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aikitarik 08-09-2005 11:06 PM

Island Hearth
I've decided to try the island hearth design Jim and James have been discussing, but wanted to clarify some quick items.

It sounds like this is a 6 inch sandwich.. 2 inches of rebar reinforced concrete and 4 inches of perlite/cement (rebar?) with a 2 inch thermal concrete island (no rebar) in the center.

Does this sound right? I am cantilevering the front shelf of the hearth about 8-12 inches, but I'm planning on adding columns and perhaps corbells, if I can find finish designs I like.



paulages 08-10-2005 10:32 AM

i believe the general consensus on the previous 'island hearth' thread was that unless your hearth is adequately supported in it's middle, there would need to be at least 3.5 in. of reinforced concrete to hold the weight accross the span between the block walls. the alternative would be to have a support in the center of said span.

one person did note that most of the weight of the dome is transfered to the point where the exterior walls rest, so if this is mostly resting on the blocks stand, it shouldn't be as much of a problem, but the hearth itself and the floor weigh enough as is. i wouldn't mess with underdoing it on something that could crumble your whole project.

that said, i would recommend the method suggested in the end of that thread:
2 inches of perlite concrete under 3.5 in. reinforced concrete under a 2 inch thermal island, surrounded by 2 inches of perlite concrete, giving a total depth of 7.5 inches (depth of a 2" x 8").

i really wish i had done this method, as i can feel the hearth heating up outside of the dome therefore tranfering heat away from the oven. the perlite concrete is easy to mix, so it's not much more work than the 5.5 inch hearth.

Alan 08-10-2005 11:17 AM

I am considering the layout shown in the "solid model" thread in this forum:

Starting at the top of the block wall:

* 4" reinforced concrete (structural support)
* 3" perlite insulating concrete (to isolate the oven from the surroundings)
* 2" non-reinforced concrete (thermal mass, essentially making the floor thickness equal to the wall thickness)
* 2.5" firebrick floor

The strucutural support layer is of course the full size of the block wall, with or without an overhang according to the aesthetic design etc.

The insulating layer and thermal layer are rectangular for simplicity but just large enough to contain the firebrick floor. Those who are willing to do more complicated forming could make these layers match the floor, and the floor itself could use firebricks cut to closely match to shape of the dome. This would remove excess thermal mass. The floor could also be cut to the shape of the inside of the dome with the dome resting on the thermal layer.

Do folks think that the insulating and thermal layers need to be surrounded by a pour of insulating concrete so that they can't shift horizontally?

paulages 08-10-2005 11:46 AM

yeah, i made a mistake in my last posting. as alan mentioned, the insulating layer would sit between the reinforced layer and the island section in this design. for the island, i would just make a circular form out of a piece of sheet metal, as i have mentioned before, and if there was worry about bonding the thermal island with the surrounding insulating layer for some reason, one could drill holes in the sheet metal.

aikitarik 08-10-2005 01:25 PM

Just don't use aluminum if your going to leave the metal in there. Aluminum and concrete don't mix well over time (fast corrosion). I'll probably just pull the aluminum form out after pouring the concrete and let a little mixing happen.

Check this out : oven base

I have structural support under most of the oven since I'm pouring the hearth on a hillside astride a retaining wall and a concrete pour. It's the front 32 inches I am wondering about. Do I need to pour some structural support there or just not worry about it?

We will be adding architectural columns and/or corbells in front of each row of blocks, but really, the only weight on there will be ME and extra bricks during construction, and I can always leave the forms on until that's done.



paulages 08-10-2005 01:45 PM

ahh, your pictures clear a lot of things up. though hearth will not be straddling the blocks stand as in the basic design, there still needs to be sufficient reinforcment to keep the hearth from cracking with the weight of the oven. you could pour a deeper footing directly underneath where the walls would lie to allieviate this potential problem. you might also consider laying rigid insulation underneath the entire slab, as is done in residential applications underneath slab floors.

aikitarik 08-10-2005 01:54 PM

Aha... now there's an interesting idea. Rigid foam insulation instead of or in addition to the perlite mix? I am coming to the conclusion that I should consider pouring a thin 3.5 inch reinforced support for the front 32 inches off of the retaining wall and then pour the hearth. Maybe by shortening my top row of blocks.

The round edge of the oven starts 30 inches back from the front of the hearth pour. The front slab is only 32 inches wide, so this means only 2 inches intrudes over the edge of the retaining wall.

So virtually all the weight of the oven will rest on the back/upper slab. Only the doorway arches with what thrust they take from the dome will be over the lower/front slab.


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