Residual and damage properties of recycled brick powder–UHPFRC after high temperature

This research explores the impact of temperature, recycled brick powder (RBP) replacement rates, and cooling methods on the mechanical characteristics of ultra-high-performance fibre-reinforced concrete (UHPFRC). In addition, the material morphology and pore structure at elevated temperatures are thoroughly examined through scanning electron microscopy, X-ray diffraction and mercury intrusion porosimetry tests. The findings reveal a pattern in the behaviour of residual compressive strength, splitting tensile strength and residual fracture energy as the target temperature increases. Initially, these properties increase and then decrease. Specifically, a threshold temperature of 400°C is identified for compressive strength. For splitting tensile strength, threshold temperatures under natural cooling and water cooling are 400°C and 200°C, respectively. The threshold temperatures for residual fracture energy at substitution rates of 30%, 0% and 50% with RBP are determined to be 400°C, 200°C and 200°C, respectively. When the RBP content is 30%, UHPFRC shows superior mechanical performance during the entire heating process. The performance under water cooling is weaker than that under natural cooling. In addition, this study also proposes the evaluation and calculation models of residual compressive strength, residual splitting tensile strength and residual fracture energy considering the influence of simulated fire temperature.