FRETTING FATIGUE IN POST-TENSIONED CONCRETE BEAMS

This paper summarizes an experimental investigation of the fatigue strength of post-tensioned concrete girders with bonded tendons. The test series included strand-in-air tests to characterize the prestressing strand used, reduced beam specimens with single-strand tendons and multiple-strand tendons, and complete beams with multiple-strand tendons. 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. Fatigue failures occurred in regions of large curvature in the vicinity of cracks. The results indicate that the expected fatigue life of prestressing strands as determined from strand-in-air tests can be substantially reduced in post-tensioned concrete applications due to fretting fatigue of the prestressing tendon. With metal ducts, the rubbing between sheath and strand greatly aggravated fretting. Plastic ducts showed marked evidence of rubbing and wear, but with single-strand tendons, fretting was not a serious problem. However, with plastic ducts and multiple strands in more than one layer, fretting occurred between layers of strands and reduced fatigue life. Design recommendations based on these and other tests are presented.