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Allowable Compressive Stress at Prestress Transfer

Schnittker, Brian A.

2008

Increasing the maximum compressive stress at prestress transfer from the current limit of 0.6f’ci in the AASHTO-LRFD Bridge Design Specifications has been the subject of several research investigations in the last decade. There are many potential benefits of increasing this limit including increased span lengths, a reduction in harped or debonded strands, and a faster turnaround time for beams in prestressed beds. In 2004, The Texas Department of Transportation initiated Project 5197 to investigate the feasibility of increasing this allowable compressive stress limit. Initially, the live load performance of 36 specimens was evaluated by Birrcher and Bayrak (TxDOT Report 5197-1, 2007). Based on the recommendations from their research, further testing on additional section types and different concrete mixture designs was performed. This thesis presents the subsequent research conducted based on recommendations of Birrcher and Bayrak (2007).

In this portion of TxDOT Project 5197, 45 Type-C beams and 10 4B28 box beams were tested to experimentally determine their cracking load. The maximum compressive stress at prestress transfer of all 55 specimens ranged from 0.56f’ci to 0.76f’ci. The Type-C beams were produced in four different fabrication plants using conventionally consolidated concrete. The 10 4B28 box beams were produced in two fabrication plants using concrete mixture designs of both self consolidating concrete as well as conventional concrete. For all specimens, measured cracking loads were compared to predicted cracking loads. The data from the 45 Type-C beams and 10 box beams were added to the 36 beams investigated by Birrcher and Bayrak (2007) to compile a comprehensive set of data from 91 specimens.

An appropriate maximum compressive stress limit was determined from the ability to accurately predict the load at which cracking occurred. As the maximum compressive stress at prestress transfer was increased, a decline in cracking load prediction accuracy was observed. For the specimens subjected to high compressive stresses at release (greater than 0.65f’ci), the concrete in the pre-compressed tensile zone was subjected to the non-linear inelastic range causing microcracking to occur. This nonlinear behavior (due to microcracking) was unaccounted for in prestress losses or standard design equations (P/A±Mc/I). Based on the analysis of the results, an increase of the allowable compressive stress limit at prestress transfer to 0.65f’ci is justified. Additionally, the use of self consolidating concrete with a maximum compressive stress of 0.65f’ci is not recommended.

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