What specific concrete mix design modifications are needed to mitigate the risk of alkali-silica reaction (ASR) in concrete pavements?

Alkali-Silica Reaction (ASR) is a chemical reaction that occurs in concrete between the alkali hydroxides in cement and certain reactive forms of silica in the aggregate. This reaction produces an expansive gel that causes cracking and deterioration of the concrete. To mitigate the risk of ASR in concrete pavements, specific modifications to the concrete mix design are necessary. One key modification is to use non-reactive aggregates. Aggregates should be tested for their reactivity potential using standardized test methods, such as ASTM C1260 (Accelerated Mortar Bar Test). If reactive aggregates must be used, supplementary cementitious materials (SCMs) should be incorporated into the mix. SCMs, such as fly ash, slag, and silica fume, react with the alkali hydroxides, reducing their availability to react with the silica in the aggregate. Fly ash and slag are byproducts of industrial processes and silica fume is a byproduct of silicon production. Using low-alkali cement is another effective strategy. Low-alkali cement contains a lower amount of alkali oxides (Na2O and K2O), reducing the potential for ASR. The alkali content of the cement should be below a specified limit, typically 0.60% by weight. Another approach is to reduce the water-to-cement ratio (w/c ratio) of the mix. A lower w/c ratio reduces the permeability of the concrete, limiting the ingress of water and alkalis, which are necessary for the ASR to occur. However, reducing the w/c ratio can also reduce the workability of the concrete, so careful consideration is needed. Lithium-based admixtures can also be used to mitigate ASR. Lithium ions react with the silica gel, preventing its expansion. However, lithium admixtures can be expensive. Proper curing practices are also important to minimize ASR. Curing helps to reduce the permeability of the concrete and limit the availability of water for the reaction. In summary, a combination of these mix design modifications, including the use of non-reactive aggregates, SCMs, low-alkali cement, low w/c ratio, lithium admixtures, and proper curing, is typically needed to effectively mitigate the risk of ASR in concrete pavements.