Galvanized RMC in Concrete Duct Banks.

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rcwilson

Senior Member
Location
Redmond, WA
My designer reviewing our company's installation specifications asked me why we spec an anti-corrosion wrapping on RGS conduits in a concrete duct bank. I assumed it is to limit corrosion of the galvanized conduit in the concrete per NEC 344.10 (B). But I recall seeing many old embedded RGS conduits without a tape coating that were not rusted out.

In your opinion, does the galvanized coating on the conduit meet the code requirement for corrosion protection by itself? And what is your typical practice for RMC embedded in concrete: to embed as is or to provide some type of additional coating?

Thanks for the input.
Bob W.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: Galvanized RMC in Concrete Duct Banks.

I don't recall ever seeing a wrapping installed over RGS, embedded in concrete or not.

I have seen people wrap EMT with duct tape at the fittings when it is in concrete. presumably to keep the concrete from seeping in. Seemed cheesey to me. but probably effective.

[ June 30, 2005, 08:05 AM: Message edited by: petersonra ]
 

sandsnow

Senior Member
Re: Galvanized RMC in Concrete Duct Banks.

RMC in simutaneous contact with earth and concrete will rust away. There is something in the UL guide somewhere about this.
RMC fully encased should not be a problem.

Some EMT connectors are marked concrete tight only when taped. Some people do it just to be safe.
 

bbb

Member
Re: Galvanized RMC in Concrete Duct Banks.

Just curious why RGS VS PVC in concrete ?
If mechanical protection needed the concrete is there. If corrosion is a concern use pvc is more resistant than RGS. If cost is concern PVC is much cheaper than RGS(material and labor).If a bonding path is wanted copper will out perform corroded threads.Also RGS causes impedance.Not trying to stir up trouble just curious.
 

rcwilson

Senior Member
Location
Redmond, WA
Re: Galvanized RMC in Concrete Duct Banks.

Why not PVC? - Good question. We do use PVC for most of our runs. But the heavy circuits we pull can damage the PVC 90's, the pull rope cuts right through the PVC elbow like a hack saw. A few years ago we standardized on using rigid conduit for any bend over 30 degrees and RGS sweeps at most corners and stub-ups.

With RGS 90's the stub-ups under the equipment or out in the plant at a boiler or pump come out of the concrete in RGS. (PVC stub ups also get damaged by our construction forces.) We had some problems in the cooling tower areas with corrosion on the conduit at the cement-air interface, I think that is where the anti-corrosion tape idea came started: tape the conduit for 6" above and below the concrete to prevent corrosion at that interface. I think that idea grew to taping all buried steel conduit.

We also have to use steel conduits for shielding on some sensitive control circuits; otherwise we go PVC as much as possible.

We always run a green EGC with every circuit, whether it is in PVC, tray, or RGS conduit. We don't trust raceway for equipment bonding.

BTW, we found a recommendation from the Steel Conduit Institute (I think that's the wrong name). They recommend taping conduit for 4" either side of a conduit to earth interface for corrosion protection and taping threads to prevent cement entry.
 

molotov27

Member
Re: Galvanized RMC in Concrete Duct Banks.

rcwilson,
out of curiosity, is the installation a power generating plant? I've heard of "wrapping" pipelines...I'm no expert on it, but here's what I know.

If you are talking about a power plants, stray currents flow in the ground and water, which cause accelerated corrosion on metal parts. Most power plants use cathodic protection systems on the premises to counteract this phenomenon. The thing is that since powerplants are next to bodies of water, the water table is pretty high, so there is a lot of water on the ground, add the stray currents and the result is a lot of corrosion. Any metal that touches water is usually stainless steel such as the traveling screens, and there are even oil containment booms specifically for service around power plants because of this. Of course the RGS will not be "touching" water, but the high water table is a concern.

If it is not a power plant, once again, the water table and soil composition might be an issue at the location.

[ July 06, 2005, 09:49 PM: Message edited by: molotov27 ]
 
Re: Galvanized RMC in Concrete Duct Banks.

We use rigid incased in concrete all the time.Whenever we stub to earth we wpar in 20 mil tape to avoid corrosion.
In water treatment plants we use OCAl which has a rubber coating already applied.
In most cases we use PVC in duck banks and incase conduits in concrete,where required.
 

Nick

Senior Member
Re: Galvanized RMC in Concrete Duct Banks.

Has anyone ever read 3m's installation instructions for their 10 and 20 mil tapes? (#50 & #51) They require a coting of pipe primer before wrapping. Can't say I have ever seen this done.
 

kentirwin

Senior Member
Location
Norfolk, VA
Re: Galvanized RMC in Concrete Duct Banks.

For what it's worth, I recall working on various military base construction projects where the specifications called for painting all RGS conduit to be installed in or under a slab with zinc chromate and then bitumastic. Since I haven't been involved with military base construction for many years, I have no idea what they do today. Perhaps the above method expired with the last Ice Age. I don't know. :roll:
 
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