On 6/20/2013 12:38 PM, Hans Hammarquist wrote:
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.
Prolonged exposure to air and moisture are needed for corrosion. The
"chemicals" in the soil along with the PH, "May" accelerate the process
considerably. Dissimilar metals will accelerate the process as will a
complete circuit of tower, guy, anchor, moist soil, tower. It's "likely
adding an insulator to the guy near the anchor will greatly reduce the
corrosion.
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.
I'm not sure of the kind of fence you are referring to. I have run
miles of page wire fence with no concrete with no resulting post
corrosion. OTOH I've seen short runs of fence that were almost holding
the posts up. The bottom end of the posts was nearly gone. Again leading
me to believe that much depended on the makeup of the soil.
These were not wet or marshy areas. I've also run miles of electric
fence, but those are really just a bunch of isolated fence posts.
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.
Now I'm going to violate all the rules.
In the past, I've put up many short towers with dirt bases and nothing
more than steel fence posts (often with a second plate welded on a foot
or so above the original) driven into the ground so the guy wire formed
at least a 90 degree (or more) angle to the post. the guy was anchored
to the posts just above ground level (maybe an inch) with a saddle).
I had 5L 20 meter KLM, 6L 15 meter KLM, and a 7L Wilson on these 40 foot
towers.
Remember the load on the guys for a 40 foot tower are quite a bit less
than a 50 footer and far less than a 60 footer.
Guying requirements go up rapidly with height.
As for the tower base on a guyed tower, it only serves two rather easy
functions. It provides support to keep the tower from sinking into the
ground and to keep the base from sliding sideways. Something easily
accomplished with a good dirt base even for a 60 foot, guyed tower IF
the soil is up to it. I never used concrete on a tower base until I put
up the 90 footer and I doubt it was more than about 5 bags of Quick
Creet. IE, Not much!
73
Roger (K8RI)
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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