Step one for today: build a frame for the hearth slab. I measured the size I want the slab to be, then cut 1x4s to length and screwed them together (put to screws in, but in the top half so that you can back the screws out once the slab is poured and knock out the boards). Then, to know where to make notches, I set the rebar in place very carefully and marked exactly where they go on the block (Only two of the four sides need the notches so that one direction of rebar can run above the other direction. Today I also had to readjust some of the grooves to go a little deeper because the top run of rebar was resting on them instead of the block). Then I took the rebar out, put the frame in, marked where the rebar will go, and then cut out the notches.
To make the vermiculite concrete, I used 6:1 vermiculite : portland cement. Make sure it is mixed very well before adding water. Also, we needed almost twice as much as we thought we needed from calculating square footage. I'm not sure why this was, I guess the water really pulled everything together.
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| Mix the dry ingredients thoroughly. |
When adding the water, I would suggest doing it with a fine spraying hose because the vermiculite will absorb a lot of water and every time you add water it washes the cement from the vermiculite. So it is good to mix the water in right away little by little so it all doesn't just go into the vermiculite. But maybe this doesn't matter.
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| Slowly adding water and mixing it in little by little. |
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| That's about right |
We poured half of the slab, then put the reinforcing wire in, and poured the rest. Usually you would not have to do this, but the wire we had had pretty small holes, so in order to insure that cement got down to the bottom sufficiently, this is what we did.
Note: The vermiculite slab is 2-inches thick. The hearth slab is 3.5-inches thick.
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| We let this sit for about an hour before pouring the hearth slab. |
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| Tying the rebar together. You can see better in this picture, the "vertical" rebar are in grooves, yet the "horizontal" rebar are simply sitting on the block. This puts the bottom rebar 1-inch from the bottom of the slab and the top rebar 1-inch from the top of the slab. |
Our concrete mix was 3:2:1 Gravel:Sand:Portland with a little extra portland for just a bit more strength, knowing that with heat, it is going to lose strength.
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| We filled the wheel barrel full full and then needed another half batch and ran so slightly short. So the edges are just slightly low, but this will not matter and does not compromise the structure at all. |
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| It was sunny and windy, so we covered it to slow the curing. Slower curing makes stronger concrete. |
So tomorrow we take the frame away and let it cure all day. Then the next day we will lay the firebrick and walls.
I realize I did not go into my concern much about the hearth slab essentially hanging on the rebar with no solid support underneath it. The vermiculite slab underneath is simply glued to the walls with no underneath support. I know it would work because I have seen it before, even on big ovens, so it should definitely work on this small oven. But I was uneasy about the concept probably because I am no mason and it just doesn't seem right to the lay person. So, in the previous days post, you see the precautions I took to make sure that slab could not go anywhere, and I felt it was alright to do this because I have also seen it before. So in short, this is me most likely completely overbuilding the oven, but it is my "just in case" approach.
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