Engineering and Construction

The sulfate resistance of concrete depends on many different factors. In general, the degree of sulfate attack of concrete will depend on the type of cement used, quality of the concrete, soluble sulfate concentration that is in contact with the concrete, and the surface preparation of the concrete
(Mather, 1968).

1. Type of cement. There is a correlation between the sulfate resistance of cement and its tricalcium aluminate content. As previously discussed, it is the chemical reaction of hydrated calcium aluminate and gypsum that forms ettringite. Therefore, limiting the tricalcium aluminate content of cement reduces the potential for the formation of ettringite. It has been stated that the tricalci- um aluminate content of the cement is the greatest single factor that influences the resistance of concrete to sulfate attack, where in general, the lower the tricalcium aluminate content, the greater the sulfate resistance (Bellport, 1968).

2. Quality of concrete. In general, the more impermeable the concrete, the more difficult for the waterborne sulfate to penetrate the concrete surface. To have a low permeability, the concrete must be dense, have high cement content, and a low water-cementitious materials ratio. Using a low water-cementitious materials ratio decreases the permeability of mature concrete (PCA, 1994). A low water-cementitious materials ratio is a requirement of ACI (1990) for concrete subjected to soluble sulfate in the soil or groundwater. For example, the maximum water-cementitious materials ratio must be equal to or less than 0.50 for concrete exposed to moderate sulfate exposure and 0.45 for concrete exposed to severe or very severe sulfate exposure (see Table 18.2). There are many other conditions that can affect the quality of the concrete. For example, a lack of proper consolidation of the concrete can result in excessive voids.

Another condition is the corrosion of reinforcement, which may crack the concrete and increase its permeability. Cracking of concrete may also occur when structural members are subjected to bending stresses. For example, the tensile stress due to a bending moment in a footing may cause the development of microcracks that increases the permeability of the concrete.

3. Concentration of sulfates. As previously mentioned, it is often the geotechnical engineer who obtains the soil or water specimens that will be tested for soluble sulfate concentration. In some cases after construction is complete, the sulfate may become concentrated on crack faces. For example, water evaporating through cracks in concrete flatwork will deposit the sulfate on the crack faces. This concentration of sulfate may cause accelerated deterioration of the concrete.

4. Surface preparation of concrete. An important factor in concrete resistance is the surface preparation, such as the amount of curing of the concrete. Curing results in a stronger and more impermeable concrete, which is better able to resist the effects of salt intrusion (PCA, 1994).