Performance of deformed steel fibers embedded in ultra-high performance concrete subjected to various pullout rates

Fiber-matrix bond properties are typically evaluated using a single-fiber pullout test. Such tests for ultra-high performance concrete (UHPC) are few in the literature and, in particular, those that focus on high pullout rates are exceedingly rare. In this research program, different types of high-strength steel fibers (twisted, straight and hooked) are embedded in two UHPC matrices with varying amounts of glass powder, which is a key ingredient of UPHC. The specimens are subjected to loading rates ranging from 0.018 mm/s (representing quasi-static loading) to 1800 mm/s (representing impact loading). Experimental results show that the pullout response of all of the fiber types exhibit progressively increasing rate sensitivity as the pullout speed increases and becomes significant during impact loading. It is most prominent in the smooth and twisted fibers and least in the hooked fibers. Additionally, scanning electron microscope studies are presented and used to explain the mechanism of rate enhancement from a microscopic perspective.