Interesting concept. It's going to be fairly expensive. In addition, concrete is a porous material, so it will need to be sealed in order to prevent staining from the constant moisture build-up. It's also a tough material, so I am not sure it's going to feel very good on your skin. Unless you...
What are the models of your units? I can try to help if you provide additional info, but your best bet is to have a couple contractors look at it. You can use Service Magic, Quality Smith, or Angie's List to find some decent local folks.