Michael Tope wrote:
>
> ----- 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.
The NEC code requires a 20 foot AWG 4 conductor, which isn't a huge
surface area, but that's an electrical safety ground requirement, and
probably only needs to handle a few hundred amps. For lightning
protection, the conductors would need to be larger. Ufer's original
bomb storage igloos had an extensive welded rebar cage with 1/2" rebar
on 12" centers, so there's a fairly large contact area.
However, the NEC ground is about 150 square inches of surface area, so
with a very large 100kA discharge, the current density would be about
700 Amps/square inch. That's not all that huge (compare it to the
current density in something like an appliance plug, where you have a 15
Amp current in a small fraction of a square inch.. or the contact area
in the "push-in" terminal on the back of a receptacle or wall switch)
In a lightning protection installation, you'd have more surface area
(considering they use 2/0 or 4/0 cabling instead of the relatively wimpy
AWG 4).
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