Impact of Field Exposure Conditions on High Strength Concrete Produced for Prestressed Bridge Girders

This is the fourth in a series of four reports that document the findings of a Texas Department of Transportation sponsored project to evaluate the allowable stresses and resistance factors for high strength concrete (HSC) prestressed bridge girders. The third phase of this research study, which is documented in this report, focused on determining the impact of different exposure conditions on the compressive and flexural strength of HSC mixtures used for prestressed girders in Texas. The allowable tensile stress at service for HSC prestressed bridge girders was evaluated based on the results from the experimental program. This phase of the project involved several key tasks: (1) an experimental study to assess the effect of several field exposure conditions on the compressive and flexural strength of HSC and development of a modified tensile stress limit based on material testing from this project, (2) a parametric study to evaluate the impact of the modified tensile stress limit derived from the material testing in this study when applied to the design of HSC U54 prestressed girders, and (3) a reliability study to determinethe probability that the tensile stress would exceed the modulus of rupture to determine the safety against flexural cracking for designs based on both the current and modified tensile stress limits when applied to HSC U54 prestressed girders. It was observed that the concrete compressive strength was not highly influenced by the variations in field exposure conditions. However, the modulus of rupture was highly influenced by variations in field exposure conditions. Exposure factors were applied to data for plant-produced samples collected in Phase 1 of this study. A modified tensile stress limit for service conditions based on a lower bound to 95 percent of the data was suggested as 7Öf'c (psi units) as compared to the current American Association of State Highway and Transportation Officials (AASHTO) allowable tensile stress limit of 6Öf'c (psiunits). When the modified tensile stress limit was used in the parametric study for U54 prestressed girders, there was only a small increase in the potential span (from 0.9 to 2.3 feet). This increase only occurs for cases where the tensile stress limit at service was the controlling limit state (when all 99 strand positions are used). Reliability indices were determined to evaluate the probability that the tensile stress would exceed the modulus of rupture. For both 0.5-inch and 0.6-inch diameter strands, most of the reliability indices for designs using the current stress limit range from 1.5 to 2.0, where as typical reliability indices for designs using the modified tensile stress limit range from 1.0 to 1.5.