Steel-Concrete Bond versus Primary Crack Opening in Reinforced Concrete Beams

Bond deterioration between steel bars and concrete in reinforced-concrete beams mostly occurs close to the primary (i.e., bending induced) cracks. To investigate bond-cracking interaction, a novel bond-slip relationship is introduced in this study, where bond parameters are a function of crack-opening displacement. Such a displacement is in turn evaluated based on the highly variable strain profiles in the concrete, through an iterative procedure. The effectiveness of the proposed approach is validated against a set of tests on RC beams well-documented in the literature, which show a clear trend for the local bond stresses to decrease close to the primary cracks. The bond-slip relationships along the bar and the maximum bond stress depend on the applied loads and on the crack pattern, with a mutual interaction that affects both the crack-opening displacement and the bond stresses, causing a nonlinear increase in the steel strains. Nevertheless, the bond-slip relationships display descending branches characterized by remarkably similar slopes within a given region, irrespective of the loads. Consequently, if the descending branch for this region is known under a particular load, the branches under other loads can be obtained by horizontally translating the known branch. The proposed approach may provide a useful tool to describe bond behavior in RC members and to understand the complex interaction among the displacements due to crack opening, bond stresses and their structural effect.