Axial stress-strain behavior of high-strength concrete confined by circular thin-walled steel tubes

The axial stress-strain behavior of high-strength concrete confined by circular thin-walled steel tube was experimentally investigated. Specifically, 18 column specimens with the key parameters of concrete compressive strength and diameter-to-thickness ratio of steel tube were tested by monotonic or cyclic axial compression. The failure mode, load versus axial displacement curves, strain and stress states of the steel tube, interaction behavior between the steel tube and the concrete core, and stress-stain characteristics of steel tube confined concrete were described and discussed in detail. The loading path affects little on the strength and stiffness of the confined concrete. As the concrete grade increases from C40 to C80, the specimens show higher brittleness with a ratio of yield to peak loads changing from 0.6 to 0.9. Due to the accumulation of frictional stresses between the steel tube and the concrete, the stress state of steel tube varies along the tube height. The equivalent confinement of steel tube was estimated by the average transverse stress which was based on the observed stress distribution. The stress-strain model proposed by Mander et al. was modified and the predicted stress-strain curves for steel tube confined concrete are in good agreement with test results.