How much will the Center hearth slabs that need to be placed atop the blocks weigh?
Each slab weighs about 375lbs - give or take a pound.
So, just to be clear those slabs are formed on the ground and then hand lifted into position on the stacked blocks? I am planning on doing this build myself, so I’m not sure about the logistics of a safe 375 lbs lift/placement.
Lean the slabs vertically against the side of the base - then two people will lift the bottom of the slab until it’s horizontal - and slide the slab into place.
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So, I have zero expertise in concrete work, but I’m wondering if it would be feasible to build a frame atop the blocks and pour the concrete slab in place.
What strikes me right away as an obstacle to that solution—even though it sounds attractive—is the “floor” in the form. You’d still have the problem of how to get that out from under nearly 400 pounds of concrete. It wouldn’t be at all difficult if there were two slabs separated by some space. Then you could unbox a slab, wrench off the sides with a demo bar, tip it inward, and slide it toward the center. But, you have three, and you have two 2x6’s adjacent to the center slab.
Add to that the issue of what happens to the void—the empty space formed by the polystyrene pieces. Using the face down casting method, you’d have to find a way to flip each of those slabs while they’re five feet in the air.
You’re casting face down so that you have a perfectly smooth, level bottom surface that lays on top of the blocks. Because your blocks were set in place using a level (right?), your hearth slabs will also be level.
I suppose that you could build a one-piece form out of cement board and 2x6, lay it on the slab, pour your concrete to a precise level, add your void forms, and ensure they are absolutely flush with the top surface. In that case you’d leave the cement board in place when you’re done. But that sounds like much more work than the three-slab, on-the-ground system, and fraught with more problems than it solves. (Also, I have no idea whether it would work!)