United States Department of Transportation
i
Suppression of ASR Through Aggregate Coating
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2013-08-01
[PDF-1.76 MB]
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Abstract:Many highways, runways, parking lots and bridges are suffering from premature deterioration due to alkali silica reaction (ASR) that takes place between the alkalis contributed primarily by the cement and a reactive form of silica from specific silicon-containing rocks or minerals in the aggregates utilized in concrete production. This produces an alkali/silica gel that, in the presence of sufficient moisture, will expand and produce stresses that damage the concrete. With time, the expansion of the gel generates internal pressure that can lead to cracking of the concrete that provides pathways for ingress of deleterious materials such as water, sulfates and chlorides to the interior of the concrete matrix. This can then lead to serious durability issues such as freeze/thaw damage, sulfate attack, or corrosion of steel or rebar. Several mitigation options for ASR exist, including the use of low-alkali cements, mineral additives, or chemical additives. All of these increase the cost of concrete: for example, the addition of lithium nitrate causes an expense of over $ 20 per cubic yard of concrete. All of these methods have another commonality – they apply the treatment globally at great expense, while the root of the ASR problem is localized surface phenomenon at the aggregate-cement interface. Therefore, the proposed solutions to the problem of ASR that were developed in this investigation targeted this interface with the aim of developing cost-effective pretreatment processes for problematic aggregates that will eliminate the alkali silicate reaction. A mineral additive was also discovered that can be added to the concrete mixture that prevents ASR expansion at as little as 1% by weight.
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