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In circumstances where concretes containing UK aggregates with reactive siliceous components are exposed to high concentrations of NaCl there is conflicting evidence as to whether salt ingress enhances susceptibility to alkali-silica reaction (ASR). There is also uncertainty over the mechanisms involved. The research undertaken was intended to elucidate these issues. Expansion tests with concrete prisms and cores immersed in 2 and 7 M NaCl solutions at 20 and 38°C showed that significant ASR expansion and cracking could be induced in specimens containing the more highly reactive UK aggregates at alkali levels well below 3 kg/m3 Na2Oeq, which is the present recommended UK limit for minimizing the risk of ASR. Even concretes containing aggregates of lower reactivity (chert-bearing gravels, etc.) were found to exhibit significant ASR expansion when exposed to NaCl solution in specimens of 3–4 kg/m3 Na2Oeq. The role of NaCl ingress in exacerbating the development of ASR in concretes of varied composition was studied by thermoanalytical techniques and petrography combined with electron probe microanalysis. The mechanisms were found to depend on several features of the environment (NaCl concentration, temperature, etc.) and the composition of the concrete (type of reactive aggregate, alkali content, cement mineralogy, etc.). A simplified reaction scheme was proposed to account for the results obtained.

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