A.Bob Kovacs, president of
Constructive Solutions in Iselin, N.J., and
moderator of the estimating forum at jlconline.com,
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
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.