DETERMINATION OF INTERFACIAL LOCAL FRACTURE ENERGY AT STEEL BAR-CONCRETE INTERFACE

Interfacial fracture energy at the steel bar-concrete interface is a very important parameter which affects the value of interfacial shear-slip and the interfacial crack propagation after debonding. In previous tests of interfacial shear stress-slip relationship between the steel bar and concrete, the measured value of shear-slip is overestimated, resulting in the overestimation of the interfacial fracture energy. However, the actual local shear stress-slip relationship can be hardly obtained based on the present test approaches. The intention of this paper is to determine the local interfacial fracture energy between steel bar and concrete by combing the analytical model with test results. For the round plain steel bars with diameters of 18 mm anchored in concrete anchorage specimens, PVC tube was used to determine different unbonded lengths denoted as the initial crack lengths. Then the maximum pull-out load of steel bar from concrete can be experimentally measured. The distributions of tensile stress in the bar and interfacial shear stress along the bonding length were expressed in closed form at different loading stages based on the deformation compatibility conditions at the interface and equilibrium conditions of forces. Thus, the maximum pull-out load can be given analytically. The results show that the analytically determined maximum pull-out load is related to the bonding strength, residual frictional stress and local fracture energy at the crack-tip region. The correlation between the crack-tip local fracture energy and initial crack length is established upon the comparison between the analytically determined maximum pull-out loads and the experimentally measured ones. The interfacial local fracture energy distribution along the bonding length is then given, and the size-independent interfacial fracture energy is 25 N/m.