What causes Alkali-aggregate reaction (AAR) and what are its consequences in concrete?

Alkali-aggregate reaction (AAR) is caused by a chemical reaction between the alkaline hydroxides in concrete pore solution and certain reactive forms of silica present in some aggregates. These reactive aggregates, when exposed to the highly alkaline environment of the concrete, react to form an expansive gel. This gel absorbs water and expands, creating internal pressure within the concrete. The consequences of AAR include cracking, expansion, and loss of strength and stiffness in the concrete. The cracking can be extensive and lead to significant deterioration of the structure. The expansion can cause misalignment of structural elements and damage to adjacent components. AAR can significantly reduce the service life of concrete structures, particularly in environments with high moisture levels and temperature fluctuations.