Do Unbonded Post-tensioned Tendons Fly Across the Job Site When They are Cut?

I just sat through yet another meeting with several people in which one individual claimed to have seen a 30 foot long post-tension cable fly out of a slab-on-grade, soar across the job site and land in front of his truck…

The problem with this is that when people tell these stories, someone believes them!

I have seen many cut tendons. I have purposely cut them for various reasons. I have drilled through them, saw cut them, torched them loose on one end, and I have yet to see the prestressing steel fly out of the concrete and become an airborne missile. And that’s not for lack of trying! Don’t misunderstand, cutting a tendon can create a dangerous situation. The force created by the sudden release of energy can cause concrete debris to break off the slab as the embedded steel hits it, and wedges can be thrown from the concrete; especially if they come out of a pocket former recess that has not been patched. But, there is just too much friction in a single strand unbonded tendon for a length of prestressing steel to shoot out of the slab and become a missile.

The Post-Tensioning Institute even published a report back in 1990 that describes the supervised demolition of a parking structure built with single strand unbonded tendons. They cut the strands at varied locations to see how far they could get the strands to shoot out of the slab, and none of them made it more than a few feet.

I am not trying to minimize the potential dangers of cutting into a post-tensioned slab, because they are definitely real. Concrete exploding at the anchors, wedges being thrown from the concrete, anchors exploding upwards when people are chipping behind them…these are all real, no need to add fake hazards to the list!

{reprinted from my old Blog - JR}

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