I use aubiose, i put a shower curtain over the top of the run so it doesn't get too wet but it soon dries out from a little dampness. It makes the poo disappear which i think is a very pleasing feature in a chicken run.
It's hemp based I think, kind of like hemp wood chip so really really absorbant, it is actually horse bedding and I get it from our horse and country shop (who also happen to sell layers pellets cheaper than anywhere else). Mine is on grass but they destroyed the grass in record time and were just on bare mud within a week. How long you had chooks? I think woodchip is better than bark chip as well.