Concrete Technology for Cast In-situ Foundations (CIRIA Report)

Licensed copy:University of Manchester, 05/11/2007, Uncontrolled Copy, © CIRIA

CIRIA C569

London 2002

Concrete technology for cast in-situ foundations

N A Henderson N J R Baldwin

L D McKibbins

D S Winsor H B Shanghavi

Century House TelfordAvenue Crowthorne Berkshire RG45 6YS Telephone 01 344 466007 Fax 01 344 466008 Email [email protected] .uk WEBSITE www.concrete.org.uk

6 Storey’s Gate Westminster London SW1P 3AU Telephone 020 7222 8891 Fax 020 7222 1708 Email [email protected] WEBSITE www.ciria.org.uk

Who we are

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For 40 years CIRIA has managed collaborative research and produced information aimed at providing best practice solutions to industry problems. CIRIA stimulates the exchange of experience across the industry and its clients, and has a reputation for publishing practical, high-quality information.

How you can join CIRIA offers several participation options that have been designed to meet different needs. These include: •

Core Programme membership - for organisations that wish to influence CIRIA's collaboratively funded research programme and obtain early access to the results.

•

Project funding - for organisations that wish to direct funds to specific projects of interest. Project funders influence the direction of the research and obtain early access to the results,

•

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Publication C569

Concrete technology for cast in-situ foundations

sharing knowledge building best practice

Errata

Readers are advised that a section of Table 5.5 on page 70 is missing. It should read: 5b:

For segmental linings and associated units for tunnels and shafts in brownfield sites for water, sewer, storage and transportation - External surface

We apologise for any inconvenience this may have caused.

Summary

This report provides the designer, the contractor, and the concrete producer with guidance on the application of in-situ concrete technology for foundation applications. Environmental issues and sustainable construction practices are also discussed. The report is based on a detailed review of published literature, consultation with experts and practitioners within the field and case studies demonstrating good practice and highlighting critical issues.

Licensed copy:University of Manchester, 05/11/2007, Uncontrolled Copy, © CIRIA

The objectives of this report are twofold: • to provide current guidance on the analysis of durability and construction-related parameters to be considered in the selection of in-situ concrete for foundation applications. • to identify current best practice on the design, specification and use of in-situ concrete in common foundation types for a range of site conditions, based on both "mature" and "new" concrete technology. HEALTH AND SAFETY

Construction activities have significant Health and Safety implications. These can be the result of the activities themselves, or can arise from the nature of the materials and the chemicals used in construction. This report does not give comprehensive coverage of the Health and Safety issues relevant to the subject it covers and other published guidance should be consulted as necessary. Concrete technology for cast in-situ foundations

Henderson, N A, Baldwin, N J R, McKibbins, L D, Winsor, D S and Shanghavi, H B Construction Industry Research and Information Association CIRIA C569

© CIRIA 2002

ISBN 0 86017 569 3

Keywords Concrete and structures, ground engineering, piling, underground construction

Reader interest

Classification

Designers, specifiers, constructors and supervisors involved in below ground in situ concrete construction. Concrete and component material suppliers

AVAILABILITY CONTENT

STATUS USER

Unrestricted Guidance on concrete technology for foundation applications Committee guided Professionals concerned with concrete foundation construction

Published by CIRIA, 6 Storey's Gate. Westminster, London SW1P 3AU. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright-holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold and/or distributed with the understanding that neither the author(s) nor the publisher is thereby engaged in rendering a specific legal or any other professional service. While every effort has been made to ensure the accuracy and completeness of the publication, no warranty or fitness is provided or implied, and the author(s) and publisher shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from its use.

CIRIA C569

Acknowledgements

The project leading to this report is part of a collaborative programme between CIRIA and the Concrete Society entitled “Concrete Techniques” and was carried out under contract by Mott MacDonald. The project was undertaken in close collaboration with a steering group and technical committee established jointly by CIRIA and the Concrete Society.

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Research team N A Henderson BSc PhD CEng MIM

Mott MacDonald

N J R Baldwin BSc MSc CEng MIM FGS

Mott MacDonald

L D McKibbins BSc MSc FGS

Mott MacDonald

D S Winsor BSc CEng MICE MIStrucE

Mott MacDonald

H B Shanghavi BSc MSc CEng MICE

Mott MacDonald

With contributions from: D S Leek BSc MSc PhD CEng MIM CGeol FGS MICorr

Mott MacDonald

Steering Group Mr RA McClelland (Chairman)

Alfred McAlpine Construction Ltd.

Dr P Bamforth

Taywood Engineering Ltd

Mr B Brown

Ready Mixed Concrete Ltd

Mr R Cather

Ove Arup Research & Development

Dr C Clear

Civil & Marine Slag Cement Ltd

Professor T A Harrison

Quarry Products Association

Dr C J Hopkins

Wardell Armstrong

Professor E Kay

Halcrow Group Ltd

Dr D S Leek

Mott MacDonald Ltd

Mr N Loudon

Highways Agency

Mr L H McCurrich

Development Consultant (formerly FOSROC)

Dr J B Newman

Imperial College of Science & Technology

Mr P F Pallett

Temporary Works Consultant

Mr R Roberts

Concrete Advisory Service

Mr D A B Thomas

HSE

Mr P Titman

Edmund Nuttall

Mr M Walker

Concrete Society

Mr P Woodhead Regions

Department of Environment, Transport & the

Dr B W Staynes (Research Manager) CIRIA

CIRIA C569

3

Technical Committee

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The project was guided and the report prepared with the help of the project technical committee. Professor E Kay (Chairman)

Sir William Halcrow & Partners

Dr P Bamforth

Taywood Engineering Ltd

Mr D A Billington

AMEC Civil Engineering Ltd

Mr N Beavor

Harris & Sutherland

Mr R Cather

Ove Arup Research & Development

Mr M Connell

Appleby Group

Mr S Crompton

Ready Mixed Concrete (UK) Ltd

Professor T A Harrison

Quarry Products Association

Mr N Loudon

Highways Agency

Dr P Nixon

Building Research Establishment

Mr M Saunders

Curtins Consulting Engineers

Professor G Somerville

British Cement Association

Mr V Troughton

Stent Foundations Ltd

Dr B W Staynes

CIRIA

Financial Support This project was funded by the Department of the Environment, Transport and the Regions (DETR) and CIRIA's Core Programme Members.

Note Recent Government reorganisation has meant that DETR responsibilities have been moved variously to the Department of Trade and Industry (DTI), the Department for the Environment, Food and Rural Affairs (DEFRA), and the Department for Transport, Local Government and the Regions (DTLR). References made to the DETR in this publication should be read in this context. For clarification, readers should contact the Department of Trade and Industry.

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CIRIA C569

Contents

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Case studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

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Glossary of terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1

Introduction 1.1

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Scope and background to report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.2 Content of report 2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Foundation Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2

Piles and diaphragm walls

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3 Pad, strip and raft foundations 3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Design and Specification Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2 Standards and codes of practice

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3 Design strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.3.1 Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.3.2 Planning, site practice and buildability . . . . . . . . . . . . . . . . . . . . 26 3.4 Environmental issues and sustainable construction . . . . . . . . . . . . . . . . 29 3.4.1 Recycled aggregates 4

Properties of Concrete

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.2 Fresh properties

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.1 Concrete design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.2.2 Specification and measurement of fresh properties . . . . . . . . . . . 33 4.2.3 Self-compacting concrete ( S C C ) . . . . . . . . . . . . . . . . . . . . . . . . 34 4.3 Hardened properties

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.3.1 Strength development and specification . . . . . . . . . . . . . . . . . . . . 36 4.3.2 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.3.3 Curing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 4.4

Effect of cement/addition type 4.4.1 Portland cement (PC)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.4.2 Ground granulated blastfurnace slag (ggbs) . . . . . . . . . . . . . . . . . 43 4.4.3 Pulverised-fuel ash (pfa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.4.4 Silica fume (sf)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.4.5 Metakaolin (mk) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

CIRIA C569

4.4.6 Portland-limestone cement (PLC) . . . . . . . . . . . . . . . . . . . . . . . . 47

4.4.7 Sulfate-resisting Portland cement (SRPC) . . . . . . . . . . . . . . . . . . 47 4.5

Effect of aggregates

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.5.1 Effect of aggregate size and type . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.5.2 Aggregates and ASR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.5.3 Aggregate and sulfate attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.6

Effect of Admixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.6.1 Retarders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.6.2 Water-reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.6.3 Superplasticisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.7 Effect of placing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

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4.7.1 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.7.2 Placing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.7.3 Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5

Deterioration of Concrete Below Ground

. . . . . . . . . . . . . . . . . . . . . . . . . 58

5.1

Introduction

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.2

Effects of concrete deterioration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.3

Role of ground conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.4

Sulfate attack

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.4.1 Conventional sulfate attack

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.4.2 Thaumasite form of sulfate attack (TSA) . . . . . . . . . . . . . . . . . . . 60

5.4.3 Sulfuric acid attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5.4.4 Delayed ettringite formation (DEF) . . . . . . . . . . . . . . . . . . . . . . . 63

5.4.5 Determination and classification of exposure conditions . . . . . . . 63 5.4.6 Guidance and specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5.4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.5

Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.5.1 Significance of cracking

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5.5.2 Minimising cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

5.6

Reinforcement corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.7 Alkali-silica reactivity (ASR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.8

Other forms of attack 5.8.1 Acid attack

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.8.2 Salt crystallisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.8.3 Industrial contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 6

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

FIGURES Figure 4.1 Permeability of drilled cores for different water/binder (cement + silica fume) ratios (approximately equal 28-day strength). . . . . . . . . . . . . . . . . 45 Figure 4.2 Carbonate ranges for concrete mix design (BRE Special Digest 1, 2001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Figure 4.3 Concrete placing by tremie pipe (Clear et al, 1999). . . . . . . . . . . . . . . . . 56 Figure 5.1 Diagram to illustrate the main types of cracking in concrete (from Concrete Society Technical Report No. 22, 1992) . . . . . . . . . . . . . . . . . . 72

CIRIA C569

TABLES Table 3.1 Review of British and European Standards, Codes of Practice and Regulations relevant to in-situ concrete foundation construction . . . . . . . . 22 Table 3.2 Requirements for recycled concrete aggregate and recycled aggregate (pr BS 5 8 0 0 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Table 4.1 Concrete requirements for diaphragm walls (Clear et al, 1999). . . . . . . . . 33 Table 4.2 Mix proportions (kg/m3) of a SCC concrete (Bartos & Grauers, 1999). . . 36

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Table 4.3 Recommended minimum pfa and ggbs contents to reduce the likelihood of thermal cracking (Concrete Society TR 40, 1991). . . . . . . . . . . . . . . . . . . 37 Table 4.4 Proposed BS EN 197-1 cement types and their acceptability in Europe and equivalent British Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Table 4.5 Summary of applications and potential effects of using pfa and ggbs in concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Table 4.6 Principle admixture types used in foundation construction, their ultimate effect and practical application and benefit (Concrete Society Report 18, 2000). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Table 5.1 Exposure environments and types of deterioration possible . . . . . . . . . . . 58 Table 5.2 Aggressive Chemical Environment for Concrete (ACEC) Classification (BRE Special Digest 1, 2001). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Table 5.3 Concrete qualities to resist chemical attack (BRE Special Digest 1, 2001). . 68 Table 5.4 Cement groups for use in Table 5.3 (BRE Special Digest 1, 2001). . . . . . . 69 Table 5.5 Design guides for different categories of buildings and structures

(BRE Special Digest 1, 2001). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

CASE STUDIES Case Study 1 Improving construction design and planning - basement project in the City of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

London, UK.

Case Study 2 Reducing peak temperatures in large pours - Manchester, UK . . . . . . . . . . . 38

Case Study 3 Construction of secant pile walls - British Library, London, UK . . . . . . . . . . 40

Case Study 4 Foundations for an industrial process plant using metakaolin . . . . . . . . . . . . 46

Case Study 5 Failure of test-piles under loading due to incorrect placement techniques . . . 56 Case Study 6

Thaumasite form of sulfate attack (TSA) in motorway bridge foundations . . 62

CIRIA C569

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GLOSSARY OF TERMS Additions

Materials such as ggbs and pfa that are added to Portland cement at the concrete mixer to obtain a combination with the properties of a composite cement.

Aggressive chemical environement for concrete (ACEC)

Defined in BRE Special Digest 1,2001 as a scheme for classifying the aggressiveness of the ground to concrete. The classification takes into account sulfate and magnesium concentrations in soil and groundwater, and also the mobility and pH of the groundwater.

Alkali-silica reaction

Disruptive expansive reaction within concrete, caused by reaction between certain types of silica in aggregate and alkalis present in the cement paste.

Autogenous healing

The natural sealing of a crack in concrete by siltation together with further hydration of cement or by the deposition of calcium hydroxide with consequent conversion to calcium carbonate.

Admixtures

Chemical compounds added to fresh concrete to modify one or more of its properties in either the fresh or hardened state.

Bleeding

The phenomenon of water migrating to the surface of fresh concrete due to settlement of particulate materials.

Blended cement

See Composite cement.

Buildability

The ease with which the design may be constructed on-site.

Cohesiveness

The degree to which fresh concrete resists segregation.

Cold-joint

Plane of weakness caused by fresh concrete being cast against unprepared stiffened concrete.

Compactibility

The ease with which the entrapped air in concrete can be expelled.

Composite cement A cement containing two or more main constituents one of which is Portland cement. Concrete, designed

Concrete for which the purchaser is responsible for specifying the required performance and the producer is responsible for selecting the concrete proportions to produce the required performance.

Concrete, prescribed

Concrete for which the purchaser is responsible for specifying the required performance and the producer is responsible for ensuring that these proportions will produce a concrete with the performance required.

Concrete, standard

Concrete selected from the restricted list given in Section 4 of BS5328: Part 2: 1991 and made with a restricted range of materials.

Construction joint A joint between two adjacent concrete pours with no allowance for movement.

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CIRIA C569

Contiguous piles

A number of individual piles emplaced in adjacent positions, with joints subsequently sealed to form a single composite structural unit, eg a diaphragm wall.

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Continuous flight Method of in-situ concrete pile placement in which concrete is auguring (CFA) placed as the hollow tube bore is retracted. Core

The innermost part of a (thick) concrete section which is generally unaffected by external ambient temperatures.

Cover

Outer part of the concrete that provides protection to the reinforcement from the external environment.

Curing

The action of preventing or minimising the loss of moisture from fresh concrete, and maintaining a satisfactory temperature so that desired properties may develop.

Deep foundations Foundations extending beyond 3 m below finished ground level, generally including piles and diaphragm walls. Delayed ettringite Denotes the formation of ettringite in a concrete, mortar or cement formation (DEF) paste that has been subjected to a temperature high enough to destroy any that was earlier present. The effect can cause expansion or cracking. It could also occur in concrete that has been heated adventitiously through the heat evolved on hydration, or from an external source during service. Design chemical class (DCC)

Defined in BRE Special Digest 1 and BS 5328 for the “qualities” of concrete required to resist chemical attack in the ground. For a given DC Class, a range of concrete compositions are given incorporating selected aggregate combinations and cement types.

Diaphragm walls

Relatively thin slab-like elements constructed from either in-situ or precast concrete or by contiguous piling methods. Normally used as retaining walls, load-bearing members and cut-off walls.

Durability

The ability to resist deterioration and retain structure and function.

Drying shrinkage The long-term reduction in size of a concrete element caused by the loss of water upon drying. Early-age thermal The reduction in length of a concrete element caused by its con traction cooling from the elevated temperatures developed during cement hydration. Ettringite

ClRlA C569

A calcium sulfo-aluminate hydrate (3CaO.AI2O3.3CaSO 4.3 1H2O). It occurs naturally as a rare mineral. In construction materials it is commonly produced by the reaction between the tri-calcium aluminate component of cement clinker (C,A) and sulfate ions. Ettringite is one of the normal products of hydration in Portland cements and this ‘primary’ ettringite forms prior to hardening of the cement paste. However, once the cement paste has hardened, subsequent formation of ettringite as a reaction product can cause disruption of the host concrete or mortar.

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Ferret’s Rule

Empirically formulated rule relating concrete strength to absolute volumes of cement, water and entrapped air.

Finishing

Mechanical operations carried out on the surface of placed concrete to achieve the required texture, appearance and serviceability.

Foundations

Those parts of a structure that are in direct contact with the ground and that transmit the load of the structure to the ground.

Fresh concrete

Concrete which has insufficient reaction between the cement and water to exhibit significant mechanical properties.

Groundgranulated blast furnace slag

A fine particulate material, a by-product of pig-iron manufacture, which exhibits latent hydraulic properties and may be used as an addition material.

Hardened concrete Concrete which exhibits mechanical properties as a result of the reaction between cement and water. Heat of hydration Heat generated by the chemical reaction of cement and water.

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Metakaolin

A highly reactive natural pozzolan formed when china clay, the mineral kaolin, is heated to a temperature between 600 and 800°C. Metakaolin may be used as an addition material in concrete.

Mobility

The ease of flow of fresh concrete.

Movement joint

A joint formed or induced to allow movement to occur in one or more degrees of freedom.

Pad foundations

Shallow foundation elements used to transmit the load of structural columns where bearing capacity is good or the structural loads to be carried are relatively light.

Permeation

Used in its looser sense, a measure of the ease with which substances, either in gaseous or liquid form, may enter and be transported through a material by processes including flow under pressure differentials, diffusion and sorption.

Piles

Relatively long, slender members, driven or bored into the ground to carry the foundation loads to depths beyond the capability of shallow foundations.

Placeability

The ease of placing of concrete on site.

Placing

All operations necessary for introducing the fresh concrete into its final position.

Plastic concrete

See Fresh concrete.

Plastic shrinkage cracks

Cracks which form due to loss of excessive moisture from concrete while in its fresh (plastic) state.

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CIRIA C569

Producer

Person or authority entering a contract to supply concrete.

Pulverisedfuel ash

Also known as “fly-ash”. A fine particulate material - a byproduct of the combustion of pulverised coal in electricity generation - which is pozzolanic and may be used as an addition material.

Raft foundations

Shallow foundation elements used to spread load transmitted by multiple elements, in conditions where bearing capacity is good or the structural loads to be carried are relatively light.

Restraint

Anything (internal or external) which prevents the free movement of concrete.

Rheological

Pertaining to the flow of concrete.

Secant piling

Method of construction by which a contiguous pile wall may be formed, involving sequential placement of piles which overlap, requiring cutting of secant sections from previously placed piles in the sequence.

Segregation

Loss of uniformity of fresh concrete mix due to separation of one or more of the constituents of the concrete.

Self-compacting concrete

High performance concrete specifically designed to compact under its own weight without any external vibration.

Setting time

The time for a cement paste to stiffen sufficiently to resist to a specified degree the penetration of a weighted needle; typically categorised into two stages. initial set and final set.

Settlement

Sedimentation or the act of heavy solid particles settling in fresh concrete.

Shallow foundations

Defined in BS 8004 as those where the depth, below finished ground level is less than 3 m, and including pad. strip and raft foundation types.

Silica fume

Also known as “microsilica” or “condensed silica fume”. A very fme particulate material, a by-product of electric-arc furnace production of silicon and ferro-silicon alloys, which is highly pozzolanic and may be used as an addition material.

Specification

Any technical requirement to which a contractor is required to conform.

Stability

See Cohesiveness.

Stiffening time

Time beyond which reworking of a concrete would be detrimental.

Strip foundations

Shallow foundation elements used to spread load transmitted by load-bearing walls or a row of columns, in conditions where bearing capacity is good or the structural loads to be carried are relatively light.

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Superplasticiser

A chemical admixture which, when added to a concrete, may produce a significant reduction in water demand and/or increase in workability.

Tensile strain capacity

The tensile strain at which a concrete will crack.

Thaumasite

A calcium silicate carbonate sulfate hydrate (CaSiO3.CaCO3.CaSO4. 15H2O). It occurs naturally as a rare mineral. In construction materials it occurs as a reaction product, either on its own, or alongside ettringite or gypsum. Thaumasite is structurally similar to ettringite and because of this, a partial solid solution between the two minerals can occur resulting in ettringite/thaumasite mixed crystals. It is commonly formed by the reaction of sulfate and carbonate ions with calcium silicate hydrates (CSH) in hardened Portland cements..

Thaumasite formation (TF)

This is thaumasite which is found in construction materials in preexisting voids and cracks but which has not caused deterioration of the concrete or mortar. It may be the precursor to TSA.

Thaumasite form of sulphate attack (TSA)

This is a form of sulfate attack in which there is significant damage to the matrix of a concrete or mortar as a consequence of replacement of cement hydrates by thaumasite.

Transporting

The conveying of fresh concrete from the point of discharge from the mixer to its final enclosed position.

Workability

The rheological properties of the fresh concrete which influence the ease with which it may be placed, compacted and finished.

CIRIA C569

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ABBREVIATIONS

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ACEC

Aggressive chemical environment for concrete

ASR

Alkali-silica reaction

CFA

Continuous flight auguring

DCC

Design chemical class

DEF

Delayed ettringite formation

ggbs

Ground granulated blastfurnace slag

mk

Metakaolin

PC

Portland cement

PLC

Portland limestone cement

Pfa

Pulverised-fuel ash

RA

Recycled aggregate

RCA

Recycled concrete aggregate

sf

Silica fume

SCC

Self-compacting concrete

SRPC

Sulfate-resisting Portland cement

SWL

Safe working load

TF

Thaumasite formation

TSA

Thaumasite for of sulfate attack

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RECOMMENDATIONS 1. In the UK the design and construction of common in-situ concrete foundations is usually undertaken in accordance with BS 8004 but it is common for combinations of other Standards and Codes of Practice to be used. Under certain circumstances these Standards do not adequately address the specific requirements of in-situ concrete for deep foundation structures. It is, therefore, recommended that particular attention should be given to the following: •

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· ·

workability: the tolerances given in BS 5328 for slump may be unacceptable for placing operations using pumps/tremie pipes. It is therefore essential that slump requirements are discussed with the concrete supplier before commencing work

cover: the specification of nominal cover to reinforcement for diaphragm walls should allow tolerances for cage installation and uneven ground surfaces both horizontal and vertical reinforcement spacing: compliance with crack width calculations to BS 5400 or BS 8110 can sometimes lead to reinforcement spacing being too close for reinforced concrete foundations. For fully embedded piles the surrounding ground provides support to the pile and distributes the loads. Conventional structural theory can, therefore, be used to design a pile as a column provided the supporting effect of the ground is considered when calculating forces applied to the pile. The overall structural strength of the soil/pile system should take this into account. Piles designed in this manner may have significantly less reinforcement (ie reduced nominal requirements for links and minimum numbers of bars) than those complying with BS 8110 or BS 5400, yet will still perform satisfactorily.

2. A thorough, well-planned and well-executed site investigation is essential. This should address both the design and construction phases of the project and include a detailed ground condition survey which could be used to assess the Aggressive Chemical Environment classification (ACEC). Where excavated sulfide-bearing clay is to be reworked and in contact with buried concrete, consideration should be given to the possibility that some sulfide may be oxidised to sulfuric acid and soluble sulfates. Detailed guidance on designing concrete to minimise the risk of chemical attack - including all forms of sulfate attack - is provided in BRE Special Digest 1 (2001). 3. The recommendations given in BRE Special Digest 1 (2001) should be taken in the context of the whole document and not taken as absolutes. The digest allows for a trade-off between different protective measures and includes a list of different additional measures to be considered. This flexibility should allow practical and economic solutions to be found for most circumstances. 4. The practical “buildability” issues pertinent to the foundation construction have to be considered. For instance, large pours require a rapid and continuous supply of suitable concrete and may necessitate measures to prevent the development of excessive peak temperatures and unacceptably large differential thermal gradients. Over-congestion of reinforcement and inappropriate placement and compaction techniques can lead to poor quality concrete and should be avoided.

5. Concrete quality, and ultimately the performance of the foundation itself, is strongly influenced by the method and quality of its placement. It is recommended that concretes should be designed to exhibit the following qualities to facilitate

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placement in deep foundations: high workability, good cohesion, self-compaction and resistance to segregation and bleeding. Workability should be included in the concrete specifications as a quality control parameter.

6. The incorporation of composite cements or combinations of Portland cement with pfa, ggbs. sf or mk should be considered to improve durability, enhance fresh properties, reduce heat generation and provide economic and environmental benefits. Admixtures, such as superplasticisers and retarders, can be used to modify the properties of the fresh concrete in a variety of ways as desired.

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7. A high level of co-operation, exchange of information and open discussion at an early stage between all parties involved in a construction is highly desirable. Such co-ordinated efforts between members of the design and construction team can have a markedly positive influence on a project, and can provide benefit in terms of efficiency. economy and innovation.

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1

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1.1

Introduction

SCOPE AND BACKGROUND TO REPORT This report focuses on the concrete technology issues relating to common buried foundation types including piles, diaphragm walls and pad, strip and raft foundations. The information contained within the report is concerned with providing guidance on the analysis and construction parameters to be considered in the selection of in-situ concrete for foundation applications and identifying best practice on the design, specification and use of in-situ concrete for a range of site conditions. Environmental issues and sustainable construction practices are also discussed. Issues relating to precast concrete foundations are not covered in this report.

BS 8004 (1986) is the current Code of Practice for foundations and provides detailed guidance on the design and construction for the normal range of buildings and engineering structures. However, it is common for combinations of standards to be used for the structural design and construction of in-situ concrete foundations in a variety of different structure types. These other codes and standards, while necessarily generalised are, however, primarily appropriate for surface and elevated construction and, in certain circumstances, they have limitations where used for in-situ concrete foundation applications. These need to be highlighted. The important considerations for the design and construction of in-situ concrete foundations include: •

assessment of exposure conditions

•

soil/structure interaction

•

fresh and hardened properties of concrete

•

concrete placing requirements

•

durability design.

Modem concrete technology affords increased flexibility to designers and specifiers of in-situ concrete to modify its fresh and hardened properties thereby enhancing placing, strength and durability. Materials such as Portland cement (PC), pulverised-fuel ash (pfa), ground granulated blast furnace slag (ggbs) and silica fume (sf) have an established track record as materials for in-situ concrete and have found use, to a greater or lesser extent, in the majority of foundation types considered in this report. While these materials represent “mature” technology, specialist materials such as selfcompacting concrete (SCC) and metakaolin (mk) have recently been developed with the potential to improve concrete placing and durability. There is limited experience of these “new” materials in the UK and they are generally outside the scope of current codes and standards. However, there are considerable potential benefits of using these materials for in-situ concrete foundation construction, and they will, therefore, be discussed in this report within the limits outlined in Section 4.4.

This report will undertake to: •

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discuss the different requirements in current codes used in the design of common foundations

ClRlA C569

• identify current best practice on the design, specification and use of in-situ concrete based on the use of“mature” and “new” concrete technology. Several case studies are included throughout the report to highlight key issues and best practice.

1.2

CONTENT OF REPORT The report is presented in sections as follows: SECTION 2: FOUNDATION TYPES

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• describes common foundation types. SECTION 3: DESIGN AND SPECIFICATION REQUIREMENTS

• Codes of Practice and Standards in relation to foundation construction providing guidance in their use are considered • practical factors such as buildability, plant requirements and access are briefly reviewed •

environmental issues and sustainable construction practices are briefly discussed.

SECTION 4: PROPERTIES OF CONCRETE

• the general properties of fresh and hardened concrete suitable for in-situ concrete foundation applications are described •

the variety of “mature” and “new” constituent materials available to achieve them are reviewed

• the effect of placing techniques used for in-situ concrete foundations are briefly discussed. SECTION 5: DETERIORATION OF CONCRETE BELOW GROUND

•

the durability of concrete below ground is considered

• the various deterioration processes are reviewed • guidance on achieving durability is provided.

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2

2.1

Foundation types

INTRODUCTION This section introduces the foundation types that are considered in this report and briefly reviews their main characteristics.

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Common foundation types fall into two broad categories: •

deep bearing elements such as piles and diaphragm walls

•

shallow bearing elements such as pad, strip and raft foundations.

The important aspects of foundation design and construction include; selection of appropriate foundation type, concrete properties and placement, reinforcement detailing, tolerances and quality control. Practical factors to consider include buildability, plant requirements and access. In subsurface works the choice of construction method is often an important consideration, particularly where there are problems such as limited space, difficult ground, tight schedules and environmental restrictions. For example, in urban situations driving of piles is commonly ruled out due to prohibitions on noise and vibration. Possible solutions in such situations include the use of “silent” hydraulic piling systems or bored contiguous or secant pile systems. Continuous flight auger (CFA) systems, in which the pile is cast as the auger is retracted, are also becoming increasingly popular. There are a number of specialist contractors able to provide techniques and equipment tailored to cope with such problematic site conditions and specific requirements, including a number of proprietary piling systems. The appropriate selection of such contractors and systems should be based upon informed engineering judgement considering the specific construction type and site conditions prevailing. Ground improvement is one other area where specialist advice and techniques may be of great value in foundation construction.

2.2

PILES AND DIAPHRAGM WALLS Piles and diaphragm walls are usually used to transmit loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. Contiguous pile, secant pile and diaphragm wall systems are also utilised as embedded retaining wall systems for deep basements and larger buried structures or a combination of both.

PILES AND CONTIGUOUS PILE-WALLS Piles are relatively long and slender members - driven or bored - into the ground to carry the foundation loads to depths beyond the capability of shallow foundations. They are also used in some situations where there is a high water-table, where construction is too restricted to allow spread footings, to resist uplift forces resulting from wind-loading, or in poor soil conditions to resist modest horizontal loads. Piles may be used in combination with raft foundations where the raft is in effect an integral

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pile cap or where the piles can limit settlement at zones of high concentrated loads. Pile diameters can range from the minipile at 150 mm to the caisson-sized 2000+ mm. The depths are typically between 6 and 35m. however, if required, rotary augered piles can be advanced to more than 50 m.

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Piles may also be used to form contiguous structural diaphragm walls by boring and emplacement of in-situ concrete, either so that their interfaces are just touching, or by the technique of secant piling. Here individual piles are bored and concreted so as to form a continuous interlocking wall that acts as a continuous and watertight structural element. The most commonly used method entails the initial placement of "female" piles at spaces of less than two pile diameters, then subsequent boring and concreting of "male"piles in the spaces between them. Emplacement of the male piles involves cutting of secant sections from each female pile, hence the name of the technique. The main advantage of these methods is that the diaphragm wall may be constructed with limited headroom (as little as 2 m). Disadvantages include the requirement for a reinforced tie-beam over the pile-heads and for a facing (usually a cement render lightly reinforced with steel mesh) to overcome irregularities in the bored surface. Recommendations concerning specification for piling are included in ICE Specifications for Piling and Embedded Retaining Walls, 1996.

DIAPHRAGM WALLS Diaphragm walls are normally used as retaining walls, load-bearing members and water cut-offs. They are most frequently cast in situ and have a number of positive features: •

they eliminate the need for temporary works when constructed before bulk excavation

•

they may be constructed to form a final structural wall

•

construction techniques have low environmental impact (minimal vibration and noise)

•

they may be constructed directly adjacent to existing structures

•

able to withstand vertical and horizontal forces according to design

•

economical method of construction.

Cast in-situ diaphragm walls may be constructed in two ways: •

in touching or interlocking bored piles (see above)

•

in trenches excavated in the ground. The trench is dug through a bentonite slurry or other fluid that temporarily supports the sides during construction. The length of the panel depends on the trench stability during the construction and the volume of concrete that can be placed in a working day. The walls are constructed in alternate panels, ranging from 4.5 m to 7.0 m in length. They are typically 500-1200mm thick and up to 30 m deep, but may be up to 100 m depth.

Where ground conditions are suitable, diaphragm wall construction can utilise a combination of precast and in-situ units, with reinforced in-situ concrete panels cast between precast posts placed at suitable centres. Such composite construction provides a great degree of flexibility but is not considered in this report. Where diaphragm walls form retaining walls to highway underpasses, the Highways Agency, Specification for Highway Works (SHW), Vol.1, Series 1600, March 1998 and

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the associated Notes for Guidance on the Specification of Highway Works, Series NG1600 set out the requirements for the concrete. Other recommendations concerning piles and diaphragm walls are given in: BS EN 1538: 2000. Execution ofspecial geotechnical work: diaphragm walls

DIN 4126 August 1986. Cmt-in-situ concrete diaphragm walls, design and construction ICE Specifications for Piling and Embedded Retaining Walls, 1996.

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2.3

PAD, STRIP AND RAFT FOUNDATIONS Pad and strip foundations are typically shallow elements used where bearing capacity is good or the structural loads to be carried are relatively light. Shallow foundations are defined in BS 8004 (1986) as those where the depth below finshed ground level is less than 3 m. Particular care is required in assessing the ground conditions for shallow foundations since the concrete is often at risk from; exposure to ground or groundwater contamination, (see Section 5.3 of this report), the effects of freeze/thaw cycles and damage from vegetation. The selection of the appropriate type of shallow foundation will depend on; the magnitude and location of the loads, the bearing capacity of the soil and the extent of settlement that can be tolerated by the structure. Raft foundations can form basement floor slabs where crack control and water-tightness are key factors in determining concrete mix design and reinforcement detailing. In masonry construction unreinforced mass concrete trench-filled footings are an economical option. Where there is the likelihood of ground problems such as heave, shrinkage and swelling, the potential for foundation damage can be minimised by the incorporation of specialist construction techniques and materials, eg void formers, slip layers etc.

PAD FOUNDATIONS Pad foundations are usually built to support loads from structural columns. Pads may be constructed as square, circular or rectangular slabs, and can be of mass or reinforced concrete. Pads can be of uniform thickness or stepped to allow for more load distribution. In some cases steel beam grillages are used as reinforcement for heavily loaded columns.

STRIP FOUNDATIONS Strip foundations are used to transfer load to the ground from load-bearing walls or from a row of columns when these are closely spaced. Usually it is more economical to construct a strip foundation whenever the clear space between square pads is less than the dimension of the pad itself. Where the strip foundation is sufficiently wide to develop bending moments in the projecting portions, the foundation should be reinforced to prevent flexural cracking.

RAFT FOUNDATIONS When the ground is of low bearing capacity or when columns are closely spaced in more than one direction, a raft foundation may be more economical than several strip foundations. A raft foundation could be a plain slab of reinforced concrete or a

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combination of slabs and beams. Raft foundations are useful in reducing the effect of differential settlement whenever there is a wide range of loads applied or if the supporting soil varies in bearing capacity across the site. The raft can bridge over localised areas of weaker soils reducing the problem of differential settlement. Where highly localised loads occur on a raft, settlement-reducing piles can be incorporated to control differential movements and reduce bending moments. Raft foundations can be constructed as part of a basement to create a hollow watertight space under a building. These are often referred to as cellular or buoyancy rafts. The function of these rafts is to spread the loads over a wider area and to reduce the total settlement using the principle of buoyancy. The uplift force of the soil and ground water displaced by the hollow basement volume reduces the net load on the soil. The combination of the stiffness of the buoyancy raft (box foundation) and the reduction in net load applied to the soil reduces the differential settlements of the structure above. Rafts forming basement slabs need to accommodate serviceability requirements for its intended grade of use including; internal environment, surface finish, durability and water-tightness (reference should be made to CIRIA Report 139 - Water resisting basements - a guide. Safeguarding new and existing basements against water and dampness, for basement requirements).

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3

Design and specification requirements

3.1

INTRODUCTlON

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This section discusses the design and specification requirements for in-situ foundation concretes. The differing requirements in current Codes and Standards used in the design of common foundations are discussed and the relevant site practice and buildability issues related to good construction are briefly reviewed. Environmental issues and sustainable construction practices are also discussed.

3.2

STANDARDS AND CODES OF PRACTICE Historically in the UK there have been several Standards and Codes of Practice used for concrete foundation construction; these and more recent European Standards are listed in Table 3.1. The structural design of foundations is normally carried out in accordance with BS 8004 (1986). This Code deals with the design and construction of foundations for the normal range of buildings and engineering structures, excluding foundations for special structures. BS 8004 includes the general design of shallow (< 3 m below finished ground), deep (> 3 m) and sub-aqueous foundations, cofferdams and caissons, and pile foundations and also provides guidance on geotechnical processes. However, although foundations in general can be designed to BS 8004 it is common for combinations of Standards to be used in design. However, in certain circumstances there are limitations to current British Standards that need to be highlighted. These relate particularly to deep foundations such as piles and diaphragm walls, where it is especially important that concrete should be designed to compact under its own weight without segregation and excessive bleeding. The new European standards, such as BS EN 1536 (2000) and BS EN 1538 (2000) give guidance on specifying concrete for bored piles and diaphragm walls, respectively. These standards begin to address the fact that in-situ foundation applications require specialised concrete specifications and are discussed further in Section 4.

Table 3.1

Review of British and European Standards, Codes of Practice and Regulations relevant to in-situ concrete foundation construction. Standard/Design Code

BS 8002: 1994 Code of Practice for Earth Retaining Structures

Key Points

Design of earth retaining structures - gravity walls - reinforced concrete walls - embedded walls - strutted excavations and cofferdams - anchorages - waterfront structures

Table continues on next page

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Standard/Design Code

Key Points

BS 8004: 1986

Design of foundations in relation to construction

Code of Practice for Foundations

Design considerations for:

- shallow foundations Pad footings Strip footings Raft footings Short piling

- deep foundations - cofferdams and caissons

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-pile foundations BS 8007: 1987

Design of basements

Design of Concrete Structures for Retaining Aqueous Liquids

Control of thermal cracking

BS 8110: Part 1: 1997

Design of:

Structural Use of Concrete - Code of Practice for Design and Construction

- pad footings -piles

- pile caps Guidance on preventing water ingress into below ground buildings

BS 8102: 1990 Code of Practice for Protection of Structures against Water from the Ground

BS 8103: 1995

Unreinforced concrete foundations:

Structural Design of Low-Rise Buildings

-strip footings

- trench fill BS 5400: Part 4: 1990

Design of bridge bases

Steel Concrete and Composite Bridges - Code of Practice for Design of Concrete Bridges

Deflection of bridge bases

Crack control in bridge bases

BS 6349: Part 1: 1984 Maritime Structures

Refers to foundation structures mentioned in other Standards

- General Criteria

BS 5328 Parts 1-4 (to be replaced by pr BS 5800)

Guides to and methods for specifying concrete

BS EN 1536: 2000

Establishes general principles for the construction of piles formed in the ground by excavation

Execution of Special Geotechnical Work - Bored Piles

Procedures for producing. transporting and testing concrete

BS EN 1538: 2000

Specifies the execution of diaphragm walls

Execution of Special Geotechnical Work - Diaphragm Walls

Guidance on:

ICE Specification for Piling and Embedded Retaining Walls, 1996

- site investigations - materials and products - execution. supervision and monitoring National industry guidance for deep foundation work in the UK

Specific examples where existing Standards do not adequately address the specific requirements of in-situ concrete for deep foundation structures are summarised below: •

CIRIA C569

workability: high workability is often a requirement for deep foundation concretes. However, when concrete is specified with a target slump to BS 5328 a tolerance on specified slump of +/- 33 per cent is allowed. Hence with a target slump of 150 mm the tolerated range is 100 mm to 200 mm. For some foundation placement methods, eg tremie concretes, this degree of variability can be unacceptable. It is therefore essential that the slump requirements are discussed and agreed with the concrete

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supplier before commencing the works. It should also be noted that the optimum method for measuring workability in high-slump concretes is the flow test (BS 1881: Part 105). See Section 4.2.2 •

cover: complex arrangements of reinforcement detail for some elements of foundation design require a higher minimum cover than that generally specified. in order to allow for tolerance in installation. For instance, the minimum cover specified by BS 8110 C1. 3.3.1.4 (for concrete cast against ground) is 75 mm, but specification of 100 mm nominal cover might be more suitable for diaphragm walls - particularly T panels - to allow tolerance for cage installation and uneven ground surfaces both horizontal and vertical

•

reinforcement spacing: compliance with crack width calculations to BS 5400 or BS 8110 can sometimes lead to reinforcement spacing being too close for reinforced concrete foundations. For fully-embedded piles the surrounding ground provides support to the pile and distributes the loads. Conventional structural theory can, therefore, be used to design a pile as a column provided the supporting effect of the ground is considered when calculating forces applied to the pile. The overall structural strength of the soil/pile system should take this into account. Piles designed in this manner may have significantly less reinforcement (ie reduced nominal requirements for links and minimum numbers of bars) than those complying with BS 8110 or BS 5400 yet will still perform satisfactorily

•

corrosion of reinforcement: this is unlikely in fully-embedded piles below the water table where there is a lack of oxygen. Reinforcement corrosion is discussed in more detail in Section 5.6

•

use of additions: the guidance given in BS 8007 (1987) regarding design of foundations for concrete structures for retaining aqueous liquids, is in places unclear and potentially confusing. Clause 6.2.1 states “...that for normal use the target mean proportion of ggbs should not exceed 50 per cent”. This clause appears, at first sight, to prohibit the use of higher proportions of ggbs in concrete. The intention of the BS 8007 clause may have been to separate “normal use” (ie preventing carbonation-induced corrosion) from “special use” (as outlined below) in a manner similar to that of the Specification for Highway Works (DOT, 1998). A “special use” may occur when hardened foundation concrete is subjected to severe sulfate exposure conditions and or is required to have a low heat of hydration in order to minimise thermal cracking.

The use of higher proportions of ggbs represents one of a number of options available to the designer under these “special” conditions and recommendations to this effect are provided in several authoritative guidance documents. The Civil Engineering Specification for the Water Industry (Water Authorities Association, 1989), recommends that 50 to 70 per cent ggbs can be used and the Thaumasite Expert Group Report (TEGR, 1999) recommends a minimum of 70 per cent ggbs for Design Chemical Classes 3,4 and 5 when using aggregates with approximately 30 to 100 per cent carbonate contents (see Sections 3.3.2 and 4.5 of this report). Similar guidance will be provided in the new BRE Special Digest 1, on sulfate and acid resistance . In addition, BS 6439 (1984) recommends using 70 per cent ggbs concrete as one of the options to resist combined attack from chlorides and sulfates. The designer should, therefore, bear in mind that the recommendations for ggbs concrete are based on “normal” use. When concrete is likely to be subject to “special” use then the guidance given in BRE Special Digest 1 on sulfate resistance should be followed. BS 8007 should therefore allow higher proportions of ggbs to be used once the concept of “special” use is clarified and understood. It should however be noted that where ground conditions are classified as Design

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Chemical Class DS-4m (ie very strong magnesium sulfate solutions) and the aggregate is low carbonate (ie Ranges B and C), the only cement recommended in the TEGR (1999) and the BRE Special Digest 1 (2001) is sulfate-resisting Portland cement (SRPC). The concepts of Design Chemical Class and aggregate Ranges are discussed in detail in Section 5.4.6.

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•

Cement content: Clause 6.3 of BS 8007 (1987) places a maximum limit of 400 kg/m3 on cement content. The purpose of the maximum cement content is to minimise the risk of early age thermal cracking by minimising the temperature fall from hydration to peak ambient. Although such a limit may be suitable for PC concretes, it is inappropriate for concrete containing more than 50 per cent ggbs. According to Henderson and Clear (2000), the maximum cement content could be in excess of 400 kg/m3 for 70 per cent ggbs concrete and this is the minimum cement content recommended for ggbs in BRE Special Digest 1 (2001). It is therefore impractical to make the maximum and minimum cement contents equal because there is no flexibility to achieve other concrete properties such as strength and workability. However, there are some field data (Bamforth, 1980) which indicate that the concrete tensile strain capacity with 70 per cent ggbs is lower than that of equivalent PC or PC/pfa concrete. With sections more than 2.5 m deep, Bamforth (1 980) estimates that the reduced tensile strain capacity of the ggbs concrete may cancel out the benefit of the reduced temperature rise. However, in sections less than 2.5 m deep, Bamforth estimates that the reduction in thermal contraction is greater than the reduction in strain capacity. This is discussed in detail in Section 4.4.2 of this report.

Over the last few years the UK construction industry has taken steps to improve the design and specification of foundation concrete and in 1996 The Institution of Civil Engineers published the Specification for piling and embedded walls (ICE, 1996). The Specification was prepared in collaboration with the DOT, Ove Arup and Partners and the Federation of Piling Specialists and it was based on the revised piling and embedded retaining wall requirements of the Highways Agency’s Specification for Highways Works (1998). The new ICE Specification represented a significant step forward for the UK construction industry, as it meant that all deep foundation works could be undertaken to a common set of rules, with the specialist contractors able to develop their own works procedures to a universally usable standard. In 1999 the Federation of Piling Specialists (FPS) published The Essential Guide to the Application for Piling and Embedded Retaining Walls (FPS, 1999). This was prepared by working groups of the FPS comprising individual specialists with extensive experience in the techniques covered in the ICE Specification. The document provides practical advice on the use of the ICE Specification and includes detailed commentary on each section with references to specific clauses. It is intended for practising engineers to promote an understanding of the practical considerations that lie behind the specified requirements.

3.3 3.3.1

DESIGN STRATEGY

.

Specification Several features are important in the design and construction of in-situ concrete foundations. Some examples of these are listed here: • there is uncertainty associated with the loading conditions where they are influenced by the behaviour of the ground. Consequently there is often a need to cater for a wide range of conditions

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•

there is likely to be some degree of uncertainty regarding the character of the chemical environment arising from limitations associated with the ground investigation and their inherent variability

•

there is a significant potential for movement in the ground and this influences the design of in-situ concrete foundations

•

the method of below ground construction is especially important, particularly feasibility and the implications for programming and costs, all of which may determine the form of the design.

Although these features are true of most structural concrete, they should be considered carefully when preparing specifications for in-situ foundation concrete. The above points also provide the concrete technologist with a number of challenges when designing and constructing in-situ concrete foundations. Concrete is specified in terms of hardened properties (ie strength) or concrete properties such as cement content and type, and water/cement ratio. However these do not directly address all of the performance criteria that may be required for in-situ foundation concrete. Regarding the fresh properties of the concrete, workability is commonly specified, although there is generally no guidance on specifying other properties such as cohesion and resistance from segregation, which are generally covered by a simple blanket statement. The specification of fresh and hardened concrete properties is discussed further in Section 4.2.

3.3.2

Planning, site practice and buildability Detailed information on the successful planning and completion of good quality foundation construction can be found in the following publications; •

Essential Guide to the ICE Specification for Piling and Embedded Retaining Walls, FPS. 1999

•

Concrete mixes -planning and design for transporting, placing and finishing, CIRIA Report 165, 1997.

•

Foundation design and construction, Tomlinson, 1995

•

Concreting diaphragm walls - Current practice sheet No 115, Clear et al. 1999

PLANNING STAGES The following is a summary of the planning stages required to ensure the successful completion of good quality concrete foundation works:

26

•

A thorough, well-planned and well-executed site investigation is essential. This should address both the design and construction phases of the project, provide detailed information on the nature of the ground and characterise its variability. Classification of subsurface conditions is necessary for concrete design and .durability assessment. However, there may be contractual difficulties where site investigations include both factual and interpretative information.

•

According to their involvement in the construction process, all relevant parties, including the designer, client and contractor, should have access to the site investigation report.

•

The method of construction should be compatible with the designer’s intentions. Where possible, early discussions between the designer and contractor are helpful in achieving this end and may avoid problems caused by inappropriate and incompatible specifications and construction techniques.

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• Tolerances quoted in the specification are usually achievable in some circumstances. However they may not be practical under the specific conditions of a given project. For example, piling under restrictive headroom may require the use of shorter tools and thereby reduce control of verticality. Eurocode EC7, Geotechnical design (1995). allows for the agreement of project-specific tolerances for piling works prior to the commencement of the works. These may take into account the design requirements. ground conditions and available equipment. Restraints stemming from the environment (eg site size, location and topography, headroom) and from buildability (eg member size, reinforcement congestion) may affect the structural requirements of the work.

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• Method statements, detailing the construction method and concretes to be used, should be provided by the contractor to enable the engineer to assess their suitability (dependent upon the contractual set-up of the project). • Discussion and agreement of the cohesion, bleed and workability requirements with the concrete supplier is essential before commencing works. Details on the methods of placement are discussed in detail in Section 4.7 of this report. • Successful completion of large pours is very much dependant on the organisation of the site works. For large continuous pours guidance is given in CINA Report 135 Concreting deep lifts and large volume pours (1995) but it is vital for ready- mixed concrete that the placement of concrete keeps pace with the rate of delivery. Concrete supplies have to be arranged in advance and all parties should be aware of the schedule of delivery and placement. Good site practice requires adequate supervision, appropriate levels of explanation to operatives and an awareness of the consequences of poor practice. As mentioned above, it is essential that in the interest of the project as a whole all parties seek to communicate all relevant data, whenever possible. The findings of the Government-appointed Construction Task Force, published in Rethinking Construction (1998) - also known as “The Egan Report” - provides a useful analysis of the problems associated with construction, the potential for improvements in management and organisation of construction projects and the associated potential benefits.

On certain projects below ground construction is already moving forward with the introduction of; an integrated approach, skills training, minimisation of waste, appropriate use of information technology, supply chain management and innovation. An example of a project where this approach has been used is given below.

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Case Study 1

-

Improving construction design and planning basement project in the City of London Summary: The construction of a large underground basement on a congested site in the City of London is a good example of how careful design and planning, with input from all main parties involved, can enhance a number of aspects of the construction.

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A high level of co-operation, exchange of information and open discussion between

all parties involved in a construction can have a markedly positive influence on a project and can provide benefit in terms of efficiency, economy and innovation. Ultimately, these benefits may be reflected in the construction itself. A good example is given in construction of a large underground basement in a highly congested site in the City of London (Goring, 1999). The following is an outline of the project’s organisation and progression.

At the start of the detailed design phase, the developer appointed a project manager who had a partnering agreement with the construction manager. Key specialist contractors were appointed by negotiation and involved in the design development at an early age. Regular meetings and workshops involving the architect, consulting engineer, services engineer and construction manager and trade contractors, were held throughout the design development phase, with briefing sessions and presentations to the client. During the detailed design phase, weekly meetings were held with the structural consulting engineers to discuss schedules of information to be provided, poursizes, reinforcement detailing, temporary works and other construction details. Detailed planning was necessary to integrate smoothly the various trade packages in the very limited space available on site. Work zones, storage areas, schedules and hand-over dates were discussed and agreed between trade contractors at planning meetings. These were marked on drawings which were signed by site managers. The basement slab was constructed independently of the pile-caps to allow steel erection to commence at an early stage of the project. The trade contractors were housed in an open-plan site-officeto encourage exchange of information. An adequate level of skilled labour was ensured by a requirement that 60 per cent of labourers be Construction Skills Certification Scheme (CSCS) cardholders, with additional training available through the unions. A draft specification for the works was made available for comment and discussion at an early stage - prior to finalisation of a price for the works. Careful review of the construction requirements, programme requirements and site restrictions allowed development of a programme for concrete pouring. Based on their previous experience of such constructions, the consulting engineers relaxed their maximum limit on the size of single slab pours from 40 m3 to 100 m3, and in some areas it was decided that pours of 200-300m3 could be accommodated without detriment to the structure’s watertightness. An afternoon seminar was held to discuss the possibilities and alternatives for the basement slab construction. This involved site and technical staff from the consulting engineer, construction manager and trade contractors, as well as the local representative of the Concrete Advisory Service. Topics discussed included design parameters, concrete technology and concrete designs, pour sizes and methods and finishes. The practical applications and benefits of using non-standard materials such as self-compacting concrete and carbon-fibre reinforced plastics for waling beams were considered. A detailed assessment of the construction allowed the use of temporary materials and for wastage to be minimised. To improve cycle-times,economy and efficiency, proprietary temporary works supports and formwork systems were used where possible.

DESIGN FOR BUILDABILITY Good site practice is likely to result in a product of higher quality and therefore of increased durability. Quality and durability may also be enhanced by increasing buildability, ie the ease with which the design can be constructed on site.

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Buildability can be influenced by the nature of the construction, complexity of detail, the construction sequence and the accessibility to the structure and other site limitations. For example, if restricted access complicates the fixing and checking of reinforcement close to formwork in a deep lift, there is likely to be a resultant variability in fixing and associated decrease in quality. Buildability issues relating to bridge foundations are discussed in CIRIA Report 165 The planning and design of concrete mixes for transporting, placing and finishing (1995). The guidance and checklists given within this publication can be used for most in-situ concrete foundation applications but differences in access, ground conditions and local restrictions will affect the buildability requirements of a specific site. The main general design issues related to buildability of foundation constructions, particularly deep foundations, can be summarised as follows:

3.4

•

a comprehensive site investigation should be undertaken to establish the current ground conditions. Any other factors that might affect the choice of construction method should be considered, eg adjacent structures and services, environmental constraints, limitations on space etc

•

foundation construction activities may also affect the ground conditions and alter drainage courses (TEGR, 1999). These issues are discussed in detail in Section 5.4.6

•

the prevailing soil and ground conditions may influence the buildability of the foundations as well as the foundations affecting the ground conditions

•

the future development of the site also needs to be considered as this will also effect buildability

•

generally, concrete for deep foundations needs to have suitable fresh properties to facilitate its placement by the techniques available on-site, ie it has to be suitably cohesive, with a low tendency to bleed and a workability which allows it to compact under its own weight without segregation (see Section 4.2.3 for details)

•

there should be a reasonable degree of flexibility in the concrete specification so as to leave the constructor free to choose and demonstrate the method of achieving the required properties

•

it is desirable to keep the geometry of foundation elements as simple as possible. In areas susceptible to deterioration (eg from sulfate and/or acid attack) the use of buried slender elements should be avoided if possible (see Section 5.4 for details)

•

the environmental issues relating to the construction technique used should be considered, with the aim of minimising the environmental impact of the works.

ENVIRONMENTAL ISSUES AND SUSTAINABLE CONSTRUCTION Consuming primary natural resources and disposing of construction waste are damaging to the environment and it is now generally accepted that such activities are no longer considered sustainable at their current levels (Dhir et al, 1998). A responsible approach to design and construction of in-situ concrete foundations, through appropriate materials selection, construction practices and re-use of existing foundations, can play a positive role in the conservation of resources and protection of the environment.

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Restrictions on factors such as noise, exhaust fumes, dust and vibration now strongly influence foundation construction (Construction Today, 1991). Continued innovation and research is required to further develop construction methods and materials that reduce environmental strain. The ability to construct a foundation system which achieves the desired result but is cheaper, faster and more environmentally friendly than that of a competitor’s proposal may be the deciding factor in winning a large construction contract (Contract Journal, 1993). Recent developments in the use of waste and by-product materials in concrete (see Section 4.4) and placing techniques (see Section 4.7) have meant that in-situ concrete foundation construction has advanced towards meeting the challenges of sustainability within the construction industry. The practical application of “new” concrete technology has the potential to make positive contributions in terms of reduction in the use of raw materials, minimisation of environmentally damaging industrial process and in the use of industrial waste products. Its application can lead to direct economic benefits for construction projects. For example: Concrete for foundation works should, where possible, make use of recycled materials, waste products such as recycled concrete aggregate (RCA), recycled aggregate (RA) and industrial by-products such as pfa and ggbs. Details on the use of RCA and RA are given below while ggbs and pfa are discussed in detail in Sections 4.4.2 and 4.4.3, respectively.

•

Consideration should be given to the use of self-compacting concrete. Although its use currently has limitations through lack of authoritative guidance and no quantitative tests for assessing its fresh properties, SCC use may be justified in terms of a number of potential benefits, including reduced environmental impact and improved health and safety on site. This is discussed in more detail in Section 4.2.3.

• Foundation construction can also play a major role in the reuse of previously occupied land by allowing construction of buildings on “brownfield" sites. The particular durability aspects associated with foundation construction on contaminated land are covered in Section 5.8.3.

• Re-utilisation of existing foundations through recycling also offers potential economic and environmental benefits.

3.4.1

Recycled aggregates The use of RCA and RA in foundation applications is limited to strip footings and mass concrete and trench fill foundations in non-aggressive soils (Design Chemical Class DC-1) in prBS 8500. The requirements for coarse RCA and RA as defined in the standard are given below in Table 3.2:

Table 3.2

Requirements for recycled concrete aggregate and recycled aggregate (prBS 5800 ) Requirement Grading - conform to Maximum content masonry, % m/m

RA

RCA

UK NA to EN 12620

UK NA to EN 12620

5

100

5

3

0.5

1

Asphalt. % m/m

5

10

Other foreign material eg glass, plastic, metals, % m/m

1

1

Sulfates, % acid soluble SO3, m/m

1

1

Maximum fines, % m/m Ultra lightweight material 1 ), % m/m

Notes 1) Material less than 1000 kg/m3

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There are several potential problems associated with the use of RCA and RA as an aggregate for concrete. These include the following: •

stricter environmental requirements, which may involve leaching tests and tests for trace elements and harmful organic compounds, to ensure the finished concrete has consistent strength and durability

•

other impurities need to be monitored and restricted in addition to limiting chloride and sulfate levels in accordance with BS 882

•

fine RCA and RA have higher leachable material contents such as sulfate (eg from gypsum plaster and/or cement paste). There are also potential problems with poor grading, which can severely affect the water demand of the concrete

•

difficulty in producing a material that conforms to existing standards

•

difficulty in guaranteeing a homogeneous supply for recycled materials.

Although the problems outlined above represent a significant barrier to the use of RCA and RA in concrete construction, there have been moves in recent years to demonstrate the wider potential of this material. Several documents have been produced that provide details on the use and specification of RCA and RA for various uses. Best practice guidance can be found in the following publications:

CIRIA C569

•

CIRIA C513 (1999)

•

BRE Digest 433 (1 998)

•

BRE Report 243 (1993).

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4

Properties of concrete

4.1

INTRODUCTION

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Modern concrete technology affords greatly increased flexibility to designers and specifiers to modify concrete’s fresh and hardened properties - enhancing certain characteristics such as strength and durability with greater economy.

This section discusses the fresh and hardened properties suitable for in-situ concrete for foundation applications. It also provides information and guidance on the selection of concrete materials, concrete specification, placing methods and construction techniques for foundation applications.

4.2

FRESH PROPERTIES After mixing, operations such as transporting, placing, compacting and finishing of fresh concrete can all considerably affect the properties of hardened concrete. Most concrete designs for in-situ foundations aim to produce fresh concrete with the following characteristics: • flowable • cohesive • stable •

compacting under its own weight (particularly for deep foundations)

•

minimal tendency for segregation and bleeding

•

able to be placed so that a continuous monolithic concrete is formed.

Where the specification identifies that such characteristics are required, the producer should be able to provide a suitable concrete through a combination ofcareful concrete design and use of appropriate additions and chemical admixtures.

4.2.1

Concrete design The requirements for workability and cohesiveness of the fresh concrete are related to the placing method, the nature of the pour and construction procedure. In order to achieve the high workability and cohesion that are frequently required, the total mass of fine material in concrete is very important in these concretes and BS EN 1538 suggests that this should be 400-500 kg/m3. Fine material is defined as £ 2-63 µ m, which includes cement, pfa and ggbs as well as the fine end of natural sand. The Standard allows a maximum w/c ratio to 0.60 and allows water-reducing, plasticising and superplasticising admixtures to be used to control bleeding or segregation, if possible.

To produce flowable concrete suitable for tremie placement and requiring no subsequent compaction, the following design guidelines are useful:

32

•

design target slumps of 150-225 mm

•

design maximum free w/c ratio of 0.50

•

use coarse aggregate/sand ratio of 1.0 to 1.2

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• use of natural rounded gravel aggregate (if these are available) •

use of composite cements

•

use of water-reducing, plasticising or superplasticising admixtures.

The use of admixtures and of additions can have significant effects on the fresh properties of the concrete and are considered in detail in Section 4.6 and Section 4.4 respectively.

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The Codes of Practice for diaphragm walls (Specification for Highway Works (1998); BS EN 1538 (2000); DIN 1428. (1986)), set out requirements for aggregate grading and cementitious content to give a concrete that compacts under its own weight without segregation. Table 4.1 summarises these, along with the range of required workability. Table 4.1

Concrete requirements for diaphragm walls (Clear et al, 1999) Specification

S H W , 1988

BS EN 1538, 2000

BS882, 1982

Minimum 40% fine aggregate of total

Tota1 aggregate grading given

150-220

160-220

-

Flov

-

520-630

550-630

Aggregate requirement

Grading M for fine aggreghate Workability at placing(mm)

Min. cement content

(kg/m’)

for given m a x aggregate size

Slump*

DIN 1428, 1986

32 mm

-

350

350

25 mm

-

370

350

20 mm

400

385

350

16mm

400

400

350

*For high workability concretes. the flow test is considered a better test and method for specification (see Section 4.2.2). For shallow in-situ concrete foundation construction, usually low slump concrete is preferable - eg in massive pours built up in layers. The performance requirements are also typically less onerous, since the nature of the elements, their positioning, geometry and orientation, are generally less problematic with respect to concrete placement. The best and most common method of compacting shallow foundation concrete is probably to use internal poker vibrators positioned with appropriate spacing and working in uncompacted concrete no deeper than the length of the metal casing of the vibrator.

4.2.2

Specification and measurement of fresh properties WORKABILITY The method of measuring workability and the acceptable tolerances should be agreed between the contractor and supplier prior to commencement of works. There are several methods available to measure the workability of concrete. These include the slump test (BS 1881: Part 102), the compacting factor test (BS 1881: Part 103) and the flow test (BS 1881: Part 105). Although the slump test is a simple and useful test for checking uniformity of fresh concrete, the high tolerances associated with the results can be unacceptable, particularly where highly workable, flowing in-situ concretes for tremie placement are concerned. Additionally the slump test measures the ‘yield” of the concrete rather than its flowability -which is frequently the most important characteristic of such concretes. The flow test - which provides a direct measure of the mobility of the fresh concrete - is considered a useful method for measurement and specification of workability for in-situ foundation concrete.

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The fresh properties of self-compacting concrete (SCC) that have to be specified and measured put it well outside the scope of current tests for workability. There are no established reliable tests that can quantitatively assess parameters such as segregation resistance (stability) and passing ability (resistance to blocking) of fresh SCC mixes. The most common method for assessing workability is the slump flow test in which the spread of the sample contained within a standard slump cone is measured instead of the slump proper. The time taken for the concrete to spread 500 mm may also be determined. Unfortunately, the test is useful only for operatives with a substantial experience of the behaviour of SCC mixtures - the numerical results alone are of little practical value. Again it is important to discuss methods of measuring workability and the acceptable tolerances with the contractor and concrete supplier. The period over which the concrete retains suitable workability - important in many foundation applications particularly for avoiding cold-joints in large pours - may also be specified. Workable time may be modified. for instance by the incorporation of admixtures (see Section 4.6). COHESIVENESS Cohesiveness, ie resistance to bleeding and segregation, is an important characteristic in high workability concretes. However, it is generally covered in specifications by a simple blanket-statement and, where in-situ foundation applications and concretes are concerned, more detailed specification may be necessary. Prior to placing it is difficult to assess the cohesivity of fresh concrete and its tendency to segregate. At this time there is no satisfactory and accepted method of testing for, or specifying, this property which is generally assessed by qualitative judgement. An indication may be obtained from careful observation of the flow test. The risk of segregation during placement may be reduced by suitable handling and placement techniques. Until satisfactory tests are developed it may be prudent to undertake site trials to establish the tendency for concretes to segregate during placing. Where necessary, the tendency for concrete to bleed may be measured. ASTM C232 (1992) is a test for assessing bleed by measuring the proportion of bleed water produced by a small sample from a test concrete. This test is. however, probably impractical for most site conditions. A European test (prEN 480-4) is currently in preparation and, although this test is intended for use in the assessment of admixtures for concrete, it is equally applicable to concrete not containing admixtures. 4.2.3

Self-compacting concrete (SCC) In recent years concrete technology has led to the development of concretes that combine additions and admixtures to produce a variety of “high performance concretes”. Such concretes have been designed specifically to exhibit enhanced performance in one or more areas, for example high workability, low heat, high strength and high durability. One type of particular relevance to in-situ foundation applications is self-compacting concrete (SCC), which is designed to achieve full compaction under its own weight without additional vibration. SCC represents “new” technology and although it has been used successfully in many projects, notably in Japan, there is currently relatively little experience and no formal guidance regarding its specification and use in the UK. However, in the future, SCC seems likely to become an important concrete technology for a range of in-situ concrete foundation applications, particularly deep foundations and those with congested reinforcement. The Japanese government is currently giving support to a plan to use SCC in half of all concrete placed in Japan by 2003 (Jones, 1999), but here are very few examples of

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SCC being used in major projects in the UK. Henderson (2000) does, however, provide a recent example of SCC’s successful use within steel columns at the Millennium Point Building in Birmingham, UK. SCC can be readily manufactured from conventional concreting materials, but requires some changes to mix proportioning to achieve the required properties. The key properties of SCC are an adequate rate of flow under self-weight, no segregation during flow and no blocking when passing through reinforcement. The first requires a low yield stress, without excessive plastic viscosity, while the second requires a reasonably high viscosity. Optimisation of this property is necessary. The no-blocking condition requires sufficient volume of the mortar fraction to lubricate the coarse aggregate hence there is the need for a high mortar content.

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The principle differences between SCC and conventional concrete are: •

lower coarse aggregate content, typically 750 to 900 kg/m3

•

lower sand content in the mortar, typically 40 to 50 per cent by volume

•

restriction of the water content to between 45 to 55 per cent by volume of the paste.

As a consequence of the above, hardened properties, in particular strength, cannot be controlled simply by varying the water binder ratio. N.B. In this case the term binder means all materials with a particle size < 75 µm. These include fine aggregate, PC, ggbs, pfa, powder fillers, limestone, sf, etc, which can all be used in SCC. For strength calculation during mix design, a modified Feret’s rule, which links strength to absolute volumes of binder, water, air and a constant, is applicable to UK materials. At present there is no agreed method to measure practically properties such as flow and resistance to segregation. There are several techniques available but all require specialist assistance to interpret the results so the currently best and universally available test to judge the workability of SCC is the slump flow test (see Section 4.2.2). The use of admixtures, probably in combination, is essential. These may include superplasticisers, air entraining admixtures, viscosity agents, retarders or preblended combinations. By balancing the proportions of PC to other materials in the binder, thermal properties can be controlled. Long-term shrinkage and creep are not significantly different from those of conventional concrete. Some of the principal advantages and limitations of SCC include: Advantages:

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•

reduced labour requirements

•

shorter concrete placement times

•

improved compaction in areas of congested reinforcement

•

potentially enhanced durability through improved compaction of cover concrete

•

expanded buildability, eg concreting deep elements in single lifts

•

environmental and health and safety benefits resulting from lack of vibration.

35

Limitations: • no formal guidance regarding specification and use •

experienced designers and operatives required to design SCC mixes and assess key properties

•

no established reliable tests for quantitatively assessing segregation resistance (stability) and passing ability (resistance to blocking) of fresh SCC mixes.

An example of a concrete design for a medium strength SCC is given in Table 4.2

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Table 4.2

Mix proportions (kg/m3) of a SCC concrete (Bartos & Grauers, 1999). PC 42.5

Crushed Granite, 5-20 mm

Fine aggregate, 0-5 mm

Free water

280

750

865

200

Limeston Powder

240

Admixture*

Flow test

4.2

650 mm

*A specialist high-performance superplasticiser was used in this mix.

4.3

HARDENED PROPERTIES

4.3.1

Strength development and specification Concrete develops strength by forming hydrates and the hydration will continue for many years provided there is water available for hydration and there is cement/additions available to react. Within these conditions, the actual rate and magnitude of concrete strength development depends on: • the basis for comparison •

the temperature and temperature history

•

cement type. class and source

• the type, source and amount of addition •

the water/cement or water/binder ratio

• the consistence. Where strength has been specified it is usually based on 28-day cube strength cured in accordance BS 1881. The FPS Report (1999) suggests that for piling construction a specified testing regime should be agreed with the concrete supplier, with tests carried out at seven and 28 days. This can also apply to other foundation concretes. However, the strength development of a cube cured in BS 1881 conditions is very different from the in-situ strength development, more so where thick sections are concerned. Ideally, especially where more critical elements and circumstances are concerned, trial mixes with temperature-matched curing may be carried out to assess the likely in-situ temperature variations and strength development of the specified concrete. An example of a project where this testing regime was carried out is given in Case Study No 2 (see Section 4.3.2). When concretes contain additions such as pfa, ggbs, sf, and mk it is likely that the strength development characteristics will differ from a similar concrete using PC only,

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and will be affected by the relative proportion of the additions. Since the use of such materials frequently reduces the rate of strength development but prolongs the period over which it occurs, samples may be tested for strength compliance at ages greater than the standard seven and 28 days - typically at 56 days. Where such a time delay is impractical, the target strength may be reduced by a suitable factor to allow compliance testing at 28 days.

Further details on the effect of the type and quantity of cement addition materials on strength development are given in Section 4.4.

4.3.2

Temperature

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Hydration of cement is an exothermic reaction and in large foundation structures the

heat dissipation is low and the temperature within the section can rise significantly. If not controlled this may have undesirable consequences, most commonly thermal cracking. High temperatures produced during early hydration may also reduce the ultimate strength, promote delayed ettringite formation (DBF), and affect other properties of the concrete (C&CA, 1976; Tyson, 1995, and Lawrence et al, 1990). In large pours especially, it may be necessary to take measures to keep temperatures within certain limits and also to avoid the development of large temperature gradients across sections. The composition of concrete has a direct impact on the early-age temperature rise. The

cement type and content of the concrete have major effects on the rate of hydration and temperature rise. The higher the cement content, the greater the heat evolved per unit volume and the greater the temperature rise. Research has shown (CIRIA Report 91, 1992), that heat of hydration can be substantially reduced in thick concrete sections by using composite cements containing high proportions of ggbs or pfa. But it should be

noted that the tensile strength and strain capacity at early ages can be reduced by an amount which is more than the benefit gained from the reduced temperature. In order to gain a net benefit from the use of additions (such as pfa and ggbs) in concrete, certain minimum amounts of addition need to be used depending on section thickness. These recommendations are summarised below in Table 4.3 and have been incorporated into Concrete Society Technical Report 40 (1991). Table 4.3

Recommended minimum pfa and ggbs contents to reduce the likelihood of thermal cracking (Concrete Society TR 40, 1991).

Section length of thickness

Minimum addition content, % Pfa

Ggbs

Up to 1 m

20

40

1 - 1. 5m

25

50

1.5 - 2 m

30

60

2 - 2.5 m

35

70

Once the heat of hydration is reduced, the requirement for crack control is less onerous, which may allow significant cost saving in reinforcing steel, particularly in massive elements. Slender, load-bearing elements, which are more heavily reinforced for structural reasons, are more able to cope with early age thermal effects because they develop less heat and steel acts as a heat sink.

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An example of a project where high proportions of ggbs where used to reduce the peak temperature of a large in-situ concrete pour is given in the following case study.

Case Study 2 Reducing peak temperatures in large pours

- Manchester,

UK

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Summary: The use of composite cement with a high proportion of GGBS was successful in reducing the peak temperature of a large concrete pour, and thereby avoiding the development of thermal cracking. This was confirmed prior to the construction by on-site trials that included temperature-matched curing of test-cubes. This road bridge over a dual carriageway required the construction of two large unreinforced concrete foundations. The west bank foundation contained 262 m3 of concrete with a minimum dimension of 5.0 m, and the east bank foundation contained 309 m3 of concrete with a minimum dimension of 5.8 m. Reduction of heat generation to minimise early-age thermal cracking, and monitoring of the concrete’s temperature and strength development during curing, were important issues in the production of such large sections. To produce a durable concrete with a low level of heat generation, the concrete was specified to include a high level of GGBS addition (a blend of 70 per cent GGBS, 30 per cent PC). A water-reducing admixture was included to provide the required level of workability whilst minimising the water/cement ratio. The main features of the concrete are summarised as follows: Grade:

C50

Minimum cement content:

365 kg/m3

Maximum water/cement ratio:

0.5

Cement used:

with 70% GGBS, 30% PC

Coarse aggregate:

1125 kg/m3 of 20-5mm limestone

Fine aggregate:

748 kg/m3

Admixture additions:

Water reducer

An on-site trial was conducted to assess the temperature rise and strength development of the specified concrete within the foundation. A 1 m3 wooden box was constructed and enclosed in dense thermal insulation material. Thermocouples were fixed within the box and it was filled with concrete delivered by the supplier and placed normally. Temperatures were logged at 30 minute intervals. One thermocouple was used to control the temperature of a temperature-matched curing bath which contained cubes cast from the same concrete as that in the box. These were tested for compressive strength along with cubes of the same concrete cured under normal conditions as detailed in BS 1881. The results of the trials were asfollows: Trial Block (temperature matched cubes)

38

Compressive strength (N/mm2)

Age/days

Temp. ºC

1 2 3 4

52.5 61.2 59.1 55.0

62.2 60.0

5 6

51.1 49.6

62.0 62.0

6.5 54.0

CIRIA C569

Block standard BS1881 Cubes (cured at 20°C) Age/days

Compressive strength (N/mm2 )

1

2.5

4

27.5

7

38.5

28

57.7

56

66.0

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The peak temperature reached by the trial block was 61.4°C at 46 hours significantly below what might be expected from an equivalent PC only concrete. In comparison with the cubes cured at normal temperatures (20°C), the compressive strength values of the temperature-matched cubes indicate the accelerated rate of strength gain associated with curing at elevated temperatures. The satisfactory results of the trials confirmed the suitability of the concrete, and after placement the concrete was insulated to avoid the development of excessive thermal gradients between its core and edges. This was achieved by covering with polythene sheeting and layers of thermal insulation; the formwork was backed by 35 mm thick thermal insulation boards. The temperature of the concrete was taken in various locations and logged at 30 minute intervals, and a thermocouple cast into the centre of one foundation was used to control the temperature of a temperature-matched curing bath containing cubes from the concrete actually placed. This allowed the strength development of the foundation concrete to be assessed as in the trials. The peak temperature recorded was 70°C (at 7.5 days) with seven and 14 day strengths of 62.1 and 64.7 N/mm2 respectively, these being comparable figures to those produced by the trials and confirming their validity and usefulness. No thermal cracking of the concrete was reported.

4.3.3

Curing In-situ concrete foundations can either be cured below ground (diaphragm walls or secant pile walls), or in conventional formwork (tunnel boxes). There are several factors that may affect the material properties of concrete cured below ground (Bamforth, 1980) which include rate of heat dissipation, bentonite effects, consolidation effects? moisture migration between soil and concrete, and chemicals in the groundwater. The understanding of how these factors may effect concrete properties is rather limited (Tyson, 1995). Concrete placed below ground may develop strength at a faster rate than the same concrete placed above ground. Bamforth (1980) has shown that strength development characteristics are affected by curing temperature, especially at an early age, so the increase in strength gain may reflect the generally higher temperatures developed in foundations compared to above-ground construction. Additionally, concrete cured below ground may benefit from a relatively prolonged period of moist-curing, increasing the hydration of its surface-skin with potential improvements in durability. The construction of the foundations for the British Library provides an example of the how in-situ concrete developed strength too quickly to allow efficient secant piling. Details are given in the following case study:

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Case Study 3 Construction of secant pile walls - British Library, London, UK Summary: The rapid early strength gain of concrete placed below ground was highlighted during construction of secant pile walls at the British Library. Modifications to the concrete design were required throughout the project in order to produce concrete that developed relatively low rates of strength gain, to facilitate pile cutting, but also to meet long term structural and durability requirements.

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Structural walls for the British Library, London were constructed using secant piling. The secant pile scheme used some 500 secant piles, 1180 mm diameter at approximately 1 m centres. The concrete was designed to produce dense, impermeable, workable concrete and be resistant to Class 3 sulfate conditions. The general approach was to use code of practice and BRE guidance but with great emphasis on implementation. The following concrete w a s originally specified: Min. 70% ggbs + 30% max. PC Max. content (7%) of C3A in the cement However, in-situ strength development for these original concretes was underestimated. At depths of 1.5 m, and below, the concrete was well insulated and rapid in-situ concrete temperatures resulted in very high strengths being developed within the first few days after casting and placing. This meant that pile cutting was difficult and so it was decided to modify the original concrete specifications and undertake trials to establish the optimum concrete mix. In order to facilitate the balance between low early strength for pile cutting and the later structural and durability requirements it was deemed adequate to specify the required minimum strength of 30 MPa, to be achieved at 56 days instead of the usual 28 days. Various modifications to the original concrete were used during the contract in attempts to reduce this early strength gain. This culminated in the use of: •

40 mm aggregate which permitted the cement content to be reduced

•

increasing the ggbs fraction from 70 per cent to 85 per cent, thus reducing the PC fraction further

•

use of a plasticiser to improve workability without the addition of water

•

acceptance of a characteristic strength at 56 days of 29 MPa, evidence from specific trials having previously shown that further strength development of this actual concrete at 90 days would carry the strength beyond 30 MPa.

Measurements of in-situ temperatures, using temperature matched curing, were carried out to help assess the effects of these changes. The secant pile wall concrete proportions for the first and last concretes are given below: Cementitious content (kg/m3) PC/ggbs ratio w/c ratio Plasticiser Max. aggregate size (mm) Design slump (mm)

First 360 3070 0.47 No 20 150

Last 340 15:85 0.48 Yes 40 150

The use of the new 15 per cent PC185 per cent GGBS concretes allowed pile cutting to be undertaken successfully. Similar concretes are now commonly used in foundation construction where there are requirements for low rates of strength gain together with high durability.

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The ambient temperature, both above and below ground, can have a significant effect on curing temperature, and therefore the weather conditions and time of year may be an important consideration in constructing in-situ foundations. Low ambient temperatures decrease the early strength development whilst high temperatures increase the early strength development. Insulation of the top surface of large pours using polythene sheeting and sand may help to avoid the development of large temperature gradients, although increased surface cracking may result. When concreting in hot weather, cooling of the mix water, or the replacement of some proportion of mix water with crushed or flaked ice, has proven highly effective in reducing peak concrete temperatures.

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4.4

EFFECT OF CEMENT/ADDlTlON TYPE There is a considerable number of cements available for use in concrete in the UK. This number is likely to increase with the adoption of the new European cement standard BS EN 197-1 in the year 2000. The cement types proposed by this new standard and their acceptability in Europe and equivalent British Standards are shown in Table 4.4. Concretes containing PC and SRPC, and additions such as pulverised-fuel ash (pfa) and ground granulated blast furnace slag (ggbs) have an established history of use in concrete foundation applications in the UK. The development of the use of pfa and ggbs in the UK has occurred mainly through direct addition at the concrete mixer rather than through composite cements. Materials such as silica fume (sf) and metakaolin (mk) and Portland limestone cement (PLC) can also be beneficial to the fresh and hardened properties of concrete used for in-situ concrete foundation applications. For these reasons, this report will be restricted to reviewing the properties of the cement and addition types mentioned above.

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41

Table 4.4

Proposed BS EN 197-1 cement types and their acceptability in Europe and equivalent British Standards

Cement

Licensed copy:University of Manchester, 05/11/2007, Uncontrolled Copy, © CIRIA

Portland Cement

BS EN 197-1 Cement notation CEM

Clinker content (º/O)

Content of other main constituents

British Standard (1)

(%)

To be Withdrawn

To co-exist

Standardising in other countries No.

I

95- 100

-

BS 12

BS 4027 (2)

18

Portland Slag Cement

II/A-S II/B-S

80-94 65-79

6-20 21-35

BS 146 (3)

-

14 13

Portland Silica Fume

II/A-D

90-94

6-10

II/A-P II/B-P

80-94 65-79

6-20 21-35

II/A-Q II/B-Q

80-94 65-79

6-20 21-35

Portland Pozzolona Cement

-

7 8

3 3

II/A-V

80-94

6-20

BS 6588

15

II/B-v

65-79

21-35

13

II/A-W

80-94

6-20

-

II/B-W

65-79

21-35

II/A-T II/B-T

80-84

6-20

-

6

65-79

21-35

II/A-L

80-94

6-20

-

13

Portland Limestone Cement

II/A-LL

80-94 65-79 65-79

6-20 21-35 21-35

BS 7583

5

II/B-L II/B-LL

Portland Composite cement

II/A-M

80-94

6-20

-

13

II/B-M

65-79

21-35

III/A III/B III/C

35-64 20-34 5-19

36-65 66-80

Pozzolanic Cement

IV/A IV/B

65-69 45-64

11-35 36-55

Composite Cement

VIA V/B

40-64 20-39

36-60 61-80

Portland Fly Ash Cement

Portland Burnt Shale Cement

Blastfurnace Cement

81-95

4 4

6

-

9 BS 146 (3)

-

-

BS 4246(4)'

15 7

BS 661 0 (5)

-

9 5 7 6

In addition to the cement types in this table. BS 1370 for low heat Portland cement. will co-exist. BS 4027 is the British Standard for CEM I cement with special sulfate-resisting properties. The low-early-strength classes (ie 42.5L, and 52.5L) in BS 146 may be retained by transfer to BS 4246. BS 4246 covers blastfurnace slag content of 50% to 85% with an early strength which will be outside the range of BS EN 197-1. 5. BS 6610 covers a strength class of 22.5N, which will be outside the range of BS EN 197-1.

1. 2. 3. 4

4.4.1

Portland cement (PC) PC (CEM I) is generally used as the basic cementitious component in cast in-situ concrete and has a long and generally successful history of use in precast concrete foundation construction. At the current time the major part of the precast industry's production still comprises PC-only concrete. It provides relatively high strength and tensile strain capacity compared with concretes made using high proportions of additions such as pfa and ggbs. PC concrete can be used in Design Chemical Classes 1 and 2 conditions but not 3,4 or 5 as recommended in BRE Special Digest SD1, 2001.

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CIRIA C569

4.4.2

Ground granulated blastfurnace slag (ggbs) Concrete Society Technical Report 40 (1990) describes in detail the fresh and hardened properties of concrete containing ggbs which is a fine particulate material and a byproduct of steel manufacture. It exhibits latent hydraulic properties and reacts in the presence of both water and lime to form insoluble compounds and with water alone to form hydration products,

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The inclusion of ggbs in concrete has several important effects on both the fresh and hardened properties of the concrete, which are often significant for in-situ concrete foundation applications. These are summarised in Table 4.5. Ggbs concrete may modify the fresh properties in terms of setting time and water demand but due to its typically uniform grading, very high proportions of ggbs may have a detrimental effect on the cohesivity and bleeding characteristics of concrete. This may lead to an increased risk of plastic settlement cracking. The rate of heat generation will be reduced leading to a lower temperature rise during hydration depending on the proportions of ggbs used. However, although high proportions of ggbs reduce the risk of thermal cracking, lower proportions are more suitable for slender members and/or cold climate conditions if early set and strength is required by construction logistics. In order to reduce the likelihood of thermal cracking minimum proportions of ggbs are recommended in Concrete Society Technical Report 40 (see Table 4.3). In the hardened state, concrete containing a significant proportion of ggbs if designed and placed correctly, is dense and impermeable. At proportions greater than 70 per cent ggbs concretes can be designed to have improved resistance to sulfate attack and delayed ettringite formation (DEF), chloride ingress and ASR. However, ggbs concrete is sensitive to freeze/thaw damage particularly at early ages. In addition, Bamforth (1980) has shown that 70 per cent ggbs concrete has a lower tensile strain capacity than that of equivalent PC and PC/pfa concrete. The slower initial strength gain of ggbs concrete may also lead to increased striking times for falsework and formwork compared with PC-only concretes. However, in large sections, typical of that used in foundations, the rate of strength gain is similar to a PC concrete.

4.4.3

Pulverised-fuel ash (pfa) Concrete Society Technical Report 40 (1990) describes in detail the fresh and hardened properties of concrete containing pulverised-fuel ash (pfa). Pfa (a type of “fly ash”) is a fine particulate material produced from the combustion of pulverised coal during the process of electric power generation. It is pozzolanic in nature, containing constituents that react with calcium hydroxide at normal temperatures to produce insoluble compounds. The potential effects of pfa on concrete properties are summarised in Table 4.5. Pfa particles are predominantly fine, glassy cenospheres that improve the workability of the concrete and decrease water demand. By allowing a reduction in water content, inclusion of pfa may allow a reduction in bleeding and segregation, and also a reduced w/c ratio. These properties enable the production of concrete that is generally cohesive and easy to place. However, the use of high proportions can increase the risk of plastic cracking. The pozzolanic reaction of pfa concrete causes it to hydrate at a slower rate than a PC-only concrete. Associated with this is a reduced rate of heat generation, leading to a lower temperature rise that may reduce the risk of early-age thermal cracking in large

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pours and thick sections. Recommendations relating the minimum pfa content to the section thickness required to reduce the likelihood of cracking is given in Table 4.3. However, the inclusion of high proportions of pfa can lead to reductions in early-age tensile strength and strain capacity by an amount that is more than the benefit gained from reduced hydration temperature. In specimens cured under standard conditions, pfa concretes exhibit lower rates of strength development up to 28 days than PC-only equivalents, however strength can continue to develop at a higher rate up to and beyond 90 days. If 28-day strengths similar to that of PC-only concrete are required, the w/c ratio of PC/pfa concrete may be reduced, or total binder content increased.

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As with PC/ggbs concrete, concretes with high proportions of pfa. if designed and placed correctly, tend to be dense and impermeable. As a result, the use of sufficient proportions of a suitable pfa can enhance resistance to sulfate attack, chloride ingress and ASR. Owens (1999) has published a technical data sheet that gives useful information on the best use of pfa in concrete.

4.4.4

Silica fume (sf) Concrete Society Report 41 (1993) describes in detail the effect silica fume (sf) has on the fi-esh and hardened properties of concrete. Silica fume (also known as microsilica, or condensed silica fume), is a by-product of the electric arc furnace production of silicon and ferro-silicon alloys. It is about 60 times finer than PC and typically contains a much higher proportion of silica. It is usually blended with PC in relatively low proportions, typically 10 per cent or less by mass of cement, but has been used at up to 15 per cent to provide high chemical resistance. However, in order to conform with BS EN 197-1 a sf cement or combination has to contain 6-10 per cent sf. Sf is an extremely fine and highly reactive material, with the ability to fill the space between large cement grains and so improve packing within the concrete. However, due to the high specific area of sf, water demand tends to increase; to counter this a superplasticiser is frequently incorporated to improve workability. Concretes containing sf exhibit more rapid early strength development than pfa and ggbs concretes - this may be an advantage in a number of applications but it may not be desirable where secant and CFA piling techniques are used or in large pours where early-age heat generation is a problem. Addition of sf can reduce the permeability of hardened concrete and, as a result, enhance durability with improved resistance to water permeation, chloride ingress and sulfate attack. An illustration of the reduction in permeability obtained by use of a PC/sf composite cement over a range of water/cement ratios is provided in Figure 4.1. The cores were taken from samples with approximately equal 28-day strengths.

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ClRlA C2569

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Figure 4.1

Permeability of drilled cores for different water/binder (cement + silica fume) ratios (approximately equal 28-day strength). [Note: MS refers to silica fume].

Experience using sf to minimise ASR with UK reactive aggregates is, at present. very limited and no recommendation can yet be offered. Further research work is required to assess how UK aggregates and binders, in combination with sf, may be used to minimise ASR. Where used in combination with ggbs or pfa, correctly designed and placed sf concretes have been found to be very resistant to sulfate attack. Sf is a relatively expensive material and requires careful handling to produce very high quality concrete. The use of sf in cast in-situ foundation concrete may be considered in special cases when high strengths or enhanced durability is required.

4.4.5

Metakaolin (mk) Metakaolin is a very effective pozzolanic material found when china clay (kaolin) is heated between 600°C and 800°C. It is available in a high state of purity (> 90 per cent) and, as with other pozzolans, can react with more than its own weight of calcium hydroxide to give new cementitious compounds. There are currently no standards or codes of practice covering the use of mk in concrete. However, the material properties required for mk in concrete are covered in an Agrement Certificate No 98/3540. Mk improves the microstructure of concrete in a similar way to other pozzolans, by pore blocking and reducing calcium hydroxide levels in the cement. The dense concrete thus produced benefits from increased durability, with improved resistance to water penetration, salts and acids and greater resistance to common forms of sulfate attack. It has been used with apparent success in some Brazilian dams to avoid

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excessive expansion caused by ASR (Saad et al, 1982). However, experience of its use in the UK to minimise the risk of ASR is very limited and therefore no recommendation can yet be offered for its use. In addition, mk is presently subject to limited availability and there is currently little guidance concerning its use. An example of a project where mk has been used to produce high quality in-situ foundation concrete is given below.

Case Study 4

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Foundations for an industrial process plant using metakaolin Summary: New concrete technology is applied to the problem of overcoming aggressive ground conditions. Metakaolin is used as a cement addition to produce a high-quality foundation concrete with enhanced durability. The construction of a foundation for an industrial process plant required concrete which was highly durable under exposure to aggressive conditions: • sulfate-bearing soils (specified Class 2 sulfate resistance)

• exposed to chloride ions and cyclic wetting and drying from exposure to seaspray To enhance the level of protection against these factors - and therefore the durability ofthe structure - a composite binder comprising PC with metakaolin was used. The concrete design details were as follows: 28-day strength: Coarse aggregate Fine aggregate: Binder: Water /binder ratio: Aggregate /binder ratio: Admixtures: Target slump:

30 MPa 478 kg/m3 20 mm crushed granite 359 kg/m3 14 mm crushed granite 358 kg/m3 10 mm crushed granite 575 kg/m3 coarse sand (Grade M) - crushed granite fines 335 kg/m3 PC 60 kg/m3 Metakaolin (15% by mass of binder) 0.50 4.5 1.4 l/m3 o f plasticiser 40 mm

Concrete from the pour was used to produce test cubes for compressive strength, density measurements, oxygen permeability and chloride penetration. The concrete was truck mixed and the mk added to the mixer along with the PC, and dispersed evenly without any difficulty. The concrete exhibited increased thixotropy, but it mobilised satisfactorily when shear force was applied. The concrete compacted well using vibrating pokers and floats, and appeared to be less susceptible to segregation caused by over-vibration than conventional PC concrete. The formwork was struck after 24 hrs and the concrete surfaces covered with plastic sheeting for a further 48 hrs. The test-cubes gave mean compressive strengths of 41.7 MPa at seven days and 61.6 MPa at 28 days, exceeding the requirements ofthe specification. Testing to evaluate the durability of the concrete was undertaken when the samples were 2.5 years of age. The results indicated average-to-low oxygen permeability (2.26 x 10-18 m2) and a high degree of resistance to chloride ingress, relative to a PC-only binder. These results indicate that the concrete produced has a low permeability, which would tend to enhance durability in such aggressive environments. An inspection of the foundation undertaken three years after construction revealed no evidence of erosion or deterioration in the concrete at that time.

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4.4.6

Portland-limestone cement (PLC)

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Most PCs contain up to five per cent limestone as a minor additional constituent. In addition, Portland limestone cement (PLC) conforming to BS 7583 (1996) is also available. BS EN 197-1 allows the use of limestone up to 35 per cent if combined with PC only. However, care should be taken when using PLCs in concrete for foundation applications. The use of PLC concrete is only permitted in Design Chemical Class 1 conditions (BRE Special Digest 1. 2001; TEGR, 1999). It is possible, though not confirmed at the present time, that the presence of limestone might promote TSA under certain conditions. Although limestone makes little or no contribution to the reactions of the cementitious phases, its physical properties can modify the behavioural characteristics of the concrete. It is relatively inexpensive and can improve the cohesion and workability of concrete. Limestone has been used in self-compacting concrete (SCC) as a filler material because it confers desirable properties to the concrete in both the fresh and hardened states, and improves economy (Bartos and Grauers, 1999). SCC is discussed further in Section 4.2.3.

4.4.7

Sulfate-resisting Portland cement (SRPC) SRPC will not be included in Part 1 of BS EN 197 but will be covered in a further part of BS EN 197. Until then, BS 4027 will continue to apply. SRPC is a form of PC with a low tricalcium aluminate (C,A) content, limited to a maximum of 3.5 per cent by mass (whereas PC usually contains about I O per cent). C3A content is a well established parameter for determining conventional sulfate resistance of PC concretes, and reduced content is recognised as conferring increased immunity from the more common types of sulfate attack. S W C is the only cement recommended in BRE Special Digest 1 for ground conditions of Design Chemical Class 4B - ie very strong magnesium sulfate, where the aggregate is low in carbonate - ie Range C (see Section 5.4.6 of this report for further details). SRPC is also resistant to DEF and low alkali SRPC minimises the risk of ASR problems. An additional advantage of SRPC in foundation applications is its relatively low heat generation compared with PC concrete. Where the foundations are thin in section and the ambient conditions are very cold, the more rapid strength gain of SRPC can be beneficial.

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Table 4.5

Summary of applications and potential effects of using pfa and ggbs in concrete Key Properties Addition type

Ultimate Effect

Water demand/

Reduced water

pfa

Practical Application and Benefit

Possible disadvantages

demand Workability ggbs

Small reduction in water demand.

Denser concrete

Reduction in cohesion at

Better compaction

very high levels of ggbs

Lower permeability

addition

increased mobility

Strength

pfa

Reduced early

Similar (or possibly

Potential reduction in

ggbs

strengths at low

slightly increased)

tensile strain

concrete temperatures

ultimate strengths at later

capacity

ages in large sections

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Setting times

pfa

Small increase in

Minimise the risk of cold

In thin sections: increase

setting time

joints in large pours*

in form work striking

Increase in setting

Useful where secondary

time dependent upon

cutting is required, eg

susceptible to damage

ggbs content

secant piling

through shock

Reduced temperature

Lower risk of thermal

time

ggbs

Heat of

pfa

hydration

rise

ggbs

Prolonged period

cracking

Reduced temperature

Suited to large-section

rise, especially at

concreting

70%+ ggbs Permeability

pfa

Reduced pore size

Decreased permeability to

ggbs

distribution, porosity

water, oxygen, chlorides

and aggressive agents

Chemical

pfa

Resistant to chemical

Used in structures

Low calcium hydroxide

resistance

ggbs

attack, including

exposed to harsh

sulfate attack, ASR. acid attack etc.

environments, eg

levels reduce level at which chlorides activate

contaminated natural

reinforcement corrosion.

ground and in sea defences

No significant improvement in resistance to attack from magnesium sulfates.**

* This risk depends on the rate of placement, dimension of the pour, and chemistry of pfa and ggbs.

** Not applicable to ggbs where concrete contains carbonate.

4.5

EFFECT OF AGGREGATES Aggregate typically comprises about 70 per cent the volume of concrete and therefore, for environmental and economic reasons, consideration should always be given to use of recycled and local materials. Concrete with the desired performance can usually be made with these materials if it is specified and designed correctly. The type and grading of coarse and fine aggregate present in concrete influences a number of its fresh properties (workability, water demand, bleeding, segregation, thermal expansion) and hardened properties (strength, stiffness, shrinkage, susceptibility to deterioration through chemical attack and abrasion). Guidance on suitability of various aggregate sizes, gradings and proportions is included in standard

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CIRIA C569

design codes, however a number of other aggregate-related properties are of particular relevance to the design and specification of foundation concrete.

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4.5.1

Effect of aggregate size and type A rounded natural gravel aggregate with a maximum nominal size of 20 mm can provide foundation concrete with the high levels of workability needed to facilitate placement and allow self-compaction. Larger maximum aggregate sizes are occasionally used for specific purposes. In combination with high proportions of ggbs, 40 mm gravel has been used to reduce the cement paste content in large pours where a reduction in heat-generation was required. However, the use of larger aggregate sizes can result in a reduction of the strain capacity of the concrete, which may more than offset the benefit of cement reduction in terms of limiting thermal cracking. Additionally. increasing maximum aggregate sizes has a negative influence on the placeability of the fresh concrete, with more demanding requirements on tremie diameters, reinforcement spacing etc. In many cases, particularly in large volume pours, it is a requirement of foundation concrete that it should exhibit a low coefficient of thermal expansion in order to limit thermally-induced cracking. This can be achieved by a combination of factors, including the utilisation of aggregates with a low coefficient of thermal expansion, most commonly limestone. Of the natural aggregates, limestone will produce concrete with the greatest resistance to cracking, and the siliceous aggregates (eg flint) the least resistant. Lightweight aggregates also exhibit highly suitable thermal properties with respect to limiting cracking, but their use necessitates an increase in cement content to retain strength grade, which may partially negate this advantage and possibly prove detrimental in other respects.

4.5.2

Aggregates and ASR The reactive silica present in some aggregate types has, under certain conditions, led to damage to concrete through disruptive alkali-silica reaction (ASR). This phenomenon is now well documented and researched and it is understood that, in practice, ASR may be prevented by: •

limiting the presence of reactive silica (usually by careful selection of aggregate used)

•

limiting the proportion of reactive alkalis available for the reaction (usually by limiting the total reactive alkali content of the cement).

Detailed guidance on minimising the risk of ASR is given in BRE Digest 330 (1999); in this document aggregate types are classified according to their potential for reactivity, with relevant recommendations regarding alkali content limits on cements (including blended cements). ASR is discussed more thoroughly in Section 5.7.

4.5.3

Aggregate and sulfate attack In most circumstances, any aggregate can be used to provide concrete with adequate resistance against all forms of sulfate attack. With the increased awareness of the thaumasite form of sulfate attack (TSA), concerns have grown over the use of aggregates containing significant quantities of carbonate in concrete for foundation applications. In light of recent cases of TSA and following the publication of the Expert Group Report on Thaumasite (TEGR, 1999). BRE Digest 363 is being updated.

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This document will provide detailed guidance on the choice of aggregate for use in buried concrete. The new Digest, BRE Special Digest 1 (2001), will contain comprehensive guidance on the design of sulfate-resisting concrete based on the TEGR (1999) and introduce the concept of aggregate classification on the basis of carbonate content and the proportion of the carbonate in the fine and coarse aggregate.

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Aggregates are classified into three Ranges A, B and C (based on “CaCO3 equivalent” as a percentage of the total mass of aggregate). These are 0-12per cent for Range C, 12-30per cent for Range B and >30 per cent for Range A if all the carbonate is in the coarse aggregate. In order to recognise the increased sensitivity to the proportion present in the fine fraction, the Aggregate Carbonate Ranges may be taken from the chart in Figure 4.2. It is the concrete producer who ultimately determines the Aggregate Carbonate Range using data from the aggregate producer on the percentage of CaCO3 equivalent in the coarse and fine fractions of aggregate. Details on TSA and the guidance given in BRE Special Digest 1 (2001) are given in Section 5.4 of this report.

4.6

EFFECT OF ADMIXTURES Admixtures are an extremely useful and increasingly widespread and important component of concrete. Their careful and judicious use may provide highly desirable practical and economic benefits. Detailed guidance on the use of chemical admixtures is provided in CIRIA Report 165 (1997) and Concrete Society Report 18 (2000). The following section provides general information on the effects of admixtures that might be usefully incorporated in concrete for foundation applications. Admixtures are available as proprietary chemical products capable of modifying the properties of concrete to which they are added. In practice, they are used to increase workability and/or increase strength. They can reduce permeability and may also be used to control setting times. The judicious use of admixtures can provide significant economic benefits in terms of savings on labour, raw materials and - less directly - on the whole-life costing of the structure in terms of increased durability. Admixtures for use in concrete are covered in BS 5075 Parts 1-3and pr EN 934 Parts 2 and 6 where they are classified according to their primary effect on the concrete (for example as accelerators, retarders, water-reducers, superplasticisers etc). For site-mixed concrete, trial mixes should be performed to assess the suitability of admixtures and more precisely determine the nature of their effects in the proposed concrete.

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CIRIA C569

CaCO3 equivalent as a percentage of total mass of aggregates

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100

80

60

Range A

40

20

Range B

10 Range C

2

0 0

10

30

20

40

50

60

70

80

90

100

% of total Ca-CO, equivalent which is the fine fraction

Figure 4.2

Carbonate ranges for concrete mix design (BRE Special Digest 1 (2001)

1. Prepare test portions of separate fine (f) and coarse (c) aggregates, as detailed in Clause 5 of BS 812: Part 102.

2. Determine the percentage of carbon dioxide ( % CO2 ) content of each test portion by a suitable method (eg. Clause 5 of BS EN 196-21:1992). 3. Convert each concentration of CO2 to CaCO, equivalent by multiplying by 100/44 to obtain values for Pcf and Pc c where:

Pcf = % CaC0 3 equivalent in the fine aggregate, Pc c

= % CaC03 equivalent in the coarse aggregate.

4. Calculate the % of CaC0 3 equivalent X in the fine aggregate as a % of the total CaC0 3 equivalent in the combined aggregates to give the x-axis value on the chart, using:

x = 100 x where F and C are the masses (kg/m3) of the fine and coarse aggregates respectively.

5. Calculate total CaCO3 equivalent T as a % of the total mass of aggregates to give the y-axis value on the chart, using:

where is a safety factor that allows for source and testing variability. The Quarry Products Association (QPA)(3)suggests using a value for of 1.15. Note: Total CaCO3 equivalent may be more than I00 per cent dire to use of safety factor and/or dire to the presence of dolomite.

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6. Identify applicable Aggregate Carbonate Range A. B or C from x-axis and y-axis values using the chart. A worked example is given in the box below.

Worked Example A Consider concrete mix with: - 350 kg/m3 cement, - 700 kg/m3 fine aggregate of which 25% is calcium carbonate (limestone)

- 1400 kg/m

3

coarse aggregate of which 50% is calcium carbonate (limestone).

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Then: F=700 kg/m3, C=1400 kg/m3, Pcf = 25%, P c c = 50%

Therefore from d) x-axis value on the chart is given by: X = 100 x ( Pcf / l00 ( F ) / { (Pcf / l00 x F) + (Pc c /l00 x C) }

=

100 x (25/100 x 700)

(25/100 x 700) + (50/100 x 1400) 100 x 175 175 + 700

= 20% and from 5 y-axis value on the chart is given by: T = x 100 x { (P c f /100 x F) + (Pc c / l00 x C) } / ( F + C )

=

x 100 x (25/100 x 700 + 50/100 x 1400)

700 + 1400

=

x 100 x (175 + 700)

2100

=

x 42%

= 48% for Q PA suggested

= 1.15

X and T plot on Figure A2.1 as Range A aggregate.

A number of admixtures are suitable for use in foundation concrete but the most frequently used are water-reducers /plasticisers, retarders and superplasticisers. The principal chemical types are indicated in Table 4.6.

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Table 4.6

Principle admixture types used in foundation construction, their ultimate effect, and practical application and benefit (Concrete Society Report 18, 2000).

Admixture Type

Ultimate Effect

Practical Application and Benefit

Water reducers/ plasticisers

• High workability of given water content.

• Denser concrete

• Higher strengths for a reduced water content at a maintained workability.

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• Same strength at a reduced cement content whilst maintaining the same w/c ratio.

4.6.1

• Stronger concrete • Improved standard deviation • Lower permeability

Retarders

• Retained workability • Extended setting times • Slower strength/time responsse • Higher ultimate strengths

• Offsetting the effects of high ambient temperatures • Prevention of cold joints between pours

Superplasticisers

• Very high workability at a given water content • Higher water reductions for a given workability

• Assist concrete placing in difficult situations eg congestedreinforcement • High early strength concrete • Time and energy savings • Non-shrink, non-bleed grouts

Retarders Retarders delay the setting of the cement paste but ultimately they have no effect on the strength or other properties of the hardened concrete. These effects are potentially useful in large pours. Delayed setting times can allow massive single pours to be carried out to produce large elements which would otherwise require segmental construction. Retarding admixtures may delay the time of onset of temperature rise and in small sections this can allow some heat to dissipate but in larger sections, once the exotherm starts, the temperature rise is rapid and - without cement reduction or partial replacement with pfa or ggbs - will reach at least the same level as an equivalent mix without admixture. If early loads of concrete in a big pour are not retarded and start to exotherm before the concreting is complete, they may cause heat-induced early stiffening of later loads leading to unexpected formation of cold joints. Great care should be exercised in the use of retarders, since overdosing may completely inhibit hardening. It should be considered that the effectiveness of retarders is reduced at high temperatures. At high temperature, premature stiffening may be due to evaporation of mix water.

4.6.2

Water-reducers Water-reducing admixtures are able to provide a moderate level (commonly 5-10 per cent, occasionally up to 15 per cent) of water reduction in the concrete. They are added in relatively low doses, and allow either a reduction in w/c ratio whilst retaining workability, or conversely an increase in workability at a given w/c ratio. Concretes containing water-reducing admixtures characteristically exhibit low segregation and improved flowability, both desirable features in foundation concretes and useful for tremie concreting. It should be noted that retarding plasticising admixtures used as water reducers or

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cement savers have little or no benefit in extending the workability retention of the concrete. Only by using them to increase initial workability will there be higher workability for a longer period. Some water-reducing admixtures have the side effect of retarding the set, particularly at higher dosages; this may be compensated for if required, although it is often of practical benefit where foundation concreting is concerned. Long-term strength is unaffected. Some entrain a small volume of air (about one per cent) which helps improve cohesion of the concrete.

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4.6.3

Super plasticisers Superplasticers are normally used either as a workability agent for the production of flowing concrete at a given w/c ratio; or a water-reducing agent for the production of high strength concrete by reducing water content (up to 30 per cent) whilst maintaining the Workability relevant to the particular application. In addition, there is an increasing tendency to design for a combination of both the water reduction and improved workability. This application is particularly useful for thick concrete sections cast below ground, allowing workable concrete with relatively low w/c ratio to be cast as monolithic sections without cold-joints, and with reduced proportions of cement leading to lower heats of hydration and decreased potential for thermal cracking. Superplasticers differ from water-reducing admixtures primarily in the greatly increased potential magnitude of their effect. Superplasticisers used in very high-slump concretes and where combined with retarders, may lead to increased bleeding. The period of high workability may be extended by repeated small additions of admixture. However, caution is necessary since overdosing may result in a number of undesirable effects, including severe retardation of the initial and final set and increased potential for bleeding and segregation. Product data sheets should contain information on the effects of overdosing. Retarding superplasticisers, able to provide extended periods of workability and a delay of set, are available, either as specially designed polymer materials, or as combinations of retarding and superplasticising admixtures.

4.7

EFFECT OF PLACING TECHNIQUES Detailed information may be found in CIRIA Reports 165 (1997) and 135 (1995) on the variety of placing techniques used in in-situ concrete foundation practice. This section briefly reviews the concrete placing techniques used for in-situ concrete foundation construction and comments on their effect on concrete properties. The placing technique is an important factor in producing good quality concrete, and the method chosen depends upon the type and depth of foundation to be placed. Placing should be undertaken with care so that dense, homogenous concrete is finally produced; concrete should always be placed as near to its final position as practicable and not allowed to flow along formwork. Segregation needs to be avoided

4.7.1

Preparation Preparation of the substrate surface before pouring is an important pre-requisite in foundation construction. In shallow foundation work, concrete is commonly poured directly onto suitably consolidated clay or loam substrate that should be free from debris and pooled water. Where the substrate is excessively dry, it should be moistened prior to pouring so as to avoid desiccation of the adjacent concrete.

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Where the substrate is highly porous, for instance sand or gravel, a layer of polythene sheeting may be used to cover its surface. If required, a thin layer of blinding concrete may be used to prevent contamination of the foundation concrete or any reinforcement from the substrate - this also provides a convenient working platform prior to pouring.

4.7.2

Placing techniques For shallow foundations there is generally no requirement for specialised placing techniques. Where reinforcement congestion is not a problem, direct delivery from a skip moved slowly along the area to be filled may give good results, although care should be taken to avoid concrete bouncing off the sides of the formwork with resultant separation of the coarse fraction and possible damage to the forms.

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For deep foundations, however, there is a variety of methods and plant available for placing, cohesive, flowing concrete. Some of the main methods are listed below; • hopper and delivery •

continuous flight augers, hollow auger systems

• tremie pipes. For long falls, tremie concreting has the advantage of improved positioning of the concrete however, as with other methods of placement, segregation may be a problem if the concrete is allowed to form a conical pile in one position. Where reinforcement allows, the pipe should be moved so as to avoid forming a conical pile. The pipe has to be at least 150 mm in diameter although a diameter of 200-250mm (or six times the maximum size of aggregate) is preferred, to ensure free flow of the concrete. The first load of concrete displaces the plug within the pipe, allowing the concrete to flow and self-compact at the base of the trench. As the level of the concrete rises. the outlet of the tremie should stay buried in the placed fresh concrete. According the BS EN 1538 (2000) and FPS (1999), the tremie pipe should remain 3 m below the surface of the concrete, unless the surface level is known, in which case it can be reduced to 2 m (see Figure 4.3). When the excavation is bentonite supported, further supply should displace the concrete present, and not free-fall through the bentonite. Intermixing with the supporting bentonite may cause lenses of the material to become trapped in the hardened concrete and impair durability. Tremie techniques cannot be relied upon to remove bentonite traces, and the adherence of bentonite to the surface of reinforcement bars can adversely affect bond strength. Residual bentonite coatings on bars may cause bond strength reductions of up to 60 per cent for plain bars and 50 per cent for deformed bars. Since the complete removal of bentonite from the bars cannot be guaranteed, this should be taken into account in the design of elements constructed in slurries. The effects of bentonite on the performance of concrete are discussed by Xanthakos (1979) and CIRIA report PG3 (1977).

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An example of the problems that can occur if concrete is incorrectly placed using a tremie pipe is given in the following case study.

Case Study 5 Failure of test-piles under loading due to incorrect concrete placement techniques Summary: An in-situ concrete bored pile failed under test loading, as did subsequently constructed test-piles. Investigation revealed that the problem was caused by incorrect concrete placement techniques (direct discharge into the bore with no tremie pipe) resulting in poor quality non-homogenous concrete at the foe of the pile. The construction of a deep basement in a town-centre location involved placement of 330 cast in situ bored piles of between 600 and 750 mm diameter, and a safe working load (SWL) of 1700 kN. The substrate was London Clay, and the piles were heavily reinforced to counter the effects of ground heave.

One preliminary test-pile was constructed, but failed suddenly at a test-load of 1.75 x SWL. A second test-pile was constructed, which again failed very suddenly under load, this time at 1.25 x SWL. Two more test-piles were constructed, one using a higher grade of concrete, the other was increased in length by 5 m. Again, both failed suddenly under testloads of 2 x SWL. In order to investigate, a man-sized shaft was excavated adjacent to the second test-pile. Direct examination revealed that the toe of the pile was highly honeycombed, comprising gravel aggregate with little or no cement paste. This had resulted in the piles acting on shaft friction only, with a minimal end-bearing element of load transfer. The segregated concrete was caused by the placement method, which involved direct discharge into the pile bore at its head from a hopper, with no delivery pipe. On discharge, concrete was running down the reinforcement and segregating, leaving the cement paste coating the steel, and the aggregate falling to the toe of the pile. The problem was successfully remedied by using a vertical rigid tremie pipe to place the concrete vertically down the bore.

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CIRIA C569

4.7.3

Precautions

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Large pours are often desirable in foundation construction, since they improve economy and minimise the number of construction joints. It is essential to ensure a rapid and continuous supply of concrete with suitable properties. Where large continuous pours are performed heat generation may be a problem (see Section 4.3.2). After placement, thermocouples can be used in order to measure the hottest and coolest parts of the concrete, and temperature gradients of over 20°C across the pour should be avoided when using flint gravel aggregates in the concrete. The use of limestone aggregates, which have a low coefficient of thermal expansion, should be considered for in-situ foundation concretes as a means of reducing the risk of early-age thermal cracking. Details on the use of limestone aggregates for use in sulfate-bearing ground conditions is provided in Sections 4.4.6 and 4.5.3. In certain circumstances, it may be possible to reduce the cement content of the concrete provided the water content is reduced in proportion (by the use of admixtures) ie the w/c ratio is maintained, any effect on strength or durability is likely to be minor (see Concrete Society Discussion Document, 1999). In many cases, problems with excessive heat generation may be overcome by choice of a suitable cement at the design stage, ie low heat or blended cements (see Section 4.4), and by certain precautions and measures taken on site, such as avoiding external restraint where possible (guidance on restraint is given in CIRIA Report 91 (1992)). Thermal and early plastic cracking of concrete may be remedied by re-vibration at the stage where it has become stiff enough to benefit from such action but before hardening has proceeded to a level where disturbance might cause disruption and reduced strength. The timing and duration of this period may be deduced from site trials. Care should be taken to avoid promotion of significant levels of settlement and bleeding.

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5

Deterioration of concrete below ground

5.1

INTRODUCTION

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Foundation concrete is susceptible to attack from aggressive agents within the ground. The most significant of these are sulfates and sulfides within the soil and/or groundwater. With the increased requirement to build on “brownfield” and contaminated land concrete is also exposed to a cocktail of different chemicals contained within the soil and detailed assessment of ground conditions is a requirement for all sub-surface construction. This section briefly reviews the deterioration mechanisms that can affect concrete below the ground and provides guidance on assessing the risks and designing concrete to resist deterioration. It is not intended as a comprehensive review of the durability of foundation concrete.

5.2

EFFECTS OF CONCRETE DETERIORATION The deterioration mechanisms associated with different exposure conditions are shown in Table 5.1.

Table 5.1

Exposure environments and types of deterioration possible Exposure environment

Deterioration mechanisms

Sulfates/Sulfides

Conventional sulfate attack

Thaumasite form of sulfate attack Chlorides Acids Flowing groundwater External alkalis Exposure to atmosphere

(TSA) Chloride-induced reinforcement corrosion Acid attack Leaching (and potential exacerbation of other deterioration mechanisms) ASR Carbonation-induced reinforcement corrosion Chloride-induced reinforcement corrosion

Moist conditions Abnormally high early temperatures Rising/mobilegroundwater

ASR Delayed ettringite formation (DEF) Conventional sulfateattack Thaumasite form of sulfate attack (TSA) Aggressive carbon dioxide attack

It should be noted that in certain environments a number of these deteriorative mechanisms may operate simultaneously, with the potential to seriously exacerbate the deterioration of subsurface concrete. Under such conditions, it is advisable to design concrete to at least meet, or possibly to exceed, the most stringent recommendations of the concrete specifications relevant to each condition.

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CIRIA C569

5.3

ROLE OF GROUND CONDITIONS

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The aggressiveness of the foundation environment is a function of: •

chemical composition of ground in the vicinity of concrete

•

chemical composition of water in the vicinity of concrete

•

height of water table

•

permeability of ground

•

rate of groundwater flow adjacent to concrete

•

quality of interface between concrete elements and the ground

•

presence of other buildings/structures

•

drainage

•

ground

.

topography

sub-surface conditions, such as position, size and slope of strata or layers.

Classification and evaluation of the ground conditions is critical when determining the type of concrete to be used in foundation works. The main documents that give guidance on assessing the role of ground conditions and assessing their relevance with respect to concrete design are listed below:

. BRE Special Digest 1 Concrete and Concrete Products in Aggressive Ground (2001)

5.4

•

Part 1 : Assessing the aggressive chemical environment

•

Part 2: Recommendations for concrete specification and additional protective measures

•

BS 5328 Concrete Part I: Guide to specrjjing concrete (1997) (under revision and to be superseded by pr BS 5800)

•

DETR,The thaumasite form of sulfate attack: risk, diagnosis, remedialworksand guidance on new construction (1999)

•

BS 5930 Code of practicefor site investigation for civil engineering (1981)

•

BRE Digest 381 Site investigation for low-rise building: trialpits (1993)

•

BRE Digest 383 Site investigation for low-rise building: soil description (1993)

•

BRE Digest 41 1 Site investigation for low-rise building: direct investigation

•

BRE Report 279 Sulfate and acid attack on concrete in the ground: recommended procedures for soil analysis (1995).

SULFATE ATTACK The risk of sulfate attack is sometimes a concern for concrete substructures in belowground environments. Therefore this section will review the current recommendations and guidance for dealing with the potential problem of sulfate attack. The types of sulfate attack discussed in this section are: • “conventional” sulfate attack •

thaumasite form of sulfate attack (TSA)

• sulfuric acid attack • delayed ettringite formation (DEF). Sulfate attack is most likely to be a problem for relatively shallow foundations in

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disturbed ground and the upper parts of deep foundations (eg pile-heads) where they are buried in sulfate-bearing soil with flowing groundwater. Footings and mass concrete completely buried in undisturbed clays are unlikely to suffer serious damage from sulfate attack unless there is a combination of a high water-table with significant sulfate levels.

5.4.1

Conventional sulfate attack

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The degradation mechanisms of concrete undergoing conventional sulfate attack are complicated, but they can be generally described as a combination of three processes: •

ingress of sulfate ions into the concrete pore water

•

reaction of sulfate ions with hydrated calcium aluminates (in particular tricalcium aluminate, C3A) forming ettringite, and/or the combination of sulfate ions with free calcium hydroxide (Ca(OH)2) forming gypsum

•

expansion and cracking of concrete with eventual strength loss and disintegration.

The expansive chemical reactions involved depend upon the nature of the sulfate present, the cement type and type of aggregate used in the concrete. Factors that influence the rate of attack are: •

the concentration and type of sulfate and the pH in the soil or groundwater

•

position of the water table and mobility of the groundwater

•

the characteristics of the concrete (degree of compaction, cement type and content, type of aggregate, w/c ratio and curing regime)

•

the form of construction

•

freeze/thaw attack is unlikely to be a problem for concrete at depth but the combination of sulfate and freeze/thaw attack represents particularly severe conditions in near-surface conditions.

The process of the conventional form sulfate attack has been the subject of extensive investigations over many years. BRE Special Digest 1 (2001) covers the subject in detail.

5.4.2

Thaumasite form of sulfate attack (TSA) Recently in the UK a few cases of the thaumasite form of sulfate attack (TSA) have been discovered in buried concrete (TEGR, 1999). The calcium silicate hydrates in the hardened Portland cements are targeted for reaction and not the calcium aluminate hydrates. This considerably impairs the strength of the hardened cement by transforming it into a “mushy” incohesive mass leading to loss of structural integrity and durability. The number of structures potentially at risk of TSA in the UK is considered to be small since it will only occur if the following combination of conditions occur:

60

•

sulfates and/or sulfides in the ground

•

significant amounts of moisture and mobile groundwater

•

carbonate, generally in the concrete aggregates

•

low exposure temperatures (generally below 15°C).

CIRIA C569

Licensed copy:University of Manchester, 05/11/2007, Uncontrolled Copy, © CIRIA

These represent primary risk factors but there are also several secondary factors that influence the occurrence and severity of TSA and its effects. These are:

.

type and quantity of cement used in concrete

•

concrete quality

•

changes in ground chemistry and water regime resulting from construction

•

type, depth and geometry of buried concrete.

Further detailed information on TSA is given in the Thaumasite Expert Group Report (TEGR, 1999). This includes guidance on the implications for existing structures and for the design and specification of new construction in the UK. The Expert Group concluded that current recommendations given in BRE 363 and BS 5328 with regard to all forms of sulfate attack needed some modifications. Section 5.4.6 of this report details how the current guidance is being updated and provided updated best practice guidance on how to minimise all forms of sulfate attack.

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Case Study 6 Thaumasite form of sulfate attack (TSA) in motorway bridge foundations Summary: This case study is taken from the Thaumasite Expert Group Report (TEGR, 1999) and provides an example of the conditions required and effects of TSA. The main problem was that the sulfate class was incorrectly assumed in the original design of the foundations and this underestimation of the aggressiveness of the soil was one of the contributing factors leading to TSA.

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Of the few currently identified examples of the thaumasite form of sulfate attack (TSA), one of the most extreme was observed in the 29-year-old foundations of a motorway bridge on the M5 motorway. These had been built using a good quality, dense and well compacted concrete with the following characteristics: •

cement content of around 370 kg/m3

•

water/cement ratio o f around 0.5

•

Portland cement type (PC) with a tri-calcium aluminate content of around 11 per cent

•

good quality natural aggregate (dolomitic limestone coarse aggregate, and a mix of limestone and quartz sand fine aggregate)

•

in-situ cube strengths in excess of 60 MPa in the unaffected area

The method of construction was, however, very significant because the buried concrete was effectively constructed in a “sump” and then surrounded by reworked Lower Lias clay, in which the sulfides (pyrites) had become oxidised to add to the reservoir of sulfates. The guidance applicable at the time of construction suggested only Class 1/2 conditions prevailing, whereas reconsideration in line with the TEGR (1999) increased the classification as much as 2 Classes Inspection of the foundations after 29 years revealed an advanced state of decomposition of the outer layers of the concrete columns, particularly in the lower parts below the level of the French drains. Here the concrete was likely to have been exposed to wet conditions with mobile groundwater for long periods of time. The outer surface zone of the concrete was severely softened to form a white mush (the cement paste having been converted almost completely to thaumasite), behind which cracks ran sub-parallel to the concrete surface, these being filled with further thaumasite. The average depth of concrete affected was around 20 mm. Chloride values at the depth of the steel were found to be high (one per cent or more by mass of cement) indicating a high risk of chloride-induced corrosion. The chlorides had been introduced by the highway drainage system, and their ingress to the steel facilitated by the deterioration of the cover concrete. In parts of the foundation exposed to undisturbed, unweathered Lower Lias clay was found to exhibit a very minor level of thaumasite formation, with no associated deterioration. Several other M5 motorway bridges of similar age were found to exhibit less severe forms of TSA, manifest either as relatively small isolated “blisters” of softened concrete (approximately 100 mm across, up to 40 mm deep), disintegration of arrises, or softening over a larger part of the face, but to a much shallower depth (up to 10 mm).

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5.4.3

Sulfuric acid attack Sulfuric acid can occur naturally in soils and ground waters and is frequently present in industrial wastes, particularly those produced by mining or coal combustion, or by the decomposition of waste, for example in sewers. Naturally occurring sulfuric acid results from the oxidation of sulfide minerals particularly pyrite which, in the presence of air and moisture pyrite, is oxidised to form ferrous sulfate and sulfuric acid. The process is greatly accelerated when aerobic bacteria are present.

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Sulfuric acid attack is particularly aggressive to concrete, and can cause considerable loss of strength and durability. and ultimately complete disintegration of the concrete. Of prime importance in reducing deterioration through acid attack is production of a dense, high quality concrete with a low permeability. Further details are included Section 5.8.1 on acid attack, and guidance on requirements for acid resistant concrete are given in BRE Special Digest 1 (2001). See Section 5.4.6 of this report.

5.4.4

Delayed ettringite formation (DEF) Delayed ettringite formation is a problem that occurs sometimes in steam-cured precast concrete elements, although it may also occur in large in-situ pours where elevated temperatures, over 7OoC, are generated and sustained, facilitated by the insulating properties of the ground. DEF is associated with concrete in moist conditions. and may develop over a period of months or even years. It can cause internal disruption and cracking in the concrete although, in many cases, the magnitude of cracking is not great and may be of relatively little significance in foundations. Under UK conditions cracking due to DEF takes between 8-20 years to occur (Lawrence et al, 1990) and all the field concretes which have cracked had alkali levels above 4.5 Kg Na(eq)/m3. Guidance on avoiding DEF is not included in the main documents dealing with other forms of sulfate attack, although recommendations for minimising the risk of DEF in precast concrete include - delaying the development of high temperatures, keeping the peak temperature below 60-70°C, and reducing rates of heat change to less than 20°C/hour (Lawrence, 1990). The chemistry of the cement is also thought to be an influencing factor, and the inclusion of composite cements is thought to provide some level of protection against DEF.

5.4.5

Determination and classification of exposure conditions Although sulfate attack is believed to be a relatively rare cause of deterioration of concrete in the UK, many structures are exposed to sulfate-bearing groundwater and soil as well as to disused industrial (brownfield) sites that can also provide a potential source of sulfates. The approach to designing concrete to function in sulfate exposures should be based on having a good knowledge of the exposure conditions and from this, select materials and detailing that offer good resistance to the expected source of attack. In light of the TEGR (1999) guidance on the determination of the design for sulfate class has been amended. Both BRE Digest 363 (1996) and BS 5328 Part 1 (1997) are being amended to take account of recommendations of the Expert Group. BRE Digest 363 will be replaced by BRE Special Digest 1 Concrete and concrete products in aggressive ground and BS 5328 Part 1 is being updated but will ultimately be replaced by pr BS 5800 the complementary British standard to BS EN 206-1 The primary classification of sites into five classes with respect to sulfates has been in

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use for many years and is retained in BRE Special Digest 1, as are the modifications to this primary classification. The modifications which relate to the aggressive chemical environment into which the concrete is to be placed (mobility and pH of groundwater and, for Sulfate Classes 4M and 5M, the magnesium ion concentration) have been incorporated into BRE Special Digest 1 to give an Aggressive Chemical Environment Classification (ACEC) for the site. This is shown in Table 5.2. An overview of the procedure needed to determine and apply the ACEC classification is set out in Figure 1 of BRE Special Digest 1 Part 2, while detailed steps for the three main categories of site are given in Figures 4 to 6 of BRE Special Digest 1 Part 1. Three Levels of Structural Performance (High, Normal and Low) are identified in BRE Special Digest 1 Part 1 depending on the type of structure. The ACEC Classification derived in Table 5.2 is for a Normal Performance Level. From 1 July 1999, for all new contracts to supply ready-mixed concrete under QSRMC certification, where the requirement is expressed in terms of Design Chemical Class (DCC), the concrete will conform to the recommendations of BE Special Digest 1. The Quarry Products Association (QPA) has issued guidance on the practical implications of TSA on buried concrete (Harrison, 1999). This provides a review of the method of determination of the DCC, and methods for specification and recommendation of additional protection measures where required (see Section 5.4.6 for further details)

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Table 5.2

Aggressive Chemical Environment for Concrete (ACEC) classification (BRE Special Digest 1, 2001)

Sulfate and magnesium Design Sulfate Class

Total

2:1 water/soil extract

Groundwater

Potential

Sulfate

for site

1

2

3

4

5

6

SO4 g/l

Mg g/l

SO4 g/1

Mg g/l

SO4

(2)

%

Static water

Mobile water

8

9

10

PH

pH

pH[4]

pH[4]

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>3.5

>65 5.5-6.5 4.5-5.5 <4.5 >5.5 >6.5

>5.5 <3.5 1.2-2.3

0.24-0.6

0.4-1.4

<5.5 5.5-6.5

<5.5

4.5-5.5

<4.5 >3.5

>5.5 >6.5

>5.5

2.4-3.7

1.5-3.0

0.7-1.2

<3.5

<5.5 5.5-6.5 <5.5

<5.5

>3.5

>5.5 >6.5

>5.5 DS-4

3.8-6.7

£1.20

3.1-6.0

£1.00

1.3-2.4

<3.5

<5.5 <6.5

<5.5 >3.5

>5.5 >6.5

>5.5 DS-4m

3.8-6.7

>1.2 [1]

3.1-6.0

>1.0[1]

1.3-2.4

<3.5

<5.5 <6.5

<5.5 DS-5

DS-5m

>6.7 >6.7

£1.20 >1.2

>6.0

£1.00

>2.4

>6.0

>1.0[11

>2.4

[1]

ACEC Class

>3.5 <3.5 >3.5 <3.5

All pH values All pH values

>5.5 <5.5 >5.5 <5.5

11

AC-1s

All pH values >5.5 <5.5

<0.24

<0.4

Mobile water

7

All pH

<1.2

Static water

for site

values DS-1

Brownfield[3]

Natural soil

All pH values All pH values

AC-1 AC-2z AC-3z AC-4z AC-1s AC-2 AC-2s AC-3z AC-4z AC-5z AC-2s AC-3 AC-3s AC-4 AC-5 AC-3s AC-4 AC-4s AC-5 AC-3s AC-4m AC-4ms AC-5m AC-4s AC-5 AC-4ms AC-5m

Notes:

Explanation of suffix symbols to ACEC Class number:

[1] The limit on water-soluble magnesium does not apply to brackish

The use of suffixes is essential as a step in the procedure for specifying

groundwater (chloride content between 12g/l and 18g/l). This allows m to be omitted from the relevant ACEC classification.

[2] Applies only to sites where concrete will be exposed to sulfate ions (SO4) which may result from the oxidation of sulfides such as pyrite. following ground disturbance. [3]

"Brownfield" is defined as sites which may contain chemical wastes remaining from previous industrial use or from imported

wastes. [4] An additional account is taken of hydrochloric and nitric acids by adjustment to sulfate content - see Section 6.1.2.

CIRIA C569

cement and concrete mixes appropriate to the ground conditions: •

Suffix s indicates that, as the water has been classified as Static, no

•

Additional Protective Measures will be necessary for concrete. Suffix z indicates that concrete primarily has to resist acid

conditions. •

Suffix m relates to the higher levels of magnesium in Sulfate

Classes 4 and 5.

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5.4.6

Guidance and specifications

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Many of the recommendations in the previous Digest which dealt with sulfate and acid attack of concrete in the ground (Digest 363) were also embodied in British Standards, notably in BS 5328: Concrete. Amendments to Parts 1 and 2 of BS 5328 will be issued following publication SDI to bring the two documents into line. It is, however, intended to replace BS 5328 by the new European Standard BS EN 206-1: Concrete and its complementary British Standard, BS 8500. on 1 December 2003. The classification in BS EN 206-1 for chemical aggressiveness is limited to natural soils and Static water conditions. For this reason, BS 8500 is following this Digest with its far wider range of application. The only proposed difference between the BS 8500 provisions and those for BS 5328 is the classification of cements. BS 8500 uses the BS EN 197-1 cements plus residual British Standards for cements not covered by BS EN 197-1. Small adjustments to the limits on composition have also been proposed to facilitate specification, eg any CEM 111 /B cement will be regarded as being sulfate-resisting. An initial classification of the sulfate conditions is made from the analysis of soil and/or groundwater samples, taking into account the possibility of oxidation of sulfides to sulfates and the magnesium sulfate concentration. The Aggressive Chemical Environment for Concrete (ACEC) classification is given in Table 2 of BRE Special Digest 1. This is repeated in Table 5.2 of this report. Modifications to this initial classification may be necessary to allow for statidmobile groundwater, type of exposure - including acidic conditions - and type of construction. The final classification following these modifications is the Design Chemical Class (DCC). Table 5.3 gives the recommendations provided in BRE Special Digest 1 for the type of cement, maximum w/c ratio and minimum cement content for each DCC and for different Aggregate Carbonate Ranges. These Ranges are explained in Section 4.5.3 of this report. The cement groups for use in Table 5.3 are given in Table 5.4. Consideration should also be given to whether additional protective measures are necessary. The need for additional protective measures depends on the DCC and on the structural performance level of the construction. The measures may include designed drainage, the use of surface protection, an increase in concrete quality and restriction of choice of Aggregate Carbonate Range. It should be emphasised that the recommendations given in BRE Special Digest 1 should be taken in the context of the whole document and not taken as absolutes. For example, the recommendation for designed drainage is not always practical - eg deep piles - and some alternative additional measure should, therefore, be used. BRE Special Digest 1 will use the concept of a trade-off between different measures and include a list of additional measures as mentioned above. This flexibility should allow practical and economic solutions to be found for most circumstances.

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The detailed guidance included in BRE Special Digest 1 will not be repeated here but a brief summary of the guidance for design and detailing of concrete structures is given below: •

assess the soil sulfate class

•

determine the Design Chemical Class (see Table 5.2 of this report)

•

design and detail the buried concrete structure taking into account the recommendations given in the relevant tables

•

specify the following - the structural grade required - the design sulfate class - any additional protection measures (eg surface protection, drainage)

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- maximum size of aggregate - the required workability •

determine if there is also a need to restrict or specify aggregates with a particular carbonate range. (Note: Until BS 5328 (1997) is amended, do not specify designated concretes in sulfate classes 3 to 5B).

In assessing ground conditions, consideration should be given to the following: •

ground-water level and mobility are influenced by seasonal variations and changes caused by the construction itself and any subsequent constructions nearby

•

where reworked clay is backfilled against a concrete surface, consider the potential for sulfates and sulfuric acid to be produced by oxidation of sulfates present

•

sulfides present in unworked, unweathered clays are not aggressive, and therefore do not require consideration in assessing the Design Chemical Class

•

sulfates are often depleted in the top metre or so of soil, which may reduce the aggressiveness of the ground conditions in this area

•

it is important to assess and consider the type of sulfates present. Even cements containing high proportions of addition materials such as ggbs and pfa, which have good resistance to sodium and calcium sulfates, are considerably less effective in resisting the action of the more aggressive magnesium sulfates.

During construction in sulfate-bearing ground, where practicable, foundations in clay should be cast without shuttering so as to avoid formation of a porous zone susceptible to filling by pore-water. Also, the re-working of clay provides the oxygen required to convert any sulfides into aggressive sulfuric acid and sulfates. Where concrete is cast against as-dug faces, design codes require provision of additional cover to ensure adequate protection of reinforcement. A list of Design Guides for different categories of buildings and structures (including foundations) is provided in BRE Special Digest 1 and these are shown in Table 5.5.

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Table 5.3

Concrete qualities to resist chemical attack (BRE Special Digest 1, 2001).

Design Chemical Class[5]

Cement or

Dense fully compacted concrete made with

aggregate conforming to

combination group[1]

BS 882 or BS 1047

Maximum free

Minimum cement content kg/m3

water/cement ratio

-

-

DC-1

No restriction

DC-2

A[4], B, C

1

340

0.5

A[4], B, C

2, 3

300

0.55

No restriction

1[2], 2, 3

300

0.55

A

2a

400

0.4

A

2b. 3

380

0.45

B, C

2, 3

340

0.5

DC-3*[7]

B

2, 3

380

0.45

DC-3**[8]

C

2, 3

380

0.45

DC-3z[3]

No restriction

1[2], 2, 3

340

0.5

A

2a

400

0.35

A

2b. 3

400

0.4

B, C

2, 3

380

0.45

DC-4*[7]

B1

2, 3

400

0.4

DC-4**[8]

C

2, 3

400

0.4

DC-4z[3]

No restriction

380

0.45

400

0.4

DC-2z[3]

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Aggregate

Carbonate Range

DC-3

DC-4

A

1, 2, 3 [2]

[2]

, 2 ,3

1

[6]

2b

,3

DC-4m B, C

3

380

0.45

DC-4m* [7]

B

3

400

0.4

DC-4m**[8]

C

3

400

0.4

Notes to Table 5.3:

[I] For cement or combination groups see Table 3. [2] Portland limestone cement may only be used in concrete specified as Design Chemical Classes DC-2. DC-2z. DC-3z. DC-4z where the sulfate classification (Design Sulfate class for the site - see Table 2 of Part 1) does not exceed DS-1. [3] Classes DC-2z. DC-3z and DC-4z apply where chemical resistance is recommended primarily to resist acid attack under conditions described in this Digest. [4] In addition to the requirement for a minimum cement content and a maximum free water/cement ratio, a minimum concrete Grade of C35 is recommended when using Range A aggregate combinations. [5] Bold horizontal lines indicate division of the Table in terms of concrete quality in respect of aggressive ground conditions. Employment of the option "Enhance concrete quality" as an Additional Protective Measure can be satisfied by using the recommendations of the next numerically higher DC Class. Where the starting point is a 'starred' DC Class, the step is to a similarly starred higher DC Class, eg DC-3** plus a 1 step enhancement of concrete quality leads to a specification of DC-4**. [6] Use of group 2b cements in DC-4m concrete requires use of Range A aggregate which has been strictly determined on

the basis of carbonate content, and not by 'declaration' (see Appendix 3).

[7] Use of a single-starred DC Class may permit the recommended number of APM to be reduced by 1 (see Note [5] of Table 7). [8] Use of a double-starred DC Class may permit the recommended number of APM to be reduced by 2 (see Note [5] of Table 7).

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Table 5.4

Cement groups for use in Table 5.3 (BRE Special Digest 1, 2001). Description

Group

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1

2

a)

Portland cement conforming to BS 12.

b)

Portland blastfurnace cements conforming to BS 146.

c)

High slag blastfurnace cement conforming to BS 4246.

d)

Portland pulverized-fuel ash cements conforming to BS 6588.

e)

Pozzolanic pulverized-fuel ash cement conforming to BS 6610.

f)

Portland limestone cement conforming to BS 7583.

g)

Combinations of Portland cement conforming to BS 12 with ggbs

h)

Combinations of Portland cement conforming to BS 12 with pulverized-fuel ash conforming to BS 3892: Part 1.

a)

Portland pulverized-fuel ash cements conforming to BS 6588 containing not less than 26% of pfa by mass of the nucleus[1] or combination of

conforming to BS 6699.

Portland cement conforming to BS 12 with pfa conforming to

BS 3892: Part 1, where there is not less than 25% pfa and not more than 40% pfa by mass of the combination. b)

3)

High slag blastfurnace cement conforming to BS 4246. containing not less than 74% slag by mass of nucleus [l] or combinations of Portland cement conforming to BS 12 with ggbs conforming to BS 6699 where there is not less than 70% ggbs and not more than 85% ggbs by mass of the combination.[2]

Sulfate-resisting Portland cement conforming to BS 4027

[1] The nucleus is the total mass of the cement constituents excluding calcium sulfate and any additives such as grinding aids. [2]

For all group 2b cements, granulated blastfurnace slag with alumina content greater than 14 per cent should only be used with Portland cement having a tricalcium aluminate (C3A) content not exceeding

10 per cent.

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Table 5.5

Design Guides for different categories of buildings and structures in Parts 3 and 4 SD 1 (BRE 1, 2001)

Category of building/ structure

Non-domestic buildings, including any building of over 3

storeys above ground level used for residential accommodation

Design Guide

la: Cast-in-situ concrete greater than 140 mm and less than 450 mm thick 1b: Cast-in-situ concrete greater than 450 mm thick 1c: Cast-in-situ concrete less than 140 mm thick

1d: Cast-in-situ concrete greater than 140 mm and less than 450 mm thick, with a hydrostatic head greater than five times the wall thickness 1e: Cast-in-situ concrete greater than 140 mm and less than 450 mm thick with a hydrostatic head greater than five times the wall thickness and concrete surface carbonated by exposure to air

1f: Cast-in-situ piles

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1g: Precast ground beams, wall units and piles - normally cured 1h: Precast ground beams, wall units and piles carbonated by exposure to air.

Foundations for low-rise domestic properties

2a: Trench fill and strip foundations

2b: Raft foundations 2c. Floor slabs and oversite concrete on fill or hardcore containing sulfate

Foundation elements of transport structures

3a: Cast-in-situ concrete greater than 140 mm and less than 450 mm thick 3b: Cast-in-situ concrete greater than 450 mm thick

3c: For retaining walls with a hydrostatic head greater than five times the wall thickness 3d: For retaining walls with a hydrostatic head greater than five times the wall thickness, and concrete surface-carbonated by exposure to air 3e: Cast-in-situ concrete used for piles 3f: Precast ground beams and wall units and piles - normally cured 3g: Precast ground beams and wall units and piles carbonated by exposure to air

Pipeline systems

4a: For external surface of concrete pipes and associated units for water and sewer services in natural ground 4b: For external surface of concrete pipes and associated units for water and sewer services in brownfield sites 4c: Internal surfaces of pipes for water and sewer services

Precast linings for tunnels and shafts

5a: For segmental linings and associated units for tunnels and shafts in natural ground for water, sewer, storage and transportation - External

surface 5b: For segmental linings and associated units for tunnels and shafts in brownfield sites for water, sewer, storage and transportation - External surface 5c: For internal surfaces of segmental linings for tunnels and shafts for water and sewer services

Precast box culverts

6a: For box culverts in natural ground - External surface 6b: For box culverts in brownfield sites - External surface

6b: For box culverts: Internal surface Precast concrete masonry blocks

7a: For precast aggregate concrete blocks used below ground 7b: For autoclaved aerated concrete (Aircrete) blocks used below ground

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CIRIA C569

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5.4.7

5.5

Summary •

There are three main types of sulfate attack that can affect foundation structures; conventional sulfate attack, the thaumasite form of sulfate attack (TSA) and sulfuric acid attack. Additionally “internal” sulfate attack may be caused by delayed ettringite formation (DEF).

•

A well-planned, detailed ground condition survey should be undertaken to assess the Design Chemical Class. Where, sulfide-bearing clay is to be reworked and in contact with buried concrete, consideration should be given to the possibility that some sulfide may be oxidised to sulfuric acid and soluble sulfates.

•

Detailed guidance on designing concrete to minimise the risk of all forms of sulfate attack is provided in BRE Special Digest 1 (2001).

•

The recommendations given in BRE Special Digest 1 should be taken in the context of the whole document and not taken as absolutes. The digest allows for a trade-off between different protective measures and includes a list of different additional measures to be considered. This flexibility should allow practical and economic solutions to be found for most circumstances.

CRACKING Cracking, as the result of restrained movement (rather than some deterioration mechanism or structural loading), may be produced as a reaction to internal and external restraints acting on the concrete. Externally the concrete can be restrained by another part of the structure, or the ground. Internal restraints develop when different parts within the same section expand or contract differentially - this may be a problem in massive pours where high temperature differentials between the core and outer concrete may occur. One or both types of restraint may be present and should be understood and considered. Figure 5.1, taken from the Concrete Society Technical Report 22, Non-structural cracks in concrete (1992), shows the types of cracks that may appear in concrete. The subject is dealt with very comprehensively in this publication. The remainder of this section will, therefore, focus on cracking in foundation applications.

5.5.1

Significance of cracking It is important to carry out a thorough investigation to determine the significance of cracking. The following may have an influence on the conclusions reached: • Cracks that result from external restraint are likely to pass through a section and tend not to close with time. The implications of such cracking are complex but cracks transverse to reinforcement are unlikely to be of concern for durability up to about 0.5 mm (ref Cement and Concrete Association Handbook to BS 8110:1985 page 206 (1987)). The implications for structural behaviour or other service requirements such as watertightness may need to be considered. • Cracks that result from internal restraint (ie temperature differentials) do not pass through the whole section and tend to close with time. The importance of limiting crack widths is related to the aggressiveness of the environment and the nature and required performance of the member. Obviously for foundation concrete the more onerous crack width restrictions relating to visual appearance are not applicable - the exception being where the top of the foundations are exposed - eg substations, service reservoirs. For further guidance on acceptable crack widths refer to CIRIA Report 91 (1992) and Concrete Society Report 22 (1992).

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71

SHRINKABLE AGGREGATES — PHYSICAL

DRYING SHRINKAGE CRAZING

AFTE R HARDENING

——

—— CORROSION OF REINFORCEMENT CHEMICAL

ALKALI-AGGREGATE REACTIONS —— CEMENT CARBONATION

—— FREEZE/THAW CYCLES

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THERMAL

EXTERNAL SEASONAL TEMPERATURE VARIATIONS ——

EARLY THERMAL CONTRACTION

TYPES OF CRACKS INTERNAL

TYPES OF _ CRACKS —— ACCIDENTAL OVERLOAD

— STRUCTURAL

TEMPERATURE GRADIENTS

—— CREEP —— DESIGN LOADS

EARLY FROST DAMAGE PLASTIC SHRINKAGE — BEFORE HARDENING

PLASTIC PLASTIC SETTLEMENT

FORMWORK MOVEMENT CONSTRUCTIONAL MOVEMENT

Figure 5.1

5.5.2

SUB-GRADE MOVEMENT

Diagram to illustrate the main types of cracking in concrete (from Concrete Society Technical Report 22, 1992)

Minimising cracking Cracking related to temperature variations, long-term shrinkage or loading, may be minimised and controlled in the design stage and during construction, by appropriate concrete specification and element detailing, including provision and positioning of movement joints and of reinforcement, if present. Further guidance on restraint and avoiding cracking is given in CIRIA Report 91 (1992) and CIRIA Report 135 (1995). The provision of movement or contraction joints is a method by which structures above ground are protected from stress concentrations caused by temperature variations or drying shrinkage; this is generally less of a requirement for foundations. Below ground the temperature variation is generally small and temperature and humidity conditions are relatively constant. In the UK, at approximately 2 m below ground the magnitude of seasonal variation is only a few degrees centigrade. In general, there is no need to accommodate any long-term thermal movement. Movement joints are potentially weak points in the structure and can cause durability problems - they should be avoided if possible in a foundation system. Guidance for the use of movement joints is found in BS 8110: Part 2: Section 8.

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CIRIA C569

5.6

REINFORCEMENT CORROSION

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Steel embedded in concrete is protected against corrosion by the high alkalinity of the concrete and by the formation of a passive oxide layer on the steel. However, chloride ions close to the steel can destroy this layer which, in the presence of moisture and oxygen, can lead to corrosion. Where corrosion is induced by the presence of chlorides, it can be concentrated in very localised areas (pitting corrosion) which may lead to rapid loss in reinforcement cross-section. Although reinforcement corrosion typically induces cracking and spalling in concrete cover with a resultant loss of concrete section, where corrosion is highly localised such effects may not be manifest. The corrosion of reinforcement in foundation concrete is unlikely to be significant below about 1 m depth where reinforced concrete is fully exposed to the ground. Where concrete is completely immersed in water or waterlogged soil, the corrosion rate is generally extremely slow due to the very slow rate of oxygen diffusion through the saturated pores of the cover concrete. However, under these conditions, and in the presence of chlorides and/or acid, anaerobic corrosion is possible which can lead to significant loss of steel section in concrete at depths greater than 1 m. Where foundation concrete is partially saturated, generally in the shallower parts of foundations, oxygen supply is less restricted and the potential for corrosion is greater. In such situations, the physical properties of the cover concrete, in particular its permeability to chloride ions, are more critical. Sulfate attack of reinforced concrete may reduce the protection afforded by concrete to the steel and may indeed provide easier paths for chloride ingress resulting in reinforcement corrosion. This was observed in some of the foundations of motorway bridges which had undergone TSA (TEGR, 1999). See Case Study 6 for further details. As with above-ground concrete the primary defence against reinforcement corrosion rests upon the provision of an adequate thickness of impermeable cover, achieved by specification of a concrete with suitable fresh and hardened properties and a high standard of on-site placement. In many situations, concrete containing ggbs or pfa additions help to satisfy these requirements if carefully designed.

5.7

ALKALI-SILICA REACTIVITY (ASR) Alkali-silica reactivity (ASR) is the most common form of alkali-aggregate reaction. It occurs when active silica constituents of the aggregate react with alkalis originating from the cement or other sources to form an alkali-silicate gel. This gel imbibes water, producing a volume expansion, which can give rise to internal stress within the concrete and may produce the following effects:

CIRIA C569

•

overall expansion of the concrete

•

fine micro-cracking through and around the expansive particles

•

visible map-type macro-cracking arising from differential expansion between small volumes of concrete within a pour

•

differential strain between reinforced and unreinforced areas of concrete which may lead to directional cracking or delamination

•

reduced elastic modulus

•

increased tensile strains in reinforcement

•

increase or decrease in bond stresses between steel and concrete, depending on the steel configuration.

73

Increased awareness of ASR, and the publication of guidance on its avoidance, has reduced the potential risk in new structures to very small proportions. ASR may be triggered by a change in the environment, eg water entry due to a failure of a waterexcluding membrane. It is therefore important to categorise applications on the basis of risk of ASR attack using the techniques given in current recommendations (Concrete Society Report 30, 1999). Practical guidance on avoidance of ASR is included in BRE Digest 330 and in BS 5328. The main features of ASR are summarised below;

• ASR attack will occur only if the following three factors are present: - a high moisture level within the concrete or an external source of water - sufficient reactive silica within the aggregate

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- a high concentration of reactive alkalis in the concrete or from another source

• The occurrence of ASR in the UK is rare and structures where it has been found pre-date the published guidance

• The foundations of low-rise structures built on non-aggressive soil using plain concrete generally have low cement contents and therefore relatively low alkali levels and are, thereby, unlikely to be at risk from ASR.

• The risk of ASR to other types of foundations is generally similar to the risk of ASR damage to concrete above ground. However, in foundation blocks with exposed tops where water can evaporate from the top surface, there is a risk of alkalis migrating through concrete and concentrating in localised areas. Not enough is known about the effects of this alkali migration to enable recommendations to be made against damage from ASR. However, CSTR 30 (1999) suggests several options including protecting concrete with tanking, using low reactivity aggregates, or reducing the alkali content permitted in the concrete or binder.

• The structural consequences of ASR will be greatest for foundations required to resist bending and tensile forces, eg pile-caps, portal frame bases, tall chimney bases and cable stays.

• Increase awareness of ASR and the publication of authoritative guidance on its detection and avoidance have now reduced the potential risk in new construction. These guidance documents should be consulted to establish if current or future buried concrete structures are at risk and what measures need to be undertaken.

5.8

OTHER FORMS OF ATTACK The variety of other forms of attack which foundation concrete may come into contact with are briefly described below. Many of these have arisen through the construction of foundation sites on brownfield and contaminated industrial sites.

5.8.1

Acid attack The action of acids on concrete is to dissolve away the cement and, in the case of aggregates with a high calcium carbonate content, much of the aggregate too. Natural groundwater is frequently acidic due to the presence of humic acid and carbonic acid, which result primarily from the decay of organic matter. These acids are not highly ionised and will not produce a pH below about 3.5. The action of carbonic acid is more complex than for humic acid. “Aggressive” carbon dioxide is only relevant to certain situations where the water is highly mobile and continually flowing over (or seeping through) the concrete.

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ClRlA C569

Diversion pipes or culverts around dams retaining moorland waters can be subject to erosion, as can poorly compacted concrete, or permeable concrete products used in foundations.

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Calcium carbonate is practically insoluble in water but the bicarbonate is soluble. Where carbon dioxide dissolves in water, carbonic acid is formed and this will react with any carbonated cement (or limestone aggregate) to form the bicarbonate which will then dissolve. This is the way swallow holes and caves are formed in limestone. With bicarbonate now in solution, a certain amount of the dissolved carbon dioxide will be needed to stabilise it. so there will be less available to attack further carbonated cement. It is this remaining portion of the dissolved carbon dioxide which is referred to as “aggressive” and a specific test procedure (see BR 279) can be used to measure it. Two levels of aggressiveness, high and low, are used in the classifications in BRE Special Digest 1 based on the method of determination and data derived from Hewlett (1998). Site investigation procedures required to detect the presence of aggressive carbon dioxide in groundwaters are discussed BRE Special Digest 1. Measures to take account of aggressive carbon dioxide for some uses of concrete are also incorporated into BRE Special Digest 1. Mineral acids are highly ionised in solution, but the only mineral acid likely to be found in natural groundwater is sulfuric acid. This acid may result from the oxidation of pyrite where there is insufficient calcium carbonate available in the ground to neutralise it by forming calcium sulfate. Some acidic conditions are known to have developed in fills derived from Carboniferous mudstone, and on one natural site in Oxford Clay (TEGR, 1999). Also there have been cases on recently drained land of pH levels less than 3.5, resulting from pyrite-bearing alluvial soils being exposed to oxygen (BRE Special Digest 1). Residual pockets of sulfuric acid may be found on sites previously used for industrial processes and, in exceptional circumstances, hydrochloric or nitric acid could be found. A detailed site investigation is required to determine the risk of attack from other acids. In situations where acid attack is thought likely to cause problems, there are a number of measures that may be taken to improve durability and reduce the risk of deterioration. The primary defence for concrete in acidic environments is that it should be of a high quality, ie well compacted, dense and impermeable. Additionally, incorporation of limestone aggregate may improve acid resistance due to the buffering capability of calcium carbonate. Where the acids are particularly concentrated, additional protection in the form of a suitable coating or sacrificial layer is required. Detailed guidance is given in BRE Special Digest 1.

5.8.2

Salt crystallisation Concrete saturated with salt solutions - eg chlorides and sulfates - can suffer from crystallisation pressure damage during periods of drying, particularly where salts are replenished by a wicking-action which concentrates salts in an area where evaporation takes place. As water evaporates from the pore solutions they become increasingly concentrated until saturation is reached. Crystals will then begin to grow within the pore space of the material; the resultant internal stresses ultimately disrupt the matrix of the material. Methods to reduce risk of crystallisation include designing concrete to be dense and impermeable to salt ingress and removing the risk of sulfates and chlorides reaching the concrete. The former may be facilitated, among other things, by the use of pfa and ggbs cement additions, while adequate drainage away from the structure will satisfy the latter.

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75

5.8.3

Industrial contaminants

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Building on ground which has been utilised by industry is potentially problematic due to the potential for ground contamination with industrial wastes and by-products, a number of which may affect concrete foundations. In addition to attack by the more common deleterious contaminants such as sulfates and acids, there is also potential for organic contaminants in the ground, such as oils and phenols, to affect the fresh properties of concrete, predominantly causing retardation or even complete inhibition of the set. More detailed information is provided in BRE Report 255 Performance of building materials in contaminated land (1994). Contaminants that are encountered include heavy metals, salts, organics (tars, oils, solvents, PCBs etc.) asbestos, acids, alkalis. sulfur compounds and gases. Combinations of phenols and sulfates can be particularly onerous. Phenol is a weak acid and reacts with the calcium hydroxide in the hardened concrete paste to form calcium phenolate. This crystallises in the pores of the concrete causing deterioration as a result of physical expansion. At phenol concentrations of less than 0.2 per cent by volume there is little or no detrimental effect to the concrete (Hardcastle. 1989). CIRIA Special Publication 78 Building on derelict land (1991), gives detailed guidance on building on derelict land. The information contained within that report will not be repeated here. However. it should be noted that, as with other sites, a well-planned and detailed site investigation is critical to identify the industrial contaminants within the ground. Concrete should be designed to be dense and have the ability to resist ingress of aggressive agents. However, under such severe exposure conditions additional measures such as surface coatings should be considered.

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6

References

AMERICAN SOCIETY FOR TESTING AND MATERIALS (1992) Standard test methods for bleeding of concrete West Conshohocken: ASTM, C232-92

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BAMFORTH, P B (1980) "In-situ measurement of the effect of partial Portland cement replacement using either fly-ash or ground granulated blastfurnace slag on the early age and long-term behaviour of mass concrete" Proceedings of Institution of Civil Engineers, part 2, vol 69, September, pp 777-800

BARTOS, P J M and GRAUERS, M (1999) "Self-compacting concrete" Concrete,April, pp 9- 13

BRE (2001) Concrete in aggressive ground Part 1: Assessing the aggressive chemical environment, 28 pp Part 2: Specifying concrete and additional protective measures, 25 pp Part 3: Design guides for common applications, 28 pp Part 4: Design guides for specific precast products, 26 pp Garston: BRE, Special Digest 1 BRE (1999) Alkali-silica reaction in concrete Part 1: Background to the guidance notes, 8 pp Part 2: Detailed guidance for new construction, 8 pp Part 3: Worked examples, 8 p p Part 4: Simplified guidance for new construction using normal reactivity aggregates, 4 pp Garston: BRE, Digest 330 BRE (1993) Site investigation for low-rise buildings: trial pits Garston: BRE, Digest 381 BRE (1993) Site investigation for low-rise buildings: soil description Garston: B E , Digest 383 BRE (1995) Site investigation for low-rise buildings: direct investigations Garston: BRE, Digest 411 BRE (1998) Collins, R Recycled aggregates Garston: BRE, Digest 433 BRE (1993) Efficient use of aggregates and bulk construction materials. Volume 1: An overview Garston: BRE, Report 243 BRE (1994) Paul, V Performance of building materials on contaminated land Garston: BRE, Report 255

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BRE (1995) Sulphate and acid attack on concrete in the ground: recommended procedure for soil analysis Garston: BRE, Report 279 BRE/DOE (1997) Teychenne, D C, Franklin, R E and Erntroy, H C Design of normal concrete mixes Garston: BRE, Report 33 1 (2nd edn) BRITISH STANDARDS INSTITUTION BS EN 197-1: (in preparation) Part 1 : Composition, specifications and conformity criteria for common cements

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EN 480: (in preparation) Part 4: Determination of bleeding BS EN 206-1: (in preparation) Part 1 : Specification, performance, production and conformity

BS 5800: (in preparation) Complementary British Standard to BS EN 206-1 BS EN 1536: 2000 Execution of special geotechnical work - bored piles

BS EN 1538: 2000 Execution of special geotechnical work - diaphragm walls ENV 1992-1-1: 1992 EC2 Eurocode 2: Design of concrete structures Part 1: General rule and rides for buildings (together with UK National Application Document) DD ENV: 1997 Eurocode 7: Geotechnical design Part 1: General rules (together with United Kingdom national application document) BS 12: 1991 Specification for Portland cements

BS 146: 1996 Portland blastfurnace cements BS 882: 1992 Specifiation for aggregate from natural sources for concrete BS 1881: Part 102: 1983 Method for determination of slump BS 1881: Part 103: 1993 Method for determination of compacting factor BS 1881: Part 105: 1984 Method for determination of flow BS 3892: Part 1: 1997 Specification for pulverised fuel-ash for use with Portland cement

BS 4027: 1996 Sulfate resisting Portland cement

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BS 4246: 1996 High-slag blastfurnace cement BS 5075 Part 1: Accelerating admixtures, retarding admixtures and water-reducing admixtures 1982 Part 3: Specification for superplasticising admixtures 1985 BS 5328 Part 1: Guide to specifying concrete (being updated) 1997 BS 5400 Part 4: Steel concrete and composite bridges - code of practice for design of concrete bridges 1990

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BS 5930: 1981 Code of practice for site-investigation for civil engineering BS 6349: 1984 Part 1: Maritime structures: general criteria BS 6588: 1996 Portland pulverised fuel-ash cements BS 6610: 1996 Pozzolanic pulverised fuel-ash cement

BS 6699: 1992 Specification for ground-granulated blastfurnace slag for use with Portland cements BS 7583: 1986 Portland limestone cement BS 8002: 1994 Code of practice for earth retaining structures BS 8004: 1986 Code of practice for foundations BS 8007: 1987 Design of concrete structures for retaining aqueous liquids BS 8110: Part 1: Structural use of concrete - code of practice for design and construction 1997 Part 2: Section 8: Movement joints 1985

BS 8102: 1990 Code of Practice for protection of structures against water from the ground BS 8103: 1995 Structural design of low-rise buildings BS 8500: 2001 Parts 1 to 4, Concrete - complementary British Standard to BS EN 206:1 CEMENT AND CONCRETE ASSOCIATION (1976) Superplasticing admixtures in concrete Report of a Joint Working Party of the Cement Admixtures Association of the Cement Admixtures and the C&CA

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CEMENT AND CONCRETE ASSOCIATION (1987) Handbook to BS 811 0: 1985 Structural use of concrete. Superplasticing admixtures in concrete Report of a Joint Working Party of the Cement Admixtures Association of the Cement Admixtures and the C&CA CIRIA (1977) Fleming, W K and Sliwinski, Z J The use and influence of bentonite in bored pile construction London: CIRIA, Piling Guide 3,94 pp

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CIRIA (1997) Hammersley, G P, Sparksman, W G, Clarke, J L, Kay E A. Slater, D and Buenfeld, N R On-site curing - influence on the durability of concrete: a review London: CIRIA. Project Report 49, 62 pp CIRIA (1991) Leach, B A and Goodger, H K Building on derelict land London: CIRIA, Special Publication 78, 232 pp CIRIA (1995) Johnson. RA, Leek, D S and Cope, M P 1995 Water-resisting basements - a guide. Safeguarding new and existing basements against water and dampness London: CIRIA, Report 139, 192 pp CIRIA (1992) Harrison T A Early-age thermal crack control in concrete London: CIRIA, Report 91,60 pp CIRIA (1995) Bamforth, P B and Price, W F Concreting deep lifts and large volume pours London: CIRIA, Report 135 (rev edn), 84 pp CIRIA (1997) Masterton, G T and Wilson, R A The planning and design of concrete mixes for transporting, placing and finishing London: CIRIA, Report 165, 84 pp CIRIA (1999) Coventry, S, Woolveridge, C and Hillier, S The reclaimed and recycled construction materials handbook London: CIRIA, C513, 208 pp CLEAR C, KINGSTON, P and DAY, R (1999) “Concreting diaphragm walls” Concrete, February, pp 33-35 CONCRETE SOCIETY (1999) The influence of cement content on the performance ofconcrete Crowthorne: Concrete Society, CS125, 48 pp CONCRETE SOCIETY (1984) Bamforth, P B Mass concrete Crowthome: Concrete Society, TR2 (also CS013), 8 pp CONCRETE SOCIETY (in prepn)

A guide to the selection of admixtures for concrete Crowthorne: Concrete Society, TR18

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CONCRETE SOCIETY (1 992) Non-structural cracks in concrete Crowthorne: Concrete Society, TR22 (3rd edn), 48 pp CONCRETE SOCIETY (1999) Alkali-silica reaction: minimising the risk of damage to concrete Crowthorne: Concrete Society, TR30 (3rd edn) CONCRETE SOCIETY (1993) Microsilica in concrete Crowthorne: Concrete Society, TR41, 56 pp

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CONCRETE SOCIETY (1991) The use of ggbs and pfa in concrete Crowthorne: Concrete Society, TR40, 144 pp CONSTRUCTION TODAY (1991) “Putting down roots” ConstructionToday, June, pp 18-19 CONTRACT JOURNAL (1990) “Piling on the major technical advances” Contract Journal, 25 January, pp 28-31 DEUTSCHES INSTITUT FÜR NORMUNG(1986) DIN 4126: Cast-in-situ diaphragm walls; design and construction Berlin: Deutsches Institut für Normung DHIR, R K, HENDERSON, N A and LIMBACHYIA, M C (eds) (1998) Sustainable construction: use of recycled concrete aggregate London: Thomas Telford, 552 pp EGAN, Sir J/DETR CONSTRUCTION TASK FORCE (1998) Rethinking construction - the report of the Construction Task Force to the Deputy Prime Minister on the scopefor improving the quality and efficiency of UK construction London: Stationery Office, 40pp FEDERATION OF PILING SPECIALISTS (FPS) (1 999) The essential guide to the ICE specification for piling and embedded retaining walls London: Thomas Telford, 104 pp FIDJESTOL, P and FREARSON, J (1994) “High performance concrete using blended and triple-blended binders” In: V M Malhotra (ed), High performance concrete, proceedings ACI International Conference, Singapore Detroit: ACI, SP-149, pp 135-157 FIGG, J W (chairman) (1989) Analysis of hardened concrete - a guide to tests, procedures and interpretation of results. Report of the Joint WorkingParty of the Concrete Society and the Society of Chemical Industry London: Concrete Society, TR32, 118 pp GORING, P G (1999) “Delivering Egan below ground" In: R K Dhir and M R Jones (eds), Innovation in concrete structures: design and construction. Proceedings of the International Conference, Dundee London: Thomas Telford, pp 167-181 HARDCASTLE, J (1989) The effect of phenol on concrete - final report London: DoE/BRE, Note No N7/89

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HARRISON, T A (1999) Practical application ofthe DETR Expert Group Report on thaumasite sulfate attack on buried concrete London: Quarry Products Association HENDERSON, N A (2000) “Self-compacting concrete at Millennium Point” Concrete,April, pp 26-27 HENDERSON, N A and CLEAR, C (2000) “BS 8007 and the use of ggbs concrete” Concrete,February, pp 32-33

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HEWLETT, P C (ed) (1998) Lea's chemistry of cement and concrete London: Edward Arnold (4th edn) HIGHWAYS AGENCY (1997) Design manual for roads and bridges (DMR) London: Stationery Office HIGHWAYS AGENCY (1998) Specification for highway works London: Stationery Office, vol 1, Series 1600, March HIGHWAYS AGENCY (1998) Notes for guidance on the specifcation for highway works London: Stationery Office, Series NG 1600 INSTITUTION OF CIVIL ENGINEERS (ICE) (1996) Specification forpiling and embedded retaining walls London: ICE, 236 pp JONES, D (1999) “Concrete paving: lessons from Japan” Concrete,vol 33, no 8, September, pp 34-35 KOSTUCH, J A, WALTERS, G V and JONES, T R (1993) “High performance concretes containing metakaolin: a review” In: R K Dhir and M R Jones (eds), Concrete 2000: economic and durable construction through excellence Vol 2, pp 1799-1811 LAWRENCE, C D (1990) “Sulfate attack on concrete” Magazine of Concrete Research, vol 42, no 153, pp 249-264 LAWRENCE, C D, DALZIEL, J A and HOBBS. D W (1990) Sulfate attack arising from delayed ettringite formation BCA Interim Technical Note 12 MAAGE, M (1984) Effect of microsilica on the durability of concrete structures Trondheim: FCB/SINTEF, Report STF 65 A84019 MALHOTRA, V M (ed) (1994) Proceedings of the ACI International Conference on High Performance Concrete, Singapore Detroit: American Concrete Institute, SP149, 884 pp MEYER, A (1979) “Experiences in the use of superplasticisers in Germany” In: Superplasticisers in Concrete Detroit: American Concrete Institute, SP62

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MOSSAZA, F and TESTOLIN, M (1980) “Latest developments in the use of admixtures for cement and concrete” I1 Cemento, vol 77, no 2 MURDOCK, L J, BROOK, K M and DEWAR, J D (1991) Concrete materials and practice London: Edward Arnold (6th edn) MURPHY, F G (1977) A survey of literature of the effect of contamination, other than rust, on the bondstrength of reinforcement London: CIRIA, Technical Note 86

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OWENS, P L (1999) “Pulverised-fue1 ash. Part 1 : origin and properties” Concrete, April, pp 27-28 SAAD, M N A, de ANDRADE, W P and PULON, V A (1982) “Properties of mass concrete containing an active pozzolan made from clay” Concrete International, July, pp 59-65

THAUMASITE EXPERT GROUP REPORT (TEGR) (1999) The thaumasite form of sulfate attack: risks, diagnosis, remedial works and guidance on new construction Report of the Thaumasite Expert Group, DETR TOMLINSON, M J (1995) Foundation design and construction Prentice Hall (6th edn) TYSON, J P (1995) Design of reinforcement in piles Crowthorne: Transport Research Laboratory, Report 144 WATER AUTHORITIES ASSOCIATION (1989) Civil engineering specification for the water industry Water Authorities Association (3rd edn) XANTHAKOS, P P (1979) Slurry walls as structural systems New York: McGraw-Hill (2nd edn) pp 855

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Core Programme members Alderburgh Ltd

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Alfred McAlpine Construction Ltd AMECPIC BAA plc Bachy Soletanche Limited Balfour Beatty Major Projects, Stent and Balvac BGP Reid Crowther Binnie Black & Veatch British Nuclear Fuels Ltd Buro Happold Engineers Limited Carillionplc Casella London Ltd Cementitious Slag Makers Association Charles Haswell and Partners Ltd Curtins Consulting Engineers plc Davis Langdon & Everest Department of Trade and Industry Dudley Engineering Consultancy Edmund Nuttall Limited EntecUKLimited Environment Agency EnvirosAspinwall Galliford Try Group plc Gardiner & Theobald Golder Associates (UK) Ltd HalcrowGroup Limited Health & Safety Executive High-Point Rendel Highways Agency HR Wallingford Ltd

Jackson Civil Engineering Jacobs GlBB Ltd KellerGround Engineering Maunsell Ltd Middlesex University M J Gleeson Group plc Morgan Sindall Group Mott MacDonald Group Ltd MWH Northumbrian Water Limited North West Water Ltd Office of Government Commerce Ove Arup Partnership Owen Williams Group Parsons Brinckerhoff Environmental Posford Haskoning Ltd Scottish and Southern Energy plc Scott Wilson SheffieldHallam University Shepherd Construction Limited SirRobert McAlpine Ltd Skanska Construction Group Ltd South Bank University Southern Water Services Ltd Taylor Woodrow Construction Thames Water Utilities Ltd University of Bradford

IMC Consulting Engineers Ltd

University of Salford URS Thorburn Colquhoun Wardell Armstrong W S Atkins Consultants Limited

lnfraco Sub-surface Ltd

Yorkshire Water Services Limited

lnnogyPIC

January 2002

THE CONCRETE SOCIETY Founded in 1966, The Concrete Society brings together all those with an interest in concrete to promote excellence in its design, construction and appearance, to encourage new ideas and innovations and to exchange knowledge and experience across all disciplines.

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TECHNICAL DEVELOPMENT CENTRE The Concrete Society is a centre of excellence for technical development of concrete, producing stateof-the-art reports, recommendations and practical guides, and initiating and undertaking R&D where appropriate. The Technical Executive Committee and its supporting specialist groups and working parties carry out the work of the Centre, with the help of the Technical Manager and his staff The Centre collaborates with overseas Concrete Societies in mutually beneficial programmes of technical development.

CONFERENCES AND EXHIBITIONS The Society organises national and international conferences and exhibitions including DTI-supported joint-venture exhibitions at major international events worldwide. The Society’s annual “Concrete Day” is a major national event in the calendar of the construction industry. CONCRETE All members receive each issue of CONCRETE, the Society’s monthly journal, with up-to-date information on all aspects of concrete including design, materials, construction techniques, quality control, equipment, maintenance and repair. MEMBERSHIP Group Membership is for firms, partnerships, government departments, local authorities, educational establishments etc. Personal Membership gives Concrete Society benefits to individuals.

REGIONS AND CLUBS The Society is organised into 22 regions and clubs, which arrange programmes of technical and social events in their areas. As well as all the technical benefits, the Society provides an ideal social forum for members to make valuable business and personal contacts. CONCRETE ADVISORY SERVICE The Concrete Advisory Service provides prompt impartial techical advice on concrete and related matters to subscribing members. The regionally based advisory staff are all Chartered Engineers and have wide experience in all aspects of the use of concrete and of providing appropriate advice. Staff of subscribing group members can tap into this vast reservoir of information on materials, technology and practice. As well as telephone advice, visits to members’ offices or sites to discuss technical matters or solve problems can also be arranged. Reports can be provided where applicable.

AWARDS Awards to publicly acknowledge excellence in concrete are made annually for completed buildings, civil engineering structures and mature structures. ENTITLEMENT TO USE DESIGNATORY LETTERS Members who have appropriate knowledge and experience may apply for the qualifying grades “Member” (MCS) and “Fellow” (FCS).

REGIONAL ADVISORY ENGINEERS NORTH OF ENGLAND AND SCOTLAND

Deryk Simpson Bolton

Tel: (01942)815546 Fax: (01942) 842533

John Plimmer

Tel: (01937) 834827

York SOUTH OF ENGLAND, WALESAND NORTHERN IRELAND

Maldwyn Enoch Tel: (01443) 237210 Cardiff Fax: (01443) 23727 1 George Barnbrook Tel: (01794) 324455 Romsey Fax: (01794) 324325 Dick Roberts Tel: (01428) 725269 Guildford Fax: (01428) 727894

The Concrete Society, Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS, UK Tel: 01344 466007, Fax: 01344 466008, Email: [email protected], Web: www.concrete.org.uk

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The Construction Directorate of the DTI supports the programme of innovation and research to improve the construction industry's performance and to promote more sustainable construction. Its main aims are to improve quality and value for money from construction, for both commercial and domestic customers, and to improve construction methods and procedures. Concrete technology for cast in-situ foundations focuses on the concrete technology issues relating to common buried foundation types including piles, diaphragm walls and pad, strip and raft foundations. This report provides guidance on the analysis and construction parameters to be considered in the selection of in-situ concrete for foundation applications and identifies best practice on the design, specification and use of in-situ concrete for a range of site conditions. The report also discusses environmental issues and sustainable construction practices. This comprehensive report provides the designer, the contractor, and the concrete producer with current guidance on the application of in-situ concrete technology for foundation applications. The report is based on a detailed review of published literature, consultation with experts and practitioners within the field, and case studies demonstrating good practice and highlighting critical issues.

ISBN 0 86017 569 3