LAUSR

Abstract The use of recycled aggregates in structural concrete is a sustainable solution to reduce the exploitation of natural resources and the harmful environmental effects of concrete waste. The present study aimed to improve the mechanical performance of recycled aggregate concrete (RAC) by incorporating hybrid steel and polypropylene fibers. Sixteen concrete mixtures with different volume fractions of steel and polypropylene fibers were tested. Mechanical properties were studied by conducting the compressive strength, splitting tensile strength, flexural strength and flexural toughness as well as impact resistance. Additionally, the microstructures of interfacial transition zone (ITZ) at aggregate/cement paste and fiber/cement paste were investigated through the scanning electron microscope (SEM). Results indicated that the incorporation of polypropylene fibers resulted in a minor change in compressive strength of RAC. Whereas, the steel fibers significantly increased compressive and flexural ductility and impact resistance especially in the mixtures made with hybrid steel and polypropylene fibers. The use of hybrid steel and polypropylene fibers exhibited an excellent coupling effect on the mechanical properties of RAC that was better than steel fiber and polypropylene fiber. Among all the mixtures with different volume fractions of steel fibers and polypropylene fibers, the hybrid fiber reinforced RAC specimen containing 1.5% steel fibers and 0.9% polypropylene fibers exhibited the best mechanical properties. SEM investigations showed that the incorporation of fibers into RAC could not only make an overall improvement in the density and uniformity of cement paste, but also improved the microstructures of ITZ. Moreover, the bonding between steel fiber and cement paste was found to be tight, and the interfacial zone was integral and dense. These observations provide insight into the benefits of different fiber reinforcement systems to the mechanical performance of RAC which is considered to have poorer quality compared with natural aggregates concrete.