Post-fire mechanical properties of high-strength steel rebars with 630 MPa nominal yield strength under different cooling regimes

Purpose This study aims to systematically investigate the post-fire mechanical behavior of 630 MPa thermomechanically rolled reinforcing bars subjected to elevated temperatures and different cooling regimes. By combining tensile testing, microstructural analysis and comparative evaluation, the study seeks to clarify temperature-dependent degradation mechanisms, quenching-induced hardening effects and the role of cooling history. Empirical prediction models are further developed to support practical post-fire assessment and engineering decision-making for high-strength reinforcement in reinforced concrete (RC) structures. Design/methodology/approach A comprehensive experimental program was conducted on 630 MPa thermomechanically rolled reinforcing bars exposed to temperatures ranging from 150°C to 1,000°C. Specimens were heated under controlled conditions and subsequently cooled using air cooling, furnace cooling or water quenching to simulate realistic post-fire scenarios. Uniaxial tensile tests were performed after cooling to ambient temperature to evaluate residual mechanical properties. Microstructural observations were used to interpret temperature- and cooling-dependent mechanical responses, and empirical models were developed for post-fire strength prediction. Findings The results indicate that 630 MPa reinforcing bars retain most of their mechanical performance below 550–600°C, while pronounced strength degradation occurs beyond this threshold due to recovery and recrystallization. Cooling history plays a critical role at high temperatures: air and furnace cooling lead to progressive softening and ductility enhancement, whereas water quenching induces significant strength hardening accompanied by severe ductility loss. The elastic modulus remains relatively insensitive to temperature and cooling conditions. Empirical prediction models provide reliable estimates within 150–850°C but should not be extrapolated beyond this range. Originality/value To the best of the authors’ knowledge, this study provides one of the first systematic post-fire experimental datasets for 630 MPa thermomechanically rolled reinforcing bars, a material increasingly used in modern RC structures but rarely addressed in existing fire research. Unlike most previous studies on plate-type high-strength steels, the work highlights the combined effects of temperature and cooling history on small-diameter rebars, particularly quenching-induced hardening and embrittlement. The proposed empirical models and engineering-oriented assessment framework offer practical value for post-fire evaluation and decision-making.