ASR is a form of concrete degradation which occurs mostly when concrete is steadily exposed to a water source. Here is a popularly available definition:

“The alkali–silica reaction (ASR) is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica, which is found in many common aggregates. This reaction causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate (C-S-H). This gel increases in volume with water, and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure. ASR can cause serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.[3]”

(“Alkali–silica Reaction.” Wikipedia. Wikimedia Foundation, 21 Apr. 2015. Web. 14 May 2015 <http://en.wikipedia.org/wiki/Alkali%E2%80%93silica_reaction>.)