TYPES OF SULFATE ATTACK AND DETERIORATION MECHANISMS

Degradation of cementitious systems exposed to sulfate salts is the result of sulfate transport through the pore system, chemical reaction with the hydration product phases present, generation of stresses due to the creation of the expansive reaction products, and the mechanical response (typically spalling and cracking) of the bulk material due to these stresses. Each component of this process plays a unique role in the ultimate response of the concrete; change the material properties relevant to any one component and the concrete performance can change dramatically. Therefore, laboratory tests of "sulfate attack'' that are based primarily on submerging the specimens in sulfate solution and then measuring some physical property, such as expansion, are effectively lumping all of these mechanisms into a single test. The result is a test that characterizes how a particular concrete performs under specific conditions. If the field conditions are different, the performance of the concrete can also be different. Therefore, to predict the resistance to sulfate attack of a concrete, it is necessary to develop a protocol that takes into account the type of exposure and separates the various mechanisms. The major degradation processes that the concrete is likely to encounter are sulfate attack, corrosion of reinforcing steel, alkali-aggregate reactions, carbonation and leaching by neutral or acidic ground waters. These degradation processes often involve the transport of moisture, ions and dissolved gases into concrete by diffusion, convection and capillary forces. Rates of degradation will be often controlled by the rate of intrusion of moisture and dissolved salts and gases into concrete.