Monotonic and Cyclic Pullout Performance of Hooked-End Super Elastic Shape Memory Alloy Fibers and Steel Fibers Embedded into Concrete

The bond characteristics between concrete and reinforcing fiber play a significant role in the performance of fiber reinforced concrete (FRC) under various loading conditions. The present work experimentally examined the pullout resistance of double hooked-end NiTi and steel fiber from 35 MPa (NS–Normal strength) and 60 MPa (HP–high-performance) concrete under monotonic and cyclic loading to assess the potential application of NiTi fiber in concrete. A novel SMA fiber fabrication method that starts with heavy cold-working the wire, then bending to the desired shape and followed by heat treatment at 350 °C was used. The heat treatment enabled higher strain recovery, shorter stresses plateau length and higher energy dissipation under cyclic loading. Individual fibers were embedded into concrete matrix with an embedment length of 30 mm. Comparison between steel and NiTi fibers on the maximum load, average and equivalent bond strength, and cyclic energy dissipation were studied. The results showed that the unique super elastic property of the NiTi shape memory fiber can be utilized by providing 5D anchorage that could be used to produce FRC with significant crack closing and energy dissipation capacity. The SMA fibers showed lower average bond strength and slightly higher equivalent bond strength than the steel counterparts. The SMA fibers outperformed the steel fibers on re-centering and energy dissipation under hysteretic pullout loading.