Fretting Fatigue in Post-Tensioned Concrete

This report summarizes an experimental investigation of fatigue strength of post-tensioned concrete girders. The test series included strand-in-air tests to characterize the prestressing strands utilized, reduced beam specimens with single strands in a tendon, reduced beam specimens with multiple strands in a tendon, and complete beams with multiple strands in a tendon. The variables explored included the effect of stress range, type of duct material, lateral contact force per unit length, and type of strand. Duct materials included both metallic and plastic sheaths. Both epoxy coated and uncoated strands were used. All specimens were tested in a cracked condition and fatigue failures occured in the regions of high curvature (and hence high lateral force) in the vicinity of cracks.

The results indicate that the expected fatigue life of prestressing strand as determined from strand-in-air tests can be substantially reduced in post-tensioned concrete applications due to fretting fatigue of the prestressing tendon. Such fretting fatigue can occur in cracked concrete sections at locations of tendon curvature. With metal ducts the rubbing between sheath and strand greatly aggravated fretting. With plastic ducts, the ducts showed marked evidence of rubbing and wearing, but with single strand tendons, fretting was not a serious problem. However, with plastic ducts and multiple strand tendons in more than one layer, fretting occurred between layers of strands and was a serious problem.

The report gives design recommendations in a form compatible with the overall AASHTO fatigue approach for steel bridge members.