Hello Brian,
A couple of thoughts..
I prefer to use to use forms that I locally construct out of
construction lumber,
2 x4's and 1/2" exterior plywood, I individually construct each of the 4
panels
with screws, but I assemble the 4 panels together with nails so I can
take apart
after initial concrete set up. I use ROHN stock so I need 2 sets of
forms, one
for a guy anchor, and the other for the base. I use a can of motor oil
painted on the
wooden forms on the concrete side, to help the cement to not stick.
If you use forms, the exact amount of concrete can be calculated and
delivered.
If you do not use forms and prefer to just pour cement in a hole, it may
be hard
to estimate how much cement should be delivered and paid for. at $55-65
a yard
for premix, cost may be a consideration.
Free standing tower usually take BIG bases that can be DEEP.. Be sure to
observe
the manufacturers recommendation. Depending on soil conditions, the hole
sides
may not be solid enough to support a form-less pour. You do not want to
contaminate the cement pour with the sides collapsing into the cement,
substantially
weakening the base.
Do not forget to use the mfg's recomendations of using steel re-bar.,
and how it is placed.
After I dig a hole, I level the bottom and compact it with a 20 lb
sledge used in a
vertical motion with the handle vertical. after compacting I level the
hole bottom again.
Then I set the fabricated form onto the compacted bottom and backfill
about a foot
to hold the form in place, Next, I check the form level on the top and
sides and
move around as need to level. if you back fill too much, the forms will
be harder
to remove after the cement sets up. depending on any number of
variables, additional
bracing may be needed between the hole sides and the outside of the
forms.
Cement gets heavy in a hurry, the form MUST be held rigid during the
pour,
and not buckle or bend the form, which may break the form.
If you do not compact the soil underneath the cement, the weight of the
cement
over time will compact the earth underneath, moving the base out of
plumb which is a BAD condition.
Be careful to keep the re-bar at least 3" from the forms during the
pour.
Move around as neccessary. Re-bar directly exposed to the surrounding
moist
soil will rust and weaken over time. Rust dimensionally expands breaking
off peices of the cured cement, exposing more re-bar to the moisture to
rust.
After the cement is set up, firm to the touch and about 4 plus hours
after
the pour, the form can be removed. I am usually so pooped that I head
for the cooler and wait till the next day to remove the forms.
I always remove the form for the next job. If you leave it buried it
will decompose
as any organic debris will and may allow the base to move out of plumb.
I would be cautious on using the hole sides as a form, unless soil
conditions
were not questionable. If the soil sides will not collapse, a formed
"cap" on the top
would be resonable.
As the cement is poured, pour it into the middle of the hole,
avoiding eroding the soil sides.
My 3 cents
Stan, WA1ECF Cape Cod, MA FN41sr
Brian Smithson wrote:
> Hi all!
>
> Forgive the bandwidth for a beginner's question...
>
> I'm planning a free-standing tower install at a QTH I haven't even
> found
> yet.
> (Moving from MA to SC). I've been studying the installations
> documented
> in various places on the internet, and I'm confused about building and
>
> handling the form for the base.
>
> It appears from the pictures I've seen that a big hole is dug, a form
> is
> built and suspended in the hole. The concrete is poured and the form
> is left in the hole and it is backfilled around the form. I've always
> heard
> that undesturbed earth is the best thing to support something like
> this,
> not backfilled.
>
> What if the form were 'short' - just a couple of feet to square off
> the top
> (visible) part and the rest of the concrete was poured against the
> bare
> undesturbed earth below? Then I'd only be backfilling the top couple
> of
> feet to neaten it up. Is the chunk so big that it doesn't really
> matter? Seems like a way to make the form smaller and easier to
> handle.
>
> What do you think?
>
> Tnx es 73!
>
> Brian Smithson, N8WRL
> brian_smithson@vds.com <mailto:brian_smithson@vds.com>
>
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