Have a look at this thread on QRZ.com.
http://www.qrz.com/ib-bin/ikonboard.cgi?s=4841084fd90540b5a1aad0c9f7dc45fc;act=ST;f=7;t=152206;st=0
MY RESPONSE:
"Everywhere I have ever read anything about steel reinforced concrete, it
has stated that in buried concrete piers the re-bar should have a minimum
cover of concrete, and that it is a real no-no for exposed re-bar to come in
contact with the earth.
The problem is electrolysis caused by chemicals in the soil, moisture
absorbed by the concrete and reaction with the exposed metal bar. The
feeble electric current generated by electrolysis may eventually set up
corrosion, causing the re-bar to swell and weaken the concrete.
Intentionally making electrical contact between the re-bar cage and a ground
rod would do exactly the same thing.
I would use ground rods isolated from the re-bar in the concrete pier. For
a normal ham type tower, the Rohn book recommends a minimum of 2" concrete
cover over the re-bar."
Don k4kyv
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