Another fine question
I know it is recommended to pour the concrete / steel and insulating layer on the same day so they cure and bond together. What is the down side if you pour the insulating layer on a different day? I can't imagine that it would move being surrounded and loaded with brick. The reason I ask is that I am playing with the idea of having a different shaped form for the insulation layer than the supporting slab.
Bond. Cement bond.
I planned on pouring my insulating layer on a different date than the hearth slab, then went with the isol board instead. I can't see any problem with pouring it seperately. I think that direction referred to when the insulating slab was underneath the reinforced slab, so the insulation would bond to the bottom side. In any case, I planned on notching the top of the reinforced slab so the perlite concrete would have something to grab onto. Isol board makes life much easier though.
I have to admit that the "pour the same day" logic goes all the way back to my experience with the ill-advised hanging hearth -- where the insulation rests under a concrete thermal layer, and the whole affair hangs by heavy 5/8" rebar on a block stand. With this design, you need to fuse the top and bottom layers, as they hang together. But that basic desgin is wrong -- and I have never seen it anywhere else in the world (other than in the BreadBuilder's book).
A much better hearth design is to place the insulating layer on top of the structural concrete support layer -- which in turn rests on top of the block stand. With that design, the insulation rests on top of the concrete support layer, and gravity takes over. The two layers do not need to fuse, and you do not need to pour the same day.
A good example of how this works is Super Isol, which simply rests on top of the concrete support layer. Another good example is our commercial oven supplier, who fills a structurally rigid metal tray with layers of solid-state insulation. No fusing required.
Hope this helps, and I will edit the Pompeii oven plans to reflect this change.
Thanks Nick and James, one less confusing issue to deal with.
I have removed the "pour the same day" text. Phew.
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