[TowerTalk] Ground rod

[David Gilbert]

Sorry, but I don't buy either of those viewpoints.

Concrete in the ground is typically loaded with moisture ... practically 
saturated with it in some parts of the country. Concrete also typically 
has lots of micro (and some not so micro) shrinkage cracks in it, which 
is why we have to put rebar in it to hold it together in the first 
place.  There is therefore ample opportunity for moisture to reach the 
rebar no matter what we do. The reason rebar doesn't rust is because the 
alkali in the concrete inhibits it ... nothing more.  The reason that we 
need to keep the rebar from poking out through the concrete is that the 
CORE of the rebar would then be able to rust from the exposed end, since 
the interior of the rebar isn't protected by the alkali.  The rust can 
progress up the interior of the rebar into the concrete and essentially 
hollow out the rebar.  I've seen that happen, and you can find pictures 
of it on the internet if you look.  Copper wire is a totally different 
story since it doesn't rust.

I also don't buy the idea that the difference in thermal expansion 
between concrete and a cooper wire is going to cause problems, since 
such crevices are going to be minor and likely no larger than naturally 
occurring cracks and crevices in the concrete itself.  In fact, I'd be 
willing to bet $100 that the average concrete slab has FAR larger voids 
in it (where moisture can condense into pockets of water) due to 
inadequate rodding of the concrete when it was poured.  Rodding is the 
vibratory action that is used to make sure that the cement and various 
sizes of aggregate (sand, gravel) flow completely around each other, but 
it is also the action that brings excess water to the surface so that it 
is not trapped inside ... later generating interstitial voids (which 
weakens the concrete) as the excess water not needed for the chemical 
reaction eventually dries out.  Unless time is spent rodding throughout 
the pour, pausing periodically to remove the excess water, significant 
water is almost guaranteed to remain trapped inside ... some of it most 
likely in "clumps" as fresh concrete was poured over surface water that 
had accumulated from early rodding.  Whatever gaps form around the 
copper wire would be trivial by comparison.

Lastly, the "steam" hypothesis for blasted concrete doesn't make sense 
to me either.  Any reference I've ever seen to it has been pure 
speculation, and doesn't fit the fact that pockets of moisture almost 
certainly exist in any concrete structure that isn't dry. And as far as 
I know dry concrete explodes just as easily due to a direct lightning 
hit as does wet concrete ... probably more so. Localized thermal shock 
makes far more sense to me.  In any case, the moisture surrounding a 
copper wire isn't going to explode anything, and the pictures I've seen 
of exploding concrete didn't involve any protruding conductor at all.

Lots of this stuff is not fully understood, and the many variables 
involved make cookie-cutter generalizations highly questionable. The 
comment that we should under no circumstances allow any conductor to 
breach the concrete-soil interface seems totally unwarranted to me ... 
it just doesn't fit the physical model as I picture it.

I have nine different #4 solid copper wires protruding from my tower 
foundation below grade ... three from the bottom to buried ground rods 
(the ground rods were driven below the surface before the concrete was 
poured) and six from the sides each running to 30 foot long runs of #4 
solid copper wire with ground rods every ten feet. I live on a hillside 
in a high lightning area and if my foundation ever explodes I'll be the 
first to tell everyone.

73,
Dave   AB7E



It allows moisture to migrate along the wire/concrete interface reducing 
the effectiveness if it doesn't connect to the rebar.  If connected to 
the rebar, moisture will also migrate along it causing corrosion.

I have seen photos on the net of broken concrete , claiming the moisture 
caused steam to form from the strike.  I do not know if that was the 
case or not.  It'll take someone a bit more knowledgeable than me to 
answer that which is a bit controversial.

73

Roger (K8RI)



On 7/16/2013 9:56 PM, Jim Lux wrote:
> small crack with capillary action. The thermal coefficient of 
> expansion is also different, so you have the possibility of the 
> crevice getting bigger and smaller with every thermal cycle.  Once you 
> get liquid water in there, then you have all sorts of potential 
> problems. Above grade, you don't get water standing at the join, so 
> the problem is less than below grade.
>
> I suppose that a AWG4 copper wire sticking out is probably less of a 
> problem than a 1/2" steel rebar. Smaller area to worry about, copper 
> is less corrodable than iron, etc.
>
> And you see small steel wires and nails sticking out (e.g. from forms 
> that have been stripped off, etc.) so it's probably not a killer.
>