Relaxation of Carbon and Stainless-Steel Threaded Bars for Posttensioning Applications
Publication: Journal of Structural Engineering
Volume 147, Issue 7
Abstract
An experimental program was conducted to investigate the stress relaxation properties of large-diameter bars [65 to 70 mm (2.5 to 2.75 in.) nominal] for posttensioning applications. This study was a component of a large-scale research program to develop a more corrosion-resistant, posttensioning bar. The bars were fabricated from 4142 quenched and tempered (QT) carbon steel and three stainless steel alloys: Type 450, Type 630, and Type 2507. The experimental program objective was to evaluate the stress relaxation properties of these various materials. Stress relaxation losses were determined for 24 specimens by measuring the force change in the specimens for 1,000 h under constant strain. The influence of several additional parameters on relaxation was also evaluated including initial stress level, galvanization on plain (black) steel, coupled bars, and the variability provided by different steel heats. Measured losses are compared with established relaxation theory and relaxation limits provided in standard specifications. The results indicate relaxation losses are dependent on the type of stainless steel alloy and, for the alloys tested, can vary of the relaxation of plain carbon steel bars. Furthermore, different heats of steel can result in significant variation in the amount of relaxation. While standard design approaches can be used for stainless steel bars, material-specific data are necessary to ensure appropriate posttensioning forces are applied over the life of the structure.
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Acknowledgments
The authors would like to thank DYWIDAG-Systems International (DSI), Alfred Benesch & Co., and the Iowa Department of Transportation for making this research possible. The technical perspectives and conclusions reported in this paper are those of the authors and the authors alone, who are responsible for the facts and accuracy of the data presented.
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Information & Authors
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Published In
Journal of Structural Engineering
Volume 147 • Issue 7 • July 2021
Copyright
© 2021 American Society of Civil Engineers.
History
Received: Jul 24, 2019
Accepted: Feb 17, 2021
Published online: Apr 27, 2021
Published in print: Jul 1, 2021
Discussion open until: Sep 27, 2021
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