Self-immunity of dam concrete to alkali-aggregate reaction

Alkali-aggregate reaction(AAR)of dam concrete is one of the decisive factors in the technical and economical consideration of many large-scale hydraulic engineering projects,however,the current test standards fail to give an objective appraisal of the impact of AAR on safety risk of practical dam structures.It is pointed out that the self-immunity of dam concrete to AAR,is contributed to by some intrinsic characteristics of dam concrete such as alkali-reactive stone powder brought in by manufactured fine aggregate with alkali-reactivity,air entrainment,and low volume ratio of cementitious paste to aggregates.This self-immunity was demonstrated by rapid mortar bar tests.The existence of 20% to 30%(by mass of binder)of alkali-reactive stone powder with a specific surface area between 358 m2/kg and 812 m2/kg decreased by 41%～58% the AAR expansion of mortar bars with potentially reactive sand-stone aggregate.An air content of 3.5% to 5% in dam concrete corresponded to about 40% decrease of AAR expansion of mortar.This kind of self-immunity should be considered in the AAR risk analysis of dam concrete in real practice,and thus performance-based test methods reflecting the characteristics of the mixture proportion of dam concrete should be developed and adopted.