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Alkali-silica reaction (ASR)
Alkali-silica reaction (ASR) is one of two forms of alkali-aggregate reaction (AAR). ASR is a chemical reaction that occurs between siliceous aggregates and ions present in concrete pore solution. The reaction forms a gel that imbibes water and expands. As a result, increases tensile stresses cause cracks to occur in the paste and through the aggregates in most cases. ASR is affected by aggregate type, the structure of concrete’s air void system, availability of moisture, and the pH of concrete’s pore solution.
How is ASR bad?
For non-structural concrete applications, ASR is an aesthetic problem. ASR typically manifests efflorescence (a white film over the outside of the concrete) that stains the surface, causes lots of cracking, may result in faulting and spalling, promotes the ingress of chlorides thereby exacerbating corrosion of any reinforcing steel, and is just plain ugly.
In structural concrete, ASR is known to reduce strengths in cases when deleterious expansion is well advanced, lower the modulus of elasticity, and increase creep. Corrosion is a structural concern, as is the susceptibility to freeze thaw as expansion and crack widths increase.
< Figure A.F3. Photo. Microscopic Image of ASR-Induced Damage to Concrete Paste and Coarse Aggregates. The image shows a polished sample of concrete. ASR identifying arrows point to cracks within the paste and one of the coarse aggregates.>
What does it look like?
ASR affects all types of concrete structures. There are documented cases of ASR in nearly every state (if not all) of the USA and all Canadian provinces. It manifests itself by:
Where else can I find more information?
Organizations such as the Federal Highway Association (FHWA), the American Concrete Institute (ACI), Portland Cement Association (PCA), American Society for Testing and Materials (ASTM), and Canadian Standards Association (CSA) are all leaders in the research and development of concrete material standards and specifications for North America. Additional information on ASR can be found within many publications from all of these credible organizations. Below are links to their home pages.
The goal of this website is to provide information specific to ASR and provide helpful reference links.