> ----- Original Message ----- From: "Jim Lux" <firstname.lastname@example.org>
> To: "jeremy-ca" <email@example.com>
> Cc: <Greenacres113@aol.com>; "David Gilbert" <firstname.lastname@example.org>;
> "N7DF" <email@example.com>; <firstname.lastname@example.org>
> Sent: Saturday, September 15, 2007 5:34 PM
> Subject: Re: [TowerTalk] re; exploding concrete
>> jeremy-ca wrote:
>>> He is not talking about gaps. The water used to form concrete is
>>> absorbed by the cement and turns into a crystaline structure. It is
>>> that structure that is conductive to some degree that is affected by
>>> the heat generated by a stroke.
>> Indeed.. but water molecules that have been dissociated from the
>> concrete being heated ( a process that I believe is negligible in a
>> lightning heating context) or are just there because the concrete is
>> "damp" aren't going to turn into steam. Even if the concrete is damp,
>> there's no spaces for liquid water to form, and hence, no large
>> expansion on the phase change to gas. There's no phase change
>> occurring at all.. it's just water molecules distributed in the pores
>> of the material (adsorbed, if you will).
> Im skeptical about steam myself except for possibly very fresh concrete.
> Since everyone else appears to be making statements such as "I think, I
> dont think, I believe, it appears, maybe, etc, I'll add one of my own.
> Since it is an unarguable fact that a welders torch can cause concrete
> to fracture (shatter, explode, take your pick) I believe (suspect at
> least) that heat is the reason in a lightning strike. Since a welders
> torch takes very little time to cause a problem, and is most likely much
> faster than it takes to melt a copper rod, then it holds that the ground
> rod does not need to be melting. Whatever temperature is generated in
> the rod and transfered to the conductive concrete is all that is
> necessary. It also holds that not all strikes have the power required
> and all concretes are not created equal.
Except that if you calculate the temperature rise in a copper or steel
rod/wire, with the kinds of currents known to exist in a lightning
stroke, it's not all that much. It just doesn't get that hot.
The only mechanism that could get that sort of heat would be if there's
a gap, and an arc, and you get localized heating. If it's fast enough,
once the metal is soft, magnetic forces tend to cause it to pinch off,
potentially extending the arc (like with stick welding). (There's some
very cool demos of this at the Deutsches Museum in Munich where they run
60 kA or so through a bunch of copper cables in parallel.. and it's also
what makes stick welding transfer the metal in little droplets)
So what this means is that the important thing isn't necessarily gaps or
cracks in the concrete, but making sure that all the components in the
the ground system is well connected and doesn't have little gaps or hot
Oh yeah, and the other thing that could lead to certain destruction
would be if you coil the cable. Run a current through a coil, and it
tries to expand, and if the current is high enough, the forces will
exceed the tension strength of the wire and it will come apart. A
transient current in a coil can also cause things in or near the coil to
be squashed and deformed and destroyed. The can crusher and quarter
shrinker are nice, spectacular versions of an industrial process called
magneforming. It doesn't even take a kiloamp to rip an aluminum can apart.
> It remains for future engineers, techs, etc to improve their knowledge
> of all the interlocking causes and effects. Right now all of us on this
> thread are involved in guesswork to one degree or another.
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