kr7x@comcast.net wrote:
> Jim / Paul / Doug and list:
>
> I have been holding back from responding to this thread as I have
> responded to this idea in the past, but I just couldn't hold back any
> more.
>
> The exploding concrete concept is mostly myth not fact. If you look
> at the referenced fig. 4 in the cited resource you see the vertical
> crack at the surface of the concrete pier. There is no spalling or
> other evidence of exploding concrete in this photo. My take is that
> this appears more like a freeze-thaw crack given the mountain-top
> location.
That's kind of what it looked like to me, but I'm not a foundations and
concrete kind of guy.
If indeed the water in the concrete flashed to steam ( a
> big stretch) the water was there due to the crack already being
> there. Concrete does not have free-water pockets incorporated in it
> if properly placed and consolidated. The water is used up in the
> chemical reaction of the concrete curing process. There is some free
> water in the micro-pores of the concrete-sand-rock matrix, but in
> miniscule amounts. If the crack is 1/8" wide then it is significant
> but to attribute it to a lighting strike with lack of before / after
> data is bad science.
There IS a report out there talking about problems with lightning scale
discharges in conductors embedded in concrete, BUT, in that case, the
bolt (which protruded from the concrete) had been improperly installed,
so there was a gap between bolt and concrete into which water
infiltrated, causing significant corrosion of the bolt, enlarging the
gap. There was a high current fault of long duration (I seem to recall
it was an arc from line to the structure attached to the bolt, started
by a lightning impulse on the line, and the breaker was slow in
opening). The paper showed pretty characteristic spalling (cone/wedge
shaped chunks blown away from the bolt) and they had sawn sections of
the concrete further in where the gap between bolts and concrete was
obvious. The theory was that someone had bumped the frame holding the
bolts in position when the concrete was partially cured. It was pushed
back to the correct position, and then they covered up the gap with some
grout when installing the structure over the bolts.
But this is more an example of why you should inspect things before
covering them up, and why "quick fixes" probably aren't the right
solution. ("Just slap some spackle on it to cover the hole" probably
isn't appropriate in a structural application)
>
> I have been engineering foundations for large power plants,
> manufacturing facilities, stacks, towers, etc. for almost 40 years.
> We have always used Ufer type grounding with an external ground grid
> and rods. All tied together with exothermic welded connections and
> 2/0 or 4/0 stranded ( heaven forbid! ) bare copper cable.
That matches what I've seen. In all the NASA facilities out at White
Sands Test Facility (across the mountains from the more famous Army
Missile Range) the lightning protection systems of all the buildings are
grounded with large stranded conductors, including the structural steel
frames. I can't imagine the hassle of trying to install 4/0 solid. The
engineer I talked to out there when I asked about why they had the
ground rod (considering there's 10 times the amount of copper connected
to the foundation, and who knows how many feet in the slab, and the
cable to the ground rod was larger in diameter than the rod) said that
it's because there's some policy that requires a ground rod, so they
just drive one in and hook it up to pass the inspection.
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