The only problem with that is that the tower base already represents a
very low impedance conductor embedded in the concrete. It is THE lowest
impedance conductor in the system. In the case of many towers (such as
the self-supporting one I will be installing), that low impedance
conductor goes almost to the bottom of the foundation. A significant
field will almost certainly be there inside the concrete during a
lightning strike. I'm not sure what kind of shunt grounding system would
be required to prevent any portion of the lightning strike from imposing
a major electric field within the concrete, but I'll bet it would be
impressive.
So in my mind, the question becomes how do you most benignly get the
resulting current out of the concrete. The Ufer seems to be an accepted
practice with good theory and decades of history behind it, and I still
don't understand why an assist from a few copper wires to ground is not
also a reasonable tactic.
73,
Dave AB7E
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
>
>
> ---------------------------------
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