Effect of heat treatment and end-hook geometry on pullout behaviour of heavily cold worked superelastic NiTi shape memory alloy fibres embedded in concrete

Abstract

In this study, superelastic fibres were fabricated from heavily cold worked shape memory alloy (SMA) wire followed by heat treatment, with the goal of enhancing fibre pullout performance and simplifying the fibre production process. The effects of annealing at 350 °C, 450 °C, and 550 °C for 20, 40, and 60 min on the tensile properties of the cold worked SMA were studied. Monotonic and cyclic single fibre pullout tests were conducted using an embedment length of 30 mm. The fibres had straight, 3D, 4D, and 5D end-hook geometries and were embedded in normal strength (NS), high performance (HP), and ultra-high-performance (UHP) concrete. The results demonstrated that heat treatment at 350 °C for 20 and 40 min produced significantly higher peak load, pullout energy, average bond strength, and equivalent bond strength values as compared to the properties of fibres exposed to other temperatures and durations. Heat treatment at 350 °C for 40 min showed the highest re-centring and cumulative energy dissipation under cyclic pullout. Pullout performance significantly increased when either mechanical anchorage or concrete strength were increased. Among the SMA fibre geometries, 5D in NS concrete, 4D and 5D in HP concrete, and 3D and 4D fibres in UHP concrete generated sufficient resistance for martensitic transformation that resulted in a significantly higher re-centring ratio and cumulative pullout energy under cyclic loading than was observed for their steel counterparts.