here's my response to your original question. Using rebar or fiber has nothing to do with the concrete cracking.

All concrete will crack over time. The challenge is to manage where it cracks. Your contractor should have installed pressure joints every 10-12 feet. The pressure joints allow for the expansion and contraction of the concrete with changes in temperature. Because the pressure joints are as thick as the thickness of your concrete, any cracking occurs at the pressure joint.

In addition to the pressure joints, concrete contractors also cut a line into the concrete about 1- 11/2 inches deep using a masonry circular saw. So when the concrete expands and contracts, any cracking that occurs does so at the bottom of where they cut- out of your sight.