Highly reactive andesite aggregates including opal or tridymite were evaluated by concrete prism test (CPT) in two ways and the suitable conditions of CPT were discussed. 18 concrete mixtures were used to examine the effect of the type and content of aggregate, the type of cementitious material. Two methods for detecting for ASR reactivity were used; RILEM AAR-4 and JASS 5N, and the latter one is of which authors revised few points. As the results, different performances of andesite depending on the size and the different required amounts of fly ash suppressing ASR depending on the mixture proportion were well demonstrated by CPT that is difficult for mortar bar test. As a curing method to avoid alkali leaching, wrapping with wet paper showed much less leaching than curing in a sealed box. From the viewpoint of easiness of operation, small size 75 × 75 × 250 mm is suitable than large size 100 × 100 × 400 mm.
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