A unified stress-strain model for LRS FRP-confined concrete columns with square and circular cross-sections

Large rupture strain (LRS) fiber-reinforced polymer (FRP) composites with an ultimate elongation of greater than 5% offer superior deformation and energy dissipation abilities over traditional carbon or glass FRP when used them in lateral confinement of concrete under the same confinement level. However, a flexible model for predicting the stress-strain relationship of concrete columns confined with LRS FRP is not yet perfectly developed, especially for noncircular columns experiencing nonuniform confining stress. Accordingly, an extensive database of LRS FRP-confined square and circular concrete columns with corner radius ratios varying from 0 to 1 was employed to establish a unified stress-strain model. This model not only includes the determinations of key points, but also attempts to define the threshold confinement level to distinguish the post-peak strain-hardening and strain-softening behaviors. The mathematical expression of the model is simple; it also avoids discontinuities in the prediction of different cross-sections. The proposed model is verified to be accurate in predicting the complete stress-strain curves of LRS FRP-confined square and circular concrete columns with hardening and softening behaviors. In addition, it outperforms the existing theoretical models in predicting the ultimate strength and the ultimate axial strain.