Concrete in aggressive ground

Transcription

1 Special Digest 1:2005 Third edition Concrete in aggressive ground BRE Construction Division

2 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. 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 construction@bre.co.uk 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: enquiries@bre.co.uk BRE publications are available from or IHS Rapidoc (BRE Bookshop) Willoughby Road Bracknell RG12 8DW Tel: Fax: brebookshop@ihsrapidoc.com Requests to copy any part of this publication should be made to the publisher: BRE Bookshop Garston, Watford WD25 9XX Tel: brebookshop@emap.com Cover photo by Graham Gaunt, courtesy of Arup SD1 BRE 2005 First published 2001 Second edition 2003 Third edition 2005 ISBN

3 iii Contents Sponsors and members of steering group vi Part A: Introduction A1 Problem of chemical attack 1 A2 Scope and structure of the guidance 1 A2.1 Types of site and chemical agents covered 1 A2.2 Readership 2 A2.3 Structure of the guidance 2 A2.4 Diagrammatic overview of ground assessment and concrete specification 2 A3 Background to guidance on sulfate attack 4 A4 Key changes since SD1: A5 Relationship between SD1:2005 and British and European Standards for concrete 5 Appendix A1 Glossary of terms 6 References: Part A 8 Part B: Chemical attack on concrete B1 General 9 B2 Principal types of chemical attack on concrete 9 B2.1 Sulfate attack 9 B2.2 Acid attack 11 B3 Other types of chemical attack on concrete 12 B3.1 Magnesium ions 12 B3.2 Ammonium ions 12 B3.3 Chloride ions 13 B3.4 Organic compounds 13 B4 Attack from aggressive carbon dioxide 14 B5 Attack from pure water 14 B6 Damage to concrete from crystallisation of salts 14 B7 Microbial contribution to chemical attack on concrete 15 References: Part B 15

4 iv Contents Part C: Assessing the aggressive chemical environment C1 General 16 C2 Principal constituents of aggressive ground and groundwater 17 C2.1 Sulfates and sulfides 17 C2.2 Acids 20 C2.3 Magnesium, calcium, sodium and potassium ions 20 C2.4 Ammonium ions 20 C2.5 Chloride ions 20 C3 Presence and mobility of groundwater 21 C3.1 Static groundwater 21 C3.2 Mobile groundwater 22 C3.3 Flowing groundwater 22 C4 Site investigation for aggressive ground conditions 23 C4.1 Introduction 23 C4.2 Desk study 23 C4.3 Site inspection (walk-over survey) 24 C4.4 Visual description of the ground 24 C4.5 Sampling and testing soils 25 C4.6 Sampling and testing groundwater 25 C5 Classification of site locations for chemicals aggressive to concrete 29 C5.1 Groundwater and soil analyses 29 C5.2 Aggressive Chemical Environment for Concrete (ACEC) classification 34 Appendix C1 Recommended test procedures for ground aggressive to concrete 36 Appendix C2 Guidance on comprehensive site investigation of sulfate ground 36 References: Part C 37 Part D: Specifying concrete for general cast-in-situ use D1 Introduction 38 D2 Changes since SD1: D3 Design process 39 D4 Selection of the DC Class and APMs 40 D4.1 Background 40 D4.2 Key factors 40 D5 Composition of concrete to resist chemical attack 41 D5.1 Background 41 D5.2 Using Table D2 42 D5.3 Cement and combination types 42 D5.4 Aggregate type 44 D6 Additional protective measures (APMs) 44 D6.1 General 44 D6.2 Enhance concrete quality (APM1) 45 D6.3 Use controlled permeability formwork (APM2) 45 D6.4 Provide surface protection (APM 3) 45 D6.5 Provide a sacrificial layer (APM4) 46 D6.6 Address drainage of site (APM5) 46 D7 Intended working life 47 D8 Contract documentation 48 References: Part D 48

5 Contents v Part E: Specifying surface-carbonated precast concrete for general use in the ground E1 Introduction 49 E2 Changes since SD1: E3 Design process 50 E3.1 Selection of the DC Class and APMs 50 E3.2 Specifying composition of concrete 50 E3.3 Additional protective measures 50 References: Part E 51 Part F: Design guides for specific precast concrete products F1 Introduction 52 F2 Procedure for using design guides 53 F3 Design guides for precast concrete pipeline systems 55 F3.1 General considerations 55 F3.2 Using Design Guide F1a for specifying concrete for pipes and associated units 56 F3.3 Using Design Guide F1b for specifying internal linings to pipes and associated units 57 F4 Precast box culverts and precast segmental linings for tunnels and shafts 58 F4.1 General considerations 58 F4.2 Using Design Guide F2a for specifying concrete for precast box culverts and segmental linings 59 F4.3 Using Design Guide F2b for specifying internal linings to precast box culverts and segmental linings 60 F5 Design guides for precast concrete masonry units 61 References: Part F 62

6 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 Chairman, Thaumasite Expert Group Department of Civil Engineering, University of Birmingham Dr C A Clear British Cement Association (BCA) Dr N J Crammond Centre for Concrete Construction, BRE Mr I Haining Costain Professor T Harrison Quarry Products Association (QPA) and BSI Committee B/517/1 Mr J C Haynes National House-Building Council (NHBC) Dr D D Higgins Cementitious Slag Makers Association (CSMA) Mr I Holton British Precast Concrete Federation / Loughborough University Mr P Livesey Castle Cement Mr T I Longworth Associate, BRE Mr N Loudon The Highways Agency Dr B K Marsh Arup Dr J D Matthews, Associate, BRE Mr A Morton Hepworth Concrete / Concrete Pipeline Systems Association (CPSA) Dr P J Nixon Associate, BRE Ms L Parker Tarmac Ltd Ms A Scothern The Concrete Centre Dr L K A Sear UK Quality Ash Association (UKQAA) Dr J F Troy Tarmac Ltd Principal Consultees Mr D Appleton Hanson Building Products Dr J C Cripps Department of Civil and Structural Engineering, University of Sheffield Mr A J Elliott Milton Precast / Box Culvert Association Dr T Grounds Tarmac Topblock Dr A Haimoni Keller Ltd / Federation of Piling Specialists (FPS) Mr R M Raymond Hughes Concrete Ltd / Concrete Pipeline Systems Association (CPSA) Mr P Rhodes RMC Mr S Wade Stent / Federation of Piling Specialists (FPS)

7 Part A Introduction 1 A1 Problem of chemical attack 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. 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. A2 Scope and structure of the guidance A2.1 Types of site and chemical agents covered 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

8 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.

9 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 Part C Stage 2 Geotechnical specialist Carry out site investigation to determine chemical conditions for concrete, including water mobility. See Part C Determine DS Class and ACEC Class for site locations using Tables C1 and C2. See Section C5 Stage 3 Designer of building or structure Determine the intended working life of proposed building or structure, and the form and use of specific concrete elements. See Section D7 Parts D, E and F 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? No 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