An Accelerated Concrete Prism Test for Evaluating Fly Ash Mitigation of Alkali-Silica Reaction in Reactive Tuff...

The concrete prism test (CPT) retains the aggregate size and specimen configuration of concrete and is widely used to evaluate measures for mitigating alkali-silica reaction (ASR). However, it requires a long test period, and stable control of moist-air curing conditions can be difficult. To improve the efficiency of concrete-scale ASR assessment, four reactive tuffs were investigated using the accelerated mortar bar test (AMBT), standard CPT, and an accelerated concrete prism test (ACPT) involving immersion in 1 mol/L NaOH at 80 °C. Grade I Class F fly ash was used at replacement levels of 0-35%, and the reaction characteristics of the tuff aggregates were examined by microscopy. With 15% fly ash, the 28 d AMBT expansions decreased from 0.212 to 0.376% to 0.036-0.050%, while the 728 d CPT expansions decreased from 0.037 to 0.064% to 0.018-0.029%. All values were below the corresponding expansion limits. The ACPT retained the concrete prism geometry and coarse aggregate grading while accelerating ASR under high-temperature alkaline exposure. The minimum fly ash replacements required to keep all tuff concretes below 0.040% were 20% at 182 d and 30% at 273 d. At 364 d, no replacement level up to 35% met this criterion for all mixtures. As the fly ash replacement increased from 0% to 15% and 35%, the visibly reacted depth of the tuff aggregate decreased from approximately 12 mm to 7 mm and 5 mm, respectively, consistent with the reduction in prism expansion. The results show that ACPT can distinguish the mitigation performance of different fly ash replacement levels within a specified test period. Its expansion criterion should therefore be linked to a fixed assessment age. The 182 d result may be used for rapid dosage selection, whereas the 273 d result provides a stricter recommendation for structures with higher durability and safety requirements.