Peak strength and ultimate strain prediction for FRP confined square and circular concrete sections

It is widely accepted that axially loaded FRP confined concrete presents significant strength and ductility increase with reference to the unconfined case. Adequate FRP confinement provides bilinear – like and hardening stress–strain behavior to concrete up to failure. Therefore, to model the mechanical behavior of adequately confined concrete two quantities are necessary: peak strength and ultimate strain. Many studies have been published regarding confinement of circular section elements; fewer studies analyze the case of square and rectangular section. Numerous predictive expressions of peak strength and ultimate strain, suitable for circular section elements, are available. Only in recent years, complete equations have been proposed to cover both circular and square sections. The aim of this work is the assessment of the performances of significant predictive expressions published in literature. Four groups of predictive expressions have been considered; they differ by the type of the predictive quantity (peak strength or ultimate strain) and by their applicability (only circular sections or both circular and square sections). For this reason a wide database has been assembled; it collects results of more than 655 compressive tests, performed on square and circular specimens confined with FRP. This comparison has been diversified as a function of cross section shape and FRP type (CFRP, GFRP, AFRP).