Unified Ultimate Axial Strain Model for Large Rupture Strain FRP–Confined Concrete Based on Energy Approach

Using large rupture strain (LRS) fiber-reinforced polymer (FRP) composites as confining material has become increasingly prominent in structural repair or retrofitting, owing to their advantageous high deformation capacity. Economic and rational usage of LRS FRP relies on displacement-based design, which requires calculation of the ultimate deformation of a member. However, prediction of the ultimate strain of LRS FRP–confined concrete is more complex and can be more inaccurate than prediction of strength, especially for structural elements under large deformation or severe damage conditions. This study proposes a unified ultimate strain model for LRS FRP–confined concrete based on an energy balance method. A unified expression form is derived using this method, providing an ultimate strain model with no restrictions on column cross section, in terms of circular, square, or oblong columns. The proposed ultimate strain model has a wider application and a better performance than other models. Furthermore, according to this paper's updated database, the characteristic points on the whole stress–strain curve can also be accurately determined. Using the new ultimate strain model for LRS FRP–confined concrete and its characteristic points, the whole entire stress–strain curve of LRS FRP–confined concrete is accurately derived.