Alkali reactivity, also known as alkali-aggregate reaction (AAR) or concrete expansion (aggregate alkali reactivity), is an important phenomenon studied in cement testing. It refers to the chemical compositions and reaction that occurs between the alkalis present in cement and certain reactive minerals in alkali aggregates used in concrete. This reaction can lead to significant expansion and cracking of concrete structures, compromising their durability and integrity over time.
The main types of alkali reactivity are alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR)
ASR is the more common and well-studied form, involving the reaction between the alkalis in cement and certain types of silica minerals in alkali aggregates, such as reactive forms of silica, opal, chalcedony, and volcanic glass. ACR, on the other hand, occurs when alkalis in cement react with certain types of carbonate minerals, like dolomite or limestone, present in alkali reactive aggregates.
These include the mortar bar test, concrete prism test, and petrographic examination of alkali aggregates. In the mortar bar test, cement-aggregate mixtures are prepared and exposed to specific curing conditions. The expansion of mortar bars is measured over time to evaluate the reactivity of the materials. Concrete prism tests involve casting concrete specimens with potentially reactive aggregates, monitoring their expansion, and assessing their performance under different environmental conditions.
By identifying reactive concrete aggregates and adjusting the alkali content in cement, engineers can mitigate the risk of alkali reactivity, ensuring the long-term performance of concrete structures.
It helps engineers make informed decisions regarding material selection and mix design to ensure the durability and longevity of concrete structures.