#11  
Old 05-25-2005, 11:59 AM
Peasant
 
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Default Another idea

Maybe the approach shown on the attached pdf would provide more security, at the expense of more concrete and more work:

I've shown a reinforced slab of regular concrete spanning the foundation. On top of that is the island slab within an insulating concrete surround.

The entire oven chamber rests on the thermal mass island, which in turn rests upon the insulating concrete, with loads pretty uniformly distributed. The exterior finish (I'm contemplating masonry so I've shown a first brick/stone on each side) rests on solid material down to the foundation. The space between chamber and exterior is filled with vermiculite, which is added as one proceeds with construction of the exterior finish wall.

I would propose (but I don't have the experience to tell whether it matters) that there be a slip plane between the oven chamber opening and the chimey proper (which is part of the exterior), so that the oven chamber and outer finish can move independently with temperature changes.

Comments/criticisms appreciated!
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  #12  
Old 05-25-2005, 02:51 PM
Peasant
 
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Default Verified compression load calculations

I agree with the compression load calculations, and went one step further:

For a round hearth sitting on an insulated slab,

Compressive strength, psi 150
Hearth diameter, in 42
Hearth area, in^2 1385
Max load if uniform, lb 207816

so "it'll be OK" seems like an understatement.

What if the loading isn't uniform? I assumed that all the load of the oven chamber rests upon a ring of width equal to the thickness of the walls. So for a 42" hearth, the ring is 42" ID and 52" OD:

Hearth wall thickness, in 5
Area under wall, in^2 738
Max load on area under wall, lb 110741

which is still plenty.

What if the entire weight of the oven rests upon the rebar, which in turn rests upon the insulated concrete, as in the orginal island slab design? This assumes that the insulated concrete provides no support other than compression beneath the rebar, like a hanging hearth. I assumed three pieces of rebar in each direction through the thermal slab, so 12 lengths in all, extending 6" outside the thermal slab:

Extension of rebar into insulated concrete, in. 6
Rebar width, in 0.5
Pieces rebar 12
Area of rebar, in 36
Max load, lb 5400

This seems close but probably OK. In reality, the insulating mix should give some support underneath. I haven't tried to account for bending load on the rebar; this is probably OK if the outer dimension of the island slab is close to the inner dimension of the foundation.
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  #13  
Old 05-25-2005, 03:22 PM
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might as well get my two cents in, just so no one tries to hang an oven on a layer of vermiculite and cement....

first, the shear strength of the vermiculite/cement layer is bound to be much less than the compressive strength. for concrete, the shear strength is only about 5% of the compressive strength.

second, if my experience is any guide, i found it fairly easy to "overwater" my vermiculite/cement mixture. i was using a concrete mixer but still the water tended to "wash" the cement to the bottom of the mixing barrel. despite my best efforts i was not able to get a consistent distribution of cement throughout the vermiculite. thus leading to the conclusion that the insulating layer will probably have areas of varying strength.
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Old 05-26-2005, 01:52 AM
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Alan,

Looking at your PDF, that would definitely do it. The bottom concrete layer holds everything up and encases the rebar, and the Island and vermiculite layers are strictly for oven perforamance. Not much more cost, but more effort.

James
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  #15  
Old 05-26-2005, 03:44 AM
Peasant
 
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Thanks for taking a look. Any thoughts on thickness? I have this rule of thumb in my head that anything concrete needs to be 3" minimum thickness (around 5x the largest aggregate) but I have no technical basis for it.

So I was thinking 4" for the support layer, 2" insulating, 3" thermal mass.
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  #16  
Old 05-26-2005, 05:19 AM
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I think 4" with rebar embedded will hold the oven and keep it from sagging. Anyone want to comment on that?

I also think you can do with only 2" of the thermal layer. Experience is showing that the non-commercial ovens just aren't driving that much heat into the floor, and it does not do you any good having the extra mass down there. Better to fully heat that layer, then stop the heat with insulation.

You end up with about a 4" thermal floor -- 2" of brick or refractory and 2" concrete.

James
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  #17  
Old 05-27-2005, 02:20 PM
Peasant
 
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Default Hmmm...pretty compelling

Well, I appreciate you putting some data up there. I am surprised that compression is not a greater risk, and I am assuming that Jim has not measured any settling on his project to date.

You're really forcing me to think here!

I'm not sure I'm equipped for that...

Michael
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  #18  
Old 06-23-2005, 01:45 AM
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Default implementation of the island design

alan,
is the attached pdf an idea, or something you have actually done? i'm curious about actual execution of this plan, and how the separate layers would be poured. are the layers all intended to be poured at once? it seems that if the rebar was in the bottom layer, and was serving as the primary support layer, the rest could be poured after this and the insulative layer just above it had set up, allowing for you to make forms to get as accurate as possible with the boundaries of the thermal and insulating layers.
i'm no engineer, so perhaps the strength lost in the layers not being totally bonded would outweigh the benefits of having the thermal layer precisely underneath the oven floor...i suppose you could make a circular form out of sheet metal, which would end up just below the hearth surface, but would more or less separate the thermal layer from the surrounding insulative layer if they were poured at once.
in other words: one would pour the reinforced layer, setting the rebar grid firmly inside it. pour the next bit of insulated concrete, then set the 42" or so round piece of metal where the floor will be. fill inside the form with thermal concrete, and around it with more insulating concrete.leave the metal just below the surface, and blend the edges together on the surface.
sorry if i'm seem like i'm splitting hairs, but my hearth comes next, and i want to benefit from the conversation in this thread and make it as efficient as possible, and doing so means figuring out what the conclusion of the questions in this thread are.
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  #19  
Old 06-23-2005, 02:54 AM
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and james,
are you suggesting that if the thermal slab in alan's PDF was 2" (along with the surrounding insulating layer), the insulating layer in the middle would be 1.5", and the reinforced concrete on bottom 3"? would 1.5" be adequate insulation under the thermal section? and would 3" be strong enough for the support layer? when i do the calculations, it adds up to a compressive strength that seems entirely impossible.
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  #20  
Old 06-23-2005, 08:10 AM
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Default The Island Hearth

I would still start this part of the conversation with the idea that the more simple insulating hearth, with 3 1/2" of rebar reinforced concrete on top of 2" of vermiculite concrete, is easy and will work great for many (most?) installations.

If you want to to the extra mile, and do the Island Hearth, I would do the following:

4" of rebar reinforced concrete at the bottom (for structural support)
2" of vermiculite concrete next (as the bottom insulation)
2" of standard concrete surrounded on all sides by 4" wide vermicuite concrete on top.

The vermiculite concrete can take the compression, so that is not an issue.

James
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