When I did the numbers acording to the ACI (american concrete institute)the span between control joints is figured it. My slab is 20 feet by 30 feet with water tubes imbedded for heating. Since it is difficuld to run piping through the joints, I eliminated them in the design which made the required steel greater.<br>The slab is 6 inches thick with an 8 inch haunch at the perimeter, admixes were fibers and super-P, 3500 psi mix. <br>I calculated the required steel 3 ways- thermal expansion method, subgrade drag, and loading, and all methods were somewhat consistent calling for 5/8" bars 12" oc. By accident, I ordered 3/4" bars of what I thought was grade 60 steel. I later found out that it was grade 40 steel. The cylinders that I had tested @ 30 days broke at only 1500 PSI. The cement company's predictable response was that the testing was handled improperly. The good news is that my slabs have NO (0) cracking at all, for almost 1 year.