Options for minimising the risk of alkali–aggregate reactions

An alkali–aggregate reaction (AAR) is one of the many deleterious processes affecting the durability of concrete infrastructure worldwide. It is the result of reactions between the alkaline pore solution in concrete and some reactive phases in the aggregate. The reactions lead to the formation of an expansive force within the concrete, which, in certain conditions, can lead to damaging expansions and cracking in the concrete. For such damaging expansion to occur, all of the following conditions must be present simultaneously: (a) a sufficiently alkaline pore solution, (b) a critical amount of reactive material and (c) a sufficient supply of water. Effective specifications to avoid damage from the above reactions are based on ensuring that at least one of the these conditions is absent. The development of the precautions against AARs around the world generally involves the determination of the necessary level of precaution required based on the aggregate's reactivity, the size of the structural element and the environment (exposure conditions), and a classification of the ‘criticality’ of the structure to build. The development and implementation of preventive actions according to the level of precaution required are then undertaken following prescriptive or performance approaches.