ASR is the most common form of alkali-aggregate reaction (AAR) in concrete; the other, much less common, form is alkali-carbonate reaction (ACR). ASR and ACR are therefore both subsets of AAR.
ASR is caused by a reaction between the hydroxyl ions in the alkaline cement pore solution in the concrete and reactive forms of silica in the aggregate (eg: chert, quartzite, opal, strained quartz crystals).
A gel is produced, which increases in volume by taking up water and so exerts an expansive pressure, resulting in failure of the concrete. In unrestrained concrete (that is, without any reinforcement), ASR causes characteristic 'map cracking' or 'Isle of Man cracking'.
Gel may be present in cracks and within aggregate particles. The best technique for the identification of ASR is the examination of concrete in thin section, using a petrographic microscope. Alternatively, polished sections of concrete can be examined by scanning electron microscopy (SEM); this has the advantage that the gel can be analysed using X-ray microanalysis in order to confirm the identification beyond any doubt.
The conditions required for ASR to occur are:
* A sufficiently high alkali content of the cement (or alkali fromother sources)
* A reactive aggregate, such as chert
* Water - ASR will not occur if there is no available water in the concrete, since alkali-silica gel formation requires water
The use of pozzolans in the concrete mix as a partial cement replacement can reduce the likelihood of ASR occurring as they reduce the alkalinity of the pore fluid.
With some aggregates, expansion due to ASR increases in proportion with the amount of reactive aggregate in the concrete. Other aggregates show what is called a “pessimum” effect; if the proportion of reactive aggregate in test mixes is varied, while other factors are kept constant, maximum concrete expansion occurs at a particular aggregate content. Higher or lower proportions of reactive aggregate will give a lower expansion.
The process of ASR is believed to be in many respects similar to the pozzolanic reaction, such as occurs normally in concrete containing pulverised fuel ash (PFA), for example. However, there is an important difference. In the pozzolanic reaction small pozzolanic particles are reacting in a Ca-rich environment, while ASR occurs in mature concrete and involves larger particles of aggregate.
