Abstract: A concrete form for pouring a polygonal column includes a plurality N of plate members arranged to form a polygonal tube having N sides, where N is an integer greater than or equal to three. The form further comprises a reinforcing framework including a plurality of ring assemblies longitudinally spaced along the tube and interconnected by a plurality of circumferentially spaced longitudinal members. Each ring assembly extends circumferentially about the tube and comprises N ring segments each having an inner surface bearing against an outer surface of a respective one of the plate members, and an outer surface formed as a circular arc. The outer surfaces of the N ring segments collectively form a substantially circular outer surface. The form also includes a plurality of clamps respectively encircling the circular outer surfaces of the ring assemblies and being releasably tightened thereabout.