Mechanical strength and drying shrinkage properties of concrete containing treated coarse recycled concrete aggregates

In recycling concrete, the crushing process leaves weak mortar particles and surface cracks throughout the recycled concrete aggregates (RCA). Thus, the process is detrimental, resulting in inferior aggregate properties. This experimental study presents a method to improve the properties of coarse RCA by modifying their surface structure through the combination of two different surface treatment methods. In this study, coarse RCA are first treated by soaking in hydrochloric (HCl) acid at 0.5 mol (M) concentration. They are then impregnated with calcium metasilicate (CM) solution to coat their surface with CM particles. The effects of both surface treatments on the properties of RCA before and after treatment are determined. Moreover, the effect of the replacement of natural coarse aggregates with 60% treated coarse RCA on the mechanical strength of concrete is evaluated. The findings of this study show that the effect of the combination of these two surface treatment methods is beneficial, as the combined methods not only modify RCA surface but also enhance RCA properties. More specifically, after treatment, the particle density, water absorption, and mechanical strength of RCA are significantly improved. Consequently, the incorporation of treated RCA in concrete results in a mechanical strength that approximates concrete prepared with natural aggregates and surpasses the strength of concrete prepared with untreated RCA. In addition, the effect use of treated RCA tends to reduce the drying shrinkage of concrete.