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Alkali-silica reaction (ASR)

What is ASR?

The History of ASR

How Does ASR Happen?

What are common methods used for detecting ASR?

So, how do you control ASR?

What does ASR look like in the field?

What is AAR?


What is ASR?

Figure A.F1. Photo. ASR-Induced Damage in Concrete Dam. This photo shows random cracking on all exposed surfaces of a concrete dam.
Figure A.F1. ASR-Induced Damage in Concrete Dam. 

Figure A.F2. Photo. ASR-Induced Damage in Concrete Element. Expansion in all Directions Results in Classic Map-Cracking. This photo shows a series of interlocking, jagged-shaped cracks on the surface of a concrete barrier.
Figure A.F2.  ASR-Induced Damage in Concrete Element. 

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.
The goal of this website is to provide information specific to ASR and provide helpful reference links.

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:

  • cracking the surface of the concrete, which may be random in direction (i.e. map or pattern cracking) or may show preferred orientation if expansion is restrained in one direction
    • If the concrete is unrestrained (for example a free standing wall, or a jersey barrier), there will be uniform expansion in all directions and this will result in "map-cracking." The term map-cracking refers to the similarity of the random crack pattern to that of the lines on a city map.
    • However, ASR tends to have a preferred crack orientation when restraint is involved. In pavements, for example, longitudinal expansion is restrained by the closing of joints. In this case, the concrete will continue to expand in the transverse and vertical directions. As a result, cracking will occur in the longitudinal direction. In prestressed structures, cracking will occur parallel to the prestressed tendons. Reinforced columns will exhibit longitudinal crack patterns as a result of ASR induced expansion.
  • discoloration around cracks
  • gel exudation from cracks
  • misalignment of adjacent sections
  • closing of joints, extrusion of joint sealant and crushing/spalling of concrete around joints
  • pop-outs over reactive aggregate particles
  • operation difficulties (e.g. jamming of sluice gates in dams)

Figure A.F4.Photos. Typical Symptoms of ASR. Six photos are shown. Top left hand corner photo shows a series of interlocking, jagged-shaped cracks on the surface of a concrete barrier, and accompanying text reads, 'Random (map or pattern) cracking'. Top right hand photo shows a concrete pavement with map cracking, and accompanying text reads, 'Preferred alignment of cracks'.Middle left hand corner photo shows the misalignment of adjacent sections of a parapet wall on a highway bridge, and accompanying text reads, 'Misalignment of adjacent sections'. Although there is no scale, the lateral misalignment could be on the order of several centimeters. Both sections contain horizontal and vertical cracks. Middle right hand corner photo shows patches of joint-sealing material along either side of the joint; one side of the joint also contains numerous interlocking cracks, and accompanying text reads, 'Extrusion of joint sealant'. Bottom left hand photo shows a close-up view of a pavement joint section that has disintegrated, and accompanying text reads, 'Crushing of concrete at joints.' Bottom right hand photo shows a close-up view of a pop-out on a concrete pavement, and accompanying text reads, 'Pop-out over reactive aggregate'.
Figure A.F4.  Typical Symptoms of ASR. 

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.

FHWA: www.fhwa.dot.gov/

ACI: www.concrete.org/general/home.asp

PCA: www.cement.org

ASTM: www.astm.org

AASHTO: www.transportation.org/

CSA: www.csa.ca/

The goal of this website is to provide information specific to ASR and provide helpful reference links.

 
Updated: 04/07/2011
 

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United States Department of Transportation - Federal Highway Administration