Well I believe most folks do use cement because it's way easier to clean. Gravel gets smelly and is a pain to bleach...
The kennels I have I use flat stone but Id do concrete if I could. But for the most part I use chain spots. For my high drive dogs it's the safest form of containment and they much prefer it over kennels any day.
"It's a nokalokapoopalotadoocanidragaleashalupus. It's a breed of mixed, strained money extorted
practically useless spin off of a once noble breed." --William Williamson