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

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.