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Finding tension cable in foundation

KRabenaldt's picture

I need to install a floor safe in a concrete slab that has tension cables running through it. What method(s) are there to find where they are. I assume there exists a metal detector suitable for the job? Anyone have a experience finding buried treasures? I am not worried about the plumbing because I will cut a new hole if any plumbing is found.

The house was built by a national builder. Would not trust any blue print data for location.

Thanks,

Kevin

(post #54231, reply #1 of 12)

I've used my kids metal detector he got for a gift from FAO shwarts. Works great for finding stuff. Used it last to find the waste drain made of cast iron under a cement slab.

(post #54231, reply #2 of 12)

I think it will be difficult to tell the difference between a tension cable and the reinforcement mesh in the slab.

(post #54231, reply #3 of 12)

Kevin,

You should be able to see the grouted pockets on the foundation, then just measure.

KK

(post #54231, reply #4 of 12)

Zircon makes some.

I've got a Bosch DMO10 that will tell you where metal is buried and how deep, within an eighth of an inch.

A repeating pattern at 6" square will be the re-wire and a stronger signal off pattern will be the cableIt cost me about 79 or 89 bucks.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #54231, reply #5 of 12)

Exactly what would a tension cable be doing in a foundation? I've only seen them in prestressed concrete panels. And why would you want to cut through a foundation or for that matter put a safe in one? If you don't know what a foundation is I wouldn't recommend cutting into what ever you think one is.

If you were arrested for being a quality builder would there be enough evidence to convict you?

(post #54231, reply #6 of 12)

I guessed it was in the foundation because the thread title is


 "Finding tension cable in foundation"


Giving advise on cutting a foundation or prestressed concrete is better left to experienced pros and those with fancy framed diplomas on the wall.


It is like the problem you had, people respond without understanding what it is they have read.


 

And I wasn't jumping down his throat just responding to a potentially dangerous situation. But I am jumping down yours... reading is fundamental

Edited 8/19/2002 7:35:46 PM ET by GEOB21


Edited 8/19/2002 7:41:34 PM ET by GEOB21

If you were arrested for being a quality builder would there be enough evidence to convict you?

(post #54231, reply #7 of 12)

Oh chit GEOB, you've touched a raw nerve now.  and those with fancy framed diplomas on the wall  Read a few messages and you'll quickly discover that most contributors here don't have much use for fancy paper, but rely more on experience and knowledge. 


To answer an earlier quextion:  most houses in Texas that I have seen are built as slab-on-grade with a basement being a very rare item.  The post tension cables are about 3/4" dia sheathed in a plastic sleeve...looks very much like an oversized stranded electrical wire.  Sometime after the slab is poured, the cables are stretched until they achieve a pre-determined tension, and then the two end sockets are grouted to keep the cable tight.  Seems to work well, at least until soemone cuts into the slab and severs the cable, then it usually whips backs and breaks through the slab. 

I'm sorry, I thought you wanted it done the right way.

(post #54231, reply #8 of 12)

Kevin:


Elcid72's description of a post tensioned slab is consistent with what is used on some soils in our area. I think laying out using the grouted pockets around the perimeter of the house and a good metal detector will reveal the location of the cables but cutting into the slab with a saw could be very dangerous. I cannot claim to be an expert in post tensioning but I have read about cables flying out the side of a foundation when cut. They use some pretty strong equipment to stretch those cables tight. It might be time to call in the experts.

(post #54231, reply #9 of 12)

Thanks for your replys.  The tension cables are an issue because I would definitely want to stay a good distance from them.  I was sure a proper metal detector was one way of locating the cables.  I wanted to see if they were other methods as well so I could verify.  If the location of the cables could not be positively located then yes I would not be cutting the slab.


The "foundation" is usually referred to here to include the footings (exterior and interior) along with the slab.  The footing would contain the structural steel re-inforcement.  The slab will "usually" contain steel in the form of mesh or rebar.  On typical houses I mostly see 5 to 6 tension cables, if they are run.


I appreciate all comments, especially when they identify safety concerns.   I should have been a little bit more specific in my description.


To cut the slab for the safe I would use a 4-1/2 inch grinder with diamond blade and cut into the slab about 1-1/2 to 2 inches deep.  The hole would be about 3 inches wider then the safe so I could later re-grout.  After the cut I would use impact chisels the rest of the depth of the slab.


Again, thanks for your replys,


Kevin

(post #54231, reply #10 of 12)

>>To cut the slab for the safe I would use a 4-1/2 inch grinder with diamond blade and cut into the slab about 1-1/2 to 2 inches deep.  The hole would be about 3 inches wider then the safe so I could later re-grout.  After the cut I would use impact chisels the rest of the depth of the slab.<<


Uh, most home slabs are 4" thick. How big of a safe are you installing?  Yu could install permanent studs into the slab, bolting the bottom of the safe to the studs.


A friend has a home in Austin, TX. It was NOT built using post tension cable, big mistake. He can "open and close" the crack in his ceiling by watering the back yard. Yes,  clay soil... lots of movement.  Funny to see folks from "up North" wonder why we have soaker hoses on timers, around the house. Land scaping is just to remind you to soak the foundation!!!  (Austin varies quite a bit in soil, so many homes are just slab on grade... others require the post tension cable... typically you can not see the cable locations once the foundation is smoothed out -- covered with a pretty layer of mortar).


As to "holes in the foundation". Not uncommon to find "sand boxes" under the shower pan. If the builder thought ahead to the owners changing out the showers in makes life much easier to change the locatoin of the drain when you put in a new shower stall.


Tom


 


 

(post #54231, reply #12 of 12)

Tom, I'm not so sure the sand box under the tub and/or shower is for future renovations, but rather it allows the plumbing sub a little wiggle room in doing the in-slab rough-in.  He (or she) can get the stub up to a pretty close location, then after the slab is poured and the walls are layed out, the drain can be adjusted an inch or so to line up correctly.  I see it frequently for toilets as well as tubs & showers.

I'm sorry, I thought you wanted it done the right way.

(post #54231, reply #11 of 12)

Why not rent a larger grinder or a concrete cutter?  Or better yet, hire a concrete cutting contractor and make it part of their scope of work to find the cables.

I'm sorry, I thought you wanted it done the right way.