United States Department of Transportation
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Mechanical integrity and sustainability of pre-stressed concrete bridge girders repaired by epoxy injection – phase 1.
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2017-02-02
[PDF-3.97 MB]
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Abstract:At present, there is a need to assess the mechanical integrity and sustainability of pre-stressed concrete beams during the entire life cycle of the built infrastructure. According to the NCHRP (Tadros et al., 2010), “further research to develop finite element modeling of the end zone of pre-tensioned members should be of value in optimizing the bursting reinforcement”. The ultimate goal of this project is to assess the mechanical integrity and sustainability of pre-stressed concrete beams during the entire life cycle of the built infrastructure, which includes crack propagation, crack reparation, repaired crack aging with possible re-opening. In Phase I, the research objectives are to: (1) Explain in which conditions the strength of cracked concrete can be recovered by epoxy injection; (2) Design the injection method for optimal mechanical performance. We explained how epoxy is used to repair cracked concrete and we summarized the governing equations of epoxy rheological models. We presented the governing equations of the Differential Stress-Induced Damage model (DSID), a Continuum Damage Mechanics model that allows predicting the propagation of cracks in three directions, according to net tension and compression criteria. The damage variable is similar to a crack density tensor. We calibrated and validated the DSID model to match stress/strain curves obtained during concrete triaxial compression tests reported in the literature. We performed DSID model sensitivity analyses with a MATLAB code. Simulations were done at the material point for tensile and compressive stress paths. Results show that cracks propagate in planes perpendicular to the maximum deviatoric stress, and that stiffness decreases in the directions in which damage increases. Simulations confirm that mechanical effects of cracks before and after reparation can be modeled by high and low damage. We designed a Finite Element model of bridge deck with ABAQUS software. The model includes the steel-reinforced concrete deck and the pre-stressed steel-reinforced girders. We applied the static loads recommended in the design standards to predict the displacement and stress fields in the structure. Concrete was first considered elastic. Simulation results highlight the high stress concentrations at the ends of the girders and at the contact between the girders and the rubber pads lying at the top of the supporting columns. Then the FEM model was modified to account for crack propagation in the concrete. The DSID model was assigned to the concrete elements located in the girder subject to maximal stress (further away from the center of the deck). Static service loads were simulated. Results highlight the capability of the model to predict the initiation and orientation of the damage in the form of vertical cracks close to the interface between the concrete and the rubber pads. Loading tests were simulated with initial vertical damage (i.e. horizontal cracks) in order to assess the effect of longitudinal cracks due to pre-stress relaxation on the deflection and internal stresses of girders. Bridge simulations were repeated with initial damage in the end zone of the most highly stressed girder in the deck. Damage configurations corresponded to typical crack patterns listed in the PCI bridge repair guidelines. In general, lower stress and higher deflections were found for higher initial damage.
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