Assessing method of shear strength between old to new concrete interface under cycling loading

The retrofitting of the existing RC structures with added concrete layer crossed by post- installed rebars is strongly related to the behavior of the interface between the new and the old concretes. The overall strength of the connection depends on several mechanisms such as dowel action, friction between the two concrete layers, and type of action. In the past, attention was given to understand these mechanisms under monotonic loading, and only limited research has been devoted to investigate the cyclic behavior of concrete layers casted at different times. In this paper common experimental test set-ups (typical push-off tests and two tests configurations that allow the application of reversed actions) have been numerically analyzed to select the most reliable configuration to assess the cyclic behavior of concrete interface. Finally, the most promising set-up has been experimentally validated considering two different boundary conditions, to define the most appropriate test configuration to assess the performance at the interface under cyclic action. The results highlight that the ratio between cyclic and monotonic strength is lower than one and that cycles at the same slip level show a progressive strength reduction. On the basis of these outcomes, it seems that static assessment cannot be assumed in practical cases when cyclic actions are crucial, and a proper evaluation of cyclic performances is needed.