Cracking stress thresholds of thermally damaged hybrid fibre reinforced concrete under compression

Identifying the cracking stress thresholds and damage processes of concrete structures exposed to high temperature conditions is crucial to advancing fire-resistant design methodologies and safety assessment protocols. This study reviewed the crack stress thresholds prediction methods and applied them to hybrid fibre-reinforced concrete. Through systematic experimental investigation, the effects of elevated temperatures (≥200 °C) and post-fire re-curing on the compression damage stress thresholds and damage evolution mechanisms of hybrid fibre reinforced concretes were analysed quantitatively. The results demonstrated that exposure to temperatures above 200 °C significantly degraded the compressive behaviour, characterized by a pronounced reduction in the stable crack propagation stage and an extension of the unstable damage phase. Notably, post-fire water re-curing is an effective rehabilitation strategy that partially mitigates thermal damage and enhances mechanical recovery; this phenomenon has not been previously documented for hybrid fibre reinforced concrete. Furthermore, post-fire water re-curing effectively prolongs the stable crack damage stage of thermally degraded specimens. The incorporation of short wave shaped steel fibres markedly increases the crack damage stress thresholds compared to polypropylene fibre-reinforced counterparts. These findings provide insights into the damage mechanics of hybrid fibre reinforced concrete under fire conditions and offer empirically validated guidelines for the developing of advanced fire-resistant concrete systems that can withstand extreme temperatures and maintain structural integrity and post-event recoverability.