Mechanical and Microstructural Performance of Concrete Incorporating Waste Tire Rubber and Recycled Steel Fibers Under Elevated Temperatures

This study investigates the thermo-mechanical and microstructural performance of concrete incorporating waste tire rubber (WR) and recycled steel fibers (WS) under elevated temperatures. Four mixtures were prepared: plain concrete (PL), rubber-modified concrete (WR5), and hybrid mixtures containing 0.4% and 0.8% steel fibers (WS0.4WR5 and WS0.8WR5). Specimens were exposed to temperatures of 400 °C, 600 °C, and 800 °C to simulate fire conditions. The results indicate that the incorporation of rubber reduces compressive strength at ambient temperature due to its lower stiffness and weak interfacial bonding. However, the addition of recycled steel fibers significantly improves crack resistance and enhances thermal stability. At 400 °C, the WS0.8WR5 mixture showed a retention rate of 92.9% (absolute strength: 44.32 MPa), compared to 72.2% for plain concrete (absolute strength: 44.11 MPa). Although the hybrid mixture has a lower ambient strength (47.68 MPa vs. 61.07 MPa), its superior retention makes it competitive in fire scenarios. Ultrasonic pulse velocity (UPV) measurements revealed a strong correlation with compressive strength degradation, confirming its effectiveness as a non-destructive indicator of internal damage. Microstructural analyses (SEM, XRD, and TGA-DTA) demonstrated that elevated temperatures lead to dehydration, phase transformation, and increased porosity, while steel fibers help maintain matrix integrity through crack-bridging mechanisms. The findings highlight a synergistic interaction between waste rubber and steel fibers, offering a sustainable and effective approach for improving the fire resistance of concrete.