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Field Monitoring of Prestress Forces in Four Box Girder Bridges Subjected to High Variation of Humidity
Author(s) and Affiliation(s):
M. Saiid Saiidi and Nathaniel Mangoba Civil Engineering Department University of Nevada, Reno, Reno, NV
Principal Investigator:
M. Saddi Saiidi
Nevada Department of Transportation
Research Start Date:
June 1, 1994
Expected Completion Date:
August 31, 1997
Research Objectives:
The dry and desert-like climate in Northwest Nevada is typically associated with high daily variation of temperature and relative humidity (RH), with the ratio of high to low values within any 24-hour period ranging from 1.5 to 3. The concern for possible consequence of the high daily range of temperature and RH on creep and shrinkage losses led to a pilot field study completed in 1994 of a post-tensioned bridge which indicated that the actual losses may be considerably higher than those predicted by AASHTO specifications. Four new post-tensioned, box-girder bridges in Northern Nevada were instrumented during construction to collect more data on the possible adverse effects of high variation in RH and temperature on prestress losses. Hence, the objective of the current study is to determine how the actual losses compare with those predicted by AASHTO and other methods, and to determine if a modification of the specifications is warranted.
Expected Products or Deliverables:
The measured variation of prestress forces over a two-year period will indicate the total time-dependent loss in each bridge. A comparison of the measured data with those determined based on the AASHTO method will establish the applicability of the results, and will determine if any modification is necessary. The comparison of shrinkage data for specimens subjected to constant RH and those subjected to variable RH will provide information on the sensitivity of the final shrinkage strain to variable RH, which can also be factored in any modifications that may be proposed.

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