Invited Speaker: Creep and Stress Relaxation of 150 ksi Threaded Bars-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines creep and stress relaxation in 150 ksi threaded bars used for ground anchors. Stress relaxation is the time-dependent loss of stress when tendon strain is held constant; creep is the time-dependent increase in strain under constant force. The research focused on creep because anchor proof and performance tests reach loads near the steel’s yield point, where creep may contribute to measured elongation and potentially cause unnecessary anchor rejections.<br /><br />ASTM A722 specifies tensile properties but does not set creep or relaxation limits. Its prescribed manufacturing process—cold stressing and stress relieving—reduces these effects. Some larger bars meet A722 tensile properties but use quench-and-temper processing instead; these are considered “A722-Like,” or special prestressing materials. The study compared A722 and A722-Like bars to assess whether their creep behavior differs.<br /><br />Funded by ADSC and DFI, the research developed a laboratory creep-testing procedure and compared results from multiple facilities. Initial testing at TUV produced erratic results, so the team selected the University of Illinois for expanded testing after its results were more consistent with those from RJ Peterman Labs. Phase II tested multiple bar sizes from suppliers.<br /><br />The results indicate that steel creep alone can approach or exceed PTI’s 10-minute creep acceptance criterion when extrapolated to a 15-foot free-stress length, while creep over a later test interval may pass but remain close to the limit. However, laboratory results may not directly represent field conditions: laboratory equipment and controls are more precise, while field hydraulic systems allow pressure fluctuations that can affect measured elongation.<br /><br />The presenters’ preliminary view is that PTI acceptance criteria generally identify problematic geotechnical creep, but steel creep should be considered when interpreting test data. Changes to PTI guidance were anticipated in a forthcoming revision.

Keywords

creep

stress relaxation

ground anchors

threaded bars

ASTM A722

prestressing steel

yield point

anchor testing

geotechnical creep

PTI acceptance criteria