How much will the hearth expand ?

[P3 Stoaker]

G'day All,
I've had a few hold ups to pouring the hearth (structural layer). but hope to get going soon. My hearth will be poured within an existing brick edge, see photo. The 2 bricks of vertical height that can be seen are for the hearth then mosaic tiles and 3 " of calsil board. The hearth will be long though as I have side compartments, so roughly 145" by 70" or 3800mm by approx 1800mm, sitting on the lower bricks.
What I'm starting to wonder is if the hearth expanding in the heat could push the bricks out and crack the outer wall? I believe the more common method of building a brick block base often has the hearth joined with metal rods down the brick block holes etc but I've not heard much about possible expanding hearths pushing out and cracking the blocks.
I'm thinking of putting some expansion foam around the edge as even a small amount of expansion will crack out the wall.
Any thoughts/comments appreciated.

[UtahBeehiver]

IMHO, As long as the oven dome wall and floor are isolated from the concrete hearth and with 3" of CaSi board, you will not see much heat transfer from the oven to the concrete hearth.

[david s]

Yes, i’d agree, as long as it’s reasonably insulated you should be able to hold your hand under the slab, so it’s expansion is not a problem. The expansion of floor and oven however, is something to consider. They will expand around 5 mm at 400C.
Do yourself a favour and cast a few holes through the centre of your supporting slab. You can use fairly dense foam like that used in a thong or polystyrene. It can be easily knocked out when the concrete sets. These holes will act as drains to provide exit for moisture elimination from the underfloor insulation.