Cyclic Stress-Strain Behavior of Concrete Confined with NiTiNb-Shape Memory Alloy Spirals

Recent studies showed that concrete confinement using shape memory alloy (SMA) spirals is a promising technique for seismic retrofitting of reinforced concrete columns that lack flexural ductility. This technique is applied by wrapping prestrained SMA wires around concrete columns and activating the confining pressure by heating the wires. This study is the first step toward understanding the behavior of NiTiNb-SMA-confined concrete both experimentally and analytically. The paper aims at investigating the cyclic behavior of NiTiNb-SMA-confined concrete, and using the test results to develop an empirical stress-strain model for NiTiNb-SMA-confined concrete. A series of uniaxial cyclic tests are performed on NiTiNb-SMA-confined concrete cylinders having different concrete strengths, confining pressures, and loading protocols. The test results show that the effectiveness of NiTiNb-SMA confinement on strength and ductility enhancement increases as active confining pressure increases; and in the studied range of normal strength concrete, the residual stress is independent of concrete strength. The study also proposes empirical equations to predict stress-strain behavior of NiTiNb-SMA-confined concrete, including the peak stress, residual stress, and ultimate stress and strain of SMA-confined concrete.