>From: Richards <firstname.lastname@example.org>
>Sent: May 29, 2008 9:29 AM
>To: Skip K3CC <email@example.com>
>Subject: Re: [TowerTalk] Tower grounding
>So... Is the rebar in the concrete to be kept separate
>from the tower metal? It merely reinforces the concrete
NEC 2005 requires that rebar be connected to the grounding system. If your
grounding system has lower impedance, it will carry most of the current.
>Thanks for the clarification. // Richards - K8JHR //
>Skip K3CC wrote:
> The water boiled in the concrete from the heat and exploded the
>> concrete bases.
if there is enough water between rebar and concrete to boil and cause spalling,
that implies that
the rebar has corroded, which implies incorrect installation of the rebar. For
instance, penetration below grade or where water can pool.
There's been a fair amount of testing and literature on this. There ARE
anecdotal examples of spalling (I recall see ing pictures of a ski lift tower
base, for instance), but it's not clear that there wasn't some sort of
installation problem (in the ski lift tower, the concrete had cracks in it
before the lightning strike, providing a path for water entry). Not to say
that spalling from lightning strikes doesn't occur, but there's a fairly good
theoretical and practical backup for the requirements for Ufer style grounds.
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