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?

