| The knowledge about the cyclic behavior and the effects of fatigue on interfaces between concrete cast at different times, subjected to shear stress, is still very limited. This aspect is particularly relevant in structures subjected to important cyclic loads, such as railway bridges. In this context, firstly, a brief state-of-the-art on the monotonic and cyclic behavior of interfaces between concretes cast at different times is presented, especially reflecting the case of a joint between a precast beam and a cast-in-place slab, where the contact surface between the old and the new concrete is usually a free surface, left without further treatment after vibration of the old concrete. Then, the test campaign, which was aimed at obtaining experimental support for modeling the behavior of such interfaces, subjected to a high number of load cycles, is described, and the corresponding results discussed and analyzed in terms of slip and crack opening, stress variations in the monotonic tests, failure mode, and number of resisting load cycles. It was further observed that the number of resisting load cycles increases as the maximum load level decreases. Based on this evidence, an experimental S-N curve was set for this type of interface.;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 author 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 were noticed when the new proposal was compared with the most common design code recommendations.;A nonlinear finite element model is developed to evaluate the behavior of a dowel bar subjected to monotonic loading and embedded on a single concrete block substrate. In the model, a "beam element" representation of the steel reinforcing bar is used, considering material plasticity, rotational degrees of freedom and flexural stiffness. The bar is connected to concrete embedment through several Winkler spring elements to simulate the deformability and strength of the concrete substrate. Comparison with results from experimental tests available in literature allowed to calibrate force-displacement relations for the nonlinear Winkler spring response during an imposed dowel displacement up to 4 mm. Finally, new coefficients are included in those relations in order to account for the influence of concrete cover and confinement imposed by stirrups.;Later, the nonlinear finite element model is extended to assess the response of a concrete interface submitted to static loading. In the model, an interface element simulates aggregate interlock of the concrete joint and the "beam element" representation of the steel reinforcing bars is used. The reinforcing bars are connected to concrete embedment through several spring elements to simulate dowel action and bond along the bar length. Considering geometric non-linearity, or kinking, effect influence of the deformed reinforcing bars, each strength mechanism is modeled separately in order to determine the corresponding contribution to the joint shear stress. Comparison with experimental results available in literature allowed to determine new stress-strain relationships for the interface aggregate interlock element for two types of interface roughness profiles: monolithic concrete cracks, and interfaces between concretes cast at different times corresponding to a free surface. These new constitutive relations are suitable to be use in the case of a concrete interface with embedded reinforcing steel bars. |
