Study on the stress-strain relationship for insufficient FRP-confined rectangular concrete columns

With the change in confinement conditions for lateral FRP composites, the stress-strain relationship curves of FRP-confined rectangular columns may or may not exhibit strain-softening. The curves with and without the strain-softening are defined as insufficient and sufficient confinements, respectively. Based on the analysis of a large number of existing test data, a critical value is proposed to judge whether or not the rectangular concrete columns are confined sufficiently. The present is focused on the stress-strain relationship of rectangular columns confined insufficiently with FRP. It is concluded that the ends of softening lines are approximately located along the same imaginary line which passes through the origin. Equations that are capable of predicting the peak stress and strain and the stiffness of the softening lines are proposed. The stress-strain relation can be expressed by a quadratic parabolic curve followed by a straight line. The predictions with the proposed equations are in good agreement with test data available in the literature. Parametric study is also conducted on the stress-strain relationship of insufficiently FRP-confined rectangular concrete columns.