Alkali Aggregate Reactions in Concrete

Alkali aggregate reaction (AAR) is a mechanism of deterioration of concrete resulting from an interaction between alkaline pore fluids in the concrete principally originating from the Portland cement, and certain types of aggregates. Although it was first identified in the USA in 1940, for many years no cases were found in the UK. In 1971, however, the reaction was found to be the cause of cracking in a concrete dam on Jersey and in 1976 was identified as the cause of cracking in the concrete bases of electricity sub-stations in South-West England. Since then, about 30 other cases have been identified, mainly in the South-West and Midlands of England. In relation to the vast amount of concrete in use, therefore, the amount of concrete affected by AAR is very small and the problem should not be allowed to get out of perspective. It still appears that the cement/aggregate combinations in use in the UK will react together only under the most adverse combinations of high cement content, high alkali in the cement and exposure to water. However, as this combination can occur in important structures the reaction cannot be ignored.