Accuracy of design code expressions for estimating longitudinal shear strength of strengthening concrete overlays

Strengthening operations of RC structures often imply the enlargement of the original cross-sections. In these cases, the shear strength of the resultant concrete-to-concrete interfaces is crucial in assuring the monolithic behavior of the strengthened members. Most concrete codes present design expressions for estimating the shear strength between concrete layers based on the shear–friction theory. However this was formulated for precast members with cast-in-place parts.Aiming to analyze the accuracy of code expressions in the strengthening situation, a research project was conducted. First, an experimental study was performed to evaluate the shear strength between a sandblasted concrete substrate and a concrete overlay, for different amounts of transverse reinforcement at the interface. Then, a numerical study was undertaken to enlarge the results’ range. Finally, these results were compared with those given by design codes and conclusions were drawn.It should be highlighted that: (a) the values of shear strength given by design code expressions are significantly different from each other; (b) in two cases, for low reinforcement ratios, these are not safe; and (c) in most cases, for high reinforcement ratios, these are too conservative.