if you are not constantly spraying the slabs down, the risk of your duckies getting bumblefoot goes up sky high... the concrete will ruff up their feet and the lack of absorbent bedding will guarantee they walk around on poop... that's how bumble starts... bumble is very hard to get rid of and keeps coming back and is expensive and painful to treat.... don't subject your ducks to this hazard if you don't have too.
my duck got bumble last spring, and had to have surgery that cost me around $400 to fix. it came back just recently and now she as to wear shoes all of the time, permanently to prevent her problems from coming back... luckily my duck is a house duck who's used to being handled all of the time, an outdoor duck will not be so keen on having shoes strapped to it, lol. even my "perfect princess" fights me every step of the way, and she even likes her shoes. lol