Development length and bond behavior of lap-spliced reinforcement in Ultra-high performance concrete beams

Ultra-high performance concrete (UHPC) is a new composite material that can significantly improve bond performance with steel bars. There is insufficient research on UHPC bond behavior under actual stress within structural members, which is critical for structure safety and economy. In this paper, 17 lap-spliced beams were tested to simulate flexural member bond conditions. The test parameters included UHPC compressive strength, cover depth, fiber content, splice length and stirrups in the spliced region. All specimens failed by the UHPC cover splitting crack. The beam without fiber experienced brittle splitting failure, while the beams with fibers experienced ductile splitting failure. The test results showed that increasing UHPC strength and cover depth could increase bond strength. The bond strength increased linearly with fiber content from 0% to 3% but decreased with 4% fiber content. Bond strength decreased linearly with splice length. Transverse reinforcement in the lap splice effectively improved bond strength; however, when the additional confinement reaches a limit, the increase in stirrups no longer improves UHPC bond stress. The experimental study established a formula for predicting UHPC bond splitting strength, considering the influence of the above parameters. The maximum limit of bond strength contributed by the stirrup was obtained. Finally, the development length calculation model applicable to UHPC was established by modifying the NC theory method. The proposed model was compared with current design methods, and the predicted values of the proposed model were closest to the experimental values.