Bond behavior of concrete reinforced with high-strength and high-toughness steel bars

To investigate the bond behavior between the ordinary concrete and high-strength and high-toughness (HSHT) steel bars with special spiral grooves on the surface, twenty-one specimens were tested by pull-out tests. The effects of test parameters, including anchorage length, the diameter of HSHT steel bars, cover thickness, stirrup ratio, anchorage form, and the number of spiral grooves, on failure modes, bond-slip curves, bond strength, the relative slip, and critical anchorage length were analyzed. The results show that increasing cover thickness and end-anchorage can significantly improve bond strength. The bond strength of specimens reinforced with HSHT steel bars with six spiral grooves is higher than that of three spiral grooves, and it gradually decreases with increasing the anchorage length. In addition, the decrease of the stirrup spacing can improve bond strength. The addition of stirrups can slow down the descending branches and make the failure modes of specimens change from splitting failure to splitting-pullout failure, indicating that the energy absorption capacity and the ductility of specimens have been improved. The models for bond strength and the slip corresponding to bond strength were established based on the existing models and experimental data. At the same time, coefficients B and D were established based on Wu's model. The evaluation shows that the proposed model can accurately calculate the bond-slip curves for ascending branches and descending branches, and it can also predict bond-slip curves corresponding to different failure modes.