UC Davis

This research investigates the consequences of two amendments by the California Department of Transportation (Caltrans) of current AASHTO (2017) guidelines related to the determination of the shear resisted by concrete in a post-tensioned (PT) girder containing grouted ducts. Experimental studies by previous researchers have either been limited to small-scale testing or comprise configurations that do not resemble typical Caltrans practice. In this study, load testing to failure of the specimens was carried out on near full-scale cross-sections that represent typical Caltrans PT girders. The experimental testing is further supplemented with numerical simulations to provide additional insight into the effect of grouted ducts in PT girders. Two large-scale specimens were fabricated to achieve the goals of the project and represented a prototype bridge from the Caltrans bridge inventory. Reinforcing details of the specimen were modified so as to induce shear failure prior to flexural yielding of the specimen. Considerable effort was dedicated to the design of a reaction system so that the imposed loading at shear failure of the specimen could be safely distributed to the strong floor of the laboratory. The primary goal of the first test was to examine the consequence of the Caltrans amendment related to the effective web width in calculating the shear resistance of a PT girder with grouted ducts whereas the second test investigated Caltrans practice of bundling more than three ducts that is currently disallowed in the AASHTO bridge design specifications. Findings from the experimental testing indicate that the shear resistance of PT girders with grouted ducts have significant reserve strength beyond the AASHTO-predicted shear capacity.

For the specimens tested in this study, the concrete contribution（Vc）to shear resistance varies between 15 – 25% of the nominal shear capacity and the difference in Vc resulting from the Caltrans amendment for establishing the effective width of the web affects the nominal shear capacity of the girder by less than 5% and consequently has a minor effect in the design of PT girders with fully grouted ducts. There was no visible distress around the duct region at the end of testing for specimen #1 indicating that the corrugated metal duct bonds well to the concrete and remains intact even at loads approaching shear failure of the girder. Minor to moderate distress was observed on the concrete surface along the duct lines for specimen #2, which experienced a dramatic shear failure. The inclination of shear cracks for both specimens during testing varied between 25 - 30 degrees, which is slightly lower that AASHTO estimates, with the angle of newly forming cracks tending to decrease with increasing load. These findings are also supported by the numerical simulations of additional girders with varying duct sizes and number of ducts.