Development and preliminarily tests of superelastic Ni-Ti SMA ribbed bars for reinforced concrete applications

The research on superelastic shape memory alloy (SMA) bars reinforced concrete members and structures has received a great deal of attention recent years. In this study, novel superelastic Ni-Ti SMA ribbed bars were developed and investigated to improve the bonding performance and to avoid the problem that the threaded or mechanical anchorage parts of the SMA plain round bars (used in many previous studies) were easy to break in advance. A comprehensive experimental program was conducted to characterize the cold-rolled superelastic NiTi SMA ribbed bars, including scanning electron microscopy to observe the micromorphology, differential scanning calorimeter tests to analyze the phase transformation temperatures, and cyclic tensile tests to examine their mechanical properties. Moreover, five reduced-scale concrete beams were prepared and tested to verify the advantages of the superelastic SMA ribbed bars as bottom longitudinal reinforcements. Results shown that the cyclic tensile response of the cold-rolled SMA ribbed bar subjected to some degree of annealing can be similar with that of the annealed SMA plain round bar. Compared with the beam specimen incorporating ECC and superelastic SMA plain round bars and the beam specimen incorporating normal concrete and superelastic SMA ribbed bars, the specimen incorporating ECC and superelastic SMA ribbed bars exhibited a significantly improved deformation ductility, ultimate bearing capacity, energy dissipation ability, self-centering capability, and cracking performance under the cyclic flexural loads. Besides, no reinforcement slippage or fracture were observed during the entire loading process in all beams, suggesting satisfactory bonding effect and uniformity.