38" vault in tropical Australia Brick Oven Photos Discuss 38" vault in tropical Australia in the Good Background Information forums; Onefella The worst that could happen IF you find that you made the wrong decision is "I told you so" ...

[nissanneill]

Onefella

The worst that could happen IF you find that you made the wrong decision is "I told you so" from Dutch.
There' always plenty of firewood up there anyway to access, even if it is from the illegal immigrant boats!

Neill

[Dutchoven]

Originally Posted by Onefella

Dutch, I hear you mate, and you have given me much to think about. I do have a bit of a dream to bake bread semi-commercially for our small town and I was resigned to long firing times for multiple bakes. I really appreciate your advice, but I think I'll still go ahead with the slab as part of the heat bank/cladding. (I hope this doesn't turn out to be too bad a decision)

Onefella
OK I see what you are thinking now...my wife and I bake for a Farmer's Market out of our Scott/Rado oven...but we have constructed a pompeii style oven for our pizzeria/bakery...if you want more heat retention from the floor my suggestion would be to increase your floor thickness by possibly using a double layer of firebricks on top of the insulation...if you poor the structural concrete slab on top of the insulating layer you will still be heating that slab to it's outer areas(once again speaking from experience)including the block walls upon which it rests...I promise not to say "I told you so" but I am seriously saying I think you will be far happier if you put the insulation on top of the concrete and the oven floor on top of the insulation...we can find ways to get more heat out of it by setting floor bricks in that insulation...what was called an "island hearth" in the orignal pompeii plans...but they are still separate from the structural layer of concrete
All the best!
Dutch

[james]

I'm not sure I have have read your plans on this, but if you place the brick on the narrow edge, your cooking floor will be 4 1/2". That's a lot, even for a lighter use commercial bread oven.

Whatever you decide to do, insulate, insulate and insulate under the thermal layer (anything that will absorb heat, including concrete).

James

[Onefella]

OK guys, you make a convincing argument. I'll probably do it the way you suggest.

So you're saying that even with the "floating" slab design where the hearth slab only touches the base blocks by the reinforcing rod and otherwise is in no way contacting the rest of the oven; it still loses too much heat into the surrounding structure?

[Dutchoven]

Originally Posted by Onefella

So you're saying that even with the "floating" slab design where the hearth slab only touches the base blocks by the reinforcing rod and otherwise is in no way contacting the rest of the oven; it still loses too much heat into the surrounding structure?

If you are doing that floating slab you would be OK but that method seems to be a very complex way of doing it when it seems simpler to me to pour a structural concrete "tabletop"...then box out for the oven's footprint and our vermicrete to it...then lay floor bricks and build your oven...now that I understand a bit more about your design you will likely be OK either way...hope I have not been too long winded...all the best
Merry Christmas!
Dutch

[dmun]

I'm no structural engineer, but it seems to me that that method of "hanging" the support slab from a few pieces of rebar is extremely suspect. I've seen enough rusted-out rebar to think it's best deep buried in concrete.

[Onefella]

Originally Posted by dmun

I'm no structural engineer, but it seems to me that that method of "hanging" the support slab from a few pieces of rebar is extremely suspect. I've seen enough rusted-out rebar to think it's best deep buried in concrete.

The 16mm deformed bar that I'm using has a shear strength of 6.8 tonnes and there are 18 'attachment' points. The gap will reduce the shear capacity slightly, but by my calculations the slab could support 2-3 tonnes before the concrete itself would crack. Probably of more concern is the 'reo' bending and pulling out of my bond beam. But it's a tried-and-tested design, so Im pretty confident it will be ok.

The exposed rebar in the 'gap' is painted with a good quality hi-temp black engine enamel. It will also be encased in vermicrete.

I've attached a diagram of how the slab is arranged.

[nissanneill]

Onefella.
tried and proven?
Well I personally wouldn't have a bar of it! The 30mm of vermiculite is way insufficient insulation to stop the stored heat in your concrete from escaping and into your block work. the whole thing is upside down (and you don't need to use the cement sheet.
The 16mm reinforcing bar will rust, no matter what you do or say, especially up in the tropics and more so with the continued heating and cooling. I don't care if you galvanise it, it will deteriorate and eventually fail.
Mate, you are on your own with this one.
That's my opinion expressed.
I wish you well with your decission.

Neill

[Onefella]

Don't right me off just yet..

I could lift the bottom sheet, remove the 'gap' formwork and have a pretty solid slab still.

So would the cladding still rest on the hearth slab, or on some new insulated bottom layer?

[nissanneill]

Onefella,
I have just drawn the construction as it is recommended and as how I did mine although it was built on a reinforced retaining wall.
You can make the base the exact size of your oven or larger allowing a work area wherever you require.

Neill