Combined Shear and Flexural Behavior of Hybrid FRP-Concrete Beams Previously Subjected to Cyclic Loading

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) were initially proposed for bridge substructures in corrosive environments in the early 1990s. Systematic studies have since demonstrated the feasibility and merits of CFFTs with or without internal mild steel reinforcement. However, the experimental database in this field is still quite limited. This paper enhances the test database through a series of monotonic bending tests on one control RC specimen and five CFFT specimens previously subjected to reverse cyclic loading. Although the control RC specimen suffered shear-flexural cracks, specimens with carbon fibers experienced flexural failure by longitudinal splitting of the FRP tube in tension and its crumpling in compression. Specimens with glass or hybrid (glass/carbon) fibers, on the other hand, all failed by local buckling of FRP with either burst crushing or crumpling cracks. The specimen with hybrid fibers had higher normalized initial stiffness primarily because of its higher FRP/concrete stiffness ratio. The tests showed that the ductility of CFFT increases with FRP rupture strain. Further synthesis of flexural strength with FRP and mild steel reinforcement indexes reveals the existence of an optimized overall reinforcement index to achieve a design moment without overconfining concrete. Finally, the study confirms that shear failure is not critical for CFFT specimens at short shear span-to-depth ratios, even with internal mild steel reinforcement, as long as the FRP architecture is designed properly.