Alkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition.
ASR is caused by a reaction between the hydroxyl ions in the alkaline cement pore solution in the concrete and reactive forms of silica in the aggregate (e.g. chert, quartzite, opal, strained quartz crystals). A gel is produced, which increases in volume by taking up water and so it exerts an expansive pressure, resulting in failure of the concrete.
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Scanning electron microscope image of chert aggregate particle with numerous internal cracks due to ASR; cracks extend into the adjacent concrete
(click to enlarge image)
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Detail of aggregate showing alkali-silica gel extruded into cracks within the concrete. Ettringite is also present within some cracks
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The conditions required for ASR to occur are:
- A sufficiently high alkali content of the cement (or alkali from other sources)
- A reactive aggregate, such as chert
- Water - ASR will not occur if there is no available water in the concrete, since alkali-silica gel formation requires water
The best way to avoid ASR is to use non-reactive aggregates, but these are not always readily available. In this case, the concrete mix designer should be made aware of the Na2O -equivalent (in %) of all products he intends to use in the concrete mix in order to ensure that Na2O equivalent value does not exceed the acceptable amount per m3 (usually set around 3.5 kg / m3).
A simpler way to reduce the risk of ASR is to incorporate a mature crystalline admixture, such as Penetron Admix, into the concrete mix. This will ensure the concrete is waterproofed in-depth and deny the ASR the necessary water for the reaction to take place. Penetron Admix has shown in a test at the MFPA-Leipzig (U 2.2/05-185) that cracks will self-heal upon when presented with water. Many other tests have proven the ability of Penetron crystals to waterproof the capillary structure inside concrete. Further, Penetron Admix is certified by the MPA-Stuttgart (90 16788/P-2008-1/Bl) to correspond to DIN V 18998 and as such has no negative influence on the embedded steel. (= U-Sign).
