A versatile model with a design framework for axially-loaded FRP-confined concrete with/without a stress reduction-recovery behavior

Given the magnitude of investigations conducted on axially-loaded FRP-confined concrete columns in the past two decades, the focus of these studies was mainly given on FRP-confined concrete with a full strain hardening behavior, categorized as Type A. Nonetheless, for a reliable design of FRP confinement arrangements imposed to concrete, it is essential to properly comprehend and accurately simulate the mechanical behavior of FRP-confined concrete considering the possibility of a stress reduction-recovery response (Type B). Accordingly, this study proposes a new unified design-oriented model to predict the stress-strain behavior of axially-loaded FRP-confined normal/high-strength concrete cylinders with either Type A or Type B. The model consists of a two-segment stress-strain function for Type A whereas it, with a smooth transition, is transformed into a three-segment stress-strain function for Type B. A new parabolic stress-strain relation was developed for the first portion of axial behavior prior to the transition zone. Based on a new analytical methodology, new formulations with a design framework were proposed to predict the slope of softening/hardening linear functions, calibrated based on a large test database of Type A and Type B specimens. Furthermore, based on a reliable numerical database, simplified formulations are proposed to calculate the stress and strain at the transition zone of the stress-strain relations. The comparative assessment of the developed design-oriented model with existing confinement models reveals the developed one has not only superior predictive performance but also a simpler calculation process, which makes it suitable for design purposes by balancing the reliability and the implementation simplicity of the model.