You'll need shavings in the winter, at least -- in North Dakota they are not going to be spending all day every day outdoors in January, to say the least Not only is the concrete really hard on their feet and leg joints, but it is also COLD. (Having exposed concrete in the coop can act as a thermal buffer and help keep the building from getting colder than necessary, but it will still be COLD for them to stand on and thus give you cold chickens.)
Shavings are not as expensive if you buy them in bulk (store somewhere high and dry under a VERY GOOD WELL TIED DOWN tarp).
Another option, that would give you a warmer although not much softer floor, might be to put down plywood over the whole floor. Not up on 2x4's - just flat on the floor. Screw battens across the joints so crud doesn't get between and beneath. If you do not paint it, it will eventually rot away, and if you don't keep the litter pretty dry it will also be a moisture reservoir; and it would not be my own choice; but it would be better than bare concrete in wintertime at least.