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Q.What precautions should you take when tearing out a post-tensioned slab? What are the risks? We are pouring a large post-tensioned slab, and while there is no demo involved at this stage, I would like to know how to proceed in case there is a plumbing mistake that requires us to cut out and repour a section of the slab.

A.Bob Kovacs, president of Constructive Solutions in Iselin, N.J., and moderator of the estimating forum at, responds: I installed dozens of post-tensioned slabs in homes in Las Vegas, and just recently finished a 10,000-cubic-yard post-tensioned parking garage. The cables are tensioned to approximately 30,000 pounds, and if you cut one the wrong way, the force can easily kill you. I've seen cut cables rip out of slabs on several occasions. One ripped open a 10-foot-long strip of slab and sliced through a shear wall. Another slammed into a dishwasher 5 feet from the hole and sliced it in half. In both cases, the geniuses who caused the damage were lucky enough to avoid harm.

In most cases, the cables can be destressed by a post-tension contractor. It's not an easy task, especially if the location of the cables isn't easy to establish, but it's the only way to proceed unless you plan to do all the demo from the cab of a very large excavator with a hydraulic hammer and can keep everyone far from the area. I wouldn't recommend that avenue, though.

Here's what I'd do in your case: Just before the pour, take photos of every slab penetration that could possibly be in the wrong location, marking dimensions to the nearest cable in each direction. Note this information on a drawing as backup in case the pictures aren't clear.

Also, spray-paint the location of the "dead" ends of the cables on the formwork. (These are the ends that, rather than running all the way to the slab edge, get embedded in the middle of the slab.) Transfer these marks to the slab before stripping the forms, so you'll be able to locate the dead ends if you need to. Mark the "live" end locations on the top of the slab before cutting the cables and patching the holes. Assuming the cables run relatively straight, this will allow you to "connect the dots" and roughly locate a cable in the slab.

We did this recently on the post-tensioned garage slab, spraying paint along the cables on the deck forms; it really helped when we had to recore a few plumbing penetrations.

Snap out all your walls before tensioning the cables, and determine what, if anything, needs to be moved. Jack up the slab now and make the moves so that you don't hit any stressed cables: Repairing an unstressed cable is far easier than repairing a stressed one. Usually you end up waiting at least a week or so after the pour to tension, so this shouldn't be a big deal.

If it's not going to cause a tripping hazard, leave the ends of the tendons hanging out for a while so that you can destress a cable later if you have to. Unfortunately, it's not usually possible to leave cables hanging, because everyone ends up tripping on them, and complains about getting grease on their pants from walking near them.