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.

中文翻译：

---

间距和成束对嵌入超高性能混凝土中的各种钢纤维的速率依赖性拉拔行为的影响

这项研究探讨了纤维几何形状，间距和加载速率对超高性能混凝土（UHPC）中钢纤维抗拉强度的影响。为此，考虑了三种不同类型的钢纤维，四种不同的纤维间距以及三种不同的加载速率，范围从0.018到740 mm / s。测试结果表明，UHPC中的单根直纤维对拉拔阻力的敏感度最高，其次是单根加捻然后再钩住的纤维。增加加载速率可以提高多根直纤维试样的粘结强度和拉拔能，但不受纤维间距的影响。较近的纤维间距对UHPC中多根钩状钢纤维的动态抗拔性有不利影响，当纤维间距和加载速率变化时，没有观察到多股加捻纤维的平均粘结强度提高。通过增加加载速率，UHPC中单根和成束的钩状和加捻的钢纤维的平均粘结强度得到了明显提高。纤维捆扎增强了UHPC中所有钢纤维的抗冲击拉力。在冲击载荷下，成束的直纤维，钩纤维和加捻纤维的最大动态增加因子分别约为3.78、1.57和1.41。