Research on the Stress-Strain Relationship of Square Concrete Columns Confined by CFRP Sheets at Low Temperatures

A total of 21 square-section concrete columns confined by carbon fiber reinforced plastic (CFRP) sheets were tested in uniaxial compression with different low temperatures and CFRP plies. The experimental stress-strain curves have three different stages. Firstly, the initial portion of the stress-strain response of confined concrete follows the path of unconfined concrete. Secondly, after exceeding the unconfined strengthen of concrete, the curve starts to form a transition zone. Finally, the CFRP sheets are fully activated and the confining stress provided by the CFRP sheets continues to increase with the applied load until failure. Within this stage, the experimental results indicate that the curve exhibits a linear behavior up to the rupture of the CFRP sheets. Based on the characteristics of experimental stress-strain curves of square concrete columns strengthened with CFRP sheets at low temperatures, Hosotani model was proposed to describe the stress-strain relationship of concrete columns confined by CFRP sheets at low temperatures. But, the meaning and calculating method of the parameters are different from that of Hosotani model, and the parameters are calculated by the methods suggested by this paper. Finally, comparison of proposed model, experimental curves and other models shows that the curves of proposed model agree well with the experimental curves. But the error of other models and the experimental curves is greater.