Bond between ribbed bars and concrete. Part 1: Modified model

The bond between ribbed bars and concrete is influenced by a number of parameters, such as the strength of the surrounding structure, the occurrence of splitting cracks, and yielding of the reinforcement. A model for three-dimensional analyses was developed earlier by the author, where the splitting stresses of the bond action were included, and the bond stress depended not only on the slip but also on the radial deformation between the reinforcement bar and the concrete. The bond model, however was shown to generate energy for some special loading-unloading sequences. This undesirable effect has led to a change in the formulation of the bond model. With the modification as described here, the model becomes equivalent to the Coulomb friction, complemented with a yield function describing the upper limit. Pull-out tests were analysed, using the modified bond model and non-linear fracture mechanics for the concrete. The tests were selected to show various types of failure: pull-out failure, splitting failure, pull-out failure after yielding of the reinforcement, rupture of the reinforcement bar and cyclic loading. The results show that the modified model is capable of predicting splitting failures, and the loss of bond if the reinforcement is yielding, as well as simulating cyclic loading in a physically reasonable way.