Last week, I wrote a post about ways post-tensioners protect tendons from corrosion. As part of that article, I mentioned that two of the major defenses for all post-tension strands were the sheathing and the coating — and that their colors were specifically chosen to contrast, so that if something did happen to the sheathing, the damage would be obvious.
While one of the specifications for sheathing is a minimum thickness (and it’s certainly tough stuff; I could wish that my car’s interior was so resilient), construction sites are notoriously tough on equipment and material. Between the bundling process at the plant, loading, transport (sometimes across several states, or even across half an ocean), unloading, un-bundling and layout, there’s a lot of sharp edges and heavy equipment — none of which give any shrift whatsoever to the designer’s desire for perfect tendons.
Fortunately, there’s a specification for repairing sheathing damage, both small and large. Both require an “approved post-tension repair tape.”
What does that mean? First, “approved” means that, prior to applying the tape, you have secured the permission of the Engineer of Record. This is not an example of “better to ask forgiveness than permission” — non-approved tape may damage the strands. Second “post-tension repair tape” has five key properties:
- Minimum width of 2″
- Elastic properties (it should have both stretch and rebound)
- Non-reactive with sheathing, coating, or prestressing steel
Basic duct tape unfortunately fails two of these categories completely — it isn’t elastic enough to qualify, and it isn’t moisture-proof enough to meet the standards required for a post-tensioned structure. Finding a duct tape repair during a site observation is never fun — invariably, in an effort to get the tape tight, the repairer has added more duct tape . . . and now, to complete the repair correctly, somebody is going to have to remove the whole mess.
In 2002, the Post-Tensioning Institute performed tests to qualify various repair tapes. Repairs were completed per the Field Procedures Manual for Unbonded Single Strand Tendons, then submerged for 24 hours. Seven tapes prevented moisture intrusion with all PTI-approved assemblies:
- 3M Scotch brand #226 black polyethylene masking tape < – EVstudio’s “Specified or Equal” product
- 3M Scotch brand #481 preservation sealing tape
- Polyken #223 waterproof duct tape
- Polyken #226 nuclear grade duct tape
- Polyken #900 (2″ wide) general utility pipeline tape
- Tape Specialties Limited #355 PVC tape
- Wimmore Tape #413
If you find that you need to repair some damaged post-tension tendon sheathing, the above list is a great place to start. Find a local supplier, and give your Engineer of Record a copy of the “Research Update – Tape Qualifications“. While there are certainly many other tapes that work, even the most cautious engineer will agree that the PTI’s testing was adequate. If you don’t have any of these tapes available, ask your Engineer what other alternatives are acceptable. And while the 2″ minimum width may may a very wide tape seem attractive, it is very difficult to wrap wide tape tightly — and, honestly, even the best tape can’t make up for wrinkles and folds.