I’ve written a great deal in the last couple of years regarding design considerations for post-tension slabs, as well as the “right” way to install and finish post-tension tendons. And while I’ve touched briefly on some of the potential pitfalls of post-tensioning, I haven’t discussed the potential impacts — and mitigation — of seriously damaged post-tension tendons.
Recently, I had a contractor call me regarding a job that had been off my desk for some time. While primary framing had been done for some time, he was wrapping up some modifications to the ground floor (a post-tension podium slab). One of these modifications was relocating some plumbing. Unfortunately, when the plumber was cutting holes for the new plumbing, he hit a pair of post-tension tendons . . . bringing me back into the job.
In a situation like this, there are five strategies that engineers and contractors pursue:
1) Measure Twice, Cut Once
I can’t stress enough the the primary defense against mishaps of this type is careful planning and preparation. The use of x-rays or Ground-Penetrating Radar (GPR) or other non-invasive tests has costs, certainly, but the cost to repair a tendon generally far exceeds the cost of locating it properly before beginning any slab modifications. This is, of course, over and above the potential hazards associated with suddenly releasing the energy in a post-tension tendon.
2) Stop Cutting!
Sometimes, when it all goes sideways, the person running the saw (or drill, or nailgun) realizes that something has gone wrong before the tendon is completely severed. If only one of the seven wires in the tendon has broken, the the Engineer of Record may allow the contractor to leave the tendon as-is. The tendon’s capacity is diminished, but after careful review of the calculations and shop drawings, it may be possible to avoid costly repairs.
3) Abandon the Tendon
Much more frequently, the tendon is totally severed. At this point, the most economical “repair” is to re-grout the stressing pockets and leave the broken tendon as-is. In this circumstance, the Engineer of Record reviews the shop drawings, stressing logs and all calculations for the project. Occasionally, there’s enough extra capacity to allow the project to continue without the cut tendon. If you’re reading this because you cut a tendon, and you haven’t called the EOR yet, don’t get your hopes up — it takes a lot of overages to make up for just one tendon.
4) Replace the Tendon
When the tendon can’t simply be abandoned, sometimes the most economical way of bringing the slab back up to the design standard is to replace the tendon completely. This is common when both ends of the tendon were originally accessible for stressing, or when the slab is congested enough (with rebar, radiant heating, tendons, conduits and embedded plates) that cutting into it is prohibitive. The cut strand is removed, and a new one pulled (or pushed) through the duct and stressed. While it’s technically possible to replace a 1/2″ diameter tendon with another 1/2″ diameter tendon, it’s always a greasy mess — most contractors prefer to replace 1/2″ strands with 7/16″ or even 3/8″ diameter strands. Careful review is required — a 3/8″ diameter strand has about 55% of the capacity, but it may be enough to avoid the more time-consuming process of a 1/2″ replacement.
5) Repair the Tendon
Lastly, it may not be economical to replace the entire tendon. This is frequently the case when one end wasn’t accessible from the beginning — it’s against an existing structure, rock face or several feet below grade — or the tendon passed through a construction joint. In this case, the contractor and engineer work together to establish a repair location, and the slab is carefully cut back to expose the damaged strand (this may be several feet from the actual break). The damaged portion is removed, and a coupler is installed to attach new strand to the existing in-place steel.
EVstudio prides itself on being a partner you can count on, even when the job isn’t going as smoothly as you would like. Don’t hesitate to tap our experience on your next project!
