My concete floor in my shed is about 6 months old. It was made in two pours of 28'x30', 6 inches deep with wire panels. No control joints were sawn in at the time. Can I still saw them now and will they still work to guide any cracks that may occur. So far no visible cracks but I am in Illinois and have not went through the winter yet.

That is some shed. You can cut the control joints but it will not guarantee no cracks will happen between the joints. Concrete always cracks not matter what you do but control joints will partially hide the issue. I have a 28x30 garage in Illinois with 4 control joints dividing the space into quarters. I now have a small crack running diagonally through on of the quarters.

This is one opinion about cracking from the Portland Cement Association.

"Why does concrete crack?

Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage. "

In highway construction, control joints in concrete pavement must be sawn within a few hours of placement (often in the middle of the night) because most cracking occurs due to shrinkage as the mix water evaporates. Water will continue to evaporate for days (some say years) but the rate of evaporation is much greater in the first few hours after placement.

Contraction will occur as temperatures fall and if there are areas where the slab cannot move toward the center of the slab, cracks could occur.

The other causes of cracking such as unstable grade, water penetration (then affected by freeze/thaw), etc will probably not be lessened by sawing joints now.

I don't think that sawing joints now will help much but probably won't cause any damage either.