Reinforcing NiTi Superelastic SMA for Concrete Structures

Superelastic (SE) shape memory alloy (SMA) is an advanced material that may be used as an alternative to conventional reinforcing steel in civil engineering structures to control residual deformations. The most common SE SMA type is an alloy of nickel and titanium (NiTi), which has been used in medical instruments and aerospace industries. A state-of-the-art review was conducted and factors that affect stress-strain behavior of SE SMA were identified. Subsequently, a simple constitutive stress-strain model was adopted from the literature, and mechanical properties for SE SMA that are of interest to structural engineering were defined. A procedure was proposed to extract mechanical properties of reinforcing SE SMA from test data obtained according to a standard test, and the range for each property was determined. Comprehensive seismic analyses of a bridge column were carried out to determine the effect of each mechanical property on the moment-curvature and force-displacement relationships. Then, a design specification was proposed for reinforcing SMA bars. The proposed SMA model was used in a finite-element computer program to simulate the seismic behavior of a 1/3-scale SMA-reinforced bridge column experiment. The simulated responses were in reasonable agreement with the test data.