Hi Brian,
I have heard some of the same things you have heard about rebar cages but I
can't actually
say I have ever read this stuff in any Rohn book I have seen. I may have read
the stuff
about keeping the rebar 3 inches from the edges of the concrete . . . Part of
it makes
some sense since rebar is never galvanized (as far as I know) and Rohn tower
legs are
galvanized, which should make a rather large difference in how fast they
deteriorate due
to rusting. There are a lot of rumors and hear-say about what Rohn says or
does not say.
I guess if I wanted to take Rohn's advice on something, I would either want to
see it in
writing in one of their books or hear it, first-hand, from one of their
engineers on the
phone. Hear-say and second-hand quotes from Rohn are not really very
dependable . . . I
like to quote the page number of the Rohn book for anything I get out of that
source so
you could look it up yourself to verify its accuracy. In some cases, Rohn
simply won't
put some of their advice in writing so second-hand quoting is the only way to
pass on the
info . . . just make sure the secondary source is a reliable one, if you can.
If the rebar really rusts away inside the base, I would worry much more about
the loss in
strength by not having the steel in there anymore than I would about any void
it might
leave.
The major reason for keeping water out of the legs is not to keep them from
rusting
(although that's not a bad reason) but to prevent the legs from splitting like
frozen
water pipes if the water freezes in them. I KNOW this happens . . . I have
seen it. What
I don't know is what happens in an area with a very high water table that is
also very
cold in the winter . . . Could the tower legs freeze and split below the
ground surface
where the problem would be invisible to anyone inspecting the tower for safety?
I rather
doubt this would happen . . . It seems to me, it would be similar to tossing
a piece of
pipe in a pond that you expect to freeze solid. If it froze solid and then
thawed, would
the pipe be split? I suspect it would not because the pipe, in that case,
would be
experiencing freezing water both inside and outside the pipe at the same time.
The frozen
water surrounding the outside of the pipe may prevent it from splitting. Maybe
this is
how it would work with water inside (and outside) of tower legs due to a high
water
table. But then, this is only my "intuitive feeling" for how this might work,
and I am
very suspicious of my "intuition" in cases like this . . . !
Stan
w7ni@easystreet.com
alsopb wrote:
> This practice recommended by Rohn has me a bit puzzled. Rebar in the
> concrete has to stop 3" from the end of the concrete on all sides.
> The reason given is that they will rust, disintegrate and form a
> cavity which weakens the base.
>
> I guess Rohn believes their towers legs never rust-- inside or
> outside.
>
> Is that true? I agree with wanting to keep water out of the legs.
>
> Also, if the water table rises above the base of the bed of rocks,
> doesn't it guarantee water being in the tower legs?
>
> 73 de Brian/K3KO
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