This study investigates the synergistic effect of steel fibres and coarse aggregates on impact behaviour of ultra-high performance fibre reinforced concrete (UHPFRC). UHPFRC matrices with a low cement content and maximum aggregate sizes of 8 mm and 25 mm are designed by using a particle packing model. Three types of steel fibres (13 mm short straight, 30 mm medium hook-ended and 60 mm long 5D) are studied in terms of the utilization efficiencies. The results show that UHPFRC with coarser aggregates tends to have a lower cement consumption but slightly weaker mechanical strength, and the largest aggregate size is suggested to be no more than 25 mm considering the reduction on flexural toughness and impact resistance. The medium and long fibres contribute to an excellent deflection/strain hardening behaviour instead of short ones. A preferential synergistic effect on impact and flexural properties is observed between the medium fibres and the finer aggregates, while the longer fibres are more compatible to the coarser aggregates. The length of steel fibre is recommended between 2 and 5 times the maximum aggregate size. The flexural strength controls the impact resistance under low-energy impact loadings, and flexural toughness determines it under relatively high-energy (beyond energy threshold) impact loadings.

本研究探讨了钢纤维和粗骨料对超高性能纤维增强混凝土（UHPFRC）冲击性能的协同作用。使用颗粒填充模型设计了水泥含量低，最大骨料尺寸为8 mm和25 mm的UHPFRC基质。研究了三种类型的钢纤维（13 mm短直，30 mm中等钩端和60 mm长5D）的利用效率。结果表明，骨料较粗的UHPFRC水泥用量较低，但机械强度稍弱，考虑到抗弯韧性和抗冲击性的降低，建议最大骨料尺寸不超过25 mm。中长纤维代替短纤维具有出色的挠曲/应变硬化性能。在中等纤维和较细的聚集体之间观察到对冲击和挠曲性能的协同效应，而较长的纤维与较粗的聚集体更相容。建议钢纤维的长度在最大骨料尺寸的2到5倍之间。弯曲强度控制着低能量冲击载荷下的抗冲击性，而弯曲韧性决定了它在相对高能量（超过能量阈值）的冲击载荷下的抗冲击性。