Thermal stability and mechanical behavior of cementitious composites reinforced with industrial and recycled tire steel fibers under elevated temperature conditions

The integration of Recycled Tire Steel Fibers (RTSF) into cement-based materials offers a promising approach to enhancing sustainable construction practices; however, research on the performance of RTSF under elevated temperatures is critically limited. This study investigates the mechanical behavior of cementitious composites reinforced with RTSF at dosages of 0.75 and 1.0 (%, vol.), in comparison to composites reinforced with industrial steel fibers (ISF) and a plain control mixture. A systematic experimental methodology was employed, wherein the composites' compressive and flexural behaviors were assessed at ambient temperature (approximately 22°C) and after exposure to elevated temperatures of 300°C and 500°C for four hours. The results indicate that ISF maintained complete mass stability under these conditions, exhibiting no mass loss, whereas RTSF experienced gradual reductions of 0.71 % and 1.00 % at 300°C and 500°C, respectively. Following a four-hour exposure to 500°C, some of the tested plain and fiber-reinforced specimens were damaged. Microscopic evaluations revealed distinct thermal cracking patterns, with RTSF retaining its metallic sheen at 300°C while ISF showed degradation. The elastic modulus of the control mix decreased significantly from 25.8 GPa at 22°C to 11.9 GPa at 500°C, while RTSF demonstrated notable reductions in modulus due to microcracking. Notably, RTSF-1.0 demonstrated the highest strength gains at 22°C, with a 0.7 % increase in compressive strength and a 157.1 % increase in flexural strength compared to the control mix, and achieved the highest flexural strength of 8.3 MPa at 300°C, indicating potential benefits in crack arrest and chloride penetration reduction. These findings provide foundational insights into the viability of RTSF for applications in fire-resistant infrastructure, which highlights the importance of optimizing fiber dosage for enhanced performance.