I would NOT use concrete- it will spall exposed to direct heat and fling shrapnel dangerously, especially with new 'crete.
Since it sounds like you can weld, the typical hand method would be to tack uprights made of flat bar/angle/channel (scraps ) to the work table or a baseplate, at a radius slightly less than proposed to accomodate springback. It is a good idea to leave some tangent on the ends. This can be done cold, given the radius stated, with a cheater tacked/clamped to the end. The start is either clamped or restrained by a stop/pin on the OD.

Thanks for your suggestion! I think I will stay away from the concrete idea....didn't even come to mind until you said something. Whew. I'm glad this can be done cold!