Unified Design Method for Flexure and Debonding in FRP Retrofitted RC Beams

Flexural retrofitting of reinforced concrete (RC) beams using fiber-reinforced polymer (FRP) plate is a common way to increase the flexural capacity. There is a lack of rational design method to determine where the strengthening plate can safely be curtailed. As a result, retrofitted beams commonly fail by debonding of the FRP plate, which occurs well before the target flexural capacity. Debonding is brittle failure; thus, ductility of the retrofitted beam needs to be ensured by debonding prevention. With sufficient ductility the ultimate strength can continuously increase beyond steel yields with the elastic behavior of the FRP strengthening plate. Debonding prevention has been accounted for empirically in most design approaches so far. The global energy balance approach (GEBA) using fracture mechanics has been proposed to determine the debonding load of an FRP-RC beam that is affected by the plate curtailment location. The GEBA results for various FRP-RC beams can be summarized using debonding contours on plots of moment capacity against the safe plate curtailment locations. The debonding contours constructed in this way for the beams with the same ratio of depth to fracture energy are virtually the same. This paper shows how GEBA can be incorporated into the design process to prevent premature debonding of the FRP plate. The method makes use of the debonding contours and derives from these simplified design charts that could be made available to designers. The retrofitting design consideration and the theoretical background of this unified design method are first explained, followed by the derivation of the conceptual design charts. Numerically correct design charts are then constructed for a wide range of design cases. Finally, a worked example is used to explain the way to apply the unified design method using design charts.