Chemical attack and aggressive ground

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The resistance of cured concrete to acid attack is largely dependent upon the quality of the concrete, although the addition of granulated blastfurnace slag (GGBS) or fly ash (pulverised-fuel ash [PFA]) increases the resistance to acids. Limestone-aggregate concrete is more vulnerable to acid attack than concretes with other aggregates. The resistance of cured concrete to chemical attack is defined by the design chemical class number, ranging from DC1 (low resistance) to DC 4 (high resistance). The required design chemical class (DC Class) of the concrete is calculated by combining the effects of the sulfate content of the ground, the nature of the groundwater and the anticipated working life of the construction (BRE Special Digest 1: 2005).

Determining the design chemical class required for concrete in a particular ground environment is a three-stage process. The first stage is to determine the design sulfate class (DS) of the site. This is a five-level classification based primarily on the sulfate content of the soil and/or groundwater. It takes into account the concentrations of calcium sulfate, also the more soluble

Design Sulfate Class

Limits of sulfate (mg/l) < 500 500 - 1500 1600 - 3000 3100 - 6000 > 6000

The next stage is to determine the aggressive chemical environment for concrete (ACEC) classification. Adverse ground conditions such as acidity (low pH), often found in brownfield sites, and/or mobile ground-water lead to a more severe ACEC classification. Static water is more benign and leads to a less severe ACEC classification. The aggressive chemical environments for concrete classes range from AC1 (the least aggressive) to AC5 (the most aggressive), and are based on a combination of the design sulfate class, groundwater mobility and pH.

The design chemical class (DC1 to DC4) defines the qualities of the concrete required to resist chemical attack. It is determined from the ACEC class of the ground together with factors relating to the concrete, such as section size and intended working life (e.g. 100 years). As there are only four design chemical classes against five ACEC classes, for the severest grade of ACEC (i.e. AC5) there are additional protective measures (APMs) which can be specified to combat the more adverse conditions. Usually APM3 (surface protection to the concrete) is appropriate for AC5 environments, but for increasing the intended working life from 50 to 100 years under the less aggressive AC3 or AC4 conditions, any one APM may be applied.

Additional protective measures (APMs) for buried concrete:

APM1 enhance the concrete quality;

APM2 use controlled permeability formwork;

APM3 provide surface protection to the concrete;

APM4 increase the thickness of the concrete as a sacrificial layer;

APM5 reduce groundwater by drainage of the site.

Careful consideration of all these additional factors is required to ensure that a suitably durable concrete, appropriate to the job, is delivered on site for use in aggressive ground and chemical environments (BRE Special Digest 1: 2005).

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