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Moisture is one of the three necessary elements for ASR to occur. The alkalis combine with the "free" silica to form a gel that consumes water. As the gel consumes the water, it expands.  The expansion results in tensile stresses that ultimately cause deleterious cracks in both the cement paste and coarse aggregates. 

The source of moisture supply varies.  Initially, moisture comes from the mix water. Over time, exposure to external sources will contribute to the internal moisture supply.  These sources include rain, lakes, rivers, sprinklers, faucets, or any other means by which water comes directly into contact with the surface of concrete.  The water or moisture from an external source permeates through the void system and infiltrates the pores.

ASR can be curbed if there is no source for additional moisture.  For example, a curb built of concrete containing ASR reactive aggregates is more likely to exhibit signs of deleterious expansion (excessive cracking) if subjected to a wet environment such as rain.  However, if the curb is sheltered in a way as to deflect any moisture from contacting the concrete’s surface, then there is no moisture to infiltrate the pore system and cause gel expansion.

The difference in reactive concrete exposed to rain compared to the same concrete under a cover...

Figure A2b.F1. Illustration. Moisture Exposure. ASR-Reactive Concrete Exposed and Sheltered from Rain. This photo shows a curb that is sheltered from rain on one side and exposed to the elements on the other side. A description and the location is listed in the top left of the illustration. 
Figure A2b.F1. Moisture Exposure.



 <Figure A2b.F2. Illustration.  Moisture Exposure.  Shows the curb with a green box enclosing the spot where the sheltered and exposed parts of the curb meet.  Figure 16b shows an enlarged photo of the spot outlined in green in 16a.>

Figure A2b.F3. Illustration. Effect of Relative Humidity on ASR Expansion in Concrete. This illustration shows graphed results from research work on the right side and an explanation of particular conditions on the left side.
Figure A2b.F3.  Effect of Relative Humidity on ASR Expansion in Concrete. 


Updated: 02/20/2015

United States Department of Transportation - Federal Highway Administration