Hans...
In my experience this is not true. I have some photos of my failed
galvanized screw anchor which I can send you. The failure was a good 3+ feet
below the ground surface, however I expect the metal rod was probably
compromised along its entire length.
The property I am on was once a working farm. I occasionally find gears and
other metal parts from farm implements I believe to date from the
1920's-1930's. This iron or steel parts are very solid and heavily rusted,
and the rust protects it from deeper corrosion (think: ocean
liner)...galvanized anchors, on the other hand, are not the same compound.
(Metallurgists please chime in). I suspect that the steel underneath the
galvanizing may be a very soft/cheap material (keep the costs down no matter
what), and once the galvanizing is compromised it does not take long (15
years in my case) for the actual metal to be totally compromised.
What you describe for poles, fence posts, etc., is true, but I suspect we
are comparing apples and oranges. And again, we should not assume that all
soils are created the same. What I have here in west central Missouri is
probably much different than soils and ground water in other parts of the
country.
As always YMMV.
73
Dan
K0DAN
-----Original Message-----
From: Hans Hammarquist
Sent: June 20, 2013 11:38
To: towertalk@contesting.com
Subject: [TowerTalk] Fwd: Screw Anchor Experience
What I have seen, it is only the portion closest to the surface that
corrodes. I have found iron pieces buried for hundred of years with only
slight corrosion. Scrap iron, stored laying on the ground, has almost
disappeared in no time. I believe it is a combination of water, "dirt", and
air that is causing this.
When installing fence with steel poles, a concrete "plate" is made around
the pole just at the surface. It is sloped on the top to allow water
run-off. If you don't do that the pole will corrode at the surface and
eventually break there.
The rebar in concrete is "protected" by the concrete but will corrode if too
close to the surface. In the same way the fence pole is "protected" by the
concrete plate.
Maybe some concrete poured around anchor can extend the life-expectancy of
the anchor.
Next: What method or how would you inspect an anchor for corrosion damages?
Just digging around it?
Hans - N2JFS
-----Original Message-----
From: Mark Robinson <markrob@mindspring.com>
To: Jim Thomson <jim.thom@telus.net>; towertalk <towertalk@contesting.com>
Sent: Thu, Jun 20, 2013 11:36 am
Subject: Re: [TowerTalk] Screw Anchor Experience
Jim wrote
## why mess with screw in anchors ? Only power company’s + telco’s use
em. Even then
they use eggs in the EHS guys. A 40’ tall utlility pole is already 6
feet into the ground. No
anodes used.
## The rohn type rod anchors are in contact with the soil, where it is
trenched, so that portion
of the rod needs to be coated with roofing tar pitch. Those GAC series
anchor rods involve
excavating a huge hole, that is then trenched. The concrete slab needs to
be poured, then the
entire mess is bakfilled with dirt.
I reply...
They weren't screw anchors. They were the Rohn supplied anchors set in
concrete per the Rohn specifcations (as you describee) BUT they are still
vulnerable to corrosion. I have read dubious accounts of using tar on the
buried metal parts - the tar can fail and then the corrosion will be
concentrated in the failed areas
My opinion/guess is that the sacrificial anodes are a better way to go but
only time will prove me right or wrong.
Mark N1UK
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