Cement based overlay for orthotropic steel bridge decks: A Multi-Scale Modeling Approach

The success of the traditional orthotropic steel bridge deck may be due to its high strength to weight ratio. However, fatigue damage has been experienced within heavily trafficked routes due to the low stiffness of this deck in combination with increasing traffic intensity and wheel pressure. This thesis investigates a system to stiffen an orthotropic steel bridge deck, using a cement-based overlay. The investigation is based on nonlinear fracture mechanics and aims to determine the performance of the bridge deck in terms of cracking behavior. The main goal of applying a cement-based overlay to an orthotropic steel bridge deck, is to increase the deck stiffness and thereby reduce the stresses in fatigue sensitive steel parts. Cracking of the cement-based overlay will have considerable influence on the composite action and durability of the system. The system has to be economically beneficial and show a good performance with regards to cracking behavior. Since cracking plays a major role on the performance, cracking behavior of the overlay is the main focus of the present thesis.

The strategy utilized in the present thesis is based on multi scale modeling, which spans from modeling and experiments on the steel-concrete interface scale, to modeling of a real size structure. The multi scale concept is utilized by identifying mechanical behavior on the steel-concrete interface scale and later applying the mechanical behavior on the structural scale. On the steel-concrete interface scale, normal cracking (Mode I) and combined normal and shear cracking (mixed mode cracking) are analyzed through experiments and modeling. The aim and outcome of the study on the interface scale, is a set of constitutive parameters which later are applied on the structural scale. The composite action between an overlay and steel plate is analyzed experimentally through small beam and plate elements with spans in the range of 0.8 to 1.0 meter. Through these tests, the numerical tools applied are verified by comparing experimental results to numerical results. Cracking between the overlay and steel plate (debonding) is also analyzed through small scale experiments and further investigated numerically. The investigation shows that debonding is initiated from a defect in the overlay, e.g. an overlay crack. Debonding initiation is observed for a certain crack width of the overlay. Significant findings on the composite elements, such as the overlay crack width which initiates debonding, are also observed when modeling a full size structure.

A set of theoretical tools have been established to analyze the performance of an orthotropic steel bridge deck stiffened with a cement-based overlay, with respect to cracking behavior. For a given design situation, it might be possible to give an estimate on the crack pattern and maximum crack width when applying a given cement-based overlay This thesis demonstrates a nonlinear investigation of a real size structure, with emphasis on the performance of the overlay system, using different cement-based materials. Effects, such as traffic load, early age shrinkage, and temperature gradients are taken into account, and it is showed that all these effects might have a significant influence on the cracking behavior. The overlay performance, e.g. the relation between the magnitude of axle load and maximum crack width, is dependent on the constitutive parameters of the overlay material. Temperature gradients and early age shrinkage, have a considerable influence on the relationship between axle load and maximum crack width. The analysis shows that cracking of the overlay, for the given structure and design regulations, might be unavoidable. Therefore, the challenge in certain design situations, might be to minimize the maximum crack width of the overlay.