How to Form a Semi Circle Concrete Patio

Outline the patio site with wood stakes and string. Hammer a wood stake into the ground at the center of the straight side of the patio. Pull a tape measure from the stake to one end of the straight side, and cut a length of string equal to this distance plus 6 inches. Tie the string to the stake with the extra 6 inches of length. Hold the other end of the string level and taut, and move it in a half circle from one side of the center stake to the other. Hammer more stakes in the ground with each foot of distance to mark the curved side of the site, and tie a long string around each stake to outline the semi-circle.

Excavate ground cover and 8 inches of topsoil from the site with a shovel. Dig straight down along the walls so the form will sit vertical against them. Go over the subsoil with a hand tamper until the loose dirt feels firm and resists force.

Stand lengths of flexible metal trim against the inner walls of the site. The trim must be 8 inches wide plus the height you want the patio raised from ground level. For instance, a patio that sits 2 inches higher than the ground requires 10-inch wide trim.

Line the straight inner wall of the site with another piece of metal trim. Overlap the edges of the straight trim behind the rounded trim to prevent concrete from spilling through.

Fill the site with a 4-inch layer of 3/4-inch angular gravel, and tamp it down. In addition to supporting the concrete and improving drainage for the patio, the gravel base helps hold the metal forms in place.

Hammer wood stakes every foot behind the outer perimeter of the metal forms. The stakes brace the temporary metal form against the pressure of wet concrete.

Mix the concrete with water in a mixer, and pour it over the circular site. Spread it with a gauge rake to work out air pockets. Drag a flat metal float over the top to smooth any rough spots and push excess concrete from high spots to low areas so the surface is even.

Dry the patio by laying breathable fabric over the top. After the patio cures for seven days, pull up the metal form and stakes.