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Instrumentation and Monitoring of High Performance Concrete Prestressed Bridge Girders
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2001-05-01
[PDF-8.47 MB]
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Edition:Final Report, January 1998-May 2001
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Abstract:Bridge A5529 in Jefferson County, Missouri was constructed using high performance concrete (HPC) prestressed I-girders. This bridge is Missouri's first high performance concrete bridge. Four of the twenty girders for Bridge A5529 were instrumented to monitor temperatures, strains, and deflections from when the girders were fabricated up to approximately one year in service. Additional laboratory experiments included tests for compression response, fracture characteristics, unrestrained creep and unrestrained shrinkage. The unrestrained creep and shrinkage tests were also performed on cylinders from a companion normal strength concrete (NSC, Bridge A5530) bridge for comparison. Thirty-six strain-gaged bars, sixty-seven thermocouples, thirty-two vibrating wire strain gages, and eight instrumented stirrups were installed in the four girders, diaphragm, and deck slab as a part of the instrumentation program. External measurements made during girder fabrication included transfer length, end slip, camber, and an infrared thermographic survey of the steel mold for surface temperature distribution. The objectives of the instrumentation program included: to study the early-age response during curing/hydration; investigate endzone stresses during prestress transfer; investigate unrestrained creep and shrinkage response of HPC used in the girders and compare them with NSC used typically; examine temperature and strain variations during storage; transport and construction; compare strains and deflections during a load test to analytical predictions; and examine strains due to daily and seasonal service temperature variations. It was concluded that cracking at girder ends could result from a combination of residual stresses due to early-age differential thermal loading and stresses from prestress transfer. Improved curing procedures and potential design modifications can minimize this potential for cracking. Unrestrained shrinkage strains in HPC were observed to be approximately 40% less than that measured for NSC under similar laboratory conditions. Total creep was 5-15% smaller for HPC compared to NSC. However, the basic creep components were nearly equal. HPC exhibited high early creep (within 60 days) which soon stabilized resulting in very little additional creep strains. The improved creep and shrinkage performance of HPC can be incorporated in design so as to allow more accurate prediction of prestress losses. The influence line of strains from a load-test (using a total truck load of 41,780 lbs) produced predictable profiles, although the overall as-built response was approximately 30-40% stiffer compared to analytical predictions (which did not include stiffness contributions from curb, railing, barriers etc.). Maximum strains from daily and seasonal temperature variations were observed to be significant, and 5-6 times than the maximum strains observed from the load-test. It would be prudent to review design procedures so that bridges of this type (continuous composite prestressed I-girder) could be explicitly designed for these levels of service thermal loading in addition to normal design loading.
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