Evaluation of Agents for Lubrication and Temporary Corrosion Protection of Post-Tensioned Tendons

In the construction of post-tensioned bridges, the increased use of precast technology has resulted in somewhat less predictable. In both cast-in-situ and precast post-tensioned bridges, as well as cable stays, there is often a need for temporary corrosion protection agents between installation and before grouting.

Historically, the solution to both the friction reduction and the temporary corrosion protection problem has been use of a single agent, 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.

There are numerous oils available, as well as several other agents often used in these applications. There is very little in terms of prior data indicating the amount of friction reduction or corrosion protection that can be expected from different oils or agents. In this 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 granite.

A series of small-scale corrosion, friction and adhesion tests were conducted to evaluate the candidate materials and compare their behavior with that of bare or unprotected prestressing strand. In the corrosion tests, the corrosion protection offered by the eleven corrosion protection agents was compared by accelerated testing in deionized water, 3.5% NaCl solutions and ambient outdoor exposure. The adhesion tests used grouted single-strand pullout tests to indicate bonding. Test conditions before grouting included bare strands, lubricated strands and lubricated but then thoroughly flushed strands. The small-scale friction tests involved comparison of the static and dynamic coefficients of friction of a single strand being pulled through a segment of galvanized post-tensioning duct with a constant normal force applied. Test conditions included both bare and lubricated strands.

Comparative data for all tests are provided, and the overall performance of the different agents was compared by using a Matrix Priority Rating System. Based on these results, four lubricants were selected for use in latter large-scale girder friction tests with multi-strand tendons, which are a part of the overall project and will be reported in CTR Report 1264-2.