Effect of coarse aggregate concentration on the bond properties between steel rebar and concrete within low and medium strength range

Most of the current rules and practices employ medium and high-strength concrete with ribbed rebar. However, there are still many structures that are of low concrete class with non-standard aggregate gradation and also contain plain rebar. In this study, to better understand the behavior of such structures, concrete mixtures with 3 different water-to-cement (W/C) ratios (0.6–0.9–1.2) and 4 different coarse aggregate concentrations (CAC) (0 %-20 %-40 %-60 %) were produced. Compressive strength (CS), elasticity modulus (MOE), and splitting tensile strength (STS) tests were performed on the concrete samples, and the stress-strain relationship was determined. Bond-slip behavior was examined on samples containing both plain and ribbed rebars. The results showed that 40 % CAC mixtures had the highest CS and 60 % CAC mixtures had the highest MOE. Adding coarse aggregate increased STS, bond strength (τ_u), and residual bond stress (τ_r). The results obtained from this study were also combined with a similar study conducted on medium and high-strength concrete, and models were proposed for CS, MOE, and τ_u, τ_r. It was determined that similar to compressive strength, the MOE was also effective on bond strength significantly. A machine learning method (random forest regression) and feature importance analysis were performed to support the results. The results revealed that CS has the most significant effect on τ_u and τ_r, but the effect of MOE and STS was also very significant. Additionally, τ_r/τ_u and the s₁ (slip value at τ_u), which is proposed as a constant value in existing studies, increased with increasing CS.