Debonded prestressed strand in prestressed beams

Principal Investigator(s):

Cathy French, Professor, Civil, Environmental and Geo-Engineering

Project summary:

End cracking has been observed in pretensioned, prestressed, concrete bridge girders as a result of stresses induced during fabrication. To avoid excessive tensile and compressive stresses at girder ends, prestressing strands are typically draped to move the center of gravity of the prestress toward the centroid of the section near the end. However, end cracking that follows the draped strand pattern has been observed. Another option to reduce end stresses is to debond a portion of the prestressing strands over a short distance at the girder ends. Although this option reduces the end stresses, a concern with the debonding option is that it may lead to long-term corrosion issues if moisture and deicing chemicals make their way into the beam end from the debonded area surrounding the strands. In addition, debonding the strands may have an impact on reducing the shear strength of girders near the end regions. The Minnesota Department of Transportation (MnDOT) is interested in exploring the use of debonded strands as a means to eliminate some of the end cracking observed during fabrication. The objective of this study is to review the literature with regard to the causes of end cracking in prestressed bridge girders and studies associated with the use of debonded strands and its potential impact on shear strength. Fabricators are being queried with regard to their strand cutting patterns and crack observations. In addition, current American Association of State Highway and Transportation Officials (AASHTO) design specifications associated with the use of debonded strands are being reviewed and compared with those of MnDOT and with states currently using debonded strands. States such as Michigan, with climate and exposure conditions similar to Minnesota, will be surveyed to determine the performance of the in-service girders with debonded strands. The potential impact of the project is to produce prestressed bridge girders with increased durability and aesthetics without compromising shear strength. Use of debonded strands could potentially lead to reduced fabrication costs.

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