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

[Alomran]

[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]

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?

[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

[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.

[Alomran]

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

Thank you for this valuable logical advice. Perhaps, you have saved my project from failure with your paragraph to which I am most grateful.
Allow me to ask...I do wonder if the heat storage bricks that contain steel in it's ingredient are designed to store heat for a very long time would retain heat as long or even longer than the bricks I have used?
Given the heat storage bricks are 40mm and they are sitting on the sand bed,, do you suggest
1- Reducing the sand bed and increasing the Hearth thickness? or leaving the sand bed as it is but adding more hearth thickness?
2- Increase the hearth by the same thickness as the dome walls?
3- Can I add another layer of heat storage bricks or do you think this material is a fast absorber of heat and a fast emitter too and should use a slower heat absorber?

Alternatively:
I have a 3cm perforate terracotta clay tiles that I can add on top of the heat storage bricks with approx 1cm air gap, do you think this is adequate if the dome walls are half a brick thick?

[Karangi Dude]

Alomran,

Try not to over think how these ovens are supposed to be when they were first built some 2000 years ago they did not have the materials that we do today they built them with whatever was available, they used things like sand as insulation which really is thermal mass except for the air in between the particles that acts like insulation so sand is neither a good thermal mass or a good insulation
There is basically two components thermal mass and insulation, thermal mass to absorb heat and insulation to help retain that heat
Any solid clay type brick is a good thermal mass the difference with a clay brick and a fire brick is that firebricks are a little more dense and don't react to thermal shock like clay bricks do
So a good Wood Fired Oven consists thermal mass and insulation under and over the entire oven
I my opinion I would remove all of the hearth material including the sand right back to the insulating material that you have used and replace it with firebricks if you can afford them or solid clay bricks laid on there side that will give you 115mm of thermal mass for your hearth
Also I would lay them in a herringbone pattern on a thin layer of sand say 5mm thick just to get them level

Cheers Doug

[Alomran]

Doug, Thank you for your valuable advise stemming from experience. I have tried to remove the heat storage bricks and the first 2 broke into pieces as they are brittle and the gaps in between the bricks are filled with filling cement. Given that the heat storage bricks may contain asbestos, I think I will leave them as they are due to health hazard and add a thin layer of clay bricks on top to make the total of approx 110mm thickness so that it is equal to that of the dome wall/thickness.
Thanks again for saving the oven!

[Karangi Dude]

Alomran,

That all sounds pretty good laying clay bricks on there flats over the storage bricks should be fine, could you please clarify your intended wall thickness

Cheers Doug

[david s]

Because there are holes in those tiles they become less dense and more insulating. The holes will also reduce their strength and perhaps make them more vulnerable to cracking. You'll soon find out. I notice that some kiln shelves are now produced in the same way, not sure of their performance relative to solid ones, but the reduced thermal mass would be an energy saver.

[Alomran]

Because there are holes in those tiles they become less dense and more insulating. The holes will also reduce their strength and perhaps make them more vulnerable to cracking. You'll soon find out. I notice that some kiln shelves are now produced in the same way, not sure of their performance relative to solid ones, but the reduced thermal mass would be an energy saver.

David, I am not going to use the pictured proferated terracotta on top of the heat storage bricks as I hadn't considered the uniform thermal mass. Instead, I will be using terracotta building bricks as a hearth. The fire bricks in the UK are expensive, the asking price is £1.65 for each plus 140 miles courier/shipping cost. This is why I have resoluted to using normal clay bricks and heat storage bricks. I have done an experiment on them and the building bricks I have seem to handle sudden heat rise as I put it on a gas stove for 40 minutes and it handled it very well.

[Alomran]

Alomran,

That all sounds pretty good laying clay bricks on there flats over the storage bricks should be fine, could you please clarify your intended wall thickness

Cheers Doug

The wall thickness on the soldier course was a blunder!. I was thinking of making a low dome rather than a sphere prior to reading your post and your published comments. My intention was to make a structural support for the loads spreading on the base of the low curvature dome on the base. At the same time, I did not have in mind the importance of the uniformity required of thermal mass. Only after you have mentioned it I became aware of it. The thickness the wall is 200mm plus I have added a clad of 70mm thick heat storage bricks facing inward to absorb heat and to reduce the huge diameter of the dome by a total of 140mm. Please bear in mind that i wanted to cook lamb and cook numerous dishes all at once when cooking for family and friends. I have been to Sardenia and saw this huge oven that they used to cook anything and everything with such flexibilty offered by the size of the oven. I am also keen on this Turkish meal that we had in Turkey and this is the real reason behind making the oven. https://www.youtube.com/watch?v=mWAuxL-ue0U
This is why I needed a large oven begging to point out that I live next to a forest with plenty of soft silver birch logs.
Cheers

[david s]

David, I am not going to use the pictured proferated terracotta on top of the heat storage bricks as I hadn't considered the uniform thermal mass. Instead, I will be using terracotta building bricks as a hearth. The fire bricks in the UK are expensive, the asking price is £1.65 for each plus 140 miles courier/shipping cost. This is why I have resoluted to using normal clay bricks and heat storage bricks. I have done an experiment on them and the building bricks I have seem to handle sudden heat rise as I put it on a gas stove for 40 minutes and it handled it very well.

There is a reason firebricks are recommended. You should get away with using solid reds (not sure about heat storage bricks) for the dome, but the floor is a different matter. Placing a brick on the gas stove for 40 mins is nothing like having a really hot fire over it for a few hours. Also doing it once will be no indication of long term serviceability. They may work fine for a year or two then start spalling. You may get away with them, but as they're not designed for a high heat application and there's no way of knowing their composition, the only way to tell is by trying them out. I suggest that if you want to go ahead with them, lay them loose on a thin bed of dry 50/50 clay/csand levelling layer, so they can be replaced if necessary.

[Karangi Dude]

If you recommend using it in building WFB, how long do you leave it for it to cure before you fire up the oven?,

There are different kinds of mortar that can be used, the common factor is there will a lot of moisture in your mortar and in the bricks being used
There are also a lot of other factors like where you are building the oven, is it under cover or outside where it will be subjected to rain etc.
If you have taken care not to let the oven get wet, I would suggest letting it dry by air and sunlight (covering it over at night) for a couple of weeks before starting your curing process
Alomran, there is a lot of discussion on curing on this forum and it is up to the individual to make a decision as to which is best.
My recommendation is not to use any flame at all by using heat beads / briquettes for several days, but as I said there is a lot of info on this forum about curing

Cheers Doug

[Alomran]

My oven is in a woodland and I have one extension cable that extends 12 0meters away from my home before getting to the WFO. Given the distance from my house, the area is very darek under trees, I am wondering if there any flood light that can installed inside the oven and designed to withsatnd high temperature?