Deterioration of Concrete Dams Due to Alkali-Aggregate Reaction

For many years engineers have been concerned over a peculiar and charact eristic type of concrete deterioration, manifested by large-scale, randompattern cracking. Such cracking is not unusual or new; in fact it is difficult to find massive concrete structures that do not display random cracking in so me degree. Most instances of pronounced pattern or map cracking, as distinguished from ordinary small-scale crazing, have been attributed to excessive internal expansion, aggravated by varying degrees of surface drying shrinkage. The mechanis ms producing internal expansion have been explained in the past on the basis of unso und aggregate or unso und cement, the latter resulting from high magnesia content or underburning and high free-lime content. Although some instances of expansive deterioration appear to be explained adequately by unso undness, mostcases, until recently, have remained enigmas and subjects for speculation only. Many concrete dams in all sections of the United States have developed evidence of this type of degeneration. Several of these cases have been caused by adverse chemical reactions between the a lkalies in Portland cement and certain siliceous mineral constituents in the aggregates.