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How long have your ducks lived with this arrangement? I would check the bottom of all of your duck's feet for bumbles to see if they are ok with the current substrate or not. If they've lived there awhile now and no one has any bumbles, then they are probably fine. If anyone has any sign of bumbles, then I would start treating those and cover all concrete areas with rubber mats or some type of bedding (wood shavings, straw, etc).
I am paranoid about bumblefoot now after treating one of my ducks this past spring, so I cover everything that might be too hard on their feet (like hard packed dirt or of course any bit of concrete). I've had no problems since then, thank goodness.