Interfacial degradation of steel fibers in fiber-reinforced mortars under high-temperature exposure: A coupled thermo-mechanical-microstructural insight

This study examines the thermal degradation of steel fiber reinforced mortars (SFRM) within the temperature range of 20 to 800°C, with emphasis on the evolution of the interfacial bond between steel fibers and the cementitious matrix. A layered four point flexural pullout configuration was introduced to measure temperature dependent interfacial shear stress under controlled loading, offering greater precision than conventional pullout methods. The work identifies a critical transition near 500°C where bond capacity declines rapidly, a mechanism that has not been fully characterized in earlier research. Mechanical evaluation included flexural strength and residual modulus, while microstructural assessment employed Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Thermogravimetric Analysis (TGA). The results show that steel fibers improve initial flexural strength and crack bridging capacity, and that the interface remains stable up to about 400°C. Above this level, shear resistance decreases due to oxidation of the fiber surface and dehydration of the surrounding matrix. At 800°C, most of the fiber cross section converts into brittle oxides, leading to complete loss of anchorage and clear interfacial separation. SEM and EDS confirm oxide layer growth and matrix vitrification, and TGA reveals mass losses approaching 65 % under oxidizing conditions. The study also shows that phosphate coatings on steel fibers lose stability above 420°C, limiting their protective function. The combined mechanical and microstructural findings clarify the processes that govern bond deterioration in steel fiber reinforced mortars at high temperature and support the development of improved reinforcement strategies for fire exposed cementitious composites.