Fun stage. Starting to do some artistic stuff that will actually show. This is the building of the hearth, done exactly how you'd do a concrete countertop. I've done the pour-in-place method before but never the upside-down mold that gives a perfectly flat surface.
Starting with the mold----this is melamine. I didn't want extreme folds in the plastic so instead of lining the entire mold with plastic I just wrapped the form walls, then screwed it all together. Then a 2x6 was cut the length of the form, and rebar wired so it would hang in the center of the concrete. 3 anchor bolts were molded into the bottom to secure the piece to the wooden base later.
After packing the concrete as best as I could, the rebar/anchor bolt rack was put in place.
Let it cure at least 24 hours. Impatience is bad---some of these forms are gonna stick, and 'green' concrete might prefer to stick to part of the form and not itself. 300 pounds minus a little is one helluva paper weight.
So I carefully stripped the forms and built the wooden base. It was very tempting to just flip it over and have a look, but I would have bent the anchor bolts. The wooden base was very solid and distributed the weight evenly on the floor. I also made certain the base was over two of the floor joists.