Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete

In this paper single fiber pull-out performance of high strength steel fibers embedded in ultra-high performance concrete (UHPC) is investigated. The research emphasis is placed on the experimental performance at various pullout rates to better understand the dynamic tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC). Based on the knowledge that crack formation is strain rate sensitive, it is hypothesized that the formation of micro-splitting cracks and the damage of cement-based matrix in the fiber tunnel are mainly attributing to the rate sensitivity. Hereby, different pull-out mechanisms of straight and mechanically bonded fibers will be examined more closely. The experimental investigation considers four types of high strength steel fibers as follows: straight smooth brass-coated with a diameter of 0.2 mm and 0.38 mm, half end hooked with a diameter of 0.38 mm and twisted fibers with an equivalent diameter of 0.3 mm. Four different pull out loading rates were applied ranging from 0.025 mm/s to 25 mm/s. The loading rate effects on maximum fiber tensile stress, use of material, pullout energy, equivalent bond strength, and average bond strength are characterized and analyzed. The test results indicate that half-hooked fibers exhibit highest loading rate sensitivity of all fibers used in this research, which might be attributed to potential matrix split cracking. Furthermore, the effect of fiber embedment angles on the loading rate sensitivity of fiber pullout behavior is investigated. Three fiber embedment angles, 0°, 20°, and 45°, are considered. The results reveal that there is a correlation between fiber embedment angle and loading rate sensitivity of fiber pullout behavior.