Engineering and Construction

Introduction
Sulfate attack of concrete is defined as a chemical and/or physical reaction between sulfates (usually in the soil or groundwater) and concrete or mortar, primarily with hydrated calcium aluminate in the cement-paste matrix, often causing deterioration (ACI, 1990). Sulfate attack of concrete occurs throughout the world, especially in arid areas, such as the southwestern United States. In arid regions, the salts are drawn up into the concrete and then deposited on the concrete surface as the groundwater evaporates. Sulfate attack of concrete can cause a physical loss of concrete and unusu- al cracking and discoloration of the concrete. Sulfate attack can also cause scaling, pitting, and etching of the concrete surface (Detwiler et al., 2000). Figures 12.1 and 12.2 show sulfate attack of a concrete patio.

Typically the geotechnical engineer obtains the representative soil or groundwater samples to be tested for sulfate content. The geotechnical engineer can analyze the soil samples or groundwater inhouse, but a more common procedure is to send the samples to a chemical laboratory for testing.

There are different methods to determine the soluble sulfate content in soil or groundwater. One method is to precipitate out and then weigh the sulfate compounds. A faster and easier method is to add barium chloride to the solution and then compare the turbidity (relative cloudiness) of barium sulfate with known concentration standards.

The geotechnical engineer is often required to provide soluble sulfate specifications and recommendations for concrete foundations. In addition to concrete foundation deterioration due to sulfate attack, there could also be deterioration of concrete or the steel reinforcement due to chloride attack and acid attack. According to AASHTO (1996), laboratory testing of soil and groundwater samples for sulfates, chloride, and pH should be sufficient to assess deterioration potential. When chemical wastes are suspected, a more thorough chemical analysis of soil and groundwater samples would be required.