Removed porcelain tile to find ceramic tile underneath on concrete. Have removed most of that but around the outside/walls, a few biggish chunks of concrete came up with the tile and I can see black (tar?) paper. Concrete looks to be 3/4 inch thick (I thought it would be more). Should I just try to remove all the concrete now or patch the holes? I need to cement board it to bring new tile up to old height. Am I creating too much work for nothing or is it better to start fresh on subfloor?