Mitigating Alkali–Silica Reaction in Concrete Containing Recycled Concrete Aggregate

Abstract

This work involves research on the use of fly ash to mitigate alkali–silica reaction (ASR) in concrete made with recycled concrete aggregate (RCA) that comes from concrete with a known ASR history. Modified ASTM C 1260 and C 1567 with different aggregate grading showed that a 25% cement substitution with a Class F fly ash controlled the expansion under 0.10% at 28 days. RCA from I-95 near Gardiner, Maine, was selected for this study. The RCA, coarse aggregate recovered from the RCA, and currently produced aggregate from the same source as the RCA were evaluated in the laboratory. The conventional ASTM C 1260/1567 14-day expansion results correlated well with ASTM C 1293 1-year criteria. The ASR-modified ASTM C 1260/1567 28-day expansion results also correlated well with ASTM C 1293 2-year failure criteria. Pore solution and thermal gravimetric analysis revealed both calcium hydroxide and alkalis are reduced by fly ash substitution. Calcium depletion alone is a sufficient condition for ASR arrest, but the substitution level, depending on the portland cement, can be as high as 60% and unusable in field concrete. Approximately 25% fly ash substitution was required to lower the pH to a range of 13.28 to 13.32, a presumed ASR threshold range. The effect of alkali reducing appears to be more pivotal than calcium consumption. Although alkalis were found to be present in the RCA, it was not shown that it was available to the pore solution. This suggests that alkali concentration in the new paste and the old RCA matrix are approximately equal and therefore in equilibrium with each other.