The National Academies

NCHRP 18-07 [Completed]

Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders

  Project Data
Funds: $300,000
Research Agency: University of Nebraska-Lincoln
Principal Investigator: Maher K. Tadros
Effective Date: 7/8/1999
Completion Date: 8/31/2002

The project provided guidelines for obtaining realistic estimates of prestress losses in high-strength pretensioned concrete bridge girders that should help achieve economical designs. The guidelines incorporate procedures that yield more accurate prediction of modulus of elasticity, shrinkage, and creep of concrete and more realistic estimates of prestress losses than those provided by the procedures contained in current specifications.

The use of high-strength concrete for pretensioned concrete bridge girders has become accepted practice by many state highway agencies because of its engineering and economic benefits. High-strength concrete permits longer girders and increased girder spacing, thus reducing total bridge cost. Design of pretensioned concrete girders requires accurate estimates of prestress losses. These losses are affected by factors such as mix design, curing, concrete strength, and service exposure conditions. Because recent research has indicated that the current provisions used for calculating prestress losses in normal-strength concrete may not provide reliable estimates for high-strength concrete bridge girders, there is a need for evaluating the applicability of the current provisions for estimating prestress losses in high-strength concrete bridge girders and for developing guidelines for better estimating these losses.

The research included a review of relevant domestic and foreign literature; identification of limitations on the methods currently used for estimating prestress losses; conducting laboratory tests for evaluating relevant properties of concrete; deriving formulas for predicting modulus of elasticity, shrinkage, and creep of concrete; and developing a "detailed" method and an "approximate" method for estimating prestress losses in pretensioned high-strength concrete bridge girders. The research also included (a) field measurements on seven full scale bridge girders in four states selected to represent a wide range of geographic and construction practices and (b) analysis of data from earlier field measurements on 31 pretensioned girders in seven states. The report gives numerical examples to illustrate use of these methods and to demonstrate that the methods developed in this research yield better estimates of prestress losses than those obtained from the current methods. The methods developed for obtaining realistic estimates of prestress losses in pretensioned high-strength concrete bridge girders seem appropriate for consideration and adoption by AASHTO as part of the LRFD Bridge Design Specifications.
NCHRP Report 496,"Prestress Losses in Pretensioned High-Strength Concrete Girders," documents the work performed in this project. The report appendices can be downloaded as an .ISO CD-ROM image (~22 MB). Here is some help on burning an .ISO CD-ROM Image.

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