Selection of optimal mix design with simultaneous use of RSF and RCA with emphasis on the initial strength of RCA

In this research, the properties of concrete made with recycled concrete aggregate (RCA) and reinforced with recycled steel fibers from waste tires (RSF) were investigated. In the production of RCA, 3 types of concrete waste with initial strength of 20, 40, and 80 MPa were used. The results of aggregates tests showed that increasing the initial strength of RCA increases the density and abrasion resistance and decreases the water absorption and amount of mortar attaching to the surface of these aggregates. In the making of specimens, 0%, 50%, and 100% of natural aggregates (NA) was replaced by RCA. In addition, the specimens were reinforced by using of 0%, 0.5%, and 1% RSF. Various tests such as slump, UPV, water absorption, compressive strength, splitting tensile strength, and flexural strength were performed on 21 mix designs. The results of these tests indicated that the using of RCA had a negative effect on the workability, quality, and mechanical properties of concrete, which can be prevented by increasing the initial strength of RCA and also using RSF. Finally, by economic analysis and optimization of mixing designs, it was concluded that it is justified to use RSF and 50% RCA with initial strength of 40 and 80 MPa, in terms of economy, resistance, workability, and quality.