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Re: [TowerTalk] From the perspective of a concrete technician

To: Michael Tope <>
Subject: Re: [TowerTalk] From the perspective of a concrete technician
From: Jim Lux <>
Date: Sat, 15 Sep 2007 14:44:56 -0700
List-post: <>
Michael Tope wrote:
> ----- Original Message ----- From: "Jim Lux" <>
> To: "N7DF" <>
> Cc: <>
> 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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