Intermediate crack debonding (i.e., IC debonding) is a common failure mode of reinforced concrete (RC) beams flexurally strengthened with a fiber-reinforced polymer (FRP) plate. Due to IC debonding failure, the tensile strength of the FRP plate cannot be fully utilized. While a number of options to suppress this failure mode have been explored, the easiest option is still to install U-jackets of an appropriate layout outside the FRP plate. This paper presents the results of an experimental study into the effectiveness of such U-jacketing on delaying or suppressing IC debonding failure, an issue which has not yet been systematically investigated so far. Eight large-scale RC beams were tested in the present study to investigate the effects of different forms of FRP U-jacketing on IC debonding failure. The test results indicated that vertical FRP U-jackets have a rather limited effect on IC debonding failure. Moreover, strong vertical FRP U-jackets may cause significant out-plan bending in the soffit plate during the deformation process of the beam, leading to the premature rupture of the FRP soffit plate. The tests also showed that inclined U-jacketing is much more effective than vertical U-jacketing, particularly when inclined U-jacketing is provided in a low moment region.
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