As someone who has had to build up a lot of stall floors for various livestock, lol.... we use pallets as a base, overlaid with sheet goods... for ease of cleaning, we usually use and OSB or other sheet good covered with scrap vinyl flooring......so far, all have held up well and if and when they do not, pallets are free !! Good luck with whatever you decide to do

Here is a link to a relative new type of heat treated decking/floor non-toxic chemical free wood. It cost a bit more but is worth it. When we built our decks 5 years ago it cost appx $2.00 per board foot (pressure treated cost $1.00 per board foot). It then had to be special ordered but now is becoming commonly available. It is rot, insect, shrink and warping resistant and comes with a 25 year warranty even when used under/on the ground.

Here is a link that might be useful: The Truely Non-Chemical Green Wood

If flooding is the problem, and otherwise the concrete floor is satisfactory, could you improve the drainage from the other pen? I was thinking of something like a French drain between the two pens which could direct the water to where it would not be a problem. Just a thought.

Think it would be cheaper (at least more cost effective)and healthier to do the drain idea. Say you build up the floor with wood, still gonna have moisture and all the goodies that come with it. Stop the water from coming in in the first place.

Dave