Steve, John & all,
Thanks for all the reminders and suggestions. In my case the forms are
required because I'm doing a pad and pier (sort of a 3D inverted T)
foundation. I tried digging it (so I wouldn't need forms or backfilling) by
undercutting the earth after first digging the 5'x5'x5' hole but anything
over a 1' undercut became very difficult. As I needed a 2' undercut (the
pad is 9'x9'), it just couldn't be done. I ended up digging out the entire
9'x9'x5' hole. The 9'x9'x1.5' pad, of course, will be poured up against the
dirt at the bottom of the hole but the 5'x5'x3.5' pier will be formed. It
will be done in a single pour. After the forms are removed and the concrete
cured for a couple weeks (my permit requires the standard four weeks before
tower assembly) I will be backfilling with a commercial backfill mix in the
first 3' of the hole. The backfill will be added in 6" lifts, watered and
machine compacted between each lift. The last lift will be native soil with
a little sand and compost added so the XYL can plant some groundcover around
the tower base. I always cover my concrete and keep it continuously wet for
a week or more depending on thickness. This will be my biggest pour so far.
Previously it was a 3' diameter by 7' deep cylindrical tower foundation that
was poured up against dirt except the top 6" or so, which were formed with
Sonotube. That base came out real nice after grinding off the sharp edge.
Not a single crack or craze of any kind.
A question for you concrete pros: how long should I leave the forms on and
how long should I wait before I backfill against the pier?
Thanks & 73 - JC, K0HPS
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of KA9OFM John Garrett
Sent: Monday, July 31, 2006 8:24 AM
To: jc-smith@comcast.net; K7LXC@aol.com; towertalk@contesting.com
Subject: Re: [TowerTalk] 80' tower in SF Bay area (update)
Why Use Forms? A lot of people ask that question but I can tell you why I
use them when I pour a tower base. The Earth sucks the moisture from the
concrete and causes it to have weak spots becase it didn't dry evenly. If
you use forms the concrete drys slowly as it should and you won't end up
with cracks later on, or at least you lower the possibility of having cracks
later on. This is provided that you have a good reinforcement inside the
concrete.
Drying time should be at leat 2 days but I always give a tower base 1 week
before I put anything up. The thickness of a base will take longer for the
center to dry so the extra time is ample to assure that you don't start
forming cracks somewhere in your new base.
John, KA9OFM
On 7/31/06, K7LXC@aol.com <K7LXC@aol.com> wrote:
>
>
> In a message dated 7/29/2006 2:56:53 P.M. Pacific Daylight Time,
> towertalk-request@contesting.com writes:
>
> > Today I'm assembling the tower base and purchasing rebar, dobies, wire
> ties,
> etc. and plywood for the forms
>
>
> What are the plywood forms for? Typically, the concrete in the
> ground should be poured against undisturbed earth. If you've got
> sloughing, then
> you might need the forms but they're tricky to backfill and tamp.
>
> Cheers,
> Steve K7LXC
> TOWER TECH
> _______________________________________________
>
>
>
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> TowerTalk@contesting.com
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>
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