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. 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.
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.