I suppose there are threads or postings on keeping chickens on concrete. I'd suggest looking for those. Certainly we'd all like to keep chickens, their coops and runs looking "pretty as a picture" and while it can be done, it takes a lot of work. I have a friend who every day removes the poop by hand wearing gloves. He has three hens. Having 12 or more might be a bit more work. A lived in coop, like a lived in house never looks as nice as the vignettes we see in magazines. It can as I said with constant maintenance. With a maid perhaps.
So no matter what you keep your chickens on in a run, expect to clean it often. A concrete floor in the run might be okay if they can range for part of the day. Other wise I don't see as there would be much to "forage" .
for·age
ˈfôrij,ˈfärij/
verb
- 1.
(of a person or animal) search widely for food or provisions.
"gulls are equipped by nature to forage for food"
noun
- 1.
bulky food such as grass or hay for horses and cattle; fodder.
synonyms: fodder, feed, food, provender
"forage for the horses"
- 2.
a wide search over an area in order to obtain something, especially food or provisions.
"the nightly forage produces things that can be sold"
synonyms: hunt, search, look, quest, rummage, scavenge
"a nightly forage for food"