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. 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. 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.
Scope and structure of the guidance
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.
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.
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.
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.