A design model for strain-softening and strain-hardening fiber reinforced elements reinforced longitudinally with steel and FRP bars

A close-form solution is developed for the prediction of the moment-curvature relationship of cross sections of fiber reinforced concrete (FRC) elements failing in bending, and reinforced longitudinally with steel and fiber reinforced polymer (FRP) bars. The FRP bars are installed with the largest possible internal arm, e.g. with the minimum concrete cover that assures the bond conditions for a sound stress transfer from FRC to the FRP bars. The model is also able of simulating the flexural strengthening contribution provided by FRP bars installed according to the near surface mounted (NSM) technique. To have good protection conditions against corrosion, the steel bars are applied with a relatively thick FRC cover. Since steel stirrups are the reinforcement with the smaller concrete cover thickness, they are the most susceptible to corrosion. In the reinforcement concept to be developed in the present research program, steel stirrups are replaced with discrete fibers. This hybrid reinforcement aims to develop high durable pre-fabricated elements that fail in bending. The proposed analytical formulation can simulate FRC with strain softening or strain hardening behavior. In the present work, the formulation is described and its predictive performance is appraised.