On Wed, 2 Jun 2010 19:54:53 -0700 (PDT), hillpc
wrote:

I'm making a home tennis court out of an asphalt paved area. I've
already bought the net and posts. The specified concrete footing for
each tennis net post is a 30" square at the bottom, tapering up over a
height of 42" into an 18" diameter circle at the top. A drawing is at
http://sportsbuilders.org/page.php?id=96&from[]=11&from[]=12&from[]=13&.
Could I use 3/4" plywood to make a form strong enough to contain this
much concrete without blowing out or breaking? I think the bigger job
may actually be cutting out a big enough square out of the asphalt
(32"?) so I can then rent a 30" auger to dig 42" deep, cut the corners
out to a square with a shovel, and then drop the empty form in, and
somehow hold it down while pouring the roughly ton of concrete into
each one. After stripping the form, backfill with dirt and repair the
asphalt. Any suggestions for me? I'm thinking readymix concrete for
the little over 1 cubic yard I need to avoid all that mixing, and it's
probably cheaper than buying eighty 80" bags of concrete mix.

As others say, I think you are overdoing it... The instructions (your
URL required some work to make it valid...) tell you what ideal should
be, but in all truth, if you followed their instructions the only
ideal thing you'd have was the cement. The rest of the court would be
questionable (flatness, quality of the surface, etc.)

I'd dig a cone shaped hole as they describe, 42" is not that bad. In
one similar job I did, I then took a heavy steel pipe (1 1/2", IIRC)
and pounded it into the earth at an angle towards the other side post.
That stiffened the entire thing so it never ever moved.