Bond Behavior of High-Strength Steel Rebar in Ultra-High-Performance Manufactured Sand Concrete: Experiment and Modelling

Manufactured sand (MS), due to its wide availability and cost-effectiveness, is used as an alternative aggregate for quartz sand (QS) in ultra-high-performance concrete (UHPC) to prepare ultra-high-performance manufactured sand concrete (UHPMC). This study aims to assess the bond behavior of 600 MPa-grade, high-strength, hot-rolled ribbed bars (HRB 600) in UHPMC. Thirty specimens were designed for the pull-out tests, taking into account several feature parameters, including MS replacement ratio (0%, 50%, 100%), water–binder ratio (0.17, 0.19, 0.21), steel fiber content (0%, 1%, 2%), and anchorage length (2d, 3d, 4d, 5d). The effects of the feature parameters on the failure mode, bond stress–slip curves, bond strength, bond-slip mechanism, and constitutive model were analyzed and illustrated. The results reveal that the pull-out specimen of UHPMC exhibits three distinct failure modes: rebar pull-out failure, UHPMC splitting failure, and splitting-pull-out failure. The bond strength increases from 46.57 MPa to 56.92 MPa when the steel fiber content increases from 0% to 2%. Additionally, a decrease in anchoring length is beneficial for improving the bond strength; as the anchoring length increases from 2d to 5d, the bonding strength decreases by 35.84%. The bond strength increases with an increase in the MS replacement ratio. As for the water–binder ratio, the bond strength presents the highest value when the water–binder ratio is 0.17. In addition, a new bond-slip constitutive model applicable to UHPMC and HRB 600 rebar, considering the MS replacement ratio, the water–binder ratio, etc., is proposed, which presents favorable prediction accuracy.