High loading-rate pullout behavior of inclined deformed steel fibers embedded in ultra-high performance concrete

Single fiber pullout tests enable a deeper understanding of the behavior of fiber reinforced cementitious materials. The vast majority of fiber pullout tests in the literature are quasi-static and conducted with fibers aligned in the loading direction. Studies that focus on dynamic or inclined pull out behavior are not common and those that combine both effects are rare. In this paper, the experimental study investigates the effects of embedment inclination and pullout rate on the behavior of high strength steel fibers embedded in an ultra-high performance concrete (UHPC) matrix. The experimental variables are fiber type (straight smooth, hooked and twisted), embedment inclination, which varies from zero (aligned with load) to 45°, and loading rate, which ranges from 0.018 mm/s (representing quasi-static loading) to 1800 mm/s (representing impact loading). Test results show that the load and energy dissipation capacities for straight smooth fibers generally increase with loading rate and inclination angle up to 45°. The hooked and twisted fibers exhibit less consistent trends and their peak load and energy dissipation capacities occur at inclination angles that range from 0° (aligned with load) to 30°. The straight smooth fibers exhibit the most sensitive response to loading rate and achieve a load capacity dynamic increase factor (DIF) as high as 2.32. The DIFs are generally less for hooked fibers and drop below 1.00 for twisted fibers, especially at higher inclination angles.