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How does ASR happen?

Understanding how ASR happens, or the mechanism of ASR, starts with a basic awareness of the materials used to make concrete. Concrete is made by combining aggregates, cementitious materials, and water.

Figure A2.F1. Photo. Concrete Components. Concrete Consists of Aggregates, Cementitious Materials, and Water. This photo depicts the equation of aggregates plus cementitious materials plus water equals concrete. Each individual component is represented by an individual picture.
Figure A2.F1. Concrete Components.

Contrary to the common assumption that concrete is completely solid; voids exist within the aggregate paste matrix.  There are several orders of magnitude to the size of the voids. The smallest of these voids are known as capillary pores.  Within these pores exists moisture (water) and a combination of ions.  The moisture can be a result of mixing water not yet used for the hydration process, or have infiltrated from an external source.  Ions are present as the result of the dissolution of the cement particles during hydration. Specifically, the following ions are found in pore solution:

  • sodium, Na+
  • potassium, K+
  • hydroxyl, OH-
  • minor amounts of
  • calcium, Ca++
  • and other ionic species

Hydroxyl ions (OH-) "attack" the reactive silica phase (SiO2) on the surface of an aggregate. The SiO2 dissolves freeing the silica into the pore solution where it combines with Na+ and K+ ions and forms an alkali-silica gel. The gel, composed predominantly of Na+, K+ and Si ions with minor amounts of Ca++ ions, absorbs water from the surrounding cement paste and expands causing internal tensile stresses and eventually leading to cracking. Watch the picture...

Figure A2.F2. Animated Illustrations. Sequence of alkali-silica reaction (ASR) in concrete. Six illustrations in total make up the animation. The first shows a jagged orange circle in the middle, labeled silicone dioxide that is surrounded by a grey square labeled Paste. Text at the top reads 'Concrete made using a reactive aggregate supply' Next, appear 2 green arrows, labeled sodium and potassium , pointed towards the silicone dioxide circle and two other teal-colored arrows, labeled hydroxide that also point towards the silicone dioxide circle. Text on both sides of the jagged orange circle reads calcium. At the top text reading ' Hydroxyl and Alkali ions from the pore solution attack the reactive aggregate' The next illustration shows the same orange-colored jagged circle, surrounded by a thick yellow line. There is text above the circle, reading '(Na, K, Ca) silicon gel form' Then, three blue arrows appear from opposing directions each labeled H2O and pointing to the yellow line. Text at the top reads 'The gel absorbs water from the surrounding paste' The next illustration shows the yellow line around the orange circle larger with accompanying text at the top reading ' The gel expands' The final illustration shows the same jagged orange circle with a much thicker yellow line surrounding it and four yellow lines radiating away from the circle. The text at the top reads 'The paste and aggregate crack.'

Figure A2.F2. Animated Illustrations. Sequence of alkali-silica reaction (ASR) in concrete.

For ASR to occur, three conditions must be met:

1. There must be a reactive silica supply
2. There must be moisture
3. There must be an alkali source

Figure A2.F3. Illustration. Triangle of ASR Necessities. There are three essential requirements for deleterious ASR. An equilateral triangle shows that the three necessary components are reactive silica, sufficient alkali, and sufficient moisture.
Figure A2.F3. Illustration. Triangle of ASR Necessities. 

Updated: 02/20/2015

United States Department of Transportation - Federal Highway Administration