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Alan 05-23-2005 06:29 AM

Question about isolated hearth slab
My other "getting started question:"

I appreciate the purpose of isolating the hearth slab from the surroundings as shown in the pdf that James posted in the Yahoo group. However, my impression was that vermiculite/cement mix isn't very strong. In the pdf, the vermiculite/cement appears to be holding the weight of the entire assembly since the regular concrete dosn't extend over the block walls. Is this the case, or is a stack of blocks under the center of the thermal slab recommended?


PizzaMan 05-23-2005 08:38 AM

I've been wondering, too.
I've also wondered about that same thing: how much will the hearth settle over time as the huge weight of the bricks and finish materials compress the vermiculite-based cement?

I may go ahead and build a hanging hearth (a la Alan Scott) unless someone can give me some clear reasons not to expect the bottom layer of the hearth sandwich to compress.

james 05-23-2005 09:46 AM

There are two separe things going on here.

1. If you pour your insulating hearth in two layers, with rebar in the concrete layer on top and the vermiculite concerete layer below, there is no risk that the vermiculite layer will compress. Jim did the vermiculite concrete compression numbers a while back, and I will find them, and post them here. I have done the hanging hearth described in the breadbuilders, and it's my opinion that it is a lot of work for no gain. I believe the idea behind hanging the hearth on the rebar was so that you could easily lift it with a forklift and move it.

2. The second thing happening is that Jim has created drawings for a hearth where the concrete layer (thermal layer) is surrounded on the sides and below by vermiculite concrete. The idea is that having vermiculite concrete on the side of the thermal layer will block heat from escaping your oven out the sides. I think there are some questions on how the rebar works with that hearth, and how the hearth safely spans the stand opening.

Let's call that the Island Hearth design.

Perhaps Jim could jump in on this one.


Alan 05-23-2005 10:02 AM

I was after point #2. I could understand compression being negligible with one uniform slab upon another. If the floor is independent of the dome (dome set around the floor) and/or the floor and dome are independent of the outer finish (as is accomplished by an insulating blanket even on an igloo, I think), then it may not even matter except for any loss of insulation.

So the question in my mind was how well the vermiculite concrete works as reinforced concrete (compressive strength and adherence to the rebar) or how well the center slab will stay suspended as a hanging floor, with stress concentrated on the vermiculite concrete where the rebar lies.

If there is concern, perhaps alternatives are (1) a stack of blocks at the center (at the expense of space/aesthetics underneath) or (2) a compromise consisting of a few inches of regular concrete around each length of rebar?

These are just ideas. I'll await advice from those who know more! Thanks.

PizzaMan 05-23-2005 10:04 AM


Hey, Jim, we're waiting....


PizzaMan 05-23-2005 12:34 PM

I dunno...
Alan, as I understand the rebar always resides within the solid concrete slab where it is very stable and solid. However, this solid slab would rest upon a much softer layer of insulating cement (vermiculite and cement).

The weight of the concrete alone is very sizable. The weight of the brick alone is enough to stress any structure. Together with whatever structure stands above and around the oven dome, this is a lot of mass to support with a lightweight, highly-compressable mixture. Unless I'm missing something, I have to believe that this will compress considerably leaving us with what would amount to a dense cusion between the tops of the block walls and the solid slab.

I'm not worried about structural failure, as the strength of the slab is more than adequate. I just wonder if this "island" is going to sink, leaving us without the supposed benefits of isolating the heat-retaining mass.

But, heck, I dunno. I certainly don't mind being wrong. I just just don't want to find out I'm right after building it as planned and putting it into use.

So Jiii-iiim, com out and plaaa-aaay! :D

Alan 05-23-2005 01:05 PM

Remember that people have day jobs :)

PizzaMan 05-23-2005 02:58 PM

Just teasin'.

I'm checking in between clients.

aka: PizzaMan

james 05-25-2005 08:52 AM

Here is the information on compression and vermiculite concrete that Jim did for the "standard" hearth desgin. I think it shows that the vermiculite layer will never get "squashed" under the weight of the oven.

I'm going to add a few thoughts on the "island" hearth next.



Actually I just did the calculation for this. If one creates a 5 1/2"
form (using 2x6 boards) around the top of the block walls and then
fills the first 2" with "standard" (1:6) vermiculite concrete followed
by a 3 1/2" layer of bag mix concrete you get both insulation &
thermal mass. The 2" of vermiculite concrete give you the insulation
effect of 40" of concrete (ASTM standards).

Of course, vermiculite concrete is weaker than regular concrete in
terms of compressive load (between 140 & 170 psi vs 2000-4000 psi). It
does however flex better without cracking when subject to tensile
stress. That's one reason you'll find it used in swimming pools. That
still leaves the issue of the crushing weight of the oven & thermal mass.

Taking a conservative 150psi for vermiculite concrete, that runs to
about 21,000psf -- a 64"x72" hearth will have a 30 sq ft load area (or
assuming you're really conservative and want to use point pressure on
the walls without regard to the overall spread -- 15 sq ft of wall top
surface). That means the vermiculite layer is capable of supporting in
compression over 150 tons. I don't think anyone has suggested that's
what everything skyward of their insulation hearth layer will weigh.

james 05-25-2005 09:03 AM

Thinking behind the Island Hearth
There are two ideas behind the Island hearth. First, that you want vermiculite on the sides of the thermal layer, and second, that 3 1/2" of concrete in the thermal layer is overkill (it rarely gets fully hot).

While we wait to hear Jim's input, I can add a few thoughts.

What if the thermal layer of the hearth (standard concrete) were 2" thick, and embedded with 1/2" rebar, in both directions, on 12" centers and that the layer was large enough to rest on the concrete block stand. The blocks are 8", so it would be possible to rest the thermal layer on the stand and still leave 4" on the sides for vermiculite concrete.

Would the rebar become a problem in the 2" thermal layer with contraction and expansion? Would a 2" concrete layer ever sag in the middle?


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