Effect of fiber spacing on dynamic pullout behavior of multiple straight steel fibers in ultra-high-performance concrete

In this study, the effects of fiber spacing and loading rate on the pullout behaviors of single and multiple straight steel fibers embedded in ultra-high-performance concrete (UHPC) were investigated. Hence, three different fiber spacings, corresponding to the fiber volume fractions of 1, 2, and 7%, and loading rates, i.e., quasi-static and impacts, were considered. Bundled fiber specimens were also fabricated and tested to examine the fiber bundling effect at both the static and impact loading conditions. The test results indicated that the pullout resistance of straight steel fibers in UHPC improved by increasing the loading rate, regardless of their number, spacing, and bundles. Approximately 33–68% greater bond strengths were obtained in the single fiber specimen compared to the multiple fiber specimens at both the static and impact loads. The fiber bundles deteriorated the static pullout resistance, whereas both positive and negative bundling effects were found on the interfacial bond strength and pullout energy, respectively, under the impact loads. The strongest rate sensitivity on the dynamic increase factors of the pullout parameters, such as bond strength, maximum fiber tensile stress, and pullout energy, was found as the fibers were bundled, followed by the single and multiple fiber specimens. For the case of multiple fiber specimens, the loading rate sensitivity increased by decreasing the fiber spacing in general, such that the multiple fibers with closer spacings, corresponding to 2 and 7% by volume, were more sensitive to the rate of loading than their counterparts with greater fiber spacings.