I was on a job that required a poured-concrete step with 6-in. radius curved corners. I’ve had good luck in the past using hardboard to form odd-shaped curves, but the material I had on hand wouldn’t bend to this tight a radius. So I used sections of 12-in.-dia. Sonotube instead.
As shown in the drawing, I cut the tube to the correct length and then cut it into quarters lengthwise. Next, I affixed a couple of pieces of 1x furring strip to the outside edges of the forms, which provided me with the nailing flanges necessary to affix the tube sections to my forms.
Robert V. Harrington, Milford, CT