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Experimental verification of the influence of time-dependent material properties on long-term bridge characteristics.
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2006-08-01
[PDF-4.67 MB]
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Abstract:Post-tensioned cast-in-place box girder bridges are commonly used in California. Losses in tension in
the steel prestressing tendons used in these bridges occur over time due to creep and shrinkage of
concrete and relaxation of the tendons. The use of existing methods in bridge specifications used to
predict these long-term losses often result in inaccurate estimate of losses leading to severe
serviceability problems. The current research program aims at developing a more precise method for
predicting the long-term prestress loss in concrete bridges. Two spans in a recently constructed posttensioned
concrete bridge were instrumented with vibrating wire strain gages to monitor the longterm
deformations and determine prestress loss. The recorded measurements of prestress loss were
compared with the available equations from several bridge specifications as well as with analytical
models developed for long-term deformations of concrete structures. To ensure accuracy in the
values of the input material parameters, creep and shrinkage of concrete were simultaneously
determined experimentally. This research has shown that shrinkage is highly dependent on
environmental influences. Additionally, measured creep and shrinkage appeared to reach asymptotes
faster than was indicated by best-fit equations, thus ultimate values may be closer to the minimum
extrapolated values. It is noted that the available specifications equations for predicting prestress loss
may not be sufficiently accurate. The analytical predictions of prestress loss exceed the monitored
values in some cases and further investigation is being continued and will be reported in the final
report for this project.
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