Alkali Reactivity Testing

Alkali Reactivity Testing

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.

To assess the alkali reactivity potential of cement, various testing methods are employed.

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.

The significance of alkali reactivity testing lies in its role in selecting appropriate cement-aggregate combinations and designing durable concrete mixtures.

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.

In conclusion, alkali reactivity testing plays a crucial role in evaluating the potential for expansion and cracking in concrete due to alkali-aggregate reactions.

It helps engineers make informed decisions regarding material selection and mix design to ensure the durability and longevity of concrete structures.

Alkali Reactive Test Method ASTM C289: Cement Testing

ALKALI REACTIVE TESTING, also known as alkali silica reaction (ASR) testing, is a critical procedure used to assess the potential reactivity of aggregates in concrete structures and chemical compositions. The alkali silica reaction refers to a chemical reaction between the alkaline solutions in concrete and reactive silica present in certain aggregates, resulting in deleterious internal expansion. The concern? Potentially deleterious internal expansion.

To conduct this test method, hardened concrete prisms or mortar bars are prepared with the reactive aggregate combinations to be tested. These specimens are then subjected to a highly alkaline solution, typically a sodium hydroxide (NaOH) solution, which simulates the concrete pore solution. Over a specified test duration, the expansion due to alkali of the specimens is monitored and measured in concrete structure.

The test results obtained from alkali reactive testing with ion concentration provide valuable information about the potential for concrete deterioration caused by ASR. Petrographic examination of the specimens can also be conducted to evaluate the mineralogical composition and assess the presence of reactive grains in known field performance.

Several test methods, such as the mortar bar method and the concrete prism test, have been developed to assess alkali reactive tests. These methods involve measuring length changes or expansion of the specimens due to alkali silica reactivity and silica reactivity of aggregates.

Alkali reactivity testing is crucial for assessing the suitability of aggregates for use in concrete mixes, as it helps prevent potential damage to concrete over time. By determining the potential reactivity of aggregates, engineers can make informed decisions in selecting appropriate materials and controlling the alkali content in cementitious mixtures. This ensures the long-term durability and performance of concrete in various field conditions. An indispensable part of aggregate testing.