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A concrete interface is a material discontinuity which requires special care in structural design and assessment. Therefore, the definition of design expressions, based on experimental testing data, must ensure the needed reliability depending on the type of structure and its use. In the present work, a new proposal for the design of concrete interfaces subjected to shear loading is presented for different roughness profile types. The proposal is characterized by 3 linear branches (for monotonic loading), an S-N curve (for cyclic loading) and is the result of a parametric analysis of existing experimental data (obtained by the authors and also from an extensive literature search) based on statistical and probabilistic methods. Design expressions were defined in order to minimize dispersion and variability of the safety factor values for each considered experimental test, and also to assure that those values are within a target range (defined according to reliability considerations). These improvements became clearer when the new proposal was compared with the most common design code recommendations. -INTRODUCTIONA concrete interface can be found in the contact area between two concretes cast at different times or in monolithic concrete crossed by a crack. The first case represents a common situation in structural rehabilitation and strengthening [1][2][3][4], and also in precast concrete structures, between the precast and cast-in-place concretes [5,6].Since it is a material discontinuity, a concrete interface is a weak surface whose strength has to be properly quantified in structural design. Therefore, the definition of design expressions from experimental testing data must ensure the needed reliability depending on the type of structure and its use [7]. To determine the reliability provided by the design expressions, a statistical and probabilistic approach has to be followed, depending on the Accepted Article www.ernst-und-sohn.de Page 3Structural Concrete number of experimental tests, the scatter of data, the parameters involved and prior statistical knowledge [8].This present paper aims to determine new design expressions for calculation of concrete interface shear strength, for different roughness profile types, considering both monotonic and cyclic load cases. The proposal is the result of a parametric analysis of existing experimental data (obtained by the authors and also from an extensive literature search), based on statistical and probabilistic methods. The new expressions are compared with the most common design code recommendations to evaluate the improvements achieved with the present study, which is performed in order to minimize variability of the safety factor values for each considered experimental test, and also to assure that those values are within a target range (which was established according to reliability considerations).Firstly, the paper presents a literature review regarding the calculation of the design shear strength of concrete interfaces. The literature review focuses on...