Nothing wrong with it if that's your only option. As you say, you wanna put a considerable depth of something atop the concrete (I would recommend an organic material, such as straw or mulch or safe garden weedings, that kind of thing) (e.t.a., after seeing previous post, yes sand is fine too, especially if they will only be in there as chicks... for adults I'd give them more like 6-10" of sand). Some of my runs are entirely on slab, just because preexisting conditions forced it, and the chickens do fine -- although IMO it is *better* to give them access to a dirt area where they can dig and dust (you can make a dusting box for a slab run but according to the chickens it isn't quite the same). But especially if this is just for chicks and they will not be spending their whole lives there, I don't personally see any problem with it at all.
And a slab *is* great protection against digging predators, rats and mud!
Good luck, have fun,
Pat