Experimental verification of the influence of time-dependent material properties on long-term bridge characteristics.

213416347PDFTech Reporthttp://worldcat.org/oclc/213416347/viewonlineFHWA/CA/ES-2006/24UCSD / SSRP-06/2059A0420CaliforniaCalifornia. Dept. of TransportationLewis, MichaelKarbhari, Vistasp M.University of California, San Diego. Dept. of Structural EngineeringUnited States. Federal Highway AdministrationUS Transportation CollectionPost-tensioned cast-in-place box girder bridges are commonly used in California. Losses in tension inthe steel prestressing tendons used in these bridges occur over time due to creep and shrinkage ofconcrete and relaxation of the tendons. The use of existing methods in bridge specifications used topredict these long-term losses often result in inaccurate estimate of losses leading to severeserviceability problems. The current research program aims at developing a more precise method forpredicting the long-term prestress loss in concrete bridges. Two spans in a recently constructed posttensionedconcrete bridge were instrumented with vibrating wire strain gages to monitor the longtermdeformations and determine prestress loss. The recorded measurements of prestress loss werecompared with the available equations from several bridge specifications as well as with analyticalmodels developed for long-term deformations of concrete structures. To ensure accuracy in thevalues of the input material parameters, creep and shrinkage of concrete were simultaneouslydetermined experimentally. This research has shown that shrinkage is highly dependent onenvironmental influences. Additionally, measured creep and shrinkage appeared to reach asymptotesfaster than was indicated by best-fit equations, thus ultimate values may be closer to the minimumextrapolated values. It is noted that the available specifications equations for predicting prestress lossmay not be sufficiently accurate. The analytical predictions of prestress loss exceed the monitoredvalues in some cases and further investigation is being continued and will be reported in the finalreport for this project