Shear Strength of a PCBT-53 Girder Fabricated with Lightweight, Self-Consolidating Concrete
Dymond, Benjamin Zachary
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The research conducted was part of a project sponsored by the Virginia Department of Transportation and the Virginia Transportation Research Council. One PCBT-53 girder was fabricated with lightweight, self-consolidating concrete. An additional composite cast-in-place lightweight concrete deck was added at the Virginia Tech Structures and Material Laboratory. The project had two specific goals. The first was to experimentally determine the shear strength of the bridge girder. The initial tests focused on the web-shear strength of the girder, and the second tests focused on the flexure-shear strength. The theoretical predictions for the web shear strength were all conservative when compared to the experimentally measured failure strength. The theoretical predictions of the flexure-shear strength were typically unconservative because during the flexure-shear test the girder reached the nominal flexural strength, and a failure occurred in the previously damaged region of the beam. Shear strength was also predicted using the design material properties. Results from these calculations suggested that the equation for the steel contribution to shear strength proposed in the NCHRP Simplified Method were unconservative. Further investigation into the results from the web-shear test showed that the maximum nominal shear strength calculated using the AASHTO LRFD Specifications was typically unconservative. Test results from this project suggested that the constant multiplier of 0.25 used in the LRFD equation for Vnmax may be too high. Further research may be needed to accurately quantify an upper limit on the shear strength. Additionally, predictions of the initial web-shear cracking load were conservative when using the AASHTO Standard Specifications and the NCHRP Simplified Method. The initial web-shear crack angle was under-predicted using the AASHTO LRFD Specifications. The second goal was to monitor the change in prestress over time (and hence the prestress loss) occurring in the PCBT-53 girder. Prestress losses were experimentally measured by vibrating wire gages (measured changes in concrete strain) and flexural load testing. Measured prestress losses were compared to a theoretical prediction calculated using the AASHTO Refined Method. The amount of prestress recorded at any given time using vibrating wire gages was greater than predictions from the AASHTO Refined method. The effective prestress measured just prior to deck placement was higher than the theoretical prediction, and the measured effective prestress at the time of testing was also higher than the theoretical effective prestressing force. The effective prestress value calculated using the flexural crack initiation method was significantly lower than the effective prestress values predicted by both the code provisions and the vibrating wire gages; however, the effective prestress value calculated using the flexural crack re-opening method corresponded very well with the effective prestress values predicted by the code provisions and measured by the vibrating wire gages. The discrepancy in the crack initiation effective prestress values may be due to prestress losses occurring between placement of the concrete and transfer of the prestress force. These losses are not taken into account when using current code provisions to estimate prestress losses. Additional research is recommended to determine if these losses occur in bulb-tee girders, and if so, to quantify them. Finally, from test results within the scope of this research project, design of prestressed bulb-tee girders with lightweight, self-consolidating concrete is practical. The current AASHTO LRFD Specifications provided conservative results when predicting the shear strength of the PCBT-53. Additionally, prestress losses in PCBT girders fabricated with lightweight, self-consolidating concrete were less than those predicted using the AASHTO Refined method.
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