Alkaliaggregate

Alkali-aggregate reaction, often shortened to AAR, is a chemical process that can occur in concrete. It involves the interaction between reactive silica in certain aggregates and alkaline pore solutions within the cement paste. This reaction produces an expansive gel, primarily alkali-silica gel.

When moisture is present, this gel absorbs water and swells, creating internal pressures within the concrete. Over time, these pressures can lead to a network of map-cracking on the concrete surface. The rate of the reaction and the extent of expansion depend on several factors, including the type and amount of reactive silica in the aggregate, the alkali content of the cement, the temperature, and the availability of moisture.

Not all aggregates are reactive, and the presence of reactive silica does not automatically mean AAR will occur. The alkali content of the cement is also a critical factor; higher alkali cements are more prone to causing AAR if reactive aggregates are present. Mitigation strategies for AAR in new concrete construction include using non-reactive aggregates, reducing the alkali content of the cement, and incorporating supplementary cementitious materials like fly ash or slag cement, which can help to reduce the availability of alkalis and consume the reactive silica. For existing structures affected by AAR, repair options are often limited and may involve surface treatments or more extensive structural repairs.