Concrete in Aggressive Ground:3rd edition

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Special Digest 1:2005 Third edition

Concrete in aggressive ground

BRE Construction Division

BRE is committed to providing impartial and authoritative information on all aspects of the built environment for clients, designers, contractors, engineers, manufacturers and owners. We make every effort to ensure the accuracy and quality of information and guidance when it is published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. BRE is the UK’s leading centre of expertise on the built environment, construction, sustainability, energy, fire and many associated issues. Contact BRE for information about its services, or for technical advice: BRE, Garston, Watford WD25 9XX Tel: 01923 664000 [email protected] www.bre.co.uk BRE publications are available from www.brebookshop.com or IHS Rapidoc (BRE Bookshop) Willoughby Road Bracknell RG12 8DW Tel: 01344 404407 Fax: 01344 714440 [email protected] Requests to copy any part of this publication should be made to the publisher: BRE Bookshop Garston, Watford WD25 9XX Tel: 01923 664761 [email protected] Cover photo by Graham Gaunt, courtesy of Arup © SD1 © BRE 2005 First published 2001 Second edition 2003 Third edition 2005 ISBN 1 86081 754 8

Acknowledgements The principal funding for the preparation of this Special Digest was provided by The Concrete Centre. A list of sponsors and members of the steering group who advised on its preparation is shown on page vi. BRE Contact details For technical enquiries or comment on the use of this Special Digest please email: [email protected]

iii

Contents

Sponsors and members of steering group

vi

Part A: Introduction A1 A2 A2.1 A2.2 A2.3 A2.4 A3 A4 A5 Appendix A1

Problem of chemical attack Scope and structure of the guidance Types of site and chemical agents covered Readership Structure of the guidance Diagrammatic overview of ground assessment and concrete specification Background to guidance on sulfate attack Key changes since SD1:2003 Relationship between SD1:2005 and British and European Standards for concrete Glossary of terms References: Part A

1 1 1 2 2 2 4 5 5 6 8

Part B: Chemical attack on concrete B1 B2 B2.1 B2.2 B3 B3.1 B3.2 B3.3 B3.4 B4 B5 B6 B7

General Principal types of chemical attack on concrete Sulfate attack Acid attack Other types of chemical attack on concrete Magnesium ions Ammonium ions Chloride ions Organic compounds Attack from aggressive carbon dioxide Attack from pure water Damage to concrete from crystallisation of salts Microbial contribution to chemical attack on concrete References: Part B

9 9 9 11 12 12 12 13 13 14 14 14 15 15

iv

Contents

Part C: Assessing the aggressive chemical environment C1 C2 C2.1 C2.2 C2.3 C2.4 C2.5 C3 C3.1 C3.2 C3.3 C4 C4.1 C4.2 C4.3 C4.4 C4.5 C4.6 C5 C5.1 C5.2 Appendix C1 Appendix C2

General Principal constituents of aggressive ground and groundwater Sulfates and sulfides Acids Magnesium, calcium, sodium and potassium ions Ammonium ions Chloride ions Presence and mobility of groundwater Static groundwater Mobile groundwater Flowing groundwater Site investigation for aggressive ground conditions Introduction Desk study Site inspection (walk-over survey) Visual description of the ground Sampling and testing soils Sampling and testing groundwater Classification of site locations for chemicals aggressive to concrete Groundwater and soil analyses Aggressive Chemical Environment for Concrete (ACEC) classification Recommended test procedures for ground aggressive to concrete Guidance on comprehensive site investigation of sulfate ground References: Part C

16 17 17 20 20 20 20 21 21 22 22 23 23 23 24 24 25 25 29 29 34 36 36 37

Part D: Specifying concrete for general cast-in-situ use D1 D2 D3 D4 D4.1 D4.2 D5 D5.1 D5.2 D5.3 D5.4 D6 D6.1 D6.2 D6.3 D6.4 D6.5 D6.6 D7 D8

Introduction Changes since SD1: 2003 Design process Selection of the DC Class and APMs Background Key factors Composition of concrete to resist chemical attack Background Using Table D2 Cement and combination types Aggregate type Additional protective measures (APMs) General Enhance concrete quality (APM1) Use controlled permeability formwork (APM2) Provide surface protection (APM 3) Provide a sacrificial layer (APM4) Address drainage of site (APM5) Intended working life Contract documentation References: Part D

38 38 39 40 40 40 41 41 42 42 44 44 44 45 45 45 46 46 47 48 48

Contents

v

Part E: Specifying surface-carbonated precast concrete for general use in the ground E1 E2 E3 E3.1 E3.2 E3.3

Introduction Changes since SD1:2003 Design process Selection of the DC Class and APMs Specifying composition of concrete Additional protective measures References: Part E

49 50 50 50 50 50 51

Part F: Design guides for specific precast concrete products F1 F2 F3 F3.1 F3.2 F3.3 F4 F4.1 F4.2 F4.3 F5

Introduction Procedure for using design guides Design guides for precast concrete pipeline systems General considerations Using Design Guide F1a for specifying concrete for pipes and associated units Using Design Guide F1b for specifying internal linings to pipes and associated units Precast box culverts and precast segmental linings for tunnels and shafts General considerations Using Design Guide F2a for specifying concrete for precast box culverts and segmental linings Using Design Guide F2b for specifying internal linings to precast box culverts and segmental linings Design guides for precast concrete masonry units References: Part F

52 53 55 55 56 57 58 58 59 60 61 62

vi

Sponsors and members of steering group

Sponsors The Concrete Centre Quarry Products Association (QPA) Cementitious Slag Makers Association (CSMA) UK Quality Ash Association (UKQAA)

Members of Steering Group Professor L A Clark Dr C A Clear Dr N J Crammond Mr I Haining Professor T Harrison Mr J C Haynes Dr D D Higgins Mr I Holton Mr P Livesey Mr T I Longworth Mr N Loudon Dr B K Marsh Dr J D Matthews, Mr A Morton Dr P J Nixon Ms L Parker Ms A Scothern Dr L K A Sear Dr J F Troy

Chairman, Thaumasite Expert Group Department of Civil Engineering, University of Birmingham British Cement Association (BCA) Centre for Concrete Construction, BRE Costain Quarry Products Association (QPA) and BSI Committee B/517/1 National House-Building Council (NHBC) Cementitious Slag Makers Association (CSMA) British Precast Concrete Federation / Loughborough University Castle Cement Associate, BRE The Highways Agency Arup Associate, BRE Hepworth Concrete / Concrete Pipeline Systems Association (CPSA) Associate, BRE Tarmac Ltd The Concrete Centre UK Quality Ash Association (UKQAA) Tarmac Ltd

Principal Consultees Mr D Appleton Dr J C Cripps Mr A J Elliott Dr T Grounds Dr A Haimoni Mr R M Raymond Mr P Rhodes Mr S Wade

Hanson Building Products Department of Civil and Structural Engineering, University of Sheffield Milton Precast / Box Culvert Association Tarmac Topblock Keller Ltd / Federation of Piling Specialists (FPS) Hughes Concrete Ltd / Concrete Pipeline Systems Association (CPSA) RMC Stent / Federation of Piling Specialists (FPS)

1

Part A

Introduction

A1 Problem of chemical attack

A2 Scope and structure of the guidance

Chemical agents that are destructive to concrete may be found in the ground. In the UK, sulfates and acids, naturally occurring in soil and groundwater, are the agents most likely to attack concrete. The effects can be serious (Figure A1) resulting in expansion and softening of the concrete to a mush. A substantial number of other substances are known to be aggressive, most resulting from human activity, but collectively these are a lesser problem as they are encountered only rarely by concrete in the ground.

A2.1 Types of site and chemical agents covered

It has been standard practice in the UK for at least six decades to design concrete for installation in the ground to be resistant to attack from commonly found chemicals, including sulfates and acids. BRE has underpinned this approach by issuing a series of guidance notes and Digests, dating back to 1939, on the causes of chemical attack and how to specify chemically resistant concrete.

SD1 provides guidance on the specification of concrete for installation in natural ground and in brownfield locations. The definition of a brownfield location adopted here is one that has been subject to industrial development, storage of chemicals, or deposition of waste, and which may contain aggressive chemicals in residual surface materials or in ground penetrated by leachates. The procedures given for ground assessment and concrete specification cover the fairly common occurrence of sulfates, sulfides and acids. They also cover the more rarely occurring aggressive carbon dioxide found in some ground and surface waters.

Consequently, most concrete installed in the ground has performed entirely satisfactorily and is expected to do so for its required working life. Occasionally, however, cases of chemical attack have come to light and have been subject to research by BRE and others. Some of these cases have been attributed to rarely occurring chemicals not specifically covered by BRE Digests: some to natural ground conditions for which there was insufficient guidance, such as occurrence of pyrite; and some to the emergence of previously unrecognised attack mechanisms, such as the thaumasite form of sulfate attack (TSA) which has been extensively reported in the last decade[1]. Guidance in BRE Digests has necessarily evolved to cater for successive adverse field findings; to take advantage of the emergence of new concrete constituents and construction methods; and to maintain harmony with newly published standards, latterly European ones. In order to be both comprehensive and flexible, Digests have tended to become longer and more complex. One objective of this third edition of Special Digest 1 (SD1) is to simplify the guidance. Other aims and changes are discussed later. Figure A1 Extreme example of sulfate attack in a 30-year-old highway bridge sub-structure exposed to wet, pyritic clay fill

2

Part A

While SD1 discusses several aggressive agents (eg ammonium salts and phenols) occasionally found in heavily contaminated ground, no specific procedures are included for dealing with these. Specialist advice should be sought if they are encountered. A2.2 Readership

SD1 provides practical guidance to ground specialists on the assessment of ground in respect of aggressiveness to concrete, and to concrete designers, contractors, specifiers and producers on the specification of concrete to resist chemical attack. A2.3 Structure of the guidance

Guidance is given in Parts B to F as follows. Part B describes modes of chemical attack and discusses the mechanisms of the principal types, including sulfate and acid attack, and the action of aggressive carbon dioxide. Part C deals with assessment of the chemical aggressiveness of the ground. It gives procedures for the determination of Design Sulfate Class (DS Class) from soluble sulfate and magnesium, and from the potential sulfate (eg from oxidation of pyrite). It shows how the DS Class together with pH and mobility of groundwater may be collectively taken into account for natural ground and brownfield sites to classify a location in terms of Aggressive Chemical Environment for Concrete Class (ACEC Class). Part D gives recommendations for the specification of concrete for general cast-in-situ use in the ground. It explains how to derive an appropriate quality of concrete, termed the Design Chemical Class (DC Class), from a consideration of the ACEC Class together with the hydraulic gradient due to groundwater, the type and thickness of the concrete element, and its intended working life. In some cases, where conditions are highly aggressive, additional protective measures (APMs) are recommended. Part D follows this with guidance on the constituents of concrete required to achieve the identified DC Class. Specification is shown as maximum free-water/cement ratio, minimum cement content and type of cement.

Part E gives recommendations for specifying surfacecarbonated precast concrete for general use in the ground. An essential requirement for compliance with this part is that surface carbonation is assured by exposure of the precast concrete to air for a minimum of 10 days after curing. Since such carbonation provides a degree of resistance to sulfate attack, the recommendations for the derivation of DC Class in respect of sulfates is relaxed by one level. Other than this, the recommendations of Part D are followed for concrete specification. Part F includes design guides for specification of specific precast concrete products, including pipeline systems, box culverts, and segmental linings for tunnels and shafts. These products are manufactured under rigorous quality control to ensure appropriate mix composition and achieve relatively low concrete permeability. Together these provide an inherently high quality in respect of chemical resistance. Consequently, a further relaxation (beyond that allowed for surface carbonation) is permitted in respect of specification of DC Class for aggressive sulfate conditions. In practice this relaxation is used to offset the general-use recommendation that a higher DC Class should be specified where concrete is of thin cross-section, or will encounter a relatively high hydraulic gradient. Part F also covers specification of precast concrete masonry units (concrete blocks) for aggressive ground conditions. The guidance is based on Design Sulfate Class rather than ACEC Class as there is currently no correlation of block performance with the latter, though work on this is ongoing. A glossary of terms is included as Appendix A1 on page 6. A2.4 Diagrammatic overview of ground assessment and concrete specification

An overview of the various procedures for ground assessment and specification of concrete is given in Figure A2. This is arranged in four stages according to the construction sector that has key responsibility. Within each of these stages, the principal tasks are shown in boxes with references to the relevant sections of SD1. While most steps are equally applicable to all uses of concrete, there is a differentiation in Stage 3 for the determination of DC Class and APM between the three categories of concrete element dealt with in Parts D, E and F.

Introduction

3

Stage 1 Designer of building or structure

Consider design options for building or structure and prepare specification for site investigation. Inform geotechnical specialist of design concept and site investigation requirements

Stage 2 Geotechnical specialist

Carry out site investigation to determine chemical conditions for concrete, including water mobility. See Part C

Part C

Determine DS Class and ACEC Class for site locations using Tables C1 and C2. See Section C5

Determine the intended working life of proposed building or structure, and the form and use of specific concrete elements. See Section D7

Stage 3 Designer of building or structure

General use of cast-in-situ concrete Find specification of concrete and APM using procedure in Part D: ● determine the DC Class and any APM from Table D1 ● adjust DC Class / APM for section thickness and hydraulic gradient ● determine options for APM from Table D4

General use of surfacecarbonated precast concrete Find specification of concrete and APM using procedure in Part E: ● determine the DC Class and any APM from Table E1 ● adjust DC Class / APM for section thickness and hydraulic gradient ● determine options for APM from Table D4

Specific precast concrete products ● Use Part F ● Determine the DC Class and APM for the concrete using Design Guides F1a, F1b, F2a, F2b, F3a, F3b

State in contract documents the DS Class and ACEC Class of the ground and the method of deriving the concrete specification (eg use of Tables C1, D1 and D2, or Table C2 and Design Guide F1a). State requirements and options for concrete specification, including: ● specified DC Class of concrete after any enhancement ● specified number and type of APM and compressive strength class of concrete ● any other requirements

Stage 4 Contractor for building or structure in liaison with any third party concrete producer

Obtain from contract documents the specified DC Class, number and type of APM, and any other design requirements for each concrete element

Formulate concrete mix design and consistence for structural element taking into account specified DC Class, strength class, availability, and cost of materials and contract requirements

Where concrete is being supplied ready-mixed, check the proposed mix for conformity to the DC Class specification

Are all requirements of design guides and contract documents met? Yes

Accept concrete mix design for specific use. Implement any APM specified for DC Class or in contract documents

Figure A2 Procedure for design of buried concrete for use in an aggressive chemical environment

No

Parts D, E and F

4 A3 Background to guidance on sulfate attack One of the key drivers for revision of BRE Digests dealing with concrete in aggressive ground since the 1990s has been a growing recognition of the occurrence of the thaumasite form of sulfate attack (TSA) in UK buildings and structures. It has long been known in the UK that concretes made with Portland cements are vulnerable to attack by sulfates in the ground. For many years it was considered that the affected components of the concrete matrix were the calcium aluminate phases and calcium hydroxide, and that the minerals formed by this attack were ettringite and gypsum. Sulfate resisting cements with low contents of calcium aluminates were made available in the UK by the 1950s to meet this ‘conventional’ form of sulfate attack. Later the benefits of using fly ash or pulverized fuel ash (pfa) and blastfurnace slag-based cements were appreciated. Guidance on designing concretes to resist conventional sulfate attack was developed in a series of BRE Digests, the most recent of which was Digest 363, Sulfate and acid resistance of concrete in the ground, first published in 1991. Since the late 1980s, however, deterioration of concrete as a result of the formation of thaumasite has become recognised as a separate form of sulfate attack. The distinguishing features of TSA are that it: ● occurs preferentially at low temperatures (below 15 °C, such as are typically found in the ground) ● requires availability of carbonate ions, in addition to sulfate ions, from sources including limestone aggregate, limestone filler in cement, and bicarbonate in groundwater ● targets the calcium silicate phases within hardened cement paste, potentially reducing concrete to a mush. A growing number of cases of TSA have been identified worldwide, although the majority have been found in the UK. In the 1990s BRE investigated three cases of TSA in the concrete foundations to domestic properties in the Cotswolds area of England[2,3]. In all three cases, the TSAaffected concrete contained carbonate-bearing (limestone) aggregates and was exposed to moderately aggressive (Class 3) sulfate conditions in a seasonally cold, wet environment. The concrete encountered in each case satisfied the recommendations of the then-current version of Digest 363. It therefore became apparent that the Digest needed to be revised to take account of the risk of TSA occurrence.

Part A Accordingly a new version of Digest 363 was issued in January 1996 which drew attention to the risk of TSA in concretes containing internal calcium carbonate and promised further guidance based on ongoing research. Subsequently, in 1998, several cases of TSA were identified in the foundations to motorway bridges in Gloucestershire. As in the previous cases, the concrete contained carbonatebearing aggregates[1,4–6]. The most severe occurrence had resulted in severe concrete deterioration to a depth of up to 50 mm, exposing steel reinforcement to corrosion[7]. The high profile of these cases ensured a co-ordinated national review, culminating in 1999 with a report from a Thaumasite Expert Group[1] set up by Government. This report gave interim guidance on specifications to minimise the risk of TSA in new construction and on the management of existing structures affected by TSA. It also gave recommendations for further research on the occurrence of TSA and mitigating measures. Following publication of the Thaumasite Expert Group report, BRE guidance was revised to incorporate the interim recommendations. This was published in 2001 as Special Digest 1, Concrete in aggressive ground. (Hereafter Special Digest 1, 2001 and other editions, will be shown as SD1:2001 etc.) There were minor revisions to the guidance in a new edition published in 2003. These were principally to bring nomenclature used for cements and combinations into line with newly published European Standards. Most of the subsequent research recommended by the Thaumasite Expert Group has been completed. Key outcomes in respect of the mechanism of TSA and concrete specification have been: ● confirmation that the carbonate required for TSA may come from a source other than aggregates or fillers in the concrete. In particular, it can come from bicarbonate dissolved in groundwater[4,8] ● clarification of the performance of various compositions for concrete recommended in SD1:2001 for aggressive sulfate conditions. Together with other findings, such as deficiencies in guidance for ground assessment, the new knowledge has prompted this major revision of SD1.

Introduction A4 Key changes since SD1:2003 Two key changes have been made to the procedure for ground assessment from SD1:2003. ● The limits of the DS Classes based on 2:1 water/soil extract tests on soil have been reduced, making this classification route more conservative. The change stems from findings of several research investigations on ground carried out by BRE and others[9–11]. Sulfate class limits based on 2:1 water/soil extract tests on soil have been found to be substantially lower than sulfate class based on sulfate in groundwater. The new limits bring sulfate classification based on 2:1 water/soil extract tests into parity with sulfate classification based on groundwater. ● High magnesium levels are not taken into account when determining the ACEC Class of natural ground in the UK. The following key changes have been made to the procedure for concrete specification. ● The recommended maximum water/cement ratio and minimum cement content have been revised, and a new classification for cements and combinations has been introduced. ● The recommended concrete quality now caters for the ever-present possibility of exposure to an external source of the carbonate required for TSA (principally bicarbonate in groundwater). The concept of ‘aggregate carbonate range’ is therefore no longer included since the revised concrete specification simultaneously caters for an internal source from carbonate in aggregates. A further consequence is that starred and double-starred concrete qualities that related to restricted aggregate carbonate content are no longer included. ● The number of APMs to be applied at higher sulfate levels has been reduced, in general by two. This follows from a higher level of confidence in the provisions for the concrete. ● The concept of ‘intended working life’ replaces that of ‘structural performance level’. This is for harmony with European standards such as BS EN 206-1. Further detail in respect of these changes is included in Parts C and D.

5 A5 Relationship between SD1:2005 and British and European Standards for concrete For several decades there has been liaison between groups responsible for guidance in BRE Digests on concrete in aggressive ground and British Standards dealing with the specification of concrete, the latest of which is BS 8500. Consequently there has been a basic harmony between these documents in respect of concrete specification for general use in the ground. In other respects the BRE Digests and British Standards have been complementary. BRE guidance has presented more background information on chemical attack, given detailed guidance on ground assessment, and included dedicated guidance for the specification of concrete in certain precast concrete products such as pipeline systems and masonry blocks. In contrast, BS guidance for concrete has integrated the provisions for resistance to chemical attack into the numerous other requirements for practical concrete specification; for example, strength class and consistence, resistance to alkali–silica reaction and chloride content in respect of corrosion of reinforcing steel. At the time of preparation of SD1:2005 the current edition of the British Standard for concrete is BS 8500: 2002. However, a revision of this is underway that will include bringing it into line with SD1:2005 in respect of resistance to aggressive ground. It is expected that an amended document will be issued for public comment in the first half of 2005, followed by publication some months later.

6 Appendix A1 Glossary of terms Additional protective measure (APM)

Additional protective measures (APMs) were first defined in SD1:2001. They comprise five options for extra measures that can be taken to protect concrete where it is considered that the basic provisions of the concrete specification might not provide adequate resistance to chemical attack for some uses of concrete (Section D6, and Table D4 on page 44). Aggregate carbonate range

A term formerly used in previous editions of SD1. It is not used in SD1:2005. Aggressive carbon dioxide

Carbon dioxide (CO2) dissolved in water essentially comprises three fractions: ● a part combined with calcium ions to form highly soluble calcium bicarbonate (Ca(HCO3)2) ● a part remaining as dissolved carbon dioxide that is needed to stabilise the calcium bicarbonate ● the remainder, forming carbonic acid (H2CO3) which can potentially attack concrete. This portion of the dissolved carbon dioxide is termed aggressive carbon dioxide. Aggressive carbon dioxide is usually only present to an appreciable extent in rather pure natural waters since in most cases, where the water contains dissolved salts, sufficient calcium carbonate is available to combine with the carbon dioxide as calcium bicarbonate. Aggressive Chemical Environment for Concrete Class (ACEC Class)

This system for the classification of aggressive ground conditions for concrete was introduced in SD1:2001. ACEC Class is derived from Design Sulfate Class, taking additional account of the type of site (natural or brownfield), and the mobility and pH of groundwater (Section C5.2, and Tables C1 and C2 on pages 31 and 32). Brownfield sites

A brownfield location is defined as a site or part of a site that has been subject to industrial development, storage of chemicals (including for agricultural use) or deposition of waste, and which may contain aggressive chemicals in residual surface materials or in ground penetrated by leachates (Section C5.1.3). Cements and combinations

Cements are pre-blended from appropriate cementitious materials and are supplied by cement manufacturers. Combinations comprise similar cementitious materials that are combined in the concrete mixer. Cements and combinations prepared from the same ingredients, taken in the same proportion, are equivalent for resistance to sulfate attack. Consequently, this Special Digest sometimes loosely uses the term ‘cement’ to cover both cements and combinations in generality or of a particular type.

Part A Conventional form of sulfate attack

This is a form of sulfate attack in which sulfate ions that have penetrated concrete react with calcium aluminate hydrate to form calcium sulfo-aluminate hydrate (ettringite), or with calcium hydroxide to form gypsum. Initially these reactions may result in non-destructive void filling. Attack is distinguished by onset of expansion and related cracking of the concrete (Section B2.1.1). Design Chemical Class (DC Class)

DC Class was introduced in SD1:2001 to define qualities of concrete that are required to resist chemical attack. It is derived from the ACEC Class but takes into account a number of other factors, including the type of concrete element and its intended working life, and any exposure to hydraulic gradient due to groundwater (Section D4 and Table D1 on page 41). Design Sulfate Class (DS Class)

This is a five-level classification for sites based principally on the sulfate content, including total potential sulfate, of ground or groundwater, or both. It is dependent on the presence or absence of substances including magnesium ions, pyrite, and, for pH less than 5.5, chloride and nitrate ions (Section C5, and Tables C1 and C2). Disturbed ground

This is, initially, natural ground that is substantially disturbed; for example, by cutting and filling to terrace a site, or by excavation and backfilling, so that air can enter and oxidise any pyrite contained therein. Simply cutting through ground without opening up the ground beyond the cut face (eg piling operations or excavation without backfill) does not generally result in disturbed ground. Enhanced concrete quality

This is an increase in concrete quality used as an APM. The necessary enhancement may be determined from Table D2 on page 42. In this table, bold horizontal lines separate the various concrete qualities in respect of aggressive ground. Using the option of enhancing concrete quality as an APM is satisfied by adopting the recommendations of the next higher DC Class (Section D6.1). Flowing groundwater

Flowing groundwater is defined in this Special Digest to cover the following two conditions (Section C3.3): ● water that percolates through the ground under a permanent head in substantial quantity and at a relatively rapid rate; say, at a velocity greater than 10 m/day where velocity equals the permeability of the ground multiplied by the hydraulic gradient ● water that is flowing in surface conduits or streams.

Introduction

7

Hydraulic gradient

Sacrificial cover layer

The hydraulic gradient across a concrete element is the difference in hydrostatic head on the two sides of the concrete, in metres, divided by section thickness, in metres. For example a 3 m head of water external to the foot of a 0.3 m thick wall of a basement results in a hydraulic gradient of 10. This is greater than the hydraulic gradient of 5 that generally calls for increased provision against chemical attack on concrete.

This is an APM for concrete that adds a further layer to a construction element to absorb aggressive chemicals. This measure will not be appropriate in circumstances where the surface of the concrete must remain sound to prevent loss of frictional resistance or settlement (eg for skin friction piles) (Section D6.5). It is additional to the nominal cover, including any allowances for casting against uneven ground. Static groundwater

Hydrostatic head

The hydrostatic head of water at a point in the ground is the height to which the water would rise in an open standpipe above that point. Intended working life

This is the period of time during which the performance of the concrete in the structure will be kept at a level compatible with the fulfilment of the performance requirements of the structure, provided it is properly maintained (BS EN 206-1). This definition has been adapted here to take some account of structural performance factors such as the consequence of serious concrete degradation and ease of repair. Two categories are defined: ‘at least 50 years’ and ‘at least 100 years’ (Section D7 and Table D1). Mobile groundwater

The term ‘mobile groundwater’ covers the following range of conditions (Section C3.2): ● water held in pores and structural discontinuities in the soil which will flow into an excavation to give a standing water level ● water which is percolating slowly through the ground; say at a velocity of less than 10 m per day ● still water in ponds, sumps, or similar accumulations. Oxidisable sulfides (OS)

This is the amount of sulfate that may result from the oxidation of pyrite or similar sulfides in the ground – most likely due to ground disturbance. OS (expressed as % SO4) can be calculated from: OS = TPS – AS where: TPS = total potential sulfate content as % SO4 AS = acid-soluble sulfate content as % SO4 See Section C5.1.2. Pyritic ground

This is ground that contains the natural sulfide, pyrite (FeS2). It is essential to take account of the additional sulfate content that might result from the oxidation of pyrite following ground disturbance (Section C5.1.2).

Static groundwater will be confined to sites where the ground is either permanently dry or contains water but is relatively impermeable. (Virtually no water movement is possible.) The mass permeability in the latter case will generally be less than 10–7 m/s. A typical example would be clayey soils with tight fissures and no included sand or silt horizons (Section C3.1). Structural performance level

A term formerly used in previous editions of SD1. It is not used in SD1:2005. Sulfate Class

This is a five-level classification of sulfate concentration that is applied to individual series of tests on soil or groundwater. Separate Sulfate Classes may be derived from the characteristic values of sulfate determined from both water-extract sulfate tests and total potential sulfate tests on soil, and from sulfate tests on groundwater (Section C5.1.1, Steps 1 and 2, and Tables C1 and C2). In general the highest of the derived Sulfate Classes is taken as the Design Sulfate Class for a location, but there may be a relaxation of this rule where the highest Sulfate Class is from total potential sulfate tests (C5.1.2, Step 8). Thaumasite form of sulfate attack (TSA)

This is a type of sulfate attack that consumes the binding calcium silicate hydrates in Portland cement, weakening the concrete and causing some expansion. To occur it requires sulfates, calcium silicate, carbonate and water. Attack is most vigorous at temperatures below 15 °C (Section B2.1.2). TPS % SO4 = 3.0 x TS % S where: TS = total sulfur determined by an appropriate laboratory procedure (Box C10 on page 35). Total potential sulfate (TPS)

TPS is an upper limit value for sulfates in the ground. It is calculated as the sum of sulfates already present in the ground, plus those that may result from oxidation of pyrite or similar minerals (Section C5.1.2). Water/cement ratio (w/c ratio)

This is the ratio of the mass of free-water in fresh concrete to the mass of the cement or combination. Free-water content is the water available for hydration of the cementitious material, this being less than the ‘total’ water content which includes water that is held within aggregates.

8 References: Part A [1] Department of Environment, Transport and the Regions. The thaumasite form of sulfate attack. Risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group. London, DETR, 1999. [2] Crammond N J and Nixon P J. Deterioration of concrete foundations piles as a result of thaumasite formation. Sixth International Conference on the Durability of Building Materials, Japan. E & F N Spon (1993), vol 1, pp 295–305. [3] Crammond N J and Halliwell M A. The thaumasite form of sulfate attack in concretes containing a source of carbonate ions. Second Symposium on Advances in Concrete Technology, ACI, 1995. SP154-19, pp 357–380. [4] Crammond N J. The thaumasite form of sulfate attack in the UK. Cement and Concrete Composites, 25 (8) 808–818.* [5] Loudon N. A review of the experience of thaumasite sulfate attack by the UK Highways Agency. Cement and Concrete Composites, 25 (8) 1051–1058*. [6] Slater D, Floyd M and Wimpenny D E. A summary of the Highways Agency thaumasite investigation in Gloucestershire: the scope of the work and main findings. Cement and Concrete Composites, 25 (8) 1067–1076*. [7] Crammond N J. The occurrence of thaumasite in modern construction – a review. Cement and Concrete Composites (2002), 24 393–402.

Part A [8] Collett G, Crammond N J, Swamy R N and Sharp J H. The role of carbon dioxide in the formation of thaumasite. Cement and Concrete Research, 34 (9) 1599–1612. [9] Floyd M. A comparison of classification for aggressive ground with thaumasite sulfate attack measured at highway structures in Gloucestershire, UK. Cement and Concrete Composites, 25 (8) 1085–1093*. [10] Longworth T I. Development of guidance on classification of sulfate-bearing ground for concrete. Concrete, 38 (2) 25–26. [11] Longworth T I. Review of guidance on testing and classification of sulfate and sulfide-bearing ground. BRE Client Report 80042. Garston, BRE, 2003 (unpublished). * Also published as a paper in Proceedings of First International Conference on Thaumasite in Cementitious Materials, BRE, June 2002. Garston, BRE Bookshop, 2002. BRE Digest 363 Sulfate and acid resistance of concrete in the ground (withdrawn) Special Digest 1 Concrete in aggressive ground (2001 and 2003 editions, withdrawn) British Standards Institution BS 8500:2002 Concrete. Complementary British Standard to BS EN 206-1 BS EN 206-1:2000 Concrete. Specification, performance, production and conformity

9

Part B

Chemical attack on concrete

B1 General This part deals first with sulfate attack and acid attack, these being the principal types of chemical attack that are of concern for concretes placed in the ground in the UK. The aggressive chemical agents responsible commonly occur in both natural ground and land contaminated by human activity. Additionally, this part identifies some of the more rarely occurring forms of chemical attack caused by high levels of chemical species such as ammonium ions, and organics such as phenols. Generally these agents are found in troublesome concentrations only in land affected by contamination. With some exceptions, specific guidance is not given in this Special Digest on protecting concrete from the action of these less common destructive agents. Generally the protective principles applied in Part D will be beneficial; for example, specifying a well compacted concrete with a low water/cement (w/c) ratio or providing a protective coating. Specialist advice should be sought when appropriate. Finally, this part explains how aggressive carbon dioxide in flowing water can be destructive to concrete. The potential for this form of attack is taken into account in Table C1 (on page 31) when assessing ground containing flowing water for concrete design. Also, the possible damaging effect of high levels of aggressive carbon dioxide are catered for in Part F in the design of specific precast concrete products such as pipeline systems. This and other parts of the Special Digest make reference to brackish water (containing up to 17 000 mg/l chloride ions) which under certain circumstances can be harmful to concrete or its steel reinforcement. Exposure of concrete to seawater (~18 000 mg/l chloride) or similarly saline groundwaters is, however, beyond the scope of this guidance.

B2 Principal types of chemical attack on concrete B2.1 Sulfate attack

B2.1.1 Background The essential agents for sulfate attack are sulfate anions (SO42–). These are transported to the concrete in various concentrations in water together with cations, the most common of which are calcium, magnesium and sodium. Where porous concrete is in contact with saturated ground, the water phase is continuous across the ground/concrete interface and sulfate ions will be readily carried into the body of the concrete. Well compacted, dense, low w/c ratio concrete in such an environment will, however, initially restrict access of the ions to the surface layer. Migration of sulfate ions from unsaturated ground into the concrete can take place by diffusion provided there is sufficient water to coat the particles of soil, but the rate will be slow and dependent on the sulfate concentration. The reactions that take place when sulfates enter the concrete matrix are complex and contentious. There is extensive research literature on the topic, including some recent collaborative books and conferences[1–4]. A simple guide is given here in order to understand the basic chemistry and resultant effects. The reactions have been demonstrated to depend on the type of cement, on the availability of reactive carbonate in, for example, the aggregate and groundwater, and on the temperature. Two separate forms of sulfate attack on Portland cement concretes are described here: ● a well-known type (commonly called the ‘conventional form of sulfate attack’) leading to the formation of ettringite and gypsum ● a more recently identified type producing thaumasite. In practice, both can operate together to some extent in buried concrete under field conditions.

10 Sulfate attack can only be diagnosed when the concrete in question is showing physical signs of degradation such as expansion (with or without notable cracking), surface erosion or softening of the cement paste matrix. The identification of abnormally high levels of sulfate (significantly greater than about 4% by weight of cement) within the surface of a visually sound concrete does not automatically imply that sulfate attack has taken place; it may only be a warning of potential attack in the future. B2.1.2 Conventional form of sulfate attack For sulfate attack to occur leading to the formation of ettringite and gypsum in susceptible concrete the following must be present: ● a source of sulfates, generally from sulfates or sulfides in the ground ● mobile groundwater ● calcium hydroxide and calcium aluminate hydrate in the cement matrix. In the highly alkaline pore solution (pH > 10) provided by the sodium, potassium and calcium hydroxides liberated during the cement hydration reactions, sulfate ions that have penetrated the hardened concrete react with calcium aluminate hydrate to form calcium sulfo-aluminate hydrate (ettringite, 3CaO.Al2O3.3CaSO4.31H2O). The formation of this mineral can be destructively expansive since it has a solid volume greater than the original constituents and it grows as myriad acicular (needle-shaped) crystals that can collectively generate high internal stresses in the concrete. In sulfate-resisting Portland cement (SRPC), the tricalcium aluminate (C3A in cement notation) level is kept to a minimum so reducing the extent of this reaction. Incoming sulfate ions may also react with calcium hydroxide (Ca(OH)2) to form gypsum (calcium sulfate dihydrate, CaSO4.2H2O). This reaction product also has a greater solid volume than the original constituents and in some cases can contribute to degradation of the concrete. If magnesium ions accompany the sulfates, they may also react with calcium hydroxide producing brucite (magnesium hydroxide, Mg(OH)2) which, because of its low solubility, precipitates out of solution, also leading to increase in solid volume. Magnesium ions may also attack calcium silicate hydrates, the principal bonding material in set concrete. Laboratory tests show that the first effect of the conventional form of sulfate attack is to increase the strength and density of the concrete as the reaction products fill the pore space. When it is filled, further ettringite formation induces expansive internal stresses in the concrete which, if greater than the tensile strength of the concrete, will expansively disrupt the affected region. This cracking, together with white crystalline accumulations, are the characteristic signs of the conventional form of sulfate attack.

Part B B2.1.3 Thaumasite form of sulfate attack (TSA) A comprehensive account of this form of sulfate attack has been given in the report of the Thaumasite Expert Group[1] and in the proceedings of a special conference[4], and so only the essentials are mentioned here. Several factors must generally be coincident for TSA to occur in susceptible concrete: ● a source of sulfates, generally from sulfates or sulfides in the ground ● the presence of mobile groundwater ● a source of calcium silicate hydrate, mostly derived from cementitious calcium silicate phases present in Portland cements ● the presence of carbonate, generally in coarse and fine concrete aggregates, as bicarbonate in groundwater or as a constituent of the cement ● low temperatures (since thaumasite formation is most active below 15 °C) ● a pH of 10.5 or greater, such as that found in the cement paste matrix of non-carbonated concrete. The availability of carbonate ions (CO32–) changes the reaction products when sulfates enter the concrete. Below about 15 °C in the presence of water at high pH, the reactions between the calcium silicate hydrate, the carbonate and the sulfate ions produce thaumasite (CaSiO3.CaCO3.CaSO4.15H2O). The calcium silicate hydrates provide the main binding agent in Portland cement, so this form of attack weakens the concrete as well as causing some expansion and, in advanced cases, the cement paste matrix is eventually reduced to a mushy, incohesive mass (as in Figure B1). Since TSA does not depend on the level of calcium aluminate hydrates, SRPC concretes can be vulnerable to this form of attack. Concretes containing ground granulated blastfurnace slag (ggbs) as part of the cement have good resistance to TSA. Concretes made with other cement types must rely on achieving very low permeability for resistance.

Figure B1 Formerly high quality concrete from a highway bridge foundation that has been severely affected by TSA. The outer 50 mm of concrete has been reduced to a mushy reaction product rich in thaumasite. White haloes of pure thaumasite can be seen around dolomite aggregate particles

Chemical attack on concrete The effect of the temperature regime on the occurrence and severity of TSA has been studied in the laboratory and field. Concrete specimens which showed no sulfate attack when immersed in a range of sulfate solutions at a normal laboratory temperature of around 20 °C were progressively more severely affected by TSA when the temperature was lowered below 15 °C. In the field, the role of temperature in the occurrence of TSA is not so well understood. However, it is likely to be a key factor since there is a variation within the critical temperature band of 5–15 °C in near-surface ground. In central and southern England, Meteorological Office data[5] indicate that the seasonal ground temperature variation progressively decreases with depth, converging to a range of about 10–12 °C at about 6 m depth. At shallow foundation depths down to 1.2 m below ground level, the typical temperature range is from a minimum of 4 °C in March to maximum of 17 °C in September. At a depth of 3 m, the temperature range is from a minimum of 8 °C in April to a maximum of 12 °C in October. As in the laboratory, it is likely that the extent of TSA will be increased at the cooler temperatures if the chemical conditions are satisfied. However, minimum ground temperatures below some types of construction will be raised significantly above natural levels due to heat loss from the building, and these sub-structures may be less prone, therefore, to TSA. Carbonation of concrete results from a reaction of carbon dioxide (CO2) from the atmosphere with the calcium hydroxide (Ca(OH)2) in the matrix of concrete. The reaction produces calcium carbonate (CaCO3) and is associated with a loss of alkalinity. The pH may drop from greater than 12 to less than 9, ultimately falling below the threshold of 10.5 that is necessary for sulfate reactions producing thaumasite. See also Box E1 on page 49.

11 B2.2 Acid attack

The acids most commonly encountered by concrete (all found in some natural groundwaters) are carbonic acid, humic acid and sulfuric acid. The first two are only moderately aggressive and will not produce a pH below about 3.5. Sulfuric acid is a highly ionised mineral acid and may result in a pH lower than 2. Other similarly aggressive mineral acids may be found occasionally in ground contaminated by industrial processes. The primary effect of any type of acid attack on concrete is the dissolution of the cement paste matrix. This weakens the affected concrete but, unlike sulfate attack, the degradation does not involve significant expansion. Neither ettringite nor thaumasite are stable in acid solution so that the main reaction product from sulfuric acid attack will be gypsum. In concrete with siliceous gravel, granite or basalt aggregate, the surface attack will produce an ‘exposed aggregate’ appearance. However, in concrete with limestone (carbonate) aggregates, the aggregate may dissolve at a rate similar to that of the cement paste and leave a smoother surface. The rate of attack depends more on the rate of water movement over the surface and on the quality of the concrete than on the type of cement or aggregate. ● Acidic groundwaters that are not mobile appear to have little effect on buried concrete. ● Mildly acidic (pH above 5.5) mobile water will attack concrete significantly but the rate of attack will be generally slow, particularly if the acids are primarily organic in origin. ● Flowing acidic water may cause rapid deterioration of concrete; therefore high quality concrete is needed. In the case of humic acid, reaction products formed on the surface of concrete are mainly insoluble and tend to impede further attack. Several cases of acid attack on concrete in the UK are described by Eglinton[6]. Occurrence of acidic ground conditions is dealt with in Section C2.2 and assessment of the ground conditions in relation to acidity and mobility of water in Section C5.

12 B3 Other types of chemical attack on concrete A large number of chemicals have been reported as attacking concrete, albeit most in the longer term or at high concentrations. For instance the Portland Cement Association in the USA[7] lists more than 100 potentially destructive inorganic and organic substances. However, the likelihood of encountering the large majority of these in the ground is low, and only the more likely ones are described in this section or are referred to in the sections on site investigation (Part C). Recently, it has been brought to the attention of BRE that nitrates in concentrations potentially harmful to concrete have occasionally been found on UK sites (eg associated with fertiliser stores). It is currently not possible to give authoritative guidance in respect of this hazard and specialist advice should be sought if encountered. B3.1 Magnesium ions

Magnesium is a common element in soil and groundwater but is generally only hazardous to concrete when the Mg2+ cation is present in high concentrations in association with certain other chemical agents, the key one being sulfate anions. Laboratory studies have found that concretes made with some cements are attacked to a greater degree by high concentrations of magnesium sulfate (MgSO4) than by equivalent concentrations of sodium sulfate. Because of this effect, recommendations for concrete specification in this and previous Digests have differentiated between low and high magnesium levels when combined with high sulfate concentrations. In practice the high magnesium levels will be found in the UK only in ground having industrial residues. Other than the above, magnesium chloride (MgCl2) is reported[8] to be especially aggressive. The action of magnesium ions in concrete is complex, but a key mechanism is the replacement of calcium in calcium silicate hydrates that form much of the cement paste. This leads to a loss of the binding properties. Formation of brucite (Mg(OH)2) and magnesium silicate hydrates is an indication of attack. The determination of magnesium ion content is a routine part of site investigation for brownfield sites and is further discussed in Section C5.1.2.

Part B B3.2 Ammonium ions

Ammonium ions (NH4+) will only be a problem to concrete in ground having chemical residues left by human activity (including in this case agriculture). Ammonium salts are reported[8] to act as cation-exchange compounds, transforming the insoluble calcium in the hardened cement paste into readily soluble calcium salts that are subsequently leached away. During the reaction, ammonia is liberated and escapes as a gas. The removal of both reaction products results in an increase in the porosity of the concrete, leaving it vulnerable to further attack. Ammonium salts are also reported to act as weak acids[8] which neutralise the alkaline hardened cement paste; the removal of the hydroxide ions results in softening and gradual decrease in strength of the concrete. In addition to the corrosive action of ammonium ions, some further deterioration may be caused by the action of the associated anions. Ammonium sulfate ((NH4)2SO4) is one of the most aggressive salts to concrete; cases of attack have been caused by spillage of the material around fertiliser stores. UK guidance is not available on the concentration of ammonium ions that can be tolerated by different types of concrete. However, BS EN 206-1, Table 2, does indicate that a level of NH4 of 15–30 mg/l should be regarded as slightly aggressive, 30–60 mg/l as moderately aggressive, and greater than 60 mg/l as highly aggressive. Because of the rarity of chemical attack attributed to ammonium ions, assessment of ammonium concentration is not specifically included in the scheme presented in this document for assessment of ground aggressive to concrete, or for guidance on specification of chemically resistant concrete. Specialist advice should be sought if the presence of ammonium ions is suspected.

Chemical attack on concrete B3.3 Chloride ions –

Chloride (Cl ) is a common anion in soil and groundwater, in most cases being associated with sodium (sodium chloride, NaCl, is common salt). However, the levels of chloride found in the ground are generally chemically innocuous; indeed, they may be beneficial since there is considerable evidence, from seawater studies, that the presence of chloride generally reduces sulfate attack. This is taken into account for brackish water in brownfield sites (12 000–17 000 mg/l chloride) in Note ‘e’ to Table C2 on page 32. No recommendations are given here for concrete exposed to seawater (~18 000 mg/l chloride). Reference should be made to BS 6349-1 for maritime structures and to BS 8500-1. While not generally causing chemical attack on concrete, chlorides originating in the ground can lead to degradation of concrete through a physical mechanism involving crystallisation of chloride salts near to the surface of the concrete. This is sometimes called salt weathering (Section B6). The risk of corrosion of embedded metals in buried concrete in non-aggressive soil is generally lower than in externally exposed concrete. However, high chloride concentrations in the ground will increase the risk of corrosion since chloride ions may migrate into the concrete and lead to a reduction in passivity at the metal surface. The recommendations for the protection of steel reinforcement in BS 8500-1 should be followed. On brownfield sites that have industrial residues, the presence of chloride ions, together with a pH below 5.5, could indicate the existence of hydrochloric acid that may cause acid attack. It will be important, therefore, to determine the amount of chloride in the soil and groundwater during site investigation, as described in Section C4. The procedure for taking account of the measured chloride content in this particular circumstance is given in Section C5.1.3. Apart from this, and the need to identify brackish and sea waters, no account is taken of chloride concentration in the procedure for concrete specification in Section D. Specialist advice should be sought if chloride levels substantially larger than 18 000 mg/l (as in seawater) are encountered; for example, related to past industrial use of land. Such high concentrations have been reported[8] as chemically affecting hardened concrete. Detrimental mechanisms include the reaction of calcium and magnesium chlorides with calcium aluminate hydrates to form chloroaluminates which may result in low-to-medium expansion of concrete.

13 B3.4 Organic compounds

Phenols are the most commonly encountered troublesome organic group. These are contaminants typically generated as by-products during the manufacture of town gas, tar and coke. The concentrations present are rarely sufficient to attack hardened concrete. However, their presence may well affect the setting of concrete through an inhibition or modification of the hydration of the cement[9]. Where insitu concrete is placed directly against ground suspected of substantial contamination by phenols, consideration should be given to the use of a barrier, such as polyethylene sheeting, as protection during the setting and hardening period. It has been reported[8] that where a phenol is present in exceptionally high concentrations (eg several thousand mg/l), it has the potential to attack hardened concrete. The phenol is said to react with calcium hydroxide in the cement paste to form calcium phenolate. This crystallises in the pores of the concrete causing deterioration as a result of physical expansion. Some organic acids will affect concrete as described in Section B2.2. In addition to naturally occurring humic acid derived from decay of organic matter, other acids (eg lactic acid, acetic acid and butyric acid) may be also produced occasionally by human activity.

14 B4 Attack from aggressive carbon dioxide Aggressive carbon dioxide comprises part of the carbon dioxide (CO2) dissolved in water that, as carbonic acid (H2CO3), has the potential to attack concrete. The carbonic acid reacts with the cement paste matrix or any limestone aggregate. A fuller explanation of the term is given in Appendix A1. The same phenomenon accounts for the formation of solution features (karst) in limestone strata. Aggressive carbon dioxide is usually only present to an appreciable extent in rather pure natural waters since, in most cases, where the water contains dissolved salts, sufficient calcium carbonate is available to combine with the carbon dioxide as harmless calcium bicarbonate. Also, the potential aggressiveness to concrete is only of concern in situations where water is continually flowing over (or seeping through) the concrete. Diversion pipes or culverts around dams retaining moorland waters containing high concentrations of aggressive carbon dioxide can be subject to erosion, as can poorly compacted concrete or permeable concrete products (eg some aggregate concrete blocks) used in foundations. Measures to take account of aggressive carbon dioxide for some uses of specific precast concrete products are incorporated into guidance in Part F. These measures are also relevant to cast-in-situ structures that are in contact with flowing water containing aggressive carbon dioxide (Table C1, Note ‘d’). Guidance is given in Section C2.2.3 on the determination of the level of aggressive carbon dioxide.

B5 Attack from pure water ‘Pure’ (or soft) water, which contains low concentrations of dissolved ions, is aggressive when it flows in quantity over a concrete surface. Concrete surfaces that are carbonated are less prone to this form of attack.

Part B B6 Damage to concrete from crystallisation of salts As well as causing chemical attack on concrete, soluble compounds originating in the ground can lead to degradation of concrete through a physical mechanism involving crystallisation of salts, usually sulfates or chlorides, near to the concrete surface. A classic scenario for this is where concrete of high permeability is partly buried in wet sulfate or chloride bearing ground and partly exposed to air. Sulfates or chlorides in solution may be drawn through the concrete by capillary suction to evaporate at or close to the free surface. Crystallisation of salts in pores close to the surface of the concrete may generate expansive stresses that disrupt the concrete, while surface salt deposits form a characteristic efflorescence. The process may be aggravated by repeated wetting and drying of the exposed concrete surface; this leads to cyclical salt precipitation and dissolution and fatigue stressing of the concrete fabric. Moreover, where crystallisation initially occurs at a relatively high temperature producing an anhydrous salt, subsequent wetting may lead to conversion to a hydrous crystalline form of substantially greater volume. A salt particularly implicated in this latter mechanism is sodium sulfate which, when subjected to alternate wetting and drying, may itself alternate between anhydrous thenardite (Na2SO4) and hydrous mirabilite (Na2SO4.10H2O) with a change in crystalline volume of some 300% and a potentially large cyclical stress change. A comprehensive discussion of the topic is included in Sulfate attack on concrete[3]. In the UK, degradation of partly buried concrete due to crystallisation of salts originating from the ground is rarely a problem. For most ground conditions, the measures recommended here to mitigate chemical attack on concrete (and in particular specified free-water/cement ratios of 0.5 or less) should also be effective against physical degradation due to crystallisation of likely salts. Further guidance for extreme ground conditions in arid areas is given in CIRIA Report C577[10].

Chemical attack on concrete B7 Microbial contribution to chemical attack on concrete The activity of micro-organisms in the ground can result in changes to the chemical environment which, in turn, can contribute indirectly to concrete attack. The most widely recognised damage of bacterial origin is the deterioration of concrete in sewers or sewage treatment works caused by bacteria that feed on sulfate in effluent, ultimately producing corrosive sulfuric acid[11]. The biochemical process is as follows. In the absence of dissolved oxygen in the sewage, anaerobic bacteria split oxygen from the sulfate ion (SO42–) generating sulfide (S2–). This immediately reacts with water to form hydrogen sulfide (H2S), a gas that rises into the air space above the sewage. Here it comes into contact with aerobic bacteria that live in microbial films on the moist surface of the pipe crown. These bacteria readily oxidize the hydrogen sulfide into sulfuric acid (H2SO4), that attacks the adjacent concrete surface. In another important process, sulfate-oxidising bacteria, such as Thiobacillus ferroxidans, help to oxidise pyrite (FeS2) in the ground producing both sulfuric acid and sulfates that subsequently lead to sulfate attack of concrete. The need to take pyrite oxidation into account where pyritic soils will be disturbed by construction is discussed in Section C5.1.2.

15 References: Part B [1] Department of Environment, Transport and the Regions. The thaumasite form of sulfate attack. Risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group. London, DETR, 1999. [2] Marchand J and Skalny J P. Materials science of concrete. Special volume: sulfate attack mechanisms. Westerville (Ohio), American Ceramic Society, 1999. [3] Skalny J P and Marchand J. Sulfate attack on concrete. London, Spon Press, 2002. [4] BRE. Procs of First International Conference on Thaumasite in Cementitious Materials, BRE, Garston, June 2002. Garston, BRE Bookshop, 2002. [5] Meteorological Office. Averages of earth temperature at depths of 30 cm and 122 cm for the United Kingdom 1931–1960. Report MO 794. London, The Stationery Office, 1968. [6] Eglinton M S. Review of concrete behaviour in acidic soils and groundwaters. CIRIA Technical Note 69. London, CIRIA, 1975. [7] Portland Cement Association. Effects of substances on concrete and guide to protective treatments. Concrete Information IS001. Skokie (Illinois), PCA, 2001. (Can also be downloaded from www.cement.org). [8] Environment Agency. Risks of contaminated land to buildings, building materials and services: a literature review. R&D Technical Report P331. Swindon, Environment Agency, 2000. [9] Paul V. Performance of building materials in contaminated land. BRE Report BR 255. Garston, BRE Bookshop, 1994. [10] Walker M. Guide to the construction of reinforced concrete in the Arabian Peninsula. Report CS136. Camberley, The Concrete Society, 2002. [11] van Mechelen T and Polder R. Ground chemistry implications for construction (Edr: A B Hawkins). Paper 5-8: Biogenic sulphuric acid attack on concrete in sewer environments, pp 511–524. Rotterdam, Balkema, 1997. British Standards Institution BS 6349-1:2000 Maritime structures. Code of practice for general criteria BS 8500-1:2002 Concrete. Complementary British Standard to BS EN 206-1. Method of specifying and guidance for the specifier BS EN 206-1:2000 Concrete. Specification, performance, production and conformity

16

Part C

Assessing the aggressive chemical environment

C1 General This part describes the occurrence of chemicals in the ground that are potentially harmful to concrete and gives procedures that lead to assessment of the Aggressive Chemical Environment for Concrete (ACEC) Class of the ground. The discussion on the occurrence of aggressive chemicals includes reference to some substances in land contaminated by human activities. The scheme for ground assessment is, however, restricted to natural ground, to ground mildly contaminated by some common manmade chemicals, and to fills derived from both of these. Ground containing excessive amounts of manmade chemicals (eg resulting in an acidity of less than pH 2.5), or rarely encountered substances, are not catered for by the ACEC classification and will require specialist investigation and assessment. The various stages in the ground assessment and decisions affecting concrete are set out in Figure A2. The detailed steps involved in ground assessment are shown in Figure C1. The terminology used in the various boxes of these figures is explained in later sections of this Special Digest. BRE Digests prior to SD1:2001 classified ground into five primary sulfate classes. They also gave incremental rules for modification of these primary classes to account for other factors that affect the severity of chemical attack, including: ● groundwater acidity and mobility ● concrete geometry, curing conditions and type of use. Sometimes, however, these recommended modifications were overlooked or incorrectly applied by designers and specifiers. A new approach to classification of aggressive ground conditions was therefore adopted in SD1:2001 that is continued with slight modification here. The derived ACEC classification (Section C5.2) takes direct account of the type of site, the sulfate concentration, and the groundwater acidity and mobility. Factors that are specific to the concrete construction (eg type of element, section thickness, curing

conditions, application of hydraulic gradient associated with groundwater and the intended working life) are taken account of separately in Parts D to F when specifying concrete quality to meet the assessed ground conditions. Differing site assessment procedures are given here for natural ground, for brownfield locations that may contain aggressive chemical residues, and pyritic ground. The procedure for the latter is specifically included owing to severe TSA found in highway sub-structures embedded in pyrite-bearing Lower Lias Clay fill; generation of sulfate due to oxidation of the pyrite following ground disturbance proved to be a major factor. Three key changes have been made to the procedure for ground assessment from SD1:2003. ● The limits of the Design Sulfate Classes based on 2:1 water/soil extract tests on soil have been reduced making this classification route more conservative (Box C7 on page 30). ● There is no need to take high magnesium levels into account for natural ground – the ‘m’ suffix Design Sulfate Classes now only apply to brownfield locations. This is because, in natural ground conditions in the UK, magnesium levels are invariably well below values that may significantly affect concrete. ● The concentrations of sulfate, magnesium and other relevant chemicals in water and water/soil extracts are expressed in mg/l instead of g/l. Minor changes include re-naming the former ‘highly mobile groundwater’ as ‘flowing water’ and catering for its presence in the Aggressive Chemical Environment for Concrete classification (Table C1 on page 31) for some types of groundwater.

Assessing the aggressive chemical environment

17

Steps

Refer to

1 Carry out desk study and walk-over of site to identify type of site (eg brownfield) and any ground conditions that may be aggressive to concrete

Sections C4.2 and C4.3

2 Carry out ground investigation to determine: ● groundwater mobility (static, mobile, flowing) ● concentrations of aggressive chemicals in soil and groundwater, including: ● sulfates, ● sulfides (especially in pyritic ground) ● water-soluble magnesium ● acids (indicators are pH, chloride and nitrate ions)

Section C3 Section C4.5 and C4.6 Section C5.1.2

3 Determine Design Sulfate Class for site or site locations

Step 3 of Sections C5.1.1, C5.1.2 and C5.1.3

4 Determine Aggressive Chemical Environment for Concrete (ACEC) Class for the site or site locations from Table C1 or C2, taking into account: ● Design Sulfate Class, ● type of site (natural ground or brownfield) ● water mobility ● pH

Section C5.2 and Tables C1 and C2

5 Proceed to concrete specification in Parts D, E and F of this Special Digest

Stage 3 of Figure A1

Figure C1 Procedure for assessing ground environments that are aggressive to concrete

C2 Principal constituents of aggressive ground and groundwater This section describes the chemical agents commonly encountered in natural ground and brownfield locations that are aggressive to concrete. It does not discuss other types of chemical activity or ground contamination. C2.1 Sulfates and sulfides

Sulfates commonly occur both in the solid part of the ground (soil, rock or fill) and in groundwater. Sulfates can also be derived by oxidation of sulfides, such as pyrite (FeS2), and by natural processes such as weathering, sometimes aided by construction activities. It is therefore necessary to consider the distribution of sulfides as well as sulfates in ground which may affect buried concrete. However, sulfides usually provide no hazard to concrete in the absence of oxygen and mobile water. Box C1 lists the main sulfur species found in the UK, most of which are either sulfates or sulfides. An overview of the role and occurrence of sulfates and sulfides is given here. A more detailed discussion is given in Chapter 3 of the Thaumasite Expert Group report[1].

C2.1.1 Natural ground In UK natural ground, sulfates most commonly occur in the form of hydrated calcium sulfate (gypsum). Significant amounts of magnesium sulfate (epsomite) and sodium sulfate (Glauber’s salt) may also be present. Calcium sulfate has limited solubility, producing a maximum concentration of sulfate in water at normal ground temperatures of about 1400 mg/l SO4. Magnesium sulfate and sodium sulfate are much more soluble; so, if present in the ground in sufficient quantities, they will Box C1 Sulfur mineral species found in UK ground Anhydrite Barytes Celestine

CaSO4 BaSO4 SrSO4

Epsomite Gypsum Jarosite Marcasite Mirabilite (Glauber’s salt) Pyrite Pyrrhotite Organic sulfur

MgSO4.7H2O CaSO4.2H2O KFe3(OH)6(SO4)2 FeS2 NaSO4.10H20 FeS2 FeS

Found in evaporite rocks Common vein mineral in rocks Rarely found (eg Mercia Mudstone) Found in evaporite rocks Common in soils and rocks Weathering product of pyrite Nodules in chalk and limestone Found in evaporite rocks Common in soils and rocks Rarely found in soils and rocks Common in peat

18

Part C

London Clay London Clay

Kimmeridge Kimmeridge ClayClay Oxford Clay Oxford Clay Lower Lias Clay Lower Lias Clay Gault Clay Gault Clay Weald Clay Weald Clay Mercia Mudstone Clay Mercia Mudstone (Keuper Marl) (Keuper Marl)

Limit ofmain main areas Limit of areas of deposits ofglacial glacial deposits

Figure C2 Principal sulfate and sulfide bearing strata in England and Wales North of the indicated line much of these strata are covered by glacial deposits which, if partly derived from the indicated strata, may also contain sulfates and sulfides

dissolve to produce sulfate concentrations many times greater. More rarely, sulfate may also be present in relatively insoluble forms, as in the mineral barite (barium sulfate). These minerals do not usually present a hazard to concrete. The likelihood of sulfates being present in natural ground depends on the geological strata, the weathering history of those strata and the groundwater flow patterns. The geological strata most likely to have substantial sulfate

concentrations are ancient sedimentary clays, including Mercia Mudstone (Keuper Marl), Lower Lias Clay (Charmouth Mudstone), Kimmeridge Clay, Oxford Clay, Wealden Clays, Gault Clay and London Clay. In addition to these, sulfates may be found in locally significant concentrations in a wide range of other natural strata ranging from Carboniferous mudstones to Recent alluvium and peat.

Assessing the aggressive chemical environment The sulfate-bearing strata of greatest national significance are shown in Figure C2. However, it is important to note that: ● in most geological deposits (the Mercia Mudstone being a notable exception) only the weathered zone (generally the upper 2–10 m) is likely to have a significant quantity of sulfates present. In most affected strata, the sulfatebearing zone can therefore be distinguished by the brown coloration characteristic of weathered clay, compared to the dark grey colour of unweathered clay, shale or mudstone that may contain sulfide minerals ● within the weathered zone, sulfate concentrations may vary substantially laterally and vertically. It is usual for the top metre or so of undisturbed ground to be very low in sulfates owing to leaching by rainfall. Also common are high concentrations of sulfates which have accumulated at the base of the tree root zone at depths of 2–3 m, and near the bottom of the weathered zone at typical depths of 3–10 m. In all of the geological strata shown on the opposite page, except Mercia Mudstone, unweathered material at some metres depth may contain sulfides, particularly pyrite. In their natural environment it may take thousands of years for these sulfides to be converted to sulfates by weathering. But sulfides can be converted relatively rapidly to sulfuric acid and sulfates if exposed to air and water by construction activities sometimes aided by bacterial action or high pH (Box C2).

Coal mining areas

19 Box C2 How sulfides are converted to sulfates in disturbed ground Construction activities (eg excavation and backfilling) that substantially disturb the ground may allow pyrite in initially unweathered geological strata to have access to air, water and bacteria. The result may be a relatively rapid oxidation of all or part of the pyrite [2]. The rate and extent of oxidation will depend on the type of pyrite and the local environmental conditions. ‘Framboidal’ pyrite that typically occurs as loosely packed clusters of 1–10 µm sized crystals in sedimentary clays is particularly prone to oxidation owing to its large surface area. In contrast, large cubic crystals found in metamorphic rocks (eg slates) are resistant to oxidation. In respect of environmental conditions, bacterial action has been widely reported as aiding the oxidation, particularly so when conditions are acidic. A relatively new finding is, however, that a pH greater than 10 (such as occurs by mixing cement or lime with soil) also accelerates pyrite oxidation [3]. A typical process of oxidation of sulfides to sulfates can be simply expressed as follows. ● In the presence of oxygen in air or groundwater, pyrite (FeS2) oxidises to form red–brown ferric oxide (Fe2O3) or yellow–brown hydrated ferric oxide (Fe(OH)3) together with sulfuric acid (H2SO4). The latter is the initial source of sulfate ions and acidity. ● If calcium carbonate (CaCO3) is present, the sulfuric acid (H2SO4) will further react with it to produce calcium sulfate which crystallises as gypsum (CaSO4.2H2O). ● In the presence of calcium, up to 1400 mg/l of sulfate ions (SO4) may remain in solution in groundwater. If there is insufficient calcium carbonate to neutralise the sulfuric acid, the groundwater may become acidic. The latter condition is rare in the most commonly encountered pyritic clays as these generally contain abundant calcium carbonate. It can, however, occur in certain strata such as some Carboniferous mudstones.

C2.1.2 Brownfield locations Fill materials found on sites, or brought in during construction, may contain substantial quantities of sulfates and occasionally sulfides. The characteristic red shale generated from the self-combustion of colliery spoil often contains variable amounts of common sulfates which originated from pyrite present in some Coal Measures strata (Figure C3). Other fill materials that may contain sulfates include accumulations of old blastfurnace slag, oil shale residues in the Lothians and clinker from the old-style chain grate power stations or from refuse incineration. Furnace bottom ash (fba) and pulverized fuel ash (pfa) from the current power generation process contain only small amounts of calcium sulfate. Some sulfates may arise from the bricks in brick rubble, but more significant quantities may be present if it has adhering plaster (containing gypsum), or if the bricks came from the demolition of old chimneys.

Figure C3 Coal mining areas of Great Britain where sulfate bearing, coal mining wastes and metal processing slags are most likely to be encountered

There also may be unusual sulfates, such as ammonium sulfate, in soil and groundwater as a result of past industrial use and agriculture.

20

Part C

C2.2 Acids

C2.3 Magnesium, calcium, sodium and potassium ions

C2.2.1 Mineral acids Sulfuric acid is the only mineral likely to be found in natural groundwater. As noted in Section C2.1.1, this acid may result from the oxidation of pyrite. Acidic conditions from the oxidation of pyrite are reported in fills derived from Carboniferous mudstone and Oxford Clay[4]. There have also been cases of pH levels less than 3.5 on recently drained marshland, resulting from pyrite-bearing peaty soils being exposed to oxygen. However, in much of the UK there is sufficient calcium carbonate available in the ground eventually to neutralise any sulfuric acid by forming calcium sulfate (gypsum).

These elements are important as they constitute the principal source of cations that support sulfate anions in solution in groundwater and collectively control the strength of sulfate solutions available to attack concrete. Additionally, the presence of magnesium inherently modifies chemical reactions in sulfate attacked concrete (Section B3.1). All four elements are prevalent in UK natural ground, but only determination of magnesium ion content is a routine part of site investigation (Section C5.1.2). C2.4 Ammonium ions

Residual pockets of sulfuric acid may be identified on sites previously used for industrial processes and, in exceptional circumstances, hydrochloric (Section B3.3) or nitric acid could also be found. C2.2.2 Humic acid Natural groundwater may be mildly acidic owing to the presence of humic acid (which results primarily from the decay of organic matter). This acid is not highly ionised and will not produce a pH below about 3.5. C2.2.3 Carbonic acid and aggressive carbon dioxide Carbonic acid (H2CO3) is a weak acid that forms when carbon dioxide dissolves in water. Rainwater is therefore the common source. As it is readily neutralised by reaction with calcium carbonate in the ground, it will generally only be encountered in relatively pure, soft waters such as those flowing from uplands of non-calcareous rock. In respect of aggressiveness to concrete, the parameter ‘aggressive carbon dioxide’ is used as a measure of the potential for water containing dissolved CO2 to dissolve calcium hydroxide and other soluble parts of the cement paste. As explained in Appendix A1 and Section B4, only part of the CO2 dissolved in groundwater is available to attack concrete as some (often most) is already utilised in bicarbonates (eg Ca(HCO3)2) and some is reserved for ‘stabilising’ these bicarbonates. Appropriate sampling and test procedures for determining aggressive carbon dioxide are given in prEN 13577:1999. Part F of this Special Digest indicates that levels greater than 15 mg/l are a potential problem to the inner surface of pipes and culverts carrying a flow of water. EN 206-1 categorises levels of 15–40 mg/l as slightly aggressive, 40–100 mg/l as moderately aggressive and greater than 100 mg/l as highly aggressive. Higher levels of aggressive carbon dioxide will be associated with low pH, but pH cannot be used as the principal indicator since pH will be affected by any presence of humic and mineral acids.

Ammonium sulfate ((NH4)2SO4) is used in agriculture as a fertiliser. However, there is no evidence that harmful concentrations of ammonium sulfate occur in ground subjected to normal use. Only rarely have potentially damaging concentrations been found (eg resulting from spillage of the material around fertiliser stores). Ammonium ions may also be present in brownfield sites subject to former industrial use, especially gasworks. Specialist advice should be sought if a high concentration of ammonium is suspected: determination is not a routine part of assessment of ground for concrete. C2.5 Chloride ions

Chloride is a common element in natural soil and groundwater, particularly in the form of sodium chloride (NaCl) or common salt. An obvious source is inland penetration by seawater. In some regions underlain by halite-bearing Mercia Mudstone, it may come from natural brine seepages. As a legacy of man’s activities, chlorides are extensively found in industrial wastes, particularly those associated with chemical production. Sodium chloride is also widely found adjacent to roads owing to its use as a de-icing salt.

Assessing the aggressive chemical environment C3 Presence and mobility of groundwater The rate of chemical attack of concrete depends on the concentration of the aggressive ions and the ease with which they can be replenished at the reaction surface in the concrete. The replenishment rate will be related both to the porosity and permeability of the soil adjacent to the concrete and to the presence and mobility of the groundwater in the surrounding area. Definitions of groundwater mobility used in this Special Digest, and procedures for establishing them, are given in the following Sections C3.1 to C3.3; see also Box C3.

21 borehole within 24 hours, the water conditions can be declared to be static. Alternatively, a standpipe piezometer can be installed; for example, by embedding it in a sand column in a borehole and sealing over the top 0.75 m with bentonite pellets. If this remains dry, or a variable head test (BS 5930) indicates a ground permeability of less than 10–7 m/s, the water conditions can be confirmed as effectively static.

‘Static groundwater’ is confined to locations where the ground is either permanently dry, or contains water but has low permeability (ie little water movement is possible). The mass permeability in the latter case will generally be less than 10–7 m/s (BS 8004, Figure 6). A typical example would be clayey soils with tight fissures and no included sand or silt horizons.

At times of low water table (generally early summer to mid-autumn), it will often be difficult to prove static groundwater conditions from seepage tests. At these times of year the absence of water at proposed concrete depths must not be taken as the sole evidence of static groundwater conditions. An appropriately qualified professional (such as a geotechnical engineer, engineering geologist or hydrologist) can be asked to prove a case for static groundwater conditions from an evaluation of the site geology and hydrology. Otherwise, for concrete design purposes, it should be assumed that the more conservative condition of mobile groundwater exists.

In winter and spring, when water tables are generally at their highest, the presence of static groundwater conditions can be established on a proposed construction site by either digging a trial pit or drilling a borehole to the intended full depth of concrete. If no water has seeped into the trial pit or

Some construction activities can greatly increase mass permeability of ground and may, therefore, change a natural site condition of static groundwater to a mobile groundwater condition. The likelihood of this happening should be considered in the ground assessment.

C3.1 Static groundwater

Box C3 Important facts to note about the presence and mobility of groundwater in relation to chemical attack of concrete ● The presence and mobility of groundwater may vary seasonally. ● Highest groundwater levels may be expected in winter and spring, the lowest in late summer. ● Groundwater mobility may vary with depth and must be established for the full depth of a concrete construction. ● Permeable silty and sandy soils in which water is present generally provide little or no resistance to the movement of water carrying dissolved chemicals. ● Accumulations of free-water (eg in a pond) will readily facilitate the movement of dissolved chemicals. ● Fissured clay and clay fills, which may have free-water present in fissures and voids, generally have a relatively low permeability that allows only slow movement of water carrying dissolved chemicals. This may be detrimental to concrete over a period of time. ● Clays in which fissures and other discontinuities are absent or are tightly closed have very low permeability but generally remain fully saturated with pore water in all seasons owing to capillary action. This pore water may allow some limited movement of chemicals by diffusion through the liquid phase, but in general the quantity of chemicals reaching the concrete will not be sufficient to cause significant chemical attack.

● Ground on, or at the foot of, slopes or retaining structures may be subject to enhanced flow of groundwater owing to the gravitational head of water. ● Civil engineering works (eg road construction, office and factory developments, and large housing developments) can disrupt natural drainage. This may affect flows in rivers and streams, and sub-surface groundwater movements and levels. In differing circumstances this may lead to increased or reduced flows. The consideration of the effects of site drainage in relation to structures and foundations is essential and, in particular, the presence of porous carrier drains which may divert water into the area of the foundations (Section D6.6). ● Care is needed to avoid concrete being exposed to aggressive conditions in a ‘sump’ environment. In several cases of serious deterioration to concrete from TSA in the foundations of highway bridges [1], a major contributory factor was that the foundations had been constructed in excavations that were subsequently backfilled with pyritic clay and also subject to ingress of water. Reminder When it is uncertain whether the groundwater is static or mobile (eg owing to lack of site data or knowledge of changes to ground permeability that may result from construction), a mobile groundwater condition should be assumed.

22

Part C

C3.2 Mobile groundwater

C3.3 Flowing groundwater

The term ‘mobile groundwater’ is defined to cover the following range of conditions: ● water held in pores and structural discontinuities in the soil, and which is free to flow into an excavation to give a standing water level. The ground permeability will generally be greater than 10–7 m/s ● water which is percolating slowly through the ground, say at a velocity of less than 10 m/day (where velocity equals permeability of the ground multiplied by the hydraulic gradient) ● still water in ponds, sumps or similar accumulations.

Groundwater is to be regarded as ‘flowing’ when it percolates through the ground under a permanent head in substantial quantity and at a relatively rapid rate; say at a velocity greater than 10 m/day. Flowing groundwater may be expected on a site that contains very permeable soil and is sloping or could be subject to a hydraulic head from a nearby hill or embankment.

The presence of mobile groundwater may be seasonal. At times of year when groundwater levels are high, mobile groundwater conditions can be confirmed by either digging a trial pit or drilling a borehole to the relevant depth and leaving it temporarily open. Surface protection is needed to prevent ingress of rainfall and for safety of personnel. If mobile water is present, there will be some seepage into the trial pit or borehole within 24 hours, but often the water intake will be much more rapid. In early summer to mid-autumn, when groundwater levels are generally low, these simple field tests may fail to detect seasonally adverse mobile groundwater conditions. However, it will often be apparent from a consideration of the site geology and hydrology that the seasonal occurrence of mobile groundwater is likely. On no account should the conclusion be drawn that groundwater conditions are static merely because there is an absence of mobile groundwater at one particular time or season. The rapidity and position of groundwater seepage detected in trial pits or boreholes does not further affect the groundwater mobility classification (except in the case of flowing water as described next). However, these seepage characteristics should be reported as part of the site investigation findings, as they may have a bearing on the type of additional protective measures (Section D6) to be adopted for protecting concrete from chemical attack.

Box C4 Possible sources of information to be considered in a desk study ● Published topographical maps ● Aerial survey photographs ● Geological maps and memoirs – particularly Drift maps ● Soil survey maps ● Site investigation records for past developments or construction on adjacent sites ● Data from the Environment Agency or local authorities on regional water levels and flooding ● Data from the British Geological Survey Further sources of information are listed in Annex B of BS 5930 and TRL Report 192[11].

Seepage flow of water into a borehole or trial pit excavated on a site below the water table (ie under a temporary head) does not necessarily prove a flowing groundwater condition. However, the condition can often be inferred from an overall consideration of the quantity and rate of flow, the type of soil and the surrounding topography. Specialist hydrological advice may be required if site evidence is meagre or difficult to interpret. For some uses of concrete, the definition of flowing groundwater is extended to cover water that is flowing in surface conduits or streams. Only in two cases is flowing water intrinsically catered for in the precautions recommended for concrete. ● In Table C1 (page 31), a step up of ACEC Class is recommended when the flowing water is potentially aggressive because it is pure or has a significant level of aggressive carbon dioxide. ● In the design guides of Part F, for specific precast concrete products, an internal lining is recommended when water and sewer services carry flowing water (not specifically groundwater) that contains a significant level of aggressive carbon dioxide. However, more generally, flowing water should be regarded as a contributory factor which can exacerbate all forms of chemical attack and be a vehicle that might transport aggressive chemicals to a site from adjacent land.

Box C5 Topics that may be relevant to desk study assessment of the risk of chemical attack ● Bedrock and superficial (drift) geology – particularly important as an indicator of pyritic ground (Section C2.1) ● Location and type of previous development, particularly of industry that might have left aggressive waste materials ● Topography from ground contours, including changes that might indicate placing of fill ● Records of flooding ● Location of existing natural and manmade drainage systems including, streams, ditches, trench drains and field drains ● Reworking of pyrite-rich clays ● Use of colliery spoil or mine waste.

Assessing the aggressive chemical environment C4 Site investigation for aggressive ground conditions

23 For all locations an appraisal should be made of the groundwater conditions and, in particular, whether concrete could be exposed to mobile or flowing groundwater (Section C3).

C4.1 Introduction

This section describes the site studies, sampling and testing needed to assess whether ground conditions are potentially aggressive to concrete. It is recommended that an investigation comprises, sequentially, a desk study, a walkover survey, and a ground investigation using trial pits and boreholes to assess visually the ground profile, and to take representative samples of soil and groundwater for chemical analysis. The chemical agents to be quantified in particular are those listed in Section C2 as commonly encountered: sulfates, sulfides, acids (pH) and magnesium. In brownfield locations, chloride and nitrate should also be quantified as respective indicators of hydrochloric and nitric acids. The presence of all of these is specifically taken into account by the ACEC classification in Section C.5. A list of recommended test methods and source documents for the chemical analysis of soils and groundwater is given in Appendix C1 on page 36. Chemical agents aggressive to concrete that are found only rarely (eg ammonium salts) should be investigated if their presence is suspected; for example, from past use of the site. However, specialist advice should be sought with regard to the detection of these hazards and to appropriate concrete specification. Where the flowing groundwater conditions described in Section C3.3 exist, the water should be tested for aggressive carbon dioxide.

The site investigation should be carried out by suitably experienced persons. (Guidance on specialist personnel is given in Digest 472.) The level of detail should be broadly related to the importance of the proposed construction, the complexity of the site and the level of assurance required for risk management. Appendix C2 on page 36 gives guidance on a more comprehensive site investigation that may be needed for a case of sulfate attack on concrete. C4.2 Desk study

An initial desk study should be carried out to identify and review relevant existing information. In particular, evidence should be sought of any aggressive chemicals and of potentially aggressive substances such as pyrite. Guidance on desk studies is given in BRE Digest 318; Boxes C4 and C5 list items specific to aggressive ground. In respect of pyrite, particular note should be taken of grey or black-coloured alluvial deposits, overconsolidated clays, mudrocks, Coal Measures, slates and schists. A list of geological formations known to contain pyrite is given in Box C6. Also listed are typical pyrite contents (by % mass) quoted for samples taken from the localities indicated. The geological formations are not necessarily confined to these locations and pyrite contents may vary substantially from indicated values.

As well as chemicals aggressive to concrete, investigation of land affected by contamination requires consideration of other hazards. These matters are beyond the scope of this Special Digest. British Standard BS 10175 and several recent reports from CIRIA and the Environment Agency[5–10] are recommended for guidance on the additional requirements required for planning, executing and interpreting site investigations, for identifying any other hazards, and for the management of the risks involved in developing contaminated land. Box C6 UK geological formations known to contain pyrite (derived from Table 3 of Thaumasite Expert Group report[1] – see for individual references) Geology Colliery spoil Carboniferous Limestone Shales Coal Measure shales Carboniferous Culm Measures Namurian mudstones Rhaetic mudstones, Westbury Formation Stonefields Slate Lower Lias Clay (Charmouth Mudstone) Upper Lias Clay (Whitby Mudstone) Whitby Shale

Location of samples UK Yorkshire & Derbyshire England Devon Derbyshire

% pyrite 0–12 5–10 0.7–1.4 2.4 0–6

S Glamorgan Gloucestershire

4–6

S W England

5–8

Northamptonshire Teesside

3–5 3–9

Geology Oxford Clay Oxford Clay Kimmeridge Clay Weald Clay Sandgate Beds Gault Clay Bracklesham Beds Headon Beds Barton Clay Bembridge Beds London Clay Recent alluvial deposits Marsh peat

Location of samples Oxfordshire E & S England Dorset Sussex S E England S England S E England S E England Hampshire Isle of Wight S E England Derbyshire Fen district

% pyrite 3–5 5–15 4 0.5–0.9 0.7–1.0

0–4

24

Part C

C4.3 Site inspection (walk-over survey)

C4.4 Visual description of the ground

The aim of the walk-over survey is to examine the surface of the site for evidence of conditions that might contribute to a chemical or hydrological environment aggressive to concrete. Particular attention should be paid to the following.

The starting point for an investigation of chemical agents in the ground that may be aggressive to concrete is a good visual description of the ground profile to the full depth of concrete construction. Laboratory testing can give precise values for chemical contents at particular locations but will not necessarily be fully representative.

● Examining any exposures of natural ground and recording details of geological materials and organic deposits such as peat (which may be acidic) ● Inspecting and recording details of any former structures and waste materials ● Comparing surface topography with previous records to check for the presence of fill, erosion or cuttings ● Considering the effects of changes to the topography as a result of new construction ● Noting the presence of slopes which, by providing a head of water, could increase water mobility ● Noting water levels, directions and rates of flow in watercourses ● Noting positions of wells or springs ● Noting the nature of vegetation in relation to soil type and wetness of the ground. Unusual green patches, reeds, rushes or willow trees often indicate wet ground. More general guidance on the walk-over survey is given in BS 5930 Annex C and BRE Digest 348. Following the walk-over survey, an assessment should be made of the presence and distribution of conditions likely to be aggressive to concrete. These data should be used to plan the ground investigation.

A visual assessment may detect local concentrations of potential hazardous minerals – such as gypsum (CaSO4.2H2O), pyrite (FeS2) and marcasite (FeS2) – and features that affect the transmission of groundwater. The soil description can be accomplished by means of trial pits or boreholes, described in BS 5930, and BRE Digests 381, 383 and 411. Guidance on the occurrence and identification of sulfate and sulfide minerals is given in Appendices A, B and C of the Highways Agency Report HA 74/00[12]. The ground description should particularly note the following features relevant to assessment of the aggressiveness of a chemical environment: ● soil particle size and composition ● soil colour: ● a dark grey or black colour of unweathered mudrocks and clays generally indicates that they originated in anaerobic conditions conducive to the formation of pyrite ● a dark grey, blue or black coloration of clay generally indicates that it is unoxidised ● brown coloration of clay generally indicates that it is in a weathered, oxidised state ● soil structure: this gives information on the state of weathering and ease of groundwater transmission ● the presence of any visible sulfate or sulfide minerals, but noting that pyrite is often finely disseminated and is not identifiable, even with a hand lens ● the presence of any visible calcium carbonate in the form of amorphous nodules, fossil fragments or calcite crystals. This can be also be detected by effervescence of the soil when it is tested with dilute hydrochloric acid (5% HCl).

Assessing the aggressive chemical environment

25

C4.5 Sampling and testing soils

C4.6 Sampling and testing groundwater

Samples for the required chemical tests can be taken using standard site investigation techniques incorporating trial pits and boreholes (BS 5930, BS 10175 and BRE Digests 381, 383 and 411). Precautions should be taken to protect site workers and site neighbours if the desk study indicates the presence of substances harmful to health. To avoid contaminating the samples, only a minimum amount of water should be added to the hole during boring, preferably none.

If there is mobile groundwater on a site, indicated by visible seepage into a trial pit or borehole, this must always be tested for aggressive chemical content. This is because groundwater may have a concentration of dissolved chemicals greater than are present in the immediately surrounding solid ground owing to transportation from a distant source.

Representative samples should be taken for sulfate classification from key depths in the ground in each test pit or borehole, bearing in mind the preliminary structural design concept and the likely distribution of sulfides as indicated in Section C2.1. One test might be considered sufficient for a house foundation to be installed at 1 m depth and two tests if it is to be founded at 3 m depth. For more substantial foundations and piles, samples should be taken at about 1–2 m intervals ensuring they include at least one from any obvious change of stratum. The number of pits or boreholes will depend mainly on the size, topography and complexity of the geology of the site as evidenced by the findings of the desk study and walk-over survey. The mass of samples required for chemical testing should be as given in BRE Report BR 279, Section 4, or BS 1377-1, Section 7; that is, 100 g for fine grained soils, 500 g for medium grained soils and 3 kg for coarse grained soils. In fine grained soils, samples should be obtained preferably by driving a tube into the ground. After extraction of the tube, the ends of the sample need to be sealed to restrict loss of moisture and intrusion of air and thereby minimise oxidation of pyrite. The samples must then be stored in a cool, dark place, at a temperature between 2 and 4 °C, and be tested as soon as possible, the maximum delay being three weeks. The conditions and duration of storage prior to testing should be recorded and given to the site appraiser together with the test data. Material selected for laboratory testing should be taken from the centres of block samples and core samples to avoid the effects of surface oxidation and contamination by different water or soil. Recommended test methods for the chemical analysis of aggressive soils are given in Appendix C1.

Groundwater samples can be obtained by collecting seepage into a trial pit or borehole. Water seeping from the base or sides of a trial pit can be collected in a container such as a clean, sealable sample jar. Care should be taken to avoid water that has entered the pit directly from rainfall or surface run-off. A note of any visible seepage and the directions from which it comes will help in a groundwater mobility assessment. In ground of lower permeability, less than 10–7 m/s (BS 8004, Figure 6), a groundwater sample can be best obtained from a standpipe piezometer installed in a borehole, backfilled with sand and sealed over the topmost metre with bentonite-cement pellets. After reaching equilibrium, the piezometer will also indicate the height of the water table. Additionally, the permeability of the ground can be determined in the piezometer by a variable head test (BS 5930). The concentration of some chemicals in groundwater (eg sulfates) may vary seasonally, probably being greatest in the late summer when groundwater is reduced in volume. Also it is possible for groundwater in boreholes to be found to contain different concentrations of soluble sulfates at different depths. In these circumstances, groundwater samples taken after the boring is completed may contain water from several different strata. Controlled procedures need to be used for obtaining, handling and storing groundwater samples. Samples of 0.5–1 litres should be obtained, stored in clean, well filled, sealed containers at low temperature to minimise changes due to bacterial action (4 °C is recommended), and analysed promptly. The acidity or alkalinity of water can be tested on site using pH test strips or a portable meter. For flowing groundwater requiring the determination of aggressive carbon dioxide, the sampling procedure given in prEN 13577 can be used. Recommended test methods for the chemical analysis of aggressive groundwater are given in Appendix C1. It is strongly recommended that groundwater mobility is assessed (referring to the definitions in Section C3) at the same time as groundwater samples are taken. Knowledge of the mobility of any groundwater on the site is an essential prerequisite for the ACEC classification of the ground. This is because groundwater determines the ease with which an aggressive chemical can have access to the concrete. Also, the mobility of any groundwater must be known for some categories of concrete construction since site drainage may need to be designed to protect concrete foundations.

26

Part C For each site location, select samples of various site materials from key depths (Sections C4.5, C4.6 and C5.1.1)

Take the highest of Results 1 and 2 as the Design Sulfate Class for the site or location (C5.1.1, Step 3)

Use Appendix C1 tests on soil samples to find: ● water-soluble sulfate mg/l SO4 in 2:1 water/soil extract ● pH in 2.5:1 water/soil extract Option 1 For static groundwater select column 5 of Table C1

Option 2 For mobile groundwater or flowing water select column 6 of Table C1

For adopted Design Sulfate Class, select row of Table C1 corresponding to characteristic pH of location. ACEC Class can be found from column 7 of Table C1

For adopted Design Sulfate Class, select row of Table C1 corresponding to characteristic pH of location. ACEC Class can be found from column 7 of Table C1

Consider all water-soluble sulfate and pH results for soil and find characteristic values for site or individual locations (C5.1.1, Steps 1(a) and 4)

Find sulfate class equivalent to characteristic values of water-soluble sulfate in soil using columns 1 and 2 of Table C1 = Result 1

Are groundwater samples available? (Note a)

Yes

No

See Section C5.1.1 for further information Table C1 is on page 31 Note a Groundwater samples should be taken and tested wherever physically possible.

Use tests in Appendix C1 on groundwater samples to determine: ● water-soluble sulfate mg/l SO4 ● pH

Consider all soluble sulfate and pH results for groundwater and find characteristic values for the site or location (C5.1.1, Steps 1(b) and 4)

Find sulfate class equivalent to characteristic values of soluble sulfate in groundwater using columns 1 and 3 of Table C1 = Result 2

Figure C4 Procedure for determining ACEC classification for locations on natural ground sites except where soils may contain pyrite

Assessing the aggressive chemical environment For each site location, select samples of various site materials from key depths (Sections C4.5, C4.6 and C5.1.2)

27

Is there a possibility of sulfides in ground (eg pyrite in unweathered clay)?

No

Yes

Use tests in Appendix C1 on soil samples to find: ● water-soluble sulfate (WSS as mg/l SO4) in 2:1 water/soil extract ● acid-soluble sulfate (AS % SO4) ● total potential sulfate (TPS % SO4) = 3 x total sulfur (TS % S) ● pH of 2.5:1 water/soil extract

Will concrete be exposed to disturbed ground in which pyrite may oxidize to sulfate? (Note b)

No

Yes

Find sulfate class equivalent to characteristic values of water-soluble sulfate in soil using columns 1 and 2 of Table C1 = Result 1

For each individual sample of the pyritic ground, subtract the result of the acid-soluble sulfate test (AS % SO4) from the result of the total potential sulfate test (TPS % SO4) to calculate the amount of oxidisable sulfides (OS % SO4); ie OS = (TPS – AS)

Are groundwater samples available? (Note a)

Are the values of OS > 0.3% SO4 for a significant number of samples?

No

No

Yes Yes

Use tests in Appendix C1 on groundwater samples to determine: ● water-soluble sulfate mg/l SO4 ● pH

Consider all soluble sulfate and pH results for groundwater and find characteristic values for the site or location (C5.1.1, Steps 1(b) and 4)

Find sulfate class equivalent to characteristic values of soluble sulfate in groundwater using columns 1 and 3 of Table C1 = Result 2

This indicates pyrite is present which may oxidise if ground is disturbed. From a consideration of total potential sulfate tests on pyritic ground, find the characteristic value of TPS (% SO4) for the site or location

Find sulfate class equivalent to characteristic value of total potential sulfate (TPS SO4%) using columns 1 and 4 of Table C1 = Result 3

See Section C5.1.2 for further information Table C1 is on page 31 Notes a Groundwater samples should be taken and tested wherever physically possible. b See Appendix A1 and Box C8 on page 30.

Take the highest of Results 1, 2 and 3 as the Design Sulfate Class. But if Result 3 is the highest, then limit to DS-4

Take the highest of Results 1 and 2 as the Design Sulfate Class

Option 1 For static groundwater, select column 5 of Table C1

Option 2 For mobile or flowing groundwater, select column 6 of Table C1

For adopted Design Sulfate Class, select row of Table C1 corresponding to characteristic pH of location. ACEC Class can be found from column 7 of Table C1

For adopted Design Sulfate Class, select row of Table C1 corresponding to characteristic pH of location. ACEC Class can be found from column 7 of Table C1

Figure C5 Procedure for determining ACEC Class for sites or locations where disturbance of pyrite bearing natural ground could result in additional sulfate

28

Part C

For each site location, select samples of various site materials from key depths (Sections C4.5, C4.6 and C5.1.3)

Take the highest of Results 1 and 2 as the basic sulfate class for the site or location

Use tests in Appendix C1 on soil samples to find: ● water-soluble sulfate mg/l SO4 in 2:1 water/soil extract ● mg/l Cl and mg/l NO3 in 2:1 water/soil extract ● pH of 2.5:1 water/soil extract

Is water-soluble sulfate > 3000 mg/l SO4?

Is pH < 5.5 for location?

No

Yes

Determine mg/l Mg as Appendix C1

Yes

Is Cl or NO3 present at location? (Note b)

No

Yes

No

Respectively for test results on soil and groundwater. Calculate SO4 equivalent of Cl (Cl x 1.35 mg/l) and SO4 equivalent of NO3 (NO3 x 0.77 mg/l) and add to corresponding characteristic values for soluble SO4. Find sulfate classes for soil and ground water equivalent to these adjusted characteristic values = Results 3 and 4. Use the highest of Results 3 and 4 to find the Design Sulfate Class for the site or location

Consider all water-soluble sulfate, Mg and pH results and find characteristic values for site or individual locations (see C5.1.3)

Find sulfate class equivalent to characteristic values of water-soluble sulfate and Mg in soil using columns 1, 2 and 3 of Table C2 = Result 1

Are groundwater samples available? (Note a)

No

Yes

Use tests in Appendix C1 on groundwater samples to determine: ● water-soluble sulfate mg/l SO4 ● mg/l Cl and mg/l NO3 ● pH

For groundwater, is SO4 ≥ 3000 mg/l?

Yes

No

Consider all sulfate, Mg and pH results for groundwater and find characteristic values for the site or location

Option 1 For static groundwater, select column 7 of Table C2

Option 2 For mobile or flowing groundwater, select column 8 of Table C2

For adopted Design Sulfate Class, select row of Table C2 corresponding to characteristic pH of location. ACEC Class can now be found from column 9 of Table C2

For adopted Design Sulfate Class, select row of Table C1 corresponding to characteristic pH of location. ACEC Class can be found now from column 9 of Table C2

Determine mg/l Mg as Appendix C1 See Section C5.1.3 for further information Table C2 is on page 32 Notes a Groundwater samples should be taken and tested wherever physically possible. b Significant values of Cl and NO3 indicate that hydrochloric and nitric acids (HCl and HNO3) may be present. These can be allowed for by adjusting the determined soluble sulfate content. A moderate presence of chlorides is not of concern provided that the pH > 5.5.

Find sulfate class equivalent to characteristic values of soluble sulfate and Mg in groundwater using columns 1, 4 and 5 of Table C2 = Result 2

Figure C6 Procedure for determining ACEC classification for locations on brownfield sites except where soils may contain pyrite

Assessing the aggressive chemical environment C5 Classification of site locations for chemicals aggressive to concrete The Aggressive Chemical Environment for Concrete (ACEC) is introduced here to take into account sulfate concentration and other factors related to the environment in which the concrete is to be placed; for example, the mobility and pH of groundwater. For the higher sulfate classes on brownfield locations, the magnesium ion concentration is also taken into account. More conservative action limits in respect of acidity have been applied to brownfield locations as compared to natural ground. C5.1 Groundwater and soil analyses

Four categories of site have been identified as requiring specific procedures for investigation of aggressive ground conditions. ● Natural ground locations except those containing pyrite. These are the most commonly encountered locations. They are described in Section C5.1.1 and Figure C4 ● Natural ground locations that contain pyrite. These are locations where the ground contains pyrite which, if disturbed, may oxidise to sulfates (Section C5.1.2 and Figure C5) ● Brownfield locations, except those containing pyrite. (Section C5.1.3 and Figure C6) ● Brownfield locations that contain pyrite (Section C5.1.4). The category of a site or individual site location should be provisionally established by desk study (Section C4.2). If no desk study has been carried out, it should be assumed that pyrite may be present in the ground and the site testing procedures given in Sections C5.1.3 and C5.1.4 be followed. The precautions recommended in this Special Digest apply only to concrete placed in ground where the pH is greater than 2.5. Only in very exceptional circumstances in the UK are pH values found below 2.5.

29 C5.1.1 Natural ground locations except those containing pyrite These are locations identified from the desk studies or preliminary ground investigations to be neither brownfield sites nor sites containing pyrite in any strata that may be encountered by construction. Several locations on a site may need to be separately classified for chemical agents aggressive to concrete if the site is extensive or the ground conditions are complex. The analytical tests required for classification (Figure C4) are water-soluble sulfate content (mg/l SO4) and pH. Classification should be carried out, wherever possible, by using samples of both soil and groundwater. For soil, the sulfate analysis, expressed as SO4, should be on a 2:1 water/soil extract and the pH analysis on a 2.5:1 water/soil extract. The chemical classification of a given site location should be carried out in the following five steps. Determine the ‘characteristic values’ for sulfate concentration in tests on (a) soil samples and (b) groundwater samples. It is important to test groundwater samples if these are obtainable from the location because groundwater is generally the agent by which aggressive chemicals reach the concrete. All samples to be used for sulfate classification should be carefully taken, handled and tested, as described in Section C4.

Step 1

(a) Soil samples If only a small number of soil samples have been tested for water-soluble sulfate using the 2:1 water/soil extract test, the highest measured sulfate concentration (mg/l SO4) should be taken as the characteristic value. However, if the water-soluble sulfate results for soil vary widely, it may be appropriate to test further samples to obtain a more representative data set. In a data set where there are five to nine results available for the location, the mean of the highest two of the sulfate test results should be taken as the characteristic value for water-soluble sulfate (mg/l SO4). In a data set where there are 10 or more results available, the mean of the highest 20% of the sulfate test results (rounded to 100 mg/l) should be taken as the characteristic value. (b) Groundwater samples The highest determined sulfate concentration (mg/l SO4) of the samples (rounded to 100 mg/l) should be taken as the characteristic value for the groundwater at a given location.

30

Part C

Box C7 Technical notes on limits for sulfate cases ● The division between Classes 2 and 3 for groundwater is related to the maximum solubility of calcium sulfate (1440 mg/l SO4). Higher sulfate concentrations in groundwater confirm the presence of more soluble sulfates, usually magnesium or sodium. Other divisions between sulfate classes are drawn somewhat arbitrarily. ● The limits of Design Sulfate Classes based on 2:1 water/soil extracts have been lowered from those in previous Digests, the correlation between old and new limits (in terms of mg/l SO4) being as follows. Sulfate class

New limits (mg/l SO4)

Old limits (mg/l SO4)

DS-1 DS-2 DS-3 DS-4 DS-5

< 500 500–1500 1600–3000 3100–6000 > 6000

< 1200 1200–2300 2400–3700 3800–6700 > 6700

The consequence of this adjustment is to make the ground classification based on soil tests more conservative; for example, some soils that were previously classified as DS-2 would now be considered as being DS-3. The change stems from findings of numerous ground investigation studies carried out by BRE and others following discoveries of the thaumasite form of sulfate attack (TSA) in the concrete foundations of highway structures[1]. In the large majority of cases, the sulfate classes based on 2:1 water/soil extract tests on soil have been found to be substantially lower than sulfate classes based on sulfate in groundwater. Not surprisingly, therefore, they were inconsistently low compared to the actual occurrence of TSA. The new limits bring sulfate classification based on 2:1 water/soil extract tests into parity with the groundwater based tests.

Determine the basic sulfate classes corresponding to the Step 1 characteristic values for groundwater using columns 1 and 3 of Table C1, and for soil using columns 1 and 2 of Table C1. Note that new limiting values are introduced in SD1:2005 for sulfate classes based on 2:1 water/soil extract tests (Box C7).

Step 2

Adopt a value for Design Sulfate Class for the site location from a consideration of the sulfate classes for groundwater and soil determined in Step 2. If only one type of sample (groundwater or soil) was tested, the class determined for this type may be taken as the Design Sulfate Class for the location. If both types of sample (groundwater and soil) were tested, the highest of the two determined sulfate classes should be taken as the Design Sulfate Class for the location.

Step 3

Determine the characteristic values for pH by considering the values obtained from tests on soil and groundwater. For both soil and groundwater, take the respective lowest measured values of pH if only a small number of samples have been tested. Otherwise take the respective means of the lowest 20% of the pH results. The characteristic value of the pH should then be taken as the lowest of the pH determinations for the soil and groundwater.

Step 4

Starting with the Design Sulfate Class, determine the ACEC Class for the site location. Chose the appropriate pH range for the assessed mobility of groundwater: static or mobile (Figure C4 and Table C1). The ACEC classification is explained further in Section C5.2.

Step 5

As well as the outcome of this classification procedure, the results of all the individual chemical analyses, including the location and depth of the samples, should be made available to the engineer and concrete specifier. C5.1.2 Natural ground locations that contain pyrite To classify site locations where ground materials (natural ground or clean fill derived from natural ground) may contain sulfides (eg pyrite), it is essential to take account of the total potential sulfate content which might result from oxidation following ground disturbance. The extra test requirements, compared with the procedure given in Section C5.1.1, are (Figure C5): (a) Determine the total sulfate content (AS % SO4) by the acid-soluble sulfate method (Appendix C1) (b) Determine the total sulfur present (TS % S) (c) Calculate the total potential sulfate content from the stoichiometric equation: TPS % SO4 = 3.0 x TS % S. This gives a conservative estimate of the total potential sulfate since any sulfur within organic matter and minerals such as barite, both of which are more inert than pyritic sulfur, are included (d) Determine, for each individual sample, the amount of oxidisable sulfides (OS as % SO4) in the suspected pyritic ground by subtracting the acidsoluble sulfates (AS % SO4) from the total potential sulfate content: OS % SO4 = TPS % SO4 – AS % SO4 If the amount of oxidisable sulfides is greater than 0.3% SO4 in a significant number of samples, pyrite is probably present. This can be confirmed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. It can also be quantified directly by the acidified chromium reduction method (TRL Report 447[13]).

Box C8 Practical notes on pyritic ground ● Concrete in pyritic ground which is initially low in soluble sulfate does not have to be designed to withstand a high potential sulfate class unless it is exposed to ground which has been disturbed to the extent that contained pyrite might oxidise and the resultant sulfate ions reach the concrete. This may prompt redesign of the structure or change to the construction process to avoid ground disturbance; for example, by using precast or cast-in-situ piles instead of constructing a spread footing within an excavation. ● The sole determination of the acid-soluble sulfate content, as employed in some recent European Standards, will not detect pyrite which might be oxidised to sulfates as a result of ground disturbance.

Assessing the aggressive chemical environment

31

Table C1 Aggressive Chemical Environment for Concrete (ACEC) classification for natural ground locations a Sulfate Groundwater Design Sulfate 2:1 water/soil Groundwater Total potential Static Mobile Class for location extract b sulfate c water water

ACEC Class for location

1

2 (SO4 mg/ l)

3 (SO4 mg/ l)

4 (SO4 %)

5 (pH)

7

DS-1

< 500

< 400

< 0.24

≥ 2.5

DS-2

500–1500

400–1400

0.24–0.6

6 (pH)

AC-1s > 5.5 d

AC-1d

2.5–5.5

AC-2z

> 3.5

AC-1s > 5.5

2.5–3.5

AC-2s 2.5–5.5

DS-3

1600–3000

1500–3000

0.7–1.2

> 3.5 > 5.5

3100–6000

1.3–2.4

> 3.5 > 5.5

> 6000

> 6000

> 2.4

AC-4 AC-4s

2.5–5.5

AC-5

≥ 2.5

AC-5

> 3.5 2.5–3.5

AC-4 AC-3s

2.5–3.5 DS-5

AC-3 AC-3s

2.5–5.5 3100–6000

AC-3z AC-2s

2.5–3.5 DS-4

AC-2

AC-4s

Notes a Applies to locations on sites that comprise either undisturbed ground that is in its natural state (ie is not brownfield – Table C2) or clean fill derived from such ground. b The limits of Design Sulfate Classes based on 2:1 water/soil extracts have been lowered relative to previous Digests (Box C7). c Applies only to locations where concrete will be exposed to sulfate ions (SO4) which may result from the oxidation of sulfides (eg pyrite) following ground disturbance (Appendix A1 and Box C8). d For flowing water that is potentially aggressive to concrete owing to high purity or an aggressive carbon dioxide level greater than 15 mg/l (Section C2.2.3), increase the ACEC Class to AC-2z. Explanation of suffix symbols to ACEC Class ● Suffix ‘s’ indicates that the water has been classified as static. ● Concrete placed in ACEC Classes that include the suffix ‘z’ primarily have to resist acid conditions and may be made with any of the cements or combinations listed in Table D2 on page 42.

The sulfide content of the ground must be taken into account if it is concluded that both: ● pyrite is present in significant amounts ● the concrete is to be exposed to disturbed ground (Appendix A1 and Box C8) which might be vulnerable to oxidation. This procedure should be done in four steps additional to those listed in Section C5.1.1. Determine the characteristic values of the total potential sulfate content for the site location from a consideration of the results of several tests on the pyritic ground. In a data set where five to nine results are available for the location, the mean of the two highest TPS values should be taken as the characteristic value (rounded to 0.1% SO4). In a data set where 10 or more TPS results are available, the mean of the highest 20% should be taken as the characteristic value.

Compare the sulfate class for total potential sulfate with the sulfate classes determined (in Section C5.1.1) for groundwater and water extract tests on soil. The highest of these sulfate classes should then be taken as the Design Sulfate Class for the site location. A limitation can be applied if the sulfate class for the total potential sulfate is initially found to be Sulfate Class 5, but sulfate classes for groundwater and the water extracts tests are Sulfate Class 3 or less. In this case, the Design Sulfate Class for the site location can be limited to DS-4.

Step 8

Step 6

Determine the sulfate class equivalent to the characteristic value of the total potential sulfate content using columns 1 and 6 of Table C2 on the next page.

The reason for this limitation is that the procedure for sulfate classification based on total potential sulfate is often highly conservative as not all the pyrite in soil will be oxidised and only a part will be taken into solution by groundwater. Some reliance is placed therefore on the findings of field studies of disturbed pyritic clay that has undergone oxidation. These have shown a maximum sulfate class for groundwater in pyritic clay subject to prolonged oxidation to be Sulfate Class 4.

Step 7

Determine the ACEC Class of the ground from the row of Table C1 that correlates first with the Design Sulfate Class, second with the water conditions, and third with the characteristic value of pH.

Step 9

32

Part C

Table C2 Aggressive Chemical Environment for Concrete (ACEC) classification for brownfield locations a Sulfate and magnesium Design Sulfate

Groundwater

2:1 water/soil extract b

Groundwater

Class for location 1

DS-1

DS-2

ACEC

Total potential Static

Mobile

Class for

sulfate c

water

location 9

water

2

3

4

5

6

7

8

(SO4 mg/ l)

(Mg mg/ l)

(SO4 mg/ l)

(Mg mg/ l)

(SO4 %)

(pH) d

(pH) d

< 0.24

≥ 2.5

< 500

< 400

500–1500

400–1400

0.24–0.6

AC-1s > 6.5 d

AC-1

5.5–6.5

AC-2z

4.5–5.5

AC-3z

2.5–4.5

AC-4z

> 5.5

AC-1s > 6.5

AC-2

5.5–6.5

AC-3z

4.5–5.5

AC-4z

2.5–5.5

AC-5z

> 6.5

AC-3

2.5–5.5

DS-3

1600–3000

1500–3000

0.7–1.2

AC-2s

> 5.5

AC-2s

2.5–5.5

DS-4

3100–6000

≤ 1200

3100–6000

≤ 1000

1.3–2.4

AC-3s 5.5–6.5

AC-4

2.5–5.5

AC-5

> 6.5

AC-4

2.5–6.5

AC-5

> 6.5

AC-4m

> 5.5

AC-3s

2.5–5.5 DS-4m

3100–6000

> 1200 e

3100–6000

> 1000 e

1.3–2.4

AC-4s

> 5.5

AC-3s

2.5–5.5 DS-5

> 6000

≤ 1200

> 6000

≤ 1000

> 2.4

> 6000

> 1200 e

> 6000

> 1000 e

> 2.4

AC-5m

≥ 2.5

AC-5

≥ 2.5

AC-5m

> 5.5 2.5–5.5

DS-5m

AC-4ms 2.5–6.5

AC-4s

> 5.5 2.5–5.5

AC-4ms

Notes a Brownfield locations are those sites, or parts of sites, that might contain chemical residues produced by or associated with industrial production (Section C5.1.3). b The limits of Design Sulfate Classes based on 2:1 water/soil extracts have been lowered from previous Digests (Box C7). c Applies only to locations where concrete will be exposed to sulfate ions (SO4), which may result from the oxidation of sulfides such as pyrite, following ground disturbance (Appendix A1 and Box C8). d An additional account is taken of hydrochloric and nitric acids by adjustment to sulfate content (Section C5.1.3). e The limit on water-soluble magnesium does not apply to brackish groundwater (chloride content between 12 000 mg/l and 17 000 mg/l). This allows ‘m’ to be omitted from the relevant ACEC classification. Seawater (chloride content about 18 000 mg/l ) and stronger brines are not covered by this table. Explanation of suffix symbols to ACEC Class ● Suffix ‘s’ indicates that the water has been classified as static. ● Concrete placed in ACEC Classes that include the suffix ‘z’ have primarily to resist acid conditions and may be made with any of the cements in Table D2 on page 42. ● Suffix ‘m’ relates to the higher levels of magnesium in Design Sulfate Classes 4 and 5.

Assessing the aggressive chemical environment C5.1.3 Brownfield locations except those containing pyrite The following points should be noted. ● A brownfield location is defined as a site or part of a site that has been subject to industrial development, storage of chemicals (including for agricultural use) or deposition of waste, and which may contain aggressive chemicals in residual surface materials or in ground penetrated by leachates. Where the history of a site is not known, it should be treated as a brownfield site until there is evidence to classify it as natural. ● The type of chemicals present on a brownfield site and their concentration, though of significance to concrete specification, may otherwise be benign and therefore not pose a risk of harm to living things (ie meriting the designation ‘contaminated land’[5 ]). A common example is the occurrence of low-to-moderate concentrations of sulfate which may be harmful to concrete, but not to organisms. ● This Special Digest does not seek to cover use of concrete in land heavily affected by contamination; for instance, the lower bound for guidance in respect of ground acidity is pH 2.5. ● This section is for sites or site locations that are known, from the desk study or a preliminary ground investigation, not to contain pyrite in any strata that may be used for construction. ● Several locations on a site may need to be separately classified for chemical agents aggressive to concrete if the site is extensive or the ground conditions are complex. The initial stages for assessment of chemical aggressiveness (Figure C6) are broadly similar to the procedure given in Steps 1 and 2 of Section C5.1.1. However, when the sulfate concentration in either the water extract or the groundwater is greater than 3000 mg/l, an additional consideration of the level of magnesium is required. The additional procedures for magnesium when applicable are: Determine the characteristic values for magnesium concentration in tests on: ● soil samples ● groundwater samples. Assess variable data in the same way as advised for sulfate.

Step 1a

Determine the ‘basic’ sulfate class for the site location using columns 1, 4 and 5 of Table C2 for groundwater, and columns 1, 2 and 3 for soil.

Step 2a

33 For ground suspected of containing mineral acids of industrial origin, an additional procedure, Step 10, is recommended prior to taking account of the mobility of the groundwater. The pH of the samples should first be considered (Figure C6). If a significant number of these are lower than pH 5.5, the amounts of chloride and nitrate (NO3) should also be determined (in mg/l) in addition to sulfate content. Substantial presence of chloride and nitrate ions on a brownfield location indicates that hydrochloric acid (HCl) and nitric acid (HNO3) respectively may be present in the ground. The effect of these acids on concrete is likely to be similar to that of sulfuric acid (Section B2.2); so, for classification purposes, their chemically equivalent sulfate concentration should be calculated and added to any actual soluble sulfates present (as SO4 mg/l) in the respective samples: SO4 equivalent of Cl = Cl x 1.35 mg/l SO4 equivalent of NO3 = NO3 x 0.77 mg/l.

Step 10

This procedure should not be used to assess the susceptibility of steel reinforcement in concrete to corrosion. No adjustment for chloride is necessary where the presence of chloride ions is from brackish water or seawater. Adjusted characteristic values of sulfate may be derived and, from these, adjusted sulfate classes for soil and groundwater be obtained. The Design Sulfate Class for the locality may be taken as the highest of these adjusted sulfate classes. The ACEC Class of the ground can then be found from the row of Table C2 that correlates first with the Design Sulfate Class, second with the water conditions, and third with the characteristic value of pH. C5.1.4 Brownfield locations containing pyrite If the desk study indicates that there is a possibility that the ground materials on the brownfield location have oxidisable sulfides (eg pyrite in unburnt colliery spoil), the additional procedures given in Steps 6 to 8 of Section C5.1.2, and Steps 1a and 2a in Section C5.1.3, should all be carried out. The Design Sulfate Class for the location should then be taken as the highest of the sulfate classes derived by the differing procedures. The ACEC Class of the ground can then be found from the row of Table C2 that correlates first with the Design Sulfate Class, second with the water conditions, and third with the characteristic value of pH.

34 Box C9 Incremental rules that have been used in Tables C1 and C2 to adjust the ACEC Class of a site for the type of ground, water mobility and pH in each Design Sulfate Class Natural ground with static groundwater ● For Design Sulfate Classes 2, 3 and 4, the ACEC Class has been decreased by 1 ● For pH > 3.5 there is no further change ● For pH < 3.5 the ACEC Class has been increased by 1 ● The suffix ‘s’ has been added. Natural ground with mobile groundwater ● For pH > 5.5 there is no change ● For pH < 5.5 the ACEC Class has been increased by 1 and suffix ‘z’ has been added. Brownfield ground with static groundwater ● For Design Sulfate Classes 2, 3 and 4, the ACEC Class has been decreased by 1 ● For pH > 5.5 there is no change ● For pH < 5.5 the ACEC Class has been decreased by 1 ● The suffix ‘s’ has been added. Brownfield ground with mobile groundwater ● For pH > 6.5 there is no change to the ACEC Class ● For pH 5.5–6.5 the ACEC Class has been increased by 1 and suffix ‘z’ has been added ● For pH 4.5–5.5 the ACEC Class has been increased by 2 and suffix ‘z’ has been added ● For pH < 4.5 the ACEC Class has been increased by 3 and suffix ‘z’ has been added. For sites with static water, two adjustments will often apply and these may cancel out.

Part C C5.2 Aggressive Chemical Environment for Concrete (ACEC) classification

The Aggressive Chemical Environment for Concrete (ACEC) classification is set out in Tables C1 for natural ground locations and C2 for brownfield locations. The process of ACEC classification of a location starts with classifying the ground into one of five Design Sulfate Classes. The route through this sulfate classification (Section C5.1 and Figures C4 to C6) depends on the type of ground location and presence or absence of substances including magnesium ions, pyrite and, for pH less than 5.5, chloride and nitrate ions. Having established the appropriate Design Sulfate Class, modifications are applied which relate to the mobility and pH of groundwater. Mobile water (Section C3.2) and low pH (Section C2.2) are both adverse ground conditions that lead to the designation of a more severe ACEC Class. Static water (Section C3.1) is a more benign condition that allows for a less severe ACEC Class. An overview of the procedure to determine and apply the ACEC classification is set out in Figure C1; detailed steps for the three main categories of site location are given in Figures C4, C5 and C6. The incremental rules that have been used in Tables C1 and C2 to adjust the ACEC Class of a site for the type of ground, water mobility and pH in each Design Sulfate Class are explained in Box C9.

Assessing the aggressive chemical environment Box C10 Recommended source documents and test methods Chemical Symbol Recommended

Soil

determinations

(unit)

pH in 2.5:1

pH

water/soil extract Soluble sulfate in 2:1

WS (mg/l SO4)

35 Recommended test methods

source documents BR 279

Electrometric method

BS 1377-3, Section 9

Electrometric method

BR 279

Procedures for gravimetric method, cation

water/soil extract

exchange, or ion chromatography BS 1377-3, Section 5

Gravimetric or ion exchange methods (Values determined as mg/l SO3 should be multiplied by 1.2)

TRL Report 447, Test 1

Sulfate extraction procedure as BS 1377-1, but ICP-AES used to determine sulfur in solution

Acid-soluble sulfate

AS (% SO4)

BR 279 BS 1377-3, Section 5

Gravimetric method Gravimetric methods (Values determined as mg/l SO3 should be multiplied by 1.2)

TRL Report 447, Test 2

Preparation and extraction of sulfate as BS 1377-3, ICP-AES used to determine sulfur in solution

Total sulfur

TS (% S)

BR 279

‘Ignition in oxygen’ method (eg with sulfur–carbon determinator)

TRL Report 447, Test 4A TRL Report 447, Test 4B

Microwave digestion method Ignition in oxygen method (eg with sulfur–carbon determinator)

Magnesium in 2:1

WMg (mg/l Mg)

BR 279

water/soil extract

Atomic absorption spectrometry (AAS) method

Commercial test lab

Sample preparation as BR 279; ICP-AES used

in-house procedure

to determine magnesium in solution

Ammonium ion

(mg/l NH4+)

BR 279

Nitrate in 2:1

(mg/l NO3)

BR 279

water/soil extract Chloride in 2:1

(mg/l Cl)

water/soil extract Groundwater

pH Soluble sulfate

BR 279 BS 1377-3, Section 7

pH GWS (mg/l SO4)

BR 279

Electrometric method

BS 1377-3, Section 9

Electrometric method

BR 279

Procedures for gravimetric method, cation exchange, or ion chromatography

BS 1377-3, Section 5

Gravimetric or ion exchange methods (Values determined as mg/l SO3 should be multiplied by 1.2)

Commercial test lab

Determination of sulfur by ICP-AES

in-house procedure Soluble magnesium

GWMg (mg/l Mg)

BR 279

Atomic absorption spectrometry (AAS) method

Commercial test lab

Determination of magnesium in solution by

in-house procedure

ICP-AES

Ammonium ion

(mg/l NH4+)

Nitrate ion

(mg/l NO3–)

BR 279

Chloride ion

(mg/l Cl –)

BR 279

BR 279

BS 1377-3, Section 7 Aggressive carbon dioxide

(mg/l CO2)

prEN 13577

36 Appendix C1 Recommended test procedures for ground aggressive to concrete UK test methods for the chemical analyses of aggressive soil and groundwater have traditionally been documented in BS 1377-3. Because this Standard did not cover some tests needed for ground investigation in respect of concrete, it was supplemented in 1995 by procedures detailed in BRE Report BR 279. This updated some test procedures covered by BS 1377-1 to include modern techniques, such as determination of sulfate in aqueous solutions by cation exchange and ion chromatography. Currently, however, both these documents are in need of further revision to accord, for example, with the latest widespread practice of determining elements in solution by using inductively coupled plasma atomic emission spectroscopy (ICP-AES) rather than gravimetric analysis. The larger test laboratories currently follow in-house procedures using this latter approach rather than BS 1377-1 and BR 279. The list of recommended test methods in Box C10 on page 35 is based on recent BRE experience and the outcome of a review of test procedures for determining sulfur species reported in TRL Report 447 [13]. This list is not exclusive, however, and other methods may be used provided they can be demonstrated to be appropriate. In respect of total sulfur determination for detection of pyrite, the preferred method is high temperature combustion of a dried and ground specimen (< 150 µm) in an appropriate instrument (eg a modern carbon–sulfur determinator). All sulfur species present are evolved as sulfur dioxide that is quantified by infrared detectors. The procedure is described in BR 279 under the ‘ignition in oxygen’ method. It is rapid (taking only a few minutes) and relatively low cost when carried out on numerous samples. Generally the procedure has been found to have an accuracy of the order of 1% provided an appropriate test procedure is used. This should include use of an appropriate accelerator (eg tungsten trioxide or vanadium pentoxide), efficient trapping of any water vapour evolved, and regular calibration using standard materials, including pyrite (Box C11).

Box C11 Warning Calculating total sulfur in specimens containing pyrite by the procedure given in Clause B.2 of BS 1047, as advocated by BS 1377-1, is not considered appropriate. It is understood that this procedure was aimed at determining the amount of complex monosulfides found in blastfurnace slag. Pyrite (FeS2) is a divalent sulfide that is typically chemically robust and is apparently not so readily dissolved by applied nitric and hydrochloric acids. Instances have been known where the total sulfur content of pyrite bearing clays was under-measured by some 50% when using the BS 1047 procedure.

Part C Appendix C2 Guidance on comprehensive site investigation of sulfate ground While the scope and procedures detailed in Sections C4 and C5 should be appropriate for site investigation leading to routine specification of durable concrete in the UK, it does not provide the full understanding of sulfate ground conditions that may be required for other applications or investigation of cases of sulfate attack on buried concrete. The principal lack in the given standard procedures is a separate determination of the likely constituent sulfates: those of calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K) metals. Knowledge of these can be important bearing in mind that magnesium, sodium and potassium sulfates are potentially more problematic than calcium sulfate owing to their high solubilities and different chemical activities, particularly in the case of magnesium sulfate which is notably aggressive to some types of concrete. If a better understanding is required, a comprehensive suite of soil and groundwater analyses can be carried out to include determining calcium, magnesium, sodium and potassium ions. Procedures for these are given in BR 279. Analysis can also be carried out by ICP-AES. Where cases of sulfate attack are being investigated and groundwater samples can be obtained, it is additionally recommended that carbonate (or bicarbonate), chloride and nitrate contents are determined. The first will provide data on a possible source of external carbonates that can fuel TSA in the absence of carbonate aggregates, while the inclusion of all three will enable an ion balance check to be made to provide an assurance that the principal constituents of the groundwater have been accurately determined. Such an ion balance check will require: (a) the quantities of the determined ions (mg/l) to be divided by their atomic weights to find their relative numbers in solution (b) the relative numbers of ions to be multiplied by their valencies as indicated in the following table: Ion Multiplier

(c)

SO42–

Cl–

–2

–1

NO3– CO32– Ca2+ Mg2+ –1

–2

+2

+1

Na+

K+

+1

+1

the algebraic sum of the result of (b) to be expressed as a percentage by dividing by the total number of ions.

Any significant out-of-balance that can not be accounted for by analytical errors may indicate the presence of some other ion which needs to be identified.

Assessing the aggressive chemical environment References: Part C [1] Department of Environment, Transport and the Regions. The thaumasite form of sulfate attack. Risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group. London, DETR, 1999. [2] Longworth T I. Contribution of construction activity to aggressive ground conditions causing the thaumasite form of sulfate attack to concrete in pyritic ground. Cement and Concrete Composites. 25 (8) 1005–1013*. [3] Higgins D D, Thomas D and Kinuthia J. Pyrite oxidation, expansion of stabilised clay and the effect of ggbs. Fourth European Symposium on the Performance of Bituminous and Hydraulic Materials in Pavements. University of Nottingham, April 2002. [4] Cripps J C and Edwards R L. Ground chemistry implications for construction (Edr: A B Hawkins). Paper 2-2: Some geotechnical problems associated with pyrite bearing mudrocks. Rotterdam, Balkema, 1997 (pp 77–88). [5] Environment Agency. Model procedures for the management of land contamination. Contaminated Land Report 11. Bristol, Environment Agency, 2004. [6] CIRIA. Remedial treatment of contaminated land. Vol III: Site investigation and assessment. CIRIA Special Publication 103. London, CIRIA, 1995. [7] Environment Agency. Guidance for safe development of housing on land affected by contamination. R & D Publication 66. Bristol, Environment Agency, 2000. [8] Environment Agency. Secondary model procedures for the development of appropriate soil sampling strategies for land contamination. R&D Technical Report P5-066/TR. Bristol, Environment Agency, 2000. [9] Environment Agency. Risks of contaminated land to buildings, building materials and services. A literature review. R&D Technical Report P331. Swindon, Environment Agency, 2000. [10] Environment Agency. Assessment of and management of risks to buildings, building materials and services from land contamination. A literature review. R&D Technical Report P5-035/TR/01. Swindon, Environment Agency, 2001. [11] Perry J and West G. Sources of information for site investigations in Britain. Transport Research Laboratory Report 192. Crowthorne, TRL, 1996. [12] Highways Agency et al. Design manual for roads and bridges. Treatment of fill and capping materials using either lime or cement or both. HA Report 74/00. London, HA, 2000. [13] Reid J M, Czerewko M A and Cripps J C. Sulfate specification for structural backfills. Transport Research Laboratory Report 447. Crowthorne, TRL, 2001. * Also published as a paper in Proceedings of First International Conference on Thaumasite in Cementitious Materials, BRE, June 2002. Garston, BRE Bookshop, 2002.

37 BRE Reports Paul V. Performance of building materials in contaminated land. BRE Report BR 255. Garston, BRE Bookshop, 1994. Bowley M J. Sulfate and acid attack on concrete in the ground: recommended procedures for soil analysis. BRE Report BR 279. Garston, BRE Bookshop, 1995. Charles J A, Chown R C, Watts K S and Fordyce G. Brownfield sites: ground related risks for building. BRE Report BR 447. Garston, BRE Bookshop, 2002.

BRE Digests 318 348 381 383 411 472

Site investigation for low-rise building: desk studies Site investigation for low-rise building: the walk-over survey Site investigation for low-rise building: trial pits Site investigation for low-rise building: soil description Site investigation for low-rise building: direct investigations Optimising ground investigation

British Standards Institution BS 1047:1983 Specification for air-cooled blast furnace slag aggregate for use in construction BS 1377-1:1990 Methods of test for soils for civil engineering purposes. General requirements and sample preparation BS 1377-3:1990 Methods of test for soils for civil engineering purposes. Chemical and electro-chemical tests BS 5930:1999 Code of practice for site investigations BS 8004:1986 Code of practice for foundations BS 10175:2001 Investigation of potentially contaminated sites. Code of practice BS EN 206-1:2000 Concrete. Specification, performance, production and conformity prEN 13577:1999 Water quality. Determination of aggressive carbon dioxide content

38

Part D

Specifying concrete for general cast-in-situ use

D1 Introduction This part provides guidance on concrete quality and any additional protective measures (APMs) required to provide resistance to chemical attack. It caters primarily for the general use of cast-in-situ concrete, but additionally will cover any precast concrete that does not meet the qualifying carbonation conditions that apply to precast concrete in Parts E and F. The starting point is the ACEC Class of the ground, derived in Part C, plus some knowledge of the type, use and geometry of the concrete element and the ground conditions to which it will be subject. Some important changes have been made to the previously published guidance. These are explained in Section D2. The overall design process is summarised in Section D3. Sections D4 to D8 give the detailed guidance.

D2 Changes since SD1: 2003 Some important changes to the way concrete quality is specified are made in this edition of Special Digest 1. These stem from a further study of occurrences of sulfate attack in concrete structures, and recent field and laboratory research (Section A3). The key changes are as follows. ● The concrete quality recommended now takes account of the possibility of an external source of carbonate. Recent research has shown that there is often sufficient bicarbonate ((HCO3)2) in groundwater to result in the thaumasite form of sulfate attack (TSA) when sulfate levels are high and the temperature low (Section B2.1.3). ● The concept of ‘aggregate carbonate range’ is no longer included. Since the concrete quality takes account of a possible external source of carbonate, it also inherently caters for an internal source from carbonate in aggregates and consequently aggregate carbonate range is redundant.

● Starred Design Chemical Classes (range B aggregates) and double-starred DC Classes (range C aggregates) are no longer valid and not included. The concept for these was dependent on the now redundant aggregate carbonate range. ● Changes have been made in the recommended maximum w/c ratio and minimum cement/combination content. These stem from the new research on the quality of concrete necessary to resist sulfate attack, including TSA. ● Changes have been made in the presentation of classification of cements/combinations. However, the basic ranking with respect to performance in sulfatebearing ground is mostly unchanged. ● The number of APMs to be applied at higher sulfate levels has been reduced, in general, by two. This follows from a higher level of confidence in the provisions for the concrete. ● The use of the concept ‘intended working life’ replaces that of ‘structural performance level’. This is for harmony with European Standards such as BS EN 206-1. ● Section width is no longer taken as a principal factor when finding a DC Class to cater for assessed ACEC conditions. Instead, footnotes call for adjustments to be made for section widths of less than 140 mm and greater than 450 mm in particular circumstances. ● No relaxation is made in respect of surface carbonation in the general use of cast-in-situ concrete. This benefit was difficult to ensure in practical conditions.

Specifying concrete for general cast-in-situ use D3 Design process The overall process of design of concrete for use in aggressive ground conditions is summarised in Figure A1 of Part A. Part D deals with Stages 3 and 4 of the overall process. Further detail is given diagrammatically in Figure D1.

39 Designer of building or structure

For each ACEC Class determined in Part C, concrete quality is specified (in Table D1 on page 41) in terms of a DC Class, taking account of intended working life, section thickness and the hydraulic gradient to which it may be subjected by groundwater.

Firstly, from a consideration of the intended structure, determine parameters. ● ACEC Class of ground from Table C1 ● Intended working life of concrete element (categories in Table D1) ● Thickness of concrete element (categories in Table D1, Notes b and c) ● Hydrostatic conditions for concrete element (Table D1, Note a)

From Table D1, determine the appropriate DC Class of concrete. ● For the assessed ACEC Class, look in the column corresponding to the required intended working life, taking account of Notes d and e ● Adjust DC Class or number of APMs up or down to take account of: ● thickness of concrete section (Notes b and c) ● hydraulic gradient due to groundwater if this exceeds 5 (Note a)

Each DC Class is prescribed in Table D2 on page 42 and follows the previous practice of defining concrete quality for each cement or combination group respectively in terms of: ● maximum free-water/cement ratio, or freewater/combination ratio ● minimum cement content or combination content. In some cases, additional protective measures (APMs) are recommended in Table D1 to further protect the concrete. The number of APMs needed increases both with higher ACEC Class of the ground and with longer intended working life required for the concrete element. The various APM options are listed in Table D4 on page 44 and discussed in Section D6.

From Table D1, find requirements for additional protective measures (APMs). ● Determine the number required ● Note any restrictions as to choice (eg instruction to use APM3)

From Table D4, guided by Section D6, select appropriate options for APM, taking account of any restrictions and engineering practicalities

Include in contract documents: ● Design Sulfate Class of ground ● ACEC Class of ground ● any estimated hydraulic gradients due to groundwater for which DC Class or number of APMs have been adjusted ● specified DC Class after optional adjustment or enhancement ● specified number of APMs after adjustment ● any restrictions or preferences in respect of APMs to be used ● any other design requirements for each concrete element Contractor and concrete producer for building or structure

Obtain from the contract documents: ● the specified DC Class ● the number and type of APM ● any other design requirements for each concrete element Formulate the concrete mix design for the element, using Table D2 to achieve the specified DC Class. Other factors will include strength class of concrete, the consistence, the availability and cost of materials, and any other contract requirements

Figure D1 Specification of concrete for general cast-in-situ use

40 D4 Selection of the DC Class and APMs D4.1 Background

Design Chemical Classes were introduced in Digest SD1:2001 as a new way of defining the ‘qualities’ of concrete that are required to resist chemical attack. Section D4 deals with the derivation (Table D1) of the DC Class from the ACEC Class of the ground (Table C1), taking into account a number of factors including the type of concrete element, its mode of exposure to the aggressive ground and the required durability. The options for limiting values of concrete required to satisfy the various DC Classes are discussed in Section D5. D4.2 Key factors

Part D ● When the hydraulic gradient is more than 5, a more cautious design is required (Table D1, Note ‘a’). Either the DC Class should be increased by one ‘step’ or an extra APM should be employed. An exception may be made where APM3 has already been selected for application, either as a mandatory measure for AC-5 level conditions or as a first ‘APM of choice’ ● Adjustments to the given ACEC Class / DC Class / APM correlations also apply when the section thickness is 140 mm or less or when it is greater than 450 mm. A more cautious design is required. The recommended approach is similar to that for high hydraulic gradient (Table D1, Note ‘b’).

140 mm or less

The key factors in using Table D1 are as follows. A relaxation of one step in the DC Class may be applied in some circumstances. Since such a relaxation implies some degree of chemical attack is acceptable, it will not be appropriate, where concrete surfaces must retain their integrity, to provide frictional resistance against the ground, as in friction piles and the bases of L-section retaining walls. For reinforced concrete, the cover should be sufficiently thick to allow for estimated surface degradation during the intended working life. Section D6.5 gives guidance on what allowance to make for surface degradation in the parallel context of providing sacrificial concrete as an APM. Greater than 450 mm

● Recommendations for concrete specification in terms of DC Class for each of the ACEC Classes are given for two categories of intended working life in Table D1. There is an obvious parity in the ACEC Class / DC Class correlations except at the AC-5 level where the DC-4 family are recommended, as no DC-5 Classes are defined. To compensate for the lack of DC-5 Classes, it is recommended that, wherever practical, APM3 (‘Provide surface protection’, page 45) should be applied to the concrete. ● APMs are also recommended in Table D1 for some other cases where a working life of ‘at least 100 years’ is required for concrete subjected to high sulfate allied to mobile groundwater conditions (ie where the ACEC Class does not have an ‘s’ suffix that indicates static groundwater). Here, any APM option of the five listed in Table D4 may be chosen provided the application advice given in Section D6 is followed. ● The given ACEC Class / DC Class / APM correlations in Table D1 apply where the hydraulic gradient across the concrete element is not greater than 5. The hydraulic gradient (difference in hydrostatic head in metres divided by section thickness in metres) will normally need to be estimated from a consideration of the likely postconstruction water levels on either side of the element. Applications of concrete that may give rise to a hydraulic gradient include ground retaining structures, and basement walls and tanks within the ground.

● The DC Classes carry the suffix ‘m’ or ‘z’ where these were part of the corresponding ACEC Class designations. Suffix notation ‘z’ indicates concretes that primarily must resist acid conditions, and ‘m’ indicates concretes that must resist high levels of magnesium sulfate. Note should be taken of these when specifying concrete composition (Section D5).

Specifying concrete for general cast-in-situ use

41

Table D1 Selection of the DC Class and the number of APMs for concrete elements where the hydraulic gradient due to groundwater is 5 or less: for general in-situ use of concrete a,b,c ACEC Class Intended working life At least 100 years (from Tables C1 and C2) At least 50 yearsd,e AC-1s, AC-1

DC-1

DC-1

AC-2s, AC-2

DC-2

DC-2

AC-2z

DC-2z

DC-2z

AC-3s

DC-3

DC-3

AC-3z

DC-3z

DC-3z

AC-3

DC-3

DC-3 + one APM of choice

AC-4s

DC-4

DC-4

AC-4z

DC-4z

DC-4z

AC-4

DC-4

DC-4 + one APM of choice

AC-4ms

DC-4m

DC-4m

AC-4m

DC-4m

DC-4m + one APM of choice

AC-5z

DC-4z + APM3 f

DC-4z + APM3 f

AC-5

DC-4 + APM3 f

AC-5m

DC-4m + APM3

DC-4 + APM3 f f

DC-4m + APM3 f

For specification of DC Class, see Table D2. For choice of additional protective measures, see Table D4. Notes a Where the hydraulic gradient across a concrete element is greater than 5, one step in DC Class or one APM over and above the number indicated in this table should be applied except where the original provisions included APM3. Where APM3 is already required, or has been selected, an extra APM is not needed. b A section thickness of 140 mm or less should be avoided in in-situ construction but, where this is not practical, apply one step higher DC Class or an extra APM except where the original provisions included APM3. Where APM3 is already required, or has been selected, an extra APM is not necessary. c Where a section thickness greater than 450 mm is used and some surface chemical attack is acceptable, a relaxation of one step in DC Class may be applied. For reinforced concrete, the cover should be sufficiently thick to allow for estimated surface degradation during the intended working life (Section D6.5). d Foundations of low-rise housing that have an intended working life of at least 100 years may be constructed with concrete selected from the column headed ‘At least 50 years’ (Section D7). e Structures with an intended working life of at least 50 years but for which the consequences of failure would be relatively serious, should be classed as having an intended working life of at least 100 years for the selection of the DC Class and APM (Section D7). f Where APM3 is not practical, see Section D6.1 for guidance. Explanation of suffix symbols to DC Class ● Concrete placed in ACEC Classes that include the suffix ‘z’ primarily must resist acid conditions and may be made with any of the cements listed in Table D2. ● Suffix ‘m’ relates to the higher levels of magnesium in DS Classes 4 and 5.

D5 Composition of concrete to resist chemical attack D5.1 Background

The main compositional factors that determine the resistance of concrete to aggressive ground are its water/cement ratio and type of cement or combination used. In the previous edition of this Special Digest, the importance of carbonate in the aggregates was stressed in relation to TSA. A source of carbonate is still considered essential for occurrence of TSA, but recent research (Section A3) has shown that sufficient carbonate can come from bicarbonate in groundwater. As a consequence, the limiting values of concrete composition make no distinction between aggregates of different carbonate contents. Recent research has also shown that resistance to sulfate attack is not a function of cement content. Concretes made with the same materials, and the same w/c ratio but different cement/combination contents, have similar sulfate resistance providing there is sufficient fine material to give a closed microstructure. However, as there is not yet any agreed method for verifying that the concrete has a closed structure, this Special Digest continues to recommend a minimum cement/combination content.

A compressive strength requirement has never formed part of BRE recommendations for sulfate resistance. However, it is recognised that the specification may need to contain a compressive strength class requirement for structural and durability purposes. Considerable recent research (Section A3) has been focused on determining what is an adequate concrete specification and performance of different cement types. The findings of this research are incorporated into the recommendations given in Table D2. The principal changes as compared with SD1:2003 are: ● the requirements for concrete made with aggregates having a medium or low carbonate content (former aggregate carbonate ranges B or C) have been increased to those given previously for concrete made with aggregates having a high carbonate content (former range A aggregates) ● the excellent performance of concrete incorporating ground granulated blastfurnace slag (ggbs) cements has been recognised and there is some relaxation of the requirements with these cements ● the mixed performance of concrete made with sulfateresisting Portland cement (SRPC) in sulfate conditions conducive to TSA has led to tightening of requirements

42

Part D

● the performance of concrete incorporating pulverized fuel ash (pfa), and fly ash cements and combinations, is still under investigation and so a conservative approach to their use is taken. The effectiveness of concretes to resist chemical attack depends to a high degree on their impermeability. Therefore, good compaction is most important. With low w/c ratios, such as those advocated here, it is probable that water reducing admixtures will be needed to achieve effective compaction. This is particularly true of concretes (eg for piling) where mechanical compaction cannot be used. D5.2 Using Table D2

For a given DC Class, specifications for concrete are shown in Table D2 in terms of maximum free-water/cement or combination ratio and minimum cement or combination content for standard aggregate sizes, and recommended types of cement or combination. The cements and combinations are in new groups, designated A to G, that are defined in Table D3. Table D2 provides a wide range of options for concrete at most DC Class levels so that, in most cases, the concrete producer can use a cement or combination which he normally has in stock.

D5.3 Cement and combination types

D5.3.1 Recommendations in Tables D2 and D3 The cements and combinations specifically recommended by this Special Digest for use in aggressive ground are listed as groups A to G in Tables D2 and D3. The groups are defined in Table D3 mainly in terms of resistance to sulfate attack. The designations used are based on those of BS EN 197-1 for cement and BS 8500 for combinations. A suffix ‘+SR’ has been added to the designations where a restriction on some element of the composition is necessary for sulfate resistance. Cements and combinations of the same composition are treated as being directly equivalent and are always grouped together. Moreover, different types (eg CEM II/B-V+SR, a fly ash cement, and CEM III/A+SR, a blastfurnace cement) that show closely similar resistance to sulfate attack are placed in the same Group – in this case, Group D. While the grouping and nomenclature differ between Table D3 and SD1:2003, in most cases the requirements of cements and combinations with respect to enhanced sulfate resistance remain unchanged.

Table D2 Concrete qualities to resist chemical attack for the general use of in-situ concrete: limiting values for composition DC Class Maximum Minimum cement or combination content (kg/m3) Recommended cement and free-water/cement

for maximum aggregate size of:

or combination ratio

≥ 40 mm

20 mm

14 mm

10 mm

combination group

DC-1

–

–

–

–

–

DC-2

0.55

300

320

340

360

D, E, F

0.50

320

340

360

380

A, G

A to G inclusive

0.45

340

360

380

380

B

0.40

360

380

380

380

C

DC-2z

0.55

300

320

340

360

A to G inclusive

DC-3

0.50

320

340

360

380

F

0.45

340

360

380

380

E

0.40

360

380

380

380

D, G

DC-3z

0.50

320

340

360

380

A to G inclusive

DC-4

0.45

340

360

380

380

F

0.40

360

380

380

380

E

0.35

380

380

380

380

D, G

DC-4z

0.45

340

360

380

380

A to G inclusive

DC-4m

0.45

340

360

380

380

F

Grouped cements and combinations A B

Cements

Combinations

CEM I, CEM II/A-D, CEM II/A-Q, CEM II/A-S, CEM II/B-S, CEM II/A-V,

CIIA-V, CIIB-V, CII-S, CIIIA, CIIIB, CIIA-D,

CEM II/B-V, CEM III/A, CEM III/B

CIIA-Q

CEM II/A-La, CEM II/A-LLa

CIIA-La, CIIA-LLa

a

a

CIIA-La, CIIA-LLa

C

CEM II/A-L , CEM II/A-LL

D

CEM II/B-V+SR, CEM III/A+SR

CIIB-V+SR, CIIIA+SR

E

CEM IV/B (V), VLH IV/B (V)

CIVB-V

F

CEM III/B+SR

CIIIB+SR

G

SRPC

–

For cement and combination types, compositional restrictions and relevant Standards, see Table D3. Note a The classification is B if the cement/combination strength class is 42,5 or higher and C if it is 32,5.

Specifying concrete for general cast-in-situ use

43

Table D3 Cements and combinations for use in Table D2 Type

Designation

Standard

Grouping with respect to sulfate resistance

Portland cement

CEM I

BS EN 197-1

Portland-silica fume cement

CEM II/A-D

BS EN 197-1

Portland-limestone cement

A A

BS EN 197-1

B or C a

CEM II/A-LL

BS EN 197-1

B a or C a

b

BS EN 197-1

A

CEM II/A-L

a

Portland-pozzolana cement

CEM II/A-Q

Portland-slag cements

CEM II/A-S

BS EN 197-1

A

CEM II/B-S

BS EN 197-1

A

Portland-fly ash cements–

Blastfurnace cements

e

CEM II/A-V

BS EN 197-1

A

CEM II/B-V c

BS EN 197-1

A

CEM II/B-V+SR d

BS EN 197-1

D

CEM III/A

BS EN 197-1

A

BS EN 197-4

A

CEM III/A+SR f

BS EN 197-1

D

BS EN 197-4

D

CEM III/B

BS EN 197-1

A

BS EN 197-4

A

BS EN 197-1

F

BS EN 197-4

F E

CEM III/B+SR f g,h

CEM IV/B (V)

BS EN 197-1

Very low heat pozzolanic cement

VLH IV/B (V)

BS EN 14216

E

Sulfate-resisting Portland cement

SRPC

BS 4027

G

CIIA-V

BS 8500-2, Annex A

A

BS 8500-2, Annex A

A

Pozzolanic cement

Combinations conforming to BS 8500-2, Annex A, manufactured in the concrete mixer from Portland cement and fly ash, pfa, ggbs or limestone fines: CEM I cement conforming to BS EN 197-1 with a mass fraction of 6 to 20 % of combination of fly ash conforming to BS EN 450 or pfa conforming to BS 3892-1 CEM I cement conforming to BS EN 197-1 with a mass fraction of 21 to 35 %

CIIB-V c d

of combination of fly ash conforming to BS EN 450 or pfa conforming to BS 3892-1

CIIB-V+SR

BS 8500-2, Annex A

D

CEM I cement conforming to BS EN 197-1 with a mass fraction of 36 to 55 %

CIVB-V

BS 8500-2, Annex A

E

CII-S

BS 8500-2, Annex A

A

CEM I cement conforming to BS EN 197-1 with a mass fraction of 36 to 65 %

CIIIA

BS 8500-2, Annex A

A

of combination of ggbs conforming to BS 6699

CIIIA+SR f

BS 8500-2, Annex A

D

CEM I cement conforming to BS EN 197-1 with a mass fraction of 66 to 80 %

CIIIB

BS 8500-2, Annex A

A

of combination fly ash conforming to BS EN 450 or pfa conforming to BS 3892-1 CEM I cement conforming to BS EN 197-1 with a mass fraction of 6 to 35 % of combination of ggbs conforming to BS 6699

of combination of ggbs conforming to BS 6699 e

f

CIIIB+SR

BS 8500-2, Annex A

F

CEM I cement conforming to BS EN 197-1 with a mass fraction of 6 to 20 %

CIIA-L

BS 8500-2, Annex A

B a or C a

of combination of limestone fines conforming to BS 7979

CIIA-LL

BS 8500-2, Annex A

B a or C a

CEM I cement conforming to BS EN 197-1 with a mass fraction of 6 to 10 %

CIIA-D

See Note j

A

CIIA-Q

See Note k

A

of combination of silica fume conforming to BS EN 13263 i CEM I cement conforming to BS EN 197-1 with a mass fraction of 6 to 20 % of combination of metakaolin conforming to an appropriate Agrément certificate Notes a The classification is B if the cement or combination strength is class 42,5 or higher and C if it is class 32,5. b Metakaolin only. c Where the fly ash or pfa content is a mass fraction of 21 to 24%. d The addition of the abbreviation ‘+SR’ denotes an additional requirement for sulfate resistance that the fly ash content should be a mass fraction of not less than 25% of the cement or combination. Where it is less than 25%, the grouping with respect to sulfate resistance is ‘A’ (Note c). e Cements or combinations with higher levels of slag than permitted in this table may be used for certain specialist applications, but no guidance is provided in this Special Digest or BS 8500. f The addition of the abbreviation ‘+SR’ denotes an additional requirement for sulfate resistance, that where the alumina content of the slag exceeds 14%, the tricalcium aluminate content of the Portland cement fraction should not exceed 10%. Where this is not the case, the grouping with respect to sulfate resistance is ‘A’. g CEM IV/A cement with siliceous fly ash should be classified as CEM II-V cement. h (V) indicates siliceous fly ash only. i Until BS EN 13263 is published, the silica fume should conform to an appropriate British Board of Agrément certificate. j These combinations are not currently covered by BS 8500-2, Annex A. However, silica fume can be used in accordance with Clause 5.2.5 of BS EN 206-1. k These combinations are not currently covered by B S 8500-2, Annex A. However, metakaolin conforming to Clause 4.4 of BS 8500-2 may be used in accordance with Clause 5.2.5 of BS EN 206-1. If the k-value concept is used, a k-value with respect to sulfate resistance of 1.0 should be used.

44 In the case of magnesium sulfate, there is some evidence from laboratory tests that certain cements, in particular those containing ggbs or pfa (or fly ash), are more susceptible to the conventional form of sulfate attack at very high concentrations of magnesium sulfate than concrete made with SRPC. Where cements containing ggbs are used in concrete that contains more than a few percent carbonate, this attack by magnesium sulfate seems to be counteracted. In contrast, in respect of TSA, concrete containing ggbs cement CEM III/B+SR or ggbs combination CIIIB+SR has a significantly better performance than concrete made with SRPC. As the typical ground temperatures in the UK are conducive to TSA, the cement and combination types for DC-4m concrete have been changed in Table D2 from SRPC to CEM III/B+SR and CIIIB+SR respectively. No restrictions on the type of cement to resist acid attack are given because the rate of erosion of concrete surfaces by natural acidic waters is affected less by the type of cement than by the quality of the concrete. Consequently, Table D3 does not differentiate between groups A to G inclusive for DC Classes with a ‘z’ suffix. D5.3.2 The expert use of special cements The expert use of special cements, such as supersulfated cement conforming to BS 4248 or calcium aluminate cement conforming to prEN 14647 (until published, refer to BS 915-2), can produce concretes with very good chemical resistance. Supersulfated cement is not currently produced in the UK, but in high quality concrete it has good sulfate resistance and a good reputation for acid resistance provided particular care is taken in the surface curing. Current research on the durability of calcium aluminate cement concrete indicates that its high sulfate and acid resistance is due in part to the formation of a resistant surface zone. Close control must be maintained over the mix proportions, temperature, curing conditions and free w/c ratio, or there will be a risk that conversion could reduce the strength and chemical resistance of the concrete. A minimum cement content of 400 kg/m3 and a total w/c ratio of not more than 0.40 should be used. Moreover, preventing the surface of the concrete from drying out during the first day of curing will ensure continued hydration and help to maintain the protective surface zone.

Part D D6 Additional protective measures (APMs) D6.1 General

The five currently recommended options for APMs are listed in Table D4. Earlier BRE Digests have always recommended using surface protection as an APM for the highest level of sulfate conditions. However, in SD1:2001, multiple protective measures (designated APMs) were introduced to compensate for a lack of field and laboratory data in combating TSA. These APMs were frequently applied in less aggressive AC-3 and AC-4 conditions. As a result of new research findings (Section A3) and the revision of guidance on the composition of concrete for given DC Classes (Section D5.1), there is greater confidence in designed concrete quality. Consequently, it has generally been possible in this Special Digest to reduce by two the number of APMs to be applied and still have robust recommendations. The APMs that are recommended for each ACEC Class and for intended working life are shown in Table D1. APMs are needed when the ground conditions are more highly aggressive or a longer intended working life is required. No APMs are generally required where the ACEC Class has a suffix ‘s’, indicating static groundwater conditions, as defined in Section C3.1. One exception, due to groundwater, is where the hydraulic gradient across a concrete element is greater than 5 (Table D1, Note ‘a’). An APM may also be needed where the concrete section thickness is 140 mm or less (Table D1, Note ‘b’). In the most aggressive conditions, Table D1 recommends providing surface protection (APM3). However, there are situations where this is not practical (Table D1, Note ‘f’); for example, for concrete used in friction piles. In this case some other protective measure needs to be found. In theory, this can be any of the other APM options since each APM is given equal status. However, engineering judgement should be used to choose the most appropriate. When the concrete is provided with surface protection (APM3 applied), no further APM is needed for a section thickness which is less than 140 mm or a hydraulic gradient due to groundwater which is greater than 5.

Calcium aluminate cements are not covered by BS 8110 or BS 8500, but recent revisions to the Building Regulations Approved Documents do not preclude their use in structural concrete provided long term properties are adequate for purpose and can be reliably predicted.

Table D4 Options available to provide additional protective measures for buried concrete Option code Additional protective measure (APM)

D5.4 Aggregate type

APM1

Enhance concrete quality (Section D6.2)

In SD1:2001, it was necessary to divide the aggregates into carbonate ranges. For the reasons given in D5.1, this is no longer necessary and the type of aggregate needs no longer be taken into consideration.

APM2

Use controlled permeability formwork (Section D6.3)

APM3

Provide surface protection (Section D6.4)

APM4

Provide a sacrificial layer (Section D6.5)

APM5

Address drainage of site (Section D6.6)

Specifying concrete for general cast-in-situ use D6.2 Enhance concrete quality (APM1)

This APM provides greater resistance to aggressive chemical conditions by increasing the specified DC Class by one step, to a higher DC Class carrying the same suffix, if present. Examples based on Table D1 are: ● a DC Class of DC-3 is initially identified together with a requirement for ‘one APM of choice’. Increasing the concrete quality to DC-4 can satisfy this requirement ● a DC Class of DC-2z is initially identified together with a section thickness of less than 140 mm. The Note ‘b’ requirement for ‘a one-step higher DC Class or an extra APM’ can be satisfied by increasing the concrete quality to DC-3z. Option APM1 is not available when the initially identified Classes from Table D1 are DC-4, DC-4z and DC-4m. D6.3 Use controlled permeability formwork (APM2)

The use of controlled permeability formwork (CPF) enhances the in-situ quality of the concrete in the cover zone relative to that achieved with conventional methods. It has been shown[1] to be able to produce a reduction in the w/c ratio of concrete close to the interface with the formwork, extending to a depth of 10–15 mm into the concrete. Concomitant modifications of porosity have also been reported which, combined with the reduction in w/c ratio, produce a very dense, low-porosity surface zone in concrete cast against CPF. Tests on this surface zone have indicated improvements in many of its properties compared with concrete cast against conventional formwork. These include improvements to durability related properties such as permeability to water and oxygen, carbonation, freeze–thaw resistance and chloride ingress. Although no comparative testing of sulfate resistance has been reported, the above improvements in durability properties strongly indicate that sulfate resistance will be likewise enhanced. The use of CPF should follow the manufacturer’s recommendations[1]. D6.4 Provide surface protection (APM3)

Two types of surface protection are considered here: coatings and water resisting barriers. Appropriately chosen and applied, initially these should completely protect concrete from aggressive chemical action and it might be thought that the quality of the concrete is not relevant. It is essential, however, that a high quality concrete is employed to cover the situation where the surface protection has been damaged and a number of years has elapsed before this is noticed and corrected.

45 D6.4.1 Coatings The main requirements for coatings for use on concrete in the ground are that they should: ● provide an impermeable barrier to water and chemicals that are harmful to concrete ● be resistant to sulfates and other deleterious chemicals ● have a neutral effect on the concrete substrate ● be resistant to mechanical damage ● be easy to apply ● have long term durability ● be cost effective. Various options for coating are discussed in The Concrete Society Technical Report No. 50[2] and in BS EN 1504-2 and BS EN 1504-10. In practice, the choice of coatings will take account of the condition and accessibility of the surface and previous practical experience. Coatings have changed over the years, with tar and cut-back bitumens being less popular, so that long term field data on currently used materials are limited. Common current choices are rubberised bitumen emulsions. These should give good protection if correctly applied. Additionally, purpose designed, polymeric based systems (eg epoxy resins) are now available. These coating systems can give exceptional performance, albeit at a higher initial cost than the earlier mentioned coatings. The risk of damage to coatings during backfill operations should be considered. Coatings must be applied in accordance with the manufacturer’s instructions and the workmanship must be of a high standard to maintain integrity. D6.4.2 Water resisting barriers The functional and practical requirements for water resisting barriers are similar to those of coatings (Section D6.4.1). Sheet materials are commonly used, including plastic and bituminous membranes. The former is usually installed before placing the concrete: a 300 µm (1200 gauge) polyethylene membrane is commonly used to line excavations for trenchfill foundations in aggressive ground or to cover a site prior to casting a raft foundation. Other types of membrane may be applied to the surface of the concrete after curing. The effectiveness of integral waterproofing agents in preventing sulfate attack has not yet been established. Various options are listed in BS EN 1504-2 and BS EN 1504-10.

46

Part D

D6.5 Provide a sacrificial layer (APM4)

D6.6 Address drainage of site (APM5)

For this APM, the thickness of concrete is increased to absorb all the aggressive chemicals in a sacrificial outer layer. The quality of this additional concrete should be equal to or higher than that of the inner concrete. This measure is not appropriate where the surface of the concrete must remain sound to prevent loss of frictional resistance or settlement (eg for skin friction piles).

The concept of this measure is to consider routes by which aggressive groundwater can reach below-ground concrete and, where necessary, to modify the drainage of the site to minimise contact between the groundwater and concrete.

The intended working life of a structure and the rate of penetration of chemicals into the concrete are the key issues that determine the required thickness of a sacrificial layer, but there are few performance data available on which to base guidance. Field investigation of severe TSA on motorway bridge sub-structures, built with Portland cement concrete containing carbonate aggregate and buried in reworked pyritic clay, showed attack to a depth of up to 50 mm in about 30 years[3]. The choice of Portland cement in these cases was based upon the sulfate content of the pyritic clay during the original site investigations. However, subsequent backfilling with the clay appeared to have led to an increase in the sulfate content to levels for which the choice of Portland cement would have been inappropriate. (See Section C5.1.2 for guidance on determining potential sulfates due to oxidation of sulfides.) In using this example of the rate of penetration of TSA as a basis for recommending a suitable thickness for a sacrificial layer of concrete, it must be borne in mind that Portland cement was used rather than one of the sulfate resisting cements listed in Table D3. The recommendations here should lead to a more accurate assessment of aggressive ground conditions and to an appropriate specification for the concrete to be used. It seems reasonable, therefore, that an additional surface protective layer of sacrificial concrete 50 mm thick would be adequate for a reinforced concrete structure with an intended working life of at least 100 years. This extra thickness of concrete should be treated as additional to the specified nominal cover, including situations where concrete is cast directly against the earth and the specified nominal cover is greater or equal to 75 mm in accordance with Clause 3.3.1.4 of BS 8110-1:1997. The additional thickness should also be ignored for the purpose of crack width calculation. If APM4 is to be adopted and blinding concrete is to be used as the APM, the blinding concrete should be at least 50 mm thick and of the same quality as the foundation construction. In general, it should be realised that some attack of this sacrificial concrete can be expected. Caution should be exercised in using this APM if attack could affect the structural integrity; for example, by introducing expansive forces or by reducing frictional forces. In respect of the latter, particular consideration should be given to the base of retaining structures that rely partially on base friction and on the design of piles that depend on side friction.

For all sites, the engineer should consider the impact of a proposed development on the ground and surface water regimes. If this APM is adopted, the engineer will need to carry out a detailed assessment of water movements before (Section C3) and after construction. As indicated below, there are various options available to reduce the risk of aggressive groundwater coming into contact with buried concrete including deemed-to-satisfy, redesigning the structure to avoid drainage problems, and constructing cutoff barriers and cut-off drains. Care is needed during construction to avoid temporary or permanent situations that increase risks. Drains should be inspected and maintained to avoid leakage close to buried concrete. There will generally be three groundwater/concrete environments to be considered in respect of addressing drainage as an APM. ● After construction, the concrete will be surrounded by relatively impermeable ground, such as undisturbed clay strata, through which there is little or no movement of groundwater. In this situation, the APM relating to site drainage is deemed to be already satisfied for concrete, provided it is not subject to a hydraulic gradient from groundwater which is greater than 5. In respect of the latter, particular consideration will be needed for structures such as basements and retaining walls that have one side exposed to air. ● In a preliminary design, impermeable natural ground surrounding the concrete will be cut through by construction excavation, thereby allowing contact of the concrete with mobile groundwater. In this case the recommended APM may often be achieved by redesigning the construction so that the concrete remains surrounded by impermeable ground that forms a barrier to movement of aggressive groundwater; for example, using a piled foundation or trenchfill foundation for a structure rather than a spread footing constructed in an open excavation. If breaching the naturally impermeable ground around concrete is unavoidable, the APM can often be achieved by resealing the possible routes (eg by trenches for service pipes) by which groundwater can reach the concrete. Alternatively, it can be achieved by designing site drainage that will conduct groundwater in trenches and excavations away from the concrete rather than towards it. As noted in Section C3, it is particularly important in aggressive ground conditions to avoid the situation where a backfilled excavation acts as a sump, ponding water against the structure. This would be particularly aggressive to concrete if the backfill contains sulfate or sulfide bearing material (eg pyritic clay).

Specifying concrete for general cast-in-situ use ● The concrete will be surrounded by relatively permeable ground. Here, the recommended APM can be achieved by installing site drainage to remove any aggressive groundwater from the vicinity of the concrete and conduct it safely away. The local authority and the Environment Agency may need to be consulted to ensure that any change in the drainage does not adversely affect surrounding land and groundwater. Particular care is needed with site drainage if there is a source of flowing groundwater on the site (Section C3.3). Large housing developments and civil engineering works, particularly road and bridge construction, often disrupt the natural drainage. The usual procedure is to accommodate identified water courses in the new site layout, or for them to be efficiently diverted. However, in permeable ground, particularly on or adjacent to slopes, many minor water channels may exist that could be a source of aggressive highly mobile water, albeit intermittently. Construction works themselves, particularly trenches for the access of services to buildings, may create further pathways for flows of groundwater.

47 D7 Intended working life In SD1:2003, recommendations for the durability of concrete in the ground used the concept of structural performance level to take into account factors such as the consequence of serious chemical attack, the ease of repair and the required working life of the structure. The use of the term structural performance level has been discontinued and replaced in Table D1 by intended working life. This alternative performance factor brings this Digest in line with BS EN 206-1 and provides for the generality of building structures to have working lives of at least 50 years and civil engineering structures of at least 100 years. Since the concept does not inherently take into account the consequence of chemical attack, it is extended by Table D1, Notes ‘d’ and ‘e’: ● to place the foundations of low-rise domestic housing in the ‘at least 50 years’ category, whatever the actual required working life. This is because the structural effects of chemical attack will generally be detected as a serviceability problem long before any instability is threatened and also will be relatively easily repaired by current underpinning techniques. Placing concrete for low-rise domestic housing in a higher performance class would result in unjustified expense in this major building sector ● to place any concrete elements – which, if they failed, would result in serious consequences – in the ‘at least 100 years’ category, whatever the actual required working life. Examples of such serious consequences could include: ● instability of a structure ● major expense owing to difficulty of repair ● spillage of hazardous materials.

48 D8 Contract documentation Clients, designers and specifiers should ensure that the recommendations of this Special Digest are included in contract documents. The preferred approach is for the designer to provide sufficient information to allow a contractor and concrete supplier to offer a package of proposals to comply with the recommendations since it could provide the basis for alternative specifications being offered that may reduce construction costs. This information should include as a minimum for each structure: ● intended working life ● DS Class of the ground ● ACEC Class of the ground ● concrete strength class ● inclusion in the construction design of any details which could be regarded as APMs (eg drainage, and permanent surface protection to concrete) ● other concrete restrictions ● other site constraints.

Part D Some contracts may alternatively opt for full prescription of the concrete requirements and APM. In this case the contract documentation should contain: ● DS Class of the ground ● ACEC Class of the ground ● DC Class of concrete ● any restriction on cement or combination group ● concrete strength class ● number and (optionally) type of APMs required ● other concrete restrictions ● other site constraints. The project specification should state clearly whether any APMs are needed that are not shown on the contract drawings and whether any particular types are required or preferred. The contract between the contractor and the concrete producer should always include: ● DC Class of concrete ● maximum aggregate size ● consistence. The contract may include also: ● strength class of concrete ● any further restrictions on cement or combination group ● any other requirements.

References: Part D [1] Price W F. Controlled permeability formwork. CIRIA Report C511. London, CIRIA, 2000. [2] The Concrete Society. Guide to surface treatments for protection and enhancement of concrete. Concrete Society Technical Report No 50. 1997. [3] Department of Environment, Transport and the Regions. The thaumasite form of sulfate attack. Risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group. London, DETR, 1999.

British Standards Institution BS 915-2:1972 Specification for high alumina cement. Metric units BS 3892-1:1997 Pulverized-fuel ash. Specification for pulverized-fuel ash for use with Portland cement (See note below) BS 4027:1996 Specification for sulfate-resisting Portland cement BS 4248:2004 Supersulfated cement BS 6699:1992 Specification for ground granulated blastfurnace slag for use with Portland cement Note BS 3892 is to be withdrawn and replaced by BS EN 450-1 and BS EN 450-2, as a harmonised European Standard, towards the end of 2005. These two parts will contain two categories for ash fineness: ● Category N, which equates to ashes conforming to the earlier version of BS EN 450:1995 ● Category S, which equates to pulverised fuel ash conforming to BS 3892-1:1997. Upon publication of the new BS EN 450-1 and BS EN 450-2, the reader should substitute the appropriate category of fly ash in SD1:2005 as required.

BS 7979:2001 Specification for limestone fines for use with Portland cement BS 8110-1:1997 Structural use of concrete. Code of practice for design and construction BS 8500-1:2002 Concrete. Complementary British Standard to BS EN 206-1. Method of specifying and guidance for the specifier BS 8500-2:2002 Concrete. Complementary British Standard to BS EN 206-1. Specification for constituent materials and concrete BS EN 197-1:2000 Cement. Composition, specifications and conformity criteria for common cements BS EN 197-4:2004 Cement. Composition, specifications and conformity criteria for low early strength blastfurnace cements BS EN 206-1:2000 Concrete. Specification, performance, production and conformity BS EN 450:1995 Fly ash for concrete. Definitions, requirements and quality control BS EN 1504-2:2004 Products and systems for the protection and repair of concrete structures. Definitions, requirements, quality control and evaluation of conformity. Surface protection systems for concrete BS EN 1504-10:2004 Products and systems for the protection and repair of concrete structures. Definitions, requirements, quality control and evaluation of conformity. Site application of products and systems and quality control of the works BS EN 13263 Silica fume for concrete (to be published) BS EN 14216:2004 Cement. Composition, specifications and conformity criteria for very low heat special cements BS EN 14647:2004 Calcium aluminate cement. Composition, specifications and conformity criteria (Draft for public comment)

49

Part E

Specifying surface-carbonated precast concrete for general use in the ground E1 Introduction This part applies to the general use in the ground of precast concrete that has assured surface carbonation. For some ACEC Classes (from Tables C1 and C2) it permits a relaxation of DC Class from those recommended in Part D for concrete for general cast-in-situ use. The permitted relaxations are applied in Table E1 (on page 51) which here replaces Table D1. Other than this, the procedure given for concrete specification in Part D is fully applicable. The requirements for using this part are: ● that the manufacturer shall have in place a manufacturing process that includes an appropriate surface carbonation stage, and also an independently audited quality control scheme that includes assurance of this stage

● that surface carbonation needs to be ensured by allowing at least 10 days exposure of the precast concrete to air after completion of normal curing and prior to delivery for burial in the ground. If these requirements are not fulfilled, using Table D1 is recommended rather than Table E1. Surface carbonation of concrete has been proved in numerous laboratory and field exposure studies to be beneficial in respect of sulfate attack since it provides a protective surface layer that is less susceptible to chemical reactions involving sulfates (Section B2.1). In practice, significant sulfate resistance will be provided by a relatively thin carbonation layer that develops on the concrete after only a few days exposure to air under appropriate conditions. A more general discussion of carbonation is included in Box E1.

Box E1 What is ‘carbonation’ Carbonation of concrete results from a reaction with carbon dioxide (CO2) from the air. The principal reaction is with the calcium hydroxide (Ca(OH)2) in the matrix of concrete, the end products being calcite (CaCO3) and water: CO2 + Ca(OH)2 → CaCO3 + H2O However, carbon dioxide will also react with calcium silicate hydrates in the matrix to form more calcite and silica gel. It is well known that the reaction results in a loss of alkalinity that is associated with the presence of calcium hydroxide (pH may drop from greater than 12 to less than 9). Because of this, carbonation of the interior of structural concrete is unwelcome (Digest 405) as it can destroy ‘metal passivity’ that results from high alkalinity leading, in turn, to corrosion of reinforcing steel bars. However, in well compacted structural concretes exposed to air, carbonation is a very slow process with a depth of carbonation of only a few millimetres in the first decade and an even slower rate subsequently. The concrete cover to embedded steel caters for this. Deep carbonation that may be detrimental to passivity can be detected by spraying a fresh fracture surface of the concrete with a phenolphthalein indicator solution (1% by mass in ethanol/distilled water solution). Phenolphthalein remains clear where concrete is carbonated, but turns pink/purple where concrete is still strongly alkaline (pH > 9.0, see BRE IP6/81).

In contrast to this unwanted consequence that can arise from deep penetration of a carbonation front, carbonation of the surface of concrete is beneficial in respect of sulfate attack. This is because it provides a protective surface layer that is resistant to chemical reactions between sulfates and the cement in the concrete; in particular those involving calcium hydroxide or that are dependent on high pH – for example, the pH of 10.5 required for formation of thaumasite (Section B2.1.3). In practice, significant sulfate resistance will be provided by a relatively thin carbonation layer that develops on the concrete after only a few days exposure to air under appropriate conditions. Ideal conditions are warm temperatures with a relative humidity of 50 to 75%. Both prolonged hot-dry conditions and wet conditions impede development of a carbonated layer. It is important to note that surface carbonation does not impart resistance to all forms of chemical attack. In particular it offers no benefit against attack from acids since these will cause dissolution of carbonates that are the end product of carbonation. Using phenolphthalein indicator is not appropriate as a proving test for surface carbonation of dense structural precast concrete: the effective layer is often too thin to be detected by this technique.

50 Designer of building or structure

Part E Firstly, from a consideration of the intended structure, determine parameters: ● ACEC Class of ground from Table C1 ● intended working life of concrete element (categories in Table E1) ● thickness of concrete element (categories in Table E1, Notes b and c) ● hydrostatic conditions for concrete element (Table E1, Note a)

From Table E1, determine the appropriate DC Class of concrete. ● For the assessed ACEC Class, look in the column corresponding to the required intended working life, taking account of Notes d and e. ● Adjust DC Class or number of APMs up or down to take account of: (a) thickness of concrete section (Notes b and c) (b) hydraulic gradient due to groundwater if this exceeds 5 (Note a)

From Table E1, find requirements for additional protective measures (APMs). ● Determine the number required ● Note any restrictions as to choice (eg instruction to use APM3)

From Table D4, guided by Section D6, select appropriate options for APMs, taking account of any restrictions and engineering practicalities

Include in the contract documents: ● DS Class of ground ● ACEC Class of ground ● any estimated hydraulic gradients due to groundwater for which DC Class or number of APMs has been adjusted ● specified DC Class after optional adjustment or enhancement ● specified number of APMs after adjustment ● any restrictions or preferences in respect of APMs to be used ● any other design requirements for each concrete element Contractor and concrete producer for building or structure

Obtain from the contract documents: ● the specified DC Class ● the number and type of APM ● any other design requirements for each concrete element Formulate the concrete mix design for the element, using Table D2 to achieve the specified DC Class. Other factors will include strength class of concrete, the consistence, the availability and cost of materials, and any other contract requirements

Figure E1 Specification of surface-carbonated precast concrete

E2 Changes since SD1:2003 The guidance of SD1:2003 followed former BRE Digests, dating back to 1991, in permitting a relaxation of concrete quality for the general use of all concrete in sulfate bearing ground when the concrete is surface-carbonated. In SD1:2005, this relaxation is limited to precast concrete that has assured surface carbonation resulting from exposure to air. For practical and enforcement reasons, there is no longer a relaxation in respect of cast-in-situ concrete elements that are surface-carbonated by exposure to air; for example, the sides of foundations constructed within an excavation that will subsequently be backfilled. Moreover, there is no relaxation unless the surface carbonation period is assured as part of the manufacturing process.

E3 Design process The design process for surface-carbonated precast concrete for use in the ground is similar to that outlined in Section D3 for the general use of cast-in-situ concrete, excepting that reference to Table D1 should be replaced by reference to Table E1. The various stages are set out in Figure E1. E3.1 Selection of the DC Class and APMs

The process of selection of the DC-class and APMs is as given in Section D4, with the exceptions that reference to Table D1 should be replaced by reference to Table E1. The key difference between Tables E1 and D1 is that a DC Class one step lower, or a reduction in number of APMs, has been applied, where appropriate, owing to the benefits of surface carbonation. This reduction has not been applied where: ● the ground is primarily acidic (ACEC Classes with a suffix ‘z’) rather than being sulfate bearing ● the ground is extremely aggressive (ACEC Classes AC-5 and AC-5m) ● a DC Class one step lower would result in loss of the ‘m’ suffix ● a DC Class would result in using DC-1 (in which no minimum cement content is specified). E3.2 Specifying composition of concrete

An appropriate composition for surface-carbonated precast concrete may be selected by using the procedure given in Section D5 for the general use of cast-in-situ concrete; Table D2 is the key guide. E3.3 Additional protective measures

Because of the declared step down of either one DC Class or one APM in Table E1, the need for an APM for precast concrete is reduced compared to the requirements for castin-situ concrete. Applicable APMs are listed in Table D4 and explained in Section D6. However, an APM other than APM3 (‘Provide surface protection’) will generally only be relevant when an extra APM is needed to cater for a high hydrostatic pressure or a section thickness less than 140 mm.

Specifying surface-carbonated precast concrete for general use in the ground

51

Table E1 Selection of the DC Class and the number of APMs for precast concrete where surface carbonation prior to ground exposure is assured and where the hydraulic gradient due to groundwater is 5 or less a,b,c ACEC Class Intended working life At least 100 years (from Tables C1 and C2) At least 50 yearsd,e AC-1s, AC-1

DC-1

DC-1

AC-2s, AC-2

DC-2

DC-2

AC-2z

DC-2z

DC-2z

AC-3s

DC-2 f

DC-3

AC-3z

DC-3z

DC-3z

AC-3

DC-2 f

DC-3 f

AC-4s

DC-3

f

DC-3 f

AC-4z

DC-4z

DC-4z

AC-4

DC-3 f

DC-4 f

AC-4ms

DC-4m

DC-4m

AC-4m

DC-4m

DC-4m f

AC-5z

DC-4z + APM3

DC-4z + APM3

AC-5

DC-4 + APM3

DC-4 + APM3

AC-5m

DC-4m + APM3

DC-4m + APM3

Notes a To qualify for application of this table, surface carbonation of the concrete shall be assured by exposing it to air for at least 10 days after completion of normal curing. This applies to any face that will later be in contact with the ground or groundwater. b Where the hydraulic gradient across a concrete element is greater than 5, one step in DC Class or one APM over and above the number indicated in this table should be applied except where the original provisions included APM3. Where APM3 is already required, or has been selected, an extra APM is not necessary. c For a section thickness of 140 mm or less, a DC Class one step higher or an extra APM should be applied except where the original provisions included APM3. Where APM3 is already required, or has been selected, a further APM is not necessary. d The foundations of any low-rise domestic housing that has an intended working life of at least 100 years may, in practice, be constructed with concrete selected (for the same ACEC Class) from the above column headed ‘at least 50 years’ (Section D7). e Structures with an intended working life of at least 50 years, but for which the consequences of failure would be relatively serious, should be classed as having an intended working life of at least 100 years for the selection of the DC Class and APM. f A DC Class one step lower than the norm has been applied here owing to the benefits of surface carbonation (not applicable in ACEC Classes with suffix ‘z’ or AC-5 and AC-5m). Explanation of suffix symbols to DC Class ● Concrete placed in ACEC Classes that include the suffix ‘z’ primarily must resist acid conditions and may be made with any of the cements listed in Table D2. ● Suffix ‘m’ relates to the higher levels of magnesium in DS Classes 4 and 5.

References: Part E BRE Digest 405 Carbonation of concrete and its effect on durability Information Paper IP6/81 Carbonation of concrete made with dense natural aggregates

52

Part F

Part F

Design guides for specific precast concrete products

F1 Introduction This part provides design guides for the following specific precast concrete products: ● pipeline systems ● box culverts and segmental linings for tunnels and shafts ● concrete masonry units (blocks). These products have dedicated guidance for using in aggressive ground owing to their distinctive characteristics. Table F1 lists the categories of product covered and the six design guides that apply to particular exposure conditions. For pipeline systems, box culverts and segmental linings, the exposure conditions include the internal flow of aggressive water or effluent when the products are used in water and sewer services.

For these same products, the design guides take into account enhanced chemical resistance that arises from inherently low permeability that results, in turn, from a manufacturing process that includes a high degree of compaction. For pipeline systems, and optionally for box culverts and segmental linings, the design guides also take into account the enhanced resistance to sulfate attack that ensues from surface carbonation. For both low permeability and surface carbonation, the specification of concrete quality in terms of DC Class is relaxed (from that recommended in Part D) provided that these properties are assured by appropriate manufacturing and quality control procedures. The latter should be independently audited by a third party certification body. For an explanation of the origin and benefits of carbonation see Box E1 on page 49.

Table F1 Design guides for specific precast concrete products Category of product Design Guide (guides are shown on pages 56 and 57, and 59 to 61) Pipeline systems and associated units to

F1a Concrete pipes and associated units for water and sewer services in the ground: external

BS EN 1916, BS EN 1917 and complementary

surface. Also for internal surfaces where protective lining is not necessary

Standards BS 5911-1, -3, -4 and -6 with

F1b Concrete pipes and associated units for water and sewer services: internal surface where

assured surface carbonation

protective lining is necessary

Precast box culverts to prEN 14844

F2a Precast box culverts and precast segmental linings, and associated units for tunnels and

Segmental linings for tunnels and shafts to

Also for internal surfaces where protective lining is not necessary

BTS/ICE Specification for tunnelling [1]

F2b Precast box culverts and precast segmental linings, and associated units for tunnels and

shafts in the ground, for water and sewer services, storage and transportation: external surface.

shafts in the ground, for water and sewer services, storage and transportation: internal surface, Optionally with or without

where protective lining is necessary

assured surface carbonation Precast concrete masonry units to

F3a Precast aggregate concrete blocks used below ground

BS 6073-1, and BS EN 771-3 and -4,

F3b Autoclaved aerated concrete (Aircrete) blocks used below ground

used in accordance with BS 5628-3 and with assured beneficial surface carbonation For any other uses not covered here, the advice of the manufacturer should be sought

Design guides for specific precast concrete products F2 Procedure for using design guides The procedure for using the design guides for pipeline systems, box culverts and segmental linings is set out in Figure F1. As with the other categories of concrete (Parts D and E), the procedure for ground assessment is in terms of ACEC Class and refers to either Table C1 for natural ground or Table C2 for brownfield sites.

53

The procedure for specification of concrete to satisfy the selected DC Class is that given in Section D5 of Part D, excepting that Table F2 (on the next page) is recommended for use as replacement for Table D2. Table F2 permits group D cements and combinations to be used at higher than ordinarily specified w/c ratio and maximum cement content to satisfy DC Class DC-4.

The procedure for selecting DC Class and APMs for a given ACEC Class likewise follows the general procedures adopted in Parts D and E, but Design Guides F1a and F2a replace Table E1. Furthermore, Design Guides F1b and F2b (that have no equivalent in Parts D and E) are introduced here to deal with specification of concrete and linings for the interior of products that will carry water or effluent.

For precast concrete masonry units, the specification procedure is different to the foregoing. It is based on ground assessment in terms of DS Class and on the proven durability of normally manufactured units. This is explained further in Section F5.

Steps

Refer to

1 Determine: ● the ACEC Class of ground ● the type of concrete product (eg component of pipeline system) ● the ACEC Class equivalent to any water or effluent to be carried internally

Previous steps as Figure A1 Tables C1 and C2 and Sections C5 and C6

2 Determine the required intended working life of the concrete element

Section D7

3 Choose the design guide appropriate to the type of concrete product and condition of exposure (eg Design Guide F1a for pipeline systems exposed to surrounding ground)

Table F1

4 Use the design guide to determine the basic DC Class of concrete and any APM required to resist the previously determined ACEC Class of ground

Design Guides F1a and F2a

5 Use the design guide to determine any need for a protective lining to the product arising from the internal flow of aggressive water or effluents

Design Guides F1b and F2b

Sections F3.2.2 and F4.2.2

6 If any APM or an internal protective lining is needed, discuss options with the product manufacturer

7 The manufacturer should use Table F2 to determine the composition of concrete required to meet the selected DC Class (enhanced to provide the APM where appropriate)

Table F2 For background, see Section D5

8 In contract documents state: ● the ACEC Class of the ground ● the ACEC class that is equivalent to any internal water of effluent ● the design guide used and its recommended basic DC Class and APM ● the DC Class specified for manufacture of the product ● the specified APM

Figure F1 Steps in the design and specification of specific precast products for use in aggressive ground or to carry aggressive liquids (excluding concrete blocks)

54

Part F

Table F2 Concrete qualities to resist chemical attack for application to specific precast concrete products only: limiting values for composition DC Class Maximum Minimum cement or combination content (kg/m3) Recommended cement and free-water/cement for maximum aggregate size of: combination group or combination ratio ≥ 40 mm 20 mm 14 mm 10 mm DC-1 DC-2

–

–

–

–

–

A to G inclusive

0.55

300

320

340

360

D, E, F

0.50

320

340

360

380

A, G

0.45

340

360

380

380

B

0.40

360

380

380

380

C

DC-2z

0.55

300

320

340

360

A to G inclusive

DC-3

0.50

320

340

360

380

F

0.45

340

360

380

380

E

0.40

360

380

380

380

D, G

DC-3z

0.50

320

340

360

380

A to G inclusive

DC-4

0.45

340

360

380

380

F

0.40

360

380

380

380

E

0.35

380

380

380

380

D, G

0.40

–

400

400

400

Da

DC-4z

0.45

340

360

380

380

A to G inclusive

DC-4m

0.45

340

360

380

380

F

Grouped cements and combinations A B

Cements

Combinations

CEM I, CEM II/A-D, CEM II/A-Q, CEM II/A-S, CEM II/B-S, CEM II/A-V,

CIIA-V, CIIB-V, CII-S, CIIIA, CIIIB, CIIA-D,

CEM II/B-V, CEM III/A, CEM III/B

CIIA-Q

CEM II/A-Lb, CEM II/A-LLb

CIIA-Lb, CIIA-LLb

b

b

CIIA-Lb, CIIA-LLb

C

CEM II/A-L , CEM II/A-LL

D

CEM II/B-V+SR, CEM III/A+SR

CIIB-V+SR, CIIIA+SR

E

CEM IV/B, VLH IV/B (V)

CIVB-V

F

CEM III/B+SR

CIIIB+SR

G

SRPC

–

For cement and combination types, compositional restrictions and relevant Standards, see Table D3. Note a This concrete quality, using cement/combinatioin group D, is recommended for use only in the manufacture of precast box culverts and precast segmental linings and associated units for tunnels and shafts. b The classification is B if the cement/combination strength class is 42,5 or higher and C if it is 32,5.

Design guides for specific precast concrete products F3 Design guides for precast concrete pipeline systems F3.1 General considerations

Design recommendations are given in Design Guides F1a and F1b for pipes, chambers and slabs conforming to the Standards listed in Box F1. Design Guide F1a deals with the general composition of concrete, as required for: ● a durable external surface when exposed to either natural or brownfield ground (as defined in Section C5.1.3) ● a durable inside surface when exposed to a range of low to moderately aggressive conditions resulting from carried waters or effluents. Design Guide F1b gives recommendations for a protective lining for the internal surfaces of pipes for a range of aggressive conditions that are aggravated by the flowing of water or effluent. Flowing waters and effluents that are potentially aggressive to concrete include: ● ‘pure’, or soft, water that is low in dissolved ions (Sections B5 and C3.3) ● water with aggressive carbon dioxide (Sections B4, C2.2.3 and C3.3) ● acidic waters containing, for example, humic acid (Sections B2.2 and C2.2) ● acidic effluents that may contain organic or mineral acids resulting from human activities (Sections B2.2 and C2.2) ● sulfate-containing effluents, notably sewage, where aggressiveness is increased by interaction with sulfate reducing bacteria. Despite their relatively thin walls and frequent subjection to a significant hydraulic gradient from groundwater, precast concrete pipes manufactured to British Standards in the UK have a long in-service record of good resistance to sulfate attack.

55

Additionally, the durability of currently manufactured pipeline systems have proved satisfactory when tested by BRE in field and laboratory trials under a range of sulfate and temperature conditions, including those known to favour the occurrence of TSA. The perceived, inherently good resistance of pipeline systems to chemical attack is attributed to two factors. ● To meet the stringent performance requirements laid down in applicable Standards (Box F1), strict control over the aggregate sizes, mix proportions and manufacturing process is necessary. High cement contents and very low free-water/cement ratios are typically achieved and results in dense concrete, of low permeability, which impedes entry of most chemicals and consequent attack ● In respect of sulfate attack, resistance is enhanced by surface carbonation. Assured industry practice to Concrete Pipeline System Association guidelines [2] specifies a storage time of at least 10 days before despatch of products to allow for effective surface carbonation. In this Special Digest, intended working life is a parameter which caters for the required performance of concrete in aggressive ground, both in respect of the length of working life and the consequence of failure (Section D7). In full compliance with the latter concept, pipeline systems are specified here only for an intended working life of at least 50 years. It is considered that upgrading of concrete quality to the ‘at least 100 years’ category will rarely be needed. The pipeline systems manufacturer should be consulted if the actual length of working life is required to be significantly in excess of the normal 50 years or the product is to be used in the rare situation where the consequences of failure could be serious.

Box F1 Standards applicable to precast concrete pipeline systems BS EN 1916:2002

Concrete pipes and fittings, unreinforced, steel fibre and reinforced

BS 5911-1:2002

Precast concrete pipes, fittings and ancillary products. Specification for unreinforced and reinforced concrete pipes (including jacking pipes) and fittings with flexible joints (complementary to BS EN 1916:2002)

BS EN 1917:2002

Concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced

BS 5911-3:2002

Precast concrete pipes, fittings and ancillary products. Specification for unreinforced and reinforced concrete manholes and soakaways (complementary to BS EN 1917:2002)

BS 5911-4:2002

Precast concrete pipes, fittings and ancillary products. Specification for unreinforced and reinforced concrete inspection chambers (complementary to BS EN 1917:2002)

BS 5911-6:2004

Concrete pipes and ancillary concrete products. Specification for road gullies and gully cover slabs

56

Part F

F3.2 Using Design Guide F1a for specifying concrete for pipes and associated units

F3.2.1 Considering the external surfaces Design Guide F1a recommends a DC Class of concrete for each ACEC Class that may be encountered by pipeline systems in the ground. The listed ACEC Classes are derived from ground assessment Table C1 (for natural ground locations) and Table C2 (for brownfield locations) with the understanding that a mobile groundwater condition is always appropriate owing to the characteristics of pipeline construction. Some of the DC Classes (Design Guide F1a, Note ‘e’) have been adjusted downwards by one step (compared with the equivalent used for general cast-in-situ use,Table D1) owing to the benefits of assured surface carbonation (Section F3.1 and Box E1). Such a reduction is not applicable in ACEC Classes with a suffix ‘z’ as these represent acid conditions against which surface carbonation provides no resistance. Nor is it applicable in ACEC Classes AC-5 and AC-5m that represent extremely aggressive conditions in which the benefits of carbonation have not been fully proven.

Design Guide F1a Specification of concrete and external APMs for pipes and associated units for water and sewer services in the ground a,b ACEC Class Design Chemical Additional protective measures (Tables C1 Classc and C2) (Table F2) (Table D4) AC-1

DC-1d

AC-2z

DC-2z d

AC-2

DC-2 d

AC-3z

DC-3z

AC-3

DC-2 e

AC-4z

DC-4z

AC-4

DC-3 e

AC-4m

DC-3 e

AC-5z

DC-4z

Apply APM3

AC-5

DC-4

Apply APM3

AC-5m

DC-4m

Apply APM3

Notes a Applicable to both natural ground and brownfield sites, and for internally carried water or effluent when considered equivalent to mobile groundwater (Section F3.2.2). b Recommended for an intended working life of at least 50 years (Section F3.1). c There is no requirement to increase the level of the DC Class for a hydraulic gradient greater than 5 or for a section thickness less than 140 mm (Section F3.2.1). d Pipes and associated units manufactured in England, Scotland and Wales to the applicable Standards (Box F1) are made of concrete that is of DC-3 or higher quality. e A DC Class one step lower than the norm has been applied here owing to the benefits of surface carbonation (not applicable in ACEC Classes with suffix ‘z’ or AC-5 and AC-5m).

The recommended DC Classes take into account all the factors that may affect durability of pipeline systems for the intended working life category of ‘at least 50 years’ as follows. ● While it is recognised that pipeline systems may often be required to withstand hydraulic gradients greater than 5, there is no need to increase the level of the DC Class or apply a further APM to cater for this. Instead a trade-off is made against the enhanced resistance to chemical attack resulting from the inherently low permeability of the product. ● For the same reason, there is no requirement to increase DC class or to apply an APM in respect of a product section thickness which is less than 140 mm. ● In practice, pipes and associated units manufactured in England, Wales and Scotland to the applicable Standards (Box F1) will employ concrete with a minimum quality of DC-3. This may not be the case in pipeline systems manufactured elsewhere. The options for composition of concrete required to satisfy the various DC Classes listed in Design Guide F1a are specified in Table F2; in respect of pipeline systems, use of this table is identical to using Table D2 which is explained in Section D5. Further to this, the quality controlled manufacturing process for pipeline systems will specifically comply with Clause 5.4.2 of BS EN 206-1:2000 which indicates that individual batches of concrete may have a w/c ratio which is up to 0.02 greater than the design maximum specified in Table F2. Additional protective measures are recommended only for pipeline systems exposed to the highest level of aggressive conditions (AC-5 family). The appropriate APM is APM3 (‘Provide surface protection’). In general, the type of surface protection will be specified by the construction designer and will be provided by the site contractor rather than the manufacturer of the pipeline system. Appropriate options are discussed in Section D6.4.

Design guides for specific precast concrete products F3.2.2 Considering the internal surfaces In some cases the water or effluent carried by the pipes will be more aggressive than the external ground environment and yet not sufficiently aggressive to merit application of an internal lining. In these cases it will be the aggressiveness of the internal environment that governs the composition of the concrete. An appropriate procedure is to consider the internally carried material as equivalent to mobile groundwater with the same sulfate concentration and pH, and find the equivalent ACEC Class from Table C2. This can then be used in Design Guide F2a to find an appropriate DC Class leading to an appropriate composition of concrete from Table F2.

57

F3.3 Using Design Guide F1b for specifying internal linings to pipes and associated units

Design Guide F1b deals with the internal surfaces of pipes and associated units that are used for carrying water or effluent. Protective linings are recommended for particular aggressive situations, protection taking precedence and replacing any specification of resistant concrete resulting from the procedure described in Section F3.2.2. Design Guide F1b lists the following factors that, taken in combination with type of water or effluent, may require the pipeline system to be lined: ● the presence of relatively high aggressive carbon dioxide (greater than 15 mg/l CO2) in relatively pure waters ● the presence of acids that result in a pH of effluent of less than 5 ● the presence of sulfate in effluent at a concentration greater than 1400 mg/l SO4 (Sulfate Class DS-3 or greater). In general, the type of protective lining will be specified by the construction designer and will be provided by the site contractor rather than the manufacturer of the pipeline system.

Design Guide F1b Specification of internal linings for concrete pipes and associated units for water and sewer services Type of pH Aggressive carbon Protective lining water or effluent

dioxide level of water or effluent (mg/l)

Natural water or

> 5.0

domestic sewage < 5.0

< 15

Lining not needed

> 15

Provide lining

< 15

Lining not needed unless sulfate level of water or effluent is more than 1400 mg/l SO4

> 15 Industrial

> 5.0

Provide lining Lining not needed unless sulfate level of water or effluent is more than 1400 mg/l SO4

< 5.0

Provide lining

Notes a Recommendations of this design guide define when some form of internal protective lining is needed for three broad categories of water and effluent. b When no internal protective lining is recommended in this design guide, the carried water or effluent may be considered equivalent to mobile groundwater of the same chemical composition and the corresponding ACEC Class found from Table C2. The composition of concrete should satisfy the recommendations for this ACEC Class and equivalent DC Class, as given in Design Guide F1a. c The composition of concrete for pipeline systems placed in aggressive ground should satisfy the recommendations for the appropriate ACEC Class and equivalent DC Class as given in Design Guide F1a. d When no lining is needed, the highest of the DC Classes indicated by the procedures of Notes b and c should be used.

58 F4 Precast box culverts and precast segmental linings for tunnels and shafts F4.1 General considerations

This section applies to precast box culverts designed and manufactured in accordance with prEN 14844, and precast segmental linings for tunnels, shafts and associated units, designed and manufactured in accordance with the BTS/ICE Specification for tunnelling[1]. The recommendations given in this section apply to these specific precast concrete products only when there is an assurance of high durability through employment of rigorous quality control procedures during the manufacturing process, audited by an independent certification body. The quality control procedures should cover the specified constitution of concrete, the process of mixing and compaction, the curing of the concrete, the achievement of low-permeability concrete and its verification by ISAT testing (Box F2), and any provision for subsequent exposure to atmospheric conditions needed to achieve surface carbonation. Use of these quality control procedures is already required by Specification for tunnelling. Where needed, precast box culverts can be supplied to meet the requirements of these procedures. In the absence of assurances of durability, the recommendations of this section will not apply. Design recommendations are given in Design Guides F2a and F2b for precast concrete box culverts, and for precast concrete segmental linings and associated units for tunnels and shafts that are variously used for carriage of water or sewage, for general transportation purposes, or for storage. Design Guide F2a deals with the general composition of concrete, as required for:

Box F2 Permeability requirements to qualify for the recommendations of Section F4 To qualify for the recommendations of Section F4, precast box culverts and precast segmental linings for tunnels, shafts and associated units are required to have concrete of low permeability. This is to be assessed by routine water absorption testing. The following procedure shall be followed. ● Sample the concrete product at a frequency of one group per month, as recommended for water absorption tests for routine inspection in Table H1 of Annex H of BS EN 1916:2002. A group is defined as a clearly identifiable collection of units, manufactured using the same process; units of different nominal sizes may be grouped together provided that the ratio of largestto-smallest nominal size is not greater than 2. ● Carry out Initial Surface Absorption Tests (ISATs) in accordance with BS 1881-208. The measured water absorption shall conform with the Low Absorption category of Table 2 of The Concrete Society Technical Report No 31[3].

Part F ● a durable external surface when exposed to either natural or brownfield ground (as defined in Section C5.1.3), a mobile groundwater condition being assumed in all cases ● a durable inside surface when exposed to a range of lowto-moderately aggressive conditions resulting from carried waters or effluents. Design Guide F2b recommends protective linings for the internal surfaces of these products for a range of aggressive conditions generated by flowing water or effluent. Flowing waters and effluents that are potentially aggressive to concrete include: ● ‘pure’, or soft, water that is low in dissolved ions (Sections B5 and C3.3) ● water with aggressive carbon dioxide (Sections B4, C2.2.3 and C3.3) ● acidic waters containing, for example, humic acid (Sections B2.2 and C2.2) ● acidic effluents that may contain organic or mineral acids resulting from human activity (Sections B2.2 and C2.2) ● sulfate-containing effluents, notably sewage, where aggressiveness is increased by interaction with sulfate reducing bacteria. In this Special Digest, intended working life is a parameter which caters for the required performance of concrete in aggressive ground, both for the length of working life and the consequence of failure (Section D7). In complying with this overall concept, the specific precast products covered by this section are specified for intended working life categories of ‘at least 50 years’ and ‘at least 100 years’. The construction designer must select the appropriate category based on the actual required working life and the presumed consequences of failure. Precast box culverts and segmental linings for tunnels and shafts manufactured according to the foregoing quality control criteria have inherently good resistance to chemical attack. As with pipeline systems, the resistance results from manufacturing processes that exercise strict control over aggregate sizes, mix proportions and degree of compaction. Dense concrete of low permeability is achieved which impedes entry of most chemicals and consequent attack. The sulfate resistance of the specific precast products covered by this section can optionally be enhanced by assured surface carbonation provided the units are exposed to air under appropriate conditions (Box E1) for a minimum of 10 days prior to despatch. Provision of assured surface carbonation allows some relaxations to be made for DC Class or APMs. The conditions where a relaxation may be applied are indicated by footnote ‘d’ in Design Guide F2a. These relaxations do not apply in ACEC Classes with a suffix ‘z’ as these represent acid conditions against which surface carbonation provides no resistance. Nor do they apply in ACEC Classes AC-5 and AC-5m that reflect extremely aggressive conditions in which the benefits of carbonation have not been fully proven.

Design guides for specific precast concrete products F4.2 Using Design Guide F2a for specifying concrete for precast box culverts and segmental linings

F4.2.1 Considering the external surfaces Design Guide F2a recommends a DC Class of concrete for each ACEC Class that may be encountered by precast box culverts and segmental linings in the ground. The listed ACEC Classes are derived from ground assessment Table C1 (for natural ground locations) and Table C2 (for brownfield locations), with the understanding that a mobile groundwater condition is always appropriate for box culverts, and tunnel and shaft construction. The recommended DC Classes take into account all the factors that may affect durability of precast box culverts and segmental linings for the stated intended working life. ● Box culverts and precast segmental linings and associated units for tunnels and shafts may often be required to withstand hydraulic gradients greater than 5. This is a condition which for the general use of precast concrete (Part E) requires an increase of one DC Class or application of an extra APM. However, for the specific precast concrete products conforming to this design guide, the requirement for a higher DC Class or APM is set aside in recognition the enhanced resistance to chemical attack resulting from inherent low permeability of the product (Section F4.1). ● For the same reason, there is no requirement to increase a DC Class or to apply an APM in respect of a section thickness of less than 140 mm.

59

Some of the DC Classes (those to which Design Guide F2a, Note ‘d’, apply) may be adjusted downwards by one step with respect to those indicated, provided that surface carbonation is assured (Section F4.1 and Box E1). This reduction is not applicable in ACEC Classes with a suffix ‘z’ as these represent acid conditions against which surface carbonation provides no resistance. Nor is it applicable in ACEC Classes AC-5 and AC-5m that represent extremely aggressive conditions in which the benefits of carbonation have not been fully proven. The options for composition of concrete required to satisfy the various DC Classes listed in Design Guide F2a are specified in Table F2; use of this table is similar to using Table D2 which is explained in Section D5. One specific difference in Table F2 is the provision of an additional composition of concrete of DC-4 quality using group D cements. This is recommended only for use with precast concrete box culverts, and for precast concrete segmental linings and associated units for tunnels and shafts. The composition permits use of a combination of higher maximum w/c ratio and higher minimum cement contents as compared with the normal DC-4 quality using group D cements. Additional protective measures are recommended for box culverts and segmental linings to provide greater assurance of durability when ground conditions are highly aggressive (ACEC Class AC-5 family) or when some less aggressive conditions are combined with an intended working life category of ‘at least 100 years’.

● In practice, segmental linings manufactured in the UK will employ concrete with a minimum quality of DC-3.

Design Guide F2a Specification of concrete and external APMs for precast concrete box culverts, precast concrete segmental linings, and associated units for tunnels and shafts used for water and sewer services, storage and transportation a ACEC Class Intended working life of at least 50 years Intended working life of at least 100 years (Tables C1 DC Class b Additional protective DC Class Additional protective and C2) (Table F2) measures (Table F2) measures (Table D4) (Table D4) AC-1 AC-2z

DC-1c DC-2z

DC-1 c c

DC-2z c

AC-2

DC-2

c

DC-2 c

AC-3z

DC-3z

DC-3z

AC-3

DC-3

d

DC-3 d

AC-4z

DC-4z

DC-4z

AC-4

DC-4 d

AC-4m

DC-4m

AC-5z

DC-4z

DC-4 d d

Apply APM3

One APM of choice One APM of choice

DC-4m d

One APM of choice

DC-4z

Apply APM3

AC-5

DC-4

Apply APM3

DC-4

Apply APM3

AC-5m

DC-4m

Apply APM3

DC-4m

Apply APM3

Notes a Applicable to both natural ground and brownfield sites, and for internally carried water and effluent when considered equivalent to mobile groundwater (Section F4.2.2). b There is no requirement to increase the level of the DC Class for a hydraulic gradient of greater than 5, or for a section thickness less than 140 mm (Section F4.2.1. c Precast box culverts, designed and manufactured in accordance with prEN 14844, and precast segmental linings for tunnels and shafts manufactured in accordance with the BTS/ ICE Specification for tunnelling [1], have a minimum quality of DC-3. d A DC Class one step lower or reduction of one APM may be applied by the designer to this indicated category if surface carbonation is assured (10 days minimum storage time to be allowed by the manufacturer before dispatch – Section F4.1 and F4.2). No reduction is permitted for categories not so indicated (Section F4.2.1).

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Part F

For the most aggressive ground conditions, the appropriate APM is APM3 (‘Provide surface protection’). In general, the type of surface protection will be specified by the construction designer and will be provided by the manufacturer or the site contractor. Appropriate options are discussed in Section D6.4. For less onerous conditions, the APM can be ‘one APM of choice’ (ie any appropriate APM from the options listed in Table D4 and described in Section D6). For segmental linings, using options APM4 (‘Provide a sacrificial layer’) and APM5 (‘Address drainage of site’), are generally inappropriate owing to the nature of tunnel and shaft construction. F4.2.2 Considering the internal surfaces In some cases the water or effluent carried by precast box culverts and segmental linings will be more aggressive than the external ground environment, and, yet, not sufficiently aggressive to merit application of an internal lining. In these cases it will be the aggressiveness of the internal environment that governs the composition of the concrete. An appropriate procedure for concrete design is to consider the internally carried material as equivalent to mobile groundwater with the same sulfate concentration and pH, and find the equivalent ACEC Class from Table C2. This can then be used in Design Guide F2a to find an appropriate DC Class leading, in turn, to an appropriate composition of concrete from Table F2.

F4.3 Using Design Guide F2b for specifying internal linings to precast box culverts and segmental linings

Design Guide F2b deals with the internal surface of box culverts and segmental linings of tunnels and shafts that are used for carrying water or effluent. Protective linings with appropriate chemical resistance are recommended for certain aggressive situations, the provision of these linings replacing any need for specification of internally chemically resistant concrete via the procedure of Section F4.2.2. Design Guide F2b lists the following factors that, taken in combination with type of water or effluent, may require the precast concrete box culverts and segmental linings to be lined: ● the presence of relatively high aggressive carbon dioxide (greater than 15 mg/l CO2) in relatively pure waters ● the presence of acids that result in a pH of effluent of less than 5 ● the presence of sulfate in effluent at a concentration greater than 1400 mg/l SO4 (Sulfate Class DS-3 or greater). In general, the type of protective lining will be specified by the construction designer and will be provided by the site contractor rather than the manufacturer of the box culverts or segmental linings.

Design Guide F2b Specification of internal linings for precast concrete box culverts, precast concrete segmental linings, and associated units for tunnels and shafts used for water and sewer services, storage and transportation Type of pH Aggressive carbon Protective lining water or effluent dioxide level of water or effluent (mg/l) Natural water or

> 5.0

domestic sewage < 5.0

< 15

Lining not needed

> 15

Provide lining

< 15

Lining not needed unless sulfate level of water or effluent is more than 1400 mg/l SO4

> 15 Industrial

> 5.0

Provide lining Lining not needed unless sulfate level of water or effluent is more than 1400 mg/l SO4

< 5.0

Provide lining

Notes a Recommendations of this design guide define when some form of internal protective lining is needed for three broad categories of water or effluent. b When no internal protective lining is recommended in this design guide, the carried water or effluent may be considered equivalent to mobile groundwater of the same chemical composition and the corresponding ACEC Class found from C2. The composition of concrete should satisfy the recommendations for this ACEC Class and equivalent DC Class, as given in Design Guide F2a. c The composition of concrete for pipeline systems placed in aggressive ground should satisfy the recommendations for the appropriate ACEC Class and equivalent DC Class as given in Design Guide F2a. d When no lining is needed, the highest of the DC Classes indicated by the procedures of Notes b and c should be used.

Design guides for specific precast concrete products

61

F5 Design guides for precast concrete masonry units

Design Guide F3a Precast aggregate concrete blocks used below ground

The precast products covered by this section are concrete blocks intended for use as masonry and which are designated ‘masonry units’ in relevant Standards BS 6073-1, BS EN 771-3 and -4, and BS 5628-3.

Design recommendations: For Design Sulfate Class DS-1 conditions Blocks conforming to BS 6073-1 or BS EN 771-3 and to BS 5628-3 should be used following the recommendations in BS 5628-3 for use below ground (Note a).

Design recommendations are given for precast aggregate concrete blocks in Design Guide F3a and for Aircrete (autoclaved aerated concrete blocks) in Design Guide F3b. Specifications in the design guides start with ground conditions classified in terms of Design Sulfate Class (Section C5, and Tables C1 and C2) rather than ACEC Class. The reason for this is that there is currently no correlation of concrete block performance with ACEC Class, though work on this is ongoing. In support of the included design guides, both types of concrete block have shown good in-service resistance to sulfate attack in ground conditions for which they are recommended. Where Design Sulfate Class DS-2 or higher conditions are reported for a site, confirmation should be sought from the site investigation findings that sulfate values apply to the depth of soil against which the blocks will be placed. Often the blocks will be in contact with only the top metre or so of the ground and, typically, this zone is leached of sulfates (Section C2.1.1). In common with other types of precast concrete products, the resistance of concrete blocks to sulfate attack benefits from surface carbonation. For one application of precast aggregate blocks, a minimum period of 10 days exposure to air to allow for surface carbonation is specified in Design Guide F3a. Such an exposure can include a period after masonry construction when the face of the blocks is exposed to air prior to burial in sulfate bearing ground. The recommendations of BS 5628-3 relating to the use of masonry units in contact with the ground should be followed; for example, Clause 5.6 in respect of durability issues, and Table 13 in respect of ‘work below or near external ground level’. Consideration may need to be given to using a sulfate resistant mortar. Where use is supported by independent third party certification, the recommendations given in the certification should be followed.

For Design Sulfate Class DS-2 and DS-3 conditions Blocks conforming to BS 6073-1 or BS EN 771-3, and which have been surface-carbonated for a minimum 10 days exposure to air between manufacture and final burial in the ground, should be used (Note a) following the recommendations in BS 5628-3 for use below ground. For conditions above Design Sulfate Class DS-3 The use of aggregate concrete blocks is not recommended. Note a In flowing groundwater conditions (definition in Section C3.3), aggregate concrete blocks should be close-textured paint quality having a low permeability.

Design Guide F3b Aircrete (autoclaved aerated concrete) blocks used below ground Design recommendations: For Design Sulfate Class DS-1 conditions Blocks conforming to BS 6073-1 or BS EN 771-4 should be used following the recommendations in BS 5628-3 for use below ground. For Design Sulfate Class DS-2 and DS-3 conditions Blocks with a density of greater than 600 kg/m3 should be used. Blocks of lower density may be used if they have been shown to perform satisfactorily under these conditions by independent third party assessment (eg BRE or BBA certification). Advice should be sought from the individual manufacturer or third party assessor. For Design Sulfate Class DS-4 conditions Some grades of blocks may be suitable for Design Sulfate Class DS-4 where they have been shown to be satisfactory by independent third party assessment (eg BRE or BBA certification). Advice should be sought from the individual manufacturer or third party assessor.

62 References: Part F [1] British Tunnelling Society and Institution of Civil Engineers. Specification for tunnelling. London, Thomas Telford Ltd, 2000. [2] Concrete Pipeline Systems Association. Technical Bulletin No 4. Leicester, CPSA, 2005. [3] The Concrete Society. Permeability testing of site concrete – A review of methods and experience. Technical Report 31. Camberley, The Concrete Society, 1988.

British Standards Institution BS 1881-208:1996 Testing concrete. Recommendations for the determination of the initial surface absorption of concrete BS 5628-3:2001 Code of practice for use of masonry. Materials and components, design and workmanship BS 5911-1:2002 Precast concrete pipes, fittings and ancillary products. Specification for unreinforced and reinforced concrete pipes (including jacking pipes) and fittings with flexible joints (complementary to BS EN 1916:2002)

Part F BS 5911-3:2002 Precast concrete pipes, fittings and ancillary products. Specification for unreinforced and reinforced concrete manholes and soakaways (complementary to BS EN 1917:2002) BS 5911-4:2002 Precast concrete pipes, fittings and ancillary products. Specification for unreinforced and reinforced concrete inspection chambers (complementary to BS EN 1917:2002) BS 5911-6:2004 Concrete pipes and ancillary concrete products. Specification for road gullies and gully cover slabs BS 6073-1:1981 Precast concrete masonry units. Specification for precast concrete masonry units BS EN 1916:2002 Concrete pipes and fittings, unreinforced, steel fibre and reinforced BS EN 1917:2002 Concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced BS EN 771-3:2003 Specification for masonry units. Aggregate concrete masonry units (Dense and light-weight aggregates) BS EN 771-4:2003 Specification for masonry units. Autoclaved aerated concrete masonry units prEN 14844:2004 Precast concrete products. Box culverts

CONCRETE IN AGGRESSIVE GROUND Chemical agents that can destroy concrete may be found in the ground. In the UK, sulfates and acids naturally occurring in soil and groundwater are the agents most likely to attack concrete. The effects can be serious leading to expansion and softening of concrete. Many other substances are aggressive, most resulting from human activity, but they present less of a problem since they only rarely come into contact with concrete in the ground This new edition of BRE Special Digest 1 (SDl:2005) updates and consolidates Parts 1 to 4 of the the previous edition published in 2003. The main changes are: l a new ranking of cements with respect to sulfate resistance l removal of the aggregate carbonate range l revision of sulfate class limits l simpler requirements for additional protective measures SDl:2005 provides practical guidance on the specification of concrete for installation in natural ground and in brownfield locations. The procedures given for ground assessment and concrete specification cover the fairly common occurrence of sulfates, sulfides and acids. They also cover the more rarely occurring aggressive carbon dioxide found in some ground and surface waters This Special Digest presents the subject in 6 parts: l Part A introduces the phenomenon of chemical attack of concrete in the ground l Part B describes modes of chemical attack and discusses the mechanisms of the principal types, including sulfate and acid attack, and the action of aggressive carbon dioxide l Part C deals with assessment of the chemical aggressiveness of the ground l Part D gives recommendations for the specification of concrete for general cast-in-situ use in the ground l Part E gives recommendations for specifying surface carbonated precast concrete for general use in the ground l Part F includes design guides for specification of specific precast concrete products, including pipeline systems, box culverts, and segmental linings for tunnels and shafts

OTHER BRE PUBLICATIONS ON CONCRETE Alkali-silica reaction in concrete (in 4 parts). Digest 330 Corrosion of steel in concrete: a review of the effect of humidity. Digest 491 Thaumasite in cementitious materials. Proceedings of the first international conference, 2002. AP147 BRE building elements. Foundations, basements and external works. Performance, diagnosis, maintenance, repair and the avoidance of defects. BR440

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SD 1 ISBN 1 86081 754 8 2005