Microstructure-Guided Optimization of Steel Fiber-Reinforced Crumb Rubber Concrete for Sustainable and Lean Construction

The development of sustainable concrete materials using recycled waste resources has gained significant attention in construction engineering. The crumb rubber obtained from end-of-life tires provides an effective approach for waste-tire utilization; however, its incorporation into concrete often reduces mechanical performance due to the lower stiffness of rubber particles and weak bonding at the rubber–cement interfacial transition zone. Therefore, understanding the microstructural mechanisms and improving the mechanical reliability of rubberized concrete are essential for its wider structural application. This study investigates the influence of steel fiber reinforcement on the mechanical and microstructural behavior of M30 grade crumb rubber concrete. Compressive strength, split tensile strength, and flexural strength tests were conducted, while scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX) analyses were performed to evaluate hydration products, interfacial characteristics, and elemental composition. Among the investigated mixtures, CRCA + 0.5% SF exhibited the highest performance under the tested conditions. At 28 days, the compressive strength increased from 43.02 MPa for the control concrete to 47.32 MPa, representing an improvement of approximately 10.0%. The split tensile strength increased from 3.62 MPa to 4.05 MPa (11.9%), while the flexural strength increased from 4.88 MPa to 5.92 MPa (21.3%). SEM observations revealed improved fiber–matrix interaction and reduced visible microstructural defects, while EDAX confirmed calcium-rich hydration products within the cementitious matrix. The findings demonstrate that an appropriate steel fiber dosage can effectively compensate for the mechanical limitations associated with crumb rubber incorporation and contribute to the development of sustainable concrete with improved engineering performance.