I think you are mixing several different concepts here, at least a
couple of which are erroneous.
Taking your anecdotal story about the Parthenon at face value, rebar
itself would rust and crack the concrete it was in because I guarantee
that concrete is more porous than marble. The more likely factor is
that the pins were made from high carbon steel, which is considerably
more prone to corrosion. Put one of your best drill bits and a piece of
rebar in a dish of water if you don't believe me.
The failed guy anchor wasn't necessarily due to electrolysis. It is
well documented that metals under mechanical stress will corrode much
faster than without the stress. The highest stress point on the anchor
rod would be right where it exited the concrete pier, and that's exactly
where I would expect to see the worst corrosion effects.
In any case, it is clear that rebar should never be allowed to penetrate
the concrete/soil interface, but NOWHERE does any code say that copper
should not, and in fact a high percentage of connections to residential
Ufer ground systems do exactly that.
Dave AB7E
On 7/18/2013 7:53 AM, Donald Chester wrote:
I agree that NOTHING should penetrate the concrete/soil interface. At a guy
anchor, make absolutely sure the anchor rod does not come anywhere near the
re-bar inside the concrete. It's not a matter of moisture creeping in, steam
cracking the concrete during a strike, etc. The problem is that if the re-bar
makes direct electrical contact with the soil, or indirectly through a copper
wire or other conductor, galvanic action sets up. IOW, a crude battery is
formed with the re-bar acting as one of the electrodes and the soil the
electrolyte. This causes corrosion of the re-bar; the corroded re-bar swells
and has nowhere to go but to crack the concrete as it expands.
I once watched a documentary on PBS about a problem with the restoration of the
Parthenon in Athens, completed early in the 20th century. A few decades later,
some of the stones began to mysteriously crack. They figured out that the
problem was steel anchor pins that were used when they re-stacked the stone
blocks. Marble is porous enough to allow moisture to creep into the stone and
in contact with the steel pins, which rusted, and the resultant swelling caused
the stones to crack. They had to go over the entire structure with an x-ray
machine, and remove all the steel pins they could find in order to arrest the
deterioration.
I have seen the effect of electrolysis at the guy anchor at a broadcast tower;
the anchor rod literally rusted in two right at the exit point from the
concrete pier and caused the top half of the tower to fail. (This was a great
benefit to me because I inherited the old base insulator when they replaced the
tower, but I'm sure their insurance company felt differently).
Lightning pulses have an extremely short rise time, so they should be treated
as RF, not the same as DC or 60 Hz a.c. The best lightning ground is not
several rods driven into the ground, but a radial system similar to what you
would use with a vertical antenna. A dozen or so 20-ft. buried radials using #8
wire or larger would be far superior to one or more 8' ground rods. Ground rods
make a good a.c. safety ground, but the best safety ground does not necessarily
make the best lightning ground, and an excellent lightning ground might not
pass a NEC inspection for a.c. power service.
My 160m vertical, 127' of Rohn 25, is mounted on that base insulator I got from
the BC station. I have 120 quarter-wave radials (133' each) made of #12 bare
soft-drawn copper wire, buried a couple of inches below the surface. I know
this was overkill, but I had the wire and at the time (30 years ago) I thought
this was absolutely necessary for minimum ground losses, although since then I
have read enough on the subject to realise I could have used half that many
radials and the additional loss would have negligible. I could have used the
rest of the wire for another radial system for another tower, or additional
verticals for 80 and 40m. I did sink 3 or 4 ground rods around the base pier to
protect the tower until I was able get the radial system in the ground. In 30
years since then, the tower has sustained several direct hits, and the only
damage has been some busted guy wire insulators and destruction of the end
insulators of an 80m dipole I have near the top of the towe
r.
No damage whatever to the tuning units at the base of the tower, or any of
the equipment in the shack, which is 140' away. I use the original BC tower
lightning gap across the base insulator and ground directly to the tower, via a
knife switch, the feed line that runs up to the dipole, whenever I leave the
place unattended or I suspect there may be a threat of a thunderstorm.
Incidentally, I have periodically inspected the radials, and after 30 years
buried in the soil, they show very little deterioration of the copper wire.
Don k4kyv
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