Homebrew Castable 21" Kent (UK)

[bamforp]

Hello all - I am a first time poster, medium term lurker planning a 21" homebrew castable oven for my son in Kent (UK). Space and budget are tight so a 21" along the lines of David S's ovens looks like a good fit for us. I think we are broadly following convention (concrete slab, brick base, 100 mm Thermalite blocks resting on lintels, homebrew dome cast over sand followed by second cast for gallery, 50 mm ceramic blanket, 15mm render and probably waterproofed with white tanking slurry). The two areas where we might diverge a little is that I am planning to use AR glass fibres in the homebrew rather than stainless needles (but will include the PP fibres) and we are also planning to use a cast 50 mm base (probably cast as two halves) as our cooking floor rather than firebricks. Are there any reasons why a cast cooking floor is a bad idea beyond aesthetics and the possibility to replace firebricks? Also, any thoughts on AR glass fibres?

Regards

Peter

[UtahBeehiver]

You will need a fire brick floor. Cast floors just do not work due to expansion and contraction of the material. AR glass fibres are not a substitute for melt extract needles.

[Nick J C]

I was also wondering if ar glass fibre would do, as presumably they never get close to melting point.... I bottled out and used stainless but would be interested in knowing if they work from others experience.... I wonder whether the heat might cause them to shatter.

[bamforp]

Hi Nick, I read this paper and my main takeaway was that glass fibres were good to at least 400C so worth a try. They also appear to have a similar stress-deflection curve to stainless needles up to 8Mpa (1160psi) and according to the study delay macrocrack propagation and reduce brittleness. Now I'm not a mechanical engineer so there's every chance I'm misinterpreting the data and it's only one study but bottom line for me given our constraints it's either glass and PP fibres or just PP fibres.

Glass fibres in refractory.pdf

[UtahBeehiver]

I suggest you reach out to David S, he is our resident cast oven expert.

[david s]

Thanks Bamforp, that's an excellent paper. Actually I've been using both stainless and AR fibres in my product for a few years now, so the paper confirms what I thought. Both fibres are quite expensive and difficult to purchase in small quantities. I really like the AR fibres to work with because they are much softer. I also now use nano fibres with refractory, but have yet to test their improvement in strength, which I have done with regular concrete which shows a 50% increase in flexural strength.

[UtahBeehiver]

So I learned something new about AR fibers.

[Gulf]

That is good to know. I have a phobia to ss needles. Both to being injected and ingested

[bamforp]

That is reassuring Dave. There was a supplier on eBay in the UK selling small quantities of the Eccotexx glass fibres so I grabbed a 100g bag for £3.50 which is enough for around 100 litres of the homebrew. That should be enough for the dome and cooking floor, assuming I go ahead with a cast floor rather than firebricks. Do you have any thoughts on the idea of using a homebrew cast floor instead of firebricks? Is it a really bad idea?

[david s]

There are 3 things holding the homebrew together, cement, lime and clay. The cement begins to fail north of 300C, the lime north of 500C while the clay does not become permanent until 573C. In addition the 500-600C range is quite unstable with materials experiencing different expansion rates. While the brew proves adequate for the dome the fire sitting directly on top of it leads to fluctuating temperature and uneven expansion. In addition the larger any casting the greater the tendency to cracking due to uneven heat. For this reason floors are better with smaller units and laid loose to allow for individual expansion and contraction.
So no the homebrew cast floor is not a good choice, a firebrick floor is better. A number of UK builders have found storage heater bricks seem to work ok, although as they’ve been designed for a different purpose some may be ok while others not, no guarantees.

[bamforp]

Thanks Dave, that makes sense. I haven't managed to source fire bricks locally yet but will keep looking otherwise I will just have to have them shipped but that gets expensive for a small quantity.

[bamforp]

Well, after almost 2 years I am getting back to building a WFO at my son's home in Kent. I managed to complete the support structure in 2020 before we descended into lockdowns and the project ground to a halt. It's built with recycled bricks with a concrete lintel across the span. It was my first attempt at a brickwork project so it is not the neatest but it is strong and it's going to be rendered in any case. Wind forward 2 years, and I finally have a window to start the next stage. I used the Google Drawings app to sketch out my ideas and I realised I could probably squeeze a 600mm diameter oven in the space available rather than the original plan for 21".

In a departure from the usual reinforced concrete base, I went with 2 layers of aerated blocks placed in alternate orientations and then glued together to form a single rigid block. The glue is a polyurethane construction foam that cures very quickly and results in a really strong bond and the blocks have pretty good thermal conductivity properties at 0.16W/mK. Obviously I am expecting weakening of the bond for the parts that are exposed to high temperatures but I am reasonably confident the structure will remain stable enough to support the weight - time will tell.

I marked out the diameters for the dome and insulation on the blocks to make sure I had the positioning correct and then laid the firebricks on a bed of soft sand with a bit of fireclay added. I went with a layout that minimised the number of cuts partly to save time and partly to reduce the amount of dust I generated on their nice newly landscaped patio.

[david s]

if you are relying on the aerated concrete as an insulator, with the cooking floor bricks directly on top you will find that they crack badly but don't disintegrate. I used them on my first mobile oven, but on rebuilding it after around 8 years use found they were not strong enough. Part of the problem was the oven bouncing along roads, but the heat also certainly contributed to their failure.A layer of calcium silicate board between them and the cooking floor would fix the problem though.

https://community.fornobravo.com/for...ion#post398266

[bamforp]

The next day built the forms for the dome entrance and some gauging tools from a sheet of 50mm polystyrene insulation. I was a little worried the styrene would be a bit crumbly but in fact it cut very cleanly with a sharp jigsaw blade and it was then very straightforward to wrap the pieces with some wrapping tape to seal the edges.

I used Google Drawings again to create a nice clean profile for the dome mouth which I then printed put and transferred to a cardboard template. I added 50mm to the 'mouth' template using a compass to give me the face profile and then taped and screwed up the various bits to create the form for the dome mouth. If you look closely at the final photo you will notice that the form is sitting on a couple of slim strips of wood to raise it off the firebricks to make it easier to release the form later on. This worked really well and I was able to remove the form in one piece once the casting had hardened.

[bamforp]

if you are relying on the aerated concrete as an insulator, with the cooking floor bricks directly on top you will find that they crack badly but don't disintegrate. I used them on my first mobile oven, but on rebuilding it after around 8 years use found they were not strong enough. Part of the problem was the oven bouncing along roads, but the heat also certainly contributed to their failure.A layer of calcium silicate board between them and the cooking floor would fix the problem though.

https://community.fornobravo.com/for...ion#post398266

Hi Dave - I did 'cook' a piece of aerated concrete in my bbq a while back to see how it coped. My highly unscientific conclusion was that whilst it lost a lot of its tensile strength it retained much of its compressive strength. In other words, if I tried to bend it, it was not difficult to break but if stood on it, it did not collapse. I am hoping that in a static oven, even if the top layer loses some tensile strength, there will be enough residual strength in the lower layer to prevent any serious structural issues. Time will tell but I agree a layer of calcium silicate board would be a belt and braces solution. I will keep the forum updated on how it performs.

By the way, I just wanted to say a big thank-you for your generosity in sharing your experience and knowledge on the forum. Your posts have been my go to source for all things related to cast ovens as I pulled my own ideas together.