Ductility improvement of high strength concrete columns using hybrid confinement

The usage of high strength concrete (HSC) in reinforced concrete (RC) bridge columns in earthquake prone regions is hindered due to the brittle nature of HSC. Passive lateral confinement does not improve the ductility efficiently since HSC exhibits low lateral expansion under axial loads. This leads to an increase in the amount of transverse steel required which causes reinforcement congestion. Concrete-filled fiber tubes (CFFT), manufactured using fiber reinforced polymer (FRP) sheets confine concrete externally overcoming reinforcement congestion problems. However, due to the passive nature of CFFT confinement, the low efficiency persists. Active confinement, applied using shape memory alloy (SMA) spirals confines concrete more efficiently than passive confinement. Hence, this research study investigates the application of active confinement using SMA spirals in the plastic hinge region of CFFTs to attain higher ductility in HSC columns. The resulting hybrid confinement in the plastic hinge region, which is a combination of passive CFFT confinement and active SMA confinement, is studied experimentally by subjecting concrete cylinders wrapped with FRP and SMA spirals to cyclic axial compression and comparing their behavior with FRP wrapped cylinders. Test results indicate that the additional active SMA confinement delays the rupture of FRP jacket. Post the FRP rupture, the SMA spirals remain intact and aid the concrete cylinders in attaining ultimate axial strains which are more than 3 times the ultimate strain of FRP confined cylinders. Next, a numerical study is undertaken to study the pushover behavior of CFFTs confined with SMA spirals in their plastic hinge region. The drift capacity of CFFT column is seen to increase significantly on addition of a limited amount of SMA.