Alkali Silica Reaction (ASR)



Alkali Silica Reaction (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. [a]

Typical indicators of ASR are random map cracking and, in advanced cases, closed joints and attendant spalled concrete. ASR requires water for the chemical reaction and so cracking usually appears in areas with a frequent supply of moisture, such as close to the waterline in piers, near the ground behind retaining walls, near joints and free edges in pavements, or in piers or columns subject to wicking action. A thin section petrographic examination can conclusively identify ASR. [b]

There are a few ways to prevent ASR. One would be to reduce the amount of reactive aggregate in the concrete mix. Another way is through the use of pozzolans which reduce the alkalinity of the pore fluid. [c]

If concrete is affected by ASR then it may be possible to treat it with soluble lithium salts that mitigate the ASR. A main difficulty is getting it into concrete within the concrete’s lifetime. [d]