Investigation of reinforcing steel rebar manufactured from local scrap at various finishing rolling temperature

Abstract

The mechanical properties of structural steel rebar are a very important factor for durable, sustainable, and resilient design of reinforced concrete structures. In developing countries, steel rebars are often manufactured from unsegregated steel scrap using manual rolling mill, which affects the quality of the final manufactured product and raises several concerns. Various earlier studies have been conducted on controlling the quality of steel rebar in various countries, yet quality issues of steel rebar are still pending in local industry. Therefore, this study aims to explore the mechanical properties of steel rebar manufactured with steel scrap having various chemical composition at different finishing rolling temperatures (FRT). Steel rebars were manufactured at an industrial kiln using various carbon contents (0.25%, 0.27% and 0.30%), replicating full-scale scenarios. Specimens for each type of steel were manufactured at FRT of 640⁰C, 680⁰C, 720⁰C, 760⁰C and 800⁰C. Various tests including tension, bending, hardness and microstructural examination were performed on the manufactured steel rebars. It was observed that by increasing the FRT, the average grain size in the steel rebar increased, leading to a decrease of the ultimate tensile strength, yield strength, and hardness. Conversely, the elongation and modulus of toughness increased with the increase in the average grain size due to a decrease in total grain boundary area owing to lower resistance to dislocation motion. By increasing the carbon content, inter-lamellar spacing between cementite strips in pearlite decreased, which increased the hardness, ultimate tensile strength, and yield strength, whereas the elongation and modulus of toughness decreased. The weight per unit length, yield, and ultimate strengths of the tested rebars satisfied the ASTM A615 limits. This study educates the local rolling mills stakeholders about possible causes of variations in mechanical properties of locally manufactured steel rebar and suggests solutions towards proper monitoring and mitigating harmful effects. The main findings of this study highlight that the manufacturing of rebars from scrap at various FRT affects the stress-strain behavior of steel rebar and its overall structural stability. Moreover, microstructural analysis assists in understanding the role of FRT on yield and ultimate strengths of rebars due to changes in the average grain size.