This study applies a plastic approach to the analysis of shear at a joint or interface in reinforced concrete. Push-off tests have been used historically to investigate combinations of shear and compression across an interface in reinforced concrete. Recent work by the authors has shown that such tests can also be modified to model combinations of shear and tension; as is often found at critical interfaces such as joints in reinforced concrete structures. New experimental results are presented for modified push-off tests subject to a range of combinations of shear and tension. These results, along with a number of historical results reported in the literature are analysed using the upper bound theory of plasticity for interface shear. It is shown that the behaviour predicted by the upper bound theory is consistent with the new experimental results for an initially uncracked concrete interface subject to combinations of shear and tension. Effectiveness factors for the plastic analysis indicated by the experimental results are proposed.