<!--[if gte mso 9]> Normal 0 false false false MicrosoftInternetExplorer4 Could somebody explain the details, since I find conflicting information on this question?

Apparently, the alkalis (OH) found in Portland cement concrete can attack certain materials.

A.)  This website states: (http://www.listentech.com/shop/product/direct-burial-cable/)

The insulation of the cable is resistant to attack from strong alkalis present in concrete unlike standard PVC insulated wire to prevent loop failure due to corrosion of the copper core.

B.)   Similarily: (http://www.ampetronic.com/Products/Other-Products/Installation-Accessories/Direct-Burial-Cables/DBC)

PVC cable can not be used for burial in concrete as the corrosive effects of alkalis present in cement based compounds will lead to failure of the loop over time

C.)  On the other hand:  (http://www.concreteconstruction.net/concrete-articles/will-pvc-react-chemically-with-concrete.aspx)

According to Bernard Erlin, the principal chemicals in concrete that could conceivably attack plastics are calcium hydroxide, sodium hydroxide and potassium hydroxide (all the hydroxide are alkalis, OH). Erlin cites a table from Handbook of Chemistry and Physics stating that rigid PVC has excellent resistance to strong alkalis.  Reference: Bernard Erlin, "Embedded Metals and Materials Other Than Reinforcing Steel," Significance of Tests and Properties of Concrete and Concrete-Making Materials, STP 169C, ASTM, West Conshohocken, Pa., 1994, p. 179.

 

I have read that Polyethylene insulation (= PE, same stuff milk cartons are made from) holds up well in concrete, but most wires at the HomeDepot or Lowes are THHN, which is usually PVC with an additional outer layer of nylon.  What happens if one would use a regular THHN wire as tracer wire directly buried into a concrete slap, running alongside the PEX tubing?  Do alkalis diffuse through the PVC insulation jacket and attack the copper itself?  Basically, what is the real corrosion mechanism of PVC-insulated copper wire directly buried in concrete, if indeed there is one?  

Let me know in case you can shed some light on this.

Regards,

Max Osterhasi

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How long do you think you will live?

Why do you want to trace the PEX?

I'm surprised that the liquid inside the PEX can't be used as a tracer wire. Perhaps with the temporary addition of salt.

Any input on the actual question, if PVC insulation is attacked by the chemistry in Portland cement?

Posted By BadgerBoilerMN on 06 May 2014 07:30 AM
How long do you think you will live?

Why do you want to trace the PEX?

Tracing PEX helps when bolting down machinery, for example.  Otherwise I might drill through the PEX tubing by accident.  Zip-tying an 18-gauge tracer wire along-side the PEX while routing it, is easy enough.  I can take digital pictures of the PEX layout with measuring tapes in place for future visual reference, but you know how it usually goes:  In 15 years time, the digital pictures are probably lost, but the concrete slab will still be there.  And a tracer wire would still be there, too, provided it wasn't eaten up by the alkali in the concrete.
Regards,
Max Osterhasi

Posted By jonr on 06 May 2014 08:13 AM
I'm surprised that the liquid inside the PEX can't be used as a tracer wire. Perhaps with the temporary addition of salt.

As far as I understand tracer wires (please correct me if wrong):  A signal generator is attached to the tracer wire, and the tracer wire carries along that signal and emits the signal like an antenna.  Then, one takes a reciever sensitive to the tracer signal, to precisely locate the tracer wire anywhere along its entire run.  Given, that the tracer wire is in close proximity of the pipe, one then knows where the pipe is.   But I've never used it myself, that's just my current understanding.  Anyhow, to use the liquid inside the pipe for the same purpose, one would have to send some detectable signal through the liquid, which might work, but I've never heard of it for locating pipes.  I guess one could even send acoustic signals, maybe similar to how they used to find submarines in WWII.  While this is an interesting point, it does not answer my initial corrosion question.
Regards,
Max Osterhasi

Looks like copper in concrete is fine as long as there are no chlorides and there is no possibility of galvanic corrosion (say between the copper and steel). I think the PVC itself is fine with concrete, but the plasticizer used may not be.

You may be able to use the aluminum layer in pex/al/pex.

Copper pipe has been used in concrete slabs for radiant, with disastrous results, it corrodes and leaks

If you are just trying to locate the PEX so you will not drill into it, consider renting a Forward Looking Infrared (FLIR) camera or just have your installer use their FLIR camera to mark the PEX locations in the area being drilled.