Fracture behaviour of steel fibre-reinforced concrete at a wide range of loading rates

Three-point bending tests on notched beams of steel fibre-reinforced concrete (SFRC) have been conducted using both a servo-hydraulic machine and a drop-weight impact device. The shape and geometry of the beams followed the RILEM recommendation, i.e., 150 mm × 150 mm in cross section, 700 mm in length, notch-depth ratio was around 1/6 and the span of the tests was kept constant 500 mm. The peak load and the fracture energy were measured over a wide range of loading rates (loading point displacement rates), spanning six orders of magnitude. Under low loading rates, from 10−3 mm/s to 100 mm/s, the tests were performed with the servo-hydraulic machine; from 102 mm/s to 103 mm/s, the drop-weight impact machine was used instead. The results show that the fracture energy and the peak load increase as the loading rate increases. Furthermore, such a trend is relatively mild under low rates. The gain of the fracture energy and peak load is around 10% compared with its quasi-static values. It could be attributed to viscous effects mainly originated by the presence of free water in voids and porous structures in the matrix, and the weak fibre pullout resistance. However, under high rates the increases in the fracture energy and the peak load are pronounced due to the inertia effect and the greater fibre pullout energy. The dynamic increase factors of the peak load and the fracture energy for the fastest loading rate are approximately 3.5 and 2.5, respectively.