Axial strengthening of thin-walled concrete-filled-steel-tube columns by circular steel jackets

• A strengthening scheme for thin-walled concrete-filled-steel-tube using steel jacketing has been proposed.
• The predicted axial load-displacement and hoop-axial strain curves agree well with test results.
• Stress lagging effect on strength and elastic modulus has been quantitatively studied.

External confinement in the form of steel rings, tie bars, spirals and FRP wraps has been widely adopted for strengthening concrete-filled-steel-tube (CFST) columns. Previous experimental and theoretical studies have proved that it can improve the strength, elastic stiffness, ductility and interface bonding of CFST columns. However, in real engineering practise, CFST columns need to be strengthened are usually under pre-compressed axial load. The stress-lagging effect between the CFST columns and external confinement due to pre-loading has not yet been justified. In this paper, external confinement in the form of circular steel jacket is proposed to improve the uni-axial behaviour of CFST columns with and without pre-compressed axial load. An experimental study, consisting of 5 hollow-steel-tubes and 10 thin-walled CFST columns was conducted to examine the effectiveness of the proposed strengthening scheme. The main parameters were the concrete cylinder strength, jacket spacing and pre-compressed axial load level. Test results revealed that the steel jacket could improve the uni-axial behaviour of CFST columns and the stress-lagging could degrade this beneficial effect. In addition, a theoretical model developed by the authors previously was adopted to predict the uni-axial behaviour of the strengthening columns. Very good agreement has been obtained between the theoretical and experimental results.