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Re: [TowerTalk] Conductive Concrete and Grounding

To: Jim Brown <>,"Tower Talk List" <>
Subject: Re: [TowerTalk] Conductive Concrete and Grounding
From: Jim Lux <>
Date: Mon, 31 Jan 2005 10:08:48 -0800
List-post: <>
At 09:02 AM 1/31/2005, Jim Brown wrote:
On Wed, 26 Jan 2005 15:48:51 -0500, Tom Rauch wrote:

>Anyone know what the resistivity per unit volume concrete

In doing research for my white paper on power and grounding, I
learned that you can buy different types of concrete designed for
either very high or very low resistivities, depending on your
application. There are, for example, mixes for use as ties on
electric transit systems that have extremely high resistivity, and
mixes designed for use in Ufers that have very low resistivity. And
there's everything in between.

Probably, the most useful thing, then, is to know the properties of the typical residential or tower foundation concrete from the local ready-mix company. Probably not high resistivity (unless you live in an area where the aggregate happens to be high resistivity), and probably not highly conductive. I doubt that any ham is going to order up special concrete one way or another.

When I posed a question on the RFI list re: concrete and grounding,
Dale, WA9ENA, who is an EMC engineer at Rockwell-Collins, responded
with his research/study from sources (including AT&T) that concrete
that becomes part of the path of a lightning hit CAN, indeed,

Can you ask him for some specific references (reports, journal articles, etc.)?
Certainly there's a huge amount of "lore" about this, and even photographs of shattered concrete next to a bolt, but some well researched information that describes the circumstances under which damage can occur would be useful. For instance, the IEEE std for substation grounding (which is vastly more concerned with shunting huge, basically continuous (compared to lightning), fault currents and switching transients) talks about concrete damage in the context of corroded conductors in the concrete.

The question is not whether, under some unlikely set of circumstances, that lightning can cause damage to concrete, but whether, in the typical ham quality installation (which, by and large, is different, than say, a commercial broadcast station, or a munitions storage depot), those circumstances are more or less likely to occur.

We'd all love to have time, money, and ability to install thousands of feet of bare 2/0 grounding cables every six inches to produce the fabled "salt water marsh on top of a hill", but, I'd guess that the vast majority of hams have something like a 6 foot 1/2" diameter rod hammered into the ground (8 foot rod with 2 feet sticking out, when you hit a rock and decided that more hammering just wasn't worth it), and a rusty old hose clamp holding the AWG16 ground wire on.

The practical medium is clearly somewhere in between, and some reasonably unbiased, well reviewed research would help determine where the "point of diminishing returns" lies.

I think the bottom line is that any concrete that could become part
of the path of a hit be engineered so that it is a relatively high
resistance part of that path, and that any Ufers (intentional, low-Z
ground elements) NOT be part of a structural system.

Of course, various building codes and respected grounding standards differ from your opinion.

Jim Brown K9YC

Jim Lux, W6RMK


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