for use in concrete as a mineral admixture. Fly ash typically is used at a replacement rate of 20–40% of ordinary Portland cement. Fly ash can reduce the rate of slump loss of concrete under hot weather conditions and this reduction in slump loss is inversely proportional to the percentage of cement that was replaced. Fly ash typically reduces the early age rate of strength gain of the concrete, but this is recovered at later ages. Due to the reduction in early age strength, fly ash has a greater potential for plastic shrinkage cracking and requires adequate curing to protect the concrete from cracking (Soroka and Ravina 1998). Ground granulated blast furnace slag (GGBFS) is a byproduct of the steel mill industry and is also used as a mineral admixture in concrete production. ASTM C989 outlines the requirements of GGBFS. GGBFS typically is used at a replacement rate of 30–70% of ordinary Portland cement. If mixed and cured properly, concrete made with GGBFS has better performance characteristics in hot weather conditions than ordinary Portland cement with respect to strength and pore structure. Concrete with GGBFS should be moist cured as quickly as possible and for a minimum of 7 days to ensure that the material achieves its benefits (Austin and Robins 1992). Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys and needs to meet the requirements of ASTM C1240. Silica fume typically is used at a replacement rate of 5–10% of ordinary Portland cement. Silica fume can be used to increase the strength of concrete and to reduce the permeability. The increase in strength causes an inverse in plastic shrinkage of the concrete. Increasing the percentage of silica fume in a concrete mix will also increase the plastic shrinkage. However, the plastic shrinkage, if controlled properly, can be permitted with no cracking in the concrete due to the early increase in strength. The fineness of the silica, as represented by its specific surface area and bulk density, is a strong indicator of its potential to increase plastic shrinkage cracking in hot weather (Al-Amoudi et al. 2006). In Florida, the typical concrete mix design for hot weather conditions will use a fly ash or GGBFS. These materials typically cost less than Portland cement and both can help reduce the amount of heat generated in the concrete mix design. When the concrete is going to be placed in a severely aggressive environment, exposed to sea water and in hot weather conditions, the typical mix design will also have silica fume as a ternary blend to provide early strength gain and reduce the amount of heat generated. Use of chemical admixtures There are many types of chemical admixtures that can provide better performance characteristics of concrete. Some of these benefits include lower

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water demand and extended periods of workability. Chemical admixtures can provide these benefits without any reduction in strengths comparable to concrete without chemical admixtures placed at lower temperatures. The effectiveness of the chemical admixtures is dependent upon the reactions with the cement they are to be used with. Any chemical admixture without a history of better performance in hot weather conditions should be evaluated prior to use (ACI 305R-99). Chemical admixtures should be added in accordance with the manufacturer’s technical data sheets. The dosage of most chemical admixtures is mix design specific and should be evaluated prior to use. Some chemical admixtures are designed to allow for an extended setcontrol of the freshly mixed concrete. Most extended set-control admixtures comply with the requirements of ASTM C494 as a Type B, retarding admixture, or Type D, water-reducing and retarding admixture. These admixtures are often referred to as hydration control admixtures and benefit the concrete in that they can temporarily stop the hydration process of both the silicate and aluminate phases in the Portland cement. These extended set-control admixtures are designed to allow for longer haul times or for additional finishing times when needed. Water-reducing admixtures are chemical admixtures designed to reduce the water:cement ratio of concrete without adversely affecting the rheological properties. Most of these water-reducing admixtures comply with the requirements of ASTM C494 as Type A, water-reducing, or Type F, highrange water-reducing mixtures. One major benefit of these materials is that they provide up to 15% of the water in a concrete mix design. Typically, water-reducing admixtures do not affect the setting time of the concrete at lower dosages, but at higher dosages can increase the setting time. Other chemical admixtures can provide high-range, water-reducing and retarding effects on the freshly mixed concrete. Most high-range, waterreducing and retarding admixtures will comply with the requirements of ASTM C494 as a Type G and ASTM C1017 as a Type II for plasticizing and retarding admixtures. These admixtures are often referred to as superplasticizers and can provide significant benefits for producing flowing concrete in hot weather concrete conditions (ACI 305R-99). Most superplasticizers are synthetic water-soluble polymers such as sulfonated naphthalene formaldehyde (SNF), sulfonated naphthalene polymer (SNP), modified sugar-free lignosulfonate polymer (MLP), and most recently polycarboxylic ether polymers (PCE). Research has shown that the type of superplasticizer affects the plastic shrinkage strain in the concrete (AlAmoudi et al. 2006). The interaction between the cement and the superplasticizer is crucial because there have been cases where the wrong combination resulted in faster slump loss and additional plastic shrinkage (Ravina and Soroka 2002).

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Most of the concrete produced in the United States will have some form of chemical admixture added to the mix design. With hot weather conditions, admixtures are typically used to control the plastic properties of the mix without any long-term strength reductions. It is not uncommon to see multiple chemical admixtures in a mix design designed for hot weather conditions. In Florida, a combination of a high-range, water-reducing and retarding admixtures will be used with a mid-range, water-reducing admixture. The high-range admixture will be kept constant in all of the production loads of concrete and the mid-range will be adjusted for changes in temperature, humidity, and absorption of the aggregates. Because of the amounts of chemical admixtures being used, a majority of the concrete facilities have concrete mix designs for the hot weather summer conditions and a different set of mix designs for winter conditions. Mix design verification process Prior to large-scale production, mix designs should be verified for use in hot weather concreting. A trial mix should be batched in accordance with ASTM C192 with a few exceptions. The batch size should be large enough to reproduce typical issues regarding the heat produced by the cementitious materials, at least 0.1 m3 [3.5 ft3]. The slump of the trial batch should be determined after the initial mixing procedure (three minutes of mixing, three minutes of rest, and two minutes of remixing). After mixing, the mixer should be stopped and covered with wet burlap or an impermeable cover material. The trial batch should remain in the mixer for at least 90 minutes or the anticipated transit time after the completion of the initial mixing procedure. During the extended mixing period, the concrete should be remixed intermittently for 30 seconds every five minutes after which it should be re-covered. At the end of the 90 minute period, the concrete should be remixed for a minimum of one minute and a slump reading should be obtained to verify that the concrete slump is within the desired range. If the slump is below the target range, water may be added to adjust the slump to achieve the desired results. If additional water is added, the concrete should be remixed for a minimum of two minutes. Other desired plastic properties (such as temperature, air content, and unit weight) should be acquired and samples should be cast to obtain hardened properties. It is important to ensure that the mix temperature is not less than 35°C [95°F] at any time during the mixing process. Plastic properties of the concrete should be within the allowable ranges or close to the desired target after water additions and final mixing. The procedure listed above is the process that is used to approve any concrete mix design for hot weather use on a Florida Department of Transportation construction project (FDOT 2007).

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The total amount of water added to the mix (including any additional water added to adjust the final slump) constitutes the design mix water content. Ensure that the total water to binder ratio does not exceed the maximum water to binder ratio desired or specified. If a procedure for cooling of the concrete is to be evaluated, it should follow the above stated mixing procedure. As an alternative to the trial mixing procedure, a full-size production batch may be used for verification of the mixture proportions, provided the required temperature levels of the concrete are obtained (ACI 305R-99).

5.2.2 Cooling of concrete Once the concrete has been proportioned to reduce the amount of heat produced, and if additional mix design adjustments do not reduce the maximum temperature, then cooling of the concrete prior to placement is necessary to achieve the desired concrete properties. One of the easiest and most cost effective ways to limit the placing temperature of concrete is to properly engineer the process for producing the concrete. There are several ways to limit the placement temperature of the concrete: selection of time and temperature of placement, use of chilled water replacement for mixing water, use of ice water for replacement of mixing water, cooling of coarse aggregates, and use of liquid nitrogen. Time and temperature of placement One of the cheapest ways to ensure cool concrete is to schedule the placement at the coolest part of the day, which is typically at night or early in the morning. The relative humidity and wind velocity may also need to be taken into account to prevent the concrete from dehydrating. The distance the concrete will be hauled should be kept as short as possible so as not to expose the concrete to additional heat and the truck staging time should be minimized to keep the concrete temperature low. Additionally, the concrete should be placed and finished as quickly as possible to minimize the prolonged exposure of concrete to the higher temperatures. Formulas given in ACI 305R-99 can be used to predict the concrete temperatures as expressed in equation 5.1 T=

0.22(TaWa + Tc Wc ) + Tw Ww + TaWwa 0.22(Wa + Wc ) + Ww + Wwa

where Ta Tc

= temperature of aggregate = temperature of cement

[5.1]

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Tw = temperature of batched mixing water from normal supply excluding ice Wa = dry mass of aggregate Wc = mass of cement Ww = mass of batched mixing water Wwa = mass of free and absorbed moisture in aggregate at Ta (Note: All temperatures are in ºC or ºF and all masses are in kg or lb) (ACI 305R-99). Concrete can expect to gain approximately 8°C [15°F] for each hundredweight of cement being used in a 0.75 m3 [1 yd3] if heat dissipation does not take place. If a concrete mix design were to contain 227 kg [500 lb] of cement and is placed at 32°C [90°F], then it can be anticipated that the concrete will increase in temperature to 82°C [180°F] within a day. With normal dissipation, the same 32°C [90°F] placed concrete could reach 60°C [140°F] before night. In the event the night time temperature dropped drastically, then it is likely that the exposed surface of the concrete could crack due to the differential temperature gradients. By reducing the initial placing temperature, the likelihood of cracking due to thermal effects is reduced (Scanlon 1997). Any large concrete placements in Florida will be placed in either the evening hours or early morning hours to reduce the concrete temperature as much as possible. This method can be very effective if the placement is not thicker than 1 m [3 ft], the materials are carefully selected, and the placement occurs just before sunrise.

Chilled water replacement for mixing water Concrete mixed with chilled water is often used to lower the temperature. The use of chilled batch water alone will lower the concrete temperature approximately 5°C [9°F]. Recently, manufacturers of heat pumps have been advertising equipment which can be used to cool the concrete as well. There are two types of heat pumps that are typically used to cool concrete: water to water and water to air. Water to water heat pumps are the most common and the most economical method used. In the event no groundwater is available, the water to air method can be used. Most heat pump modules can cool 75 m3 [100 yd3] of concrete in a twelve-hour period. If a higher rate is necessary, then additional modules are typically employed. Heat pumps which are used to cool the concrete for hot weather applications can also be used to heat concrete for cold weather applications. In Florida, this method of cooling concrete has not been used very often because of the initial capital costs and because typically the concrete temperature needs to be lowered at a higher rate than can be obtained with heat pumps.

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Ice water replacement of mixing water The most commonly used method for reducing the temperature of the concrete is to replace a portion of the mixing water with ice. This has two benefits: first, it lowers the mix water temperature and secondly, it lowers the mix temperature of the concrete by extracting heat during the phase change from ice to water. Ice can be used to substitute up to 80% of the batch water. Typically the amount of cooling that is achieved is limited to about 11°C [20°F]. Ice can be added directly into a ready-mix truck or premixed at the plant in large tanks with the mixing water (Sumodjo 2005). In order to ensure correct proportioning, the ice must be weighed and introduced into the mixing operation in finely graded sizes. This process typically requires a large capital investment due to the cost of ice, transportation, refrigerated storage, and handling. If blocks of ice are used, then a crusher/ slinger unit will also be required to finely crush a block of ice and blow it into the mixer. An alternative is to establish an ice plant near the concrete facility. Typical ice plants can produce 40 metric tons [44 tons] of ice in a 24-hour period. Formulas given in ACI 305R-99 can be used to predict the concrete temperatures as expressed in equations 5.2 and 5.3. With ice (SI units): T=

0.22(TaWa + Tc Wc ) + Tw Ww 0.22(Wa + Wc ) + Ww + Wi + Wwa TaWwa − Wi (79.6 − 0.5Ti ) + 0.22(Wa + Wc ) + Ww + Wi + Wwa

[5.2]

With ice (in.-lb units): T=

0.22(TaWa + Tc Wc ) + Tw Ww 0.22(Wa + Wc ) + Ww + Wi + Wwa 79.6Ww TaWwa − Wi (128 − 0.5Ti ) + 0.22(Wa + Wc ) + Ww + Wi + Wwa

[5.3]

where Ti = temperature of ice Wi = mass of ice (Note: The temperature of free and absorbed water on the aggregate is assumed to be the same temperature as the aggregate; all temperatures are in ºC or ºF) (ACI 305R-99). The use of ice to replace part of the mixing water is rather frequent. Some of the benefits of using ice to lower the placing temperature are: it can be purchased when needed, large quantities are available, and the amount of ice can be increased or decreased depending on the temperatures through-

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out the day. The ice can be added at a second staging area in the concrete production facility so as to not slow down the amount of concrete being delivered to a project site. Cooling of coarse aggregates Coarse aggregates typically make up 60 to 80% of a concrete mix design. Thus, the use of aggregates with a decreased temperature can have significant effects on the concrete placing temperature. Reducing the coarse aggregate temperature by 1°C [2°F] typically lowers the placing temperature of the concrete by 0.5°C [1°F]. Typical processes used to lower aggregate temperatures include: sprinkling, air blast or water chilled soaking. When smaller amounts of temperature reduction are desired, sprinkling the coarse aggregate with water can lower the concrete placing temperatures by 5°C [9°F]. This process relies on the evaporative cooling of the coarse aggregate utilizing only enough water to keep the stockpile wet, not saturated. Use of chilled water is unnecessary, as the heat loss is a result of evaporation. This is one of the more economical methods to cool concrete but has a limited amount of usage (Sumodjo 2005). Another method used for cooling the aggregates is by blowing air through the moist aggregates. The air flow can bring the aggregate to within 1°C [2°F] of the wet bulb temperature and will enhance evaporative cooling. The effectiveness of this method depends upon the ambient temperatures, relative humidity, and velocity of the air flow through the coarse aggregates. By adding a chilled air unit instead of air at ambient temperatures, the air cooling system can reduce the concrete placing temperature by 13.5°C [25°F]. The typical amount of coarse aggregates that can be air cooled is about 150 m3 [200 yd3] per hour. The main drawback is a relatively high installation cost compared to other methods (ACI 305R-99). If large temperature reductions are needed, then soaking the coarse aggregates in chilled water is required. The effectiveness of this method relies on the concrete facility being able to contain the coarse aggregate in a bin or silo so that the material is cooled in a short period of time. Care must be taken to ensure that all of the material is evenly inundated to provide a consistent slump of concrete from load to load. To obtain this consistency, modifications to the aggregate scooping device might be needed to allow the aggregate to drain properly. This process can produce concrete with as high as a 20°C [35°F] reduction in concrete placing temperatures. The process requires a large capital investment because dedicated bins or silos are required, as are installation of a chiller, and modifications to the scooping device. For a large project that will require a continuous amount of cooled concrete for placing in a hot weather environment, this procedure will typically be the most cost effective. This process was recently used at a concrete

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production facility supplying concrete to the Escambia Bay Bridges that were destroyed by Hurricane Ivan. The replacement bridges will take three years to complete and will mainly require hot weather concrete to be placed. Examples of the coarse aggregate soaking tanks and modified frontend loader are given in Figs 5.1 and 5.2. Other types of liquids and gases have been used on certain projects such as dry ice (solid CO2) to cool the aggregates. Dry ice is solid at temperatures of approximately −78.5°C [−110°F] and has a latent heat of 137 kcal/kg. At −20°C [−4°F] and with a pressure of 20 bars [2.0 MPa], dry ice is in a liquid state called liquefied carbonic acid gas (LCAG). LCAG is used to cool

5.1 Modified front-end loader.

5.2 Coarse aggregate soaking tanks.

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aggregates as it is sprayed onto the aggregate piles at a rate of 32 kg [70.5 lb] per minute. Dry ice is limited to cooling aggregate piles because adding the dry ice to the concrete would result in carbonation and loss of workability. After the dry ice has successfully cooled the aggregate stock piles, exhausting the carbonic acid gas from the mixer becomes critical prior to the addition of any other materials. Research has shown that the use of dry ice can result in decreasing the temperature of the concrete by up to 12°C [20°F] (Takeuchi et al. 1993). Liquid nitrogen for cooling concrete Liquid nitrogen has also been used as a method for cooling concrete for over twenty years. Liquid nitrogen (LN) is an inert cryogenic fluid with a temperature of −196°C [−320°F]. LN is injected directly into the batch water storage tank, aggregate, or mixer via lances to lower the temperature of the concrete as much as practical without freezing. LN can be stored at the batch plant or on the project site and if used on the project site itself, then repeated cooling of the concrete and greater control of the concrete temperature is possible. LN can be set up at a project or plant within a few days and can supplement other cooling methods to achieve a reduction in concrete temperature when necessary (Beaver 2004). LN is produced by compressing and cooling nitrogen gas to a point below its evaporation point of about −196°C [−320°F]. Typically, LN is injected directly into the mixer without making changes to the mixer itself. LN is injected into the mixer with a lance, which can move in and out of the ready-mix truck using a pneumatic cylinder. Typically, the lance can move vertically and horizontally, to allow the operator to position it correctly. The flow of LN through the lance is controlled by a pneumatically operated ball valve. The entire sequence of inserting the LN into the mixer is computer controlled to assist the operator in positioning the truck properly and when the cooling is complete. Typically, when the lance is inserted into the mixer it appears as though there is a release of toxic fumes, but in reality, this is just inert nitrogen and water vapor boiling off. The rotation of the mixing drum is essential when the LN is injected to the concrete to prevent nitrogen pools from accumulating within the mixer. In the event that too much LN is injected into the mixing drum, the mixing drum can become damaged or even ruptured. LN can cool concrete by more than 12°C [20°F] and has been used to obtain concrete with a placing temperature as low as 2°C [35°F]. The LN cooling process is flexible enough that it can be used for readjustment of the concrete in the field if certain processes are in place and the LN is being used at the project site. This method requires additional mixing time and the mixer is partially sealed to minimize the loss of coolant. The primary

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drawback to the use of LN for cooling concrete is the initial cost set up compared to other cooling methods. All material safety data sheets (MSDS) need to be read and understood by all personnel using LN (Sumodjo 2005). LN is not used very often and is typically reserved for use when all other options of cooling concrete have been exhausted. The Texas Department of Transportation has started using this technology with varying degrees of success.

5.2.3 Effects on plastic properties Once the concrete has been placed into the forms and is still in a plastic state, the high ambient temperature at the surface of the concrete can still cause problems. There are four main areas that have to be addressed to ensure that the concrete has adequate quality at early ages. These four areas include rate of slump loss, setting time of concrete, plastic shrinkage cracking, and proper curing. The stiffening of fresh concrete and the associated slump loss are mainly caused by the hydration of the cement. Evaporation or absorption of the mixing water into the aggregate may also reduce the amount of free water in the concrete, hindering the proper hydration of the cement. The hydration rate of cement increases with rise of temperature and generally follows the Arrhenius equation. When mixing concrete in hot weather, cement will hydrate at an accelerated rate, have a shorter setting time, and a higher rate of slump loss. The rate of slump loss is increased with a reduced amount of water available (Soroka and Ravina 1998). The increased rate of slump loss is typically resolved by adding mineral or chemical admixtures to the mix, and properly evaluating the mix design prior to placing it in the field with a trial batch. Decrease in time of set results in greater difficulty with transporting, handling, compacting, finishing, and additional need for cold joints. The initial setting time of the concrete is reduced due to increase of ambient temperature, decrease of ambient humidity, and increase of air/wind velocity. The same conditions also decrease the final setting time of the concrete (Ahmadi 2000). Most adverse field conditions can be overcome by making the following provisions: • • •

Placing concrete in the cooler part of the day and extending the time of setting in a condition that is workable. The use of a water fog spray nozzle to keep the air cool and the field humidity as high as possible (ACI 305R-99). Shading the concrete surface during exposure to solar radiation and high wind velocity can reduce the rate of evaporation on the concrete surface by 50% or more (Hasanain et al. 1989).

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Plastic shrinkage cracking typically occurs on the surface of freshly placed concrete when the surface stresses exceed the tensile strength of the concrete, and is caused by the evaporation of water from the surface of the concrete. Plastic shrinkage cracks can be identified as having a spacing of 0.3 to 1.0 m [1 to 3 ft] apart and normally do not extend to the free edge of the concrete. When the bleed water on the concrete surface evaporates, loss of water from beneath the concrete surface will occur and plastic shrinkage cracking takes place (Hasanain et al. 1989). ACI Committee 305 suggests an evaporation rate of 1.0 kg/m2/h [0.2 lb/ft2/h] or less so as not to exceed the amount of bleed water produced and thus induce plastic shrinkage cracking: h ER = ⎡t 2.5 × × a2.5 ⎤ × (1 + 0.4 × v) × 10 −6 [5.4] ⎣⎢ ⎦⎥ 100 where

(

ER t h a v

= = = = =

)

evaporation rate (lb/ft2/h) temperature of concrete (°F) ambient humidity (%) ambient air temperature (°F) wind velocity (mph)

Concrete mixtures incorporating slag, fly ash, silica fume, high cement contents, finely ground cements, high air contents, or super-plasticizers typically bleed at a rate within the limits recommended by ACI Committee 305. Therefore, some agencies have lowered the allowable evaporation rate for these types of mixes or require additional fogging systems to create additional humidity near the surface of the concrete to reduce the evaporation rate (Hover 2006). In some cases, revibrating before floating can close the plastic shrinkage cracking in large placements. When floating the surface, plastic shrinkage cracks can also be closed by striking the surface on each side of the crack before the concrete has reached final set (ACI 305R-99). The purpose of curing concrete is to maintain a proper amount of moisture within concrete during the early ages to develop the desirable properties at later ages. From the strength gain curve for concrete, it may be seen that the early age strength gains are much more rapid than at later ages (Fig. 5.3). As such, the concrete benefits from curing at early ages. Research has shown that concrete can lose 43% of its strength if not cured properly when placed in hot weather with a low humidity. The most effective curing operations are applied to the concrete as quickly as possible after initial set. Delaying the application of the curing material by as little as one day in hot weather can drastically reduce the later age properties of the concrete. For hot weather applications, it is recommended that the concrete be cured for at least seven days.

128

Developments in the formulation and reinforcement of concrete Compressive strength gain curve compared to 360-day results 10000 Compressive strength (psi)

13% 7%

8000 22%

6000 4000 58%

2000 0 0

28

56

84 112 140 168 196 224 252 280 308 336 364 Time (days) Raw data

Predicted

5.3 Strength gain curve for concrete.

There are two types of curing process available for use in hot weather. The first is to apply water to the surface of the concrete through ponding, sprinkling, spraying, or saturated materials (burlap, rugs, sand, straw, or cotton mats). The second is to prevent excessive loss of water through impervious paper or plastic sheets, membrane-forming curing compounds or evaporation reducers (Al-Ani and Al-Zaiway 1988). A typical practice in Florida is to use a combination of these two methods such that membrane-forming curing compound is applied to the concrete until it has reached initial set. Once the initial set has been obtained, then a layer of burlap mats can be applied to insure no surface water is lost. Recently, there was a bridge deck in Florida where a contractor did not take the weather conditions into account. The concrete was designed with too short a set time and was not cured properly (see Figs 5.4 and 5.5). The bridge deck suffered severe cracking and had to be completely removed and replaced at the contractor’s cost.

5.2.4 Effects on hardened properties Despite the use of chemical and mineral admixtures, cooling the concrete and ensuring proper curing has taken place; concrete exposed to hot weather at early ages may still be prone to reduced performance. In the event that the long-term performance of hardened concrete properties is not taken into account, the structure may be subject to reduced life cycle or decreased load capacity. There are four main areas to be addressed with respect to hardened concrete at later ages: compressive strength, flexural/tensile strength, modulus of elasticity and durability.

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5.4 Concrete bridge deck in Florida.

5.5 Severe cracking in the concrete bridge deck in Florida.

Owing to the environmental conditions associated with hot weather concreting, the amount of evaporation of water from the fresh concrete results in a lower effective water content. As previously stated, the lower effective water content usually results in a concrete that is less workable, and additional water may be needed on site for additional workability and to reduce slump loss. Upon the introduction of water to concrete on site, there is an increase in the water to binder ratio and capillary porosity. An estimate for the compressive strength of the concrete placed and cured in hot weather conditions, (fct)T, can be calculated from the following equation:

130

Developments in the formulation and reinforcement of concrete (fct)T = −14.15 + 7.06 lnT + (9.80 − 0.125T)lnT

[5.5]

where (fct)T = compressive strength of the concrete in MPa t = age of compressive strength testing in days T = temperature of the concrete in °C (Zivkovic 1992) This phenomenon, in which the increase in concrete placing temperature decreases the compressive strength of the concrete, has been confirmed in the field. The Tandy Center in downtown Fort Worth, Texas, showed a strength reduction of about 2.1 MPa [300 psi] for every 5.6°C [10°F] increase in placing temperature. The study involved samples which were cured in laboratory conditions with fairly uniform temperature and humidity after placement compared to samples cured in the field (Dodson and Rajagopalan 1979). Not only is compressive strength reduced with a rise in concrete temperature, but the modulus of rupture and splitting tensile properties are also reduced due to an increase in placing temperature. The reduction in modulus of rupture and splitting tensile strength is typically attributed to lower effective water, higher water to binder ratio, and improper curing. The modulus of rupture of concrete placed and cured in hot weather conditions, (fr)T, can be calculated from the following equation: (fr)T = (1.24 − 0.01Tc fr ≤ 0.935fr

24˚C ≤ Tc ≤ 45˚C

[5.6]

where fr = modulus of rupture of concrete placed and cured at approximately 24°C [75°F] Tc = temperature of concrete placed and cured in hot weather conditions. The splitting tensile strength of concrete placed and cured in hot weather conditions, (fst)T, can be calculated from the following equation: (fst)T = (1.132 − 0.00552Tc)fst ≤ 0.955fst

24˚C ≤ Tc ≤ 45˚C

[5.7]

where fst = splitting tensile of concrete placed and cured at approximately 24°C [75°F] Tc = temperature of concrete placed and cured in hot weather conditions (Abbasi and Al-Tayyib 1990). The modulus of elasticity can be reduced by as much as 17.5% when the concrete is prepared and cured at 45°C [115°F]. The modulus of elasticity of concrete placed and cured in hot weather conditions, (Ec)T, can be calculated from the following equation:

(Ec)T = (1.4775 − 0.0145Tc)Ec

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131

35˚C ≤ Tc ≤ 45˚C

[5.8]

where Ec = modulus of elasticity of concrete placed and cured at approximately 26°C [78°F] Tc = temperature of concrete placed and cured in hot weather conditions (Abbasi and Al-Tayyib 1990). Durability of hardened concrete is affected by hot weather conditions and can drastically reduce the service life of a structure. One of the main causes of deterioration of a structure is corrosion of steel reinforcement especially in aggressive environments. The corrosion of steel due to salt exposure is found to increase drastically with higher humidity. Areas that are exposed to salt water environments are at a higher risk for corrosion than dry areas, even with both exposed to hot weather conditions. The diffusion coefficient of reinforced concrete exposed to salt water is increased as compared to airborne salts. This increase in diffusion coefficient is mainly due to the increase in capillary porosity, plastic shrinkage cracking, and higher water to binder ratio. The service life of a concrete structure exposed to hot weather can be extended with lower water to binder ratio and increase in the cover of reinforcing steel (Morinaga 1992).

5.3

Future trends

5.3.1 Initiation of cracking One of the largest problems that affects the durability of concrete is cracking of the concrete. Currently, considerable research is being devoted to the prediction of early age cracking in concrete. Some of these techniques use completely saturated samples, other testing techniques use sealed samples, and still other tests are being developed for the surface tension of the concrete. Regardless of the test type, the environmental exposure conditions of the in-situ concrete should be accounted for prior to development. Currently, there is a lot of promising ongoing research focusing on the prediction of plastic shrinkage in concrete materials (Sant et al. 2006, Schindler 2004).

5.3.2 Concrete modeling Modeling of concrete properties is becoming a key area for future research. Some of the better known models are Stadium® developed by Materials Service Life, HYPERCON developed by the National Institute of

132

Developments in the formulation and reinforcement of concrete

Standards and Technology (NIST) and LIFE-365 developed by E.C. Bentz and M.D.A. Thomas. There are several advantages and disadvantages with any type of modeling software package. The main advantage of a modeling software package is that it can give the user predicted performance characteristics in a short amount of time and without extensive laboratory work. The major disadvantage is that they are often misused and based on assumptions not always associated with real world events. As technology advances and more data is inputted into models in a holistic approach, these modeling software packages should become more valuable and could be used for predicting the service life of a structure. One future initiative of predictive modeling could be used to evaluate a mix design for the appropriate materials and the best time to place the concrete.

5.3.3 Nondestructive evaluation techniques Recent advancements in evaluating the data collected from nondestructive techniques is starting to show some real promise for examining an existing structure. Some of the more advanced research has been coming out of the Federal Institute for Materials Research and Testing (BAM) in Berlin, Germany. One of the most promising current research initiatives has been the development of data fusion techniques based on information from different types of nondestructive tests applied to concrete structures. Perhaps a future initiative might incorporate the use of nondestructive techniques on freshly placed concrete to predict consolidation, potential for plastic shrinkage cracking, cover of steel reinforcement and density of concrete.

5.3.4 Materials specialty engineers In recent years, there has been an initiative taken by the concrete industry to incorporate the relatively large amount of information regarding cement and concrete material properties being generated on a daily basis. New tests are being developed which aid end users in determining concrete properties, new models are being developed to predict service life, and new techniques are being evaluated to determine in-situ properties. What is also becoming apparent is that without proper guidance from an individual who knows the limitations of different tests, models and evaluation techniques, a poor quality concrete can still be placed without anyone’s knowledge until the structure starts to fail. There has been more demand for individuals who specialize in the understanding of local materials to produce a structure that will meet the owner’s performance requirements. Without these

Hot weather concreting

133

individuals, it will become increasingly difficult to obtain the performance requirements desired by the owner of the structure.

5.4

Sources of further information and advice

5.4.1 American Concrete Institute One of the preferred resources for information in North America is the American Concrete Institute. ACI produces different types of documents for mix design calculations, placing, curing, and testing of concrete. One group in particular that specializes in hot weather concreting is Committee 305 (Hot Weather Concrete), and is currently chaired by James Cornell.

5.4.2 Japan Concrete Institute One of the preferred resources for information in Asia is the Japan Concrete Institute. JCI has several research committees investigating issues pertaining to setting of concrete, material durability, and cracking of concrete. One group in particular that looked at concrete under hot weather conditions is Committee JCI-TC-033A (Technical Committee on Performance Evaluation of Concrete under Natural Weathering Conditions) and was chaired by Noboru Saeki. Another group that is looking at the time-dependent behavior of concrete is Committee JJCI-TC061A (Technical Committee on timedependent behavior of cement-based materials) and is chaired by Tada-aki Tanabe.

5.4.3 RILEM One of the preferred resources for information in Europe is the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM). RILEM has several different technical committees researching current issues to advance the materials used for construction. One group that is looking at early age cracking of concrete is Committee 195-DTD (Recommendation for test methods for autogenous deformation and thermal dilation of early age concrete) and is chaired by Tor Ame Hammer. Another group that is looking at concrete at high temperatures is Committee 200-HTC (Mechanical concrete properties at high temperature – Modelling and applications) and is chaired by Ulrich Schneider. One last group that is also looking at the durability of concrete cracking is Committee CCD (Concrete cracking and its relation to durability: Integrating material properties with structural performance) and is chaired by Jason Weiss.

134

Developments in the formulation and reinforcement of concrete

5.5

References

AASHTO M 85: Standard Specification for Portland Cement, American Association of State Highway and Transportation Officials (2007). Abbasi A F, Al-Tayyib A J (1990), ‘Effect of hot weather on pulse velocity and modulus of elasticity of concrete.’ Materials and Structures, Vol. 23, Number 5, 334–340. ACI Committee 305 (1999), ‘Hot Weather Concreting (ACI 305R-99),’ American Concrete Institute, Farmington Hills, Michigan, 20 pages. Ahmadi B H (2000), ‘Initial and final setting time of concrete in hot weather.’ Materials and Structures, Vol. 33, Number 8, 511–514. Al-Amoudi O S B, Maslehuddin M, Abiola T O (2006), ‘Effect of type and dosage of silica fume on plastic shrinkage in concrete to hot weather.’ Construction and Building Materials, Vol. 18, Number 10, 737–743. Al-Ani S H, Al-Zaiway M A K (1988), ‘The effect of curing period and curing delay on concrete in hot weather.’ Materials and Structures, Vol. 21, Number 3, May, 205–212. ASTM C1157-03: Standard Performance Specification for Hydraulic Cement, ASTM International (2003). ASTM C494/C494M-05a: Standard Specification for Chemical Admixtures for Concrete, ASTM International (2005). ASTM C618-05: Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM International (2005). ASTM C1240-05: Standard Specification for Silica Fume Used in Cementitious Mixtures, ASTM International (2005). ASTM C989-06: Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars, ASTM International (2006). ASTM C150-07: Standard Specification for Portland Cement, ASTM International (2007). ASTM C192/C192M-07: Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory, ASTM International (2007). ASTM C595-07: Standard Specification for Blended Hydraulic Cements, ASTM International (2007). ASTM C1017/C1017M-07: Standard Specification for Chemical Admixtures for Use in Producing Flowing Concrete, ASTM International (2007). Austin S A, Robins P J (1992), ‘Performance of Slag Concrete in Hot Climates’, in Walker M J, Concrete in Hot Climates, London, E & F N Spon, 129– 139. Beaver W (2004), ‘Liquid Nitrogen for Concrete Cooling.’ Concrete International, Vol. 26, Issue 9, 93–95. Dodson C J, Rajagopalan K S (1979), ‘Field Tests Verify Temperature Effects on Concrete Strength.’ Concrete International, Vol. 1, Issue 12, 26–30. Duggan C R, Scott J F (1989), ‘Alternative cement for hot climates.’ Concrete. Journal of the Concrete Society, London, Vol. 20, Number 2, 18–20. ENV197-1: Cement. Composition, specifications and conformity criteria for low heat common cements, European Standards (2000). FDOT – Florida Department of Transportation (2007), Standard Specification for Road and Bridge Construction.Tallahassee, Florida, 926 pages.

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Hasanain G S, Khallaf T A, Mahmood K (1989), ‘Water Evaporation from Freshly Placed Concrete Surfaces in Hot Weather.’ Cement and Concrete Research, Vol. 19, Number 3, 465–475. Hover K C (2006), ‘Evaporation of Water from Concrete Surfaces.’ ACI Materials Journal, Vol. 103, Number 5, September–October, 384–389. Morinaga S (1992), ‘Life prediction of reinforced concrete structures in hot and salt-laden environments’, in Walker M J, Concrete in Hot Climates, London, E & F N Spon, 129–139. Owens P L (1992), ‘The Selection of Hydraulic Cements to Satisfy the Requirements for Concrete Construction in Hot Climates’, in Walker M J, Concrete in Hot Climates, London, E & F N Spon, 187–197. Ravina D, Soroka I (2002), ‘Admixture Effects on Hot-Weather Concrete’ Concrete International, Vol. 24, Issue 5, 34–38. Sant G, Lura P, Weiss J (2006), ‘Measurement of Volume Change in Cementitious Materials at Early Ages: Review of Testing Protocols and Interpretation of Results.’ 85th Annual Transportation Research Board Meeting, January 22–26, Washington, D.C., 19 pages. Scanlon J (1997), ‘Controlling Concrete During Hot and Cold Weather’ Concrete International, Vol. 19, Issue 6, June, 52–58. Schindler, A K (2004), ‘Effect of Temperature on the Hydration of Cementitious Materials.’ ACI Materials Journal, Vol. 101, Number 1, 72–81. Soroka I, Ravina D (1998), ‘Hot Weather Concreting with Admixtures.’ Cement and Concrete Composites, Vol. 20, Issue 2–3, 129–136 Sumodjo F (2005), ‘Precooling Mass Concrete: Liquid Nitrogen Proved to be the Best Alternative on the San Francisco-Oakland Bay Bridge.’ Concrete Construction, Vol. 50, Number 8, Aug., 36–40. Takeuchi H, Tsuji Y, Nanni A (1993), ‘Concrete Precooling Method by Means of Dry Ice’ Concrete International, Vol. 15, Issue 11, Nov., 52–56. Zivkovic S D (1992), ‘The Effect of Increase Temperature on Fresh and Hardened Concrete’, in Walker M J, Concrete in Hot Climates, London, E & F N Spon, 129–139.

6 Underwater concrete A K AL-TAMIMI, College of Engineering, The American University of Sharjah, United Arab Emirates

6.1

Introduction

Underwater concrete “UWC” is one special type of high performance concrete used in the past, present, and in the foreseeable future as long as there is need to construct bridges, with foundations in soil with high water levels, and almost all off- and on-shore structures. The term high performance concrete refers to concrete that performs particularly well in at least three key performance indicators: strength, workability, and service life. Therefore, underwater concrete should meet these performance criteria and it remains a viable and economic choice for consultants and contractors. UWC requires special and careful monitoring during all stages of construction; i.e. special considerations for selecting the right materials, specialized apparatus for the quality control, design, and methods of construction. Underwater concrete was specially designed to enhance constructability and performance in water environments. Using the underwater concrete technique may avoid engineers using the “old-style” of construction by isolating the water, and therefore, minimize interruption to plant operation and result in high savings. “UWC” is a highly flowable concrete that can spread into place under its own weight and achieve good compaction in the absence of vibration, without exhibiting defects due to segregation and bleeding. Underwater concrete technology has developed dramatically in recent years, so that the mix can be proportioned to ensure high fluidity as well as high resistance of washout and segregation. The construction of a wide range of structures including bridge piers, harbors, sea and river defences over many decades, and the development of offshore oilfields, has required placement of concrete underwater. This process can be successfully carried out and sound, high-quality concrete can be produced if sufficient attention is paid to the concrete mix design and the production method applied. 136

Underwater concrete

137

The stability of fresh concrete depends on the rheological properties and placement conditions. It can be characterized by the concrete resistance to washout, segregation and bleeding and is affected by the mix proportioning, aggregate shape and gradation, admixtures, vibration and placement conditions. The differential velocity at the interface between the freshly cast concrete and surrounding water can erode some cement and other fines. Such erosion can increase the turbidity and contamination of the surrounding water, and impair strength and durability, as well as bond to reinforcement steel and existing surfaces. The improvement of the in-situ properties of underwater concrete is related to the enhancement in washout resistance [1–3]. A superplasticizer (SP) is used to ensure high fluidity and reduce the water/powder ratio (W/ P). An anti-washout admixture (AWA) is incorporated to enhance the yield value and viscosity of the mix and hence the washout resistance and segregation resistance [1, 4]. The majority of AWAs are water-soluble polymers that increase the yield value and viscosity of cement paste and concrete [4, 5]. A statistical design approach was used to establish statistical models and to provide an efficient means of evaluating the influence of key mix variables on the fresh and hardened concrete characteristics that affect the performance of underwater concrete [6, 7]. The derived models include mixes with 380 to 600 kg/m3 of powder, W/P ratios of 0.34 to 0.46, sand/ aggregate ratios of 0.42 to 0.50, as well as AWA and SP dosages varying between 0.005% and 0.265% and 0.05% and 2.65%, respectively. The slump flow and the washout resistance are influenced, in order of importance, by the concentrations of AWA and cement, then by the water/cement ratio and dosage of SP, and various combinations of these parameters. The sand/ aggregate ratio had a secondary effect on these properties [6, 7].

6.2

Development of underwater concrete

Underwater concrete (UWC) continues to flow with time under its own weight before it starts to harden, unlike ordinary fresh concrete, which usually assumes its stable shape very rapidly. The commonly used standard tests for workability of ordinary fresh concrete, such as the slump test or the flow (spread) test, are inadequate. Flowing concrete usually drops to one-third or less of the original height in the slump test, and the result is a collapsed heap, the height of which is likely to be determined by the angle of repose of the largest particles. Therefore, visual measurement of collapsed concrete, or simply the height of the slumped concrete, does not differentiate the characteristics of two cohesive concretes. Moreover, even if a measurable slump is obtained, the sample will continue to settle and show increasing slump with time. The DIN flow table, which had been developed in Germany as a workability test for ordinary concretes and

138

Developments in the formulation and reinforcement of concrete

adopted as the British Standard test (BS 1881:1984), is no more satisfactory than the slump test. The test is intended to measure a bulk property of the concrete, but the end-point condition for the flowing concrete (510 mm spread) can only be achieved by assuming that the concrete spreads into a disc of 21 mm thickness, equal to the size of the largest particles, and clearly not representative of the bulk. The test is also operator-sensitive (manual jolting of the base plate, perfectly level position). Many other tests have thus been developed to assess UWC, and they will be described briefly in the following section.

6.3

Quality control of underwater concrete

6.3.1 Flow/spread test There have been several tests proposed based on this principle. The version of the test described here was originally developed in Germany by Graf [8] in the 1930s (Fig. 6.1). The test measures the spread of a sample of fresh concrete after it has been molded into the shape of a truncated cone and allowed to slump following the removal of the mold. The slumped concrete is then subjected to a controlled amount of jolting. The term “spread” test appears to be more appropriate than “flow” in order to avoid confusion with other “flow” tests. The “spread” describes much better the principle of the test in which the sample spreads in all directions. Flow of concrete tends to imply moving or “flowing” in one direction, restrained within a container or pipe. The test had originally been aimed at the assessment of workability of medium range concrete mixes and remains in use for such purposes in

130

700

6.1 Spread/flow table test apparatus [8].

200

40

700

200

Underwater concrete

139

several European countries. It is widely used in Germany, its country of origin. The test can also be used for fresh mixes of high and very high workability, where collapsed slumps are recorded. This capability has increased the use of the test for assessment of superplasticized and other special flowing fresh mixes. The apparatus consists of a flat, square (700 mm × 700 mm) plywood top plate, which has its upper surface lined with a metal sheet at least 1.5 mm thick. Center-lines at 90 degrees are engraved on to the surface of the metal lining together with a concentric circle of 200 mm in diameter. The mass of the top plate should be within 16 kg ± 1 kg. The top plate is attached to a bottom plate by hinges along one side. The top plate is fitted with a handle at the center of the edge opposite to the hinged side. The handle is used for lifting the top plate; however, the height of the lift is restricted to 40 mm ± 1 mm by metal retainers. The bottom plate extends forward by at least 120 mm along the side with the handle to provide a foothold. Spread values in the range of 450 to 600 mm (18 to 24 in.) were recommended for underwater concrete used in drilled shaft construction [9] and values of 550 to 650 mm (21 to 26 in.) were reported in underwater concrete repair [10].

6.3.2 The Orimet test The Orimet was developed by Bartos [11, 12] specifically as a method for a rapid assessment of very highly workable, flowing fresh concrete mixes on construction sites. The test is based on the principle of an orifice rheometer which is applied to fresh concrete. The Orimet test is applicable to fresh concrete mixes of very high workability, preferably mixes for which the result of the slump test is greater than 150 mm or which record a collapse slump. The test is used for specifications of workability (mobility) of fresh concrete mixes, for the compliance with specifications and for a rapid check of adjustments of mix proportions/admixtures on construction sites where very high workability of a fresh mix has to be maintained. It is particularly suitable for superplasticized and other flowing mixes. The Orimet consists of a vertical casting pipe fitted with an interchangeable orifice at its lower end. A quick-release trap door is used to close the orifice. The basic Orimet is provided with an orifice having an 80 mm internal diameter which is appropriate for assessment of concrete mixes of aggregate size not exceeding 20 mm. Depending on the composition of the mix and the workability required, orifices of other sizes, usually from 70 mm to 90 mm in diameter can be fitted instead. The casting pipe, the orifice and the trap door mechanism are supported by an integral tripod which folds back to facilitate transport (Fig. 6.2). A sample of at least 7.5 liters of fresh mix is required.

Developments in the formulation and reinforcement of concrete

60

1090

600

980

140

6.2 Orimet tester: basic dimensions [11].

The same sample can be re-tested rapidly. Normal assessment requires at least two, preferably three samples to be tested. The Orimet test includes two stages: 1) Go or not Go for underwater concrete mixes, and 2) Recommended value of 3–5 seconds for good underwater concrete [12].

6.3.3 The washout-resistance test The washout-resistance test was developed at the University of Paisley [13, 14], particularly as a method for the assessment of the non-dispersability of fresh concrete placed underwater. The test is based on the principle of evaluating non-dispersability by direct contact of fresh concrete with water (Fig. 6.3). The washout-resistance test is applicable to fresh concrete mixes of any level of workability conditions, to evaluate their suitability for an underwater application. The test assembly consists of a barrel containing water (30 l) with a pipe and a spray head connected at the bottom. The test sample is placed on a

Underwater concrete

141

6.3 Washout test [15].

frame, which is freely suspended on an electronic balance. The balance is supported on a bench. The test is simple. A sample of a concrete mix is put into a mold on the plate. The mold is removed and the plate with the sample is placed on the frame suspended from the balance. The tap on the pipe connected to water tank is turned on. Water from the spray head washes out the sample until the tank is empty. A computer connected to the electronic balance records the whole washout process. The test produces diagrams showing the loss of mass during the test. The measurement recorded directly from the balance at any moment during the test is the mass of the sample and of the plate resting on it plus the mass and pressure of water which is poured on the sample. The net amount of lost material is the direct measurement from the balance minus the effect of the pressure and weight of the poured water. The end result is the loss of material expressed as the percentage loss of the original sample, accompanied by graphs of the washout mass during the test, and visual assessment of the sample after test. The variability of the washout results inherently increases when a greater mass has been washed out from the original sample. However, the test is able to recognize clearly the dosage of underwater admixtures and the suitability of fresh concrete to be placed underwater. The advantages of this test are: • • • •

good simulation of concrete-water interaction in practice a simple test procedure highly sensitive to washout resistance acceptable mass of the test samples (1 kg).

However, the apparatus is not designed for use on site. The apparatus is expected to be used by concrete laboratories and companies producing underwater concrete admixtures. Washout values of 1% to 6.6% are recommended for underwater concrete [15].

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Developments in the formulation and reinforcement of concrete

6.3.4 The plunge test The plunge test originates from Belgium where it was used by the University of Ghent for an indication of how the fresh concrete mix will perform when either dropped through water or subjected to flow of water over it. The plunge test is applicable to fresh concrete mixes from low to medium workability. A high workability mix will tend to flow out from the wire basket. The plunge test (Fig. 6.4) consists of a vertical transparent plastic pipe embedded at the bottom into a concrete-filled container to provide a seal around the pipe and to form a stable base (Fig. 6.4(a)). A test sample is placed in a wire basket (Fig. 6.4(b)), which is plunged during the test through water in the plastic pipe (Fig. 6.4(c)). The actual test arrangement and the size of the equipment has varied from one research center to another [16]. A sample of 4–30 kg is required. The sample cannot be retested. The test requires at least two samples to be tested. Plunge test case study at the American University of Sharjah – 2007 The loss in mass of the concrete in the receiving container is equal to M1 − M2 (M1 – mass of concrete before test; M2 – mass of concrete after test). Washout or loss of mass of sample, expressed as a percentage of the initial mass of the sample is given by the following formula: D=

M1 − M 2 × 100 M1

(where D = washout, %)

Major problems with this test are: • •

•

Variation of the rate of fall of the basket full of concrete through the water. Variation of the size of the basket and the diameter of the holes. Also the wire basket retains the sample, therefore the test can show a lesser “washout” than in practice. Fresh concrete placed underwater is expected to have very high workability. In this case parts of the mix will often flow out from the basket without any washout and the measurement of the washout becomes unreliable.

Using the plunge test, washout results in the range between 2% and 3% were found acceptable to produce underwater concrete [17].

6.3.5 The filling ability test The test has been developed by the Technology Research Center of the Taisei Corporation in Japan [18]. The test is used to measure the filling ability of a new type of concrete called Self Compacting Concrete (SCC).

Underwater concrete Plastic pipe 200 mm OD × 3 mm wall Supporting wire

20

Steel reinforcing collar 20 mm wide × 3 mm thick

120 150 dia. Expanded metal wire mesh ‘Expamet mesh hex 2’

1.7 m

315 dia. 20 L

Plastic container

320

(a)

280 dia.

Dense impervious concrete to provide seal around pipe and to form stable base

(b)

(c)

6.4 Plunge test examples [16].

143

144

Developments in the formulation and reinforcement of concrete Middle wall

2400Pa

Sliding door

45 cm Reinforcing bars (D13 mm)

Concrete 59 cm

R1 R2

14 cm 14 cm

Filling height

14 cm

28 cm

4@5 cm=20 cm

6.5 Apparatus for evaluating filling ability [18].

SCC has excellent deformability and high resistance to segregation, meaning that it can be placed in heavily reinforced formwork without the need for vibrators. The test is suitable for high workability mixes such as SCC. The apparatus consists of a vessel, which is divided by a middle wall into two rooms, shown by R1 and R2 in Fig. 6.5. At the bottom of the wall is an opening, which is controlled by means of a sliding door. Deformed reinforcing bars with nominal diameters of 13 mm are installed at the opening with center-to-center spacing of 50 mm – creating a clear spacing of 35 mm between the bars. To conduct the test, the concrete sample is placed in R1. A small amount of pressure – about 2400 Pa – is applied to the concrete to assist its flowing ability. The sliding door is then moved upwards to allow the concrete sample to flow through the opening and then through and around the reinforcing bars and fill section R2 of the vessel. The filling height of the concrete in R2 is then measured and recorded as the filling ability of the mix. A difference of 0–30 mm between R1 and R2 is recommended to produce typical good quality underwater concrete [19].

6.4

Application/case study

6.4.1 Design of UWC mixtures [20] UWC mixtures were designed using a Portland cement, and coarse aggregate consisting of round natural quartz and sandstone particles with a nominal aggregate size of 20 mm. Well-graded quartzite sand with a finesse modulus of 2.74 was also employed. The relative density values of the coarse aggregate and sand were 2.50 and 2.56, respectively, and their absorption rates were 1.7% and 1%, respectively.

Underwater concrete

145

A new generation copolymer-based SP was used which has a solid content and specific gravity of 30% and 1.11, respectively. This SP was developed for self-compacting concrete. The SP was used at dosages varying from 0.2 to 2.1%, by mass of cement. Welan gum was selected as the AWA. Welan gum is a high molecular-weight, water-soluble polysaccharide obtained through a controlled microbial fermentation [5]. It is used to increase the viscosity of mixing water, and hence that of the cement paste. The powderbased welan gum was mixed with part of the mixing water, 10% solution, using a high-shear mixer. This was done to prevent the AWA from continuing its hydration during mixing and agitation. All mixes were prepared in 25-liter batches and mixed in a drum mixer. The mixing sequence consisted of homogenizing the sand and coarse aggregate for 30 s, then adding 50% of the mixing water in 15 s. After mixing for 2–3 min, the mixer was stopped for 5 min while the contents were covered. The cement was then added along with the remaining solution of water and SP. The AWA was added last. The concrete was then mixed for a further 3 min. The workability of the concrete was evaluated using the slump test. Because of the viscous nature of concrete containing an AWA, the readings of the measurement were delayed for one minute following the removal of the slump cone. The test consisted of determining the mass loss of a fresh concrete sample weighing 2.0 ± 0.2 kg which was placed in a perforated basket and allowed to freely fall three times through a 1.7 m-high column of water [21]. The Orimet test described earlier was used for determination of the flow time of a fresh concrete mix [14]. The Orimet was provided with an orifice which reduced the internal diameter from 120 mm within the casting pipe to 90 mm at the end of the orifice. A sample of at least 7.5 l of fresh mix was used. The flow time was measured from the time at which the trap door was opened until the flow of the concrete from the orifice was finished. Three measurements of the time flow were determined. The study was concentrated on determining the effects of SP, AWA dosage and cement content on the slump, flow time and the washout loss and also the degree of variation between washout loss and slump. As summarized in Table 6.1, the investigated mixtures were prepared with a fixed W/CM of 0.43 corresponding to high-quality underwater concrete. These mixtures were made with 100% Portland cement which varied from 420 to 520 kg/m3. The concentrations of AWA were 0.02% and 0.13%, by mass of cementitious materials, corresponding to relatively low and medium dosages used in underwater concrete. The dosage of SP used varied from 0.40 to 1.8%. Results of the fresh properties of the 16 mixes used in this investigation are given in Table 6.1. Mixes 1-A, 1-B, 1-C and 1-D are replicates to determine the variability of slump, flow time and washout loss obtained by the plunge test. This mix was made with 0.43 W/CM, 470 kg/m3 of cement and

0.075

0.075

190 9.3 3.3

1.10

1.10

190 9.5 3.0

955

955

Slump (mm) Flow time (s) Washout after 3 drops (%)

0.43 470 202 695

0.43 470 202 695

W/C Cement (kg/m3) Water (kg/m3) Fine aggregate (kg/m3) Coarse aggregate (kg/m3) Superplasticizer (%) Welan gum (% of cement mass)

1-B

1-A

Mix

190 9.5 3.0

0.075

1.10

955

0.43 470 202 695

1-C

180 9.0 3.1

0.075

1.10

955

0.43 470 202 695

1-D

40 none 5.6

0.02

0.40

1012

0.43 420 181 736

2

Table 6.1 Mix proportioning and test results of mixes

0 none 3.6

0.13

0.40

1012

0.43 420 181 736

3

0.02

0.2

898

0.43 520 224 653

5

180 110 1.47 6.0 10.5 8.7

0.02

0.40

898

0.43 520 224 653

4

90 10 2.0

0.13

0.40

898

0.43 520 224 653

6

190 14 6.6

0.02

1.80

1012

0.43 420 181 736

7

240 10 7.8

0.02

2.1

1012

0.43 420 181 736

8

80 none 4.2

0.13

1.80

1012

0.43 420 181 736

9

190 8 5.6

0.43 420 181 736 1012 2.1 0.13

10

265 2.9 11.2

0.43 520 224 653 898 1.80 0.02

11

220 2.4 4.7

0.43 520 224 653 898 1.80 0.13

12

260 2.0 6.5

0.43 520 224 653 898 2.1 0.13

13

Underwater concrete

147

Table 6.2 Repeatability of test parameters

Mean (N = 4) Coefficient of variation Estimate error (95% confidence limit) Relative error

300

Slump

Flow time

Washout loss

188 mm 2.7% 5 mm

9.3 s 2.5% 0.24 s

3.1% 4.6% 0.14%

2.7%

2.6%

4.5%

SP = 1.8%

420 kg/m3

Slump (mm)

250 200

520 kg/m3 SP = 1.8%

SP = 0.4%

150 SP = 0.4%

100 50

0 slump 0

AWA = 0.02%

AWA = 0.13%

6.6 Variations of slump with concentrations of SP and AWA, and cement content.

contained 1.1% and 0.075% of SP and AWA, respectively. Table 6.2 shows the mean measured results of the four replicate mixes, coefficients of variations, as well as the standard errors with 95% confidence limit for each of the three measured properties. The relative errors for slump and flow time are lower and shown to be limited to approximately 2.7%. On the other hand, the relative error for washout loss was 4.5% indicating the greater degree of experimental error for the washout resistance test. Figure 6.6 illustrates the effect of increasing SP and AWA concentrations and the dosage of cement on the slump. For any given dosage of AWA, an increase in the dosage of SP or cement content resulted in a substantial increase in workability. For example, with 0.02% of welan gum, concrete made with 420 kg/m3 of cement exhibited an increase of slump from 40 mm to 190 mm when the SP dosage was increased from 0.4 to 1.8%. The increase of AWA for a fixed dosage of SP and cement content resulted in a drop in slump. For example, for the mixture made with 520 kg/m3 of cement and containing 0.4% of SP, the increase in AWA dosage from 0.02% to 0.13% led to a reduction of slump from 180 mm to 90 mm. For all mixtures, an increase in cement content resulted in an improvement in workability. For a constant dosage of 0.02% of AWA and 1.8% of SP, the slump increased

148

Developments in the formulation and reinforcement of concrete

from 190 mm to 265 mm when the cement content increased from 420 kg/m3 to 520 kg/m3. The washout loss is affected by the concentration of AWA and SP, and the cement content. Figure 6.7 presents the variation of washout loss with different dosages of AWA, SP and different cement contents. For any given concrete, the increase in SP dosage and cement content increased the washout mass loss. In fact, the increase in fluidity due to higher additions of SP resulted in an increase in washout mass loss regardless of the AWA content. However, an increase in AWA concentration resulted in a reduction of washout mass loss. The improved resistance to water dilution of concrete containing AWA is due in part to the ability of the polymer to retain some of the mixing water. The AWA polymers also become adsorbed onto cement grains along with imbibing and fixing part of the mixing water, resulting in further retention of suspended cement particles. For example, for mixtures made with 0.02% welan gum and 420 and 520 kg/m3 of cement, the increase of SP from 0.4% to 1.8% led to increases in washout mass loss of 18% and 7%, respectively. For fixed dosages of SP of 0.4 and 1.8%, the increase of cement content from 420 to 520 kg/m3 resulted in increases in washout mass loss of 88% and 70%, respectively. The effect of increasing SP, AWA and cement content on flow time is shown in Fig. 6.8. An increase in SP resulted in a reduction of flow time for fixed contents of cement and SP. However, an increase in AWA concentration led to an increase of flow time for mixes made with constant SP and cement contents. In fact, for a given concentration of SP and cement content, the increase in AWA content increases the viscosity of concrete and therefore the flow time increased. For example, for the concrete made with 520 kg/m3 and containing 0.13% of AWA, the increase of SP from 0.4% to 1.8% resulted in significant reduction of flow time (from 10 s to 2.4 s). For the mixture containing 0.02% of AWA and 1.8% of SP, increas-

Washout mass loss (%)

14 12

420 kg/m3 520 kg/m3

SP = 1.8% SP = 0.4%

10 8 6

SP = 1.8% SP = 0.4%

4 2 0 AWA = 0.02%

AWA = 0.13%

6.7 Variations of washout loss with concentrations of SP and AWA and cement content.

Underwater concrete

149

20

Flow time (s)

420 kg/m3 15

520 kg/m3

SP = 1.8% SP = 0.4%

10 5

SP = 1.8%

SP = 1.8%

SP = 0.4% 0 AWA = 0.02%

AWA = 0.13%

6.8 Variations of flow time with concentrations of SP and AWA and cement.

Washout loss (%)

12

C = 420 kg/m3 AWA = 0.02% C = 520 kg/m3 8 Increasing AWA = 0.02% SP 6

10

4 AWA = 0.13%

2

AWA = 0.13%

0 0

50

100

150 200 Slump (mm)

250

300

6.9 Changes in washout loss vs. slump for concretes made with different AWA concentrations and cement content.

ing the cement content from 420 to 520 kg/m3 resulted in a substantial reduction in flow time (14 s to 2.9 s). Figure 6.9 presents the relationship between washout loss and slump of various mixtures containing 0.02% and 0.13% welan gum and 420 and 520 kg/m3 of cement. For a given concrete, an increase in slump is shown to increase washout loss. With 180 mm of slump, the lower washout mass loss (3.8%) occurred with concrete made with a high dosage of AWA (0.13%) and 520 kg/m3 of cement. For similar slump and 0.13% of AWA, the concrete made with 420 kg/m3 cement exhibited more washout loss (5.6%). However, the decrease of AWA content from 0.13% to 0.02% of concrete made with 520 kg/m3 resulted in increase in washout loss from 3.8% to 10.5% for the same slump. The washout mass loss can decrease with an increase of AWA concentration, despite the additional SP content required to maintain a given fluidity. The relationship between slump and flow time is presented in Fig. 6.10. The increase in slump resulted in a reduction in flow time. Concrete made

150

Developments in the formulation and reinforcement of concrete 70

C = 420 kg/m3 C = 520 kg/m3

Flow time (s)

60 50

Increasing SP

40

AWA = 0.13%

30 20

AWA = 0.13%

AWA = 0.02%

10 AWA = 0.02%

0 0

50

100

150 200 Slump (mm)

250

300

6.10 Changes in flow time vs. slump for concretes made with different AWA concentrations and cement content.

with 520 kg/m3 of cement content exhibited lower flow time. For a fixed slump, the increase in AWA concentration increased flow time, which directly affected the viscosity of the concrete. The degree of water retention, and therefore the remaining free water needed to lubricate the concrete, increases with the dosage of AWA, which acts on the aqueous phase. The experimental error in slump and flow time is shown to be limited to 2.6%. On the other hand, the relative error of washout loss was higher than 4.5%. This showed that the slump and flow time are influenced by the dosage of superplasticizer (SP), the cement content and the concentration of anti-washout admixture (AWA). The washout loss is affected by the concentration of AWA, cement content and the dosage of SP. The washout resistance is enhanced by the increase in AWA concentration and reduction of SP dosage. The increase in the SP dosage for a given AWA and cement increases the slump and mass loss by washout. However, for a fixed dosage of SP, the increase in AWA dosage reduces the slump and mass loss by washout. It also showed that for any given concentration of AWA, the addition of SP enhanced fluidity which was reflected by a reduction in flow time and an increase in slump.

6.4.2 Placement methods of UWC Several methods have been applied to place fresh concrete in underwater environments as shown in the following list. The Tremi and Pump methods are the most effective and practical used in recent construction: • • •

Tremi Pump Bottom dump buckets

Underwater concrete • •

151

Toggle bags Bag work.

In order to make the most of underwater concrete construction, the concrete should be produced properly, choosing the right composition, tested well in the production plant and before placing using the appropriate quality control mentioned above. The fresh concrete should be placed continuously to avoid inferior quality produced by an interrupted casting. The site condition, cost, and experience of workmanship will dictate the suitable methods. The Tremi and Pump methods are very common practices, however their procedures are different. The Tremi method operates by means of gravity flow while a pump applies pressure to cast concrete in its final location. The Tremi consists of a rigid pipe with hopper fixed on top of it to collect the concrete and force it down by gravity pressure. Placing enough continuous fresh concrete on the hopper overcomes the friction developed between the internal wall of the Tremi and the concrete to ensure uninterrupted flow of concrete in the pipe. The Pump method is more effective when speedy process is required in severe environments; however, it has also some drawbacks which should be taken into consideration. Both the Tremi and Pump methods have produced good quality underwater concrete; however, it was reported that the Tremi method is more practical in construction for two reasons [10, 22]: 1.

The Tremi produces a more uniform rate of flow than the Pump where the concrete exists at uncontrolled high speed causing great disturbance to the concrete that has already been poured. 2. Owing to the difference between the pump pressure and the fast speed of concrete flow, a vacuum will be created in the pump line. This vacuum will affect the composition of the mix and produce segregation. Concrete consultants and contractors should be familiar with the applications of these two methods and the consequences of their use when deciding which is the appropriate method to adopt.

6.5

References

1 Sonebi, M. “Development of high-performance, self-compacting concrete for underwater repair applications”, Ph.D. Thesis, Université de Sherbrooke, Canada (Sep. 1997) 420 p. 2 Yamaguchi, M., Tsuchida, T. and Toyoizumi, H. “Development of high-viscosity underwater concrete for marine structures”, Marine Concrete, International Conference on Concrete in the Marine Environment, Concrete Society (Sep. 1986) 235–245. 3 Khayat, K. H., Gerwick, B. C. and Hester, W. T. “Self-levelling and stiff consolidated concretes for casting high-performance flat slabs in water”, ACI Concrete International: Design and Construction (15) 8 (1993) 36–43.

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Developments in the formulation and reinforcement of concrete

4 Khayat, K. H. “Effects of anti-washout admixtures on fresh concrete properties”, ACI Materials Journal (92) 2 (March 1995) 164–171. 5 Ghio, V. A., Monteirio, P. J. M. and Gjørv, O. E. “Effect of polysaccharide gums on fresh concrete properties”, ACI Materials Journal (91) 6 (Nov. 1994) 602–606. 6 Khayat, K. H., Sonebi, M., Yahia, A. and Skaggs, C. B. “Statistical models to predict flowability, washout resistance and strength of underwater concrete”, in Production Methods and Workability of Concrete, Glasgow, June 1996 (E & FN Spon, London, 1996) 463–481. 7 Khayat, K. H., Yahia, A. and Sonebi, M. “Applications of statistical models for proportioning underwater concrete”, Fourth International Conference on Recent Advances in Concrete Technology, Supplementary Papers, Japan, (June 1998) 95–113. 8 Graf, O. “Experiments on the behaviour of reinforcement in concrete of various compositions”, Deutscher Ausschuss für Eisenbeton, 71 (1933), 37–60. 9 Brown, D., Bailey, J. and Schindler, A. “The use of self-consolidating concrete for drilled shaft construction”, Proc. Geo Construction QA/QC Conf., 2005, pp. 437–448. 10 Yao, S. X. and Gerwick, B. C. “Underwater Concrete Part II: Proper mixture proportioning and Underwater concrete Part III”, Concrete International, February and March 2004. 11 Bartos, P. “Workability of flowing concrete: assessment by a free orifice rheometer”, Concrete, 1978 (12) 28–30. 12 Sonebi, M. and Bartos, P. J. “Filling ability and plastic settlement of selfcompacting concrete”, Materials and Structures, (35) 8 (2002) 462–469. 13 Ceza, M. and Bartos, P. J. M. “Development of an Apparatus for Testing the Washout Resistance of Underwater Concrete Mixtures”. ACI Concrete in Marine Environment, Proceedings Third CANMET/ACI International Conference, Canada, 1996, pp. 111–126. 14 Bartos, P. Orifice rheometer as a test for flowing concrete, in developments in the use of superplasticizers (W.M.Malhortra, Ed.) ACI SP-68, USA, June 1982, 467–682. 15 Ceza, M. and Bartos, P. J. M. “Assessment of washout resistance of a fresh concrete by the MC-1 test, Production methods and workability of concrete, P.J.M. Bartos et al., 1996, E & FN Spon, pp. 399–413. 16 Neeley, B. D. “Technical report of USA Army Engineering-Waterway Experiment Station”. Evaluation of Concrete Mixtures for use in Underwater Repairs, 1988. 17 Sonebi, M., Tamimi, A. K. and Bartos, P. J. M. “Application of factorial models to predict the effect of anti-washout admixture, superplasticizer and cement on slump, flow time and washout resistance of underwater concrete”, Materials and Structures, (33) 5 (2000) 317–323. 18 Haykawa, M. “Development and Application of Super Workable Concrete”, Proc of Intl. RILEM Workshop on Special Concretes Workability and Mixing, Paisley, 1993, edited by Prof. Bartos, pp. 183–190. 19 EFNARC, Specification and guidelines for self-compacting concrete, February, 2002. 20 Sonebi, M., Tamimi, A. and Bell, D. “Analysis of the performance of fresh underwater concrete produced with polysaccharide gum and superplasticizers using

Underwater concrete

153

plunge and the Orimet tests”, Proceedings of 14th International Conference on Building Materials, Ibausil, Vol. 1, Weimar, Germany, pp. 147–156, Sept. 2000. 21 CRD C61: “Test Method for Determining the Resistance of Freshly-Mixed Concrete to Washing Out in Water”, US Army Experiment Station, Handbook for Concrete, Vicksburg, Mississippi, Dec. 1989, 3 p. 22 Netherlands Committee for Concrete Research, Underwater concrete, HERON, 1973.

7 Fibrous concrete reinforcement S MINDESS, University of British Columbia, Canada

7.1

Introduction

Plain concrete is a brittle material, with low tensile strength and strain capacities. The use of short, discontinuous fibres to strengthen and toughen such materials, which are much weaker in tension than in compression, goes back to ancient times. Probably the oldest written account of such a composite material (clay bricks reinforced with straw), occurs in Exodus 5:6–7: “And Pharaoh commanded the same day the task-masters of the people, and their officers, saying: ‘Ye shall no more give the people straw to make bricks, as heretofore: let them go and gather straw for themselves.’ ”

In modern times, the use of fibres to reinforce cementitious materials goes back to about 1900, when the invention of the Hatschek process enabled the production of asbestos cement. Over the past forty years, there has been a steady increase in the use of fibres in cement and concrete. Today, about 100 million cubic metres of fibre reinforced concrete are produced annually: • • • •

60% – slabs on grade 25% – fibre shotcrete 5% – precast members 10% – miscellaneous other applications.

However, the lack of truly structural applications should be noted; the reasons for this will be discussed later. Fibres are not added to concrete to increase its strength, though some modest increases in strength may occur. The main role of the fibres is to bridge across the matrix cracks that develop as concrete is loaded, and thus to provide some post-cracking ductility. Fibres should not be considered as a replacement for conventional reinforcing bars, even though in some applications this may be the case. They are, in fact, complementary methods of 154

Fibrous concrete reinforcement

155

reinforcing concrete, and there are many applications in which they should be used together.

7.2

How do fibres work?

The mechanical behaviour of fibre reinforced concrete (FRC) depends largely on the interactions between the fibres and the brittle concrete matrix: physical and chemical adhesion; friction; and mechanical anchorage induced by complex fibre geometry or by deformations or other treatments on the fibre surface. The “first generation” steel fibres, produced by shearing thin sheets of steel, were not very efficient, because they were too smooth to bond well with the matrix. Subsequently, many different fibre geometries (Fig. 7.1) were developed to improve the mechanical anchorage, which is the most important of the bonding mechanisms. Surface treatments of the synthetic (mostly polypropylene) fibres have been similarly employed to improve the fibre-matrix bond. As FRC is stressed (either by external loads or by shrinkage or thermal stresses), there is initially elastic stress transfer between the fibres and the matrix. Because the fibres and the matrix have very different elastic moduli, shear stresses develop at the fibre/matrix interface. When the shear stress at the interface is exceeded, debonding gradually begins to occur, and frictional shear stresses become the dominant stress transfer mechanism. At some point during this gradual transition from elastic to frictional stress transfer, some cracking of the matrix occurs, and some frictional slip occurs in the debonded areas. Of course, we are primarily interested in how the fibres in FRC inhibit crack extension once the matrix has cracked, i.e., how they behave in the post-cracking zone. This is governed primarily by the nature of the pull-

Crimped or non-straight

Hooked Button end

Indented Twisted polygonal

7.1 Some types of available steel fibres.

156

Developments in the formulation and reinforcement of concrete

out of the fibres from the matrix. It must be emphasized that failure by fibre pull-out is much the preferred mode of failure of FRC; much more energy is consumed in pulling the fibres out of the matrix than in breaking them. It is possible to define a critical length, lc, at which the fibres break rather than pulling out. This must be taken into account when designing or choosing fibres for a particular application. In a properly designed FRC, following the appearance of the first crack, a process of multiple cracking begins, in which the brittle matrix cracks into successively smaller segments (held together by the fibres bridging these cracks). This leads to toughening of the composite. The crack width and crack spacing during this process can be controlled by proper selection of the fibres and the matrix. As stated above, straight, smooth fibres cannot develop sufficient adhesional or frictional bond to be efficient; thus, essentially all of the fibres presently used in practice are deformed in some way (Fig. 7.1), or are surface treated, to increase the bonding with the matrix. This turns out to be much more significant than the fibre length in controlling the degree of bonding.

7.3

Types of fibres

A number of different types of fibres have been developed for use specifically with concrete, though it must be remembered that within each fibre type there are a number of different producers and fibre geometries, leading to different fibre properties. They may be classified as follows: Steel fibres may be produced by cutting wires, shearing sheets, or from a hot-melt extract, and are still the most commonly used fibres. As shown in Fig. 7.1, they are almost always deformed in some way to enhance the fibre-matrix bond. They have been found to be extremely durable in concrete, even though they may rust visibly when exposed at the concrete surface. In some cases, where surface rusting is unacceptable, or in very aggressive environments (e.g., refractory applications) stainless steel fibres may be used. Glass fibres are produced by drawing molten glass in the form of fine filaments through a special bushing. Typically, 204 filaments are drawn simultaneously, and after solidification these are formed into a single strand. Ordinary E-glass (soda-lime glass) fibres and A-glass fibres (borosilicate glass) are not stable in the highly alkaline concrete environment. For use in concrete, alkali resistant glass fibres, typically containing about 16–20% zirconia must be used. Asbestos fibres have been used since about 1900 in the manufacture of asbestos cement pipes, roofing materials, and other building components.

Fibrous concrete reinforcement

157

They have a particular affinity for the cement matrix, and are very effective as reinforcement. Unfortunately, there are significant health risks associated with the production of the asbestos fibres themselves, and so they have largely been replaced with other types of fibres, primarily cellulose fibres. Synthetic fibres have become very common in the last few years. Unlike the fibres mentioned above, they have a significantly lower elastic modulus than does the concrete matrix. At relatively low fibre volumes (<0.5%), they are effective mostly for the control of plastic shrinkage cracking. At higher fibre volumes, or with some of the new high performance synthetic fibres, significant toughening and strengthening may also be achieved. The most common synthetic fibres are polypropylene or polypropylenepolyethylene blends; some nylon fibres are also used. Carbon fibres and Aramid (Kevlar) fibres are high modulus fibres that are very effective in FRC, but which are currently too expensive to be used extensively. High strength acrylic fibres and polyvinyl alcohol (PVA) fibres have recently been introduced as replacements for asbestos fibres, but their use is still limited to specialized applications. A variety of natural organic fibres are also sometimes used in concrete, primarily for the production of low-cost housing elements in developing countries. These low modulus fibres, such as sisal, jute, coir, elephant grass and sugar cane bagasse tend to deteriorate in damp or alkaline environments, and must be specially treated for use in concrete. However, cellulose fibres, derived from wood pulp, which are stiffer and stronger than the other natural fibres, are now being used very extensively as a replacement for asbestos fibres, though they too need special processing for use in FRC.

7.4

Mix proportioning, fabrication and placement

For the fibre volumes generally used for ordinary FRC, the procedures for mix proportioning of concretes containing these different fibres are generally the same as those used for plain concrete, though rather more “trial and error” is usually involved when fibres are incorporated. Generally, the addition of fibres reduces the concrete workability; this may be compensated for by increasing the ratio of fine-to-coarse aggregate, or by the addition of more pozzolanic material. It should also be noted that fibre concretes are more difficult to compact than plain concretes. FRC can be produced in much the same way as plain concrete, using the same equipment and procedures. However, care must be taken to ensure that the fibres are uniformly dispersed, by avoiding “balling” or clumping of the fibres. This requires some attention to the way in which the fibres are introduced into the concrete, and adequate mixing. In the field, FRC

158

Developments in the formulation and reinforcement of concrete

appears to be stiff compared to plain concrete, but a properly designed mix will flow readily under vibration. FRC can be pumped, and can also be placed as fibre reinforced shotcrete (see Chapter 4).

7.5

What do fibres do?

7.5.1 Toughness It must be emphasized again that, at the fibre volumes normally used (<1.0%), the fibres are not added to improve the strength; their principal role is to bridge across the cracks that develop in concrete as it is stressed. If the fibres are well bonded to the matrix, and if they are sufficiently stiff and strong, they will permit the FRC to sustain significant loads over relatively large deformations in the post-cracking (or strain softening) stage. That is, the fibres will provide some post-cracking “ductility” or toughness to the composite. The higher the fibre volume, and the more efficient the fibres, the more the toughness will increase. There is, unfortunately, still no general agreement on an unambiguous method to quantify this behaviour, particularly in such a way as to introduce it into standard building codes. According to Mindess et al. (2003), any toughness or residual strength parameter used for the specification or quality control of FRC should, ideally, satisfy the following criteria: • •

• •

•

It should have a physical meaning that is readily understandable. The “end-point” used in the calculation of toughness parameters should represent the most severe serviceability conditions anticipated for any particular application. The variability inherent in any measurement of concrete properties should be acceptably low. It should be able to quantify some important aspect of FRC behaviour (strength, toughness, crack resistance) and should reflect some characteristics of the load vs. deflection curve. It should be largely independent of specimen size and geometry.

Unfortunately, none of the many test methods that have so far been standardized meet these criteria, in large part because neither strength nor the shape of the load vs. deflection curve are themselves fundamental concrete properties. Indeed, the different tests may often give conflicting results when compared with each other. Probably the most useful of the currently available tests is ASTM C1609: Standard Test Method for Flexural Performance of Fibre-Reinforced Concrete (Using Beam with Third-Point Loading). In this test, a small FRC beam (100 mm × 100 mm × 350 mm) is tested in flexure under third-point loading, and the load vs. deflection curve is recorded. The residual strengths

Bending load (kN)

Fibrous concrete reinforcement

159

Curves shown are for a design flexural strength = 5 MPa 20 Design load (Pd) 15 75%Pd Level IV 10 50%Pd 45%Pd Level III 30%Pd 30%Pd Level II 5 15%Pd 15%Pd Level I 5%Pd 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Net midspan deflection (mm)

7.2 Template approach to specifying toughness in terms of residual strengths.

at various mid-point deflections (in the post-peak part of the curve) are determined directly from the load vs. deflection curve. In addition, a toughness parameter may be calculated as the area under the load vs. deflection curve out to any specified deflection. This test appears to be sensitive to different fibre types and volumes. A related way of characterizing toughness, the template approach, was suggested by Morgan et al. (1995). This utilizes the same test arrangement as in ASTM C1609. However, it does not involve the calculation of a toughness parameter per se. Rather, toughness performance levels, such as those shown in Fig. 7.2, are used. The actual load vs. deflection curve is then compared to the template to see whether the FRC conforms to the specified toughness performance level. The real advantage of this method is that it is not sensitive to the precise shape of the load vs. deflection curve, which is difficult to determine, particularly at small deflections. In this approach, the shape of the curve up to a deflection of 0.5 mm is not taken into consideration at all. It must be emphasized that these two methods of characterizing FRC, as well as all of the other proposed methods, are empirical in nature, and are thus not directly comparable. They all violate one or more of the criteria outlined above (Mindess et al. 2003), and thus are of limited usefulness in providing design values for FRC. Indeed, this lack of a commonly agreed upon method for characterizing the performance of FRC is one major factor that has inhibited the truly structural use of this composite material.

7.5.2 Impact resistance As stated earlier, under static loading, fibres (at the addition rates normally used in commercial practice, that is at volume fractions less than

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Developments in the formulation and reinforcement of concrete

about 1%) do not contribute much to the strength of the concrete, though small improvements in tensile and flexural strengths may be found. Under impact loading, however, fibres may well increase both the strength (compressive, tensile and flexural) and the toughness (or fracture energy) of the composite, compared to the behaviour of the plain matrix under similar loading conditions. Of course, the increases obtained depend upon the fibre volume, the fibre geometry and the matrix strength, since these parameters largely determine whether the fibres will break, or be pulled out of the matrix. The reasons for the improved FRC behaviour under impact loading are not completely understood. In part, this may be due to the fact that FRC becomes increasingly strain rate sensitive at higher fibre contents and higher fibre aspect ratios. This is generally attributed to the strain rate sensitivity of the fibre-matrix bond strength (Naaman and Gopalaratnam 1983). Similarly, adding fibres to the concrete matrix will significantly improve the bond between the concrete and conventional steel reinforcing bars under impact loading. Again, the composite exhibits a higher bond stress, becomes more ductile and absorbs more energy (Yan and Mindess 1994, 2001). The effects of fibres are even more dramatic when the concrete is subjected to impact while under lateral confinement. As has been shown in detail by Sukontasukkul et al. (2001), when compression specimens are laterally confined, the mode of failure changes from the normal shear cone type to a columnar or vertical splitting type, accompanied by increases in strength and strain at peak load. When beams or plates are laterally confined, the failure mode gradually changes from flexure to shear as the degree of confinement increases, again accompanied by increases in strength and toughness. Higher confining stresses and/or higher fibre contents lead to higher energy absorption by the specimen. Unfortunately, it is not possible to predict the behaviour of FRC under high loading rates from static tests. The problem is further complicated by the fact that, depending on the particular FRC system and the strain rate, the failure mechanisms may be quite different. FRC systems may also be subjected to very different strain rates, ranging from about 10−6 s−1 to about 106 s−1, depending on the source of the dynamic event. Thus, because of the enormous range of possible strain rates, and the complexity of the FRC system itself, the high strain rate and impact properties of FRC are still poorly understood. However, based on a great deal of empirical evidence, we can say with certainty that fibres can be very effective in improving the impact resistance of concrete. We are simply unable to quantify these improvements in an unambiguous manner.

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161

7.5.3 Shrinkage Up to fibre volumes of about 1%, fibres have little effect upon the drying shrinkage of concrete, though they tend to reduce the resulting crack widths considerably. However, fibres can be very effective in reducing plastic shrinkage cracking; indeed, this is one of the principal applications of synthetic fibres.

7.6

High performance fibre reinforced concrete

The discussion above has dealt primarily with “ordinary” FRC. However, there are a number of more sophisticated FRC systems that have recently been developed. While they are only just beginning to be used in practice, it is worth describing them briefly, to demonstrate how far the limits of FRC technology may be extended.

7.6.1 Engineered cementitious composites (ECC) One way to minimize crack widths in concrete is through the use of engineered cementitious composites (ECC) as developed by Li (2003, 2005) and by Li and Stang (2004). ECC is a fibre reinforced cementitious composite, containing typically about 2% fibres by volume. Using a micromechanics-based approach to the mix design, involving careful matching of the matrix strength and the fibre pull-out strength, it has been possible to achieve ductility values of up to 3% in direct tension. This material can be placed in many ways – by ordinary casting techniques, as self-consolidating concrete, and by shotcreting. Because of its ductility, and the fact that it keeps crack widths small (Fig. 7.3), this material can lead to more durable structures and better sustainability, even though the initial costs can be substantial.

7.6.2 Ultra high strength FRC A family of materials with very high fibre contents, very high strengths, and very high durability are now beginning to appear. The common features of these materials are very low water : binder ratios, the use of silica fume and superplasticizers, high fibre contents, severe limitations on the maximum aggregate size (often less than 1 or 2 mm), careful control of the particle size distribution of all of the solid materials in the mix, and tight quality control in their production, placement and curing. Not surprisingly, these materials are very expensive, though they are beginning to find a place in certain specialized applications. Some examples of these materials are:

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Developments in the formulation and reinforcement of concrete 100

5 ECC

80

3

60

2

40

1

20

Crack width w (μm)

Tensile stress s (MPa)

4

Concrete strain 10 times expanded 0

0

1

2

3

4

5

6

0

Strain e (%)

7.3 Typical tensile stress–strain curve and crack width development of ECC (Li 2005).

DUCTAL®: This material consist of fine aggregate (<2 mm), crushed quartz, silica fume, and, of course, cement, water and superplasticizer, and up to 2% by volume of fibres. When the fibres are steel, compressive strengths of about 150 to 180 MPa may be achieved, with flexural strengths of about 32 MPa. When polypropylene fibres are used instead, these strengths are reduced by about 25%. BSI®-CERACEM was recently used to construct the toll gate roofs for the new Millau viaduct in the south of France (Thibaux et al. 2004). With about 2.5% of steel fibres it achieved a compressive strength of 165 MPa, and a tensile strength of 8.8 MPa. It was also self-consolidating. CEMTECmultiscale® has much higher cement and fibre content than the two previous materials, though the underlying principles remain the same. This material can achieve flexural strengths of 60 MPa, and has a very low permeability (Parent and Rossi 2004). Typical mix proportions for two of these materials are given in Table 7.1. CRC (Compact Reinforced Composite): This material is made with a very low water : binder ratio (∼0.16 or less) and contains from 2 to 6% steel fibres, with matrix compressive strengths ranging from 140 to 400 MPa. It is different from the materials mentioned above in that it is combined with closely spaced conventional steel reinforcement. It is used primarily for precast elements, but can also be use in cast-in-place construction (Aarup 2004).

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163

Table 7.1 Composition of two high performance fibre reinforced concretes Material

DUCTAL® (kg/m3)

CEMTECmultiscale® (kg/m3)

Portland cement Silica fume Crushed quartz Sand Water Fibers Superplasticizer

710 230 210 1020 140 40–160a 10

1050.1 268.1 – 514.3 180.3 858b 44

a b

Either steel or polypropylene fibers (13 mm × 0.20 mm). A mixture of three different geometries of steel fibres.

It should be noted that while all of these materials are promoted largely on the basis of their compressive strength (since this is still the principal obsession of structural engineers), it is their high impermeability and durability that are probably of greater importance in the long run. In spite of their high initial costs, this family of materials will become more important as sustainability considerations are embraced by the industry.

7.7

Hybrid fibre systems

The combination of two or more different types of fibres (different fibre types and/or geometries) is becoming more common, with the aim of optimizing overall system behaviour. The intention is that the performance of these hybrid systems would exceed that induced by each fibre type alone. That is, there would be a synergy. Banthia and Gupta (2004) classified these synergies into three groups, depending on the mechanisms involved: 1. Hybrids based on the fibre constitutive response, in which one fibre is stronger and stiffer and provides strength, while the other is more ductile and provides toughness at high strains. 2. Hybrids based on fibre dimensions, where one fibre is very small and provides microcrack control at early stages of loading; the other fibre is larger, to provide a bridging mechanism across macrocracks. 3. Hybrids based on fibre function, where one type of fibre provides strength or toughness in the hardened composite, while the second type provides fresh mix properties suitable for processing. These concepts have been applied both for thin sheet FRC (particularly for asbestos-cement replacement), and for high performance–high ductility systems, with fibre volumes of from 2 to 10%.

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Developments in the formulation and reinforcement of concrete

In composites with only one type of fibre, high modulus fibres tend to increase strength with only modest improvements in toughness, while low modulus fibres tend to increase the toughness, with little or no improvement in strength. However, it has been shown in many studies that a judicious combination of the two fibre types can lead to a composite in which the disadvantages of the two fibre types are offset, and only their advantages are displayed.

7.8

Applications of fibre reinforced concrete

The applications of FRC are limited only by the imagination of the engineers and architects who specify these materials. The general areas in which FRC is currently being used include the following.

7.8.1 Structural applications As indicated earlier, fibres are not yet widely used in structural applications, because of the difficulty in introducing FRC into the various national building codes. However, in spite of this, fibres are being used in conjunction with conventional steel reinforcement. The fibres act in three ways: 1.

They permit at least a portion of the tensile strength of the FRC to be used in design, since the matrix no longer loses its entire load-carrying capacity at first crack. 2. They improve the bond between the reinforcing bars and the matrix. 3. They control crack widths. Fibres are particularly useful in increasing the shear capacity of a structural member, and it is in this regard that FRC will first be introduced into the 2008 ACI 318. Fibres can also contribute to the flexural strength. FRC should be particularly effective in design for seismic and blast loading.

7.8.2 Pavements and slabs on grade Pavements and slabs on grade are now responsible for about 60% of the total production of FRC. A number of rational design methods have been developed for FRC pavements and slabs on grade, using both steel and synthetic fibres.

7.8.3 Self-compacting FRC Even though fibres generally reduce the workability of concrete, it is nonetheless possible to design FRC mixes that are self-compacting. This does

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165

require careful mix design and quality control. Particular attention must be paid to the total particle size distribution of the mix, and to the selection of both chemical and mineral admixtures.

7.9

Concluding remarks

We now know how to “tailor-make” FRC for a wide variety of applications, and FRC has become very much a mainstream construction material. However, its uses are still primarily in non-structural applications (industrial floors, thin-sheet materials, fibre shotcrete tunnel linings, and so on). FRC is rarely mentioned in modern building codes, which of course greatly inhibits its use in structural applications. A massive amount of research has been carried out during the last forty years, and countless papers and books have been published. (For instance, the recent edition of Bentur and Mindess, Fibre Reinforced Cementitious Materials (2007) contains over 1150 carefully selected references.) Unfortunately, this has not resulted in the sort of information required for routine structural design. Part of the problem is that, in North America and in most other parts of the world, structural design in concrete is almost entirely strength based. However, at the fibre volumes generally used in practice (<1%), fibres have little effect on the concrete strength; their purpose is to make the material appear to be more “ductile”, by providing a degree of post-cracking load bearing capacity. Design codes such as ACI 318: Building Code Requirements for Reinforced Concrete simply have not recognized this post-peak behaviour. (To be fair, there is now a proposal to mention FRC specifically in the 2008 edition of ACI 318, though only with regard to its contribution towards shear resistance.) On the other hand, one should not simply blame the structural engineers for this state of affairs. We in the concrete materials research community are equally remiss. While we have focused on the fundamental properties of FRC, the mechanisms underlying FRC behaviour, and how to produce ever more exotic FRC composites, we have not put sufficient effort into developing appropriate methods of characterizing the behaviour of FRC in a manner that can be quantified unambiguously and thus used in design. The relatively few current test methods, such as they are, are not suitable for this purpose. For properties such as impact resistance, which is becoming an increasingly important design consideration, we can neither characterize FRC behaviour nor test this behaviour in a consistent and theoretically sound manner. Until the materials engineers and the structural engineers begin to work together to solve these problems, FRC will be unable to take its rightful place as a useful, modern, high technology structural material.

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7.10

References

Aarup, B. [2004]. CRC – a special fibre reinforced high performance concrete. In Advances in Concrete through Science and Engineering, RILEM Publications, Bagneux, France, 2004, CD-ROM Paper No. 13, Hybrid-Fiber Session. Banthia, N. and Gupta, R. [2004]. Hybrid fibre reinforced concrete (HyFRC): fiber synergy in high strength matrices. Materials and Structures (RILEM), Vol. 37, pp. 707–716. Bentur, A. and Mindess, S. [2007]. Fibre Reinforced Cementitious Composites, Taylor & Francis, London and New York, 601 p. Li, V.C. [2003]. On engineered cementitious composites (ECC) – A review of the material and its applications. Journal of Advanced Concrete Technology, 1, pp. 215–230. Li, V.C. [2005]. Engineered cementitious composites. In Construction Materials, Proceedings of ConMat ’05 and Mindess Symposium, N. Banthia, T. Uomoto, A. Bentur and S.P. Shah, eds. University of British Columbia, Vancouver, CD-ROM. Li, V.C. and Stang, H. [2004]. Elevating FRC material ductility to infrastructure durability. In Fibre-Reinforced Concrete, BEFIB 2004, M. di Prisco, R. Felicetti and G.A. Plizzari, eds. RILEM Proceedings PRO 39, RILEM Publications, Bagneux, France, Vol. 1, pp. 171–186. Mindess, S., Young, J.F. and Darwin, D. [2003]. Concrete, 2nd edn. Prentice-Hall, Upper Saddle River, New Jersey, USA. Morgan, D.R., Mindess, S. and Chen, L. [1995]. Testing and specifying toughness for fibre reinforced concrete and shotcrete. In N. Banthia and S. Mindess (eds.), Fibre Reinforced Concrete: Modern Developments. University of British Columbia, Vancouver, Canada, pp. 29–50. Naaman, A.E. and Gopalaratnam, V.S. [1983]. Impact properties of steel fibre reinforced concrete in bending. International Journal of Cement Composites and Lightweight Concrete, Vol. 5, No. 4, pp. 225–237. Parent, E. and Rossi, P. [2004]. A new multi-scale cement composite for civil engineering and building construction fields. In Advances in Concrete through Science and Engineering, RILEM Publications, Bagneux, France, CD-ROM Paper No. 14, Hybrid-Fiber Session. Sukontasukkul, P., Mindess, S., Banthia, N. and Mikami, T. [2001]. Impact resistance of laterally confined fiber reinforced concrete plates. Materials and Structures (RILEM), Vol. 34, pp. 612–618. Thibaux, T., Hajar, Z., Simon, A. and Chanut, S. [2004]. Construction of an ultrahigh-performance fibre-reinforced concrete thin-shelled structure over the Millau viaduct toll gates. In Fibre-Reinforced Concrete, BEFIB 2004, M. di Prisco, R. Felicetti and G.A. Plizzari, eds. RILEM Proceedings PRO 39, RILEM Publications, Bagneux, France, Vol. 2, pp. 1183–1192. Yan, C. and Mindess, S. [1994]. Bond between epoxy coated reinforcing bars and concrete under impact loading. Canadian Journal of Civil Engineering. Vol. 21, No. 1, pp. 89–100. Yan, C. and Mindess, S. [2001]. Bond between concrete and steel reinforcing bars under impact loading. In Brittle Material Composites 3, A. H. Brandt and I. H. Marshall (eds.), Elsevier Applied Science, pp. 318–327.

8 Lightweight concrete T W BREMNER, University of New Brunswick, Canada

8.1

Introduction

Weight is the bane of most construction materials and this is particularly true for concrete (density ∼2400 kg/m3). Fortunately, lower density concrete can be made using lightweight aggregates that have within their mass an array of vesicules or air voids that render these aggregates of significantly lower density than normalweight aggregates. When these lower density aggregates are incorporated in a concrete mixture they can produce a structural grade concrete with a density of about 1850 kg/m3. In addition to the very obvious reduction in mass, the other advantages that lightweight concrete can provide include increased thermal insulation, extended moist curing and increased durability. A brief description of the methods of manufacturing the various types of lightweight aggregates will be covered, with emphasis on recent technical developments. The way in which the production process creates desirable properties in the resultant product itself, and in the concrete made from it will be explained. A brief review of how various countries use this product will be given. This chapter concludes with sources of further information about lightweight concrete and advice for its more effective use. When a critical comparison of the rewards and liabilities associated with the use of lightweight concrete reveals that the lightweight concrete is a preferred material, it should not be construed as a negative inference for normalweight concrete. It is simply that lightweight concrete in some instances has enhanced capabilities that fully justify its use. Lightweight aggregates made from natural deposits of volcanic materials such as pumice, sintered fly ash, and special deposits of silica-rich shale, clay and slate heated to about 1150°C are the main focus of this work. Other low density aggregates, for example polystyrene beads and cold bonded fly ash, will only be discussed briefly because they are not associated with reinforced concrete. Aerated or foamed concrete, another form of low density concrete that results when a very high amount of entrained air is 167

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Developments in the formulation and reinforcement of concrete

incorporated into a mortar matrix, is covered in another part of this book.

8.1.1 Terminology The term “lightweight concrete” which is preferred by the American Concrete Institute (ACI) will be used rather than the more scientifically correct term of lower density concrete (1). The Institute also defines normalweight as one word. ACI Committee 213 “Lightweight Aggregates and Lightweight Concrete” defines “lightweight concrete” as being concrete made with lightweight coarse aggregates and normalweight fine aggregates with possibly some lightweight fine aggregates. “All lightweight concrete” is concrete made with the fine and coarse aggregates all being lightweight. This ACI terminology will be used throughout this chapter (2).

8.1.2 Nature of lightweight concrete To fully appreciate lightweight concrete it is essential to understand the intrinsic nature of lightweight aggregates and how they influence the properties of concrete made from them. These special aggregates have within their mass an array of vesicules or air voids which can be seen in Fig. 8.1. The size, spacing and degree of interconnection of the vesicules make these aggregates capable of producing concrete with special properties. A reduction in the density of the concrete is well appreciated by the structural designer, since the weight of the concrete frequently makes up well over half of the dead load in a structure. Of course an increase in vesicularization results in a significant decrease in particle strength, but this does not prevent the lightweight concrete from being used for high strength applications (3). The reason is that normalweight aggregates are from 4 to 6 times stiffer than the surrounding mortar matrix which generates large stress concentrations in the concrete when subjected to load (4). With lightweight concrete the stiffness of the aggregate closely matches the stiffness of the matrix so stress concentrations are eliminated or at least greatly reduced. Figure 8.2 shows the maximum and minimum stress distributions around spherical inclusions that represent a stiff inclusion (Ea = 5Em) on the left and a flexible inclusion on the right (Em = 5Ea). Figure 8.3 shows the stress concentrations in terms of the maximum and minimum principal stresses for various ratios of stiffness of inclusion to stiffness of matrix. The low stress concentrations in the lightweight system explain why a high strength concrete can be made from a relatively low strength particle. However, it should be recognized that there is a strength ceiling characteristic of each type of aggregate above which increased strength generally

Lightweight concrete

(a) Expanded clay – 10 μm

(b) Expanded slag – 200 μm

(c) Sintered fly ash – 10 μm

8.1 Microstructures of lightweight aggregate.

169

170

Developments in the formulation and reinforcement of concrete Unit compressive stress at infinity Em Ea

+1.667

0.333

1.667

+0.333 (a) Maximum principal stress

+0.333

–0.333 –0.351

0 (b) Minimum principal stress

0

σ1 (Compressive stress)

Compression

σ3 (Tensile stress) Tension

(c) Orientation of σ1 and σ3, σ3 is to plane of Figure (causing splitting in a plane to plane of Figure) Stiff inclusion (Ea = 5Em)

Orientation of σ1 and σ3, σ3 is to plane of Figure (causing splitting in a plane through inclusion) Flexible inclusion (Em = 5Ea)

8.2 Principal stress around spherical inclusion.

cannot be achieved without substantially higher cement contents in the mixture as can be seen in Fig. 8.4 (4). The reduced stress concentration around lightweight aggregates substantially reduces the amount of microcracking in the lightweight concrete under applied stresses as compared to normalweight concrete in a similar

Lightweight concrete Structural normalweight concrete

Structural lightweight concrete

2.0

1.0 0.5 0

Inclusion and matrix Inclusion

Matrix

–0.5 –1.0

Matrix

(Hole)

1.5

(Rigid inclusion)

Tension stress compression

2.5

171

α

10

6

4

2 Log10 Ea

1

0.2

0.5

0.1

0

Em

8.3 Maximum and minimum principal stress for a spherical inclusion subjected to unit compressive stress at infinity.

Normalweight concrete

Aggregate strength Concrete strength

1.0

Lightweight concrete

X Binder content

8.4 Optimum content is “X” for most efficient use of binder.

condition. Less microcracking, in turn reduces the permeability of the concrete and enhances its ability to protect embedded steel reinforcement from corrosion. Sugiyama et al. found that when lightweight concrete was subjected to applied compressive stress, the onset of microcracking was delayed until about 80% of the ultimate compressive stress was reached, whereas with normalweight concrete the microcracking started before 60%

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of the ultimate compressive stress was applied. This was shown by the enhanced stress level at which permeability was noted to increase and the level at which the stress–strain curve became non linear for lightweight as compared to normalweight concrete (5). Examination with an optical microscope of polished surfaces of hardened concrete taken from field structures reveals significantly less microcracking in lightweight concrete as compared to normalweight concrete, indicating that service loads, thermal stresses and other volume changing influences have less effect on the permeability of lightweight concrete (6). Also, during the passage of the shale, clay or slate particles through the rotary kiln, they are heated to temperatures that activate their silica rich surfaces so that they act as pozzolans that can substantially improve the nature of the aggregate–cement paste interface (7). Conversely in normalweight concrete a weak boundary layer is usually noted at the interface which, combined with the normal stress concentration from the large mismatch of the stiffness between the inclusion and the matrix, causes premature failure. By enhancing the interfacial bond strength between the inclusion and the matrix, microcracking is further reduced thereby enhancing the overall performance of the lightweight concrete. A comparison of the effect of concrete density on the stress–strain curve of concrete (see Fig. 8.5) indicates two aspects that the designer must address. Firstly, in the elastic range the stiffness of the lightweight concrete is less than the stiffness of the normalweight concrete. This is covered in the various design codes so that normal deflection limits for structural components can be met. Secondly, the area under the stress–strain curve is less for lightweight concrete. To compensate for this the codes may require additional confinement reinforcement in the form of closer spacing of stirrup reinforcement so that the necessary ductility at ultimate load can be met. Codes also require enhanced bond length of reinforcement and increased amounts of shear reinforcement when lightweight concrete is used. Fortunately the intrinsic advantage of using vesicular aggregate usually outweighs these factors.

8.1.3 History of lightweight concrete The use of lightweight aggregates for construction dates from antiquity when builders realized that vesicular aggregates like pumice and scoria from volcanic activity could be used more easily in the building process than dense normalweight aggregates. In addition to being lighter in weight these deposits tended to break up into manageable sizes. The reason for this is that the vesicular aggregates tend to be an order of magnitude less thermally conductive than the surrounding volcanic material, and as a result develop much steeper thermal gradients within their mass after they start

45 40 35 30 25 20 15 10 5 0

300 mm

Compressive stress in MPa

Lightweight concrete Insulating concrete

100

ρ = 1.5 kg/dm3 0

0.5

1

1.5 2 2.5 Strain ε in ‰

3

3.5

4

45 40 35 30 25 20 15 10 5 0

300 mm

Compressive stress in MPa

A – Stress–strain diagram of concrete with a density of 1500 kg/m3

Lightweight concrete ρ = 1.7 kg/dm3

100

0

0.5

1

1.5 2 2.5 Strain ε in ‰

3

3.5

4

45 40 35 30 25 20 15 10 5 0

300 mm

Compressive stress in MPa

B – Stress–strain diagram of concrete with a density of 1700 kg/m3 ρ = 2.3 kg/dm3

100

Normalweight concrete 0

0.5

1

1.5 2 2.5 3 3.5 4 Strain ε in ‰ C – Stress–strain diagram of concrete with a density of 2300 kg/m3

8.5 Note the increase in area under the stress–strain diagram as density of the concrete increases.

173

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Developments in the formulation and reinforcement of concrete

to cool in the air. The steep thermal gradients in the now solidified mass cause thermal cracking so that manageable sizes of light rock particles are produced. What was convenient to early man is also advantageous to modern builders; in most deposits in Australia material is excavated with a front end loader and with a modest amount of crushing and size classification can be used directly for making lightweight concrete. The port of Cosa is on the west coast of Italy about 50 km north of where the Tiber flows into the Mediterranean, and some time shortly after 273 bc the Romans used natural deposits of vesicular aggregate exclusively in the concrete for their harbor piers. Although they had a well-graded aggregate on the beach that would be suitable for making concrete to today’s standards, they went inland in a northeasterly direction to the Volcine Geological Complex that had a deposit of pumice which they transported some 44 km to the construction site at Cosa. To go with their lime binder they went to the Bay of Naples about 100 km to the south to get volcanic ash from Mount Vesuvius. Their wisdom in selecting materials paid off. Their piers stand today, resisting wave action in a very exposed location in the Mediterranean Sea and, had siltation not occurred, could serve a useful purpose today (8). The Roman Emperor Hadrian was a man accused of continually changing his mind, but in one case at least, with very positive results. With lightweight aggregates used in building the 43.3 m Pantheon dome, completed in ad 128, he varied the density of the concrete, uniformly decreasing the density of pumice particles in the concrete going up the dome. More dense pumice was used at the base where the greatest strength was required and less dense pumice at the top of the dome (7). Amazingly the Pantheon continues to be used to this day for religious purposes as Hadrian had intended, unlike the Colosseum which also used lightweight aggregates in the concrete, but is now no longer used for its original purpose. Geological deposits of pumice and other vesicular aggregates continue to be used to this day. No doubt these modern-day lightweight concretes will continue to serve as well as the above examples, clearly indicating that a design service life of 2000 years can be expected. Natural deposits of lightweight aggregates were found to be too variable in degree of expansion to provide the high strength concrete needed by the building industry at the beginning of the 20th century. To achieve high strength concrete a Kansas City contractor, Mr Stephen J. Hayde, initially experimented with crushed reject bricks, and found that these “bloaters” (clay bricks that had been accidentally heated too rapidly in the kiln), made a useful aggregate for concrete. His simple experiments led him to pass particles of selected deposits of silica-rich shale, clay or slate through a rotary kiln similar to that used to make cement clinker. He found that when heated to about 1150°C these materials bloated as a result of evolved gases

Lightweight concrete

175

within the material and, fortunately, that this distended shape was retained upon cooling. While the raw material had a particle relative density of about 2.65 the existing material had a density of about 1.45 (9). By carefully grading the material entering the kiln, he found that after expansion a material that met the appropriate grading requirement for concrete could be obtained with minor crushing and sieving. The material produced was called Haydite and a firm with that name continues to produce essentially the same type of aggregate that is now also being produced by many plants in many countries around the world. Because the material is still made in the same way and is still expanded to about the same degree, there is a service record of good performance for almost a century. Mr Hayde, as well as being an able materials engineer, had sufficient entrepreneurial skills to patent the process and to license it to other producers. One of his astute moves was to offer it free of charge to the US Government for use in the First World War effort. As a result extensive research was done on this new material so that it could be used in the construction of concrete ships. Some of these ships are still afloat and continue to serve a useful purpose at Powell River, B.C., Canada. Thanks to the US Government’s involvement, at the end of the war industry was provided with design tools to be used in building bridges and buildings. Throughout the 20th century the industry grew rapidly. Currently the North American production of aggregates is relatively stable being between five and six million cubic metres per year. As with the cement industry in developed countries the amount produced each year is relatively constant (10). In Denmark, the firm of LECA® (Light Expanded Clay Aggregate) introduced expanded clay aggregates to Europe immediately before the Second World War and has 35 kilns in operation worldwide with a capacity of six million m3 per year (11). In Germany LIAPOR® started production in 1967 and has four plants producing aggregates with a particle density of from 0.45 to 1.55. Their clay comes from a deposit from the earliest part of the Jurassic age some 180 million years ago (the “liea” age hence the name Liapor). They dry, pulverize and then pelletize it followed by passage through a two kiln system that allows full control over particle size, shape, compressive strength and density. In addition to the structural lightweight concrete that uses material with a particle density of 1.55, LIAPOR® uses the lower density material for masonry units, horticulture and loose insulation. Fly ash from a coal-fired electric generating station can be pelletized and the residual unburnt carbon in the fly ash ignited to cause sintering of the pellets making an aggregate suitable for structural concrete. In about 1960, LYTAG® in the UK perfected this technology to produce a product with a particle relative density of about 1.9 which was suitable for high strength structural concrete applications. But because the density was not sufficiently

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Developments in the formulation and reinforcement of concrete

lower than that of normalweight aggregates, the industry failed to grow, or to develop in North America despite several rather expensive attempts to do so. Current research is underway to make the product lighter and results are promising (12). One plant operating in Poland produces a particle with a relative density of 1.45 for a particle size of 14 mm to 5 mm, but no details are available as to how they achieve this low density or if high strength lightweight concrete can be made with it. In China research has been done on all of the various methods of making lightweight aggregates but no information is available about production other than that there is one plant making sintered fly ash lightweight aggregates. One of the major problems is that most fly ashes used to make lightweight aggregates contain much more carbon than is needed. With carbon content higher than the 5 to 7% required, the excess heat in some instances has destroyed the plant. Modern equipment is available that can economically lower the carbon content in the fly ash, so this should not now be a problem. Molten slag from a blast furnace can be either air cooled or granulated and then crushed to make a rather heavy normalweight aggregate with a particle density of about 2.9 as compared to a natural aggregate of about 2.65. Various methods have been perfected to produce an expanded slag and one of the most successful methods is to direct the flow of molten slag as it comes directly from the blast furnace onto a rotating finned drum. The drum flings the slag up in to the air where it solidifies and rains down as discrete spherical particles of a relative density as low as 1.7. This technology was perfected about 1960 by a Mr R.P. Cotsworth in Hamilton, Canada while he was employed by National Slag Ltd. That company is now part of Lafarge Canada Ltd and has plants in Hamilton and Chicago (13). The Canadian plant produces about 400,000 m3 per year with 85% going for masonry units, 15% for structural uses and 5% for miscellaneous uses. One very favorable aspect of both sintered fly ash and pelletized slag lightweight aggregates is that the energy expenditure is very low compared to the products made with a rotary kiln. Also in both of these methods the release of greenhouse gases is lower.

8.2

Applications/case studies

8.2.1 Structural applications Lightweight aggregates can be used to make high strength concrete and designers have known this from their first introduction to concrete construction. During the First World War designers of ships and barges used rotary kiln produced lightweight aggregates to make concrete that was twice the strength of that normally used at the time for general construction

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177

(14). The ship USS Selma was built in 1919. Over three decades later when cores from Selma were taken and tested, the concrete had strength in excess of 50 MPa. Builders of major structures worldwide have taken advantage of the weight reduction achieved through the use of lightweight concrete. One of the first major applications of lightweight concrete in high rise buildings was the 28-story South Western Bell Telephone building built in 1929 in Kansas City of which the upper 14 stories were of lightweight concrete. Since then many buildings have been built with lightweight concrete including the NatWest Tower and the Canary Wharf Building in London, UK. In the Canary Wharf Building lightweight concrete made with sintered fly ash was used for the floor slabs (15). In Toronto, Canada the Toronto Dominion Center that is 230 m high and has 56 floors used expanded slag aggregates for the floor slabs and the masonry infilling walls (16). Certain characteristics of lightweight concrete make it the material of choice where unusual construction needs require specialized or unique solutions. The 60-story Nations Bank Building in North Carolina, USA used pre-soaked expanded shale lightweight aggregate in 117 mm thick slabs that were supported on post-tensioned concrete beams at 3.0 m centers. The lightweight concrete floor system was used to minimize dead weight and to achieve a 3-hour fire rating. With the pre-soaked lightweight aggregates it was possible to pump the concrete from street level up 250 m to the top floor using one concrete pump at street level (17). Lightweight concrete is durable and is enormously useful for repairing and refurbishing ageing infrastructure. In the USA the first bridge made using lightweight concrete was built about 1922 and since then many bridges have been successfully constructed with this material (9). In 1985 the US Federal Highway Administration employed T.Y. Lin International of San Francisco to review this work and to prepare a state-of-the-art report on the use of “Lightweight Concrete in the Design, Construction, and Maintenance of Bridges” (18). This company examined the condition of existing lightweight bridges, reviewed the current design procedures and came to the conclusion that the “successful experience with the lightweight-concrete roadway decks for the bridges described in their report was due to good project specifications, attention to quality control, the use of trained personnel, and an effective maintenance program”. It is crucial that these four factors be priorities in all concrete construction to obtain good long-term performance. T.Y. Lin International found “Lightweight-concrete is being used to produce an economical solution in rehabilitating and upgrading existing bridges, especially where they involve an increase in the load rating or a widening of the roadway.” Holm has documented applications where rehabilitation and widening of highway bridge and viaduct structures were economically carried out primarily because the existing footings and

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columns could be used to support a much wider deck when the previous normalweight concrete deck was replaced with a lightweight one (19). The positive weight to mass ratio gained through the use of lightweight concrete has resulted in new and novel construction techniques. The US Army Corps of Engineers used “float-in and in-the-wet technology” to build the new Braddock Dam in Pennsylvania. To accomplish this project they used “lightweight concrete to make a dam float” (19). Also the dam functions as a lock on the Mississippi River and was floated 44 km downstream to the site where it was allowed to settle down onto a previously placed pile foundation. Design specifications were a maximum unit weight of 2000 kg/m3 and a minimum strength of 35 MPa at 28 days. Lightweight concrete has been used extensively for offshore oil platforms that are built at an onshore graving dock or in a fjord where deep water is close to shore. When completed, these platforms are towed to a permanent location to be set on the ocean floor. In the case of the Hibernia Platform, to reduce draft a controlled density concrete was made with a unit weight of 2160 kg/m3. This was achieved by using equal proportions of lightweight and normalweight coarse aggregates for the concrete mixture. Tarsuit Caisson Retaining Island, Draugen and Troll platforms are examples where the construction was done on or close to land, towed in place and then bottom founded. All have performed well and should serve as models for future construction where concreting and fitting out can be done in convenient locations followed by towing to operating sites (20). The superior strength to weight ratio of lightweight concrete is well recognized, making it the preferred material for floating concrete structures. Service records from the First World War ships to the oil platforms of the 1990s serve to substantiate its good long-term performance. Such structures as the Heidrun Platform, built in 1996 with a density of 1940 kg/m3 and strength of 70 MPa, provide the assurance necessary to specify the use of this material for new marine applications. For example, floating LNG terminals built with lightweight concrete hulls incorporating offshore gas storage, facilities to liquefy the gas, and terminal facilities for regasification and storage before piping the gas to shore are in the foreseeable future (21).

8.3

Production of lightweight concrete

Unlike normalweight concrete, which is usually made from deposits of granular aggregates created by glacial action and subjected to water transport, lightweight aggregates such as expanded clay, shale and slate are manufactured in a controlled way from specifically selected large deposits of raw material that tend to be uniform in composition. This means the intrinsic variability within normalweight concrete mixtures is greatly

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Table 8.1 Bulk density and particle density of typical structural grade lightweight aggregate (5–20 mm size) Aggregate type

Particle density (dry)

Cold bonded fly ash Sintered fly ash

1.6–1.9 1.6–1.9 1.3–1.5 1.4–1.5 1.7–1.9 1.3–1.5 1.4–1.5 0.6–1.6

Expanded shale Expanded slag Expanded clay

US UK Poland US Canada (old) Canada (new) US Germany

Table 8.2 Concrete density of typical insulating grade lightweight aggregate Aggregate type

Air dry concrete bulk density

Typical compressive strength

Perlite Vermiculite Expanded polystyrene

490 480 800

f ′c = 1.6 MPa f ′c = 0.7 MPa f ′c = 1 MPa

reduced in comparison to those made with lightweight aggregates. This uniformity enables producers of structural lightweight aggregates to provide detailed guidance on concrete mixture proportions to the purchasers of their aggregates. Typical ranges of particle density for the various types of commercially available lightweight aggregate are given in Table 8.1. In Table 8.2 concrete densities for typical insulating structure grade lightweight aggregate are given and because of their low strength, they have very limited structural uses. For the various types of lightweight aggregate, the local suppliers should be contacted for the properties of materials that they produce. Typical high strength concrete mixtures are given in Table 8.3 indicating that high performance concrete can be achieved through the use of commercially available lightweight aggregates (22–25). Normally for most domestic and industrial construction much lower strengths are adequate in which the required compressive strength is less than the particle strength (Fig. 8.4). In this case mixture proportioning is greatly simplified in that typical normalweight concrete mixtures can be used as reference in preparing lightweight concrete mixtures by simply replacing normalweight aggregate with lightweight aggregate on a volume basis. Most commercial lightweight concrete mixtures contain about 5% entrained air and have a slump that is reduced by the same ratio as the density of the final concrete mixture has been reduced. For example a 2400 kg/m3 normalweight

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Table 8.3 Typical mixture proportions for various strengths of modified density and high performance structural lightweight concrete Units

Specified Strength Cement Silica fume FA Sand CA LtWt W/C Density Slump Air content Strength 28 days Tensile splitting stress Modulus of elasticity Strength base

Ref 22 Stovset

Ref 23 Nordhordland

Ref 24 Sandhornoya Bridge

Ref 25 Hibernia

55

50

55

430 35 630 570 0.35 1881 200 – – –

400 25 575 650 0.35 1850 200 – 59.8 –

– – – –

kg/m2 mm % MPa MPa

425 30 685 520 0.35 1924 200 5 64.5 –

GPa

22.0

21.0

–

30.5

Cube

Cube

Cube

Cylinder

kg/m3 kg/m3 kg/m3 kg/m3

69

0.33 2170 210 2.1 79.9 5.87

concrete mixture with a slump of 120 mm becomes 90 mm slump with a lightweight concrete with a density of 1800 kg/m3, both of which have essentially equal workability and the same compressive strength at 28 days. Creep and shrinkage of lightweight concrete are covered in References 26 and 27, and the values given are surprisingly similar to those for normalweight concrete. A possible reason for this is that the high localized stress concentrations in the concrete matrix in the vicinity of the more rigid normalweight aggregates give rise to high permanent dimensional changes. With the more uniform stress distribution in lightweight concrete the high stresses are not present. In both cases the dimensionally stable normalweight and lightweight aggregates are effective in resisting the dimensional changes of the volumetrically unstable cement paste matrix. Unfortunately, little resistance to dimensional change of the cement paste matrix is afforded by cold bonded aggregates and by very low density expanded polystyrene, perlite, or vermiculite aggregate, a factor that must be considered by designers. With cold bonded fly ash (fly ash bonded with a cementitous material), both the aggregate inclusion and the matrix are subject to creep and shrinkage, and as a result the primary application for these products is in making concrete masonry units where special attention can be paid to incorporating additional contraction joints in masonry walls (28).

Lightweight concrete

8.4

181

Future trends

The high growth rate of the industry in the past eight decades can be expected to continue as designers now, more than ever, fully appreciate the intrinsic advantages of using low density material for structural purposes. In the past cost was the main limiting factor; however, the designer must now take a holistic approach including social concerns and environmental factors. Such factors as low volatile release rate from building materials and the desirability of using inherently non-combustible materials rather than fire retardant treated combustible materials puts lightweight concrete and concrete masonry in a very auspicious position for future growth. Taking into account the energy to create and greenhouse gas emission during manufacture of typical non-combustible materials and relating that to allowable stress and thermal resistance per unit thickness, concrete ranks well above other materials. Where credit is given for reduced dead load and enhanced thermal resistance, lightweight concrete is the preferred material (22). For this reason the industry should prosper in the foreseeable future in spite of the projected reduced rate of growth of the cement industry in the developed world (10). In China and India as well as in much of the developing world, there are clear signs of an awakening to environmental problems and in particular to global warming and the associated problem of the rise of ocean levels. China has experimented with essentially all types of raw materials that can be used to make expanded aggregates including clay, fly ash, shale, and slate as well as most methods of manufacture. The results of their research are likely to have a profound effect both in that country and around the world. Siltation of canals is a major problem in many countries including in China. Research has shown that material dredged from these canals, which for health reasons is hard to dispose of, can be used safely by first extruding it into pellets and then introducing it into a high temperature rotary kiln, thereby making a high quality lightweight aggregate (29). In its more populated areas India is not well endowed with good aggregate sources but has fly ash from power generating stations, a ready source of material to make lightweight aggregates. Extensive research is being carried out at various centers, including at the Indian Institute of Technology in Madras, to make a stronger and more highly expanded aggregate than is available using current manufacturing procedures. A particular advantage of lightweight aggregates derived from fly ash is that worldwide immense supplies of coal are available at low cost. This coal is usually transported to population centers so the resulting coal ash when made into lightweight aggregates is close to the potential user. In India, as well as in most other places where coal is burned, the ash

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Developments in the formulation and reinforcement of concrete

produced is in excess of the need for it as a supplementary cementing material. This surplus ash in an urban environment causes a disposal problem unless it is processed into lightweight aggregates which should find a ready market. Most of the larger countries in the developing world are exporters of steel but have limited energy resources. By taking the molten slag directly from the iron blast furnace to a pelletizing machine, lightweight aggregates are produced with essentially a negligible input of energy. Industries in Canada and Russia have perfected this process and have been doing this for the past several decades (30). To have a viable industry producing lightweight aggregates there needs to be a base load market to sustain the industry through the vicissitudes of the structural market. The building industry is noted for its boom and bust behavior, a factor causing many well-managed firms to come to grief. In the past two decades the following markets have been developed and old ones been reinvigorated to produce an important base load for the industry: 1.

2.

3. 4.

5.

6. 7.

8.

Masons, because of an aging workforce and back problems caused by lifting the heavy normalweight units, prefer a much lighter block that can be made by using some or all lightweight aggregate in the masonry block mixture. In some markets this consumes about 80% of their production. Horticultural uses of lightweight aggregate to absorb and slowly release water and liquid nutrients can reduce labor costs as well as minimize weight, a useful factor in green roofs. Highway fills where load balancing is needed over low bearing capacity soils. Vehicle arrest pads of loose lightweight aggregates are used at the ends of runways and at run off ramps at the bottom of steep hills in mountainous regions. Water treatment facilities use vesicular crushed aggregates to provide more exposed surface area for vegetative growth which leads to even greater reduction in phosphate content in the wastewater than when normalweight aggregates are used. Lightweight aggregates are used in asphalt mixtures to produce high friction surfaces. In winter skid-free surfaces are produced when lightweight aggregates are saturated with salt water and used to replace normalweight sand. Delayed release of the salt from the vesicular aggregates provides either ice removal or helps key the aggregates into the ice surface. Self desiccation in high cementitious content concrete mixtures can be prevented by incorporating some highly saturated lightweight aggregates into the concrete mixture.

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183

In conclusion, the successful development of all of these new markets is vitally important if we are to have a healthy industry producing a suitable grade of lightweight aggregate for the structural market. At this time only a few of these markets are being effectively promoted by any given producer. To accomplish this, all of the above uses need to be promoted on a worldwide basis, and conferences such as the broadly based Second International Congress of Lightweight Concrete held in London in 1980 are good models of the type of events that need to be held. The closely focused conferences on structural lightweight concrete held in the recent past generally did not serve this purpose.

8.5

Sources of further information and advice

Internationally relevant information on lightweight concrete covering all types of structural lightweight concrete can be found in the American Concrete Institute (ACI) (31) Committee 213 publications. The two most relevant are the “ACI 213R (2003) Guide for Structural Lightweight Aggregate Concrete” and “ACI 211.2 (1991) Standard Practice for Selecting Proportions for Structural Lightweight Concrete.” These ACI publications make reference to ASTM C330 “Standard Specification for Lightweight Aggregates for Structural Concrete” and ASTM STP 169D Chapter 46 “Lightweight Concrete and Aggregates” in “Significance of Tests and Properties of Concrete and Concrete Making Materials” (32). A supplement to the European Model Code – 90 (33) has been prepared by the FIB Task Group 8.1 Lightweight Aggregate Concrete which provides important information on the Structural Design Requirements for lightweight aggregate concrete. Particular attention is paid to modification of the stress–strain diagram in compression for use in flexural members, efficiency of confining reinforcement, allowable bearing strength as well as anchorage and splicing of reinforcement. The supplement states “that there is no need for extra concrete cover to achieve the required corrosion protection of the reinforcement than for normalweight concrete”. The reason for this is the high elastic compatibility of both components minimizing the microcracking and the fact that “the cement paste penetrates into the pores of the aggregate surface”. The supplementary document concludes with the observation that the lightweight aggregates are pozzolanic and that because of the absorptive nature of lightweight aggregates, bleed water does not accumulate under the aggregates as is noted in normalweight aggregates and as a result “it is usually not possible to determine the transition zone” using a scanning electron microscope. The Expanded Shale Clay and Slate Institute (ESCSI) is an organization of most of the lightweight aggregate producers in the US, with associate

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Developments in the formulation and reinforcement of concrete

members from Germany and Japan. ESCSI maintains a web site (www. escsi.org) listing the names of its members, their addresses and web sites. Posted on the web site are copies of their technical data sheets and promotional material as well as copies of refereed technical papers that can be downloaded free of charge (34).

8.6

References

1 ACI 213R-03, “Guide for Structural Lightweight Aggregate Concrete,” (2003), pp. 38. 2 Holm, T.A. and Bremner, T.W., (1990), “70 Year Performance Record for High Strength Structural Lightweight Concrete”, ASCE Materials Engineering Congress, Denver CO, August 13–15, pp. 15. 3 Bremner, T.W. and Holm, T.A., (1986), “Elastic Compatibility and the Behavior of Concrete”, ACI Journal, Proceedings, 83 (2), 244–250. 4 Bremner, T.W. and Holm, T.A., (1995), “High Performance Lightweight Concretes”, in Proceedings of the Second CANMET/ACI International Symposium on Advances in Concrete Technology, Ed. V.M. Malhotra, ACI SP Series, Las Vegas, June 11–14, pp. 1–20. 5 Sugiyama, T., Tsuji, Y., Bremner, T.W. and Holm, T.A., (1995), “Chloride Permeability of Concrete under Compressive Stress”, Proceedings of the 1st International Conference on Concrete under Severe Conditions, Eds. K. Sakai, N. Banthia and O.E. Gjørv, Sapporo, Japan, August 2–4, Vol. 2, pp. 1389– 1398. 6 Selih, J., Sousa, A.C.M. and Bremner, T.W., (1992), “Numerical Simulation of Water Migration in Concrete”, Advanced Computational Methods in Heat Transfer II, Vol. 2, Natural/Forced Connection and Combustion Simulation, Eds. L.C. Wrobel, C.A. Bribbia, and A.J. Nowak, Elsvier Applied Science, Amsterdam, pp. 349–366. 7 Bremner, T.W., Holm, T.A. and Stepanova, V.F., (1994), “Lightweight Concrete – A Proven Material for Two Millennia”, Proc. of Advances in Cement and Concrete, University of New Hampshire, Eds. S.L. Sarkar and M.W. Grutzeck, Durham, NH, July 24–29, pp. 37–51. 8 McCann, A.M., (1987), The Roman Port and Fisherie of Cosa, Princeton University Press, Princeton, New Jersey, 353 pp. 9 Expanded Shale Clay and Slate Institute (ESCSI). (1971), “Lightweight Concrete – History, Applications, Economics”, Salt Lake City, Utah, USA, 44 pp. 10 Harder, J., (2005), “Outlook on the Cement Industry in 2010”, ZKG International, Cement Kalk Gips, Vol. 58, No. 1, pp. 24–32. 11 www.leca.dk, last accessed April 23, 2007. 12 Ramamurthy, K. and Harikrishnan, K.I., (2006), “Influence of Binders on Properties of Sintered Fly Ash Aggregates’, Cement and Concrete Composites, 28, 33–38. 13 www.nationalslagassoc.org, “Pelletized Lightweight Slag Aggregate”, paper by J.J. Emery, 1980, last accessed April 30, 2007. 14 Holm, T.A. and Bremner, T.W., (1994), “High-Strength Lightweight Aggregate concrete”, in High-Performance Concrete and Applications, ed. S.P. Shaw and S.H. Ahmad, Edward Arnold, London, pp. 341–374.

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15 www.liapor.com, last accessed April 22, 2007. 16 www.aviewoncities.comm/building/torontodominionbanktower.htm Last accessed April 22, 2007. 17 Expanded Shale Clay and Slate Institute (ESCSI), 1996, “Pumping Structural Lightweight Concrete”, Salt Lake City, Utah, 4pp. 18 Federal Highway Administration, (1985), “Criteria for Designing Lightweight Concrete Bridge”. Report No. FHWA/RD-85/045, McLean, VA. 19 Tasillo, C.L.,Neeley, D.B.D. and Bombich, A.A., (2004), “Lightweight Concrete Makes Concrete Float,” in High-Performance Structural Lightweight Concrete, John P. Ries and Thomas A. Holm, eds., pp. 101–130. 20 Holm, T.A. and Bremner, T.W., (2004), “State-of-the-Art Report on High Strength Low Density Concrete for Applications in Severe Marine Environments,” U.S. Army Corps of Engineers, Engineering Research and Development Centre, pp. 110. 21 www.dnv.com/energy/news/Offshoreconcretestructuresareback.asp,lastaccessed April 26, 2007. 22 Johnsen, H. et al. (1995), “Construction of the Stovset Free Cantilever Bridge and the Nordhordland Cable Staged Bridge, International Symposium on Structural Lightweight Aggregate Concrete, Sandefjord, Norway, June. 23 Heimdal, E. and Ronnenberg, H., (1995), “Production of High Strength LWAC – The views of a ready mix producer”, International Symposium on Structural Lightweight Aggregate Concrete, Sandefjord, Norway, June. 24 Fergestad, S., (1996), Bridges built with Lightweight Concrete”, Proceedings of International Symposium on Lightweight Bridges, Sponsored by CALTRANS, Sacramento, CA. 25 Hoff, G.C., Walum, R., Weng J.K. and Nunez, R.A., (1995), “The Use of Structural Lightweight Aggregate in Offshore Concrete Platforms”, International Symposium on Structural Lightweight Aggregate Concrete, Sandefjord, Norway, pp. 349–362. 26 Jones, T.R. and Stephenson, H.K., (1957), “Properties of Lightweight Concrete related to Prestressing”, in World Conference on Prestressed Concrete Inc., Eds. Kelley, Scordelis and Zollman, San Francisco, July–August, pp. A6–1 to A6–12. 27 Rogers, Grover, L., (1957), “On the Creep and Shrinkage Characteristics of Solite Concretes”, in World Conference on Prestressed Concrete, Eds. Kelley, Scordelis and Zollman, San Francisco, July–August, pp. 2–1 to 2–5. 28 Bremner, T.W., Ries, J.P. and White, W.H., (2007), “Achieving Sustainability with Lightweight Concrete”, in Proceedings of Special Session in Honour of Professor Giacomo, editors Marconi, N. Kraws, T. Naik, P. Claisse and M. Sadeghi-Pouya, Coventry, UK, June 11–13, pp. 12–17. 29 Collins, R.J., (1980), “Dredged Silt as Raw Material for the Construction Industry”, Resource Recovery and Conservation, Vol. 4, pp. 337–362. 30 Bremner, T.W., (2005), “Lightweight Concrete – An International Perspective,” Plenary Paper For Second All Russian Conference on Concrete and Reinforced Concrete, Moscow, Russia, 6–9 September. 31 American Concrete Institute, P.O. Box 9094, Farmington Hills, Michigan, 483339094, USA.

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32 ASTM International (www.astm.org), 100 Barr Harbor Drive, PO Box C-700, West Conshohocken, PA 19428-2959, USA. 33 Supplement to the European Model Code – 90, Fib Task Group 81, Lightweight Aggregate Concrete, Fib Secretariat, Case Postale 88, Ch-1015 Lausanne, Switzerland. 34 Expanding Shale, Clag and Slate Institute (ESCSI), Suite 102-2225 Murray Holliday Road, Salt Lake City, UT, 84117, USA.

9 Self-compacting concrete (SCC) M GEIKER, Technical University of Denmark, Denmark

9.1

Significance of self-compacting concrete

In many aspects self-compacting concrete (SCC), (“self-consolidating concrete” in North America) can be considered the concrete of the future. SCC is a family of tailored concretes with special engineered properties in the fresh state. SCC flows into the formwork and around even complicated reinforcement arrangements under its own weight. Thus, SCC is not vibrated like conventional concrete. This drastically improves the working environment during construction, the productivity, and potentially improves the homogeneity and quality of the concrete. In addition SCC provides larger architectural freedom in structural design. Highly flowable concretes have for many years been used for underwater concreting of unreinforced or lightly reinforced low grade structural elements. The availability of new types of admixtures started the development of SCC in Japan in the 1980s. SCC is especially popular in Europe; in 2005, 70% of the precast concrete in the Netherlands was SCC, while 30% of the ready mix concrete produced in Denmark was SCC (Nielsen et al. 2007a). Productivity The use of SCC reduced the construction time for the anchorages of the Akashi-Kaikyo Suspension Bridge, Japan, from 2.5 years to 2 years. It is estimated that productivity in the building industry will be improved by 5– 10% by the successful use of SCC. For horizontal castings the estimated reduction in man hours is as large as 50% (Nielsen 2007c). Working environment The working environment is improved through the reduction by more than half of the noise, from about 95 dB(A) to 85 dB(A) at building sites and even more at precast plants. The most significant improvement of the 187

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Developments in the formulation and reinforcement of concrete

working environment is achieved for vertical castings, where the ergonomics during casting and compaction of conventional concrete are extremely stressful (Nielsen 2007a). The present chapter describes selected properties of SCC. The properties and use of SCC are illustrated through a few case histories, and future trends are briefly described. The chapter concludes with a list of sources of further information.

9.1.1 Definitions For the purpose of this chapter the definitions given in Day et al. (2005) apply, see Table 9.1.

9.2

Selected properties of self-compacting concrete

SCC is characterised by its ability to flow into the formwork and even around complicated reinforcement arrangements under its own weight and without segregating. Therefore, the key properties of fresh SCC are filling ability (and rheological properties), passing ability and segregation resistance, see Table 9.1. Until the fluidity of the cast SCC is reduced, either due to thixotropic (Table 9.1) behaviour or hydration, the SCC will affect the formwork pressure and the air void stability; thus, these issues are also discussed. Most emphasis is placed on filling ability and rheological properties as this area is most advanced. The composition of SCC varies much between countries, as illustrated by the variations in powder and water contents (Wallevik 2003a), as shown in Table 9.2. This is due to a combination of tradition and availability of resources. The differences in composition of SCC affect many of the engineering properties, such as strength, shrinkage, creep and density. The influence of the constituent materials on these engineering properties may be found in standard textbooks on concrete technology and concrete design.

9.2.1 Fresh SCC – a suspension and composite material In the fresh state, i.e. before hydration takes place and reaction products are formed, concrete consists of a granular material (particles) embedded in a liquid (matrix), forming a wet granular material (suspension) with particle sizes ranging from the sub-micron scale to the centimetre scale. The properties of the fresh SCC can, to a large extent, be explained by the volume fraction and properties of the granular material and the matrix, i.e. by composite models. Depending on the scale at which the phenomena are explained and to what extent the interactions between the granular phase and possible particles in the matrix phase can be neglected, the

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189

Table 9.1 List of definitions according to Day et al. (2005) Term

Definition

Addition

Finely divided inorganic material used in concrete in order to improve certain properties or to achieve special properties. Material added in small quantities during the mixing process of concrete related to the mass of cement to modify the properties of fresh or hardened concrete. The combined cement and hydraulic addition. A measure of the ease by which fresh concrete can be placed. The ability of concrete to flow into and fill completely all spaces within the formwork, under its own weight. The flow of fresh concrete when not restricted by formwork and/or reinforcement. The fraction of concrete paste plus those aggregates less than 4 mm. The ability of concrete to flow through tight openings such as spaces between steel reinforcing bars without segregating or blocking. The fraction of concrete comprising powder, water, air and admixtures where applicable. Material of particle size smaller than 0.125 mm, it includes this size fraction in cement, additions and aggregates. The capacity of concrete to retain its fresh properties when small variations in the properties or quantities of the constituent materials occur. The ability of a concrete to remain homogeneous while in its fresh state; during transport and placing, i.e. in dynamic conditions, and after placing, i.e. in static conditions. Concrete that is able to flow under its own weight and completely fill the formwork, while maintaining homogeneity even in the presence of congested reinforcement, and then consolidate without the need for vibrating compaction. The property of a material (e.g., fresh concrete) to rapidly lose fluidity when allowed to rest undisturbed but to regain its fluidity when energy is applied. Concrete characterised by the need to be vibrated to achieve full compaction. The resistance to flow of a material once flow has started (an abbreviation of the term plastic viscosity). Admixture added to fresh concrete to achieve cohesion and segregation resistance. The stress or force needed to initiate flow.

Admixture

Binder Consistence Filling ability

Flowability Mortar Passing ability

Paste Powder (fines)

Robustness

Segregation resistance (stability)

Self-compacting concrete (Selfconsolidating concrete) (SCC) Thixotropy

(Traditional) vibrated concrete Viscosity Viscosity modifying admixture (VMA) Yield stress

Reproduced from Technical Report 62: Self-compacting Concrete: a Review. Published by The Concrete Society and available to purchase from the Concrete Bookshop (www.concretebookshop.com; tel: 0700 460 7777).

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Developments in the formulation and reinforcement of concrete

Table 9.2 Very rough estimation of compositions and rheological properties of SCC in selected countries (Wallevik 2003a) Country

Powder (kg/m3)

Water (kg/m3)

Yield value (Pa)

Plastic viscosity (Pa s)

Sweden The Netherlands Japan France Switzerland Norway Iceland Denmark UK Germany USA

>550 >550 >550 ? <450 <450 <450 <450 >500 >500 >500

180 190 170 ? 200 170 180 160 210 180 190

0–30 0–10 0–30 0–10 0–50 10–50 10–50 30–60 10–50 0–10 0–20

50–100 60–120 50–120 >60 110–20 30–45 20–40 <40 50–80 60–90 40–120

Paste (powder in water)

Paste (powder in water)

Mortar (sand in paste)

Concrete (stones in mortar)

Concrete (aggregates in paste)

9.1 Sketch of fresh paste, mortar and concrete as granular material (particles) in a liquid (matrix) forming a suspension. See Table 9.3 for all combinations.

matrix phase may be considered to be either the water, the paste or the mortar (Fig. 9.1 and Table 9.3). To simplify the models and the computations, the homogeneous phase (the matrix) should be considered to contain as large a portion of the suspension as possible.

9.2.2 Filling ability and rheological properties The flow characteristics of SCC – and other materials – can be described by rheology. Several rheological models have been proposed. Vibrated con-

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Table 9.3 Granular material (particles) in a liquid (matrix) forming the suspensions: paste, mortar and concrete Matrix

Paste

Mortar

Concrete

Water Paste Mortar

Powder – –

Powder and sand Sand –

Powder, sand and stones Sand and stones Stones

Shear stress (Pa)

120 100 80

Newton

60

Bingham

40

Herschel-Buckley

20 0 0

50 Shear rate

100 (s–1)

9.2 The Newton (t = hg ) , Bingham (t = t 0 + mg ) and the HerschelBuckley (t = t 0 + Ag B ) models; parameters selected: η = 1; τ0 = 20, μ = 0.8; A = 2.5; B = 0.75 (after Banfill, 2006).

cretes and many SCCs flow according to the Bingham model, which contains two rheological parameters: yield stress and plastic viscosity: t = t 0 + mg

9.1

where τ and τ0 are the stress and yield stress of the suspension, μ is the plastic viscosity and g˙ is the shear rate. SCC may also exhibit shear thickening behaviour according to the Herschel-Buckley model (de Larrard et al. 1998). Sometimes the apparent Herschel-Buckley behaviour may be explained by a lack of equilibrium during testing (Geiker et al. 2002a). The Bingham and the Herschel-Buckley models are together with the simplest rheological model – the Newton model – illustrated in Fig. 9.2. For SCC to flow by its own weight both the yield stress and the viscosity must be low; however, too low a viscosity causes instability. Typical rheological properties of SCC are yield stress: 0–60 Pa (∼30 Pa) and viscosity: 20– 120 Pa s (Table 9.2 and Fig. 9.3). The more solids a suspension contains, the less fluent but also the more stable it will be. A minimum amount of water is needed to obtain flow of cement paste, approximately 50% volume. Examples of volume fraction of

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Developments in the formulation and reinforcement of concrete

Yield stress (Pa)

80 60 Restricted flow Envelope

40

Typical envelope

20 Segregation

0

0

50

100

Plastic viscosity (Pa s)

9.3 Suggested approximate envelope of yield stress and plastic viscosity for SCC. Restricted flow is observed for concretes with high rheological properties, whereas segregation is observed for concretes with low rheological property (after Wallevik 2002).

solids, ϕw/c, at varying w/c are ϕ0.25 = 0.56, ϕ0.30 = 0.51, ϕ0.40 = 0.44, ϕ0.50 = 0.39, ϕ0.60 = 0.35 (assumed density of cement 3150 kg/m3). The effect of volume fraction and maximum volume fraction on viscosity is often described using the Krieger-Dougherty equation (Krieger and Dougherty 1959): h hmatrix

f ⎞ ⎛ = ⎜1− ⎟ ⎝ f max ⎠

−[h ]f max

9.2

where η is the viscosity of the suspension, ηmatrix is the viscosity of the base medium, ϕ is the volume fraction of particles in the suspension, ϕmax is the maximum volume fraction of particles in the suspension and (η) is the intrinsic viscosity of the matrix, which is 2.5 for spheres. The KriegerDougherty equation is based on the assumption of a Newtonian (i.e., without a yield stress) and homogeneous matrix. The more angular and elongated the particles are, the higher the viscosity (and yield stress) and less fluid the suspension (Geiker et al. 2002b). A similar equation was proposed by Coussot and co-workers (Ildefonse et al. 1997) for the yield stress of a suspension, where non-colloidal forces act between the particles and there is a gap between particles and possible particles in the matrix, i.e. the matrix is considered homogeneous: t0 t 0,matrix

f ⎞ ⎛ = ⎜1− ⎟ ⎝ f max ⎠

−m

9.3

where τ0 is the yield stress of the suspension, τ0,matrix is the yield stress of the matrix, and m is a constant; m = 1 for f < 0.6 and a broad particle size distributions. Tomosawa and co-workers proposed models for viscosity and yield stress of cement based materials taking into account the grading of the aggregates (Oh et al. 1999). Other composite models for the effect of aggregates on

Self-compacting concrete (SCC)

193

the flow of concrete have been proposed (de Larrard 1999). Taking into account the interparticle forces that occur in super-plasticised cement paste, Flatt (2004) proposed that the yield stress be calculated from: t 0 = m1

(f - f 0 )2 f max (f max − f )

9.4

where m1 is a function of the particle size distribution and f0 is the percolation solid fraction. The flow of paste containing colloidal particles is improved by breaking down agglomerates of the fine particles and stabilising the deflocculated particles; this is done by mixing and addition of plasticisers. The effect of plasticisers and mixing intensity on flow properties of colloidal suspensions is dealt with in Wallevik (2003b). Other models for the thixotropic behaviour of concrete are also becoming available (Roussel 2006a). The flowability – and the resistance to segregation – depend on hydrodynamics, interparticle forces and the difference in gravity of the particles. The controlling effects vary, depending on particle size, particle concentration and flow rate (Coussot and Ancey 1999). Differences between flow of SCC with low solid fraction and conventional concretes may be explained by the possible presence of frictional forces (Roussel 2006b). Requirements for workability depend on form geometry and casting technique. Besides rheological measurements, several empirical methods are applied to characterise the filling ability of SCC; see Section 9.5.2 for further information.

9.2.3 Passing ability An insufficient passing ability can be caused by poor filling ability or poor segregation resistance. But even if these requirements are fulfilled, insufficient passing ability can be due to blocking of aggregates in narrow paths. Blocking develops easily when the size of aggregate is large relative to the size of the opening, if the total content of the aggregate is high and also if the shape of the particles deviates from spherical, for example crushed aggregate. Passing ability is, for example, dealt with in Billberg et al. (2004). Several empirical methods are applied to characterise the passing ability of SCC; see Section 9.5.2 for further sources of information.

9.2.4 Resistance to segregation Segregation significantly reduces the concrete quality, which subsequently leads to problems during the service life of the structure. Segregation is caused by the differences in gravity of the constituent materials. Sedimentation may take place either during flow due to stress gradients (dynamic segregation) or as settlement due to gravity (static segregation).

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Developments in the formulation and reinforcement of concrete

The sedimentation rate of a single particle, i.e. the worst case, can be calculated from Stoke’s law. The larger the density differences between particle and matrix, the larger the particle and the lower the viscosity, the higher the sedimentation rate. Only limited fundamental research on segregation in SCC has been carried out (Wallevik 2003b, Bethmont et al. 2003, Roussel 2006c). To achieve cohesion and segregation resistance, viscosity modifying admixtures (VMA, see Table 9.1) are sometimes added to fresh concrete (Khayat 2003). Also, the addition of fine particles has been proposed to facilitate robustness with regard to sedimentation. Palygorskite with an aspect ratio (length : diameter) of more than 35, is claimed to produce highly stable SCC at the addition rate of 2–10% in combination with a superplasticiser (EP 1 152 994 B1).

9.2.5 Formwork pressure The high workability of SCC invites high casting rates. However, casting walls at high rates may result in large formwork pressures. The formwork pressure is affected by the casting rate and the concrete type, but the inlet position and the geometrical dimensions of the formwork may also have an influence (Billberg 2003, Khayat et al. 2005). The current ACI Manual of Concrete Practice does not specifically address SCC, but recommends that unless a method based on appropriate experimental data is available, formwork should be designed to withstand the full hydrostatic pressure. That is, assuming a concrete density of 2400 kg/m3, the formwork pressure may be up to 24 kPa per m in height. Ovarlaz and Roussel (2006) recently proposed a physical model for the prediction of lateral stress exerted by SCC on formwork. They explained why the lateral stress is equal to hydrostatic pressure when the casting rate is high or when the concrete is injected from the bottom. The model takes into account the possible thixotropic behaviour of the concrete through a flocculation (restructuring) coefficient.

9.2.6 Air void stability Casting walls at high rates may also result in large pressures at the bottom of the form and subsequent compression of the air voids. Laboratory investigations suggest that the pressure related changes of the air void structure may be estimated by using Boyle-Mariotte’s law: p ⋅ V = constant

9.5

where p is the pressure and V is the volume. Full-scale wall castings revealed that factors other than the form pressure also influence the air void

Self-compacting concrete (SCC)

195

Cumulative air content (%)

8 7 6 5

Top

4

Middle

3

Bottom

2 1 0 10

100

1000

10000

Void size (micron)

Cumulative air content (%)

8 7 6 5

Top

4

Middle

3

Bottom

2 1 0 10

100

1000

10000

Void size (micron)

9.4 Estimated (dotted lines) and measured air void distribution according to EN 480-11 in full-scale wall casting (3.9 m high, cast with air entrained SCC from a ready mix plant). Top: Concrete poured 0.5 m above surface. Bottom: Form filled from bottom (after Jensen et al. 2005).

structure, e.g. the method of placement, where form filling from the bottom was found to reduce the content of larger pores, as shown in Fig. 9.4 (Jensen et al. 2005).

9.3

Applications/case studies

A few case histories are presented here. The first two cases are demonstration projects and illustrate some of the challenges experienced in connection with both horizontal and vertical castings; the third case describes the use of SCC in concrete filled steel tubes (CFT). The use of SCC in vertical castings is still much less common than the use of SCC in horizontal castings

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Developments in the formulation and reinforcement of concrete

such as floors, and these cases were (among others) selected to support the use of SCC in vertical castings. Information on economy and productivity is not dealt with here; some information on this was given in the introduction. Other case histories can be found in ACI (2007) and Day et al. (2005) and Proceedings of the various international conferences on SCC.

9.3.1 Case 1: small bridge over new motorway at Give, Denmark, 2006–2007 Design and supervision of the construction were undertaken by Gimsing & Madsen A/S on behalf of the Danish Road Directorate. The bridge is a two-span 63 m long post-tensioned concrete bridge. The bridge served as a demonstration project in connection with a 3.5 million USD R&D project exploiting the possibilities of SCC in Denmark. The Danish Road Directorate concrete specifications were adjusted with regard to control of fresh concrete properties and trial testing. The concrete contained three binders: rapid hardening Portland cement, fly ash and silica fume (Table 9.4). Pre-testing included full-scale trials of casting techniques and performance testing with regard to durability. Challenges and outcomes are summarised in Table 9.5 and Fig. 9.5. According to Nielsen et al. (2007a) SCC has great potential for use in foundations as well as vertical and highly reinforced structural elements such as

Table 9.4 Composition of the first trial mix, and mixes for abutments and deck of small SCC bridge over new motorway at Give, Denmark, cast 2006–2007. For comparison, a traditional Danish concrete mix for extra aggressive environment is also listed (kg/m3) (after Nielsen et al. 2007b)

Low alkali sulphate resistant Portland cement CEM I 42.5 N Rapid hardening Portland cement CEM 52.5 R Fly ash Microsilica Water Air CP 326 1:1 Conplast 212 Structuro A1510 Glenimum SKY 525 Sand 04 mm, E Stones 4–8 mm, E Stones 8–16 mm, E

Traditional E40

First trial mix

Abutment/ columns

Bridge deck

406

360

–

–

–

–

381

380

81 18 169 0.71 3.5 – – 603 302 668

82 12 152 0.09 – – 0.95 718 290 691

86 12 176 0.06 2.88 4.56 – 626 277 697

87 12 166 0.34 3.4 5.7 – 618 274 700

Self-compacting concrete (SCC)

197

Table 9.5 Challenges and outcomes experienced in connection with the casting of a highway bridge in SCC (after Nielsen 2007b) Challenge

Outcome

Robust composition of SCC

Sensitive to variations of constituent materials and external conditions as weather and casting stops (1 h transport time OK) The concrete was sensitive to rain during casting One hour transport acceptable (max 30°C experienced)

Long distance between concrete plant and construction site Contradictive requirements to workability a) Low flowability to allow for establishment of slope b) High flowability to allow proper form filling c) High segregation resistance d) Pumpability

Finish of upper surface of deck Finish of form surfaces

a) OK if slump flow 500–550 mm. Slope of 3% was established b) Partly OK (see finish of form surfaces) c) OK, even with 10 m flow distance d) Partly OK; change of cement type and admixtures during pretesting, see Table 9.4 Manual floating needed after levelling Risk of visible spacers and imprints of formwork, see Fig. 9.5

9.5 Small SCC bridge over new motorway at Give. Left: casting and finishing of bridge deck. Middle and right: Examples of casting and improper compaction around spacers (courtesy of left: SCC-Konsortiet, Denmark, middle and right: Aalborg Portland Group).

columns and abutments, but may not be optimal for structural elements such as bridge decks, which have strict requirements for the finish of the slightly sloped upper surface.

9.3.2 Case 2: walls in basement, Danish Broadcasting Corporation, 2005–2006 Design and supervision were undertaken by MT Højgaard A/S for the Danish Broadcasting Corporation. The case comprises five lightly (88 kg/ m3) reinforced concrete wall elements each 4 m high, 5 m long and 0.5 m

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Developments in the formulation and reinforcement of concrete 0.20 V1 and V5

4.0

Inlet 0.50 m above free surface

V2 and V4 Inlet 0.50 m below free surface

0.50

0.50

Plane of symmetry V3 4.0

Inlet y x

0.20

z

4.0

Inlet fixed 0.20 m above bottom

0.50

(m)

0.20 5.0

4.80

0.20

9.6 The formwork geometry of five full-scale walls (V1–V5) applying different filling methods. During casting of walls V2, V4 and V1, V5, the initial position of the inlet was 0.20 m and 0.50 m above the bottom, respectively, and the inlet was gradually lifted during the casting to maintain the position shown relative to the surface. Wall V3 was cast with an inlet fixed relative to the form, 0.20 m above the bottom (Thrane et al. 2007).

wide. Besides acting as walls in a basement, the wall elements served as a demonstration project in connection with a 3.5 million USD R&D project and experimental verification of numerical simulation of form filling in connection with a recent PhD project (Thrane 2007a). The aim of the fullscale wall castings was to obtain experience on (a) the relationships between the fresh concrete workability, casting technique and the form filling behaviour, (b) form pressure, (c) surface finish, and (d) air void structure and frost resistance (Section 9.2.6). Numerical modelling was used to study the flow patterns in order to provide a qualitative means of understanding flow induced segregation and the development of surface air voids. The testing was carried out using three different inlet positions and two different SCCs. The walls are referred to as V1–V5. During filling, the inlet was either movable relative to the free surface: 0.50 m below or 0.50 m above, or fixed relative to the form, 0.20 m above the bottom (Fig. 9.6). Some information on the concretes is given in Table 9.6. The rheological properties of the concretes were determined using a prototype of a measuring device (4C Rheometer) (Thrane and Pade 2005). Blocking was not observed during form filling and complete form filling was obtained. Form geometry and reinforcement configuration did not prevent high casting rates of up to 25 m/hour. However, casting rates may have to be lowered due to finish and form pressure. Segregation was initiated when the concrete was forced sharply upwards (against gravity). It was speculated

Self-compacting concrete (SCC)

199

Table 9.6 Environmental class, target binder content and w/b, and measured rheological properties of the concretes used for the wall castings (after Thrane 2007b) Unit Environmental class Binder content w/b Plastic viscosity Yield stress

kg/m3 Pa s Pa

Walls V1–V3

Walls V4 and V5

Extra aggressive 439 0.33 60 20

Moderate 327 0.49 35 60 and 45

that flow-induced segregation may be traced back to the combined effect of shear-induced particle migration and gravity-induced segregation due to differences in density. The concrete mix with higher yield stress was less prone to exhibit flow-induced segregation. Although not observed for the investigated concretes, increased plastic viscosity is also expected to improve the resistance to segregation. With respect to surface quality, high shear rates at the form surface contributed positively in terms of reducing the number of air voids (Thrane et al. 2007). As discussed in Section 9.2.6, a reduction in air content is to be expected in fresh concrete exposed to high form pressure. According to DS 2426, concrete exposed to a combination of salt and frost should have air void content in hardened concrete larger than 3.5% and spacing factor smaller than 0.20. All tested samples fulfilled these requirements except three cores from the bottom of V1, where the total air void content was 3.4%. All concretes showed satisfactory frost resistance according to SS 13 72 44 (Thrane 2006).

9.3.3 Case 3: Mori Tower Roppongi Hills, completed 2003 The introduction of self-compacting concrete initiated the development of construction methologies such as CFT, which stands for “concrete filled steel tubes”. SCC is poured into steel tubes which form the frame of the building. The most important achievement of CFT is the saving of construction time due to the significantly improved speed of casting. According to Danzinger (2007) the concrete must fulfil the requirements summarised in Table 9.7 to successfully implement the CFT technology. For the actual job, a 60 MPa concrete was cast into steel tubes of 2 m diameter and 120 m height. The concrete was made from a low heat cement, 165 kg/m3 water and 1.55% (by weight of cement) polycarboxylate based superplasticiser, and had a w/c at 0.32. This concrete had a slump flow of 68 cm, decreasing to 64 cm after 120 min; a T50 (time to 50 cm spread) at 5–7 s and 28-day compressive strength of 92 MPa (Danzinger 2007).

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Developments in the formulation and reinforcement of concrete

Table 9.7 Requirements to successfully implement the concrete filled steel tubes (CFT) technology according to Danzinger (2007) Property

Requirement

Variability Flowability

Every batch must fulfil the requirements 100% Slump-flow 650 ± 50 mm and t500 less than 8 s for min 120 min (excluding pumping) Not the smallest sign of separation Pumpable up to 400 m in one go

Stability Pumpability

9.4

Future trends

This section addresses the major challenges and opportunities of SCC: sustainability, robustness and compatibility of constituent materials, modelling of flow and virtual mix design.

9.4.1 Sustainability A challenge for the entire concrete industry is to improve the sustainability of concrete structures. Sustainable development is defined by the World Business Council for Sustainable Development (WBCSD) as “forms of progress that meet the needs of the present without compromising the ability of future generations to meet their needs” (www.wbcsd.ch). Considering concrete structures, their entire service life needs to be considered; they should be made from renewable resources and cause low emissions of pollutants (e.g., CO2) as well as low energy use during construction, operation and maintenance, and demolition. Some SCCs contain large amounts of powder (see Table 9.2 and Table 9.1 for definition of powder), and in some countries the powder is mainly cement. As for other types of concrete, an expected future trend is to decrease the cement content in particular and powder content in general. Increasingly, packing programmes for optimisation of the aggregate grading and minimisation of the paste content are becoming a mix design tool. As mentioned in the introduction, SCC potentially improves the productivity, work environment and the quality of the hardened concrete; all contributing to a sustainable development. SCC is therefore expected to be the concrete of the future.

9.4.2 Robustness and compatibility of constituent materials Robustness, i.e. the capacity of concrete to retain its fresh properties when small variations in the properties or quantities of the constituent

Self-compacting concrete (SCC)

201

materials occur, is central for the success of SCC. SCC is generally more sensitive to variations in content and properties of the constituent materials than conventional concretes. For instance, some concrete producers have experienced difficulties in controlling the moisture content of the aggregates sufficiently. It is anticipated that robustness of mixes will be facilitated both via improved procedures and mix design. Establishment of a so-called “workability window” during pre-testing may assist in the selection of mixes which are less sensitive to variations in the content and properties of the constituent materials (Kordts and Breit 2003). Concrete is primarily made from local materials and there are limitations to possible mix compositions; addition of limited amounts of VMAs and fines such as palygorskite improves the resistance to segregation (Section 9.2.4). Only about half of the water needed to obtain sufficient workability is needed for the hydration of the cement. Unreacted water appears as pores which decrease both strength and durability. To limit the water content in SCC, super-plasticisers are used. The performance of super-plasticisers depends, amongst other things, on the cement chemistry and the mixing schedule. Super-plasticisers may be intercalated (adsorbed) in the calcium aluminate phases and thus lost for dispersion purposes; also sulphate ions have been found to decrease the efficiency of some polycarboxylates due to competitive adsorption (Flatt et al. 2004). It is anticipated that improved knowledge on the compatibility of constituent materials will be developed and brought into practice.

9.4.3 Modelling of flow and virtual mix design An overview of computational methods for modelling flow of concrete can be found in Roussel et al. (2007). It has recently been demonstrated that a complete framework consisting of numerical, single fluid flow modelling and rheological testing and characterisation can be established, yielding consistent results with the full-scale form filling of SCC (Thrane 2007a) (Fig. 9.7). However, a major obstacle for further application of SCC is the lack

Blowholes

Dead zone, Blowholes

Red SCC

Dead zone, Blowholes

9.7 Left: simulated velocity field and particle paths in the time interval 250 s. Right: observed flow behaviour in the experiment (dead zones in lower corners, particle path of the red SCC, and surface quality “blowholes” in all corners except above inlet) (Thrane 2007a).

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Developments in the formulation and reinforcement of concrete

of understanding of the form filling process, leading from time to time to problems such as segregation. Segregation significantly reduces the concrete quality, which subsequently leads to problems during the service life of the structure. An objective of future research will thus be to improve the basic understanding of the flow behaviour of SCC and, through simulation, to enable prediction of particle segregation. Another objective will be to improve the engineering tools used in concrete mix design, both to ensure filling ability and to prevent segregation in a given flow regime.

9.5

Sources of further information and advice

9.5.1 Guidelines European guidelines for self-compacting concrete were prepared in 2005 based on a review of best practice by five European organisations: BIBM, Cembureau, ERMCO, EFCA and EFNARC. The Guidelines represent a state-of-the-art document addressed to specifiers, designers, purchasers, producers and users who wish to enhance their expertise and use of SCC. The proposed specifications and related test methods for ready-mixed and site mixed concrete are presented in a pre-normative format, intended to facilitate standardisation at the European level (Biasioli 2005). Earlier guidelines were, amongst others, prepared by the Norwegian Concrete Association (NB 2002), the Swedish Concrete Association (SB 2002), the Japan Society of Civil Engineers (Omoto and Ozawa 1999), and the Concrete Society UK (Day et al. 2005). Also, international organisations such as ACI (2007) and RILEM (Skarendahl and Petersson 2000, Bartos et al. 2002, Skarendahl and Billberg 2006) have prepared state-of-the-art reports and recommendations. Special attention should be paid to the report of RILEM TC 188-CSC (Skarendahl and Billberg 2006) and the parts of the report of ACI TC 237 report and the European Guidelines which address the processes of construction (execution). In recent years several web pages on SCC have been established; the reader is encouraged to search the internet for these; for example http:// www.SelfConsolidatingConcrete.org, www.nordicscc.net; www.selvkomprimerendebetong.no; www.VoSCC.dk; www.SCC-Konsortiet.dk.

9.5.2 Standardisation Several methods have been proposed to characterise filling ability, viscosity, passing ability and segregation resistance of SCC (Table 9.8). Table 9.9 summarises the present (2007) state of standardisation within the American and European standardisation bodies, ASTM and CEN.

Self-compacting concrete (SCC)

203

Table 9.8 Selected methods for tests for SCC (after Day et al. 2005) Test

Property Filling ability

Viscosity

Passing ability

Segregation resistance

Slump flow J-ring

Total spread Total spread1

t500 time t500 time1

(Paste rim) –

Kajima box V-funnel Orimet O-funnel L-box Penetration Sieve stability Settlement column

Flow time – – – – – –

– Flow time2 Flow time2 Flow time2 – – –

– Stop height, total flow3 Visual (Blocking at orifice) (Blocking at orifice) (Blocking at orifice) Blocking ratio3 – –

–

–

–

– – – – – Depth Percent passing 5 mm Segregation ratio

1

If OK passing ability If no blocking at orifice 3 If OK filling ability 2

Table 9.9 Standards for SCC (2007) Test

Standardisation body ASTM

Sampling Slump flow J-ring Kajima box V-funnel Orimet O-funnel L-box Penetration Sieve segregation Settlement column

C 1611 2005 C 1621 2006 (fig. 1 under discussion 2006)

CEN EN 12350-1 Final draft prEN 12350-8:2000x Final draft prEN 12350-12:2000x

Final draft prEN 12350-9:2000x

Stopped 2006

Final draft prEN 12350-10:2000x Final draft prEN 12350-11:2000x

C 1610 Column segregation

Other than tests Definitions Others

Terminology proposed to include in existing testing to allow their use with SCC (June 2006)

prEN 13670 §8.5 Informative annex to prEN 13670 Definition of extreme mix compositions

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Developments in the formulation and reinforcement of concrete

Table 9.10 Preliminary guidelines for consistence, viscosity, passing ability and segregation classes and tolerances on target values when testing according to EN (Table 9.8) (after prEN 206-9:2007-05 and where mentioned Day et al. 2005) Consistence class

SF1

SF2

SF3

Tolerances on target values

Slump flow (mm)

550 to 650

660 to 750

760 to 850

±50

Viscosity class

VS1/VF1

VS2/VF2

VS3

Tolerances on target values

t500 (s)

≤2

3 to 6

>6

±1

V-funnel (s)

<9

9 to 25

±3 if target <9 ±5 if target ≥9

Passing ability class

PA1/PJ1

PA2/PJ2

Tolerances on target values

L-box ratio

≥0.80 with 2 rebars

≥0.80 with 3 rebars

−0.05 (Day et al. 2005)

J-ring step (mm)

≤10 with 16 rebars

≤10 with 16 rebars

Segregation class

SR1

SR2

Tolerances on target values

Segretation resistance (%)

≤20

≤15

–

SCC is not covered by the European materials standard for concrete, EN 206-1, and will not be before its revision in 2010. Because of this a parallel standard: prEN 206-9 “Concrete Part 9: Additional Rules for Self-compacting Concrete (SCC)” has been drafted. Only testing of slump flow will be required, while other tests will be optional. The draft standard prEN 2069:2007-05 states the requirements for consistence (workability) in the form of consistence classes (Table 9.10). Proposed tolerances when using target values are also given in the table.

9.6

References

ACI Committee 237 (2007) 237R-07 Self-Consolidating Concrete, Farmington Hills, American Concrete Institute. Banfill P F G (2006), “Rheology of fresh cement and concrete” (eds. D.M. Bindius and K. Walters) Rheology Reviews, ISBN 0-9547414-4-7, 61–130. Bartos P J M, Sonebi M, Tamimi A K (eds) (2002), Workability and Rheology of Fresh Concrete: Compendium of Tests – Report of RILEM TC 145-WSM, RILEM, ISBN: 2-912143-32-2, e-ISBN: 2351580435.

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Bethmont S, Schwarzentruber L, Stefani C, Leroy R, “Defining the stability criterion of sphere suspended in a cement paste: A way to study the segregation risk in self-compacting concrete (scc)”, 3rd int RILEM symp Self-Compacting Concrete, Reykjavik, RILEM, 2003, 94–105. Biasioli F, “The European guidelines for self-compacting concrete”, 18th int cong Betonwerk Fertigteil Techniek (BFT), Amsterdam 2005 – available at http://www. efnarc.org/. Billberg P, “Form pressure generated by self-compacting concrete”, 3rd int RILEM symp Self-Compacting Concrete, Reykjavik, RILEM, 2003, 271–280. Billberg P, Petersson Ö, Westerholm M, Wüstholz T, Reinhardt H-W, Summary Report on Work package 3.2, Test Methods for Passing Ability, Growth Contract No. G6RD-CT-2001-00580, Draft version 2004-10-14, available from http://www. civeng.ucl.ac.uk/research/concrete/Testing-SCC/WP3.2-Summary%20report.pdf, accessed 15 June 2007. Coussot O and Ancey C (1999), “Rheophysical classification of concentrated suspensions and granular pastes”. Physical Review E, 59 (4) 4445– 4457. Danzinger M (2007), personal communication. Day R, Holton I, Domone P, Bartos P (2005), Self-compacting Concrete – A review, Technical Report No. 62, Surrey, Concrete Society, ISBN 1 904482 19 8. de Larrard F (1999), Concrete mixture proportioning. A scientific approach, London, E FN Spon. de Larrard F, Ferraris C, Sedran T (1998), “Fresh concrete, a Herschel-Buckley material”, Mater. Struct., 31, 494–498. EP 1 152 994 B1, Concrete containing superplasticiser and palygorskite, international publication number WO 00/035824. Flatt R J (2004), “Towards a prediction of superplasticized concrete rheology”, Mater. Struct., 37, 289–300. Flatt R J, Martys N, Bergström L (2004), “The Rheology of Cementitious Materials”, MRS Bulletin, Materials Research Society, 29 (5) 314–318. Geiker M R, Brandl M, Thrane L N, Bager D H, Wallevik O (2002a), “On the effect of measuring procedure on the apparent rheological properties of selfcompacting concrete”, Cem Concr Res, 32, 1791–1795. Geiker M R, Brandl M, Thrane L N, Nielsen L F (2002b) “On the effect of coarse aggregate fraction and shape on the rheological properties of self-compacting concrete”, Cement, Concrete and Aggregates, 24 (1), 3–6. Ildefonse, B, Allain C, Coussot C (Eds) (1997), Des grands écoulements naturels à la dynamique du tas de sable: introduction aux suspensions en géologie et en physique, Pairis, Editions Cemagref. Jensen M V, Hasholt M T, Geiker M R, “The effect of form pressure on the air void structure of SCC”. 4th int RILEM symp Self-Compacting Concrete, Chicago, Hanley Wood, 2005, 327–332, ISBN 0-924659-64-5. Khayat K (2003), “Effect of viscosity-modifying admixture-superplasticizer combination on flow properties of SCC equivalent mortar”, 3rd int RILEM symp Self-Compacting Concrete, Reykjavik, RILEM, 2003, 369–385. Khayat K H, Assad J, Mesbah H, Lessard M (2005), “Effect of section width and casting rate on variations of formwork pressure of self-compacting concrete”, Mater. Struct., 38, 73–78.

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Kordts S, Breit W, “Controlling the workability properties of self-compacting concrete used as ready mixed concrete”, 3rd int RILEM symp Self-Compacting Concrete, Reykjavik, RILEM, 2003, 220–231. Krieger I M, Dougherty T J (1959), “A mechanism for non-Newtonian flow of suspensions of rigid spheres”, Transactions of the Society of Rheology, III, 137– 152. NB (2002) Guidelines for Production and Use of Self-Compacting Concrete, Publication no. 29. Oslo, Norwegian Concrete Association. Nielsen C V, “Improved working environment from using SCC”, int conf Sustainability in the cement and concrete industry, Lillehammer, Norsk Betongforening, 2007a, available from www.sustainableconcrete.no. Nielsen C V, “Danmarks første vejbro i selvkompakterende beton”, Meeting Spektakulære projekter, Copenhagen, Danish Concrete Society, 2007b. Nielsen C V, Personal communication, 2007c. Nielsen C V, Glavind M, Gredsted L, Hansen, C N, “SCC a technical breakthrough and a success for the Danish concrete industry”, 5th int RILEM symp SelfCompacting Concrete, Ghent, RILEM, 2007a. Nielsen C V, Thrane L N, Pade C (2007b), SCC demobro, Danish Technological Institute, available from http://www.scc-konsortiet.dk/18794. Oh S G, Noguchi T, Tomosawa F, “Towards mix design for rheology of selfcompacting concrete”, 1st int RILEM symp Self-Compacting Concrete, Stockholm, RILEM, 1999, 361–372. Omoto T and Ozawa K (eds) (1999), Recommendation for Self-Consolidating Concrete, JSCE Concrete Engineering Series 31, Japan Society of Civil Engineers. Ovarlaz G and Roussel N (2006), “A physical model for the prediction of lateral stress exerted by self-compacting concrete on formwork”, Mater. Struct., 39, 269– 279. Roussel N (2006a), “A thixotropy model for fresh fluid concretes: Theory, validation and applications”, Cem Concr Res, 36, 1797–1806. Roussel N (2006b), Personal communication. Roussel N (2006c), “A theoretical frame to study the stability of fresh concrete”, Mater. Struct., 39, 81–91. Roussel N, Geiker M R, Dufour F, Thrane L N, Szabo P (2007), “Computational modeling of concrete flow: General overview”, Cem Concr Res, 37, 1298– 1307. SB (2002), Self-Compacting Concrete – Recommendations for use. Concrete Report no 10 (2002. Stockholm, Swedish Concrete Association.) Skarendahl Å and Billberg P (eds) (2006), Casting of Self Compacting Concrete – Final Report of RILEM TC 188-CSC, RILEM, ISBN: 2-35158-001-X, e-ISBN: 2912143985. Skarendahl Å and Petersson Ö (eds) (2000), Self-Compacting Concrete – State-ofthe-Art Report of RILEM TC 174-SCC, RILEM, ISBN: 2-912143-23-3, e-ISBN: 2912143594. Thrane L N, (2006), “Experiences from Vertical Full Scale Castings with SCC”, Nordic SCC Workshop, Copenhagen. Thrane, L N (2007a), “Form filling with self-compacting concrete”, PhD thesis, Department of Civil Enginering, Technical University of Denmark.

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Thrane L N (2007b), Formfyldning med SCC, DR Byen, SCC-Konsortiet, delprojekt P33, Danish Technological Institute, available from http://www.scc-konsortiet. dk/18794. Thrane L N, Pade C, “Determination of Bingham Rheological Parameters of SCC using On-line Video Image Analysis of Automatic Slump Flow Testing”, int conf Nordic Concrete Research, Sandefjord, 2005, 92–94. Thrane L N, Stang H, Geiker M R, “Flow Induced Segregation in Full Scale Castings with SCC”, 5th int symp Self-Compacting Concrete, Ghent, RILEM, 2007. Wallevik O H (2002), “Practical description of rheology of SCC”, SF Day of the Our World of Concrete, Singapore, from 42. Wallevik O H (2003a), “Rheology – a scientific approach to develop self-compacting concrete”, 3rd int RILEM symp Self-Compacting Concrete, Reykjavik, RILEM, 23–31. Wallevik J E (2003b), Rheology of particle suspensions; fresh concrete, mortar and cement paste with various types of lignosulphonates, PhD Thesis, Department of Structural Engineering, Norwegian University of Science and Technology.

10 Recycled materials in concrete C MEYER, Columbia University, USA

10.1

Introduction

Concrete is by far the most important building material. Worldwide, more than 10 billion tons are produced each year – in North America alone, about two tons for every man, woman and child. The reasons for this overwhelming popularity are well known. If properly designed and produced, concrete has excellent mechanical and durability properties. It is moldable, adaptable, relatively fire resistant, generally available, and affordable. Maybe its most intriguing characteristic is the fact that it is an engineered material, which means it can be engineered to satisfy almost any reasonable set of performance specifications, more so than any other material currently available. But this popularity comes with a significant price, which is all too often overlooked: alone for the sheer volumes produced each year, concrete has an enormous impact on the environment. First, there are the vast amounts of natural resources needed to produce those billions of tons of concrete each year. Then, it is known that the production of each ton of Portland cement releases almost one ton of carbon dioxide into the atmosphere. Worldwide, the cement industry alone is estimated to be responsible for about 7% of all CO2 generated (Malhotra, 2000). The production of Portland cement is also very energy intensive. Then, the production of concrete requires large amounts of water, which is particularly burdensome in those regions of the earth that are not blessed with an abundance of fresh water. Finally, the demolition and disposal of concrete structures, pavements, etc., creates another environmental burden. Construction and demolition debris contributes a considerable fraction of solid waste in developed countries, and concrete constitutes its largest single component. The items listed above seem to indicate that the concrete industry has become a victim of its own success and is therefore now faced with tremendous challenges. But the situation is not as bad as it appears, because concrete is inherently a very environmentally friendly material, as can be demonstrated readily with a realistic life cycle cost analysis and considering 208

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the embedded energy (VanGeem and Marceau, 2002; Marceau and VanGeem, 2002). The challenges listed above are more a result of the fact that Portland cement is not particularly environmentally friendly. One could therefore reduce these challenges to the following simple formula: use as much concrete, but with as little Portland cement as possible. More specifically, the potential tools and strategies to meet the environmental challenges can be summarized as follows: 1.

2.

3.

4.

5.

Increase the use of supplementary cementitious materials, especially those that are byproducts of industrial processes, such as fly ash and ground granulated blast furnace slag. Use recycled materials in place of natural resources. Since aggregate constitutes the bulk of concrete, an effective recycling strategy will have to incorporate the substitution of recycled for virgin materials to make the industry more sustainable. Improve durability. For example, by doubling the service life of our structures, we can cut in half the amount of materials needed for their replacement. Improve mechanical and other properties. An increase in mechanical strength and similar properties can lead to a reduction of materials needed. For example, doubling the concrete strength for compressioncontrolled members may cut the required amount of material in half. Reuse wash water. The recycling of wash water is readily achieved in practice and already required by law in some countries.

It is appropriate to mention in this context that concrete has a largely unnoticed positive effect on the environment in that it actually absorbs large quantities of carbon dioxide from the atmosphere through the wellknown carbonation process. The emphasis of this chapter will be on how the use of recycled materials can achieve the objectives listed above. Most promising appears to be the use of supplementary cementitious materials such as fly ash and slag. A considerable body of knowledge exists already and is an indicator that, worldwide, a significant reduction of Portland cement per unit volume of concrete can be achieved in the near and not so near future. Only a brief overview of the state of the art will be given here. Next, some of the more important recycled materials will be discussed that have been proposed for use in concrete. These are derived from a variety of solid waste streams that would need to be disposed of otherwise, usually in landfills. Candidate materials vary from recycled concrete and post-consumer glass to crumb rubber from tires, plastics, wood wastes, and even farm wastes. Also, the use of short, randomly distributed fibers has become widespread in the industry. The use of recycled carpet fibers and tire-derived steel fiber reinforcement has been proposed as replacement for

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fibers made of virgin materials, but the state of the art has not yet led to their widespread use.

10.2

Fly ash

The cementitious properties of fly ash have been known for some time (Mindess et al., 2003). However, its widespread use was made possible only after large amounts of the material had become available, that is after clean air regulations forced power plants to install scrubbers and electrostatic precipitators to trap the fine particles, which earlier went up the smokestacks and into the environment. The utilization rates of fly ash vary greatly from country to country, from as low as 3.5% for India to as high as 93.7% for Hong Kong (Malhotra, 2000). Hong Kong has such a high utilization rate presumably because it receives its coal from a single source of highquality material. Fly ash is an important pozzolan, which means that in itself it possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide to form a material with cementitious properties (ACI 116, 2000). It has a number of advantages compared with regular Portland cement. First, because of the delayed and different chemical reaction, the heat of hydration is lower, which makes fly ash a popular cement substitute for mass structures. Mehta et al. reported on the construction of a massive foundation slab for a temple in Kauai, Hawaii, “built to last 1000 years” (Asselanis and Mehta, 2001; Mehta and Langley 2000), i.e. to remain crack free basically forever. The foundation was constructed in two slabs of 36 × 17 × 0.61 m (117 × 56 × 2 ft) dimensions without any reinforcing steel and construction joints. By replacing 57% of the Portland cement by Class F fly ash, the temperature increase during hydration could be kept below 15°C (27°F), and because the cooling of the slabs was carefully controlled, thermal stresses remained below the cracking strength of the young concrete. An investigation of the microstructure revealed that the fly ash replacement increased the homogeneity of the paste microstructure by removal of calcium hydroxide from the hydration products. The development of such high volume fly ash concrete mix designs is typically attributed to Malhotra (1999), who developed mixes with over 60% cement replacement by fly ash. Possibly the most important advantage of fly ash is the fact that it is a byproduct of coal combustion, i.e. it would be a waste product to be disposed of at great cost, if it were not used beneficially. Moreover, concrete produced with fly ash can have better properties than concrete produced without it. In other words, fly ash adds value. It is widely available, namely wherever coal is being burned. Moreover, fly ash is generally less expensive than Portland cement, in addition to all of the other advantages it offers.

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There are a number of situations where the beneficial properties of fly ash can be utilized. Corinaldesi et al. (2001) have found that fly ash improves the pore structure of concrete, in particular macro pores that may be encountered in mixes utilizing recycled concrete aggregate. Consequently, a combination of recycled concrete aggregate and fly ash can result in mixes with superior mechanical properties. Particles of Class F fly ash are typically of the same size as those of Portland cement Type I, i.e. less than 40 μm (passing sieve #325). A new generation of ultra fine fly ash with a mean particle size of 30 μm has recently been studied in the UK (Kandie and Byars, 2007) and was found to have significantly higher pozzolanicity than all other UK fly ashes, with greatly reduced water demand and air content. The relatively slow rate of strength development of fly ash concrete is a disadvantage in applications where high early strength is required. But in many situations, especially involving mass concrete structures such as dams and heavy foundations, which are not loaded to their design values until months if not years after their placement, it is quite common to specify 90-day strengths instead of the conventional 28-day strengths. If normal strength development is critical, accelerators are available to speed up the hydration rates of fly ash concrete mixes (Shi and Day, 1995; Shi, 1998). A more serious problem is posed by the need for quality control. The physical and chemical properties of fly ash can vary considerably from power plant to power plant, primarily because of the differences in the sources of coal. In particular, high loss of ignition, the result of incomplete combustion processes, can lead to unacceptable levels of carbon content. The wide variety of chemical composition and quality poses challenges to the industry, which may manifest themselves in such innocuous appearing aspects as color. One concrete block manufacturer we have worked with decided to discontinue the use of fly ash, because he could not control the color of his product, and customers generally demand products of uniform color. But the fly ash industry has improved the quality control in recent years and developed technologies to separate unburned residues.

10.3

Ground granulated blast furnace slag (GGBFS)

As the name implies, GGBFS is a byproduct of the steel industry. It is the glassy granular material formed when molten blast-furnace slag is rapidly chilled, as by immersion in water (ACI 233, 1995). Its cementitious properties have been known for some time. The first recorded production of Portland blast-furnace slag cement was in Germany in 1892, and since the 1950s, use of GGBFS as a separate cementitious material has become widespread in many different countries (ACI 233, 1995). Because of its generally beneficial properties, such slag is not only used as partial Portland

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Table 10.1 Ranges of typical chemical compositions of ordinary Portland cement, fly ash, and blast furnace slag (percent by mass) Chemical constituents (as oxides)

Ordinary Portland cement

Fly ash, Type F

Blast furnace slag

SiO2 Al2O3 CaO MgO S Fe2O3 MnO

17–25 3–8 60–67 0.5–4

>5 20–30 <5

0.5–6

<20

32–42 7–16 32–45 5–15 0.7–2.2 0.1–1.5 0.2–1.0

cement replacement, but also as aggregate. In Table 10.1, the ranges of chemical composition of North American blast-furnace slags are compared with those of fly ash and ordinary Portland cement. Because the calcium to silicate ratio of the CSH formed from GGBFS is lower than that formed from Portland cement, GGBFS is able to chemically bind a larger amount of alkali in the CSH. The optimum cement replacement level is often quoted to be about 50% and sometimes as high as 70 and 80%. Like fly ash, GGBFS also improves many mechanical and durability properties of concrete and generates less heat of hydration. For example, recently the 9-ft thick foundation slab for a water treatment plant in New York City was constructed using 70% slag and 30% Portland cement Type II. One of the major design objectives was to minimize temperature differentials without the installation of a potentially costly internal cooling system and thereby guarantee a crack-free structure. In many situations it is now being recommended to use a so-called ternary system, that is, a blend of ordinary Portland cement, fly ash, and GGBFS. The cost of slag is generally of the same order as that of Portland cement. Primarily because of its known beneficial properties, customers are willing to pay as much for the slag as for the cement it replaces. Although the steel industry probably generates the largest amount of slag, several other metallurgical slags are produced today that are still being mostly stockpiled, landfilled, or “downcycled” into low-value applications such as road bases. Such disposal methods carry their environmental costs, especially since these materials often contain toxic metals. For example, to produce one ton of copper, approximately 2.2 to 3 tons of copper slag are generated as a by-product, resulting in about four million tons per year in the United States alone (Collins and Ciesielski, 1994; Ayano and Sakata, 2000). Recent studies have shown that such slag can be used beneficially in concrete applications (Behnoud, 2005). Mehta (2000) suggests that the

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concrete industry offers ideal conditions for the beneficial use of such slags and ashes because the harmful metals can be immobilized and safely incorporated into the hydration products of cement.

10.4

Recycled concrete

Construction and demolition waste (C&D waste) constitutes a major portion of all generated solid waste. In the European Union it is estimated that 200 to 300 million tons of C&D waste are generated annually, and concrete accounts for more than half of this amount (Lauritzen, 2004). The numbers for the United States, with comparable population size and level of development, are similar. The traditional ways of disposing of these large amounts of waste used to be to dump them in landfills. However, suitable landfill capacities are getting fewer all the time. This is particularly true in Japan, where the remaining landfill capacity has been estimated to last for only a few more years (Kasai, 2004). Coupled with the increasing scarcity of suitable aggregate, the pressure is particularly severe on the Japanese construction industry to find ways of substituting recycled concrete aggregate (RCA) for natural aggregate. That is the reason why Japan is a leader in developing processes of and standards for the use of recycled C&D waste in general and concrete in particular (Sakai, 2007). In Europe, where the shortage of suitable aggregate is not as acute, most of the recycled C&D debris is used for road base or sub-base material (Hansen and Lauritzen, 2004). Since such material is generally less expensive or “valuable” than high-quality concrete aggregate, such uses constitute a form of downcycling. The technical problems of incorporating RCA into new concrete mixes are well known and have been addressed through research (ACI 555, 2001; Hansen, 1992). Most of these are attributed to the large amount of fines found in recycled concrete. A recent study (Sarhat, 2007) suggests that this problem is also solvable. Recycled aggregates have generally lower densities than the original material used, because of the cement mortar that remains attached to the aggregate particles (De Pauw, 1981). This is also the main reason for the larger water absorption of RCA compared with that of virgin aggregate. Another source of concern is the variety of contaminants that can be found in recycled concrete as a result of demolition of existing structures, such as plaster, soil, wood, gypsum, asphalt, and rubber. Since even small amounts of such contaminants can severely degrade the strength or durability of the concrete made with them, upper limits for allowable volume percentages have been established (Table 10.2). Most reductions in strength found for concrete made with recycled coarse aggregate were in the range from 5 to 24%, compared with concrete made with virgin aggregate. When both coarse and fine aggregate were obtained

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Table 10.2 Limiting amounts of deleterious substances for recycled aggregate (Sakai, 2007) Category

Deleterious substances

Limits (mass %)

A B C D E F Total

Tile, brick, ceramics, asphalt Glass Plaster Inorganic substances other than plaster Plastics Wood, paper

2.0 0.5 0.1 0.5 0.5 0.1 3.0

from recycled concrete, the strength reductions ranged from 15 to 40%, compared with concrete made with only naturally occurring materials. Thus, most of the strength loss is thought to be due to the portion of the RCA that is smaller than 2 mm (Hansen, 1992). RCA also causes a reduction in elastic modulus, larger creep and shrinkage deformations, as well as higher permeability of concrete. In sum, concrete produced with RCA is generally of lower quality. Also of concern is the large quality variability of RCA obtained from different sources. One study found variations in 28-day compressive strength from 4600 to 7100 psi (31.7 to 49.1 MPa) when concrete with identical mixture proportions was produced using recycled concrete from different sources (De Pauw, 1981). As a result, greater standard deviations are to be used when preparing concrete mix proportions. This increases the cost of the concrete. In spite of the quality issues, which can be overcome, the primary reason why RCA is not used more widely, especially in the United States, is economics. Creating “clean” concrete aggregate, i.e., separating it from other construction debris such as wood, asphalt, brick, and other contaminants, crushing and grading it to specification, is generally more expensive than quarrying virgin aggregate. There are a number of factors, though, that can change the economics. First, there is the cost of transportation of both the C&D debris from the demolition site to the nearest suitable landfill and of the virgin aggregate from its source to the construction site. Since transportation constitutes a major cost item for bulk materials like aggregate, the transportation costs can easily tip the balance, such that manufactured RCA becomes more economical than virgin aggregate. The second factor is the cost of land-filling C&D debris, which has a tendency of increasing faster than the rate of inflation, especially in areas of increasingly scarce suitable landfills. The third and probably most decisive factor is the intervention of governmental authority. In Europe and Japan, governments do

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not shy away from such intervention, often in a heavy-handed way, by either demanding directly the use of RCA or indirectly by increasing tipping fees (as is being done in Great Britain, for example). In the United States, governmental authorities used to tend more towards letting market forces prevail. However, the situation is changing. Prodded by a public attuned more and more to the demands of sustainable development, local, State, and Federal agencies are increasingly promoting, if not demanding the use of recycled materials (for example, within the context of Green Buildings), especially for projects that are supported partially or fully with public funds (for example, in New York City as well as New York State). It should also be noted that not all applications require high-performance concrete. Although RCA is often considered with suspicion, it may be quite acceptable for many applications, and if higher performance specifications are to be met, a blend of virgin and recycled aggregate may make economic and technical sense. One major success story in the US is the recycling of Denver’s former Stapleton International Airport (Yelton, 2004). Instead of hauling the 6.5 million tons of concrete and hardscape (enough aggregate to build the Hoover Dam) to landfills, the Recycled Materials Company, Inc., was able to recycle or reuse all of this material. The company claims this project to be the world’s largest recycling project, and it completed it at no cost to the City of Denver within six years.

10.5

Recycled waste glass

Each year, more than 41 billion glass containers are produced in the US, and over 11 million tons of glass are discarded by American households. Only about 27% of these amounts are currently recycled, primarily to produce new bottles (Kirby, 1993). The glass industry typically takes back only clear glass for such purposes, primarily because most post-consumer glass is not color-sorted. Thus the bulk of it is landfilled as waste glass, at great cost to tax payers. In New York City, for example, it is estimated that glass constitutes about 6% of all solid waste, and its disposal costs City taxpayers some $60 million each year. There is no shortage of proposals for secondary uses of waste glass. A comprehensive survey of these has been prepared by Reindl (2003). Most of these uses, however, constitute downcycling, i.e. the value of the material for its secondary use is less than in its original form. Examples of such lower-value uses are applications as “sand” or “gravel” for fill, drainage, filtration, road base, pipe bedding, and sand blasting. At times, several transportation departments have used glass as partial replacement of aggregate for asphalt paving (“glasphalt”), but for various reasons, this application never became widespread.

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The processing of post-consumer glass involves curbside collection. In large metropolitan areas, the cost of separate glass collection can be significant, because the recycling “culture” in the United States is not as developed as, for example, in many European countries, where consumers have been depositing bottles into special recycling bins for decades, usually sorted by color, and the recycling rates in some countries approach 100%. If the glass is to be used as an aggregate for concrete, it is subject to similar specifications as natural aggregates, as described in ASTM C33 (2003). For example, the requirement that the aggregate shall be free of injurious amounts of organic impurities implies that the glass be washed properly. If the glass is not already separated by color at the collection point, it may be thus sorted either manually (if the pieces are not too small) or by automatic equipment, which is offered by several manufacturers. There are several reasons why sorting glass by color increases its value and therefore justifies the expense. First, glass manufacturers are more likely to accept the cullet for remelting if it is clear (known as “flint”) or contains only small amounts of colored glass. Second, the use of colored glass aggregate can lead to special esthetic effects, with the potential of adding considerable value to the concrete end product. The fact that much of the post-consumer glass is already broken when collected is one cause that limits its use for some applications. Crushing the glass reduces its volume considerably and therefore lowers its transportation cost, and it is necessary if the glass is to be used as aggregate for concrete. It is important that the glass be crushed using high-velocity impact equipment to avoid sharp edges which would make its handling hazardous. Several manufacturers are offering such equipment. If properly crushed, the aggregate can be handled just like ordinary sand and crushed stone without the danger of injury. The glass dust generated by the crushing operation has not been shown to present a quantifiable health hazard. However, prudence calls for collection of the dust at the source. Not only does this measure prevent air pollution, the very fine glass powder can fetch a retail price of several hundred dollars per ton. Secondary markets for such glass powder exist in the optics, paint, and other industries. Very finely ground glass particles (below 10 μm) have also been shown to have pozzolanic properties and can serve as an excellent filler material to produce high-performance concrete (Jin, 1998; Byars et al., 2004; Shao et al., 2000). The use of glass as an aggregate for concrete had already been contemplated decades ago (Phillips and Chan, 1972; Johnson, 1974), but the socalled “alkali-silica reaction” (ASR) caused an insurmountable problem at that time. ASR is a phenomenon that is now well known in the concrete industry, because it can also occur with natural aggregates that contain certain kinds of reactive (amorphous) silica. In the presence of moisture, the resulting ASR gel swells and can cause severe concrete cracking. This

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is a long-term problem that may manifest itself in concrete after years of seemingly satisfactory service. The complexity of this phenomenon makes it difficult to predetermine a priori whether a specific aggregate is potentially reactive or not. If soda lime glass, the common material of household and beverage containers, is used as aggregate in concrete, not much uncertainty exists that ASR-induced damage is to be expected, provided there is sufficient moisture available to drive the reaction. Figure 10.1 summarizes the effects of both the aggregate particle size and the glass color on the expansion of mortar bars containing various percentages of glass aggregate, tested according to ASTM C1260 (1994), which sets an expansion of 0.1% as the 14-day limit, beyond which the aggregate is suspected to be reactive. The results indicate the presence of a pessimum particle size, which for clear glass is mesh size #16. Glass particles passing mesh size #100 cause less expansion than the reference aggregate, a slightly reactive Long Island sand. This result points to one solution of the ASR problem in practice, namely to grind the glass fine enough to pass mesh size #100. Figure 10.1 also demonstrates the large differences in reactivity between glasses of different color. Clear glass was found to be the most reactive, followed by amber glass, whereas green glass caused less expansion than even the reference aggregate. This surprising finding was explained by the presence of chromium-oxide that manufacturers add for the green color (Jin et al., 2000). There are various tools available to counteract the detrimental effects of ASR. These can be summarized as follows: Grind the glass fine enough to pass at least US standard mesh #100. Use certain mineral admixtures such as metakaolin, fly ash, or slag. 0.50 Relative expansion (%)

1. 2.

0.45 0.40

10% clear glass

0.35 0.30

10% amber glass 10% green glass

0.25 0.20 0.15 0.10

Ref.

0.05 0.00 #4

#8

#16

#30

#50

#100

Pan

Aggregate size (Sieve no.)

10.1 Expansion of mortar bars with 10% glass aggregates of different size and colour (Jin, 1998).

218 3. 4. 5. 6. 7.

Developments in the formulation and reinforcement of concrete Apply a protective coating to the glass (e.g., zirconium, as for AR glass fibers). Modify the glass chemistry to make it less reactive. Seal the concrete to prevent moisture ingress. Use a low-alkali cement. Develop a special ASR-resistant cement.

A special word of caution is in order, however. ASR is an extremely complex phenomenon. Even small changes in glass chemistry can make large differences. Each concrete product and glass source needs to be evaluated and tested thoroughly to ensure an acceptable quality and durability. Moreover, accelerated tests such as the ASTM C1260 (1994) test may prove insufficient to guarantee durability, and it may be necessary to conduct longer-term tests such as the ASTM C1293 (1995) for additional assurance that ASR will not be a serious problem. Finally, replacing natural aggregate with glass aggregate has significant repercussions on the mix design and concrete production technology, in particular if fully automated production processes are used. Glass aggregate is considerably different from natural aggregates for a number of reasons. First, it is a manufactured material, therefore its chemical composition is generally known, although the chemistry can vary widely between different kinds of glass (beverage containers, window glass, neon tubes, windshields, to name a few) and between different producers. Its chemical, physical and mechanical properties are also different from those of natural aggregate because of its amorphous nature. If glass is considered for use as aggregate in concrete, the following properties are of particular interest. Glass has basically zero water absorption capacity. For the design of a concrete mix for a specific application, this is an advantage, because the water absorption capacity and therefore water content is no longer a variable or even unknown, as is the case with most natural aggregates. Because of the lack of water absorption and the smooth surfaces of glass particles, the flow properties of fresh concrete with glass aggregate are clearly better than those of natural aggregate concrete. This means that either improved workability can be achieved or, for a given workability, a lower water-cement ratio can be used, with resulting improvements in mechanical strength and durability properties, without the assistance of a superplasticizer. Another advantage of glass is its excellent hardness and abrasion resistance, which makes it a suitable aggregate for paving stones, floor tiles, and other applications subject to high wear and tear. The durability and chemical stability of glass are proverbial. The pozzolanic properties of finely ground glass powder have already been mentioned. Somewhat related to

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these is the suitability of glass powder as a filler. It is possible to produce very high-strength and durable concrete with glass aggregate and glass powder as filler. Probably the most intriguing property of glass if used as a concrete aggregate is its esthetic potential. The combinations of different colors offer basically unlimited possibilities for decorative and architectural concrete applications. A key is to use white cement instead of regular Portland cement, because it requires much smaller quantities of (relatively expensive) color pigments. The possibilities of light reflections and refractions, together with the various color combinations give architects and other design professionals an important novel tool to experiment with. The potential applications are adding a value to post-consumer glass that is uncorrelated with processing and production costs. Since glass is relatively inexpensive to manufacture, it is even conceivable that specialty glasses can be produced economically, for example, with special colors or special effects for particular applications. Some of the resulting economic aspects will be discussed later. A final advantage of using post-consumer glass as aggregate for concrete is the environmental aspect, because it has the potential of a noticeable impact on the solid waste streams of major metropolitan areas. If a LEED (Leadership in Energy and Environmental Design) rating of the US Green Building Council is the goal (USGBC, 2007), the recycled material content may qualify a project for extra LEED-points. As for applications, it is useful to draw a distinction between commodity products and value-added products. The main purpose of using crushed glass as aggregate for commodity products is to divert as much glass as possible from the waste stream into beneficial use applications. However, the markets for commodity products, such as paving stones and concrete masonry units are typically very competitive, with low profit margins. Therefore, the economic benefit of substituting glass for natural aggregate is marginal at best, because the glass does need to be cleaned, crushed and graded to specifications, and the producer needs to have a dependable source of glass. If the added cost of ASR-suppressing admixtures is to be avoided, the glass needs to be ground sufficiently fine. But in this case it is invisible to the naked eye so that the potential esthetic advantages of glass cannot be utilized. In value-added products, the purpose of the glass substitution is to exploit the special properties of the glass and thereby add value to a material that otherwise would be a waste product – the exact opposite of downcycling. If the glass is sorted by color and this is coordinated with the color of the cement matrix, novel esthetic effects can be achieved, which can be further enhanced with appropriate surface treatments. Surface textures can range from highly polished surfaces, for example for tiles or tabletop counters, to

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exposed aggregate surfaces for building façade elements. In order to be visible, glass particles need to be of a certain minimum size, for example, size #8 or #4. But glass particles of this size are also most vulnerable to ASR and therefore require effective countermeasures. Production technologies that utilize higher moisture contents than those used to produce concrete blocks and paving stones are often referred to as wet technology. This is generally used for a wide variety of precast concrete products, including some that are produced in fully automated facilities, such as the terrazzo tiles manufactured by the Wausau Tile Company in Wausau, Wisconsin. Although the mix designs utilize higher moisture contents than is common for dry technologies, the development of an appropriate production technology should similarly recognize the differences between glass and natural aggregates. For example, the zero water absorption of glass improves the mix rheology and is likely to affect the choice of other admixtures. Terrazzo tiles can be categorized as a value-added product, because the improvements in mechanical and other properties coupled with the variety of possible color combinations add so much value to the end product that the market will bear a higher price. Special esthetic effects can be achieved with color-sorted glass. Architects or designers can help coordinate the colors of glass aggregate and cement matrix. Also the choice of surface texture and treatment may benefit from specialists trained in the visual arts. There is a wide variety of architectural and decorative concrete applications that could be produced with glass aggregate. For example, there are architectural concrete blocks, building façade elements, wall tiles, panels, partitions, stair treads, table top counters, benches, window sills, planters, trash receptacles, etc.

10.6

Recycled tires

It has been estimated that in the United States alone, over 300 million scrap tires are stockpiled, with almost an equal amount added each year. The disposal of these large numbers of used tires poses a serious environmental problem. Not only are tire dumps unsightly. They also pose significant health hazards as breeding grounds for mosquitoes as well as fire hazards. Some tire fires have been reported to burn for months and even years (Taha et al., 2008; Dhir et al., 2001). Therefore the disposal of tires in regular landfills is often prohibited. One unfortunate consequence is an increase in illegal dumping of scrap tires, with their accompanying environmental problems. Probably the most meaningful method of recycling used tires is to reuse them after retreading. The barriers to such reuse due to public perception are well known, but latest research and industry efforts promise an increase

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in such reuse (Brown et al., 2001; Brodsky, 2001). Yet, the most common disposal method of old tires seems to be to burn them for the production of steam and electricity or heat. The use of tires as alternative fuel in cement kilns is widespread throughout the US and Europe (Davies and Worthington, 2001). But their value as fuel is considerably less than that of the original material, so that such a use constitutes another example of downcycling. A different use of scrap tires is in hot mix asphalt or as crumb rubber for modifying binders in asphalt pavements (Nelson and Hossain, 2001; Amirkhanian, 2001; Navarro et al., 2005). Although some of these and other applications have been more or less successful, they either result in too much loss in value, or they do not generate enough volume to make a noticeable dent in the existing stockpiles of scrap tires. This leaves use of tire rubber as an ingredient in concrete production as a major viable alternative. From a strictly economic viewpoint, a simple replacement of fine or coarse aggregate still implies a certain degree of downcycling, unless specific properties of the rubber can be exploited that natural sand and gravel or crushed stone do not have. The most common ways of recycling rubber in cement composites and concrete is to use it as shredded, chipped, ground, or crumb rubber, with sizes ranging from shredded pieces as large as 450 mm to powder particles as small as 75 μm. Because of the large differences between Young’s moduli of rubber and cement matrix, major differences in the mechanical properties are to be expected between concrete with conventional natural aggregate and with rubber containing concrete. Most significant is the loss in compressive and tensile strength as well as stiffness, with increasing rubber content. The strength loss, which can be as high as 80% (El-Dieb et al., 2001; Eldin and Senouci, 1993), is to be expected, since the rubber particles not only constitute weak inclusions, they also are responsible for significant tensile stresses in the cement matrix, which lead to earlier cracking and failure. On the other hand, the rubber particles have a restraining effect on crack propagation, which leads to a significant increase in strain capacity, ductility, and energy absorption capacity (Taha et al., 2008; El-Dieb et al., 2001). Other potential advantages of the rubber derive from its sound absorption as well as thermal properties. However, the value added by the use of rubber particles is usually insufficient to offset the loss in value as a tire. It has also been proposed to exploit the energy absorption potential of rubber with the production of shock absorbing elements. However, due to the incompatible Young’s moduli of rubber and concrete matrix, for such composites to dissipate large amounts of energy, they have to undergo large deformations, in which case actual impact loads are likely to inflict damage to the concrete matrix with resulting progressive deterioration of its mechanical properties, especially under repeated load application.

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Of more significant value than the rubber can be the tire derived steel. It has been suggested to use such scrap steel as fiber reinforcement in concrete such as slurry infiltrated concrete (SIFCON) (Pilakoutas and Strube, 2001; Tsoi and Meyer, 2007). Although there is the potential of beneficial use of tire rubber and tire derived steel in concrete, more research is needed before such uses make economic sense in the larger context of sustainable development.

10.7

Recycled plastics

It has been estimated that in 2002, almost 4 million tons of plastic bottles were produced in the United States, of which only 21% were recycled. Plastics come in many different forms and chemical formulations. This complicates the recycling process as well as their use in concrete production. Because the different types of plastics are typically commingled, it is barely economical to separate them in volume. Many plastics can be recycled back into blank feedstock to be used as input for thermosetting or plastic manufacturing. However, the quality is lower and less uniform than that of virgin material, therefore manufacturers generally prefer to downcycle post-consumer plastics into alternative uses such as plastic lumber. De-polymerization or chemically breaking plastics down to their virgin components is not possible with currently available technologies, therefore the main option for recycling is to grind up the material and use it in other forms. A major obstacle for the use of recycled plastic in concrete is the poor bond between the plastic particles and the cement matrix. In one particular study (Al-Manaseer and Dalal, 1997) shredded plastic from car bumpers was used as partial replacement of coarse aggregate, from 10 to 50% by volume. The compressive strength reduction for 10% replacement was 34% and for 50% coarse aggregate replacement, the strength reduction was 67%. Although some of this strength reduction could be attributed to the low water absorption of the plastic which increased the effective water-cement ratio, the main cause for the reduction in strength as well as Young’s modulus seemed to be due to the poor bond between untreated plastic and concrete matrix. Several other studies have arrived at basically the same conclusion, namely that the straight substitution of recycled plastic for natural aggregate causes a drop in strength and other mechanical properties of concrete (Siddique, 2008). Most techniques to incorporate recycled plastics in concrete focus on replacing fine aggregate with plastic fines. There exist several patented processes to treat the plastic particles thermally or otherwise to improve the bond properties.

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Further research is needed to develop methods to replace larger coarse aggregate with recycled plastic. This goal could be accomplished by different processing techniques such as foaming to engineer a change in the performance of the concrete. This may or may not improve the bond to the concrete. Alternative methods of integrating plastic into concrete could be developed, including void filling and foaming without bonding. One possibility is to combine a foaming agent with the use of bioplastic as a coating of plastic aggregate (Hagerman and Meyer, 2008). Bioplastic in its most elementary form is an agricultural waste product (starch). If mixed with water and some oil for workability it can easily biodegrade in warm wet environments and is therefore highly unstable. When the plastic aggregate is introduced to the wet concrete, the bioplastic coating begins to biodegrade. This process is accelerated by any heat of hydration during curing of the concrete. Once the bioplastic has degraded sufficiently, a chemical foaming agent is activated and causes bubbles to form in the concrete. Additionally, the aggregate can be made easily pliable or extremely rigid. The aggregates’ bond to the concrete can be varied and designed. All of these features of plastic’s incorporation into concrete are engineering problems – as the aggregate itself becomes an engineered product within the concrete matrix. Compared with recycled glass, the chemical interaction with the concrete matrix is benign in its simplest form. One of the most promising aspects of using recycled plastic in concrete (whether raw, modified, or in a bioplastic composition) is the potential change in the visual appearance of the aggregate and concrete matrix. For example, the plastic in the aggregate can be exposed or hidden. The visual impact on the concrete translates into a slight change in the surface color of the mix, as can be seen, for example, in the Plascrete blocks produced by Conigliaro Industries (2007), which consist of commingled waste plastic used as aggregate, at compressive strengths ranging from 300 to 1700 psi.

10.8

Other recycled materials

Numerous other materials have been proposed as substitutes for conventional ingredients of concrete. Here the focus is on those materials that are byproducts, i.e. products that are produced in the course of or as a result of other things (ACI 213, 2003) and that are more commonly referred to as waste materials. Most important among these are ashes of many different kinds. Fly ash, resulting from coal combustion, has already been discussed in Section 10.2. But there are other kinds of ashes with more or less pronounced pozzolanic properties that lend themselves to partial replacement of Portland cement.

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Rice husk ash (RHA) is the residue from burning rice husk, an agricultural byproduct of the production of rice. For every 1000 kg of rice paddy milled, about 50 kg of rice husk ash is produced (Siddique, 2008), which translates into millions of tons worldwide each year. The RHA has been shown to have valuable cementitious properties and therefore has been proposed as a supplementary cementitious material (Mehta, 1992; Zhang and Malhotra, 1996; Nehdi et al., 2003). The combustion of wood results in about 6–10% ash, the characteristics of which vary widely with the type of wood, its cleanliness, the combustion temperature, etc. Typical wood burnt for fuel at pulp and paper mills and wood products industries may consist of sawdust, wood chips, bark, and saw mill scraps, etc. (Siddique, 2008). The suitability of the ash as a cementitious material has been shown (Naik, 1999; Naik and Kraus, 2000). Most metropolitan areas in the United States are facing major solid waste disposal problems. This is particularly true for New York City, which probably generates more solid waste than any other city in the world, including those that are much bigger. One of the means of its disposal is to burn it in so-called waste-to-energy facilities. However, the disposal of the ash even in conventional landfills is problematic because the fly ash in particular is typically considered a hazardous material as it may contain unacceptable levels of toxic elements. One way to circumvent this problem is to mix the fly ash and bottom ash such that the toxicity level of the blend is below the acceptable limit. But rather than landfilling such ash, it is possible to exploit its cementitious properties, while encapsulating the heavy metals in the ash in such a way that they cannot leach out. However, before such technologies can be applied in actual practice, additional research is needed. In particular, the question of public acceptance needs to be addressed. There are materials other than ashes that have been shown to be suitable ingredients for concrete production. In the United States, 100 million tons of sand are used in foundries for the production of steel and other metals. Most of these foundry sands are discarded and available to be recycled (Siddique, 2008). Naik et al. (2004) have shown that such foundry sands are suitable for the production of concrete. The Port Authority of New York and New Jersey needs to dredge about three million cubic meters of sediment each year to keep shipping lanes open and also to deepen them to accommodate the new larger vessels. As long as the Port Authority was able to dump the material in the open ocean, the disposal costs were minimal. But since national legislation and international treaties are prohibiting such ocean dumping, the material has to be deposited in engineered landfills at great cost, because much of it is highly contaminated with heavy metals, dioxins, PCBs, oils, etc. It is vital for the

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Port Authority that the disposal costs be drastically reduced. Similar problems are faced by many other world ports. Treatment methods are already available, which render the material suitable for concrete production, because the heavy metals can be encapsulated chemically such that they cannot leach out (Millrath et al., 2001a, 2001b). But the economics of such treatment methods are complicated by numerous factors, not all of which are of a technical nature. Fiber-reinforced concrete is increasingly used throughout the industry. The addition of large numbers of short, uniformly dispersed fibers has the effect of modifying the properties of the concrete matrix. The main benefits are improved ductility and energy dissipation capacity, which have been thoroughly documented in the literature. Maybe even more significant is the role that fibers play in controlling the cracking of the concrete matrix. By preventing cracks from opening up, the permeability of concrete can be preserved, which translates into improved durability. The most common types of fiber are steel and polypropylene, and alkali-resistant glass fibers are widespread in the precast concrete industry. All of these fibers are usually manufactured out of virgin material. However, studies have been reported on substituting fibers manufactured out of recycled carpets. Millions of tons of old carpets need to be disposed of each year, constituting another sizeable fraction of solid waste. Since carpet fibers are typically made of nylon, recycled fibers have been shown to improve some mechanical properties of concrete (Meyer et al., 2002).

10.9

Future trends

The future of using recycled materials in concrete will be governed primarily by economic factors, just as the present is and the past has been. First of all, in a free market economy the price of a service or commodity is determined by supply and demand. But government can and regularly does intervene with incentives (for example, in the form of tax write-offs) and disincentives, such as fees, penalties, or outright prohibition, if this is considered to be in the best interest of the public. Maybe equally important is a general shift in public attitude. Whereas Europeans and Japanese have long been used to material shortages, Americans have been raised much more on the principles of conspicuous consumption and wasteful use of natural resources. But that is now changing. The first Earth Day of 1970 is often considered to be the birthday of the environmental movement in the United States. But “environmentalists” have long been considered to be on the fringes of society. Yet in recent years the concerns about the dangers of climate change and global warming have become so commonplace that the principal demands of sustainable development are now becoming more and more mainstream. Major industries

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and developers are signing on, not necessarily out of principle, but because of purely economic considerations. In the construction industry, the signs of change are most visible in the success of the Green Building movement, with the most conspicuous example being the US Green Building Council’s LEED rating system (Leadership in Energy and Environmental Design) (USGBC, 2007), which has been experiencing exponential growth in recent years, both in the number of professional members and the number of buildings that were registered. Under these changing circumstances, the use of recycled materials is becoming more and more a way of life. The most significant recent development was the recognition by developers that “building green” will positively affect the bottom line, in addition to the tangible and intangible benefits to be derived from good publicity. Although many green building features require initial investments of a few percent beyond the costs for conventional buildings, the payback periods are typically only a few years. In New York City, developers have been able to charge higher rents for both residential and commercial units in certified Green Buildings. In addition, the substitution of recycled materials for virgin materials is an important component of sustainable development. There are obviously costs associated with recycling, such as collection, processing, transporting, and the required associated capital investments. On the other hand, materials that are not reused or recycled will have to be disposed of somehow, and suitable landfill capacities are getting more and more sparse, and tipping fees increase faster than the rate of general inflation. An important factor in the economics of recycling is the cost of the materials that are being replaced. Are we replacing sand, which is literally dirtcheap, or is the objective to replace marble chips that are being imported from Italy at high cost? This is where the question of “beneficiation” arises, i.e. the process of adding value. The key challenge is to identify special properties inherent in recycled materials that can be exploited and thereby generate added value. Another important driver in a free-market economy is competition or the lack thereof. For example, right now, there are relatively few recyclers in the US who are specializing in the processing of post-consumer glass. As a result, those who are doing it get paid by municipalities to take the glass off their hands and then can sell the processed glass for a handsome profit. It is to be expected that increased competition will bring down the price of recycled glass in the near future. And of course there is the final economic driver that may be even more powerful: namely the profit motive. If people don’t think they can earn a reasonable profit doing something, they won’t do it. This applies to all kinds of recycled materials.

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10.10 References ACI Committee 116 (2000), “Cement and Concrete Terminology”, American Concrete Institute Report ACI 116R-00, Farmington Hills, MI. ACI Committee 213 (2003), “Guide to Structural Lightweight-Aggregate Concrete”, American Concrete Institute Report ACI 213R-03, Farmington Hills, MI. ACI Committee 233 (1995), “Ground Granulated Blast-Furnace Slag as a Cementitious Constituent in Concrete”, American Concrete Institute Report ACI 233R-95, Farmington Hills, MI. ACI Committee 555 (2001), “Removal and Reuse of Hardened Concrete”, American Concrete Institute Report ACI 555R-01, Farmington Hills, MI. Al-Manaseer A A and Dalal T R (1997), “Concrete Containing Plastic Aggregate”, Concrete International, August, 47–52. Amirkhanian S N (2001), “Utilization of Crumb Rubber in Asphaltic Concrete Mixtures – South Carolina’s Experience”, Research Report, South Carolina Department of Transportation. Asselanis J and Mehta P K (2001), “Microstructure of Concrete from a Crack-Free Structure Designed to Last a Thousand Years”, 3rd CANMET/ACI Int. Symposium on Sustainable Development of Cement and Concrete, Malhotra V M, ed., American Concrete Institute, Special Publication SP-202, 349–358. ASTM C33 (2003), “Standard Specifications for Concrete Aggregates”, ASTM, West Conshohocken, PA. ASTM C1293 (1995), “Standard Test Method for Coarse Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction”, ASTM, West Conshohocken, PA. ASTM C1260 (1994), “Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)”, ASTM, West Conshohocken, PA. Ayano T and Sakata K (2000), “Durability of Concrete with Copper Slag Fine Aggregate”, American Concrete Institute, Special Publication SP-192, 141– 158. Behnoud A (2005), “Effects of High Temperatures on High-Strength Concretes Incorporating Copper Slag Aggregates”, Proceedings, 7th International Symposium on High-Performance Concrete, Washington, D.C. Brodsky H (2001), “The Important Role Retreads Can Play in Reducing the Scrap Tyre Problem”, in Recycling and Use of Used Tyres, Dhir, R.K. et al., eds., Thomas Telford, London. Brown K M, Cumming R, Morzek J R, and Terrebonne P (2001), “Scrap Tire Disposal: Three Principles for Policy of Choice”, Natural Resources Journal 41(1) 9–22. Byars E A, Zhu H Y, and Morales B (2004), “Conglasscrete I”, Final Report to The Waste & Resources Action Programme, University of Sheffield. Collins R J and Ciesielski S K (1994), “Recycling and Use of Waste Materials and By-Products in Highway Construction”, National Cooperative Highway Research Program Synthesis of Highway Practice No. 199, Transportation Research Board, Washington, D.C. Conigliaro Industries (2007), http://www.conigliaro.com/products/plascrete.cfm, 71 Waverly Street, Framingham, MA 01702. Corinaldesi V, Tittarelli F, Coppola L, and Moriconi G (2001), “Feasibility and Performance of Recycled Aggregate in Concrete Containing Fly Ash for

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Sustainable Buildings”, 3rd CANMET/ACI Int. Symposium on Sustainable Development of Cement and Concrete, Malhotra V M, ed., American Concrete Institute, Special Publication SP-202, 161–180. Davies R W and Worthington G S (2001), “Use of Scrap Tyre as a Fuel in the Cement Manufacturing Process”, in Recycling and Use of Used Tyres, Dhir, R.K. et al., eds., Thomas Telford, London. De Pauw C (1981), “Fragmentation and Recycling of Reinforced Concrete – Some Research Results”, in Adhesion Problems in the Recycling of Concrete, NATO Conference Series IV (Materials Science), Chapter 5.3.2, Plenum Press, New York, 331–317. Dhir R K, Limbachiya M C, and Paine K A, eds. (2001), Recycling and Use of Used Tyres, Thomas Telford, London. El-Dieb A S, Abdel-Wahab M M, and Abdel-Hameed M E (2001), “Concrete Using Tire Rubber Particles as Aggregate, in Recycling and Use of Used Tyres, Dhir, R.K. et al., eds., Thomas Telford, London. Eldin N N and Senouci A B (1993), “Tire Rubber Particles as Concrete Aggregate”, Journal of Materials in Civil Engineering 5(4) 478–498. Hagerman J and Meyer C (2008), “Recycled Plastic as a Novel Aggregate”, in Concrete With Recycled Materials, ACI Committee 555, Report under review. Hansen T C, ed. (1992), Recycling of Demolished Concrete and Masonry, RILEM Report 6, Chapman and Hall, London. Hansen T C and Lauritzen E K (2004), “Concrete Waste in a Global Perspective”, in Recycling Concrete and Other Materials for Sustainable Development, Liu T C and Meyer C, eds., American Concrete Institute, Special Publication SP-219, 35–45. Jin W (1998), “Alkali-Silica Reaction in Concrete – A Chemo-Physico-Mechanical Approach”, PhD Dissertation, Columbia University, New York. Jin W, Meyer C, and Baxter S (2000), “Glascrete – Concrete with Glass Aggregate”, ACI Materials Journal, March–April 2000. Johnson C D (1974), “Waste Glass as Coarse Aggregate for Concrete”, Journal of Testing and Evaluation 2(5) 344–350. Kandie B K T and Byars E A (2007), “Ultra Fine Fly Ash Concrete”, in Sustainable Construction Materials and Technologies, Chun Y-M et al., eds., Taylor & Francis, London, 121–130. Kasai Y (2004), “Recent Trends in Recycling of Concrete Waste and Use of Recycled Aggregate Concrete in Japan”, in Recycling Concrete and Other Materials for Sustainable Development, Liu T C and Meyer C, eds., ACI Special Publication SP-219, 11–33. Kirby B (1993), “Secondary Markets for Post-consumer Glass”, Resource Recycling, June 1993. Lauritzen E K (2004), “Recycling Concrete – An Overview of Challenges and Opportunities”, in Recycling Concrete and Other Materials for Sustainable Development, Liu T C and Meyer C, eds., American Concrete Institute, Special Publication SP-219, 1–10. Malhotra V M (1999), “Making Concrete Greener with Fly Ash”, Indian Concrete Journal 73, 609–614. Malhotra V M (2000), “Role of Supplementary Cementing Materials in Reducing Greenhouse Gas Emissions”, in Gjørv O E and Sakai K, eds., Concrete Technology

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for a Sustainable Development in the 21st Century, London, E & FN Spon, 226–235. Marceau M L and VanGeem M G (2002), “Life Cycle Assessment of an Insulating Concrete Form House Compared to a Wood Frame House”, PCA R&D Serial No. 2571, Portland Cement Association, Skokie, IL. Mehta P K (1992), “Rice Husk Ash – A Unique Supplementary Cementing Material”, Proc., Int. Symposium on Advances in Concrete Technology, Athens, Greece, 113–122. Mehta P K (2000), “Concrete Technology for Sustainable Development – Overview of Essential Elements”, in Concrete Technology for a Sustainable Development in the 21st Century, Gjørv O E and Sakai K, eds., London, E & FN Spon, 83–94. Mehta P K and Langley W S (2000), “Monolith Foundation: Built to Last a ‘1000 Years’ ”, Concrete International, July, 27–32. Meyer C, Shimanovich S, and Vilkner G (2002), “Precast Concrete Wall Panels with Glass Concrete”, Final Report to New York State Energy Research and Development Authority, Rep. No. 03-01, Albany, NY. Millrath K, Kozlova S, Shimanovich S, and Meyer C (2001a), “Beneficial Use of Dredged Material”, Progress Report prepared for Echo Environmental, Inc., Columbia University, New York, NY, Feb. 2001. Millrath K, Kozlova S, Shimanovich S, and Meyer C (2001b), “Beneficial Use of Dredged Material 2”, Progress Report prepared for Echo Environmental, Inc., Columbia University, New York, NY, Dec. 2001. Mindess S, Young J F, and Darwin D (2003), Concrete, 2nd edn, Prentice Hall, Englewood Cliffs, NJ. Naik T R (1999), “Tests of Wood Ash as a Potential Source for Construction Materials”, Report No. CBU-1999-09, UWM Center for By-Products Utilization, Department of Civil Engineering and Mechanics, University of Wisconsin – Milwaukee, Milwaukee. Naik T R and Kraus R N (2000), “Use of Wood Ash for Structural Concrete and Flowable CLSM”, Report No. CBU-2000-31, UWM Center for By-Products Utilization, Department of Civil Engineering and Mechanics, University of Wisconsin – Milwaukee, Milwaukee. Final Report Submitted to the University of Wisconsin System, Solid Waste Management and Research Program. Naik T R, Kraus R N, Chun Y M, Ramme W B, and Siddique R (2004), “Precast Concrete Products Using Industrial By-Products”, ACI Materials Journal 101(3) 199–206. Navarro F J, Partal P, Martinez-Boza F, and Gallegos C (2005), “Influence of Crumb Rubber Concentration on the Rheological Behavior of Crumb Rubber Modified Bitumen”, Energy and Fuels 19, 1984–1990. Nehdi M, Duquette J, and Damatty A E (2003), “Performance of Rice Husk Ash Produced Using a New Technology as a Mineral Admixture in Concrete”, Cement and Concrete Research 33, 1203–1210. Nelson R G and Hossain A S M M (2001), “An Energetic and Economic Analysis of Using Scrap Tyres for Electricity Generation and Cement Manufacturing”, in Recycling and Use of Used Tyres, Dhir, R.K. et al., eds., Thomas Telford, London. Phillips J C and Chan D S (1972), “Refuse Glass Aggregate in Portland Cement Concrete”, Proceedings, 3rd Mineral Waste Utilization Symposium, Chicago, March 1972, U.S. Bureau of Mines and IIT Research Institute.

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Pilakoutas K and Strube R (2001), “Re-Use of Tyres Fibers in Concrete”, in Recycling and Use of Used Tyres, Dhir, R.K. et al., eds., Thomas Telford, London. Reindl J (2003), “Reuse/Recycling of Glass Cullet for Non-Container Uses”, Dane County Department of Public Works, Madison, WI. Sakai K (2007), “Contributions of the Concrete Industry Toward Sustainable Development”, in Sustainable Construction Materials and Technologies, Chun YM et al., eds., Taylor & Francis, London, 1–10. Sarhat S R (2007), “An Experimental Investigation on the Viability of Using Fine Concrete Recycled Aggregate in Concrete Production”, in Sustainable Construction Materials and Technologies, Chun Y-M et al., eds., Taylor and Francis, London, 53–58. Shao Y, Lefort T, Moras S, and Rodriguez D (2000) “Studies on Concrete Containing Ground Waste Glass”, Cement and Concrete Research 30 (2000), 91–100. Shi C (1998), “Pozzolanic Reaction and Microstructure of Chemical Activated Lime-Fly Ash Pastes”, ACI Materials Journal Sept.–Oct., 537–545. Shi C and Day R L (1995), “Acceleration of the Reactivity of Fly Ash by Chemical Activation”, Cement and Concrete Research 25(1), 15–21. Siddique R (2008), Waste Materials and By-Products in Concrete, Springer, Berlin. Taha M R, El-Dieb A S, and Nehdi M (2008), “Recycling Tire Rubber in CementBased Materials”, in Concrete With Recycled Materials, ACI Committee 555, Report under review. Tsoi C and Meyer C (2007) “Tire-Derived Steel as a Substitute for Virgin Steel in Fiber-Reinforced Concrete”, Report to 2007 Intel Science Talent Search, Columbia University, New York. USGBC (2007) “LEED Rating System”, Version 3.0, U.S. Green Building Council, Washington, D.C. VanGeem M and Marceau M L (2002), “Using Concrete to Maximize LEEDS Points”, Concrete International, November, 69–73. Yelton R (2004), “Concrete Recycling Takes Off”, The Concrete Producer, September, 28–31. Zhang M H and Malhotra V M (1996), “High-Performance Concrete Incorporating Rice Husk Ash as a Supplementary Cementing Material”, ACI Materials Journal 93(6), 629–636.

11 Foamed concrete V BINDIGANAVILE and M HOSEINI, University of Alberta, Canada

11.1

Introduction

About two thousand years ago, the Romans were making a concrete mix consisting of small gravel and coarse sand mixed together with hot lime and water. They realized that by adding animal blood into the mix and agitating it, small air bubbles were formed making the mix more workable and durable. There is also evidence that earlier still, the Egyptians used a similar technology some 5000 years ago to engineer air entrainment [1]. No significant advance in this type of material was made until the early 1900s when the manufacture of highly air entrained cement based materials began commercially in Sweden and Denmark and the first Portland cement based foams in the present era were patented in 1923 by Axel Eriksson [2, 3]. In this process, the entrained gas was produced by the creation of hydrogen gas (using powdered aluminum or hydrogen peroxide) in a slurry mix made alkaline by the inclusion of Portland cement, and at times, lime. Since the end of the Second World War, its usage has grown worldwide and considerable advances have been made in the production technology. Cement based cellular composites are now produced by introducing air in one of three ways: (1) by the addition of relatively large amounts of powerful air entraining agents; (2) through chemical admixtures which release gas bubbles during the mixing process [2, 4] or (3) by the addition of foaming agents. Accordingly, these low density cement based composites are classified as (1) highly air entrained concrete, (2) foamed concrete, and (3) aerated concrete. Because of their cellular microstructure, these types of concrete, in general, are called cellular concrete. This chapter deals primarily with the production and properties of foamed concrete, and only a brief introduction to the other types is presented here. 231

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11.2

Definitions and classifications

According to ACI 523.2R the material, which is commonly referred to as cellular or aerated concrete, is defined as [5]: “A lightweight product consisting of Portland cement and/or lime with siliceous fine material, such as sand, slag, or fly ash, mixed with water to form a paste that has a homogeneous void or cell structure. The cellular structure is attained essentially by the inclusion of macroscopic voids resulting from a gasreleasing chemical reaction or the mechanical incorporation of air or other gases (autoclave curing is usually employed)”.

The British Cement Association has defined foamed concrete as “a lightweight material produced by incorporating a preformed foam, into a base mix of cement paste or mortar, using a standard or proprietary mixing plant”. The entrapped air bubbles reduce the density of the base mix and have a strong plasticizing effect on it [6]. Typically the mix composition in foamed concrete is made up of cementitious materials, sand, water and entrained air, so that it contains no coarse aggregate. It is thus perhaps more closely related to mortar, some researchers have called it highly air entrained cement sand slurry [2]. Cox [7, 8] further states that this product is not created by foaming ordinary concrete. Rather, the pores are introduced by agitating air with a foaming agent diluted with water, thus creating a mechanically manufactured foam. This foam is then carefully blended with the cement slurry or the base mix [7]. Alternatively, Kearsley has defined it as a cementitious material in which a minimum of 20% of the volume consists of foam that is entrained into the plastic mortar [9, 10].

11.3

Materials

As mentioned before, foamed concrete consists of Portland cement, water, a foaming agent, and/or possibly other fine materials. In addition, cementreplacing materials, mineral and chemical admixtures have been successfully used in foamed concrete. As suggested by ACI 523.1R-06 [11], all of the admixtures must be compatible with the stable foam within a specific mixture.

11.3.1 Portland cement Portland cement is the main cementitious component of foamed concrete. It has been used at dosages varying from as high as 1400 kg/m3 to as low as 75 kg/m3 but in practice, usually between 300 and 500 kg/m3 [3, 6, 11, 12]. In addition to normal Portland cement, rapid hardening Portland cement, high alumina cement and calcium sulphoaluminate cements have also been

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used in foamed concrete to reduce its setting time and improve the early strength [3]. Using geocements and alkaline Portland cement is reported to improve the fire resistance of foamed concrete [13].

11.3.2 Mineral admixtures Depending on the application, cement-replacing materials such as fly ash (FA), ground granulated blast-furnace slag (GGBFS), solid wastes and silica fume (SF) have been added to foamed concrete [1, 3, 12, 14–18]. In addition, Proshin et al. [19] used mineral admixtures in the form of fine-crushed carbonate or quartz sands. Similarly, Lee and Hung [14] investigated the use of solid wastes such as rice husk ash (as a pozzolanic admixture), expanded polystyrene (as a lightweight aggregate), and paper sludge (which contains fragments of paper fibers that serve as reinforcement) in foamed concrete. Cement may be successfully replaced with fly ash (up to 80%) and several studies report its effect on the properties of foamed concrete [12, 15, 18]. Further Kearsley and Wainwright [15] examined the effect of incorporating unclassified ash, – so called by the South Africa Standard (SABS) [20], since approximately 40% of the particles have a particle size exceeding 45 μm (this criterion is similar to ASTM C618-05, which limits the maximum amount of fly ash retained when wet-sieved on the 45 μm (No. 325) sieve to 34% [21]). Their research indicates that large volumes of fly ash can be used in foamed concrete. Although the high ash content results in a decrease in the early strength, the long-term strength was improved by replacing up to 75% of cement with fly ash. The trends observed for classified ash, where according to SABS [20] approximately 12.5% of the particles have a particle size less than 45 μm, and the unclassified ash were similar [16, 18, 22]. Ground granulated blast-furnace slag (GGBFS) has also been added to Portland cement at levels between 30 and 50% by cement mass. Silica fume (SF) has been incorporated into foamed concrete at up to 10% by mass of cement and was found to be effective in improving the compressive strength of the mixes with a low percentage of foam (up to 30%) without affecting the stability of the air void system. However, in those mixes with a high volume of foam (>30%), the effect of silica fume was not significant [3, 17]. When using admixtures, particular care must be given to factors such as economy, consistency, mix stability and their contribution to strength in deciding upon their suitability. For instance, the use of a high volume of fly ash results in the de-stabilization of the mix, but this can be prevented by using foam stabilizers. Also, a fly ash with a high loss on ignition (i.e., high carbon content) may adversely affect the preformed foam by causing an increase in its density and consequent loss of yield [23].

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Developments in the formulation and reinforcement of concrete

11.3.3 Aggregates In general, coarse aggregate is not used in the production of foamed concrete. The use of fine aggregates with a maximum particle size of up to 5 mm is recommended. However, the fine aggregate fraction can be partially or fully replaced with recycled or secondary materials, including fly ash, lime, chalk, crushed concrete, granite dust, recycled glass, expanded polystyrene granules and materials arising from demolition [3, 14, 17, 24]. Of course, the aggregate phase may be replaced with more air bubbles. Thus, it is also not uncommon to completely exclude the fine aggregate fraction.

11.3.4 Foaming agents There are two principal methods of producing foamed concrete namely, (i) the pre-foamed method and (ii) the mixing foam method [1, 25]. The foams that are used may be either synthetic or protein-based and are available from proprietary sources. As mentioned earlier, the Egyptians and the Romans used animal blood to entrain air into concrete. These days, refined animal products form the basis for protein based foams. On the other hand, synthetic foams are made of amine and amine oxides, naphthalene sulphonate formaldehyde condensates, etc. Some of these products can contain one or more substances classified as dangerous or hazardous to the environment. Hence caution must be exercised when using these products, especially those based on formaldehyde condensates, butyl carbitol, and glycol ethers [26]. The protein based foaming agents result in a stronger and a more closed-cell bubble structure while the synthetic ones yield greater expansion and thus lower density [5, 27]. In addition, the protein based foams permit the inclusion of greater amounts of air and also provide a more stable air void network [28]. Typically, protein based foams have a shorter shelf life (up to 6 months) while synthetic foaming agents can last up to a year in storage. The foam itself has no chemical action in concrete. The pre-formed foam that is blended with the base materials to produce foamed concrete can be divided into two categories: wet foam and dry foam. The wet foam is produced by spraying a solution of the foaming agent (usually synthetic) and water over a fine mesh which results in a network of bubbles ranging from 2–5 mm in diameter. The wet foam has a large loose bubble structure and although relatively stable, it is not recommended for the production of low density (below 1100 kg/m3) foamed concrete. It is also not suitable for pumping over long distances or for pouring to great depths. The dry foam is similar in appearance to shaving foam and has a bubble size distribution much smaller than wet foam (less than 1 mm) and is

Foamed concrete

235

extremely stable. While synthetic foams are easier to handle and can be stored longer, they are less susceptible to extremes of temperature. In addition, they are less expensive and they require less energy to produce. On the other hand, foams based on animal protein can produce stronger concretes. This is because the foaming agents based on animal protein possess the ability to take on water and hold it within the protein structure. During the cement hydration process, this water is released from the foam and is readily available to the cement particles. This results in a network of hydration products around the air bubbles ensuring a strong microstructure [1]. ASTM C796-04 [29] and ASTM C869-06 [30] introduce standard test methods and standard specifications respectively for foaming agents used in the making of pre-formed foam for cellular concrete.

11.4

Mix design

There is no standard method for designing a foamed concrete mix. The design philosophy differs from that for regular concrete in that within foamed concrete technology, the mix proportions are chosen, not only for a specified compressive strength, but also for a specified density. As with normal concrete, the greater the air content, the lower the strength. As expected, foamed concrete has characteristically a much lower strength than normal concrete. Again, as with normal concrete, the strength of foamed concrete is related to its cement and water content. However, in addition, with foam concrete, the type and content of the foaming agent has a considerable effect on properties of both the fresh and the hardened material. Using mineral admixtures such as FA and GGBFS will also result in a significant change in both fresh and hardened properties [1, 3, 12, 14–16, 22]. Furthermore, just as the water-cement ratio holds relevance to normal concrete technology, some foam concrete mixes are designed based on the aggregate-cement ratio and/or the sand-cement ratio [3, 31, 32]. Based on the method proposed by Kearsley and Mostert [16] for designing foamed concrete, a target casting density, sand-cement and (if applicable) fly ash-cement ratios are chosen and the water requirement is calculated. Using these values and the relative densities of the constituent materials, the mass of the cement and the volume of foam that should be added to obtain the required density can be determined. Kearsley also proposed some equations for calculating the mix proportions based on establishing two variables, the cement content and the foam content, and then solving the following equations: pm = x + x(w / c) + x(a / c) + x( s / c) + x(a / c)(w / a) + x( s / c)(w / s) + RDf Vf

11.1

236

Developments in the formulation and reinforcement of concrete 1000 =

x x(a / c) x( s / c) + x(w / c) + + RDc RDa RDs + x(a / c)(w / a) + x( s / c)(w /ss) + V1

11.2

where: pm = target casting density (kg/m3) x = cement content (kg/m3) w/c = water/cement ratio a/c = ash/cement ratio RDf = relative density of foam RDc = relative density of cement

s/c = sand/cement ratio w/a = water/ash ratio w/s = water/sand ratio Vf = volume of foam (l) RDa = relative density of ash RDs = relative density of sand

The results were reportedly within 5% of the target density and show the suitability of this method.

11.5

Production of foamed concrete

As mentioned earlier, the entrained air void network can be produced by the generation of hydrogen gas as a result of chemical reactions of aluminum powder in a slurry made alkaline by the inclusion of Portland cement and sometimes also lime. These reactions are such that the aluminum powder, reacting with calcium hydroxide and water, releases hydrogen. The hydrogen gas in turn foams the raw mix to double the volume (with gas bubbles up to 1/8 inch in diameter). At the end of the foaming process, the hydrogen escapes to the atmosphere and is replaced by air. In this method, after casting and the initial set, the material is then cured under steam [180 to 210°C] at a very high pressure i.e. “autoclaved” for a specific amount of time to produce the final micro/macro-structure. This method was the earliest modern means of producing cement based foams and was introduced in Sweden and Denmark in the 1920s. Nowadays, foamed concrete is produced by the addition of foaming agents to the concrete mix. This method has been in usage since the 1980s. As mentioned earlier, the two basic methods of producing foamed concrete by using foaming agents are the pre-formed foam and the mixing foam methods. In the pre-formed foam method, the foaming agent is mixed with a part of the mix-water in a foam generator and aerated to form the foam and then is forced at a high pressure through the foaming lance before being added to the mix (Fig. 11.1). The pre-formed foam method thus consists of an aqueous surfactant solution and compressed air [3]. In the mixing-foam method, the foaming agent is mixed with the matrix as a part of the constituent materials, i.e. cement, water and fine aggregates. In general, the production of foamed concrete is most-commonly via the pre-formed foam method. It can be divided into three stages: (1) prepara-

Foamed concrete

237

Water + foam concentrate Compressed air

Finished foam Foam solution

11.1 Production of foam via the pre-form method [26].

tion of the mortar, (2) preparation of the foam from a pre-mixed foaming agent and (3) generation of the foam using compressed air [31]. For convenience and accuracy, the foam generator should be calibrated prior to mixing so that the calculated quantity of foam required for a mix can be converted to the more easily understood “duration” of foam generation. For a given air content or volume of air, if the bubbles are too large, there will not be enough of them present to properly protect the paste. Large bubbles are also less stable and hence more likely to collapse while the concrete is being mixed, transported, placed and if necessary, vibrated. If too much air is lost during these operations, the remaining air voids may lead to a performance below that expected from the resultant foamed concrete [26].

11.6

Properties of foamed concrete

As with any cement based product, the characteristics of foamed concrete depend strongly upon its mix composition. Nevertheless, some general properties may be identified [1]: (1) high strength-to-weight ratio, (2) low coefficient of permeability, (3) low water absorption, (4) good freeze-thaw resistance, (5) rigid well bonded microstructure, (6) low shrinkage, (7) thermally insulating, (8) shock absorption capacity, and (9) non susceptibility to breakdown from hydrocarbons, bacteria or u-v radiation. The behavior observed for normal concrete does not necessarily hold true for foamed concrete; for most properties it would be unwise to assume, without experimental proof, behavioral trends for foamed concrete as mere extensions of our knowledge of regular concrete.

11.6.1 Properties of fresh foamed concrete Workability and water demand In its fresh state, foamed concrete is a free flowing, self-compacting and self-leveling material and therefore is expected to yield a collapse slump but it is known to exhibit a thixotropic behavior [6]. It is easy to pump, and

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Developments in the formulation and reinforcement of concrete

flows into the most restricted and irregular of cavities [3, 6]. In its visual appearance, fresh foam concrete looks like a thin grey mousse or grayish milkshake. The effect of mixing time is very important. Usually, the greater the mixing time, the more the entrained air. However, where the maximum air content (a critical limit) has been reached; further mixing may cause the loss of entrained air. In other words, increasing the mix time will produce a higher air content but when a critical air content is exceeded, any further mixing causes a drop in the air content. This behavior depends upon the amount, type and efficiency of the foam or the air entraining agent [28]. Since the slump test is not an appropriate workability measure for values in excess of about 200 mm, the water demand of the constituent materials used in foamed concrete should be determined using a flow table test [16, 28]. In the UK, the workability is evaluated by using the dropping ball consistency test as per BS 4551 [28]. The water-cement ratio (w/c) is typically in the range of 0.4 to 0.8 depending on the mix composition, consistency requirements, the use of chemical admixtures and the foam stability [3, 33]. The water-cement ratio should not be less than 0.35 before the introduction of the foam. Too little water in the mix might cause the cement to draw its moisture requirement from the foam, causing the latter to collapse partly or in full [32]. In general, as for normal concrete, greater foamed concrete spreads are obtained with higher water-cement ratios and the consistency is reduced by a drop in the concrete plastic density. This is perhaps due to the lower self-weight [3]. Due to the presence of bubbles and the absence of coarse aggregate, foamed concrete in general has a higher consistency with no segregation or bleeding [26]. However, it has been reported that some foam instability and mix segregation occur when incorporating GGBFS as a mineral admixture in foamed concrete [3]. Kearsley and Mostert [16] found that if small volumes of sand (less than 25%) are added to the fly ash, no additional water is required to adjust the water content and it remains suitable for use in foamed concrete. On the other hand, in mixes that contain sand in excess of 25% (by volume), the water requirement is seen to increase dramatically. The replacement of sand with coarse fly ash is seen to significantly reduce the yield values in shear flow, and any increase in the plastic density leads to a corresponding drop in the plastic viscosity [3]. The flow behavior of foamed concrete depends mainly on the foam volume and it is reported that an increase in the foam volume results in a drop in the flow [24, 34] (Figs 11.2 and 11.3). Moreover, the effect of cement content on the amount of air entrained in mixes that already contain an air entraining agent is substantial and studies show that an increase in the cement content leads to a drop in the air content [28].

Foamed concrete

239

80 Cement:coarse sand 1:1

75

Cement:fine sand 1:1

70

Cement:fly ash:fine sand 1:0.5:0.5

65

Cement:fly ash 1:1

Flow %

60 55 50 45 40 35 30 25 5

10

15

20

25

30

35

40

45

50

Foam volume %

11.2 Effect of foam content on the flow in foamed concrete [24]. 95

coarse sand (with foam)

85 75

fine sand (with foam)

65

fine sand-fly ash (with foam)

55

fly ash (with foam)

45 fly ash (without foam) 35 25 950 1050 1150 1250 1350 1450

fine sand-fly ash (without foam)

11.3 Variation of percentage of flow with and without foam [34].

Density Most properties and applications of foamed concrete are dependent on its density. Naturally, density is very important in the application of foamed concrete when the weight itself is a structural issue. The dry density of foamed concrete can be as low as 48 kg/m3 to as high as 1800 kg/m3 [3, 15, 26]. Kearsley and Mostert [16] found a linear relationship between fresh density and the dry density of foamed concrete for different mix designs including different ash contents. They proposed the following linear equation for calculating the required casting density, ρcast, for a range of dry densities, ρdry, between 600 kg/m3 and 1200 kg/m3:

240

Developments in the formulation and reinforcement of concrete ρcast = 1.034ρdry + 101.96

11.3

Heat of hydration Like most solids with a cellular microstructure, foamed concrete is a very good thermal insulator. Jones and McCarthy [3] noticed that by changing the density from 1400 kg/m3 to 1000 kg/m3, the core-to-surface temperature differential in a trench rose from 10°C to 20°C, while the greatest temperature at the top of the foot of the trench for 1400 kg/m3 and 1000 kg/m3 dry density of foamed concrete, were 45°C and 50°C, respectively. It has also been observed that by decreasing the density, the rate of temperature decline is slower due to the greater insulating ability of the lower density foamed concrete [3]. Use of fly ash (either as a cement replacement material or as a fine aggregate), is very effective in decreasing the peak temperature and the rate of temperature rise in foamed concrete (Fig. 11.4) [35]. Curing As the concrete hardens, the bubbles disintegrate or transform, and in the process release their water to be absorbed into the cement matrix. Not only 100 1000 kg/m3 plastic density 600 kg/m3 cement content Sand fine aggregate Ambient = 20±2°C

90 80

Reduction in peak temperature 6 °C

14 °C

20 °C

Degrees C

70 PC 60 PC/20%FA 50

3.25 hr 3.75 hr

40

PC/25%FA 5 hr

30

PC/30%FA

Retardation of peak

20 1

10 Time after casting, hours (log scale)

100

11.4 Influence of fly ash on the temperature development in foamed concrete at 1000 kg/m3 [35].

Foamed concrete

241

does this aid in the hydration process, it also creates air voids out of the network of pores previously filled with water. Thus, there is less need to keep the concrete damp during curing, as is normally necessary with conventional concrete [26]. Higher strengths have been obtained with aircuring in comparison to sealed or water cured samples [3]. On the other hand, for mixes containing fly ash, the long-term strength gain is seen to be higher for well-cured samples [15]. Kearsley and Mostert [10] investigated the effect of different curing regimes on the properties of foamed concrete with fly ash. Based on their results a high temperature curing regime can significantly increase the rate of strength gain of mixtures containing a high volume of fly ash but it results in a lower ultimate strength. However, the curing period required was significantly lower (less than 3 days) due to the high ambient temperature.

11.6.2 Properties of hardened foamed concrete Compressive strength The compressive strength of foamed concrete is mainly influenced by its density and it decreases exponentially with a decrease in the foamed concrete density. It can be as low as 0.34 MPa and as high as over 20 MPa [1, 3, 16, 24, 31, 33]. Ultimate strengths of more than 50 MPa have also been achieved by the application of fly ash in higher density foamed concrete (1500 kg/m3) [15]. It is usually desirable to obtain the highest possible strength at the lowest possible density. Kearsley and Wainwright [36] concluded that as with normal concrete, there is a correlation between the porosity and the compressive strength of foamed concrete and a decrease in the concrete porosity results in an increase in its strength. They also proposed a relationship between the concrete strength and its porosity at a given w/c ratio (as described on pp. 244–5). Higher compressive strength may be obtained by reducing the volume of voids required to obtain a given foamed concrete density. This is done by choosing low density constituent materials for manufacturing the foamed concrete [10]. Since the compressive strength of foamed concrete is mainly a function of its density, the filler, fly ash and cement content do not seem to have a significant effect on the compressive strength [15, 16]. However, Papayianni and Milud [12] showed that high calcium fly ash increased the compressive strength of foamed concrete. They studied the compressive strength of foamed concrete with high calcium fly ash replacement up to 70%. The results indicate that with an increase in the fly ash content, the compressive strength increases compared to the reference foamed concrete with no

Developments in the formulation and reinforcement of concrete 100 mm cube strength (MPa)

242

2.5 PC/60%FA

2

PC/40%FA 1.5 1

PC/30%FA REF.PC

0.5

PC/70%FA

0

0

20

40

60

80

Age (days)

11.5 Strength development in foamed concrete containing fly ash [12].

fly ash. This increase continued even at 90 days of maturity. They concluded that the higher water retention in fly ash (two times that of cement) in combination with its pozzolanic reactivity seems to contribute to the superior performance of fly ash as a binder in cellular concrete (Fig. 11.5) [12]. Moreover, based on the work by Kearsley and Wainwright [22], when using a coarser fly ash, high fly ash content results in a decrease in the early strength while the long-term strength is seen to improve. The study reports an optimum fly ash content for maximum strength after one year at nearly 60% of the cementitious materials content [22]. When cementitious fillers are used in foamed concrete, it is reported that the compressive strength continues to increase in the long term [35]. Using some other fine aggregates such as lime and recycled glass appears to have little or no effect on the compressive strength [3]. Water-reducing chemical admixtures tend to cause instability in the foam and consequently are not normally used. In addition, in foamed concrete, small changes in the water-cement ratio do not influence the strength in the way expected for normal weight concrete. Foamed concrete is characterized by its plastic density [24]. In other words, the volume of the voids is an important determinant of strength as well as the water-cement ratio and it is often the defining parameter. This is particularly true in the case of the more highly air entrained mixes [32]. Moreover, it has been observed that increasing the water content results in an increase in the strength, and the effect of void content seems to counteract the effect of the water-cement ratio on the strength of foamed concrete. This is because foamed concrete is usually designed based on a desired density; the low density of water reduces the need to add foam to reach a target density [16]. Thus, the long-held rule of thumb of concrete

Foamed concrete

243

technology, namely, the compressive strength being inversely proportional to the water-cement ratio, is somewhat turned on its head in the case of foamed concrete. According to Nehdi et al. [37], the compressive strength of foamed concrete does not depend upon the water-cement ratio, rather it is mostly affected by the foam content. The compressive strength of lower density foamed concrete can be increased to equal that of higher density foamed concrete by increasing the cement content [31]. On the other hand, it is reported that higher sandcement ratios (s/c), result in a lower compressive strength [31, 32]. As mentioned before, the compressive strength of foamed concrete is influenced by the type of foaming agent used and it is observed that proteinbased foams increase the compressive strength of foamed concrete more than synthetic foams [3], primarily through the creation of a closed cell network. It should be noted that in comparing the properties of foamed concrete, the type of foaming agent is important and only those mixes with the same type of foaming agents should be compared. Modulus of elasticity As expected for a system not containing coarse aggregates, the static modulus of elasticity of foamed concrete is lower than that of normal concrete, and is in the range of 1000 MPa to 8000 MPa for dry densities between 500 kg/m3 to 1500 kg/m3 [3, 6, 35]. Jones and McCarthy [35] obtained equations for predicting the modulus of elasticity, E, of foamed concrete with various fine aggregates as follows: Sand fine aggregates: E = 0.42 fc 1.18

11.4

FAcoarse fine aggregate: E = 0.99 fc

11.5

0.67

where E is the static modulus of elasticity (in kN/mm2) and, fc is the 100 mm cube strength after sealed-curing (in N/mm2). Wee et al. [32] found from both experimental and numerical studies, that the inclusion of air bubbles in foamed concrete had a greater effect on the compressive strength than on the modulus of elasticity. Thermal properties Foamed concrete has a low thermal conductivity which makes it a good insulating material. This is mainly because of its cellular structure. The thermal resistance (as a measure of insulation) ranges from R = 2 to R = 4. In comparison, regular concrete typically has a thermal resistance below R = 1.

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Developments in the formulation and reinforcement of concrete

The values for thermal conductivity of foamed concrete are typically 5% to 30% of those measured for normal concrete. They range between 0.1 and 0.7 W/mK for dry densities between 600 to 1600 kg/m3 and 0.23 and 0.42 W/mK at 1000 to 1200 kg/m3 dry densities that reduces with decreasing dry density of concrete [3, 24]. ACI 523.2R-96 introduces the design values for low-density concretes (densities between 320 kg/m3 and 800 kg/m3) as between 0.09 W/mk and 0.2 W/mK for oven dry densities and 0.12 W/mk and 0.26 W/mk for air dry densities, respectively [5]. A note of caution: if foamed concrete is used in elements with high volume-to-surface ratio, the low thermal conductivity of this material can lead to an increase in the core temperature due to the heat of hydration and may cause cracking. Porosity The pore structure of foamed concrete consists of gel pores, capillary pores and air voids (air entrained and entrapped pores). As foamed concrete is self-compacting and self-flowing, the possibility of entrapped air is negligible. Wee et al. [32], Kearsley and Wainwright [36] and Hoff [38], developed strength-porosity relationships for foamed concrete that show the effect of porosity on its compressive strength. Their models show that a decrease in the concrete porosity results in an increase in the strength (Fig. 11.6). Kearsley and Wainwright [36] found that the best equation that fits their result (Fig. 11.6) can be expressed as follows: fc = 981e−7.43 p

11.6

Compressive strength (MPa)

100 90 80

a/c=0

70

pfa/c=1

60

pfa/c=2

50

pfa/c=3

40 30

poz/c=1

20

poz/c=2

10

poz/c=3

0 0

0.1

0.2

0.5 0.3 0.4 Measured porosity

0.6

0.7

0.8

11.6 Effect of porosity on the compressive strength of foamed concrete after 365 days [36].

Foamed concrete

245

where: fc is the compressive strength of foamed concrete and p is the porosity. Similarly, Wee et al. [32] have proposed a relationship between the concrete strength and its porosity at a given w/c ratio as: s = 1.262s p (1 − A)2.962

11.7

Here s, sp, and A are the compressive strength of foamed concrete, the compressive strength of the cement paste, and the air content, respectively. Nambiar and Ramamurthy [25] measured the air-void network of foam concrete and reported that the volume, size and spacing of the voids, together influence the density and the mechanical properties of cement based foams but on the other hand, the shape of the air voids does not influence the strength [25, 32]. Moreover, concrete with a higher air content tends to contain larger air-voids, especially at air content of over 40% [39]. The fine fractions influence the air-void network and the use of fly ash is seen to result in a more uniform distribution of air voids and is recommended over using fine sands [25]. Fire resistance In regular concrete, the loss of strength due to high temperature is influenced primarily by the type of cement and the type of aggregate. Foamed concrete is non-combustible and its fire resistance is very good and shows better performance than does normal weight concrete at lower temperatures. This response improves with lower densities of foamed concrete. Kearsley and Mostert [9] investigated the fire resistance of foamed concrete with high alumina cement and a coarse grained fly ash. They observed that the type of fly ash, aggregate and cement can influence the fire resistance. While foamed concrete containing hydraulic cement can withstand temperatures as high as 800°C, those mixes that contain cement with an Al2O3/CaO ratio higher than 2 and andulusite aggregates, can withstand temperatures as high as 1450°C without showing any signs of damage. Moreover, foamed concretes with a density of 500 kg/m3 and based on alkaline Portland cements have been shown to be fire resistant with a residual compressive strength as high as five times the strength measured before firing [13]. However, at higher temperatures, cement based foams undergo excessive shrinkage and research is ongoing in this area [3]. Shrinkage Since foamed concrete has a relatively high paste content and no coarse aggregate, it will shrink more than normal concrete. It is interesting that no

246

Developments in the formulation and reinforcement of concrete

plastic shrinkage is reported in foamed concrete but the drying shrinkage for this type of cementitious material is high, with values normally between 0.1% and 0.35%. The lower the density of the cement based foam, the higher the shrinkage strain [3, 6]. Papayianni and Milud [12] studied the drying shrinkage of foamed concrete with high calcium fly ash replacement up to 70% by mass of cement. The replacement of cement by this type of FA decreased the drying shrinkage from about 1800 microstrains (for concrete without fly ash) to about 1200 microstrains for foamed concrete containing 60% fly ash. They also observed that the higher the strength, the lower the shrinkage. Similar results on reducing the drying shrinkage of foamed concrete by using coarse grained fly ash have been reported by Kearsley [18]. While ACI 523.2R-96 [5] limits the average drying shrinkage of cellular concretes to 0.2%, the drying shrinkage for some typical foamed concrete with sand-cement ratio of 2, water-binder ratio between 0.65 to 0.70 and a dry density around 1500–1600 kg/m3 have been reported as being much lower (less than 0.09%) [14]. Water absorption The water absorption of foamed concrete depends on its density (which itself is a function of the foam content and the mix design). When represented as a percentage by mass, the absorption of water increases with a decrease in the density and can be as low as 15% for a density of 1800 kg/m3 and as high as 35% for density of 700 kg/m3 [32, 34]. The presence of aggregates has a bearing on the absorption for identical water-cement ratios. Water absorption tests on foam concrete resulted in higher values for mixes that contained fly ash instead of fine sand [3, 35]. Nambiar and Ramamurthy [34] proposed that the water absorption of foamed concrete should be represented in kg/m3 of foamed concrete rather than as a percentage of original weight and by this expression, it increases with a reduction in the overall density. Since the absorption of water is mainly influenced by the paste, this trend is found because of the relatively lower paste volume for lower densities, which results in a smaller capillary pore volume. Similar results were obtained by Bagheri et al. [40]. They showed that the increase in the absorption for lower densities is due to the lower weight of the material itself and not necessarily due to a higher value of absorbed water. They also concluded that stating the absorption on the basis of volume will yield almost similar results at equal water-cement ratios regardless of the density of the foam concrete [40]. Permeability The permeability indices of foamed concrete (including air permeability, oxygen permeability, and water permeability) are known to increase with

Foamed concrete

247

a drop in the density. This increase in permeability is also faster than that seen for regular concrete [3, 14, 16, 33, 41]. However, Kearsley and her associates [18, 33] have reported that the oxygen permeability of foamed concrete with a density of 1500 kg/m3, was less than that of a normal weight concrete with a compressive strength of 25 MPa. Thus, it appears that high density foamed concrete could be at least as durable as normal concrete. The average coefficient of water permeability of some typical foamed concrete samples as measured in accordance with ISO/DIS 7031 [42] was in the order of 10−10 m/s where the dry density of those samples was in the range of 1500–1600 kg/m3 [14]. Freeze–thaw resistance Since the air-void network in foamed concrete provides the additional space required for hydraulic and osmotic pressures during freezing, it has a very good freeze–thaw resistance [3]. Jones and Giannakou [43] examined the freeze–thaw resistance of different foamed concrete mixes with sand or coarse fly ash as fine aggregates and also after replacing part of the cement with fine fly ash. Their study was according to Procedure B of the ASTM standard test method for resistance of concrete to rapid freezing and thawing (ASTM C666-97) [44]. The results show that using a fine fly ash as a supplementary cement admixture has no effect on the freeze–thaw resistance of foamed concrete. Moreover, while the mixes with a dry density of 1000 kg/m3 showed more expansion during the test than those with a density of 1400 kg/m3, they also exhibited more compressive strength at the end. This is probably due to the larger volume of pores available, that allows cementitious foams to accommodate the expansive forces more efficiently (Table 11.1). A method for assessment of the freeze–thaw resistance of pre-foamed cellular concrete has been introduced by Tikalsky et al. [27] based on the ASTM C666 test method. The results of their study show that the depth of absorption is key to developing a freeze–thaw-resistant foamed concrete. They also observed that the compressive strength, the depth of initial penetration, the absorption, and the absorption rate are all important variables in producing foamed concrete that is resistant to repeated cycles of freezing and thawing. On the other hand, it appears that density and permeability are not significant parameters in determining freeze–thaw resistance. Walkability Walkability is a term developed to describe the ability of controlled low strength cement composites to sustain normal construction pedestrian traffic without damage. It is judged by examining the surface distress, and in foamed concrete it improves with an increase in the density. ACI

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Table 11.1 Influence of alternating freeze/thaw cycles on linear expansion and corresponding compressive strength (taken on 100 mm cubes) for 1000 and 1400 kg/m3 foamed concretes (sealed-cured specimens) [43] Plastic density (kg/m3)

Cement type

1000

PC

1400

PC

1000

PC/30% FAfine

a b

Fines type

Sand Sand/FAcoarse FAcoarse Sand Sand/FAcoarse FAcoarse Sand FAcoarse

Resultant deterioration of test samples Expansion after 100 days (μstrain)

Strength of F/T specimens, % of 56d referenceb strength

460 446 295 278 at 63da 219 350 at 56da 480 330

83.5 107.6 97 77.8 132.8 118.4 82.4 372.2

Tests stopped as specimens had fractured Sealed-cured specimens

523.1R-06 [11] suggests that when heavy construction traffic is expected, the surface of the foamed concrete should be protected with wooden boards. Other issues in durability Chloride ingress Limited tests have shown that the performance of foamed concrete against chloride ingress is equivalent to that of a normal strength, normal weight concrete of 25 MPa compressive strength. The resistance to corrosion, as found from using an impressed current method, increases with a decrease in the density [33]. Carbonation Jones and McCarthy [35] investigated the resistance to carbonation of foamed concrete to see whether foamed concrete could be reinforced with carbon steel and exposed to the outdoor environment (Fig. 11.7). Their results show that foamed concrete has a poor resistance against carbonation with significantly higher carbonation than normal weight concrete at the same maturity.

11.7

Fiber reinforced foamed concrete

Since conventional foamed concrete (without any form of reinforcement) is brittle, addition of fibers – usually short fibers of glass, carbon or polymer

Foamed concrete

249

Carbonation depth (mm)

25

20 Density:1800 kg/m3, Fine agg: Sand 15 Density:1400 kg/m3, Fine agg: FA coarse

10

5

Density:1800 kg/m3, Fine agg: FA coarse

0

Density:1400 kg/m3, Fine agg: Sand 0

2

4

6

8

10

12

14

16

Test age (weeks)

11.7 Carbonation resistance in foamed concrete as seen for different types of fine aggregate [35].

– serves to mitigate the brittle nature of the material by imparting post cracking strength and toughness to the composite [3, 12, 35, 45]. The response of fiber reinforced cementitious foams is significantly influenced by the fibers and the low-density void structure of the cellular concrete matrix [45]. The more numerous pores have a finer air void network at lower densities. Papayianni and Milud [12] investigated cracking under compression in foamed concrete that contained only Portland cement and a mix with 30% high calcium fly ash as cement replacement. They added polypropylene fibers to the mixtures at a dosage rate of 1.3 kg/m3 (0.15% volume fraction) to reduce cracking. Similarly, polypropylene fibers have been used in foamed concrete structural elements to improve their compressive and shear strength [3, 35] and are known to bond well with the paste phase of the matrix. However, these fibers were seen to reduce the flowability and lessen the self-compacting property of foamed concrete. Not surprisingly, fiber reinforced foamed concrete is also used in the production of shock-absorbing concrete. In this case both the cell structure and the fiber reinforcement contribute to prevent fracture during impact loading on the composite. The cavities in the foam concrete limit the propagation of cracks upon impact through progressive collapse of the cell walls, while the fiber bridges the cracks that form [46]. Thus, the impulsive loading causes only localized damage. Zollo and Hays [45] investigated foamed concrete under high energy rapid rates of loading. This produced only localized damage in fiber reinforced cellular concretes. They concluded that this is likely due in part to the effect that the fibers have on the fracture toughness of fiber reinforced materials, and in part due to the residual tensile

250

Developments in the formulation and reinforcement of concrete

strength imparted by the fiber. The impact response is also significantly influenced by the low density void structure of the cellular concrete matrix. Lee and Liang [47] proposed a model to predict the overall elastic behavior and damage evolution in fiber reinforced cellular concrete. In their model, the material damage is assumed to occur by an interfacial fiber debonding mechanism and the nucleation of micro voids in the cement matrix. The fiber interfacial debonding was simulated by using a micromechanical damage model. This model had a good agreement with experimental results for both fiber reinforced regular concrete and fiber reinforced foamed concrete.

11.8

Applications

Due to its favorable properties including a very low density, low thermal conductance, superior flowability, self-compacting nature, and given the ease of manufacture and its relatively low cost, foamed concrete has found applications in many areas. Applications for foamed concrete include cavity filling and insulation (for lower densities) on the one hand and structural applications on the other (for higher densities). As it is capable of flowing under its own weight, it is an ideal material for voids (old sewers, basements, ducts, storage tanks and voids under roadways caused by heavy rain). Other applications of foamed concrete include (1) trench reinstatement, (2) road sub-base, (3) thermal and acoustic insulation, (4) production of lightweight blocks and pre-cast panels, (5) fire insulation, (6) soil stabilization and (7) shock absorbing barriers for airports and regular traffic. In North America, interest in foamed concrete has been widespread in all parts of Canada, the US, and Mexico. It is equally popular in other parts of the world, especially those regions with a housing shortage, or that have been subject to adverse weather, hurricanes, earthquakes, etc. In North America, the overall demand appears to be equal to the actual production, with demand being more from the southern US and regions with longer construction seasons. Particularly in Canada, cement based foam has been used for tunnel annulus grouting, flowable fills and other geotechnical applications over the years, but a keen interest and a tremendous expansion of use has happened over the past five years. This increased interest appears to be due in part to dramatic increases in the costs of other lightweight building materials, especially wood and dry wall and in part to the environmental issues associated with forestry or gypsum based products. An additional factor in favor of foamed concrete has been the manufacturing and environmental cost associated with cement production, as it allows for large volumes of supplementary cementing admixtures.

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251

Building product applications of cellular concrete have been used in Europe for over 50 years but have caught on in the US and Canada more recently. These include cellular wall panels, blocks, architectural items, and void fillers. In Canada, foamed concrete has been applied in the provinces of British Columbia, Alberta, Manitoba, Ontario and Quebec. In the US due to its warmer climate (as well as in the Canadian province of British Columbia), the largest use of foamed concrete is as a lightweight engineered fill. On the other hand, in the colder regions of the Canadian prairies and in Canada’s Northern Territories, cement based foam is used for its thermal properties. Foamed concrete is used in order to prevent frost heave in roads, under concrete paving, to insulate shallow foundation systems, to prevent frost jacking of shallow piles, to prevent frost heave under pile caps, to act as backfill under buried oil field modules, for tank support and to reduce the temperature under hot oil tanks. It is also useful to reduce the thermal gradient and the thermal stress in hot concrete pits and thus insulate shallow placements. As a filler it is used as a grout to fill abandoned pipes and fill voids under slabs. The annual market size for foamed concrete in the UK is estimated around 250,000–300,000 m3 (this excludes one very large mine stabilization project) [3, 28]. Data available for Western Canada indicates a market size of approximately 50,000 m3 per year. In the Middle East, the superior thermal insulation and lightweight nature of foamed concrete is seen as suited to reducing the adverse effect of earthquakes. In Holland, such foams are employed as road sub-base since the load to be carried is low. Because of its low density, when used in the construction of bridge abutments, it does not impose large lateral loads. Thus, significant savings in cost can be achieved by reducing the thickness of the walls and the size of the foundations. Engineers continue to use and find other applications for foamed concrete. In addition, it can easily be excavated if necessary and hence is more labor friendly at times of repair and rehabilitation.

11.9

Research needs

Although it has been extensively investigated during the 1960s and 1970s [48], several factors necessitate a fresh look at the science and technology of cement based foams. These include: (1) the development of advanced foaming agents, chemical and mineral admixtures, and reinforcement, (2) a growing environmental awareness regarding cement based products, (3) applications involving extreme loading, temperature, and environment, and (4) the ongoing quest for building materials with superior strength-toweight characteristics. Therefore, notwithstanding the attributes listed in

252

Developments in the formulation and reinforcement of concrete

the preceding discussion, foamed concrete has a significant potential for additional utilization, which can be achieved through further research in a number of areas. There is an ongoing need to develop chemical admixtures that do not affect the foam stability or cause segregation in the mix. For instance, superplasticizers that maintain the air void network will facilitate the addition of fibers and lightweight fillers to improve both the strength and the fracture toughness without raising the overall density. In addition, other chemical admixtures such as accelerators and retarders must be made compatible with the foaming agents to allow for larger volume of pours, with a reduced heat of hydration. There is a need to study the engineering properties of foamed concrete in greater detail. In particular, the modulus of elasticity, Poisson’s ratio and creep must be characterized to aid in structural design. In addition, the coefficient of thermal expansion and the specific heat of foamed concrete should be charted for a range of densities and composition. Although it has a good fire resistance, foamed concrete undergoes excessive shrinkage at very high temperatures and the reasons are unclear [49]. Research is needed to understand the mechanisms underlying this behavior. Our understanding of the properties of fiber reinforced cementitious foams is still very limited. In addition, there is a vast potential for the use of foamed concrete in combination with other materials such as fiber reinforced plastics, structural and non-structural coatings and in sandwich systems. Very little is known about the strain rate sensitivity of low density cement based systems. Clearly, more research is needed in order to describe the constitutive response of cement based foams and their composites under static and dynamic loading and varied environmental conditions.

11.10 Acknowledgements The authors would like to thank Dr J. Li and Mr E.R. Brooks (CematrixTM, Canada), Mr J. Timbrell (Lightconcrete LLC, USA), Mr G.J. Colaizzi (Goodson & Associates, USA), Mr L.A. Legatski (Elastizell Corporation of America, USA) and Dr K. Sobolev (Universidad Autónoma de Nuevo León, Mexico) for valuable discussions that contributed to the writing of this chapter.

11.11 References 1 Aldrige, D., (2005), “Introduction to Foamed Concrete: What, Why, How?”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 1–14. 2 Beningfield, N., Gaimster, R., and Griffin, P., (2005), “Investigation into the Air Void Characteristics of Foamed Concrete”, Proceedings: International

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4

5 6 7

8 9

10

11 12

13

14

15

16

17

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Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 51–60. Jones, M.R. and McCarthy, A., (2005), “Behavior and Assessment of Foamed Concrete for Construction Application”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 61–88. Goual, M.S., Bali, A., de Barquin, F., Dheilly R.M., and Quéneudec, M., (2006), “Isothermal Moisture Properties of Clayey Cellular Concretes Elaborated from Clayey Waste, Cement and Aluminium Powder”, Cement and Concrete Research, Vol. 36, pp. 1768–1776. ACI 523.2R-96, (1996), “Guide for Precast Cellular Concrete Floor, Roof, and Wall Units”, ACI Committee 523. British Cement Association (1994), “Foamed Concrete: Compositions and Properties”, Cambery, UK. Cox, L.S., (2005), “Major Road and Bridge Projects with Foam Concrete”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 105–112. Cox, L.S. and van Dijk, S., (2002), “Foam Concrete: A Different Kind of Mix”, Concrete (UK), Vol. 36, no. 2, February, pp. 54–55. Kearsley, E.P. and Mostert, H.F., (2005), “The Use of Foamed Concrete in Refractories” Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 89–96. Kearsley, E.P. and Mostert, H.F., (2005), “Opportunities for Expanding the Use of Foamed Concrete in the Construction Industry”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 143–154. ACI 523.1R-06, (2006), “Guide for Cast-in Place Low Density Cellular Concrete”, ACI Committee 523. Papayianni, I. and Milud, I.A., (2005), “Production of Foamed Concrete with High Calcium Fly Ash”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 23–28. Krivenko, P.V., Kovalchuk, G.Y., and Kovalchuk, O.Y., (2005), “Heat Resistant Cellular Concrete Based on Alkaline Cements”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 97–104. Lee, Y.L. and Hung, Y.T., (2005), “Exploitation of Solid Wastes in Foamed Concrete: Challenges Ahead”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 15–22. Kearsley, E.P. and Wainwright, P.J., (2001), “The Effect of High Fly Ash Content on the Compressive Strength of Foamed Concrete”, Cement and Concrete Research, Vol. 31, pp. 105–112. Kearsley, E.P. and Mostert, H.F., (2005), “Designing Mix Composition of Foamed Concrete with Fly Ash Contents”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 29–36. Kearsley, E.P. (1996), “The Use of Foamcrete for Affordable Development in Third World Countries: Appropriate Concrete Technology”, Proceedings:

254

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19

20 21 22 23 24

25 26 27

28

29 30 31

32

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International Conference on Concrete in the Service of Mankind, (Eds. RK Dhir & MJ McCarthy), London (UK), pp. 233–243. Kearsley, E.P., (1999), “Just Foamed Concrete: An Overview”, Proceedings: International Conference on Specialist Techniques and Materials for Concrete Construction, (Eds. RK Dhir & NA Henderson) University of Dundee, Scotland, September 8–10, pp. 227–237. Proshin, A.P., Beregovoi, V.A., Beregovoi, A.M., and Eremkin, A.I., (2005), “Unautoclaved Foam Concrete and its Construction Adapted to Regional Conditions”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 113–120. SABS 1491: Part II, (1989) “Portland Cement Extenders Part II: Fly Ash”, South African Standards. ASTM C618–05, (2005), “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete”, ASTM International. Kearsley, E.P. and Wainwright, P.J., (2002), “Ash Content for Optimum Strength of Foamed Concrete”, Cement and Concrete Research, Vol. 32, pp. 241–246. Jones, M.R. and McCarthy, A., (2005), “Utilization of Unprocessed Low-Lime Fly Ash in Foamed Concrete”, Fuel, Vol. 84, no. 11, pp. 1398–1409. Jones, M.R. and McCarthy, A., (2006), “Heat of Hydration in Foamed Concrete: Effect of Mix Constituents and Plastic Density”, Cement and Concrete Research, Vol. 36, pp. 1032–1041. Nambiar, E.K.K. and Ramamurthy, K., (2007), “Air-Void Characterization of Foam Concrete”, Cement and Concrete Research, Vol. 37, pp. 221–230. Timbrell, J., (2007), “Private Communication”, Light Concrete LLC. Tikalsky, P.J., Pospisil, J., and MacDonald, W., (2004), “A Method for Assessment of the Freeze–Thaw Resistance of Preformed Foam Cellular Concrete”, Cement and Concrete Research, Vol. 34, pp. 889–893. Beningfield, N., Gaimster, R., and Griffin P., (2005), “Investigation into the Air Void Characteristics of Foamed Concrete”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 51–60. ASTM C796–04, (2004), “Standard Test Method for Foaming Agents for Use in Producing Cellular Concrete Using Preformed Foam”, ASTM International. ASTM C869–06, (2006), “Standard Specification for Foaming Agents Used in Making Preformed Foam for Cellular Concrete”, ASTM International. Hamidah, M.S., Azmi, I., Ruslan, M.R.A., Kartini, K., and Fadhil, N.M., (2005), “Optimization of Foamed Concrete Mix of Different Sand-Cement Ratio and Curing Conditions”, Proceedings: International Conference on the Use of Foamed Concrete in Construction, University of Dundee, Scotland, July 5, pp. 37–44. Wee, T-H, Babu, D.S., Tamilselvan, T., and Lim, H-S, (2006), “Air-Void System of Foamed Concrete and its Effect on Mechanical Properties”, ACI Material Journal, Vol. 103, no. 1, pp. 45–52. Kearsley, E.P. and Booysens, P.J. (1998), “Reinforced Foamed Concrete: Can it be Durable?” Concrete Beton, no. 91, November, pp. 5–9. Nambiar, E.K.K. and Ramamurthy, K., (2006), “Influence of Filler Type on the Properties of Foam Concrete”, Cement & Concrete Composites, Vol. 28, pp. 475–480.

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35 Jones, M.R. and McCarthy, A., (2005), “Preliminary Views on the Potential of Foamed Concrete as a Structural Material”, Magazine of Concrete Research, Vol. 57, no. 1, pp 21–31. 36 Kearsley, E.P. and Wainwright, P.J., (2002), “The Effect of Porosity on the Strength of Foamed Concrete”, Cement and Concrete Research, Vol. 32, pp. 233–239. 37 Nehdi, M., Khan, A., and Lo, K.Y., (2003), “Development of Deformable Protective System for Underground Infrastructure using Cellular Grouts”, ACI Materials Journal, Vol. 99, no. 5, pp. 490–498. 38 Hoff, G.C., (1972), “Porosity-Strength Considerations for Cellular Concrete”, Cement and Concrete Research, Vol. 2, pp. 91–100. 39 Babu, D.S., Wee, T-H and Tamilselvan, T., (2005), “Mechanical Properties of Foamed Concrete with and without Aggregates”, Proceedings (CD-ROM): ConMat ’05 and Mindess Symposium, Vancouver, August 2005. 40 Bagheri, T., Parhizkar, A.M., and Ghasemi, R., (1999), “Foam Concrete: Properties and Application Areas”, Proceedings: Annual Conference of the Canadian Society of Civil Engineering, Regina, Canada, June 2–5, pp. 101–110. 41 Kearsley, E.P. and Wainwright, P.J., (2001), “Porosity and Permeability of Foamed Concrete”, Cement and Concrete Research, Vol. 31, pp. 805–812. 42 ISO/DIS 7031, (1983), “Concrete Hardened: Determination of the Depth of Penetration of Water under Pressure”, International Standards Organization. 43 Jones, M.R. and Giannakou, A., (2004), “Thermally Insulating Foundations and Ground Slabs Using Highly-Foamed Concrete”, Journal of ASTM International, Vol. 1, no. 6, pp. 100–112. 44 ASTM C666-97, (1997), “Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing”, ASTM International. 45 Zollo, R.F. and Hays, C.D., (1998), “Engineering Material Properties of a Fiber Reinforced Cellular Concrete”, ACI Material Journal, Vol. 95, no. 5, pp. 631–635. 46 Weiss Jr., C.A., Tom, J.G., and Malone, P.G., (2005), “Foamed Fiber-Reinforced Concrete as a Construction Material for Live-Fire Training Ranges”, Proceedings (CD-ROM): ConMat ’05 and Mindess Symposium, Vancouver, August 2005. 47 Lee, H.K. and Liang, Z., (2004), “Computational Modeling of the Response and Damage Behavior of Fiber Reinforced Cellular Concrete”, Computers and Structures, Vol. 82, pp. 581–592. 48 Hoff, G.C., (2003), “Discussion of ‘Development of Deformable Protective System for Underground Infrastructure using Cellular Grouts’, by Nehdi, Khan and Lo”, ACI Materials Journal, Vol. 100, no. 4, pp. 350–351. 49 Sach, J. and Seifert, H., (1999), “Foamed Concrete Technology: Possibilities for Thermal Insulation at High Temperatures”, CFI Forum of Technology, DKG 76, no. 9, pp. 23–30.

12 Polymer concrete Y OHAMA, Nihon University, Japan

12.1

Introduction

Polymer concrete is the composite material made by fully replacing the cement hydrate binders of conventional cement concrete with polymer binders or liquid resins, and is a kind of concrete-polymer composite. For hardening of polymer concrete, most liquid resins such as thermosetting resins, methacrylic resins and tar-modified resins are polymerized at ambient or room temperature. The binder phase for polymer concrete consists only of polymers, and does not contain any cement hydrates. The aggregates are strongly bound to each other by polymeric binders. The advantages and disadvantages of polymeric binders are directly given to the polymer concrete. Accordingly, in comparison with ordinary cement concrete, its properties such as strength, adhesion, watertightness, chemical resistance, freeze-thaw durability and abrasion resistance are generally improved to a great extent by polymer replacement. Since the bond between polymeric binders and aggregates is very strong, its strength properties depend on those of the aggregates. On the other hand, its poor thermal and fire resistance and its large temperature dependence of mechanical properties are disadvantages due to the undesirable properties of the polymer matrix phases. Therefore, the glass transition point (or temperature) of the polymer matrix phases should be noted from the viewpoint of such thermal properties. Thermoplastic resins generally retain their practical properties at temperatures below the glass transition point and lose them at temperatures exceeding the point, beginning to thermally decompose at somewhat higher temperatures. The practical temperature range of the thermoplastic resins may be improved by the addition of suitable cross-linking monomers or comonomers having higher glass transition points. Thermosetting resins do not commonly show a glass transition point, and retain their mechanical properties up to the thermal decomposition temperature. Such essential disadvantages of the polymer concrete can be considerably improved by 256

Polymer concrete

257

controlling the necessary polymeric binder content by volume to a minimum. The history of the research and development of polymer concrete is relatively short compared with that of conventional cement concrete. The early research and development of the polymer concrete was done mainly in the Soviet Union (currently, Russia),1 the United States,2 Germany3 and Japan4 in the late 1950s to the early 1960s.

12.2

Production techniques for polymer concrete

The process technology of polymer concrete is much the same as that of conventional cement concrete except for the use of liquid resins for polymeric binders and curing methods. Materials for polymer concrete are the liquid resins as shown in Fig. 12.1, fillers such as ground calcium carbonate and silica flour, the same fine and coarse aggregates as those used for cement concrete, etc. The most common types of the liquid resins are unsaturated polyester resin (i.e., polyester-styrene system), methacrylic resin and epoxy resin. For the part 20 years, the application of recycled monomers or polymers to liquid resins has been tried.5–9 As the liquid resins themselves cannot set or harden, proper initiators (or hardeners) and promoters are selected, and added to the liquid resins at the mixing of the polymer concrete. The working life and hardening time of the polymer concrete can be controlled by suitably selecting the type and content of the initiators (or hardeners) and promoters. Since the moisture in and on fillers and aggregates has a deleterious effect on the hardening reactions of the liquid resins and reduces the bond between most polymeric binders and fillers or aggregates, the fillers and aggregates are used after drying to suitable moisture content of less than 0.5 or 1.0%. As the strength properties of the polymer concrete are governed by those of the aggregates (as mentioned above), the use of aggregates with high strength is required to make

Thermosetting resin Liquid resin Tar-modified resin Methacrylic resin

Unsaturated Orthophthalate-type polyester (UP) Isophthalate-type Epoxy (EP) Furan (Furfural-acetone, etc.) Vinyl ester (VE) Polyurethane (PUR) Phenol (PF)

Shrinkage-type Nonshrinkage-type

Tar-epoxy Tar-urethane Methyl methacrylate (MMA) Glycerol methyl methacrylate-styrene

12.1 Classification of liquid resins for polymer concrete.

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Developments in the formulation and reinforcement of concrete

high-strength polymer concrete. The interfacial bond between polymeric binders and aggregates is generally improved by using silane coupling agents, which are directly added to the liquid resins prior to mixing. In the structural applications of polymer concrete, mild steel bars (for conventional cement concrete), PC bars (i.e., high-strength steel bars for prestressed concrete), and FRP (fiber-reinforced plastics) rods are used to reinforce the structural members, and steel fibers, glass fibers, etc., as mixable reinforcements are done to reinforce the polymer concrete itself. In general, the manufacturing process of polymer concrete is divided into two types: precast and cast-in-place application systems. The precast application system at factories is widely employed under the present conditions. For purpose of reducing the cost of polymer concrete and improving its strength, durability and other properties, it is most important to find out the effective mix proportions of the liquid resins and aggregates. Although the liquid resins are toxic and flammable, the use of the safety procedures that have been well established for them allows them to be handled without undue difficulty. The common procedures for the mix design or mix proportioning of polymer concrete are as follows: (1)

Arrange aggregate gradation to give the lowest possible void volume or close-packed state by mixing the aggregates with different size distributions on the basis of continuous or gap grading. (2) Mix the aggregates that have the lowest possible void volume with a liquid resin to make polymer concrete. (3) Check up on the basic properties such as workability, bleeding or segregation, and strength of the polymer concrete, and decide the optimum mix proportions. Regardless of the liquid resin type, the typical mix proportions (by mass) of the polymer concrete widely used are liquid resins to fillers to fine/coarse aggregates mixtures = 1 : (1 to 1.5) : (8 to 8.5). The liquid resin content is normally around 10% (mass fraction). The working life and hardening time of any polymer concrete are determined according to its applications and ambient temperatures, and they can be controlled by the suitable selection of the type and content of the initiators (or hardeners) or promoters as stated above. The manufacture and execution of the polymer concrete are made by use of almost the same mixers and execution tools as those for cement concrete. To avoid heterogeneous mixing due to the rapidhardening reaction and high-viscosity binder formulations, the polymer concrete must be mechanically mixed using mixing equipment; mixing should never be done by hand. Mixing equipment is classified into two types: discontinuous or batch mixers such as forced mixing-type mixers used for conventional cement concrete, and continuous mixers such as “Respecta” and “Concrete Mobile”. The former are widely used throughout the world.

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259

The latter are refined continuous mixers with automatically variable metering of the materials used. After mixing, the fresh polymer concrete is placed and finished in a manner similar to conventional cement concrete within its working life. Depending on its applications, the polymer concrete is cast into the molds or forms treated with effective mold-release agents such as silicone wax or greases, and fluoro resins. In the manufacture of precast products using the polymer concrete, the following three types of molding processes are applicable:10 (1) (2) (3)

Casting process in which the polymer concrete is cast into molds or forms, and consolidated by using suitable vibrators. Centrifugal molding process in which the polymer concrete is placed in rotary cylindrical molds, followed by applying centrifugal force. Hot press molding process in which the polymer concrete is placed in hot press molds, and pressed under heating.

Molding process selection depends on the type of the liquid resin used, the shape, size and output of products. Because polymer concrete generally has an excellent adhesion to various materials, all the equipment and tools such as mixers, trowels, shovels and vibrators should be cleaned immediately after use or at least within their working life. The use of the mold-release agents is indispensable for the molds or forms. Polymer concrete can generally be placed in a temperature range of 0 to 50°C. Polymer concrete using methacrylic resin-based binders can exceptionally be placed at temperatures of −20 to −25°C. In the applications of the polymer concrete, polymerization and curing processes are usually accomplished through an initiator-promoter system. The polymer concrete is subjected to ambient temperature cure, heat cure or combined ambient temperature/heat cure. As the hardening of the polymer concrete is obstructed by moisture, the polymer concrete during curing should be protected from the effect of moisture. It is most important that the developments of mass production systems for precast polymer concrete and of automated application systems for cast-in-place polymer concrete should aim at cost reduction and that a good balance between economy and performance should be achieved.

12.3

Practical applications, recycling and quality standards

In Japan and Europe, polymer concrete was already the dominant construction material in the 1970s; in the United States, polymer concrete became the dominant construction material in the 1980s. Polymer concrete is currently used as a common construction material in various applications globally because of its high performance, multifunctionality and sustain-

260

Developments in the formulation and reinforcement of concrete

ability compared with conventional cement concrete. At present, polymer concrete using mainly unsaturated polyester resin and methacrylic resin as liquid resin is the most widely used for various structural and nonstructural precast products, and it has few cast-in-place applications in Japan. Table 12.1 lists the applications of polymer concrete in Japan. In the United States, polymer concrete is chiefly used as patching materials for repair work and overlays for bridge decks in cast-in-place applications, and in precast applications as shown in Table 12.2.11,12 In Europe, precast applications such as machine tool structures, building panels, utility boxes and underground junction boxes of the polymer concrete are common, and the development of its machine tool structures is especially active in Germany and Switzerland.

12.3.1 Recycling and reuse of industrial waste for polymer concrete In recent years, various waste such as waste polymers, waste woods, waste tires and rice husks have been discharged in large quantities from various industrial fields, and effective recycling or reusing countermeasures against Table 12.1 Applications of polymer concrete in Japan Application

Location of work

Structural precast products

Manholes and handholes for telecommunication cable lines, electric power cable lines and gas pipelines, prefabricated cellars or stockrooms, tunnel liner segments for telecommunication cable lines and sewerage, pipes for sewage, hot spring water and seawater, piles for port or hot spring construction, FRPreinforced frames or panels for buildings, machine tool structures, e.g. beds and saddles, etc.

Nonstructural precast products

Gutter covers, U-shaped gutters, footpath panels, permanent forms for checkdams with acidic water and offshore or marine structures, terrazzo tiles and panels, and large-sized or curved decorative panels for buildings, partition wall panels, sinks, counters, washstands, bathtubs, septic tanks, electrolytic tanks, works of art, e.g. carved statues and objets d’art, tombs for Buddhists, etc.

Cast-in-place applications

Spillway coverings in dams, protective linings of stilling basins in hydroelectric power stations, coverings of checkdams, foundations of buildings in hot spring areas, acid-proof linings for erosion control dams with acidic water, patch materials for damaged concrete structures, overlays for pavement repairs, overlay strengthening for bridge decks, drainage pavements using porous polymer concrete, etc.

Polymer concrete

261

Table 12.2 Applications of polymer concrete in the United States Application

Location of work

Precast applications

Transportation applications such as railroad crossings, railroad ties, median barriers, etc. Structural and building panels Sewer pipes, equipment vaults, drainage channels, etc. Corrosion-resistant tiles, bricks and linings Small water-flow control structures Stair treads and nosings Nonconductive, nonmagnetic support structures for electrical equipment Manhole structures and shims Components for the animal-feeding industry Large-scale preinsulated wall panels for segmental building construction Electrical insulators Machine tool bases

Cast-in-place applications

Patching materials for reinforced concrete structures Overlays for reinforced concrete structures in the transportation industry

Table 12.3 Industrial wastes recyclable or reusable as polymeric binders (liquid resins) for polymer concrete Type of industrial waste

Recycling or reusing technique

Waste polystyrene (EPS)

Manufacture of EPS solutions by dissolving waste EPS in vinyl monomers such as styrene and methyl methacrylate → Utilization of EPS solutions as liquid resins and as shrinkage-reducing agents for polyester concrete

Waste polyethylene terephthalate (PET)

Manufacture of unsaturated polyester resins for liquid resins by chemical recycling of waste PET

Waste thermoplastic resins

Manufacture of reclaimed (or regenerated) thermoplastic resins for polymeric binders by melting waste thermoplastic resins under heating → Manufacture of polymer concrete products with reclaimed thermoplastic resins (1) Bars, rods, plates and piles, specified by JIS K 6931:1991 (Reclaimed plastics bars, rods, plates and piles) (2) Piles for survey and boundary, specified by JIS K 6932:1991 (Reclaimed plastics piles for survey and boundary) (3) Inspection chambers and covers for rainwater, specified by JIS A 5731:2002 (Recycled plastics inspection chambers and covers for rainwater)

262

Developments in the formulation and reinforcement of concrete

industrial waste have been strongly requested internationally. Against such a social background, polymer concrete is considered to be a two-phase composite which is composed of “polymeric binder” and “aggregate”, and the recyclability and reusability of the industrial wastes as the polymeric binder and the aggregate are examined in Japan.13 Tables 12.3 and 12.4 show the industrial wastes recyclable or reusable as polymeric binders, and the aggregates and fillers for polymer concrete, respectively.13 For the purpose of effectively utilizing industrial waste for the manufacture of polymer concrete, we can use the appropriate combinations of the polymer binders and the aggregates and fillers as shown in Tables 12.3 and 12.4. Figure 12.2 illustrates the typical manufacturing process for Table 12.4 Industrial waste recyclable or reusable as aggregates and fillers for polymer concrete Type of industrial waste aggregate and filler

Recycling or reusing technique

Recycled aggregates

Granular materials made by crushing waste hardened polymer concrete from precast polymer concrete factories

Blast-furnace slag aggregates and fillers

Granular materials made by crushing blast-furnace slag from the iron industry

Coal ash aggregates and fillers

Artificial lightweight aggregates made by granulation of coal ash, and fly ash for fillers from thermal power stations

Waste glass fine aggregates

Granular materials (cullet) made by crushing waste glass (e.g., glass bottles), and foam glass made by sintering cullet

Waste wood aggregates

Wood wools, wood flakes, wood chips, wood powders made by flaking, chipping or chopping waste woods from the construction industry, lumber industry and forestry

Waste plastics aggregates and fillers

Granular materials and powders made by crushing waste plastics (1) Waste fiber-reinforced plastics (FRP) aggregates and fillers from FRP bathtubs and fishing boats (2) Waste PET chips from waste PET bottles (3) Waste polypropylene and polyethylene chips from packaging films (4) Waste polyvinyl chloride (PVC) chips from PVC films for agriculture

Waste rubber aggregates

Granular, powdered or chip-like materials by crushing waste car tires

Rice-husk aggregates

Rice-husking or -hulling in rice crop

Polymer concrete Waste EPS

EPS Crosslinking agent Styrene

Crushing of EPS

Filler

263

Dissolution of EPS

Liquid resin

Coupling Promoter Initiator agent

Binder

Fine Coarse aggregate aggregate

Dry mixing

Polymer concrete

Curing

Demolding

Mixing of polymer concrete

Casting of polymer concrete

12.2 Manufacturing process for precast polymer concrete products using EPS-styrene solution by casting method.

precast polymer concrete products using EPS-styrene solutions as liquid resins.13

12.3.2 Standardization work Since the early research and development of polymer concrete in the late 1950s to the early 1960s, their research and development has actively been carried out. As a result, polymer concrete is one of the most common construction materials at present in advanced countries such as the United States, Germany and Japan. Standardization work on the quality requirements, test methods and executions for the polymer concrete has been in progress in such advanced countries. Tables 12.5, 12.6 and 12.7 list the international and national standards, and recommendations for polymer concrete, which have been published up to the present time.

12.4

Future trends

Polymers in concrete or concrete-polymer composites including polymermodified concrete, polymer concrete and polymer-impregnated concrete have been developed for over the past 50 years. Polymer-impregnated

Table 12.5 International and national standards, and recommendations (guides) for polymer concrete (Part 1) Type of standard or recommendation (guide)

Number and title of standard or recommendation (guide)

Quality requirements

JIS A 5350:1991 JIS A 5731:2002 JIS K 6931:1991 JIS K 6932:1991 ASTM C 267-01

Test methods

ASTM C 413-01

ASTM C 531-00 (2005)

ASTM C 579-01

ASTM C 580-02

ASTM C 905-01

DIN 512901:1991 DIN 512902:1991

DIN 512903:1991

DIN 512904:1991

JIS A 1181:2005

(Fiberglass reinforced plashic mortar pipes) (Recycled plastics inspection chambers and covers for rainwater) (Reclaimed plastics bars, rods, plates and piles) (Reclaimed plastics piles for survey and boundary) Standard test methods for chemical resistance of mortars, grouts, and monolithic surfacings, and polymer concretes Standard test method for absorption of chemical-resistant mortars, grouts, monolithic surfacings, and polymer concretes Standard test method for linear shrinkage and coefficient of thermal expansion of chemical-resistant mortars, grouts, monolithic surfacings, and polymer concretes Standard test method for compressive strength of chemical-resistant mortars, grouts, monolithic surfacings, and polymer concretes Standard test method for flexural strength and modulus of elasticity of chemicalresistant mortars, grouts, monolithic surfacings, and polymer concretes Standard test methods for apparent density of chemical-resistant mortars, grouts, monolithic surfacings, and polymer concretes Testing of polymer concretes (reactive resin concretes) for mechanical engineering purposes; terminology Testing of polymer concretes (reactive resin concretes) for mechanical engineering purposes; testing of binders, fillers and reactive resin compounds Testing of polymer concretes (reactive resin concretes) for mechanical engineering purposes; testing of separately manufactured specimens Testing of polymer concretes (reactive resin concretes) for mechanical engineering purposes; in-process testing and testing of final parts (Test methods for polymer concrete)

Polymer concrete

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Table 12.6 International and national standards, and recommendations (guides) for polymer concrete (Part 2) Type of standard or recommendation (guide)

Number and title of standard or recommendation (guide)

Test methods

RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) Recommendations for test methods for concrete-polymer composites (1995) PC-1 Method of making samples of polymer concrete and mortar in the laboratory PC-2 Method of making polymer concrete and mortar specimens PC-3 Method of test for slump and flow of fresh polymer concrete and mortar PC-4 Determining methods for working life of fresh polymer concrete and mortar PC-5 Method of test for compressive strength of polymer concrete and mortar PC-6 Method of test for splitting tensile strength of polymer concrete and mortar PC-7 Method of test for flexural strength of polymer concrete and mortar PC-8 Method of test for static elastic modulus of polymer concrete and mortar PC-9 Method of test for adhesion of polymer concrete and mortar to cement concrete PC-10 Method of test for bond strength of polymer concrete and mortar to reinforcing steel bars PC-11 Method of test for water absorption of polymer concrete and mortar PC-12 Method of test for chemical resistance of polymer concrete and mortar PC-13 Method of test for coefficient of thermal expansion of polymer concrete and mortar American Concrete Institute (ACI) ACI 548.7-04 Test method for load capacity of polymer concrete underground utility structures

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Developments in the formulation and reinforcement of concrete

Table 12.7 International and national standards, and recommendations (guides) for polymer concrete (Part 3) Type of standard or recommendation (guide)

Number and title of standard or recommendation (guide)

Practice or execution

Architectural Institute of Japan (AIJ) AIJ Recommendation for practice of concrete-polymer composites (2001) The Society of Materials Science, Japan (JSMS) JSMS Guide for mix design of polyester resin concrete (1992) JSMS Recommendation for structural design of polymer concrete (2006) ACI 548.1R-97 (98) Guide for the use of polymers in concrete Federal Ministry for Transport, The Federal Länder Technical Committee, Bridge and Structural Engineering (Germany) TP BE-PC Technical test regulations for concrete replacement systems using reactive resin mortar/reactive resin concrete (PC) (1990) TL BE-PC Technical delivery conditions for concrete replacement systems using reactive resin mortar/reactive resin concrete (PC) (1990)

concrete, which was developed with great promise has already disappeared from the construction industry except for a few applications because of its bad cost-performance balance. The concrete-composite industry seems to have matured. Although polymer concrete is currently used as a common construction material in various applications globally, it appears to have reached a plateau in terms of new research and development and in the consumption of materials.14 As the binders for polymer concrete are only liquid resins such as unsaturated polymer resin, epoxy resin, methacrylic resin and polyurethane, the future growth of polymer concrete may depend considerably on the relative cost of the liquid resins.15 An important raw material for any liquid resin is petroleum. With an increase in the cost of petroleum in the present world economic trends, the likelihood of the liquid resin becoming more expensive seems a foregone conclusion.15 This will provide many difficulties for the future development or growth of polymer concrete. In order to regain the momentum of the development of polymer concrete in the future, significant advances or innovations in the materials process technology and applications will be needed as follows:15

Polymer concrete (1)

267

Materials a. Lower-cost liquid resins from recyclable or reusable industrial wastes such as waste EPS, PET and thermoplastic resins (see Table 12.3). At present, the cost of recycled or reused liquid resins is always less than that of virgin liquid resins. With increased emphasis on saving resources in sustainable development, the cost is likely to be reduced. b. Liquid resins that are chemically designed to be easily recycled or reused after their use. The development of such liquid resins is also requested from the viewpoint of saving resources. c. Liquid resins with a low volatile monomer content or low-odor liquid resins from the viewpoint of ecological safety in sustainable development. d. Liquid resins that are hardened to high-performance polymers with low setting shrinkage, small temperature dependence of mechanical properties, good weatherability and incombustibility. e. Liquid resins that are chemically designed to make lowtemperature curing (at −20 to −30°C) and underwater placing possible or able to use wet aggregates. f. Lower-cost aggregates and fillers from recyclable or reusable industrial wastes such as recycled polymer concrete aggregates (from polymer concrete products factories), blast-furnace slags, coal ashes, waste glass, waste wood, waste plastics, waste tires and rice-husks (see Table 12.4). g. High-strength aggregates for high-strength polymer concrete products, which are expected to have a high strength-to-mass ratio in structural applications. (2) Process technology a. Improved batching or mixing processes for manufacturing denser, void-free polymer concrete.15 b. Improved mix proportioning or mix design methods for more efficient aggregate close-packing with the objective of requiring less liquid resin content but providing adequate workability15 because of the cost reduction of polymer concrete. (3) Applications a. Rapid, user-friendly, reduced labor systems for more efficient use of polymer concrete in various applications, in order to make it cost-competitive and safer to install.15 b. Replacements with polymer concrete for metal castings. The higher strength-to-mass ratio, much easier fabrication, reduced machining and improved insulation of the polymer concrete offer many advantages for various new applications.15

268

Developments in the formulation and reinforcement of concrete c.

Containment or encapsulation vessels using polymer concrete for toxic and/or hazardous wastes such as heavy metals and radioactive wastes.15 d. Components or members using intelligent polymer concrete with crack-autohealing (or crack-self-healing) or deflection-autostraightening function. Examples of such components or members are containment or encapsulation vessels that could autoheal cracks; beams that could autostraighten excessive deflection; and foundation supports that could autoadjust for differential settlement.15

12.5

Sources of further information and advice

The publications which can be recommended for the further information of polymer concrete include the following: Chandra, S. and Ohama, Y. (1994). Polymers in Concrete. CRC Press, Boca Raton. ACI Committee 548 (1996). Polymer Concrete – Structural Applications – State-ofthe-Art Report, ACI 548. 6R-96. American Concrete Institute, Farmington Hills. Kaeding, A.O. and Prusinski, R.C. Eds. (2003). Polymers in Concrete: The First Thirty Years, SP-214. American Concrete Institute, Farmington Hills.

12.6

References

1 Itinskii V I, Oster-Volkov N N and Kamenskii I V, Plastic-concrete in dam construction, Sov Plast, 1962, 9, 59–61. 2 Simpson W C, Sommer H J, Griffin R L and Miles T K, Epoxy Asphalt concrete for airfield pavements, J Air Transp Div, Proc Am Soc Civ Eng, 1960, 86(AT1), 57–70. 3 Liesegang H, Plastics in concrete, Plastics, 1962, 27(297), 62–64. 4 Murai N and Mizuno S, Thermosetting plastic swelled with grainy fillers (plastic concrete), Rev Electrical Commun Lab, 1961, 9(9/10), 581–588. 5 Ohama Y, Demura K, Kobayashi T and Dholakia C G, Properties of polymer mortars using reclaimed methyl methacrylate, Mater Eng, 1989, 1(1), 97–104. 6 Rebei K S and Fowler D W, “Properties of plain and reinforced polymer concretes made with recycled PET”, Proc Int RILEM Workshop Disposal and Recycling of Organic and Polymeric Construction Materials, London, E & FN Spon, 1995, 3–11. 7 Choi N W and Ohama Y, “Development of new polymer mortars using styrene solution of waste expanded polystyrene”, Proc 4th Asia Sym Polymers in Concrete, Chuncheon (Korea), Kangwon National University, 2003, 125–133. 8 Choi N W, Moroka A and Ohama Y, “Properties of polymer mortars using methyl methacrylate solutions of waste expanded polystyrene”, Proc 4th Asia Sym Polymers in Concrete, Chuncheon (Korea), Kangwon National University, 2003, 105–113.

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269

9 Jo B W, Park S K and Kim C H, “Mechanical properties of polyester polymer concrete using recycled polyethylene terephthalate”, ACI Mater J, 2006, 103(2), 219–225. 10 Hibino M, Imai T, Kido M and Matsushita H, “Properties of polymer concrete and the structural use II: Process technology of polymer concrete (in Japanese)”, J Soc Mater Sci J, 2005, 54(10), 1087–1098. 11 ACI Committee 548, Polymer Concrete – Structural Applications – State-of-theArt Report, ACI 548. 6R-96, American Concrete Institute, Farmington Hills, 1996. 12 ACI Committee 548, Guide for the Use of Polymers in Concrete, ACI 548. IR-97, American Concrete Institute, Farmington Hills, 1997. 13 Ohama Y, Trends and prospects in uses of industrial and municipal wastes as concrete materials in Japan (in Japanese), Cement Concrete (Semento Konkurito), 2003, 678, 1–8. 14 Fowler D W, State of the art in concrete polymer materials in the U.S., Proc 12th Int Congr Polymers in Concrete, Vol. 1, Chuncheon (Korea), Kangwon National University, 2007, 29–36. 15 Fowler D W, “Polymers in concrete: Where have we been and where are we going?”, Polymers in Concrete: The First Thirty Years, SP-214, American Concrete Institute, Farmington Hills, 2003, 111–117.

13 Future developments in concrete L Czarnecki, Warsaw University of Technology, Poland, W Kurdowski, Institute of Mineral Building Materials, Poland and S Mindess, University of British Columbia, Canada

13.1

Introduction

Forecasting future development is a difficult task, and for obvious reasons entails a considerable degree of uncertainty. There are no logical premises for anticipating the future on the basis of past experiences. “The fact that in the past the future was similar to the past does not mean that this will indeed be the case in future” [1]. Neither does induction, as an attempt at transforming partial information into full information, have any justification in the theory of information. The future is difficult to forecast, but forecasting is nevertheless necessary [2]. Those whose objectives are unclear do not know how to use the driving force of change for their own development [3]. As Seneca the Elder once stated, “No wind is beneficial to a sailor who does not know his port of destination.” In general, forecasts are formulated on the basis of the expectations of specialists engaged in the development of a particular field. This also holds true for concrete. In recent years, a number of publications dealing with the future of concrete have appeared [4–8]. The present study constitutes a further element of this cycle of publications. The authors have concentrated on tendencies shaping the development of concrete; these may turn out to be more or less permanent in the future. However, substantiated practical conclusions may be drawn from these for at most the next 30 years [3, 7]; the future thereafter appears steadily less clear. Concrete is a very special subject for forecasting. It is characterized by a very short period of workability (i.e., mixing, transporting and placing) followed by a very long useful life in the hardened state. The workability of a concrete mix ranges from a few dozen minutes to a few hours, while its serviceability after hardening should be at least 50 years. The period of service life is the state in which the concrete in a structure satisfies the performance requirements for the structure in question. Concrete structures generally become a permanent element of the landscape. Concrete is the oldest artificial material currently used in technology. The premise 270

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Table 13.1 Annual production of some common materials Concrete

∼6 billion cubic metres ∼13 billion tonnes

Portland cement Salt Sugar Oil

∼1.6 billion tonnes ∼200 million tonnes ∼135 million tonnes ∼5.2 billion tonnes

that concrete does have a future is supported by its long history, which stretches back over 8000 years (Syria, 6500 bc), with nearly 200 years of “modern” concrete (dating from the Aspdin patent for Portland cement in 1824). Materials (4th place) and structures (9th place) are classified amongst the 12 most dynamically developing areas of the greatest significance for society [3]. This ranking of “innovative importance” is typical: materials before structures. Progress in materials continues to anticipate the development of structures. Currently, concrete is the most widely used material (Table 13.1) of all those created by man, and is second only to water in its use. The annual per capita production of concrete is almost 1 cubic metre. Thus, any forecasts for the future of concrete concern one of the basic building blocks of contemporary civilization.

13.2

Does concrete have a future?

In the years 2003–2004, the Polish journal “Construction – Technologies – Architecture” was the forum of a discussion focusing on the topic “Why concrete does have a future” [9]. All of those participating in the discussion were absolutely certain that concrete does have a future, and would remain the world’s pre-eminent construction material if not forever, then at least as far as they could foresee. However, certain conditions should be met for this to occur. Concrete must undergo constant change and development, as has been the case to date. Progress in concrete technologies should constitute an inherent element of sustainable development. For instance, the history of admixtures shows that even at very small addition rates, they may enormously affect the technical properties of concrete. The statement that “concrete is a material of the future for building the world” is based on the rational notion that concrete has an enormous, though still incompletely identified, “modification potential.” Hand in hand with innovations in the way in which we produce and use concrete, we must not forget the human factors involved. As Brandt [10] has stated, “it is not concrete that should change, but people!” It is difficult to overrate the importance of developing

272

Developments in the formulation and reinforcement of concrete

and improving professional staff. The path leading to the concrete technology of the future should be based on knowledge, and this knowledge must be developed.

13.3

General factors influencing the development of concrete

Annually, more than five thousand concrete-related articles are published around the world [11]. A recent internet search using the YahooTM search engine revealed about 1.78 million entries related to “Portland cement concrete.” Yet, despite the immense quantities of concrete produced everywhere in the world, it remains a much under-appreciated material. For instance, in the European White Book on basic research in materials science [12], the term “concrete” appears only once! However, the role of concrete globally, particularly in its consumption of natural resources, cannot be over-estimated. Annually, concrete production requires about 10 billion tonnes of aggregate and about 0.8 billion tonnes of water (about 0.5% of the total consumption of water). As well, about 500 billion MJ of energy are consumed. Of particular concern, the production of Portland cement generates about 5–7% of global CO2 emissions: the production of one tonne of concrete leads to the emission of about one tonne of CO2. The future development of an industry that consumes so much material and energy cannot remain ignored by society, particularly when concrete also constitutes a response to the basic needs of society in terms of its civilian (and military) infrastructure. As populations continue to grow, and the standard of living in much of the developing world continues to rise, the demand for concrete can only continue to increase. The challenge to do this in an environmentally responsible way is truly daunting. However, it must be pointed out that much has already been done to reduce the environmental impact of the concrete industry. For instance, the amount of energy required to produce a unit mass of cement has been reduced by about 50% over the past two decades. At the same time, the emission of kiln dust has fallen approximately forty-fold.1 The cement industry utilizes certain wastes (e.g., automobile tires and other alternative fuels) to generate energy, and also uses the industrial wastes generated by other industries, such as slag from the production of steel, and fly ash derived from coal-burning plants used to produce electricity. The binders used in concrete now contain up to 30% of alternative cementing materials, such as fly ash, slags, waste gypsum, and so on, which decreases the demand 1

In this context, it should be noted that Joseph Aspdin, the man who took out the first patent on Portland cement, was arrested on the island of Portland for “publically raising dust” when using his cement kiln [13]!

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273

for Portland cement clinkers, and much more can be done in this area. Finally, the extensive use of water-reducing admixtures has made it possible to decrease the water demand by up to about 15%. The concrete development curve shown in Fig. 13.1 presents the progress in both concrete quality (represented by the increase in the compressive strength of concrete over time), and concrete quantity (denoted by the increase in concrete production). Clearly, the driving force behind this process is demographic growth. However, this begs the question – will the situation be similar in future? Will progress continue to be exponential and characterized by an upward trend? Will it still be possible to describe both quantitative and qualitative growth by the same curve, or will the ordinates of strength and production volume begin to separate? In the near future, demographic growth will exert a considerable influence on the quantitative development of concrete; greater geographical differentiation will surely occur. Even now, Asia produces three times more cement than Europe (Fig. 13.2). The aging European continent will continue to develop its infrastructure and repair and modernize existing resources, while concrete in Asia will be used largely for new construction (dams, bridges, high rise structures, and so on). It should be noted that merely increasing the mechanical strength of concrete is unlikely to lead to significant reductions in cement production. Even now – if it is necessary and appropriate – we can obtain extremely high strength concretes (fc ≥ 600 MPa) [14]. While this may lead to some reductions in the size of structural elements, this would be offset by the

Ductal

250

10000

World population [bln]

6

5

4

0

Compressive strength, f [MPa]

w/c = 0.25 7

8000

200 w/c = 0.30

6000

150 RPC w/c = 0.35 100

50

Microfibers 4000

Polycarboxylic superplasticizers Silica fumes w/c = 0.60 Fibers w/c = 0.7 Air-entraining

0 1750

1800

Plasticizers 1850 1900

13.1 Concrete development curve [9].

w/c = 0.4

2000

w/c = 0.5 Superplasticizers

1950

2000

0 2050

World concrete production [106 t]

8

274

Developments in the formulation and reinforcement of concrete 1800

1994

1995

1996 1997

1998

1600 1400 1200 1000

71 187 187

292 292

76

79

186 186

195 195

288 288

277 277

881

918

81

87

213 213

226 226

281 281

293 293

950

934

1999

2000

89

93

230 230

231

304 304

311

973

1001

800 600 400

810 810

200 0 Asia

Europe

Americas

Others

13.2 Global consumption of Portland cement (million tonnes).

much higher cement contents of these high strength concretes. Improvements in concrete durability (i.e., in the effective life of a concrete structure) would be much more effective in reducing cement consumption. The ultimate goal of sustainable development is a “closed circuit of materials”, that is, complete recycling [3]. Experience gathered during the rebuilding of Berlin shows [15] that even just after the Second World War “debris concrete” was used, and there is now increasing interest in using recycled concrete as aggregate for new concrete construction. Concrete obtained through demolition work will thus not result in the generation of more solid waste, but will be used to produce more concrete. In terms of multiple utilization, this could eventually equate concrete with steel. The recycling of concrete could, ideally, also lead to favorable changes in the emission balance of CO2. The process of crushing of concrete will result in the considerable development of the new surfaces in the recycled concrete aggregate, and thus lead to a considerable increase in the intensity of carbonation (Fig. 13.3). It is anticipated that up to 90% of the CO2 emitted during the production of cement may be consumed by this concrete over a hundred-year cycle (including one recycling operation [13, 16]). Thus, the circulation of carbon dioxide in the cement-concrete industry would also remain nearly totally closed. Another limitation on the quantity of concrete produced may involve the lack of water, which binds irretrievably during cement hydration. Many of the current developments in concrete technology depend upon the reduction of the water-cement ratio, due to the use of various water reducing agents. Their application has led to the possibility of reducing the w/c ratio to less than 0.38, which was, until fairly recently, considered as the limiting value. It may be assumed that in future developments of concrete technol-

Future developments in concrete

275

1.0

0.8

0.6

0.4

0.2

0

-0.2

-0.4

Ca aft rbon er i rec zatio yc n lin g

Ca du rbon rin iza g u tio sa n ge

co Fue mb l us tio n

ind ing Gr

Ca lcin atio n

a)

-0.6

-0.8

-1.0

kg of CO2 / kg of cement b) 100

Degree of carbonation

80

60

40

20

0

20

40 60 Years

80

10

13.3 (a) The emission balance of CO2 during the manufacture and use of concrete; (b) the forecast of carbonation taking into consideration recycling; source: Nordic Project [13, 16].

ogy, this value will tend towards 0.15, assuming that the non-hydrated part of cement grains will function as very effective microfillers. The net water savings will, however, be considerably lower, due to the consequent increase in the cement content of such concretes. (In the current generation of ultra high strength concretes, (e.g., DUCTALl, CEMTEC), the w/c ratio is less than half as high as that of ordinary concrete, while the cement content is more than twice as high [7].) Over the past decade, the price of drinking water has risen sharply, and will continue to rise. However, an alleviating factor will be the utilization of water from industrial processes, and in

276

Developments in the formulation and reinforcement of concrete

particular of water recovered during the production of concrete and the washing of aggregate. The above observations lead us to the conclusion that, in terms of quantity, we should expect a decrease in the rate of growth of concrete production. This slow-down will be rather mild, for demographic factors will continue to exert pressure on the concrete industry, though with varying intensities in different regions of the world. The decrease in the rapidity of growth will, however, probably not apply to the production of cement. The concretes of the future are likely to contain more cementing material, even though the proportion of mineral additions to the Portland cement will increase. A similar conclusion may be drawn with reference to qualitative improvements in concrete, expressed as compression strength. On the whole, technical and economic arguments point towards the attainment of optimal strength for any project, rather than towards the breaking of records in this regard. In general, we would expect a departure from the current move to produce ever higher strength concrete in favor of producing instead “high performance concrete” with much greater emphasis on the rheology and durability of such concretes.

13.4

Functional concrete

Traditionally, concrete quality was assessed primarily in terms of its compressive strength; it was assumed that making concrete “stronger” made it better in all respects. However, as it became apparent that this assumption was flawed, attention began to turn away from a mere reliance on strength, to considerations of the usefulness (in the broad meaning of the term) of a particular concrete, represented by properties such as: watertightness, resistance to abrasion, freeze-thaw resistance, and resistance to chemical aggression. These properties were then referred to the anticipated time of usage – i.e., the durability (or life cycle) of the concrete. This has led to the ever more frequent utilization of concrete with not only high strength, but also high durability [17], often referred to as high performance concrete (HPC). More recently, with the development of self-compacting concrete (SCC), the rheological properties of the fresh concrete have also become important design considerations. This change in the approach to the design and execution of concrete construction [18] is particularly evident in the European standard relating to concrete, such as EN 206. Unfortunately, practice in North America still lags behind in these areas. In the future, the market share of defined performance concrete [19], that is concrete with performance values specific to a particular project, will continue to grow, as the industry becomes more sophisticated both in defining concrete properties of importance and in their measurement. It remains a challenge to formulate performance criteria for various applications; in

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other words, a set of properties and values of critical importance for specific applications. Obviously, the fundamental requirements that must be met include: structural safety; fire protection; safety of use; health and environmental protection; noise and vibration protection, and energy and thermal savings, and overall economy [20]. The durability requirements make it imperative to ensure the usability of the structure throughout the period of service life – at minimum 50 years. Concrete construction should be made subordinate to the principles of sustainable development. This means that contemporary needs will have to be satisfied without endangering the opportunities of future generations. Currently, most concrete specifications are prescriptive in nature; that is materials, mixture proportions and construction methods are strictly defined. However, this approach does not encourage innovation, or even the optimum use of the available materials. There is thus a move towards the establishment of performance specifications, in which the concrete is defined on the basis of its technical properties in both the fresh and hardened states, and not in terms of its composition. The new European Standard EN 206-1 creates such an opportunity (Fig. 13.4). In this context, concrete of the future would appear to be a “better” concrete: a concrete with known properties, but with better selected values. The move to a more sustainable concrete industry will also require “the production of good-quality concrete from inferior raw materials2 and with a lower cement content” [7]. In the light of Neville’s [21] well-known statement, that “good and bad concrete may be obtained from the very same ingredients”, this is not impossible, although difficult. It will require the

Application of concrete

Requirements / / Properties

- Transport of the mix

Designed concrete

FIE

R

Designing of concrete

SP EC I

SP

EC

IFI

ER

- Using - method of placement of - Placement reinforcement - Curing - Compacting

Components / / Composition

Prescribed concrete

13.4 General inference diagram leading to the definition of designed concrete or prescribed concrete [18]. 2

This will also include waste materials.

278

Developments in the formulation and reinforcement of concrete

development and improvement of design and material optimization methods, including a better understanding and utilization of synergy mechanisms (the beneficial interactions amongst various ingredients). This will increase the role of material modeling (microstructure – material properties relationships) in “virtual” concrete laboratories [22], making it possible to explain the results of experiments and helping to plan new undertakings. Once suitable models are developed, computer simulations will in part replace experiments. The optimization of composition will also make it necessary to produce concrete mixes with tighter control of material proportions, and lower allowable variability of technological parameters. This will also lead to the necessity of considering the particle size distribution of the entire concrete mixture, from the maximum aggregate size down to the finest microfillers. This concept has already been successfully applied in the development of the ultra high strength “reactive powder concretes”. Of course, all of this will necessitate further developments in production control. Based on the foregoing, then, it may be predicted that the concrete of the future is likely to have the following characteristics: • • • • • •

It will be a “better” concrete, but made with inferior raw materials. It will be designed on the basis of performance rather than prescriptive specifications. High strength and high performance concretes will be more common, even for “ordinary” applications. It will contain more industrial waste materials, including recycled concrete aggregate. It will be more durable. Unfortunately, it will probably also be more expensive.

As well, work will continue on producing concretes with much higher ratios of tensile to compressive strength, through the utilization of fibers and/or polymers. There will also be more utilization of textile reinforced concrete, that is, concrete reinforced with specially designed meshes of fibers. These materials have high inherent strengths, and possess a considerable amount of ductility. They are particularly suited for applications such as protective walls (alas, an important consideration in today’s world). Indeed, survivability, the ability to survive a terrorist attack or a natural catastrophe, is becoming a more frequent requirement for structural materials. Currently, this objective can only be formulated in “performance” requirements; there are no prescriptive specifications for survivability. The changes in concrete technology mentioned above are all now underway, at least to some degree. In addition, however, there are other, more “futuristic” concepts under consideration. These include self-repairing concretes, in which the concrete will be able to sense damage, and then begin

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13.5 Transparent concrete, LiTraCon – concrete enabling the passage of solar light; source: www.techeblog.com.

to repair it (to be discussed further below); and self-cleaning concretes, in which the concrete will be able to shed the layers of dirt and grime that inevitably form on exposed concrete surfaces. Unexpectedly, there has even been developed a transparent (!) concrete, which enables the passage of ordinary solar light. This concrete [23] contains dispersed optical fibers of varying diameters, which facilitate the passage of light through concrete blocks with thicknesses of up to several tens of centimeters (Fig. 13.5). There have also been apparently serious studies of so-called “lunar concrete” [24], which discuss the problems of eventually making concrete on the moon, despite the temperature extremes and the complete lack of water!

13.5

Nanocement and nanoconcrete

A hydrated cement paste, which contains hydrates and pores identifiable on a nanoscale (Fig. 13.6) constitutes a most promising point of departure for nanomodification [25] using both reactive and non-reactive nanoparticles. Recently, the new European Research Network NANOCEM has been created. Twenty-five universities and 12 industrial laboratories have been successfully gathered together, under the coordination of Dr Karen Scrivener of EPFL (Ecole Polytechnique Federale de Lausanne). The project title “Fundamental understanding of cementitious materials for improved chemical, physical and esthetic performance” presents an ambitious goal [26]. However, it is clear that cement and concrete have considerable potential for intelligent modification through manipulation of their nanostructure.

280

Developments in the formulation and reinforcement of concrete Entrapped air void Hexagonal crystals of Ca(OH)2 or low sulfate in cement paste Interparticle spacing between C-S-H sheets

Max. spacing of entrained air for durability to frost action

Capillary voids Aggregation of C-S-H particles

0.001 μm 1nm

0.01 μm 10 nm

0.1 μm 100 nm

1 μm 1000 nm

Entrained air bubbles

10 μm 104 nm

100 μm 105 nm

1 mm 106 nm

10 mm 107 nm

13.6 Dimensional range of solid and pores in a hydrated cement paste (after P. Monteiro, Univ. of California, Berkeley; courtesy of K. Scrivener) [27].

The history of the development of concrete technology shows that the reduction of the water-cement ratio through the utilization of various plasticizers is accompanied by ever more subtle technological changes; for instance, it is the use of modern plasticizers that has enabled us to introduce ever finer particles into the concrete (Fig. 13.7). For example, the size of a sand grain is about 40,000 nm, while a particle of silica fume has a size of only about 200 nm. Logically, the next step would be the introduction of precipitated silica (∼50 nm), and then nanosilica (∼5 nm). As a consequence, the interphase surface and accompanying interfacial phenomena would increase significantly [28]; quantum effects may also occur (<10 nm) [29]. A wide range of nanomodifiers may be used, including such materials as carbon nanotubes [30]. It is expected that a self-compacting concrete with ultrahigh strength and increased durability will eventually be obtained. The concept of concrete containing microcapsules filled with an epoxy resin, known as “self-repairing concrete”, has been developed by Ohama and his colleagues [31]. The epoxy resin is distributed in the concrete mix through intensive mixing. If the concrete is cracked, the resin from the capsule is freed and contacts the surface of the crack and, under the influence of the alkaline environment, hardens (Fig. 13.8), thus consolidating the crack. In the case of polymer-cement concrete, due to the considerable cost of the polymer, it is very important to ensure its rational location in the concrete (Fig. 13.9). This purpose is served by nanomonitoring [32]. Within this same group of issues we may consider research into the model of percolation transformation, understood as the “non-continuity– continuity” passage during formation of the polymer chain in concrete [33]. Bacillus pasteurii bacteria may be considered as “vivoactive” nanomodifiers; these would repair a concrete scratch by precipitating the calcium carbonate [34].

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A, m2/kg N-C

1000000

N-SiO2

HSC / HPC

100000

SiO2 precipitated

OC

10000

Metakaolin Portland cement

1000 100

Silica fume Fine mineral additives

Fly ash Meal

10 Sand 1 Coarse aggregate

0.1 0.01

108

107

106

105

104

103

102

10

1

Grain size, nm

13.7 The proper surface depending on the grain size (according to [28]): OC – ordinary concrete, HSC – high strength concrete, HPC – high performance concrete, N-C – nanoconcrete.

Aggregate Cement matrix Hardened epoxy resin Unhardened resin

The microcracks appear following the loading

The microcracks are filled up with the resin

13.8 The self-repair mechanism of epoxy-cement according to Y. Ohama [31].

282

Developments in the formulation and reinforcement of concrete

13.9 The bridging of cracks in concrete by the polymer; a cracked cement matrix (left), the bridging effect of the polymer (right).

13.6

Concluding remarks

In this chapter, we have tried to provide an analysis of the main tendencies shaping the development of concrete and their anticipated consequences for the future of this material. The concrete of the future will be a much more sophisticated product. One of the most important requirements for the future is the research and engineering personnel, whose future is closely connected with that of concrete. There is a need for a holistic approach to the material engineering of concrete, addressed simultaneously by research, practice and education in this area. This should lead to the development of an international strategy of research into concrete production engineering. The future of concrete should be based on knowledge, and this knowledge must be developed.

13.7

References

1 S. Lem: Summa technologiae. Interart, Warszawa, 1996. 2 L. Czarnecki: Basis for a system of development trends recognition in Building Materials Engineering. Prace Instytutu Techniki Budowlanej, 2 (2005) [in Polish]. 3 Delphi – Study on Development in Science and Technology. www.isi.fraunhofer. de. 4 A.J. Boyd, S. Mindess, J.P. Skalny (eds.): Materials Science of Concrete: Cement and Concrete – Trends and Challenges; Special Volume. Wiley, 2006. 5 Road Map 2030: The US Concrete Industry Technology Road Map. Strategic Development Council, USA, 2002. 6 P. Hewlett: Future of Concrete – Significant Trends and Changes. Conference Concrete Days, Wisła, Poland, 2004. 7 A. Bentur, A. Katz, S. Mindess: The Future of Concrete: the Vision and Challenges. Cement Wapno Beton, 2 (2006). 8 W. Kurdowski: The Future of Concrete. Scientific Conference Concrete and Prefabrication, Jadwisin, Poland, 2006. 9 L. Czarnecki: Why concrete does have a future. Budownictwo, Technologie, Architektura (Construction, Technologies, Architecture), 3 (2003) [in Polish].

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10 A.M. Brandt: Concrete does have a future, but . . . ibid., 1 (2004). 11 European Network of Building Research Institutes: Future Needs for European Construction R&D. www.enbri.org. 12 G. Wegner: Soft materials and polymers: strategies for future areas of basic materials science. In European White Book on Fundamental Research in Materials Science, Max-Planck-Institut für Metallforschung Stuttgart, Max-PlanckGesellschaft, Stuttgart, 2001. 13 C. Nielsen: Concrete Production – Best Available Technologies. Seminar “Challenge for Sustainable Construction: the Concrete Approach”, Warsaw, Poland, 2006. 14 P. Richard, M.H. Cheyrezy: Reactive Powder Concrete With High Ductility and 200–800 MPa Compressive Strength. In Concrete Technology Past, Present and Future (P.K. Mehta ed.), SP-144, American Concrete Institute, 1994. 15 M. Maultzsch: The utilization of processed building rubble – a contribution to sustainability in concrete technology. Conference Concrete Days, Szczyrk, Poland, 2002. 16 M. Glavind: Properties for RTD Identified by Eco-Serve and ECTP, Focus Area Materials, ibid. 17 A.C Aitcin: High-Performance Concrete. E & FN Spon, London, 1998. 18 L. Czarnecki et al.: Concrete according to EN 206-1 – a commentary. Polski Cement, Cracov, 2004 [in Polish]. 19 J. Walraven: From High Strength, through High Performance, to Defined Performance Concrete. Conference High Strength/High Performance Concrete, Leipzig, 2002. 20 European Directive 89/106/EEC Construction Products of 1998-12-21. 21 A.M. Neville: Properties of Concrete. Pearson Education Ltd, London, 1995. 22 E. Garboczi: The Past, Present and Future of the Computational Materials Science. www.ciks.cbt.nist.gov/~garbocz. 23 Concrete Nation: Bright Future for Ancient Materials. Science News Online, 1 (2005). 24 J. Wickman: Using Lunar Soil for Propellants & Concrete. Ad Astra, 2 (2002). 25 L. Czarnecki: Nanotechnology – a challenge for Building Materials Engineering. Inzynieria i Budownictwo, 9 (2006) [in Polish]. 26 K. Scrivener, E. Gartner: Nano work for cement giants. Materials World, 9 (2006), 26–28. 27 P. Monteiro: Portland Cement. www.ce.berkeley.edu/~paulmont/CE60New/ cement.pdf. 28 K. Sobolev, M. Ferrada-Gutierrez: How nanotechnology can change the concrete world: Part 2. American Ceramic Society Bulletin, 11 (2005). 29 A. Porro: Nanoscience and nanotechnology in construction materials. 2nd Symposium on Nanotechnology in Construction Materials, Labein, 2005. 30 A. Cwirzen: Self-compacting ultra high strength concrete: nanotubes in concrete. TKK-project, Warsaw University of Technology, Warsaw, Poland, 2006. 31 T. Katsuhata, Y. Ohama, K. Demura: Investigation of microcracks selfrepair function of polymer-modified mortars using epoxy resin without hardeners. 10th International Congress on Polymers in Concrete, Hawaii, USA, 2001.

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32 L. Czarnecki, H. Schorn: Nanomonitoring of polymer-cement concrete microstructure. International Journal of Restoration of Buildings and Monuments, 3 (2007). 33 P. Lukowski: Continuity threshold of the polymer phase in polymer-cement composites. Archives of Civil Engineering 2/3, 2008. 34 C. Rodriguez-Navarro, M. Rodriguez-Gallego, K. Ben Chekroun, M.T. Gonzalez-Muñoz: Conservation of ornamental stone by myxococcus xanthusinduced carbonate biomineralization. Applied and Environmental Microbiology, 4 (2003).

Index

abrasion resistance 80, 88 acrylic-based coatings/finishes 9, 10 acrylic fibres 157 added value 219–20, 226 adhesion, shear based on 30, 31 admixtures 189, 271 chemical 117–19, 252 mineral see mineral admixtures aerated concrete 231 see also foamed concrete aesthetic effects 219, 220 aggregate velocity 103 aggregates characteristics of heavy aggregates 44–5 fine aggregates for foamed concrete 234 glass 216–19 hot weather concreting and cooling of coarse aggregates 123–5 lightweight see vesicular aggregates recycled 262 in shotcrete 99–100 aggregate size and rebound 102–3, 106, 107 air cooling system 123 air voids foamed concrete see foamed concrete lightweight aggregates see vesicular aggregates stability in SCC 188, 194–5, 199 Akashi-Kaikyo Suspension Bridge 187 alkali-silica reaction (ASR) 216–18 ‘Allen’ clay 50 American Cement Institute (ACI) 46–7

American Concrete Institute (ACI) 8, 133 annual global production of concrete 271, 272 anti-washout mixtures (AWAs) 137, 145–50 Aramid (Kevlar) fibres 157 asbestos fibres 154, 156–7 ashes 223–4, 262 fly ash see fly ash Asheville three-storey AAC shear-wall hotel 14–33 Asia 273, 274 ASTM specifications for AAC construction 13–14 standards for SCC 202, 203 attrition milling 53–4 autoclaved aerated concrete (AAC) 1–43 applications 4–6 design of three-storey shear-wall hotel 14–33 choice of design criteria 14–16 design of exterior walls for gravity plus out-of-plane wind 26–8 design of floor diaphragms for in-plane actions 28–33 design for gravity plus earthquake loads 24–6 design steps 14 summary of design procedure 16–24 development of seismic design factors for ductile AAC shearwall structures 41

285

286

Index

earthquake performance of AAC shear-wall structures 33–41 elements 2 construction details for 10–11 handling, erection and construction with 8–9 fabrication process 2–3 historical background 1–2 materials used in 2 seismic design of AAC structures 11–14 strength classes 3, 4 structural design of AAC elements 6–11 typical dimensions of AAC units 4 Autoclaved Aerated Concrete Products Association (AACPA) 1–2 Bacillus pasteurii 280 bentonite 54 binders 54 Bingham model 190–1 bioplastic coating 223 blended hydraulic cements 115–16 bloaters (rejected clay bricks) 174 boron-modified DUAGG 51, 53 Boyle-Mariotte’s law 194 Braddock Dam 178 bridges cracking in hot weather concrete 128, 129 high-strength concrete 81–3, 84, 85, 90, 92 lightweight concrete 177–8 SCC 196–7 BSI-CERACEM 162 Cd (displacement amplification factor) 12–13, 41 calcination 54 Canada 250–1 canal dredgings 181 Canary Wharf Building 177 cantilever bridges 82 carbon dioxide emissions 208, 272, 274, 275 carbon fibres 157 carbonation 209, 274, 275 resistance of foamed concrete 248, 249 carpet fibres, recycled 225 cast-in-place polymer concrete 258 applications 260, 261

casting process 259 cellulose fibres 157 cement 276 content impact of varying in UWC 146, 147–50 and rebound in shotcrete 106–8 environmental impact of producing 208 Portland cements see Portland cements selection for hot weather concreting 115–16 CEMTECmultiscale 162, 163 CEN standards for SCC 202, 203 centrifugal moulding process 259 ceramic tiles 10 chemical admixtures foamed concrete 252 hot weather concreting 117–19 chilled water 121 China 181 chloride penetration 80 foamed concrete 248 offshore structures 90–1, 93 cladding, AAC 5, 6 clay 50 coal ash aggregates and fillers 262 see also fly ash coefficient of restitution 105–6 cold bonded fly ash 180 Colosseum 174 colour, glass and 216, 217, 219 commodity products 219 compact reinforced composite (CRC) 162 compatibility of constituent materials 200–1 composite models 188–90 compression struts 31–3 compressive strength 80, 273, 276 and abrasion resistance 88 DUCRETE 60–1, 64–5 foamed concrete 241–3, 244–5 hot weather concrete 128–30 and porosity of cement paste 89 concrete and demolition waste (C&D waste) 213–15 concrete development curve 273 concrete filled steel tubes (CFT) 199–200 consistence classes 204 contaminants 213, 214

Index cooling 120–6 of coarse aggregates 123–5 replacement of mixing water 121–3 using liquid nitrogen 125–6 copolymer-based superplasticizer 145–50 copper slag 212 corrosion hot weather concrete reinforced with steel 131 North Sea offshore structures 90–1 resistance of DUCRETE 64–6 Cosa harbour 174 cost 64 recycling 214–15, 226 crack-autohealing 268, 278–9, 280–2 cracking AAC shear walls 35, 38, 39, 40 alkali-silica reaction 216–17 fibre reinforced concrete crack width development in ECC 161, 162 foamed 249 multiple cracking 156 hot weather concrete 127, 128, 129 initiation of cracking 131 microcracking in lightweight concrete 170–2 plastic shrinkage cracking 127, 161 creep 80, 180 critical length 156 crushing of glass 216 curing foamed concrete 240–1 polymer concrete 259 proper curing and hot weather concreting 127–8, 129 Danish Broadcasting Corporation 197–9 defined performance concrete 276–9 deflection-auto-straightening 268 delayed expansion 116 Denmark 196–9 density DUCRETE 60–1 fresh foamed concrete 239–40 lightweight concrete 179 and stress-strain curve 172, 173 Denver Stapleton International Airport 215 depleted uranium 47 depleted uranium oxide 52–3

287

design seismic design of AAC structures 11–14 structural design of AAC elements 6–11 three-storey AAC shear-wall hotel 14–33 design base shear 23 design response spectrum determining 17, 19 key ordinates of 17, 18–19 developing countries 181–2 DIN flow table 137–8 displacement amplification factor (Cd) 12–13, 41 displacement ductilities 39, 41 Draugen Platform 178 dredging sediment 181, 224–5 drift ratios 38, 39, 41 dry foam 234–5 dry ice 124–5 dry milling 54 dry-process shotcrete 98–9, 100, 101–2, 109–10 rebound 102–8, 109 drying shrinkage 161, 245–6 DSP (densified with small particles) materials 89 DUAGG 47–58, 71 compositions 48–51 fabrication process 52–5 process flow diagram 48 properties 55–8 DUCRETE 58–66, 67 corrosion resistance results 64–6 ‘Over Pack’ shielding 60, 61 physical properties 60–1 proportioning, mixing and placing 58–60 radiation-shielding properties 61–4 repository overpacks 67–70 spent nuclear fuel casks for shipping 67, 68 toughened for shields 71–2 DUCTAL 162, 163 ductile AAC shear-walls development of seismic design factors 41 testing 34, 35 durability 80, 209, 274, 276 glass-aggregate concrete 218–19 hot weather concreting 131

288

Index

probability-based durability design 91–3 DuraCrete 92 earthquake loads 24–6 earthquake resistance mechanism of AAC structures 12 performance of AAC shear-wall structures 34–41 see also seismic design Egyptians 231 Ekofisk Tank 85, 86, 87 elasticity 80 modulus of 102, 130–1, 243 electrical installations 9 energy absorption 108–9, 221 used in cement production 272 Energy Solutions VSC-24 cask 73, 74 engineered cementitious composites (ECC) 161, 162 environmental impact of concrete production 208–9, 272–3 environmental movement 225–6 epoxy resin 280, 281 Escambia Bay Bridges 124 Europe 213, 216, 260, 273, 274 evaporation rate 127 extended set-control admixtures 118 exterior finishes 9–10 factored design shear 23 fibre reinforced concrete (FRC) 154–66 applications 164–5 foamed concrete 248–50, 252 high performance FRC 161–3 hybrid fibre systems 163–4 mechanical behaviour 155–6 mix proportioning, fabrication and placement 157–8 recycled fibres 225 role of fibres 158–61 shotcrete fibre orientation 101 fibre rebound 108 toughness and impact resistance 108–11 types of fibre 156–7 filling ability 142–4, 188, 189, 190–3 standardisation 202, 203 test 142–4 fire, design for 15

fire resistance 245 Flamsperse 54 flexural capacity 25–6, 27–8 flexural reinforcement 34, 36, 37 flexural toughness 110 flexure, floor diaphragms design for 29–30 floating bridges 82 floating concrete structures 178 floor diaphragms 28–33 flow 189 fresh foamed concrete 238–9 modelling 201–2 test 137, 138–9 flow time 145–50 fly ash 116–17, 210–11, 212 foamed concrete 233 compressive strength 241–2 lightweight aggregates from 169, 175–6, 181–2 foamed concrete 167–8, 231–55 applications 250–1 definitions and classifications 232 fibre reinforced 248–50, 252 materials 232–5 mix design 235–6 production of 236–7 properties 237–48 fresh foamed concrete 237–41 hardened foamed concrete 241–8, 249 research needs 251–2 foaming agents 234–5, 236, 243 combined with bioplastic as coating of plastic aggregate 223 force-reduction factor, seismic (R) 12– 13, 41 formwork pressure 188, 194 foundation slabs 210, 212 foundry sands 224 freeze-thaw durability 80, 247, 248 frictional shear stresses 155 functional concrete 276–9 future developments 270–84 functional concrete 276–9 general factors influencing development of concrete 272–6 nanocement and nanoconcrete 279–82 gamma rays 61–2 Give motorway bridge, Denmark 196–7

Index glass, recycled 215–20, 226, 262 glass dust/powder 216 glass fibres 156 glass transition point 256 GNB CONSTOR cask 67, 68 gravity load 16 design of AAC shear-wall hotel 24–8 design of exterior walls for gravity plus out-of-plane wind 26–8 design for gravity plus earthquake loads 24–6 Green Building movement 226 Griffith’s theory for rupture of brittle materials 79 ground granulated blast furnace slag (GGBFS) 117, 211–13, 233 gunite 98 see also dry-process shotcrete gypsum board 10 half value layer thickness 62–3 Hatschek process 154 Haydite 174–5 heat of hydration 115, 116, 240 heat pumps 121 heavy aggregates 44–5 Heidrun Platform 86, 87, 178 Helgelandsbrua Bridge 84, 85 Herschel-Buckley model 191 Hibernia Platform 178 high-density and radiation-shielding concrete 44–78 applications/case studies 47 definition 44 DUAGG 47–58, 71 DUCRETE 58–66, 67 future trends 66–72 repository overpacks 67–70 requirements for radiation-shielding concrete 45–7 shields 71–2 spent nuclear fuel casks 46–7, 63–4, 66, 67, 68, 70–1 high-level waste (HLW) containers 66–7 repository overpacks 67–70 high-performance concrete (HPC) 79, 80, 136, 276 high-performance FRC 161–3 see also high-strength concrete; underwater concrete high-range, water-reducing admixtures 118, 119

289

high-rise buildings AAC cladding 5, 6 high-strength concrete 81, 82, 83, 90 lightweight concrete 177 high-strength concrete (HSC) 79–97, 273–4 applications 80–9 bridges 81–3, 84, 85, 90, 92 general applications 80–1 high-rise buildings 81, 82, 83, 90 offshore structures 83–7, 90–1 special applications 88–9 future trends 90–3 general 90–2 probability-based durability design 91–3 highly air entrained concrete 231 see also foamed concrete highways see roads Holland 251 Holtec dry cask system 73 hot press moulding process 259 hot weather concreting 114–35 applications/case studies 114–31 cooling 120–6 hardened properties 128–31 material selection and mix design evaluation 115–20 plastic properties 126–8, 129 future trends 131–3 concrete modelling 131–2 initiation of cracking 131 materials speciality engineers 132–3 nondestructive evaluation techniques 132 hotels 5 design of three-storey AAC shearwall hotel 14–33 housing 5 hybrid fibre systems 163–4 hydrated cement paste 279, 280 hydration, heat of 115, 116, 240 hydration control admixtures 118, 273 hydraulic cements 115–16 hydrogen gas 231, 236 HYPERCON model 131–2 ice water 122–3 impact factor 106 impact resistance fibre reinforced foamed concrete 249–50

290

Index

FRC 159–60 shotcrete 108–11 in-plane flexural design 25–6 India 181–2 industrial waste 272–3 recycling and polymer concrete 260–3 initiators (hardeners) 257 interfacial fibre debonding 250 interior finishes 10 Japan 213, 259, 260 Japan Concrete Institute (JCI) 133 Kauai temple, Hawaii 210 kinematics 102–8 Krieger-Dougherty equation 192 lateral load 16–17 leaching 65–6 lead 62, 63 LECA 175 LEED rating system 226 LIAPOR 175 LIFE-365 model 132 lightweight aggregates see vesicular aggregates lightweight concrete 167–86 applications/case studies 176–8 defining 168 future trends 181–3 history of 172–6 nature of 168–72, 173 production of 178–80 Lin, T.Y., International 177 liquid nitrogen (LN) 125–6 liquid resins 256, 257, 266, 267 LiTraCon transparent concrete 279 load-bearing wall panels 11 loaded nodes 31, 32 lunar concrete 279 LYTAG 175–6 magnetite 62, 63 markets for lightweight aggregates 182–3 masonry design 6–8 Masonry Standards Joint Committee (MSJC) 6–7 masonry veneer 10 materials speciality engineers 132–3 maximum considered response acceleration 17, 18

MDF (macro defect free) materials 89 metal mats 72 methacrylic resins 256, 257 microcracking 170–2 Middle East 251 mineral admixtures fly ash see fly ash foamed concrete 233 GGBFS 117, 211–13, 233 hot weather concreting 116–17 shotcrete 102–3 silica fume 102, 103, 106, 107, 117, 233 mix design DUCRETE 58–60 foamed concrete 235–6 FRC 157–8 hot weather concreting 115–20 verification process 119–20 lightweight concrete 179–80 polymer concrete 258, 267 shotcrete 99–101 UWC 144–50 virtual 201–2 mixers 258–9 mixing foam method 234, 236 mixing time 238 modelling flow 201–2 functional concrete 278 software packages 131–2 moment diagrams 23 Mori Tower Roppongi Hills 199–200 moulding processes 259 movement joints 14 multiple cracking 156 NANOCEM 279 nanocement 279–82 nanoconcrete 279–82 Nations Bank Building 177 natural organic fibres 157 NatWest Tower 177 neutron shields 62–3, 66–7 neutrons 44, 61–3 New York City 224 water treatment plant 212 Newton model 191 nondestructive evaluation techniques 132 North American basalts 49 North Sea offshore structures 83–7, 90–1

Index Norway bridges 82–3, 84, 85 offshore structures 83–7 Oslo Harbour project 92, 93 offshore structures 83–7, 90–1, 178 Orimet test 139–40, 145 Oslo Harbour project 92, 93 overstrength, system 17, 21 oxygen permeability 246–7 Pacific Nuclear 46 panel bond beam joint 30, 31 panel-to-panel joint 30, 31 Pantheon 174 parge coat 10 particle size 280, 281 of aggregates and rebound 102–3, 106, 107 glass recycling 217 particle velocity 103 passing ability 188, 189, 193 standardisation 202, 203, 204 penetration modelling of rebound 104–5 resistance 98–9, 105–6 performance specifications 277 permeability 246–7 petroleum 266 placement AAC elements 8–9 DUCRETE 58–60 FRC 157–8 methods for UWC 150–1 polymer concrete 259 shotcrete 99–100 time and temperature of placement of hot weather concrete 120–1 plan structural irregularities 17, 21 Plascrete blocks 223 plaster 10 plastic shrinkage cracking 127, 161 plastic viscosity see viscosity plastics, recycled 222–3, 262 plumbing installations 9 plunge test 142, 143 polyethylene terephthalate (PET) 261 polymer concrete 256–69, 280, 282 applications 259–60, 261, 267–8 future trends 263–8 necessary innovations 266–8 production techniques 257–9, 267

291

recycling and reuse of industrial waste 260–3 standardisation work 263, 264–6 polymer fibre-reinforced DUCRETE 72 polymer-impregnated concrete 263–6 polymer-modified stuccos/paints/finishes 9 polypropylene fibres 157 polystyrene (EPS) 261, 262–3 polyvinyl alcohol (PVA) fibres 157 porosity of cement paste and compressive strength 89 foamed concrete 241, 244–5 Port Authority of New York and New Jersey 224–5 Portland cements 208, 209, 212 foamed concrete 232–3 hot weather concreting 115, 116 precast polymer concrete 258, 259 applications 260, 261 recycled materials 262–3 pre-formed foam method 234, 236–7 prescriptive specifications 277 probability-based durability design 91–3 productivity 187 protein-based foams 234–5 pull-out, fibre 155–6 pumice 172–4 pump method of placement 150–1 quality control fly ash 211 UWC 138–44 R (seismic force-reduction factor) 12– 13, 41 radiation-shielding concrete see highdensity and radiation-shielding concrete RBMK spent fuel casks 70–1 reaction, in modelling of rebound 104–5 reactive powder concretes (RPCs) 89, 278 rebound 102–8, 109 recycled concrete aggregate (RCA) 213–15 recycled materials 208–30 concrete 213–15, 274 fly ash see fly ash future trends 225–6

292

Index

GGBFS 117, 211–13, 233 glass 215–20, 226, 262 other recycled materials 223–5 plastics 222–3, 262 in polymer concrete 260–3 tyres 220–2, 262 redundancy 17, 21–2 repository overpacks 67–70 retarding admixtures 118 RFNC-VNIIEF 70–1 rice husk ash (RHA) 224, 262 RILEM 133 Rion-Antirion Bridge 92 roads 88, 164, 182 bridge over motorway 196–7 robustness 189, 200–1 rock-bursts 108, 110–11 Romans 174, 231 rubber, recycled 220–2, 262 rupture, modulus of 130 Russia 70–1 scaling resistance 80 scoria 172–4 segregation 193–4, 202 segregation resistance 188, 189, 193–4 standardisation 202, 203, 204 seismic base shear 22 seismic design 11–14 ASTM specifications 13–14 basic earthquake resistance mechanism 12 three-storey AAC shear-wall hotel 14–33 seismic design category 17, 20 seismic design factors development for ductile AAC shearwall structures 41 for ductile AAC shear-wall structures in the US 12–13 seismic importance factor 16, 17 seismic load effect 17, 21–4 seismic use group 16, 17 self-cleaning concretes 279 self-compacting concrete (SCC) 142–4, 187–207, 276 applications/case studies 195–200 definitions 188, 189 FRC 164–5 future trends 200–2 guidelines 202 productivity and 187 properties 188–95

air void stability 188, 199, 194–5 filling ability 142–4, 188, 189, 190– 3, 202, 203 formwork pressure 188, 194 fresh SCC 188–90, 191 passing ability 188, 189, 193, 202, 203, 204 rheological properties 188, 190–3 segregation resistance 188, 189, 193–4, 202, 203, 204 standardisation 202–4 working environment 187–8 self-repairing concretes 268, 278–9, 280–2 Selma, USS 177 service life 270 setting time 126 shear 30–3 shear capacity 26, 28 shear-wall structures, AAC 12 earthquake performance 34–41 seismic design factors 12–13 development of 41 three-storey shear-wall hotel 14–33 shields 71–2 shipping casks 67, 68 ships 176–7 shock-absorbing concrete 249–50 shooting consistency 108 shotcrete (sprayed concrete) 98–113 kinematics and rebound 102–8 mix proportioning and process implications 99–101 strength and stiffness 101–2 toughness, impact resistance and fibre reinforcement 108–11 shrinkage 80 foamed concrete 245–6 FRC 161 lightweight concrete 180 Sierra Nuclear 46 cask 67, 68 silane coupling agents 258 silica fume 102, 103, 106, 107, 117, 233 silicate phase in DUAGG 56–7 SIMCON 72 site class 16, 17–18 skid-free surfaces 182 slabs on grade 164 slag 262 GGBFS 117, 211–13, 233 lightweight aggregates from 169, 176, 182

Index slump loss and hot weather concreting 126 test 137, 145 UWC 145–50 soaking of coarse aggregate 123–4 solid waste disposal 224 South Western Bell Telephone Building 177 spent nuclear fuel storage casks 46–7, 66 DUCRETE 63–4 GNB CONSTOR cask 67, 68 RBMK casks 70–1 shipping casks 67, 68 spherical inclusions, stress around 168–70, 171 splitting tensile strength 130 sprayed concrete see shotcrete spread/flow test 137, 138–9 Stadium model 131 stainless steel 62 fibres 156 standardisation polymer concrete 263, 264–6 SCC 202–4 Stapleton International Airport, Denver 215 steel 62–3, 66–7 corrosion in offshore structures 90–1 from recycled tyres 222 reinforcement in hot weather concrete and corrosion 131 steel fibres 72, 155, 156 steel-polyethylene composite 62, 63 stiffness see elasticity Stoke’s law 194 Storseisund Bridge 84 strength 80, 273–4, 276 AAC strength classes 3, 4 compressive see compressive strength and curing of hot weather concrete 127, 128 lightweight concrete 168–70, 171 reduction in RCA 213–14 reduction and recycled plastics 222 shotcrete 101–2 strength interaction diagram 25 structural design 6–11 Subsurface Disposal Area (SDA) Lake Bed Soil 49–50 studded tyres 88

293

superplasticizers 118 foamed concrete 252 SCC 201 UWC 137, 145–50 survivability 278 suspensions 188–90, 191 sustainability 200, 274 synergies 163 synthetic basalts 48–50, 52–3 synthetic fibres 157 synthetic foams 234–5 system overstrength 17, 21 Tandy Center 130 tar-modified resins 256, 257 Tarsuit Caisson Retaining Island 178 temperature, placement 120–1 template approach to toughness 159 temple foundation slab 210 tension ties 31–3 ternary system 212 terrazzo tiles 220 Texas University tests on AAC shearwall structures 34–41 textile reinforced concrete 278 thermal conductivity 243–4 thermal resistance 243 thermoplastic resins 256 waste-recycling 261 thermosetting resins 256, 257 third-point loading test 158–9 three-storey AAC shear-wall hotel 14–33 time flow time 145–50 placement of hot weather concrete 120–1 setting time 126 titanium oxide phase in DUAGG 56, 57–8 Toronto Dominion Center 177 toughness FRC 158–9 shotcrete 108–11 toughened DUCRETE 71–2 transparent concrete 279 transverse wall in-plane flexural design for earthquake loads 25–6 shear design for earthquake loads 24–5 Tremi method 150–1 Troll platforms 86, 87, 178

294

Index

truss model 30–3 two-storey AAC assemblage 35–41 tyres recycled 220–2, 262 studded 88 ultra high strength FRC 161–3 underwater concrete (UWC) 136–53 application/case study 144–51 mix design 144–50 placement methods 150–1 development 137–8 quality control 138–44 United States (US) 216, 225 AAC 1–2, 6–8 ASTM specifications 13–14 design and construction provisions for AAC masonry 6–8 design provisions for reinforced AAC panels 8 integrated design context 6 seismic design factors 12–13 applications of foamed concrete 250–1 applications of polymer concrete 259, 260, 261 depleted uranium 47 high-rise buildings and HSC 81, 82, 83 lightweight aggregates 175 recycled concrete 215 unloaded nodes 31, 32 uranium 62, 63 DUCRETE and leaching of 65–6 uranium oxide phase in DUAGG 55 uranium titanium phase in DUAGG 56, 57 value-added products 219–20, 226 vehicle arrest pads 182 vertical structural irregularities 17, 21 vesicular aggregates 167, 168, 169, 172–7 future trends 181–3 markets for 182–3 particle density 179 virtual mix design 201–2 viscosity modifying admixtures (VMA) 189, 194 viscosity 189, 190–3 standardisation 202, 203, 204 vivoactive nanomodifiers 280

walkability 247–8 walls SCC and basement walls 197–9 shear-wall structures see shear-wall structures washout loss 141, 142, 145–50 washout-resistance test 140–1 waste-to-energy facilities 224 water foamed concrete water absorption 246 water demand and fresh foamed concrete 237–9 water permeability 246–7 glass aggregate and lack of absorption 218 hot weather concreting chilled water replacement of mixing water 121 design mix water content 120 ice water replacement of mixing water 122–3 limitations on concrete production 274–6 recycling wash water 209, 275–6 resistance to penetration 14 water-cement ratio 80, 274–6 foamed concrete 238 compressive strength 242–3 and strength of shotcrete 101, 102 water-reducing admixtures 118, 242, 273 water treatment facilities 182, 212 waterproof membrane 10 Wausau Tile Company 220 welan gum 145–50 wet attrition milling 53–4 wet foam 234 wet-process shotcrete 98–9, 100, 102, 109–10 wet technology 220 wind, out-of-plane 26–8 wood ash 224, 262 workability 145, 270 consistence classes for SCC 204 flow/spread test 138–9 fresh foamed concrete 237–9 UWC 145–50 workability window 201 working environment 187–8 yield stress 189, 190–3