Mechanism of rate dependent behaviour of ultra-high performance fibre reinforced concrete containing coarse aggregates under flexural loading

Coarse aggregates are often eliminated in ultra-high performance fibre reinforced concrete (UHPFRC) for the sake of homogeneity, however, this causes an impairment on impact resistance. The flexural performance of UHPFRC with coarse aggregates under different loading rates (0.2, 20 and 200 mm/min) is investigated here to clarify the flexure and energy absorption mechanism. The flexural behavior and crack propagation are measured, meanwhile, the fracture of coarse aggregates and the surface morphology of steel fiber are analysed. The results show the energy absorption tends to be more rate dependent than the first crack stress and flexural strength. An increase of crack propagation speed and multiple cracks are observed at higher loading rates. The percentage of fracture across coarse aggregate is 23%, 32% and 58% at loading rates of 0.2, 20 and 200 mm/min, respectively. Further, a rate-dependent model for predicting the fracture of coarse aggregates is proposed. The present results contribute to designing UHPFRC with enhanced flexural performance under different loading rates.