Thermo-mechanical degradation of fibre–matrix bond and flexural behaviour in steel-fibre-reinforced concrete after elevated-temperature exposure

The thermo-mechanical degradation of fibre–matrix bond and the flexural behaviour of steel-fibre-reinforced concrete (SFRC) specimens subjected to elevated temperatures (200°C, 400°C and 600°C) were investigated. Fibre pull-out and four-point bending tests were conducted independently to characterise the temperature-dependent degradation of interfacial bond and flexural response. The results showed that moderate heating (400°C) enhanced the bond strength and pull-out energy to, respectively, 158% and 188% of their room-temperature (20°C) values, owing to frictional strengthening and preserved matrix integrity. Severe deterioration occurred at 600°C, and both indices decreased to approximately 63% of their reference values owing to microcracking and chemical bond loss. Flexural strength decreased progressively with temperature, retaining 42.8% for SFRC and 24.7% for plain concrete at 600°C. Despite matrix damage, the SFRC exhibited stable ductility and higher fracture toughness, confirming the sustained efficiency of fibre bridging under thermal degradation. These findings provide insights into the temperature-dependent fibre–matrix bond degradation and corresponding flexural response of SFRC within the investigated temperature range of 200–600°C.