Here is an excerpt from PolyPhaser's technical document titled Document
Number TD1028: An interconnected concrete tower base can help the tower
Concrete is a fair conductor and can be used safely and effectively to
augment the tower grounding system. The concrete's ability to quickly absorb
moisture and release it slowly over a long period of time makes this
possible. The pH of the released moisture in turn enhances the conductivity
of the surrounding soil.
It is a common misconception to think that a lightning strike will blow up a
concrete pad. However, consider first, a myth-perpetuating case of an
improperly designed earth ground system where the tower leg "J"-bolts are
imbedded directly into the concrete pad. In this case, due to the poor
nature of the tower ground system, each of these J-bolts will actually share
a significant amount of strike current which in turn will flow through the
concrete. Since the surface area interface between the J-bolts and the
concrete is small, and the surge current density from the strike very large,
the corresponding heat generated by the energy transfer can turn the
concrete moisture into steam and possibly crack the pad. We have only seen
this happen once on a mountain top in the Nevada desert. However, a few
poorly implemented occurrences can give a valuable technique a bad
[mailto:firstname.lastname@example.org] On Behalf Of David Robbins K1TTT
Sent: Wednesday, January 26, 2005 3:05 PM
To: 'Tower Talk List'
Subject: RE: [TowerTalk] Conductive Concrete and Grounding
> >It *will* cause concrete to explode exactly like wood, trees, etc., i.e.
> the sudden vaporization of the moisture within causing a violent
> expansion thereof.
> A popular belief but not backed up with any evidence to my knowledge.
Where I work we have blown chunks out of concrete utility poles we were
testing with artificial lightning. And it is common for chimneys that are
hit to suffer damage. I have also heard of pieces being blown out of
concrete footings under high voltage power lines, but there is some question
of the cause... the energy dumped into the ground from the follow on current
after flashing over an insulator can be much higher than that from the
lightning that actually caused the flashover.
I don't have the conductivity figures handy but they are out on the web
somewhere, google for concrete conductivity and you should find it. But in
general it is better than poor soil or rock, enough so that it is not
uncommon to encase ground rods in concrete to improve conductivity in really
bad cases. There are also modified concrete formulas using carbon fiber or
fly ash that are used for improving grounds... and also for things like
resistively heated sidewalks and runways... just pour the concrete with some
electrodes in it and pump through a bunch of current, instant hot concrete
and bye bye ice and snow.
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
TowerTalk mailing list
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list