Essentially, I'd like to put down a concrete pad for a patio, but leave a couple of circular areas within that patio unpaved (for planting shrubs).
I'm familiar with the techniques for using dimensional lumber for angular concrete pads, and even using plywood for curved ones, but is there a clever trick or something for creating say a 3' diameter unpaved area within a concrete pad aside from trying to bend plywood planks into a circle?
Metal landscape edging (the kind that's held in with stakes) should work fairly well. You could either just leave it there or cover it in plastic and remove it after the concrete sets (personally, I'd leave it).
This can be done using Bender Board. You should be able to find it in both wood and recycled plastic.
When using it to form concrete make sure you use enough stakes to hold the weight of the concrete.
What about a Rubbermaid container? The kind they have to hold ice and drinks/kegs seems about the size you want.
Use a upside down 5 gallon bucket, paint can, or anything that is deep enough to not get buried in the concrete. They use several old 5 gallon buckets leftover from construction in our basement.
A wooden barrel has a 21"-24" diameter at the head. You can usually buy a half barrel at a good nursery/garden center. The advantage is that you can easily dismantle it after the concrete has cured by knocking the staves inward.
I'd look for something else cheap that is about the correct size, cover it with plastic sheeting and use it to keep the concrete out. Ideally it would be something flexible (or breakable :-)) so you can get it out when you are done.
Why not first install some nice edging (of brick or otherwise) then concrete up to the edging.
John, you undoubtedly found a solution some years ago, but this kind of situation will likely surface again for other DIY's. I'd suggest considering using Sonotubes, if they are readily available. Yes, they typically are used for making tall columns (I've used them for building 9 foot high, 1 foot diameter pillars in an earlier backyard of mine), but they also have been used for concrete footings. The only difference in this latter instance is that you are pouring the concrete outside the Sonotube, rather than inside. The end result will be a well-defined, beautiful circle.
Sonotubes can be purchased in short lengths (expect the cost/foot to be higher for small height pieces, rather than for purchasing a full-size Sonotube), as well as different diameters. Ask your local supplier (checking availability at HD Supply or White Cap would be good starting place) to either cut exact dimensions for you, or buy a long enough piece and cut it yourself, marking it first and then cutting it carefully with an electric jig saw, or keyhole saw.
Stake the Sonotube in place along the inside. making certain that you already have in place any irrigation line and elbow (for the riser--that can be inserted following your pour) for the plant going in the circle.
Staking can be with 1/2" diameter rebar, with lateral rebar pieces tied to the verticals, so as to hold the top of the tube in place. This assumes also that you have a firm base for the bottom of the Sonotube, that won't sink when dampening the ground prior to the pour (to extend the curing of the concrete, rather than having its moisture sucked quickly out by dry ground).
Hope this helps out future concrete pad endeavors!
