Dave,
My experience agrees with yours in that grounding a tower through the
concrete base is a non issue. All my 5 towers are grounded below the 4 ft
cube concrete base for over 35 years with no visible problems.
Doug
"Think of all the ways you can hurt yourself laughing."
-----Original Message-----
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)
Well Skip ole boy that is misinformation. All my towers (5) are grounded
through the concrete base (4 ft cube) for over 35 years and have been hit by
lightning and the bases are FINE.
Doug
I'm not sure I understand the thread however, NEVER EVER, ground anything
into the concrete base !!!!!!! NEVER use any ground rods in concrete !!!!
If you would get a lightening strike, the current would carry into the
concrete and boil off the water. When this happens the concrete will
explode !!!
If you must ground a tower base I use 4 rods spaced about 4 ft from the
base, Cad weld the wire onto the rod and make a physical connection to the
tower on all legs. Bond the wire with at least 2 clamps onto the tower base
using a conductive paste . Years ago I would pour Copper Sulfate around the
tower to aid in the ground conductivity. As a point check the tightness of
the clamps every few months
it's not the best but who can afford the grounding system that works de Skip
K3CC
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.
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