Modelling short-term tension stiffening in tension members

Recent research has questioned the validity of bond–slip relationships owing to the ‘necessity for the concrete to be able to slip past the ribs on the bar’. Conversely, it is suggested in this paper that bond–slip models are simply a numerical device that relates bond stress to the relative displacements between the concrete, which surrounds the internal cracking zone, and steel. This paper shows that the short-term response of a reinforced concrete tension member with a central reinforcement bar is predicted well by a one-dimensional non-linear finite-element analysis incorporating a trilinear idealisation of the Model Code 1990 bond–slip relationship. The authors' analysis is significant since it is shown to predict many of the trends in behaviour observed in tests associated with tension stiffening and cracking. Different approaches are adopted in Eurocode 2 (EC2) (2004) for modelling tension stiffening in the calculation of crack width and mean axial extension. These approaches are reviewed in the light of the authors' finite-element analysis and existing experimental data. The EC2 models for mean axial strain are shown to be reasonable but the crack spacing formula overestimates crack spacing in the tension members considered in this study.