Behaviour of shape-modified columns confined with carbon fibre-reinforced polymer under eccentric loading

Fibre Reinforced Polymer (FRP) has been popularly used to improve the loadcarrying capacity and ductility of columns. Early research studies have indicated that confinement enhancement of FRP wrapped circular columns is more efficient than that of square/rectangular RC columns. Most existing columns, however, are square/rectangular in cross-section. This study presents a retrofitting method for square reinforced concrete (RC) columns using shape modification and FRP wrapping. The cross-section of RC columns is changed from a square to a circle before confining them with FRP to maximise the confining pressure of FRP. Sixteen square RC columns (150 mm in side length by 800 mm in height) were cast with 40 MPa concrete and divided into four groups. The specimens of the first group were tested without further modification and their four specimens served as a reference group. Each column from the twelve remaining square RC columns in the second, third and fourth groups were bonded with four pieces of segmental circular concrete covers, which had nominal compressive strength of 40 MPa, 80 MPa and 100 MPa, respectively. The shape-modified columns were then wrapped with three layers of Carbon FRP (CFRP). From each group, three of the four columns were tested under different eccentric loadings including 0 mm (concentric), 25 mm, 50 mm eccentricity, and the remaining specimen in each group was tested under flexural bending to failure. The effectiveness of the proposed retrofitting using shape modification and CFRP wrapping was investigated. A theoretical study was then implemented taking into account the interaction between the internal transverse steel reinforcement and the external FRP jacket. The analytical values were validated with the experimental data and good agreement was found. Conclusions are drawn and recommendations for future research are proposed.