Exploration of UHPC applications for Montana bridges
Starke, James Gerald
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The following research project explores bridge applications of ultra-high performance concrete (UHPC). Bridge deterioration is a problem across Montana and UHPC overlays and patching/repairing have been found to be viable alternatives to bridge replacement. The current study began with a literature review on research, specifications, and implementation projects of UHPC bridge deck overlays. A report from FHWA was highlighted that summarized the results of previous overlay and repair projects, and developed their own recommendations. A material-level evaluation was performed on three UHPC mixes, primarily focusing on workability, compressive strength, tensile strength, and tension and shear bond strengths. All three UHPCs exhibited adequate behavior and the resultant properties were above recommendations from ACI for concrete repair and overlay applications. Based on the material-level evaluation results, a thixotropic version of Ductal was chosen for subsequent structural testing. Five slab test specimens were designed and constructed to model a deck section from an existing bridge in Montana. The testing and specimens were designed to determine the effects that including a UHPC overlay, overlay thickness, and substrate concrete strength have on the ultimate moment capacity. The slabs consisted of one control slab, two slabs with varying UHPC overlay depths, one with weak substrate concrete, and one tested to emulate a negative moment region on a bridge deck. The testing demonstrated that including a UHPC overlay increased the ultimate moment capacity of the slabs, even with a weak substrate concrete, but cause the slabs to fail in shear rather than concrete crushing. Additionally, the results imply that a weak deck strengthened with a thin UHPC overlay will respond similarly to a deck composed of much stronger normal concrete. The tensile capacity of the UHPC plays a large role in the overall strength and stiffness of a slab subjected to a negative moment and the tensile strength should be included in capacity calculations, as recommended by FHWA. Overall, the results are promising and shed light on how a UHPC overlay may contribute to the overall strength of an existing bridge deck if implemented in a future overlay project in Montana.