The confining stress in FRP (fiber reinforced polymer) confined concrete is dependent on the lateral strain
of the concrete but the lateral strain of the concrete is dependent on the confining stress. Hence, the confining
stress and lateral strain are inter-related and have to be evaluated at the same time. However, up to
now, such evaluation has remained a difficult task. Consequently, the variation of the confining stress
during loading and the effectiveness of the FRP confinement at various stages of loading are still not fully
understood. Herein, a theoretical model for evaluating the lateral strain, confining stress and axial stress
in FRP confined concrete is developed. The theoretical model is first applied to analyze the FRP confined
concrete specimens tested by others to verify its accuracy and then used to perform a parametric study
on the effects of confining stiffness. Based on the parametric study, the two minimum values of confining
stiffness needed to achieve two different levels of ductility at various concrete strengths have been evaluated
for design guidance.