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Tendon Encapsulation and Durability

In an earlier article regarding the parts of a post-tension tendon, I discussed the specification for the grease and sheathing being designed to keep water away from the high-strength steel that makes up the structural portion of the tendon.  That is not an accident – after physical damage (such as drilling or hole-sawing), the most common cause of tendon failure is corrosion.

In addition to the manufacturing and material standards laid out by the PTI for the strand itself, a great deal of ingenuity has been devoted to designing anchor systems that prevent water from reaching the strand.  When I began my career, these systems were seen as “going the extra mile” – they were deployed in extremely aggressive environments, such as parking decks (which are exposed to deicing salts in winter, and weather year-round) and balconies.  However, as time has gone on, the incremental cost to provide “encapsulated” systems has become a very minor part of the cost of a post-tensioning system overall.

In 2011, the PTI issued Addendum #3 to the specification for Unbonded Single Strand Tendons, which has since been incorporated into the 2017 version of the specification.  To ensure maximum durability for post-tension structures and protect the public, all elevated applications of post-tensioning are now required to have encapsulation systems to protect them from water.

The complete encapsulation system employed by Advanced Post-Tension is a defense in depth.

  1. Tendons are packaged in a corrosion-protecting grease.
  2. The tendon sheathing is continuous, with no seams or joints at any point on any tendon. The sheathing material is a heavy-duty plastic selected to withstand shipping and jobsite placement.
  3. The grease is colored dark amber. The sheathing is brightly-colored, so that any damage that would allow grease to escape (and water to enter) is immediately apparent to field personnel and inspectors.
  4. The anchor castings are contained within a seamless plastic molding with structural openings for the tendon and the stressing jack.
  5. On the tendon side of the anchor, there is a plastic tube that positively locks with the back of the anchor encapsulation. This tube forms a watertight seal with the tendon sheathing, and is designed to have no voids where water might accumulate.
  6. On the jack side of the anchor, once the wedges have been set, a cap is installed. This cap positively locks with the anchor encapsulation.  The cap has a grease reservoir that immediately floods the wedges with corrosion-protecting grease when the cap is installed.

The overall assembly is tested by immersing it in water for 24 hours with a pressure surcharge of 10 psi – the equivalent of 23 feet of water.  No leaks were detected.  Third-party product testing information is available upon request.

Advanced Post-Tension believes that quality products mean maximum durability and customer value.  Ask how we can help your project succeed today!

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