A dirt floor would work better, than concrete, for outside. cement (smooth) may work, but gonna need lot more work to keep up. key is dryness usually, but the rain washes poop into soil, and sun dries quick so can be tossed on or mixed into compost pile. some use pea gravel or wood...
Concrete is gonna be really hard to clean unless completely smooth n you don't mind scraping n hosing off a lot. just put one inch mesh on bottom of coop (helps let poop etc fall through and keeps pigeons cleaner n dryer), n keep pen one n a half to four feet off ground. Homing Pigeons are...