Seismic retrofit for RC columns by NiTi and NiTiNb SMA wires

This paper conducted several flexural tests of RC (reinforced concrete) columns retrofitted by SMA (shape memory alloy) wires. The RC columns with lap spliced reinforcements at the base show less ductile behaviour than those with continuous reinforcements. SMA wire jackets can increase the ductility of the RC columns with lap spliced reinforcements and thus this study used the two kinds of SMA wires to retrofit such RC columns; the first one is NiTi SMA wire that remains in martensite at room temperature and the other is NiTiNb SMA wire that is austenite at room temperature. Both the SMA wires show shape memory effect and provide active confinement for concrete. The SMA wires were prestrained to 7% strain and released. At the state, some residual strain remained and can be recovered by heating. The prestrained SMA wires were wrapped around the plastic hinge region of the RC columns and then the temperature of the wires raised by heating. Then, the SMA wires induced an active confinement on the RC columns. Each wire was measured for recovery and residual stress to obtain the information of active confinement. This active confinement and the resistance of the SMA wires increased the ductile behaviour of the RC columns in lateral direction. The parameters in this study are the kinds of SMA and the pitch of the wires. All types of the specimens retrofitted the SMA wires showed good ductile behaviour without degrading the flexural strength