Before you break out your jackhammer, use a locating device to find those post-tension cables. The investment could save your life.
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While they were initially used in areas where soil movement was a concern, post-tension cables (tendons composed of steel wires embedded in concrete and then tensioned after the concrete is hardened) are being used around the country on projects ranging from single-family and multi-family residential to commercial and institutional buildings and parking structures.
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.
Post-tension cables are being designed into a variety of construction sites including single-family and multifamily housing as well as commercial and institutional buildings.
"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.
If you are looking for more details, kindly visit post tension system solutions.
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.
"This article was originally posted on ww.reevesjournal.com."
Looking to spruce up your backyard with a nice patio? Want a sturdy foundation for your next tool shed? Consider having a concrete slab as your base. Concrete slabs are inexpensive, long-lasting and easy to install—as well as multi-purpose.
Key Takeaways
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Find A General ContractorNote: For the sake of this cost estimate, exact dimensions (in feet) have been given to reflect the different price points. Costs may be lower or higher depending on the size of the slab. All costs have been calculated at a rate of $6 per square foot.
A concrete slab is composed of wet cement mixed with additional materials (usually gravel). This mix is poured onto a gravel bed bordered by a wooden frame, the latter typically made from standard lumber like 2x4s or 2x6s. Once the concrete has cured, this frame—also known as the formwork—is removed, leaving behind a fully shaped piece of concrete—the slab.
Concrete slabs are known for being extremely durable. If properly installed and maintained, they can easily last 100 years or more. Though the name “slab” brings to mind an uninteresting flat gray surface, in truth, concrete slabs offer a lot of design flexibility—from the shape of the formwork to any texturizing or colorizing done after the concrete has been poured.
As a reminder, cement and concrete refer to two different types of material despite often being used interchangeably. Concrete is a composite substance that includes cement and is more durable.
If you’re considering getting a concrete slab in your home, knowing how much it will cost is essential. The first thing to consider when pricing out a concrete job is the square footage of the space you cover. You’ll also want to know what slab thickness you’re looking for. Finally, it’s crucial to consider the difficulty of the project itself when budgeting for this project.
Concrete is a relatively cost-effective material when compared to stone or brick. Anticipate spending about $6 per square foot for the raw material, ranging from around $4 to $8 per square foot, depending on variables like labor and the thickness of the concrete.
Thickness A standard concrete slab will typically be 4 or 6 inches thick, but this can be adjusted depending on the needs of your project. Thickness is key in determining the final cost of the project: the thicker the slab, the more expensive it will be. A 4-inch slab, for example, can cost anything from $0.50 to $1 less than a 6-inch slab.
Concrete slab projects can vary significantly in cost. Depending on the square footage, thickness and finishes you choose, the cost of your project can range from simple to highly complex. For instance, installing a concrete slab for your driveway, patio or garage will typically require more materials and labor than installing one for your shed.
Though not all of these factors may be relevant to your situation, they each have the potential to significantly change the overall cost of your project.
It may be necessary to make the edge of your concrete slab thicker, depending on the end design and function of the slab itself. The weight of the concrete is also a factor here, since you do not want the frame buckling under the pour. Expect to pay an additional $1 to $2 per square foot.
If you aren’t fond of the classic concrete slab look, there are a variety of texturizing and colorizing options that you can use to finish your slab. Adding any kind of finish and design—whether it be stenciling, staining or engraving—can bump up your final cost from $8 to $18 per square foot. If you desire a fully bespoke or advanced design, you may pay even more.
For the most part, the typical price range of $4 to $8 per square foot includes labor. However, if your project requires the use of specialized equipment—such as any machines needed for grading—the cost can skyrocket. For example, a single pump truck (used to pour concrete if your build location is too difficult for a standard concrete truck to access) can tack on around $900. Keep this in mind especially if you are doing a DIY install, since you will need to rent or buy any equipment you do not have.
Depending on your specifications or contractor recommendations, your project may require the use of additional materials. Things like a vapor barrier, a Styrofoam layer or using wire mesh will add between $0.30 to $2 per square foot. When mixing concrete, you may also be charged extra if you require more cement than normal.
Dirt work and grading refers to any preparation of the substrate that needs to occur before laying the formwork and pouring the concrete. This can be as simple as leveling the surface, or as complicated as consulting with a structural engineer on how to build the slab on a slope. Some contractors will include grading in their services at no additional cost, but this is highly dependent on the complexity of the site and its requirements.
Concrete is a sturdy building material known for its longevity and durability. Still, it is also susceptible to degradation over time due to various factors, including weather conditions, regular usage and subpar construction. Here are some signs that could suggest the necessity of concrete replacement:
Concrete slabs are designed to serve a specific purpose and accommodate different structural requirements. Here are some common types of concrete slabs:
With careful planning and preparation, it is entirely possible to install a simple concrete slab by yourself. The process is time and labor intensive, so set aside at least a week to complete it—which should cover everything from preparing the form to allowing the concrete to set and cure. Keep in mind that while the installation itself is relatively straightforward, there are lots of little factors and steps that may make the process more hassle than it’s worth.
Details like permit requirements, leveling and grading the sub-surface, properly staking and bracing the frame and smoothing the concrete after it has been poured can all affect the final build of your slab. Any finishing you want to do should also be thoroughly researched so that you do not apply a finish too early or late in the curing process.
Wet concrete is also considered toxic and can cause chemical burns, so take the appropriate safety precautions to make sure that you are not exposing your skin and eyes.
Though DIY is cheaper overall than hiring a professional service, depending on the complexity and size of your project, it may not be worth the personal time and energy spent. Consult with a professional contractor to receive a project estimate and compare it to your DIY estimate before you make a final decision.
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