Impact resistance of hybrid fibre-reinforced ultra-high performance concrete beams after high-temperature exposure

This study examines the impact resistance of reinforced ultra-high performance concrete (UHPC) beams incorporating steel and polypropylene fibres after exposure to elevated temperatures. To evaluate the effect of thermally induced damage on structural impact resistance, three UHPC beams were tested, of which two beams were exposed to elevated temperatures of 600 °C and 1000 °C, respectively. The residual impact resistance was then examined using a free-fall hammer test, in which a 641 kg hammer was dropped at mid-span from heights of 0.25 m and 1.0 m, depending on the thermal damage level. Although material-level tests confirmed that incorporating PP fibres effectively prevents explosive spalling of UHPC under high temperatures, spalling was still observed in structural beam tests. When subjected to higher heating rates, thermal spalling became more pronounced on the beam. Nonetheless, the beams largely retained their flexural behaviour and sustained only minor damage under severe impact after high-temperature exposure. Refined numerical models were developed to supplement the experimental findings. UHPC material was modelled with a plasticity-based constitutive model capable of capturing triaxial stress states, post-peak softening, strain-rate sensitivity, and thermal degradation. The simulations showed good agreement with the experimental results, although the absence of the random thermal spalling in the model led to underestimation of deflections at elevated temperatures. • UHPC beam showed evident thermal spalling under elevated temperature • Thermal spalling was more evident with the increase of heating rate • The beams retained their flexural behaviour and sustained only minor damage under severe impact after high-temperature exposure • The absence of the random thermal spalling in the numerical model led to underestimation of deflections