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QUESTION: How do you repair a post tension cable break?

Thanks to Mr. Bob Coffeen fo the Enclave of Naples for posting this

ANSWER: Contributed by Matt Nolton, CEO, P.E., C.B.I.E.

The simple answer is very carefully with a firm experienced with post-tension cable repairs.

The longer answer is post-tension cable repairs are a straight forward process albeit involved and tedious.

Typically a post-tension cable is compromised when some type of restoration, renovation, or remedial work is being performed on a post-tensioned reinforced structure. Post-tension cables can also be compromised when the original construction was performed poorly or maintenance of the exterior of a building is not performed on a regular basis. In either of these cases the ends of the cables at the edges of concrete decks are allowed to corrode. Without proper maintenance the corrosion can advance to the point where cables or end anchors fail.

First some of the basics about post-tension reinforcement. Post-tension reinforcement are stranded cables that are contained typically in a plastic sheath. The cables are utilized frequently in the elevated concrete decks and sometimes in the ground floor slabs of condominium structures (including the associated parking garages). Post-tension cables were introduced into residential high- rise construction as a means to reduce the thickness of the elevated concrete decks, to widen spans between column supports, and to speed up the construction process.

Concrete has a very high compressive strength (Typ. 3,000 to over 10,000 psi) but very low tensile strength (Typ. 300 to 400 psi). Post tension cables provide tensile strength to the concrete. To provide this tensile strength, they are stressed under significant tension loads following casting of the deck (thus the term post-tensioned). Therefore, when a post-tension cable fails it is an aggressive catastrophic failure typically shooting out the end of the building and blowing out the concrete on either the top or the bottom of the deck.

Repair of a damaged post-tension cable is performed by typically exposing one or both ends of the damaged cable at the deck edges and at one or multiple locations where the cable breeched the concrete deck. A new cable or portion of a cable is used to replace the damaged cable.

If the entire cable is replaced, the old cable is removed from the plastic sheathing within the concrete deck and a new cable is installed through the sheathing. The affected sheathing and the concrete are then repaired, allowed to cure, and the cable is stressed and the ends patched.

If only a portion of a cable is replaced, the connection of the new cable to the older cable is accomplished with a post-tension cable splice chuck then the cable is repaired following the same procedure listed above.

Thank you for all the questions, keep them coming on any hot topic that concerns you!!!

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