They layed plastic over everything to prevent moisture from seeping up out of the ground into the concrete when it is eventually poured. Then they spread sand over everything to make it as even as possible. The sand kind fo filled in all of the depressions and made the whole spot as close to even as it could be. Then they laid these styrofoam pads around the edges to help with heat retention. As mentioned in a previous post, radiant floor heating tends to lose a lot of heat out of the edges of the slab and the styrofoam should help with heat retention.
Here's Keith, our General Contractor, Mike Hammer, and some of the MMAPers installing a manifold for the radiant floor heating. As you can see, in the photo above, after spreading the sand and laying out the styrofoam pads they crss-crossed a grid of rebar across the entire area.
Then they began laying out the radiant floor heating on top of the rebar matrix.