Synergistic Effects of Fiber Inclination, Geometry, and Thermal Treatment on Fe-SMA Fiber Pullout Resistance in High-Performance Concrete

Iron-based shape memory alloy (Fe-SMA) fibers can enhance cementitious composites through both crack-bridging and thermally activated recovery stresses. Since fiber pull-out governs load transfer at the microscale, understanding the combined effects of fiber geometry, inclination, and thermal treatment is essential. This study experimentally investigated the pull-out behavior of hooked-end Fe-SMA fibers embedded in high-performance concrete (HPC). A total of 54 ASTM C307-type briquette specimens were tested using single-hook (3D) and double-hook (4D) fibers at inclination angles of 60∘, 75∘, and 90∘ under ambient, 100 ∘C, and 200 ∘C conditions. Additional flexural, compressive, and direct tensile tests were conducted on plain HPC exposed to the same thermal regime. At ambient temperature, 4D fibers showed 50–70% higher peak pull-out forces than 3D fibers. Heating to 100 ∘C further increased pull-out resistance by about 6–17%, and the 4D-60-100 configuration achieved the highest performance. In contrast, exposure to 200 ∘C reduced pull-out resistance by about 5–12% below ambient values. Overall, a 60∘ inclination generally provided the best response, while 90∘ produced the lowest. The results confirm that moderate thermal activation combined with double-hook geometry is the most effective strategy for maximizing Fe-SMA fiber–matrix load transfer in HPC.