Allowable compressive stress at prestress transfer.
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2008-12-01
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Abstract:In 2004, The Texas Department of Transportation initiated Project 5197 to investigate the feasibility of
increasing the allowable compressive stress limit at prestress transfer. Initially, the live load performance of 36
specimens was evaluated by Birrcher and Bayrak (TxDOT Report 5197-1, 2007). Report 5197-4 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 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 non-linear 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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