Investigation on the high-temperature thermal damage mechanism of hybrid steel-polypropylene fiber reinforced UHPC using X-ray computed tomography

To investigate the influence mechanism of hybrid fibers on the high temperature damage of ultra-high performance concrete (UHPC), this study employed micro-scale X-ray computed tomography (X-CT) to evaluate the high temperature damage process of UHPC reinforced by steel fiber (SF) alone and hybrid SF and polypropylene fiber (PPF). The results show that the hybrid fibers (1.6% SF + 0.6% PPF) can significantly enhance the crack resistance and thermal damage resistance of UHPC at high temperature. Compared to the porosity at 25 °C, the porosity of UHPC at 600 °C increases by 0.68% for the control group, 1.14% for the SF-reinforced group, and only 0.26% for the hybrid fiber-reinforced group. The hybrid fiber-reinforced UHPC exhibits a smaller increase in porosity, a slower degree of crack propagation, and a higher retention of compressive strength after high temperature damage compared to UHPC without fiber and SF-only reinforced UHPC.