> 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
As for the water in the crystalline structure, I doubt that it is of
sufficient volume to make an appreciable amount of steam. Even if you do
heat concrete up to a temperature high enough to dissociate the
molecules (which is unlikely, since you've got a lot of mass there.. see
previous calculations for copper wire/strap.. concrete's not all that
different in terms of specific heat), the water molecules would be in
the interstices of the matrix, not collected in the form of liquid water
which can then boil.
Also, the decomposition temperature for concrete is quite high
(thousands of degrees?) and I think the rebar would melt before there
was significant decomposition. Concrete is basically artificial rock,
lightning strikes will fuse sand/soil (see, e.g., fulgurites), but it's
more of a sintering process than turning rock into lava.. You can stand
up on top of Half Dome or Mt Whitney, both places that have had numerous
lightning strikes, and I'd venture that you'll see no evidence of those
strikes on the rock itself.
Yes, you can take a torch to concrete and it will crack and spall. Been
there, done that, soldering copper pipe on the garage floor as a kid.
Heck, build a big hot fire on concrete (or rock) and it will do this.
However, it's more likely from the thermal stresses, than from "boiling
water". The classic turquoise mining process of building a fire against
the rock face and then throwing water on it is relying on the thermal
shock to fracture the rock, not the water flashing to steam.
This is also not to say that it's inconceivable that a lightning stroke
could cause a steam explosion in rock or concrete. It's just unlikely in
a properly constructed concrete structure. Steam explosions do occur in
things like trees, because there's an actual water bearing layer right
below the bark. It's like it was designed for the purpose. Likewise, a
suitably fine conductor (AWG 30 or so) will explode (and destroy
whatever the wire is inside) with a fast 10-100 kA pulse, but that
depends on the energy being greater than the energy required to melt and
boil the material in the wire, and that just isn't going to happen with
something AWG 12 or bigger.
use a knife or thin saw to slit a 2x4 3 feet long and lay a 30AWG wire
in the groove. Charge a low inductance 30 uF capacitor to 20 kV or so,
and discharge it through the wire. That's an explosion.. the wood will
shatter. The discharge is probably around 50kA in 20 microseconds or so,
which isn't far off from a lightning stroke. Do the same with AWG 12
wire.. the wire melts in pieces, but doesn't explode. Cast it in
concrete and I'll bet it's the same. I've done with plaster of paris,
which is similar to concrete in the sense of it hardening from a
chemical reaction with the water.
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