Post Tension Slab Tendon Location....Undeterminable? Can Superman See them?

[bigmig]

I have a project where a client wants to hang some new equipment from an existing post tension concrete system.
In one area the system is a flat slab. In another area it is a Tee type system. The proposed equipment is
replacing equipment of idential weight.

We want to make sure we don't drill through post tension tendons in the event we have add some more attachements.
We do not have any access to the slab ends and don't have any clue if the tendons run in the perpendicular or parallel direction.

Has anyone used either X ray or Metal Detecting equipment to find post tension tendons in slab systems like this?
The slabs can only be accessed from below (they have a roof surface over them).

Thank you.

 

[phamENG]

Never used it inverted like that, but yes - ground penetrating radar is typically used.

 

[phamENG]

Check with your local testing lab. Our's has a handy portable unit they can get into tight spaces and their techs are top notch.

 

[Ingenuity]

Yes, I have GPR scanned lots of slab soffits to detect PT and rebar. It is a bit of a pain-in-the neck, literally, as you are working overhead. And have to to interpret the graphic results on the screen as 'inverted' as bottom rebar/tendons are shown on 'top' of the screen.

GPR is the preferred equipment to do this. Accurate, safe and portable. X-ray won't work for your application as you need access to both surfaces, and has safety issues. Metal-detecting equipment is often just a toy (aka stud finder).

If it is a PT flat slab, it is most probably that there are tendons running parallel AND perpendicular (i.e. banded and uniform, in North America etc).

 

[msquared48]

Just had a client use X-Ray to locate the steel in a Tilt-up wall.

Mike McCann, PE, SE (WA, HI)

 

[STrctPono]

Yes. We have in house ground penetrating radar equipment and find mild rebar and PT. Ingenuity is right, scanning upside down is a bit of a pain but can be done. If the surface isn't nice and flat or there are formwork joints that make it uneven then it can produce erratic results. A big smooth sheet of acrylic can help if you can find some way to pin it to the surface. You normally need a large area to scan. Scanning small areas does not produce the best results.

 

[Canuck65]

I have quite a bit of experience in locating PT tendons in reinforced concrete slabs. It is relatively easy to do with a covermeter. My go to in the day was Protovale Rebar Plus. It helps if you have an idea of the layout of the tendons from original design. A covermeter can also give you an idea of depth. GPR is also a candidate. While Xray will work, I am of the opinion that both covermeter and GPR are easier and cheaper and accurate enough for your purposes. Beams can be more difficult due to congestion of the tendons/rebar.

 

[BAretired]

Superman can see them provided there's no kryptonite around.

BA

 

[Agent666]

Generally portable systems like hilti has will do about 200mm of depth, so if that suits your needs it's much easier to scan from above.

It's generally very cheap, in this part of the world it would be a couple of hundred dollars to get a guy out for an hour or so and do 5-6 locations up to 1.5m square.

Some hire places might even have the less sophisticated versions for much cheaper rates.

 

[Ron]

We use GPR for this. Yes, it can be used from the bottom of the slab. We have magnetic devices also (Hilti and Profos) but they are of limited use for PT tendons.