Abstract
This study examines the effects of fiber geometry, spacing, and loading rate on the pullout resistance of steel fibers in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, four different fiber spacings, and three different loading rates ranging from 0.018 to 740 mm/s were considered. Test results indicated that the single straight fiber in UHPC was most rate sensitive for pullout resistance, followed by the single twisted and then hooked fibers. The bond strengths and pullout energy of specimens with multiple straight fibers were improved by increasing the loading rate but were not affected by fiber spacing. Closer fiber spacing had a detrimental effect on the dynamic pullout resistance of multiple hooked steel fibers in UHPC, while no enhancement of average bond strength of multiple twisted fibers was observed as fiber spacing and loading rate varied. The average bond strengths of single and bundled hooked and twisted steel fibers in UHPC were clearly improved by increasing the loading rate. Bundling of fibers enhanced the impact pullout resistance of all the steel fibers in UHPC. The highest dynamic increase factors for the bundled straight, hooked, and twisted fibers were approximately 3.78, 1.57, and 1.41, respectively, at the impact loads.
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