Pouring a circle of concrete requires a firm form. If the circle is to cover a large area, the sides of the form will need to be braced to prevent bulging or malformation of the planned shape. Concrete forms are usually made of wood; but wood does not readily bend into a circular shape. The correct approach for creating a form for a concrete circle will be influenced by the size and purpose of the object.
Commercial Poly Forms
Pre-made industrial poly-forms may be the ideal form when working with circles with a three foot radius or larger. Although somewhat expensive, they are reusable, which would make them a good investment for small contractors.
Plywood can be used for large circles where the arc is wide and gentle. They can be scored on the back with cuts about 2/3 the depth of the board, spaced about every half inch along its length. This will increase the flexibility of the wood as the cut spaces will expand as pressure is applied to bow the board.
Poured slabs the surface of which will be level with the ground can be poured directly into a carefully dug space in the earth. Clay soil is ideal for this because it will hold its shape without slumping into the dug-out space.
Cut-out Stacked Wood
Arcs can also be created using two-inch thick boards that are stacked in sections, somewhat like a Chinese puzzle or bricks. By layering them, and staggering the way the pieces are stacked together, a smooth curve can be achieved that is sturdy and resistant to bowing. The wood pieces should be clamped or screwed together, and will still need bracing before the concrete is poured into the mold.
Small concrete circles, such as might be used for paving stones, can be poured into small plastic commercial forms or into cake pans. Be sure to oil the form well before pouring! You should be able to reuse these forms several times.