Question about isolated hearth slab

[james]

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

[PizzaMan]

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

[paulages]

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.

[paulages]

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.

[james]

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

[paulages]

i think i'll probably go with the simpler version...the extra labor mixing the extra concrete to make an 8" slab instead of a 5.5 in. is reason enough, and using 2" x 6" lumber makes the form building easy.

[ColonelCorn76]

Ooops. Ignore this.

[james]

I've been thinking about how to make the Island Hearth work with a 2x8 form. The layout could be:

4" concrete bottom with rebar
1.75" vermiculite layer
1.75" concrete Island

Let us know the first time someone builds this -- whomever you are out there. :-)

James

[Fabio Allodoli]

Stress bearing load.

Hello: I very new to this web site, and in about a week and half I'll be building an outdoor kitchen, with the 42" Pompeii oven. I have been reading this intresting web site and specifically this subject, I to would like to make some comments.
I have built bridges, but timber type, however stress and supports was the major concerns since we had to deal with several factors. Having said that, I remeber when I lived in Florence Italy, my dad worked for the city and my grand father worked for the King of Italy at that time, shared and remeber stories that they shared about the Ponte Vecchio. Now I do not know how many people have been there, but I am sure most have seen pictures of the bridge.
The Ponte was build 3 or 4 centuries ago, and the Arno rivers runs through it. There has been many terrible floods in througout the centuries, and bridges have been washed away, except one... You got it the Ponte Vecchio. The reasons, it has withstood the floods is because the construction style. It was built almost in the same manner as the Pompey oven. I'll refer to the center of the oven as the cupola. The key to the strengt of the cupola is the inter locking bricks at angles, once the mortar has been filled in, the amount of weight you can put on top of the oven is incredabily high, but the bearing stress and load factor, is in the outer edges, and at the base of the walls. Which brings me to my opinion regarding the amount of strength the base plattform can hold.
If you take the 42"Id oven with an estimate 52"Od of the oven wall, the main weight of the oven is in the outer width od the oven wall. Now the 52"Od is the outer wall, is in the outer edges of the base foundation or harth, which now the base of the is closer to the outer edge of the entire harth, and the majority of the weight the stress factor has been reduced to outer edge of the hart and the main supporting walls of the oven foundation. Yes there is weight in the center but that weight due to the design has been transfered to the outside edges. To finish the Ponte Vecchio story, once all the bricks and stones carefully cut and placed, and the outer retaining walls where built, the fill was mostly sand with some clay to hold the sand in place. To make the bridge strong and carry the top bearing weight, the builders used large carts with oxen to pull accross the bridge, over and over, and as the bricks and stoned started to interlock with eachother more sand and clay was added and the procedure with the oxen continued untill the bridge was settled to where the builders was satisfied, and no more fill was needed. As matter of fact many historical churches and building made with cupola type tops or archways are made that way for that reason.
I am not trying to give anyone history lessons but those bridges and buildings still stand today.
I want to thank the person that introduced the Pompeii oven. This is why I building my oven in this fashion, I had forgotten about the mentioned story and why I liked the design of this oven.
I also want to thank all that are taking and active part in this construction, it has been very helpfull to me, and I look forward in seeying more pictures and remarks. I will be posting pictures and dimensions when I am done, with my construction, and BBQ kitchen.
P.S. I used own a wholesale bakery, and food cannot be beat when cooked with convection ovens, but my ovens where gas and electric, which brick ovens operate in a similiar manner. When I finished and baked some food, maybe I'll post some baked goody formulas.

Ciao.
Fabio.

[ColonelCorn76]

Hi Fabio,
Nice historical reference. It's amazing what the ancient Romans & Greeks were able to accomplish with arches. I got the inspiration for the Pompeii design from James (at FornoBravo) who had lots of local (Italy) ovens which he took photos of. But to validate the design I took my inspiration from Brunelleshci's Dome. In the early 15th century he won the competition to build the dome on Florence's Santa Maria del Fiore (the cathedral had been started a 100 years before but the original architects didn't have any idea how to build such a large dome so trusted that by the time the building got that far that God would provide the answer). The cathedral's dome is still the largest free-standing masonry dome in the world -- over 500 years after it was completed!

Of course I did a lot of engineering stress calculations using today's mechanical engineering principles & material data but all they did was confirm the inherent strength in the shape of the dome we're using.

Jim

[Fabio Allodoli]

Pompeii

Hello Jim.
Thank you for your comments. I was 12 when I came to the States, and have not been back since, but my wife and I are planning on going to Italy next year.
I have a couple of cousins in Florence and one in Rome. One of my cousin in Florence starts up small grocery stroes, and stocks them with the best of food and sells them.
Where you in the military and are you an engineer. Do you have any ideas or pictures of the flute for the Pompeii! I have seen very little pictures on flutes and the proper spacing from the end of the end of the oven top to the edge of the entry way toward the front. If the flute opening is larger than required will this cause to much vacuum and suck the hot air out. I have an opportunity to get some pre-fab clay type flute sections but they are 16"ID ova and do not know if they will work.
Any suggestions, on what is the best solution or what type of flute to use, or make out of fire bicks.

Thank you Jim.

Fabio.

[ColonelCorn76]

Hello Jim.
Where you in the military and are you an engineer. Do you have any ideas or pictures of the flute for the Pompeii! I have seen very little pictures on flutes and the proper spacing from the end of the end of the oven top to the edge of the entry way toward the front. If the flute opening is larger than required will this cause to much vacuum and suck the hot air out. I have an opportunity to get some pre-fab clay type flute sections but they are 16"ID ova and do not know if they will work.
Any suggestions, on what is the best solution or what type of flute to use, or make out of fire bicks.

Thank you Jim.

Fabio.

Hi Fabio -- I trained as an engineer but wound up managing software development companies. That's probably why I tend to tackle projects like this with a fairly analytical & research oriented approach.

I've done some drawings for a vent hood topped with a clay flue insert that I'll post. I've also got some pictures of how it goes together in my Yahoo photo albums (photos.yahoo.com/colonelcorn76).

I built a steel vent hood that tapers from 20" at the base (top of the door height) to about 6x10ish (fits inside the rounded corners of an 8x13 clay flue section). The clay flue section is a 3' rectangular one that is stock at masonry suppliers and is used to line chimneys. It ran about $15. On top of this I've got a chimney cap & spark arrestor. Inside the "house" enclosing my oven is filled with loose vermiculite insulation.

The flue sections you've got should work fine. You won't find a noticeable vacuum effect and as a rule of thumb larger is better than smaller so you can get a good draft.

Jim