Slip hardening behavior of bundled steel fibers in ultra-high performance concrete

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.