"extrapolation" may have been a poor word choice, but my reasoning is
that copper pipes are not buried without special precautions for
domestic water service because of corrosion. Another reason is that the
galvanic cells and/or stray currents that exist in soils can cause
electrolytic erosion. Copper clad ground rods are known to have finite
life, sometimes quite short in certain soils. Some concrete mixtures
were a disaster with buried radiant heating copper tubes and that
application now very rarely uses copper in concrete. There are
discussions on this reflector that AM broadcast station copper ground
screens disappear over time. Copper in air lasts a long time, not so in
certain soils or electrolytic environments.
So, why expose a Ufer ground lead to what might be an environment that
significantly shortens its life, particularly if it isn't possible to
inspect it? As I have read the Ufer literature and codes, steel rebar
is the conductor, not buried copper wire, and the connection is to a
protected rebar exit point from the concrete.
wikipedia re Ufer: "Ufer's original grounding scheme used copper encased
in concrete. However, the high pH of concrete often causes the copper to
chip and flake. For this reason, steel is often used instead of copper."
see http://www.psihq.com/iread/ufergrnd.htm and
http://en.wikipedia.org/wiki/Ufer_ground
Grant KZ1W
On 7/17/2013 10:36 AM, David Gilbert wrote:
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.
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