Factors affecting friction losses in multi-strand post-tensioning tendons including the effect of emulsifiable oils

Abstract

Emulsifiable oils are sometimes used in post-tensioned construction to provide temporary corrosion protection to the tendons in the period between stressing and grouting. It has been common practice to flush tendons with water to remove the oil, and then use compressed air to remove the water from the ducts. Often environmental concerns make disposal of the flushed oils difficult and costly. In addition, significant voids, pockets of water, and corroded strands have been found. It is believed that flushing with water and compressed air does not completely remove either the oil or the water, and such a practice is now strongly discouraged. Leaving the oil will certainly affect the bond strength of the tendon and that can be a negative effect. However, previous research has shown that emulsifiable oils reduce the friction losses and this can be a positive effect. The objective of this research is to quantify the effects of lubrication with emulsifiable oils on the friction losses of multi-strand tendons. In addition, the effects of curvature, duct material, and the time between lubrication and stressing were investigated. Damage to the inside of the duct caused by stressing of the tendon was also studied. Large-scale friction tests were performed using multi-strand tendons in three different conditions: dry (unlubricated), freshly oiled, and one day after oiling. These tests were performed with three different duct materials: steel pipe, corrugated galvanized steel duct, and high density polyethylene (HDPE) duct. The tests were repeated using two different radii of curvature. Results from these tests indicated that the coefficient of friction was independent of the radius of curvature. However, different coefficients of friction were found for each duct material. The coefficient of friction found for steel pipes and galvanized steel ducts agree with those recommended in the literature. This was not the case for HPDE ducts. In this case the coefficient found was substantially lower than that recommended. Lubrication of the tendons reduced the coefficient of friction. However, this reduction was sometimes eliminated if a one day delay occurred between oiling and stressing. No significant damage was observed in either HDPE ducts or galvanized steel ducts.