Three exposure classes (XA1, XA2 and XA3) are described in BS EN 206-1 (2000) according to the aggressiveness of attack on concrete in natural soils and groundwater (corresponding to slightly, moderately and highly aggressive respectively). The chemicals taken into account are sulfate, H+ ions and also, in the case of groundwater, CO2, Mg2+ and NH4 + ions. The Standard gives guid- ance on the maximum w/c ratios, minimum strength class and minimum cement contents for the different exposure classes.
More recently BRE Special Digest 1 (2005) has updated its guidance on the specification of concrete in aggressive ground. This includes attack on concrete by acids, as well as by sulfates, in natural ground and brownfield locations. The acids covered by the Digest are those most commonly found in natural ground- water, viz. sulfuric acid, humic acid and carbonic acid. As mentioned in Section 4.5 of this chapter, sites are classified according to the Aggressive Chemical Environment for Concrete (ACEC) which takes into account sulfate concen- tration, potential concentrations of sulfate based on pyrite/sulfide present, pH, magnesium ion concentration and mobility of groundwater. Additionally, HCl, HNO3, and aggressive CO2 can be incorporated into determining the ACEC Class. Based on this class and other factors (e.g., type of concrete element, intended working life, exposure to hydraulic gradient of groundwater), the BRE SD1 (2005) gives guidance on the Design Chemical (DC) Class, which defines the quality (maximum w/c ratios and minimum cement (or combination) contents) of concrete needed to resist chemical attack, and also on additional protective measures that can be applied. Concrete placed in ACEC Classes given the suffix `z’ needs to resist acid conditions. Since concrete quality is the main factor in determining acid resistance, no restrictions are placed on the use of any of the cement types listed in the BRE SD1 (2005). Recommendations are also given for the use of protective linings for the internal surfaces of precast concrete pipes, box culverts and segmental linings for tunnels and shafts under a range of aggressive conditions exacerbated by flowing water or effluents, such as relatively pure water with aggressive CO2 at concentrations >15 mg/l, acidic solutions of pH < 5 and effluents, particularly sewage, containing sulfate at concentrations > 1400 mg/l.
Specialist advice is needed where concrete is exposed to other less frequently occurring chemicals. BS 5502-21 (1990) gives guidance on selection of con- cretes to be exposed to attack by milk, silage, slurry and other agricultural agents. A guide to the effect of many different substances on concrete and advice on selecting protective treatments is given by the Portland Cement Association (2001).
