Axial stress-strain model for CFRP confined concrete cylindrical columns with and without preloading

Carbon Fiber Reinforced Polymer (CFRP) is a well-known composite material for retrofitting existing structures. Many studies have investigated various aspects of CFRP-retrofitted concrete columns, resulting in the development of stress-strain models. An important aspect that has been neglected in most of the proposed models is the consideration of existing gravity loads prior to retrofitting. In other words, although in practice columns are retrofitted while sustaining service loads, the effect of pre-existing stress induced by these loads is often neglected in most proposed models. In this study, an extensive database of CFRP-retrofitted columns tested without and with preloading is compiled and analyzed. Furthermore, the influence of different approaches (i.e., using direct tensile test results of FRP coupons, manufacturer data sheet, or the actual rupture strain observed during testing) for drawing the axial stress-strain curve is discussed and confinement effectiveness coefficient is modified for non-preloaded confined concrete columns. Additionally, a new design-oriented model for CFRP-retrofitted cylinder columns adjustable for different preloading ratios, is proposed and validated against test results of preloaded columns.