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Release Methodology of Prestressing Strands
Author(s) and Affiliation(s):
Jeffrey Kannel, Catherine French, and Henryk Stolarski Department of Civil Engineering University of Minnesota, Minneapolis, MN
Principal Investigator:
Catherine French
Minnesota Department of Transportation
Research Start Date:
November 1994
Expected Completion Date:
May 1996
Research Objectives:
Cracks have been observed to form in the ends of pretensioned bridge girders when using the flame-cutting technique to release the girders from the precasting bed. Because some of the cracks were observed to remain open following the full release of the girders. the effectiveness of the bond between the strands and the concrete in the end regions of the girder was questioned. The objective of this study was to investigate methods to eliminate these cracks which included changes in the release methodology (strand cutting pattern) and debonding some of the strands in the end regions. An FEM model using a mesh of three-dimensional continuum elements was used to model the end regions of the girders to investigate methods to eliminate the end cracking. The most promising techniques were implemented in the field. The girder ends were instrumented to compare the field observations with the analytical results.
Expected Products or Deliverables:
As a result of the investigation, a fabricator has already implemented some of the recommended changes to the strand release order, which have proven to dramatically reduce the end cracking observed at the precast plant. A final report is being submitted to the Minnesota Department of Transportation.

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