Configuration and Size Effects on Bond Stress-Slip and Failure Modes of RC Connections

Bond slip of RC structures occurs wherever primary cracks are present. These cracks usually appear at the beam-column and column-foundation joints, for example, when structures are subjected to seismic loadings. As a result, it is imperative to consider bond slip in performing an accurate evaluation of the response of RC structures. The distributions of stress and strain in an embedded rebar vary depending on the geometry, such as concrete cover and embedment length, which are referred to here as configuration effects. Because bond stress and bond slip can be defined as functions of stress and strain of the embedded rebar, respectively, the relationships of bond stress and bond slip also differ. This, in turn, leads to different crack patterns: splitting cracks, shearing-off failures, or no cracks in the concrete. These differing crack patterns also cause distinct governing failure modes involving splitting failure, shearing-off failure, and bar fracture, respectively. Consequently, the relationships of bond stress and bond slip should be defined considering configuration effects, and a method should be developed to predict the governing failure mode for given geometric conditions. Furthermore, because cracks are the main sources of size effects, bond stress and bond slip should be evaluated carefully given that splitting cracks and shearing-off failures are tied to bond stress and bond slip. To investigate size effects of an embedded rebar and configuration effects of concrete cover and embedment length, a series of pullout tests was performed. In each test, a pullout force was applied to the rebar, which was embedded in a concrete cylinder. To examine size effects, three different sizes of rebar and associated concrete cylinder specimens were selected, whereas to address configuration effects, four different geometric settings for the concrete cylinders were considered. Based on the experimental program, an analytical stacked hollow cones (SHC) model was developed to evaluate the relationships of bond stress and bond slip at the failure states by assuming that imaginary concrete cones, which represent the transferring paths of forces generated by ribs of a rebar to the surrounding concrete, are supported by imaginary tension annuli formed at the cracked section. A method to predict the governing failure modes for given geometric conditions is proposed using critical energy release rates when cracks occur in the concrete. Meanwhile, a bond capacity model is proposed for prediction in the case of bar fracture for which no cracks develop in the concrete. Depending on the rebar-cylinder geometry, these size effects can vary. The bond stresses follow the general size effect law, with the strength of structures decreasing as size increases. For bond slip, however, the short-embedment cases follow the general size effect law, whereas the long-embedment cases do not. Scaling dimensionality factors, which depend on the geometric configuration, are established to capture the size effects. The proposed analytical model, along with the appropriate scaling dimensionality factors, then is able to predict the bond stress–bond slip relationship at failure states considering size effects and the governing failure modes for given geometric configurations. Experimental data obtained from the literature also are used to verify further the proposed analytical model. Although all this provides encouraging results that emphasize the importance of configuration and size effects on bond stress and bond slip, a more comprehensive experimental program is recommended as future work to refine the proposed models and to establish model parameters with increased confidence.