Some very large chunks if you can get them would be ideal. I agree with Bill that a mix is best, but some huge pieces that would create large crevices would be a good idea. I like the look of natural rock compared to concrete, but I think concrete makes better fish habitat compared to natural rock. A lot of my research and personal experience with grass shrimp leads me to believe they will heavily utilize your concrete piles. If you do end up stocking them, this will be a big bonus for the RES that will no doubt frequent the concrete piles.
