Influence of steel fiber content on the rate-dependent flexural performance of ultra-high performance concrete with coarse aggregates

Steel fiber is widely used to improve the flexural performance of ultra-high performance concrete (UHPC), yet the synergistic effect of steel fiber and coarse aggregates in UHPC containing coarse aggregates (UHPC-CA) has not been elucidated. Here, the influence of fiber content on the rate-dependent flexural performance of UHPC-CA is investigated from the view of the synergistic effect of steel fiber and coarse aggregates. Results reveal that the increasing pull-out force due to the increase of the crack mouth opening speed at a higher loading rate is responsible for the flexural curve type transition of UHPC-CA. The first crack stress is insensitive to the fiber content. However, the dynamic increase factors (DIFs) of the flexural strength and energy absorption capability of UHPC-CA decrease with increasing fiber content, which is attributed to a lower crack propagation speed of UHPC-CA with a higher fiber content. A model concerning the crack propagation speed and the critical angle for the fracture of coarse aggregates is proposed.