Anti Freeze For Ice Melt

I'm not sure what would be the ideal spacing for the PEX, but I think that you may be better off not having the PEX at the bottom of 6" of concrete. If I was doing it for myself, I would probably put a 2' x 2' grid of #4 rebar in the slab, with the bottom course of rebar sitting up on 1-1/2" or 2" rebar chairs. Then I would tie down the PEX to the top of the rebar. As long as the PEX is tied down well it would still be plenty deep to make (crack) control cuts in the slab if you were planning on doing that.

i found with my ice melt system you need to melt it as its happening (snowing) or it will melt just whats touching the concrete and then create an air space and not melt the rest. the antifreeze i used is cryo-tec-100. it worked better when it was water but had to leave running 24/7

Keep the pex within 2" of the top for best results. Why such a thick slab? That's about 3 yards of concrete! The more mass, the longer it will take to warm from a cold start. 3-1/2- 4" maybe thicken the edges a bit for some footing, should be plenty.

The more tube, and the tighter the spacing the better the response also. 6" on center would be a good idea for a small slab it does not add much cost to tighten the spacing. It also eliminates streaking when the system starts up.

One loop 300" long would be about 6" on center. Consider 5/8 or 3/4" to flow more btus. Generally you want 100 BTU/ square foot for a good working melt slab. Commercial, critical use slabs may be as much as 200 BTU/ sq. ft. output.

Propylene glycol PG. Look for a hydronic glycol that has inhibitors to protect your pump, HX, piping, etc. Pressurize the system to 10- 15 psi and include an expansion tank as the fluid will expand and contract with the wide temperature fluctuations. The expansion tank assures you always have pressure on the pump.

8 inch spacing 4 inch concrete