Retraining my dogs to potty on concrete in outdoor kennel

Question
We just moved and installed a nice outdoor kennel for our two shelties to be able to go in and out while we are gone.  We did this so they would go out and go potty on their own but it seems as though they do not want to go on the concrete.  They hold it until I can let them out on the grass like they are used to.  We need to train them to go on the concrete.  We now have to be away for extended periods of time and we built this for them.  What we have now is a dog door out a bedroom in the lower level into an enclosed chain link fence kennel run about 10'X 4' in size with a concrete floor.  I don't want to come home to the room all messed up or the hold it all day.  Any sugestions?

Answer
You could try getting a small, plastic kiddie pool from your local K-mart or Wal-mart, and filling it about halfway with dirt, for them to use the bathroom in out in the kennel. Drill a bunch of holes in the bottom of the pool first, so that the liquids will be able to drain out (otherwise you'll end up with a nasty, nasty mess). After a few weeks, you can try removing the makeshift 'litter box' and see if they will go to the bathroom on the concrete.