Reducing Friction Losses in Monolithic and Segmental Bridge Tendons

In the construction of post-tensioned bridges, the increased use of precast technology has resulted in somewhat tighter radii of curvature and greater total angle changes. Both factors make friction losses during stressing higher and somewhat less predictable. Various design recommendations have suggested different values. There have not been any reported tests of actual friction losses in segmentally cast girders.

Historically, the solution to the friction reduction problem has been use of a lubricant, often an emulsifiable oil applied to the surface of the tendon or stay. The agent is usually flushed immediately before grouting. Particularly in bonded post-tensioned girders, it is essential that any residues of these agents not diminish the bond between the strand and the grout. The tests showed flushing was ineffective and emulsifiable oils are potentially harmful in bonded tendon applications.

In a preliminary study, thirteen agents were identified as practical candidates for tendon lubrication and/or temporary corrosion protection. Ten were emulsifiable oils, one was a sodium silicate solution, one was a soap and one was powdered graphite. The powdered graphite was the best lubricating agent but is not a corrosion inhibitor.