Enlargement of original cross-sections or replacement of defective concrete layers with new concrete are usual situations in strengthening operations of reinforced concrete structures. In these situations, the shear strength between concrete cast at different times is crucial for the monolithic behavior of the strengthened members. Most design standards for concrete structures present design procedure for estimating the shear resistance between concrete layers based on the shear friction theory. The study includes three-dimensional and two-dimensional finite element model (FEM) analysis for calculation of shear stresses and comparison of three different code approaches, i.e. BS8110, ACI 318 and EN 1992, for determination of design shear resistance at an interface between concrete cast at different ages of a pile cap supported on precast concrete piles. Based on the results of the analysis carried out, it can be stated that complicated three dimensional finite element model analysis is not always essential for analysis of structures, which are having complex geometrical shapes. It is possible to transform three-dimensional problems to a simplified two-dimensional problem based on the level of accuracy required. For the selected surface characteristics and r/f percentage, the estimated design shear resistance based on recommendations of EN-1992-1-1-2004 was found be lower than the corresponding estimated value based on ACI 318M-11 recommendations. It was further observed that BS 8110-1-1997 recommendations gives the highest value for the design shear resistance independent of r/f percentage provided. EN-1992-1-1-2004 can be used to compare contribution of concrete interface roughness and interface reinforcement on design shear resistance without any limitation of design shear stress as specified in ACI 318M-11. Furthermore, the EN-1992-1-1-2004 recommends a conservative value for design shear resistance compared to other two standards
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