2 meter diameter New build oven with pumice reinforced with Basalt Rebar

[david s]

As your dome is massively thick and appears to be a hemisphere, you shouldn't need any buttressing. The strength of any unreinforced concrete is determined by the aggregate used and the proportion of cement in the mix. Eg. A 10:1 vermicrete can be easily scratched away, but is firm enough to render/stucco onto. A standard concrete mix with basalt aggregate replaced by vermiculite, perlite or pumice will have its strength approximately halved.

I hope you have a large forest nearby, you'll need lots of wood for that monster oven.

[Karangi Dude]

Hi Alomran,

I see that you have used some sort of paver for the hearth it looks like it is only 40mm thick, it concerns me what you have placed them on is that a thick sand base? with an oven that size and with walls that thick I would suggest that your oven could be out of balance. Most ovens have walls 115mm thick and hearth 75mm thick, if you have walls 220mm thick then I would suggest that the hearth would need to be at least 115mm or 150mm. With your walls being so thick it will take a lot of time and wood to get the oven up to temp that would then make your hearth very hot in comparison the your walls and then you would have a problem with the hearth cooling to quick.
Alomran I have had some experience with larger ovens see; Karangi Dudes 48inch Oven (https://community.fornobravo.com/for...-s-48inch-oven) I have worked on and cooked in many kinds of ovens wether they be igloo or barrel ovens and the one thing that is most important is to have your oven balanced heat wise (having the right amount of thermal mass and the right amount of insulation for the walls and the hearth)

Cheers Doug

[Alomran]

I'm confused by what you mean or intend by the use of "basalt rebar". Do you mean steel reinforcing bar in conjunction with basalt aggregate concrete? If so be careful if steel rebar is used in an area that gets pretty hot as it is likely to cause cracking of the surrounding refractory or concrete.

David: I was going to create a fire-concrete dome from fire cement reinforced by Basalt rebar and lined with heater storage bricks. However, after few experiments, the idea is risky. So, I am thinking for using the basalt rebar to reinforce the pumice outside the dome to hold the base of the dome. I am uncertain if the pumice will hold the dome external forces at the soldier course level.
The question is shall I risk it and reinforce with normal concrete> or shall I use reinforced pumice with Basalt rebar?

[david s]

I'm confused by what you mean or intend by the use of "basalt rebar". Do you mean steel reinforcing bar in conjunction with basalt aggregate concrete? If so be careful if steel rebar is used in an area that gets pretty hot as it is likely to cause cracking of the surrounding refractory or concrete.

[Alomran]

[RandyJ]

While what you essay there is true I don't think you have a lot to worry about from one exposure. I hope you were wet cutting the bricks. And if so you should have not had huge plumes of dust. If this is true then it was just a learning experience that should scare you.

Randy

[Alomran]

WARNING:

Heater storage bricks manufactured prior to 1984 contain asbestos which causes Mesothelioma cancer. A single exposure to the dust of asbestos could cause lung cancer. Symptoms could be hidden for 10-40 years then the cancer kicks off with aggressive symptoms. https://www.cancer.gov/about-cancer/...tos-fact-sheet
I have exposed myself by cutting the heater storage bricks then read the text!
If however you acquire the heater storage bricks manufactured after 1984, then you should be OK.

[Alomran]

Following 6 PRP treatment on my both elbows and shoulders, the oven adventure is commenced! Please forgive the harsh details and the roughness as this is the first time for me to lay a brick and mix morter. The lines are not perfectly straight, but the fun is great!
I am pretty convinced that using engineering bricks is far better than using fire bricks as PeterBennete has published on another page the following:

High Duty, when the alumina content is high it makes the bricks more resistant to very high temperatures and erosion/hot corrosion. The reason is that the other constituent in them is silica. On the net you can find the alumino-silicate phase diagram, essentially at the high alumina end the brick's melting point is extremely high 1828C, whereas once the silica content is above 30% this drop down to a mere 1587C (and stays there unless fluxed lower by other contaminating oxides, e.g. calcia).

So that is the duty side of things clarified, a non-starter for cooking ovens, but what about when in direct contact with the fire, what is the max. temp?

Heat retention is a completely different matter, here we are talking about thermal mass and thermal conductivity. If you want the brick to store a lot of heat you want it to have a high thermal mass (and not lose it by conducting it away to cooler regions), in simple physics terms you want it to have a high specific heat capacity, i.e. the heat/energy required to raise a unit mass, Kg by 1 degree, now here is where densification comes in. If the product is quite dense it will have a higher thermal mass per unit volume than a less dense version. Now replacing alumina by silica or vice versa will [B]not[B] necessarily improve or reduce the thermal mass, it depends on their relative specific heat capacities (& how dense the brick is made when you consider this on a volumetric rather than a weight basis, as noted above). My guess is that their specific heat capacities are probably quite similar, so probably a non-issue?

From tables, an engineering brick has a specific heat capacity of about 1.0 kJ/KgK, whereas a firebrick has a specific heat capacity of about 1.05 kJ/KgK.

So unless you plan to melt steel the high duty firebrick could be a bit of an overkill, or is it?

I have updated another post on this type of topic with info on typical thermal conductivities of firebricks, engineering bricks and a couple of red /buff common bricks (not concrete common bricks). Not an issue if you choose the right brick.

I hope this is helpful and helps clears up the various issues involved (but I don't know the max duty temps.)

[Alomran]

I have just researched the use of lime worldwide. I found it puzzling that by mixing only mud & lime you can build as high as 15 floors and even a minaret that goes as high as 53 meters as they did in Yemen for centuries

[jonv]

It applies to the inner opening into the dome, which is where the door is placed for retained heat cooking.

[Alomran]

David: Thank you for the information.
Is the 63% formula for an opening in proportion to the dome's size applies to the door opening or to the opening leading to the gas/smoke chamber?

[staestc]

The inside of the dome needs to be fire brick with as little mortar showing as possible to reduce the chance of mortar crumbling/cracking/falling out, etc. Using lessor bricks can lead to issues in that they are not as conductive as fire bricks are and they do not have the heat capacity of fire brick. Out side of the fire brick you can certainly increase the thermal mass with additional refractory mortar, more bricks, whatever you want. The upside of additional thermal capacity is that the oven will stay hot for a longer time, which is useful if you are doing multiple loads of bread and the like. The downside is that it takes more fuel and time to get it up to temperature for use, so you have to start your fire much earlier that with an oven with lower thermal capacity. For most people you need little additional thermal mass than that provided by the fire brick in the dome itself.

[Alomran]

If Victorian red bricks are used in a way whereby the width of the dome is 22cm, that would increase the thermal mass. What negative side this would have? Isn't it wise to make the walls thick in order to store the heat inside the bricks?

[Alomran]

Staestc: Great, many thanks, I appreciate your feedback.

[staestc]

1- If the pizza oven heath is extended to the outside of oven, doesn't this action create a thermal bridge to the outside? Should I stop the heath made of the heater storage bricks to the door?
2- How do I insulate the oven? how thick is the ceramic blanket I should use? Is there a need for rock-wool as well as ceramic blanket as well?
3- How thick should the Leca/ pumice/perelite layer light concrete insulative external layer be?
4- I am using a pumice mixed with cement as an insulative layer for the pizza oven heath, Is 10cm thick is adequate? Upon which a heater storage brick of 5cm will be positioned. Is this more than adequate?
6- Is it worth using an aluminium foil then a 1cm layer of sand between the pumice and the heater storage bricks?

I'll take a shot at answering your questions, but I am by no means an expert. I have yet to build my oven but have studied a lot. I am sure those way more experienced than I will chime in and correct me!

1 - Are you talking about the oven floor in this question? Fire brick are indeed conductive, which is why you want them as your floor and inner layer of your dome. Yes, extending the floor out side of the dome does allow some heat loss through conduction, but it is very minor. You have way more heat being lost up the flue. Having the floor extend outside the dome to a point where temperatures allow you to use common brick, granite, slate, etc. seems to be the approach most accepted.

2 - The most common and apparently easiest method is to use 3 layers of 1 inch thick ceramic blanket. A lot of folks that are building a full enclosure around their domes fill the remainder of dead space with vermiculite or perlite just for good measure, but it is likely not necessary. See the host site's .pdf for the insulating blanket they sell at the link below. Some ovens have been guild using rock wool (gypsum), but it is not as efficient an insulator I believe.

3 - Not sure about this one. Most builds I have been through look like 1-3 inches.

4 - Yes, 10 cm is roughly 4 inches and that is pretty much an accepted thickness of the insulating concrete on top of the normal concrete stand. Note you will have rigid insulation board on top of that as well, on which your fire brick or fire tile floor will rest.

5 - Have not seen it done and do not think it is necessary. Aluminium will survive the temperatures and it probably would not hurt anything, but if you are going with rigid insulation board anyway you would not need the foil.

Like I said, don't take anything I said as gospel as I am still learning too! Cheers