Fire Performance of Hybrid Fibre Reinforced Concrete with Steel and Sodium Carbonate-Treated Natural Fibres Under Standard Fire Exposure

This study investigates the fire behaviour of hybrid fibre reinforced concrete (HFRC) incorporating steel fibres combined with coir, polypropylene (PP), ramie, and sisal fibres under the ISO 834 standard fire curve. Concrete specimens were exposed to fire for 15, 30, and 60 min, and their residual compressive, split tensile, and flexural strengths were evaluated. Mass loss and a Thermal Performance Index (TPI) were determined to quantify fire-induced deterioration, while thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were performed to characterize the thermal, phase, and microstructural changes after fire exposure. The results showed that all concrete mixtures experienced progressive reductions in mechanical properties with increasing fire exposure duration. After 60 min of fire exposure, the compressive strength reductions were 45.9%, 42.9%, 37.9%, 45.1%, and 43.8% for CC, SPP, SCO, SSI, and SRA, respectively. The SCO mixture retained 62.1% of its compressive strength, exhibited the lowest mass loss (8.2%), and achieved the highest Thermal Performance Index (7.57). The TGA, XRD, and SEM analyses indicated lower thermal and microstructural deterioration in the hybrid fibre reinforced mixtures than in the control concrete. The findings demonstrate that hybrid fibre reinforcement improves the residual performance of concrete subjected to standard fire exposure.