[TowerTalk] Conductivity of cement

David Gilbert xdavid at cis-broadband.com
Sun Sep 7 04:21:08 EDT 2008

Actually, I think I was a bit skewed in my reply to Larry.  Although I 
personally have no problem with connecting a rebar cage to ground 
outside of the concrete, Larry didn't actually suggest that the tower 
itself and the rebar shouldn't be connected internally.  After I reread 
Larry's posting I decided that a portion of my reply was mixing apples 
with oranges.  I'd still bet, though, that the problem with exploding 
wellhead pads was that the rebar in them was not grounded to the well 
casing itself.

The way to bring a ground connection into the concrete would be with 
copper.  That's the way most Ufer grounds are connected to the 
electrical service panel, at least here in the Southwest.  The wire 
exits the footing below grade and is brought up to the panel, but the 
rebar itself still needs to be embedded a few inches within the concrete 
to avoid a corrosion path.  The rebar gets wet anyway since concrete is 
so porous, but the high alkalinity prevents corrosion.  Copper doesn't 
really need that same protection except in highly acidic soils, and even 
then the corrosion would occur outside the concrete and not within it.

Sorry for adding to the confusion.

Dave   AB7E

Roger (K8RI) wrote:
> David Gilbert wrote:
>> <snip>
>> Besides, I would bet that every one of those exploding pads occurred 
>> because the rebar was NOT connected to the wellhead casing.  I 
>> suggest you head to the Polyphaser web site for a more authoritative 
>> recommendation ... check out their tech note on Ufer grounds for a 
>> tower foundation.
> I have a bit of a problem reconciling the regulations that the rebar 
> be bonded to "what ever" outside the concrete with the statement I've 
> often seen that the rebar should not penetrate the surface of the 
> concrete due to corrosion.  In either case we end up violating something.
> 73
> Roger (K8RI)

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