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Re: [TowerTalk] rebar

Subject: Re: [TowerTalk] rebar
From: jim Jarvis <>
Date: Thu, 29 May 2008 16:35:17 -0400
List-post: <">>
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  
This can happen even when a rebar cage is tied together and grounded
reasonably well.

In the case of the two foundations which I saw fail, both had  
properly installed
RF and dissipative ground systems around the foundation.   They STILL  
wound up
with a conductive path through the foundation.   In one case, it  
split the 4x4' base
almost cleanly in half, leaving a 1" scorched gap.

In both cases, they were insulated towers with pin-pier bases, 120  
radials, and 6" copper
strap from the lightning ball-gap, around the foundation, to the  
ground system.  Both were
also illuminated, using RF isolation transformers to provide the 110  
for lighting.

I hadn't considered rebar oxidation-expansion which Dave mentions.    
Although my notion
of the increased IR losses with time and corrosion would include that  

I no longer recall what I posted, and what was back-channel private  
chat.   My preference is
for lightning ground around the slab, not through it... and visible,  
so I can maintain it.   If I were
dropping rohn 25 or 45 into a hole,  I'd probably do both... provide  
down-bottom grounding
before pouring the concrete,  and topside, to divert strike current  
away from the tower.
That's essentially what we did with the WMGQ-FM 500' self-supporter.



quoth  Dave   AB7E:

I could be wrong, but I don't think it is the water infiltration by
itself that causes the problem.  Concrete is inherently porous and there
are typically thousands of microcracks in it anyway.  In the open, steel
progressively rusts in the presence of moisture because iron oxide
requires a significantly greater volume than the iron beneath it,
causing the oxide flakes away from the iron which then exposes more bare
iron.  Some oxides are somewhat self-limiting, but iron oxide isn't one
of them.  When contained within  an envelope of concrete, however, rebar
is semi-protected from rusting by the high alkalinity of the cement.  I
believe the problem with the larger cracks is that where they reach the
rebar, the steel is no longer "protected" by the alkalinity of the
cement at that spot and the steel is free to rust and expand, making the
problem progressive.

Jim Lux wrote:

> Somewhere in between, you'll have a failed structure that is  
> structurally OK today, but won't have the life or longevity in the  
> future.  That's the situation where water infiltrates and causes  
> corrosion of the rebar, then, perhaps lightning induced spalling.
> Jim, W6RMK


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