Enhancing spalling prevention of self‐compacting concrete at elevated temperatures: Incorporation of steel‐ PVA fibers

Self‐compacting concrete (SCC) is widely used for its excellent workability, but its dense internal structure causes spalling at elevated temperatures, which poses a critical risk to structural integrity and engineering safety. This study proposes a synergistic reinforcement scheme of steel and polyvinyl alcohol (PVA) fibers, with steel fiber volume fractions of 0, 0.4%, 0.8%, 1.2%, 1.4% and PVA fiber volume fractions of 0.1%, 0.2%, 0.3%. The effects of single‐doped and hybrid fibers on the high‐temperature spalling resistance of SCC were investigated via high‐temperature tests, gas permeability measurements, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). Results show that ≥0.1 vol.% PVA fibers completely prevent SCC spalling under engineering high‐temperature conditions, compared with 100% spalling of the reference group; 1.4% single‐doped steel fibers reduce the spalling degree to 4.32%, providing practical dosage references. SCC gas permeability remains acceptable below 400°C but degrades sharply above 600°C, and higher fiber content effectively retards this degradation to maintain steam‐release capacity. 400°C is the critical temperature for pore‐structure degradation, where thermal‐stress concentration accelerates interfacial microcracking and impairs structural performance. Microscopic analyses reveal that PVA fibers melt at ~230°C to form interconnected steam‐venting channels with slight pore coarsening, while steel fibers restrain crack propagation via three‐dimensional bridging for complementary interfacial protection. A temperature‐dependent two‐stage synergistic mechanism is proposed: steel fibers control cracks below 400°C, and PVA fibers realize steam‐pressure relief above 400°C. This greatly improves the high‐temperature spalling resistance of SCC and offers theoretical–experimental support for its fire‐resistant engineering design.