Engineering and Construction

The soil or groundwater samples should be obtained after the site has been graded and the location of the proposed building is known. For the planned construction of shallow foundations, near surface soil samples should be obtained for the sulfate testing. For the planned construction of deep foundations that consist of concrete piles or piers, soil and groundwater samples for sulfate testing should be taken at various depths that encompass the entire length of the concrete foundation elements.

The geotechnical engineer should select representative soil samples for testing. If the site contains clean sand or gravel, then these types of soil often have low sulfate contents because of their low capillary rise and high permeability that enables any sulfate to be washed from the soil. But clays can have a higher sulfate content because of their high capillary rise that enables them to draw up sulfate bearing groundwater and their low permeability that prevents the sulfate from being washed from the soil.

Sulfate concentrations in the soil and groundwater can vary throughout the year. For example, the highest concentration of sulfates in the soil and groundwater will tend to occur at the end of the dry season or after a long dry spell. Likewise, the lowest concentration of sulfates will occur at the end of the rainy season, when the sulfate has been diluted or partially flushed out of the soil. The geotechnical engineer should recognize that sampling of soil or groundwater at the end of a heavy rainfall season will most likely be unrepresentative of the most severe condition.

Soil that contains gypsum should always be tested for sulfate content. This is because soluble sulfate (SO4) is released as gypsum (Ca SO4⋅ 2H2O) weathers. Gypsum is an evaporate and can rapidly weather upon exposure to air and water.

There are many environments that could lead to the chemical attack of concrete. For example, if the site had been previously used as a farm, there may be fertilizers or animal wastes in the soil that will have a detrimental effect on concrete. A corrosive environment, such as drainage of acid mine water, will also lead to the deterioration of concrete. Another corrosive environment is seawater, which has both moderate soluble sulfate content and a high salt content that can attack concrete through the process of chemical reactions involving sulfate attack and deterioration of concrete due to the physical growth of salt crystals.

As indicated in Table 18.2, of the types of Portland cements, the most resistant cement is type V, in which the tricalcium aluminate content must be less than 5 percent. Depending on the percentage of soluble sulfate (SO4) in the soil or groundwater, a certain cement type is required as indicated in Table 18.2. In addition, minimum water-cementitious ratios and minimum unconfined compressive strengths f'C are required depending on the amount of sulfate in the soil or groundwater (see Table 18.2).