[TowerTalk] rebar

[Jim Lux]

-----Original Message-----
>From: jim Jarvis <jimjarvis at optonline.net>
>Sent: May 29, 2008 1:35 PM
>To: towertalk at contesting.com
>Subject: Re: [TowerTalk] rebar
>
>We appear to be converging on some agreement:
>
>It was my impression that most concrete cracking initiated by  
>lightning strikes
>was caused by explosively expanding steam, from the water in the  
>concrete.
>This can happen even when a rebar cage is tied together and grounded
>reasonably well.

The failure analysis literature says that this only happens if there's water between rebar and concrete, not just water absorbed into the concrete.  Mind you, in my youth, I've spalled concrete in my parent's garage by building a (very) hot fire on top of the surface (much to their dismay).  But that was the layer of finish coat on top of the lower concrete, and it was a very hot, very localized heat (not quite thermite, but close), so the thermal stresses were very high.  I could see surface spalling occurring from lightning traveling over the surface of the concrete.  Typical dissipation per unit length for a stroke in air is around 100kJ/meter, and I would expect a similar sort of dissipation for a surface/creeping discharge.  On the other hand, this would be surface spalling, not wholesale fracture.