Fibre-Reinforced Concrete at Elevated Temperatures: Damage Mechanisms, Spalling, and Residual Performance

Fibre-reinforced concrete exhibits complex deterioration when exposed to elevated temperatures because of physicochemical degradation, moisture migration, thermal incompatibility, pore-pressure development, and progressive cracking. Previous reviews have largely addressed temperature-dependent mechanical properties, residual performance, or spalling behaviour separately, while the connections between fibre-specific thermal mechanisms, residual mechanical performance, cracking and spalling, and hybrid-fibre interactions remain insufficiently integrated. This review addresses this gap through a mechanism-based comparison of concrete reinforced with steel, polypropylene (PP), glass, and hybrid fibres. Relevant peer-reviewed studies were identified through targeted keyword searches covering fibre-reinforced concrete, elevated-temperature exposure, residual mechanical properties, cracking, spalling, and hybrid-fibre effects, and were subsequently synthesised according to fibre type and dominant high-temperature mechanism. Based on this synthesis, distinct fibre-specific functions under elevated-temperature exposure were identified. Steel fibres primarily improve crack bridging, stress transfer, and post-cracking toughness, although their effectiveness decreases as the fibre-matrix interface deteriorates at severe temperatures. PP fibres mainly enhance vapour transport by promoting pore connectivity after thermal softening and melting, thereby reducing pore-pressure accumulation and explosive spalling. Glass fibres can restrain microcrack development and contribute to residual strength within suitable temperature ranges, but their high-temperature performance is more dependent on fibre and matrix characteristics. Hybrid fibre systems, particularly steel-PP combinations, provide complementary benefits by integrating pore-pressure relief with crack control and residual load transfer. The effects of fibre dosage and geometry, matrix properties, heating regime, moisture condition, and restraint are also discussed. The review highlights the need for fibre design strategies that balance spalling resistance, residual strength, stiffness, and post-cracking performance rather than optimizing a single property.