Defusing the Bomb
Proponents say they save money because the stronger structure allows for a thinner slab, thus reducing the amount of concrete. They also claim other savings, including less labor during construction.
"When you have soil movement on the bottom of your foundation, post tension tends to hold the slab together, rather than have it crack or buckle," says Clint Sundeen, field service technician for Suncoast Post-Tension Inc., a supplier of fabricated unbonded post-tensioning materials and services.
While it is open to debate as to whether post tensioning is worth the possible price reduction and additional support, there is no question that there are some lethal dangers involved when working around them. For a plumbing contractor, who has to cut into a post-tension slab in order to repair or replace pipe, tread lightly. In addition to the possibility of getting seriously injured, post-tension ignorance has cost many a contractor a pretty penny.
"Most plumbers out there don't even know what a post-tension cable is," says Tim Downs, technical training supervisor for American Leak Detection in Santa Ana, Calif. "They'll come in with their saws and just start cutting because they don't know. It's starting to become more of a common word now, but the cat's not quite out of the bag yet."
Downs himself once snapped a cable several years ago. "The very first one that I ran across I didn't know what it was, and I just took the saw out and cut right through. Since then, I've learned quite a bit more."
Dick Bonin, a marketing representative for The Western Group, a company that performs post-tension tendon and concrete repairs, said when one of them snaps it's like a .44 Magnum going off. "These tendons carry over 30,000 psi. You are taking a 100-foot, half-inch wire rope and stretching it close to 10 inches like a rubber band. These things can cut a hole in a car. They can come out of the concrete and blow concrete for two blocks if you don't watch it."
Bonin has seen the destruction in vivid detail. "I've looked at some pictures that would scare the hell out of you," he says, "where the wires have gone through the concrete and gone through walls and doors. They [the cables] are under tremendous pressures and can be very dangerous."
Bonin's comments are echoed by Arnie Rodio, owner of Pacesetter Plumbing in Lancaster, Calif. "There's a story about the cables coming out of slabs and cutting a guy's foot off."
A broken cable can pop straight up out of the slab or penetrate out of the side. Rodio said he has seen cables that had shot out of buildings some six or seven feet. "If you were standing next to it [when it broke] it would cut you in half."
In another instance, a worker was ripping away with a jackhammer and struck a cable. "The whole house jumped. It didn't do any damage to the house and it didn't hurt the guy, but it absolutely gave him a heart attack. It cost something like $1,200 to re-stretch that cable. You have to go in and fix the sleeve and re-stretch the cable and re-torque it."
Depending on the damage, Sundeen said repairs could amount to $20,000.
So, how should a plumber work around these cables? Carefully. It can have all of the thrills of defusing a bomb.
George Moschopoulos, a project engineer for Suncoast, says contractors can find the cables by reading the post-tension plans or as-built plans. "Usually, the tendons are very expressly dimensioned and you should be able to locate them with a fairly high degree of accuracy."
If the slab was poured recently, Moschopoulos says, you might be able to figure out the alignment of a certain tendon by looking at the slab edge and seeing where the anchorage is. "Of course, a lot of times if the house is already inhabited, you won't have that because there's landscaping or backfill that won't allow you accesses to where the anchorages are."
Sundeen suggests that jackhammer work be performed deliberately. Stay three or four inches away from the center mark of the tendon on either side.
In order to preserve the family lineage, he also recommends that you work sidesaddle. "I'd recommend staying to either side of the cable, not straddling it."
Bonin suggested using a metal detector along with the post tension plans. "Try to locate the stressing ends, the anchors, if you can't find the exact locations (with the plans). They are usually at the edge of the slab, a little round pocket-looking thing about two inches around in the anchors. Use that and a Pachometer (a metal locator) and you map it out. It's kind of tough in the newer buildings because there's so much rebar in the slab."
If you don't have a Pachometer, he said you may wish to use more finesse by utilizing a chipping hammer, not a jackhammer. "It's not that hard," he said. "You can see the cable before you snap it."
He said his company has repaired several cables broken by plumbers and electricians. Bonin strongly recommends that plumbers hire an outside contractor to do the job of locating the cables. The Western Group hires specialists that use X-rays.
While in the past he has used plans to locate cables, Rodio does not recommend it. "It's not totally accurate. They can be off two, three, four inches. If they're like an 18-inch grid, and you're off two or three inches on either side and you punch a hole, you can bust one."
He said he has used a metal detector to find the wires, but it's much easier to hire a subcontractor to do the job. "To me economically, I'd just rather pay a guy who is a specialist. He does it faster, quicker and has a lot better equipment than I want to buy."
According to Alex Tarussov, senior applications specialist for Geophysical Services Systems Inc., the most commonly used system is X-rays. His firm produces Structurescan, a ground penetrating radar. "The problems with X-ray equipment is that it's expensive equipment, it's slow and it's dangerous to your health," he said. "You need to evacuate the building before doing it. Ground penetrating radar gives you immediate results and it is completely harmless. It gives a visual image of what's inside concrete and it can see metal objects, as well as non-metal objects."
Cost range for Structurescan is $20,000 to $30,000, while an X-ray machine would approach $50,000. "Also, with X-rays, you need a lot of consumable material," said Sundeen. "You need film, radioactive sources and chemicals. There's a lot of operating costs. With the radar, once you have it, that's it."
The important point is that contractors take the initiative to use some type of locating device before breaking out the jackhammer. "I worked on a post-tension hotel once where the general contractor was too cheap to spend a few bucks for X-rays. His technique was to drill a cluster of small holes, gradually making a large hole. He didn't hit any cables, but he did hit a conduit containing the power to a bank of elevators. It shut them down for a couple of days. So much for saving a couple of bucks," explains Scott Denny, president of Frank Denny Plumbing in Menlo Park, Calif.
Downs has been busy doing two or three post-tensioned slab jobs per week. He uses a magnetic machine called a Micro Cover Meter, which is produced by ELE International. It has never failed him. Purchase cost is about $2,500. It not only locates the cable, it tells how deep it is and its thickness.
According to Buzz Teter, research and development specialist for American Leak Detection at the corporate offices in Palm Springs, Calif., ground-penetrating radar is the No. 1 industry choice for locating cables. He said Sensors and Software, a firm based in Mississauga, Ontario, Canada, has a 1,000 Hertz model that is made especially for mapping slabs. Cost: $20,000.
"I look at GPR equipment all the time and they seem to be the only ones out there who are modifying their equipment to be able to take a three dimensional picture of these slabs and what's in them," said Teter. "From what I can tell so far these are the only guys who are stepping up to the plate here."
Other firms he has investigated include Mala and Verneer, plus several others that are located in Denmark.
The GPR products interface with a computer, which produces a map of what's inside the entire slab. For the jobs American Leak Detection has been getting, this is overkill, said Teter. They only need to locate the wires in a few selected locations.
As post-tension slabs continue to be poured across the country, plumbers are learning more about how to work around the cables confidently and safely. The proverbial bottom line is tread lightly. Even if you get a clear reading on what's underneath, take your time with that jackhammer. Whether you hire a contractor or locate the cables on your own, post tension does not have to mean post-mortem.