Resistance of recycled aggregate concrete containing low- and high-volume fly ash against the combined action of freeze–thaw cycles and sulfate attack

Abstract

The present study investigated the effect of the combined action of freeze–thaw (F–T) cycles and sulfate attack on the resistance of concrete containing low-volume fly ash (LVFA) and high-volume fly ash (HVFA) made with coarse recycled concrete aggregates (CRCAs). Concretes with a water–binder ratio of 0.50 containing fly ash (FA; LVFA and HVFA) and CRCA (i.e., 0%, 20%, 50% and 100% also by weight) as a replacement for coarse natural aggregates (CNAs) were exposed to water, 5% sodium sulfate solution and 5% magnesium sulfate solution under F–T cycles. The performance, including residual compressive strength, relative dynamic modulus of elasticity and concrete microstructure, was evaluated after being subjected to certain F–T cycles in sulfate solutions. Results indicated that the resistance of the concrete mixtures to the combined F–T cycles and sulfate attack increased with the increase in CRCA content as CNA replacement. Compared with the concrete without FA, the LVFA-based concrete showed excellent improvement in the resistance to the combined action of F–T cycles and sulfate attack; however, the HVFA-based concrete had an adverse effect on the resistance. Concrete deterioration was attributed to the interaction between F–T and sulfate attack. Moreover, the resistance of LVFA- and HVFA-based concretes against the combined F–T and sulfate attack increased during the entire test when the concretes were subjected to F–T cycles in 5% sodium sulfate solution. The sulfate attack exerted more positive effects than negative on the F–T cycles. However, the resistance of LVFA- and HVFA-based concretes against the combined F–T and sulfate attack increased during the initial F–T cycles and then decreased in the 5% magnesium sulfate solution. The 5% sodium sulfate solution produced similar improvements in the F–T resistance of the LVFA- and HVFA-based concretes, whereas the 5% magnesium sulfate solution evidently reduced the F–T resistance of the concrete with HVFA than that with LVFA.