Evaluation of the Mechanical Performance of Hybrid Concrete Incorporating Steel Fiber and Recycled Coarse Aggregate with a Design Compressive Strength of 30 MPa

Abstract

The increasing demand for sustainable construction materials has accelerated the utilization of recycled aggregates to minimize natural resource depletion and construction and demolition waste. However, the inferior mechanical properties of Recycled Coarse Aggregate (RCA) concrete, primarily caused by adhered old mortar and a weakened interfacial transition zone (ITZ), remain a significant challenge for structural applications. This study evaluates the mechanical performance of hybrid concrete incorporating RCA and steel fiber with a target compressive strength of 30 MPa. An experimental program was conducted using RCA replacement levels of 0%, 25%, 50%, 75%, and 100%, combined with steel fiber contents of 0%, 0.5%, 1.0%, and 1.5% by concrete volume. Fresh concrete properties were assessed through slump and fresh density tests, while hardened concrete was evaluated using compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity tests. The results revealed that increasing RCA content and steel fiber dosage reduced workability, with slump values decreasing from 95 mm to 65 mm. Nevertheless, the optimum performance was achieved with 50% RCA and 1.0% steel fiber, producing a compressive strength of 33.2 MPa, splitting tensile strength of 3.62 MPa, flexural strength of 5.18 MPa, and modulus of elasticity of 29.2 GPa. Higher RCA replacement levels (≥75%) resulted in reduced mechanical performance due to increased porosity and weaker ITZ characteristics. Overall, the synergistic combination of moderate RCA replacement and steel fiber effectively enhances the mechanical behavior of recycled aggregate concrete while supporting sustainable construction and circular economy principles.