Q.: We were asked to build a curved concrete retaining wall for a homeowner. The curve of the wall has a radius of 4 feet. Can we produce this curve using job-built forms?
A.: Forming curved surfaces can be accomplished by using plywood, which can be bent to the desired radius. However, the plywood must be thick enough to support the load, or pressure, of wet concrete yet thin enough to bend properly.
Plywood can be bent two ways: the strong way and the weak way. Bending plywood so that the grain is perpendicular to the supporting studs is the strong way. Bending it so the grain runs parallel to the supports is the weak way. Though bending plywood the strong way provides more support, it's more difficult to achieve a tight radius.
To produce a bend with a radius of 4 feet, try using 3/8-inch-thick plywood with the grain running parallel to the supports. (North American plywood has the grain running the long direction of the panel.) Saw kerfing, or cutting notches partially through the plywood sheet, will allow you to use thicker sheets of plywood while meeting the required radius. For best results, provide smoothly curved supporting members. This can be done by cutting 2x stock to the proper radius. Remember to provide additional studs and place concrete in smaller lifts, since the sheathing is being used the weak way.