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CCEER-15-02: Post-Tensioning Duct Air Pressure Testing Effects on Web Cracking
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Nevada Department of Transportation (NDOT) post-tensioned concrete bridges have experienced web cracking near the post-tensioning ducts during the construction process. The ducts were air pressure tested to ensure the duct can successfully be grouted for a bonded post-tensioned system. During the air pressure testing, tendon stressing and grouting, web cracks form that cause delamination of the concrete in the web that lead to repairs. The research presented in this thesis focused on recreating the web cracking issue to understand why the problem exists and to create a solution for the web cracking. The project consisted of four main components: recreating the web cracking using three large-scale experimental beams, updated design details to minimize/eliminate web cracking in three large-scale experimental beams, an extensive parametric study, and develop design recommendations. Beam studies included the design and construction of six large-scale (0.7) concrete bridge beams. Three beams studied the effects of curvature on web cracking using existing NDOT details and three beams updated design details such as increased spacing between post-tensioning ducts and inclusion of duct tie reinforcement bars between post-tensioning ducts. All beams were designed with the same dimensions and tested to NDOT standards. Results indicate that the greatest impact on web cracking was the air pressure testing. Post-tensioning curvature also has an effect on web cracking. Increasing the duct spacing and inclusion of duct tie reinforcement bars increased the performance of the beams. A detailed parametric study using the ATENA nonlinear analysis software investigated various design parameters including current NDOT details. Various parameters included web width, duct spacing, duct tie reinforcement, number of ducts and location of duct within the web. Through the experimental beams and the analytical studies, design equations were developed to reduce web cracking. Recommendations regarding additional reinforcement, duct layout and duct details were developed to reduce damage and increase the performance of post-tensioned bridge beams.
Report No. CCEER-15-02