Push-off shear strength with inadequately anchored interface reinforcement

An essential requirement for composite construction, consisting of precast and in situ concrete elements, is that the two component parts act together as a single unit under all load conditions. It is unclear how or even if; this is achieved when the reinforcement crossing an interfacial shear plane is inadequately anchored. In order to provide experimental data, 63 single-bar push-off shear tests have been performed with anchorage lengths less than those currently required. The anchorage lengths ranged from 2.5 to 20 bar diameters. Two bar types and two covers were tested. The results indicate that the interface shear strength increases as the bond length increases. This has been attributed to an increase in the concrete-to-concrete friction, brought about by an increase in the pressure normal to the interface. In order to predict the shear strength a modified shear-friction equation is proposed, where the coefficient of friction is a function of the anchorage length of the reinforcement crossing the interface. This approach is also used to predict the residual strength found to remain after-failure. Comparisons are also made with predictions from the UK concrete bridge assessment code (BD44/95) and it is shown that this code is conservative.