[TowerTalk] Conductive Concrete and Grounding

[Jim Lux]

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
> >is?
>
>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,
>explode.

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