Bond behavior between concrete and thermoplastic GFRP bars with novel surface features

Compared with conventional thermoset-based GFRP reinforcement, thermoplastic GFRP bars offer distinct benefits, most notably their recyclability and their ability to be reshaped under heat and external forces—features that closely resemble the behavior of steel reinforcement. Since the bond performance of thermoplastic GFRP bars in concrete largely depends on surface geometry, this study introduces an innovative surface-felt wrapping technique integrated during bar manufacturing to enhance bond characteristics. To evaluate bond performance, 48 pull-out concrete-thermoplastic GFRP bar bond joint specimens were tested. Key variables included surface geometry, concrete type, bond length, and bar diameter. The results revealed that the proposed surface treatment enabled thermoplastic GFRP bars to achieve average bond strengths surpassing 7.6 MPa at a bond length of three times of the bar diameter, thereby meeting the ASTM D7957 requirements. The bond strength was found to increase with concrete strength but decline with larger bond length and bar diameter. Furthermore, two analytical models, the modified BPE model (mBPE) and the Cosenza-Manfredi-Realfonzo (CMR) model, were adopted to capture the bond stress-slip relationship, both demonstrating strong agreement with the experimental results. • TP bars with novel surface show bond strength > 7.6 MPa at 3d length. • Bond strength decreases with bar diameter in C30 but the same trend is not found in UHPC. • Modified BPE and CMR models fit TP bar bond-slip behavior well. • Surface-felt treatment enhances mechanical interlock with concrete.