Some data from Concrete Society:
"Moist concrete behaves as an electrolyte with resistivity of up to 100
ohm-m. Air dried concrete has a resistivity in the order of 10 000
ohm-m, whilst oven-dry concrete has a resistivity in the order of 100
000 000 ohm-m." So isn't likely that wet concrete is so conductive the
energy dissipated is low and dry concrete is so dry it can't make steam
or conduct much current? Also consider that concrete is sufficiently
conductive that NEC now requires all residential AC outlets over bare
concrete floors to be GFI protected.
As a composite material then consider how the rebar matrix reduces the
point to point conductivity much more. (I'll leave it to others to
figure that out). Then consider how much area is in a Ufer foundation in
contact with earth. In my case, about 250 sq ft for a tower base and
720 sq ft for a foundation. Both are wet year round. Compare to a 10'
5/8" ground rod at 1.6 sq ft of area. So the relative current density
at the earth interface is 2+ orders of magnitude less for my tower base,
assuming the current is uniformly distributed by the rebar matrix (which
it isn't since rebar has inductance, but there is a lot in parallel).
I think one idea of a Ufer is to distribute (20' minimum length buried
conductor) over enough volume in contact with the rebar so as to prevent
very high current densities. Codes or policies that require Ufer's
(grounded rebar) for power plants (IEEE), ammunition bunkers (DOD), and
explosive storage (ATF) facilities (& NFPA) were convincing to me of the
robustness of the engineering. see
http://www.wbdg.org/FFC/DOD/UFC/ufc_4_420_01_2015.pdf as a publicly
available spec. Some interesting requirements are specified. Bond
everything together.
So, I'm also a skeptic about exploding concrete being common or a
problem to worry about. I'd like to see a forensic analysis of
fractured concrete that proved lightning was the cause, rather than salt
water corrosion or improper grounding/construction or urban myth.
Grant KZ1W
On 6/26/2017 11:31 AM, Patrick Greenlee wrote:
There seems to be an urban myth in popular circulation about
foundations being subject to being blown to bits or at least cracked
open sufficiently for them to fail due to lightning. If this were a
real threat wouldn't it be common experience with Ufer grounds? Can
anyone provide a reference to a properly documented incident where
lightning blew apart a foundation?
I'd be happy to abandon my current thinking and get on board with the
lightning blows up concrete folks it there were sufficient factual
evidence.
Patrick NJ5G
On 6/26/17 11:23 AM, Clif Keely via TowerTalk wrote:
Reading through some of the comments here recently have me trying to
recall some of the comments I have read over the years. I seem to
recall several that spoke to using 2 or 3 ground rods on a tower
which might not be a bad idea. I should think one of the reasons
would be to keep the discharge energy from making steam within the
concrete base and that explosive burst of steam, producing a lot of
cracking within the block and with that reducing it's ability as a
solid base. I have no research to support this but offer it only as
a thought. For myself if I hear thunder I disconnect antenna and
power cables. After that I keep my fingers crossed as I think
lightning will do what ever it bloody well wishes.
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