This study introduces a novel steel fiber arrangement, termed “bundled fibers,” where multiple high-strength steel wires are twisted into short, discontinuous fibers. The bond behavior of these bundled fibers in ultra-high performance concrete (UHPC) was evaluated through single-fiber pullout tests. Four variations of bundled fibers, consisting of two to five wires, were tested across three embedment lengths (3.3 mm, 4.9 mm, and 6.5 mm). Results indicate that increasing embedment length and bundling more wires enhance maximum pullout load, fiber stress, pullout energy, and bond strength, although slip capacity decreases with more wires. Compared to equivalent numbers of straight fibers, bundled fibers demonstrated superior performance in all pullout parameters and outperformed five common steel fiber geometries (straight, striated, wavy, hooked, twisted) in bond strength and slip capacity. A new parameter introduced to quantify slip hardening addresses a gap in the literature, with bundled fibers showing a higher degree of slip hardening due to torsion-induced frictional bonding. Additionally, bundled fibers reduce fiber agglomeration, highlighting their potential for developing high energy-absorbing UHPC.

中文翻译：

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超高性能混凝土中成束钢纤维的滑移硬化行为


本研究介绍了一种称为“束状纤维”的新型钢纤维排列，其中多根高强度钢丝被绞合成短的不连续纤维。通过单纤维拉拔测试评估了这些束束纤维在超高性能混凝土 （UHPC） 中的粘合性能。在三种嵌入长度（3.3 毫米、4.9 毫米和 6.5 毫米）上测试了四种由两到五根电线组成的束状光纤变体。结果表明，增加埋入长度和捆绑更多电线可以提高最大拉拔载荷、纤维应力、拉拔能量和粘合强度，尽管滑移能力会随着电线的增加而降低。与同等数量的直纤维相比，成束纤维在所有拉拔参数中都表现出卓越的性能，并且在粘合强度和滑爽能力方面优于五种常见的钢纤维几何形状（直、条纹、波浪、钩状、扭曲）。引入一个用于量化滑移硬化的新参数解决了文献中的一个空白，由于扭转引起的摩擦粘合，成束纤维显示出更高程度的滑移硬化。此外，成束纤维减少了纤维团聚，突出了它们开发高能量吸收 UHPC 的潜力。