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
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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.
Go to local concrete supply place- they have premade plastic forms that look like an upside-down cone.Dig the hole, put the form in, stick a little rebar in, fill with concrete, insert post base, done. The form stays in the hole. Lots less work than building a form, and with the price of plywood these days, probably about as cheap. Still gotta patch the hole, though.
I'm no tennis expert, but isn't most parking-grade asphalt a lousy court surface? Not flat, not smooth, etc. The balls will bounce funny, etc. Or are you going to put a sand bed and astro-turf/rubber mat surface above it?
Is there a reason you can't use the hole as the form? I'd use a Dingo with an 18" auger, then switch to a 12" auger and/or a post hole digger to finish. That avoids all the problems associated with trying to backfill, compact the soil, and fix the asphalt (and then fix it again after the disturbed dirt settles).
If you really want to have fun, you can get a cement mixer attachment for the dingo. It'll mix and pour 200lb (dry) batches.
Yeah, I'm no expert either, and this is at least a start. I don't know how much use it will actually get; time will tell. The most experienced person in my family has only 2 years of high school tennis experience. But I understand that people do commonly use asphalt, and the more rigorous people put something on top, such as some kind of sanded latex coating or other surfaces.
You don't need a form if you are digging a hole in the ground. I'd just auger a 30" hole straight down and then fill it with concrete. Having a slightly bigger top is not going to be an issue. Just set the posts a little towards the insides.
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.
From what I have heard, it is to keep the concrete from pushing up during freezing weather. That's also why it recommended to dig holes below the frost line.
Surface area against the ground to counteract the tension force (induced moment arm) at the top from the net. I don't have a computation nor reference at hand but it's a non-trivial force...
This is yet another construction detail concerning net and post. Resoning given for shape of concrete in footing: john Section II.L. - Net and Net Post Equipment
1.0 Post Foundations Post foundations should be not less than 18" in diameter at the top, not less than 30" at the bottom, and not less than 42" in depth. An extended concrete base at the bottom of the foundation, shaped like a foot pointing in the direction of the opposing net post, will increase the foundation's resistance to stress and strain of torque in the direction of force. Foundations should be so constructed as to provide a distance of 33' on a singles court and 42' on a doubles court, measured from center of post to center of post. Concrete for foundations should use well-graded rock, gravel or stone mixed in ratios attaining a compressive strength of not less than
3,500 lbs. per square inch at the 28th day after pouring. For asphalt courts, the top of the concrete foundation should be round to prevent radial cracking.
2.0 Net Posts and Sleeves Net posts may be galvanized steel or aluminum. They may be installed in sleeves or installed permanently in foundations. Tennis post ground sleeves may be steel, aluminum or PVC. Circular posts should have an outside diameter of not less than 2 - 7/8", nor greater than 6", while square posts should not be less than 3" across, nor greater than 6". The net post shall project no more than 1" above the top of the net cord. Minimum yield strength is 1,100 lbs., with a minimum of 1,500 lbs. tensile strength. Mechanical tensioning devices (worm gear, ratchet reel, or screw-type) are to be limited in the amount of force applied to the net post, not to exceed
1/2 post yield strength. Posts and post sleeves should be set 42' apart for a doubles court, measured from the center of one post to the center of the other. For tournament use, it is recommended that a second set of net post sleeves be supplied 33' (center to center) apart for singles play. Posts should be set plumb and true so as to support the net at a height of 42" above the court surface.
3.0 Center Strap Anchor The ground anchor should be made from a strong, non-corrosive metal pipe not less than 10" in length, 1 5/8" o.d. minimum.
A non-corrosive 1/4" o.d. pin is centered through the pipe 1/4" to
3/8" below the opening for the purposes of attaching a center strap hook.
A center strap anchor should be set in concrete footings measuring 12" x 12" x 12". The base of the footing should be slightly larger (15" x 15") to avoid the possibility of heaving due to freeze/thaw action. The top of a concrete footing set in an asphalt court should be round to minimize radical cracking. The cross pin in the ground anchor should be flush with the court and parallel to the net.
Tapering or haunching a foundation is common for all sorts of foundations. It's like adding a diagonal brace underground and makes it harder to pull out (like a dovetail joint).
After taping big pieces of cardboard together to make a fullsize mockup of the spec concrete footing (3 1/2 foot tall pyramid), I can see it'd be quite an impressive mass that would do all the things you have all mentioned. Thanks. And be a huge job. So now I'm looking at using a guy wire to the top of the post to provide the resistance to being pulled in (as volleyball nets and other applications do), instead of that giant buried hunk of concrete. (I also printed out a number of US patents for alternatives to buried footings for sports nets. Some interesting ideas these people have had.) By modifying an Oz-Post spike (a nice system for putting up fences and decks without digging footing holes; I did a 600 foot wood fence with these a few years ago), I might even be able to do this whole job with minimal, if any, actual digging. The weight of the tennis post itself and half the net is pretty small, less than 50 pounds, though if I use a guy wire, that will end up pulling down on the post as well, adding to the apparent "weight" needing to be supported. The angle of the guy wire (its length to some kind of stake) determines this. The longer, the less downpull, but I don't need people tripping over guy wires in my yard. I'm mocking this up as well to see if guy wires can be lived with.
If you make the bottom wider than the top, you have something of an anchor to prevent frost heave. It can't push out of the ground. I was a little confused by the "upside down cone" mentioned earlier. I believe you want to put it in "right side up" (wider base at the bottom). That way when the ground shifts, it won't tilt or move. But, as far as forms are concerned, I agree with the other poster that they may not be necessary. If you dig a deep enough hole with a wide base, it should be stable. Leaving forms in place after you pour the concrete, or even having smooth sides on the concrete from forming, may actually allow greater movement. Irregularities in the concrete might help it to grip the earth better. Plus, if you are going to to remove forms, you would have disturbed soil around it, which is less stable for the post to anchor.
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