Robotic rebar insertion and grouting for reinforcement of 3D printed concrete: Technique development and bond behavior characterization

This study explores vertical insertion of steel rebars into 3D printed concrete (3DPC) using a robotic arm, enabling reinforcement during the printing process. A robotic system comprising a mobile base, a 6-axis robotic arm, a force/torque sensor, and a gripper was employed, alongside a dedicated algorithm for self-calibrating surface detection and force-controlled insertion. To enhance bond performance, grouting with epoxy or high-flowability cementitious materials was investigated. A total of 19 specimens were fabricated for the pull-out test, while 3 specimens were reserved for visual inspections. Pull-out tests identified four failure modes: concrete splitting, rebar pull-out with or without splitting, and rebar fracture. Steel cable confinement prevented splitting failure and increased bond strengths. Grouting improved bond strength up to 24–42%, whereas 0.5 vol.% PVA fibers in the printed matrix reduced bond performance by 11.55%. Confined specimens with 100 mm bond lengths and grouting exhibited rebar fracture without concrete damage, indicating sufficient bond capacity for structural applications. Visual inspections of specimens indicated that non-grouted specimens exhibited minor voids in the upper region, while grouting effectively filled voids along the insertion path. Existing bond–slip models were evaluated for robotically inserted rebars, showing reasonable predictions for splitting failure but limited accuracy for pull-out failure. An analytical model was developed based on thick-walled cylinder and fictitious crack models. Overall, robotic rebar insertion (combined with grouting) achieved an acceptable bond performance by observing rebar fracture during the pull-out test, demonstrating the feasibility and potential of this automated reinforcement strategy in 3DPC.