Behaviour of hybrid polypropylene and steel fibre reinforced ultra-high performance concrete beams against single and repeated impact loading

The high cost of steel fibres limited the application of ultra-high performance concrete (UHPC) in practical construction. To balance the performance and cost, hybrid polypropylene and steel fibre reinforced UHPC was proposed. The material properties of UHPC that contained 2 vol% polypropylene fibre and 1 vol% steel fibre were measured by the four-point bending tests and uniaxial compression tests. Three large-sized hybrid fibre reinforced UHPC beam specimens were tested against repeated and single impact loading. The weight of the drop hammer was 641 kg and the impact location was at the mid-span, the impact velocity varied from 3.13 to 4.43 m/s. The UHPC beams showed the flexural response in all the impact scenarios. With the same total impact energy, single impact loading was noted to be less hazardous than the repeated impact loadings. Based on the test results, a concrete numerical model of hybrid fibre reinforced UHPC was developed. The bond-slip relationship between UHPC and steel bars was incorporated into the structural model. Compared to the simulation considering the bond-slip relationship, the simulation with perfect bonding assumption exhibited an overall smaller mid-span displacement, where the maximum and residual displacement were 11.41% and 9.43% smaller, respectively. The numerical drop weight tests were reproduced and validated with experimental data. Finally, the failure mechanism and energy absorption of the test specimen under the repeated impact loading scenario was analysed.