Flexural Strength and Ductility of FRP-Plated RC Beams: Fundamental Mechanics Incorporating Local and Global IC Debonding

Reinforced concrete (RC) beams and slabs are frequently strengthened or stiffened in flexure by adhesively bonding fiber-reinforced polymer (FRP) plates to their surfaces using a strain-based moment-curvature design technique. This design technique is generally based on the intermediate crack (IC) debonding strain of the FRP reinforcement, that is, on the start of IC debonding; from this analysis it is often deduced that FRP plating is ineffective at the ultimate limit state because FRP debonding occurs before yield of the steel reinforcement. In this paper, it is shown that the strain-based approach is generally a lower bound at the ultimate limit state. Instead, a displacement-based approach is described that shows that FRP plated beams can be designed to achieve a higher strength than that of the RC beam by itself no matter when IC debonding first occurs. The mechanics of the analysis approach developed here treat the FRP debonded plate as a FRP prestressing tendon with a force equal to the IC debonding force. Consequently all FRP plated beams have the potential to achieve strengths greater than that of the unplated beam specifically when designed for ductility, which makes the system much more effective at the ultimate limit state. This paper describes, in a form suitable for the development of numerical solutions, the fundamental mechanics that control local IC debonding at a section or segment as well as global IC debonding along a member. It is shown how FRP plates and their extent of plating can be chosen through mechanics to increase the strength and if necessary the ductility of a member as well as allowing for both stable and unstable debonding.