----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "N7DF" <n7df@yahoo.com>
Cc: <towertalk@contesting.com>
Sent: Saturday, September 15, 2007 7:38 AM
Subject: Re: [TowerTalk] From the perspective of a concrete technician
N7DF wrote:
> I guess I might as well jump in here on the subject of concrete exploding
> under lightning stresses.
>
> First of all I am a certified concrete technician with nearly 30 years
> experience and have designed numerous concrete structures, including tower
> bases and water tank foundations.
>
> Concrete is a hydrated crystalline material. The crystal structure
> combines several inorganic elements with water molecules. When concrete
> hardens it does not “dry” it becomes rigid as the crystal structure forms.
> None of the water in the concrete mixture is lost. The crystallization
> process is very slow. About 60% will be completed in a week and nearly
> 90% in a month. Theoretically it never reaches completion.
>
> The point is, concrete contains water; a lot of it, in the crystal
> structure. It also contains metallic elements. When a sufficiently
> powerful electric field is imposed on the crystal structure it begins to
> break down and becomes conductive. The water molecules in its structure
> begin to be released and will flash into water vapor. As one person
> stated, this can happen in 40 year old concrete that is as dry as a bone.
>
> The final analysis is that you must not, under any circumstance, permit
> an electric current to run through the concrete. Even if a good conductor
> is embedded in the concrete, a lightning strike will cause current to flow
> in the concrete immediately in contact with the conductor and the
> degradation process will take place.
>
> Ground rods and conductors should always be placed outside the concrete
> foundation. QED
>
"This all sounds interesting, but is not borne out by theory, analysis,
or actual experience. Surely you are aware of the concrete used in
electric railways which is specifically designed to have high
conductivity? Oddly, electric railways also use concrete with especially
low conductivity. Sort of depends on the specific application, I suppose.
And, of course, your statement that currents must not be allowed to flow
in concrete is entirely at a variance with the almost universal
*requirement* in modern building codes for concrete encased grounding
electrodes."
Jim, Larry, et al:
I haven't followed this entire thread, so I apologize if this has already
been
discussed. Is there anything in the code about electrode surface area in
contact with the concrete? I mean if you put enough current density
through any conductor or semicondutor it will suffer damage, but if you
make the electrode surface area large enough, the current density won't
ever get high enough to damage anything. Seems to me that the encased
electode in a UFER ground would tend to have a pretty large surface
area.
73 Mike, W4EF....................
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