Part A: Introduction - BRE projects website

1 SD1:2005 part A Draft 1 v14 : 16/2/051 part A: IntroductionA1. The problem of chemical attackChemical agents that are destructive to concrete maybe found in the ground. In the UK, sulfates and acids,naturally occurring in soil and groundwater, are theagents most likely to attack concrete. The effects canbe serious (see Figure A1) resulting in expansion andsoftening of the concrete to a mush. A substantialnumber of other substances are known to beaggressive, most resulting from activities of man, butcollectively these are a lesser problem as they areencountered only rarely by concrete in the has been standard practice in the UK for at least sixdecades to design concrete for installation in theground to be resistant to attack from commonly foundchemicals, including sulfates and acids. BRE haveunderpinned this approach by issuing a series ofguidance notes and Digests, dating back to 1939, onthe causes of chemical attack and how to specifychemically resistant , the large majority of concrete installedin the ground has performed entirely satisfactorily andis expected to do so for its required working , however, cases of chemical attack havecome to light and have been subject to research byBRE and others.

2 Some of these cases have beenattributed to rarely occurring chemicals not specificallycovered by BRE Digests; some to natural groundconditions for which there was insufficient guidance,such as occurrence of pyrite; and some to theemergence of previously unrecognised attackmechanisms, such as the thaumasite form of sulfateattack (TSA), which has been extensively reported inthe last decade. [1]Guidance in BRE Digests has necessarily evolved tocater for successive adverse field findings, but also totake advantage of the emergence of new concreteconstituents and construction methods, and maintainharmony with newly published standards, latterlyEuropean ones. In order to be both comprehensive andflexible, Digests have tended to become longer andmore complex. One objective of this third edition ofSpecial Digest 1 (SD1) is to simplify the aims and changes are discussed later. A2.

3 Scope and structure Types of site and chemical agent coveredSD1 provides guidance on the specification ofconcrete for installation in natural ground andbrownfield locations. The definition of a brownfieldlocation adopted here is one that has been subjectto industrial development, storage of chemicals, ordeposition of waste, and which may containaggressive chemicals in residual surface materialsor in ground penetrated by leachates. Theprocedures given for ground assessment andconcrete specification cover the fairly commonoccurrence of sulfates, sulfides and acids. Theyalso cover the more rarely occurring aggressivecarbon dioxide found in some ground and A1: Severe sulfate attack in a 30-year-oldhighway bridge sub-structure exposed to wet,pyritic clay :2005 part A Draft 1 v14 : 16/2/052 While SD1 discusses several aggressive agents, suchas ammonium salts and phenols, occasionally found inheavily contaminated ground, no specific proceduresare included for dealing with these.

4 Specialist adviceshould be sought if they are ReadershipSD1 provides practical guidance to ground specialistson the assessment of ground in respect ofaggressiveness to concrete, and to concrete designers,contractors, specifiers and producers on thespecification of concrete to resist chemical attack. Structure of the guidanceGuidance is given in Parts B to F as follows: part B describes modes of chemical attack anddiscusses the mechanisms of the principal types,including sulfate and acid attack, and the action ofaggressive carbon dioxide. part C deals with assessment of the chemicalaggressiveness of the ground. It gives procedures forthe determination of Design Sulfate Class from solublesulfate and magnesium and from the potential sulfate(eg from oxidation of pyrite). It shows how the DesignSulfate Class together with pH and mobility ofgroundwater may collectively be taken into account fornatural ground and brownfield sites to classify alocation in terms of Aggressive Chemical Environmentfor Concrete (ACEC) class.

5 part D gives recommendations for the specification ofconcrete for general cast-in-situ use in the ground. Itexplains how to derive an appropriate quality ofconcrete, termed the Design Chemical Class (DCClass), from a consideration of the ACEC classtogether with the hydraulic gradient due togroundwater, the type and thickness of the concreteelement, and its Intended Working Life. In some cases,where conditions are highly aggressive, AdditionalProtective Measures (APM) are D follows this with guidance on the constituents ofconcrete required to achieve the identified DC is in terms of maximum free water/cementratio, minimum cement content and type of E gives recommendations for specifying surface-carbonated precast concrete for general use in theground. An essential requirement for compliance withthis part is that surface-carbonation is assured byexposure of the precast concrete to air for aminimum of 10 days after curing.

6 Since suchcarbonation provides a degree of resistance tosulfate attack, the recommendations for thederivation of DC Class in respect of sulfates isrelaxed by one level. Other than this, therecommendations of part D are followed forconcrete F includes Design Guides for specification ofspecific precast concrete products, includingpipeline systems, box culverts, and segmentallinings for tunnels and shafts. These products aremanufactured under rigorous quality control toensure appropriate mix composition and achieverelatively low concrete permeability. Together theseprovide an inherently high quality in respect ofchemical resistance. Consequently, a furtherrelaxation (beyond that allowed for surface-carbonation) is permitted in respect of specificationof DC Class for aggressive sulfate conditions. Inpractice this relaxation is used to offset the general-use recommendation that a higher DC Class shouldbe specified where concrete is of thin cross-section,or will encounter a relatively high hydraulic F also covers specification of precast concretemasonry units (concrete blocks) for aggressiveground conditions.

7 The guidance is based onDesign Sulfate Class rather than ACEC Class asthere is currently no correlation of blockperformance with the latter, though work on this glossary of terms is included as Appendix Diagrammatic overview of groundassessment and concrete specificationAn overview of the various procedures for groundassessment and specification of concrete is given inFigure A2. This is arranged in four stages accordingto the construction sector that has keyresponsibility. Within each of these stages theprincipal tasks are shown in boxes, with referencesto the relevant Sections of SD1. Whilst most stepsare equally applicable to all uses of concrete, thereis a differentiation in Stage 3 for the determinationof DC Class and APM between the three categoriesof concrete element dealt with in Parts D, E and F. SD1:2005 part A Draft 1 v14 : 16/2/053 Stage 1 Geotechnical specialistStage 2 Stage 3 Carry out site investigation to determine chemical conditions for concrete, including water mobility.

8 See part C. General use of surface-carbonatedprecast concreteSpecific precast concrete productsConsider design options for building or structureand prepare specification for site investigation. Inform geotechnical specialist of design concept and site investigation requirements. Parts D, E and F part CDesigner of building / structure Determine the Intended Working Lifeof proposed building or structures and theform and use of specific concrete Section DS Class and ACEC Classfor site locations using Tables C1 and C2. See Section C5. Figure A2: Procedure for design of buried concrete for use in an aggressive chemical environment. - Use part Determine the DC Class and APM for the concrete using Design Guides F1a, F1b, F2a, F2b, F3a, 4 Obtain from Contract Documents the specified DC Class, number and type of APM, and any other design requirements for each concrete element.

9 Are all requirements of Design Guide and Contract Documents met?Accept concrete mix design for specific any APM specified for DC Classor in Contract Documents. Designer ofbuilding / structure Where concrete is being supplied ready-mixed, check the proposed mix for conformity to the DC-Class in Contract Documents the DS Class & 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 of part F. State requirements and options for concrete specification, including: - specifed DC Class of concrete after any enhancement, - specified number and type of APM and compressive strength class of concrete, - any other requirements. 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.

10 Contractor for building/ structure (in liaison with any third-party concrete producer)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 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 use of cast-in-situ concreteSD1:2005 part A Draft 1 v14 : 16/2/054A3. Background to guidance on sulfate attackOne of the key drivers for revision of BRE Digestsdealing with concrete in aggressive ground since the1990s has been a growing recognition of theoccurrence of the thaumasite form of sulfate attack(TSA) in UK buildings and has long been known in the UK that concretes madewith Portland cements are vulnerable to attack bysulfates in the ground.