I put rigid foam insulation panels under the rebar, and had that concrete with fibres added. I'm sure it's common practice, but I had small lines cut to take the stress and if there were going to be cracks they'd happen inside the cuts and not be visible. I have been really pleased with the results as far as cracking and reasonably warm (no heated floor). I put in 200amp service, and break out 100 to send to the house for mine. I also used scissor trusses as I could only go so high up, and wouldn't have room in the attic for any real storage. The scissor trusses give you easily enough room to stand a sheet of plywood and move around and I can still hang garden furniture in the high points for the winter in the non work area. Make sure to wire for at least 20amp plugs so tools don't keep blowing breakers. The wire, breakers and outlets add a little more expense, but worth the trouble!
brian waz