I think you're a little off on this one, Steve.
In construction of all concrete framed buildings, pours separated by
weeks and having rebar in common are all over the place, and have to
be. They occur at the boundary of floors and columns.
The floor above cannot be poured until the one below has hardened to
the strength to carry the weight of the one above.
There are simple, common, well-documented steps that are always taken
at pour boundaries by professionals, no more mysterious than the
vibrator used to eliminate voids in the pour.
That it's not bandied about much on Towertalk merely reflects that
double pours are commonly not needed for amateur towers, not that
there's anything wrong with them.
The base for my Trylon was designed by a fellow that does building
foundations. It was done in two pours so that we could stand the base
and bottom section on a firm surface, shim it to vertical and tie the
base directly into rebar aligned to the desired orientation (one face
edge on to 45 degrees).
When the rest of the base was poured, the bottom section was securely
positioned by angled rebar across the square and fastened to the base
legs, no possibility of movement, and an automatic UFER ground.
The layers were joined by a generous coating of some nasty epoxy stuff
put down the morning of the second pour.
The base itself was an interesting 11 cubic yard over-engineered
design that forces the base to be lifted out of the ground and kick
sideways into undisturbed hardpan in order to be overturned. The tower
could be ripped off the base by a tornado, but the base is never going
anywhere. If I move, the base will be marketed as a pad for an
incinerator or large outdoor barbeque.
When I asked the guy about the double pour and seepage, he just looked
at me like I was crazy, and my contractor son-in-law gave me one of
those shut-up-before-you-embarrass-me looks.
Just because there's a wrong way to do something doesn't mean it
shouldn't be used.
73, Guy.
----- Original Message -----
From: <K7LXC@aol.com>
To: <wz7i@arrl.net>; <towertalk@contesting.com>
Sent: Sunday, December 01, 2002 11:04 AM
Subject: Re: [Towertalk] Pouring base of tower
> In a message dated 11/30/02 4:51:50 PM Pacific Standard Time,
wz7i@arrl.net
> writes:
>
> > I am planning the installation of a self-supporting tower. My
contractor
> > asked me a question to which I don't know the answer. Is there
any reason
> > why he cannot pour the base in two pours as long as they are tied
together
> > by the rebar cage? The plans call for the foundation to be 9 x 9
at the
> > base and then 5 x 5 at the surface. He would prefer to pour the
18" thick
> > 9 x 9 foot section before the 5 x 5 foot section. Any reason why
not?
>
> It should be in one pour so that the base is totally bonded
together. If
> the bottom pour has 'gone off' and hardened, you wind up with two
separate
> layers.
>
> Sometimes there are two pours if you have to have two trucks
worth of
> concrete but they are poured consecutively with little or no time
delay.
>
> Are you looking at something like a pedestal base where it's not
a cubic
> hole? Did you take the manufacturer's specs or have a PE design it?
Sometimes
> manufacturer's specs are pretty impractical (i.e. Trylon's undercut
base, US
> Tower's deep skinny holes) so it'd probably be easier if you could
> reconfigure it for simplicity's sake. The problem is that the bases
are
> designed by engineers sitting in an office somewhere and they never
have had
> to install one of their (stupid) designs.
>
> What kind of contractor wouldn't know the answer to your
question?
>
> Cheers,
> Steve K7LXC
> TOWER TECH
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