Influence of Fly Ash as Cement Replacement on the Properties of Recycled Aggregate Concrete

TLDR

The use of high percentages of recycled aggregates in concrete would usually worsen the concrete properties. The study investigates how adding Class F fly ash mitigates the negative effects of high recycled aggregate content on concrete properties. Concrete mixes were prepared at water‑to‑binder ratios of 0.45 and 0.55, varying recycled aggregate from 0 % to 100 % and fly ash from 0 % to 35 % of cement. Increasing recycled aggregate lowered strength, modulus, and chloride resistance while raising shrinkage and creep, but replacing 25–35 % of cement with fly ash improved chloride resistance and reduced shrinkage and creep, offering a practical means to offset the negative effects of recycled aggregates.

Abstract

The use of high percentages of recycled aggregates in concrete would usually worsen the concrete properties. This paper tries to address the deficiency of the use of recycled aggregates by systematically presenting results on the influence of incorporating Class F fly ash on concrete properties. In this study, two series of concrete mixtures were prepared with water-to-binder (W/B) ratios of 0.45 and 0.55. The recycled aggregate was used as 0, 20, 50, and 100% by weight replacements of natural aggregate. In addition, fly ash was used as 0, 25, and 35% by weight replacements of cement. The results showed that the compressive strengths, tensile strengths, and static modulus of elasticity values of the concrete at all ages decreased as the recycled aggregate and the fly ash contents increased. Further, an increase in the recycled aggregate content decreased the resistance to chloride ion penetration and increased the drying shrinkage and creep of concrete. Nevertheless, the use of fly ash as a substitute for cement improved the resistance to chloride ion penetration and decreased the drying shrinkage and creep of the recycled aggregate concrete. The results showed that one of the practical ways to utilize a high percentage of recycled aggregate in structural concrete is by incorporating 25–35% of fly ash as some of the drawbacks induced by the use of recycled aggregates in concrete could be minimized.