Dowel behaviour of rebars under combined axial and shear actions

The resource of shear resistance provided by the dowel mechanism of rebar in reinforced concrete (RC) structures can significantly affected by the simultaneous presence of axial loading. This occurs for example of plastic hinges of seismic resistant structures. In fact, at load reversals in cycles of large deformation demand, rebars are subjected to combined axial and shear loading, particularly in those section where the shear transfer via aggregate interlocking is jeopardized by the opening of the crack throughout the entire section depth. Thus, a reliable assessment of the shear capacity of dowels under combined shear and axial load is required to check the element shear resistance. The paper describes the results of a specific experimental campaign on rebar dowels subjected to shear loading in presence of different levels of axial load. Both smooth and ribbed rebar dowels were investigated. A marked reduction of the dowel shear strength and stiffness in presence of increasing axial loading was experimentally observed, only partially compensated by the kinking effect. The latter was found to characterize the entire resource of dowel capacity when the axial load was close to the rebar yielding strength. The paper proposes an analytical model, adapted from others available in the literature, to predict the dowel shear-displacement response accounting for the applied axial load. The model helps the understanding of the dowel response, is suitable for hand calculation and can easily assist the dowel design. The experimental response was quite well captured for smooth dowels, while the prediction was less accurate for ribbed ones. Future refinements may address the local damage induce by the rebar pull-out, typical of ribbed rebars, which is neglected in present form of the model.