Another attempt at a brickless oven build with HomeBrew

[dannok]

Hey Chicharron,

I plan on doing something very similar in the next couple of weeks, although I'll likely be building a sand form and casting in 3-4 pieces to enable dismantling when we move. Worst case - the thing lasts a season or two and breaks when I move it but will only have to replace the dome.

I also hope to cast a vermicrete shell that can be lifted off the main dome, we'll see how that goes... I know my build will be experimental like yours but hey someone's gotta try something different every now and then!

Have you constructed a base yet or have you got an idea of what you're doing? Also how much homebrew mix did you end up needing for your dome size (700mm ext? and what wall thickness did you end up with?)

Cheers

[Chicharron]

Hey dannok,

Yeah I feel most of the builds are very traditional and very similar (also very professionally executed). Nothing wrong with that, I would do that too if I owned a property and/or had the resources. I just find difficult to believe there's not a lot more people in a similar situation and needing something much lighter and easier to move and take with them.

I made mine 600mm internal and 50mm dome wall (I did 30mm for the entrance wall which I found harder to do than the 50mm one) hence the 700mm external, although I must admit I found it very hard to keep the consistent 50mm throughout. What I did was I had to poke the concrete very frequently only to find I was under 50 every time, so I then had to cover the hole I'd just poked.

I'll get you the exact figures when I have look at my notes back home but from memory my calculations gave me I needed around 56 Lts (56,000 CC) of mixture. I'll get you the exact numbers in KG of dry mix.

I haven't made the base yet but my idea was to build it modular as well. So, my plan is to build a 140mm high "Container" made out of wood and pour 100mm of Percrete (5-6:1 mix) and put the 40mm firebricks on top of that. Then sit the dome on top of that without "Gluing it" not sure how I'm going to do that yet. And then sit the whole thing in either pallets or build a wooden base.

I was also thinking on casting the vermicrete shell so that it can be separated but I'm just not sure how that would work and how to achieve that. What's your idea?

Cheers

[dannok]

Excellent cheers.

Thinking along similar lines with the base, although I might use 600x200mm 100mm thick hebel blocks in place of the vermicrete slab. As for the shell, I was just going to wing it when it comes time but probably use foil and cardboard over the inner dome once it has cured. Then form up the vermicrete ~50mm thick, removing the cardboard after its dry and leaving a void ~5-10mm thick between inner and outer dome to allow expansion etc

[Chicharron]

Mi initial idea was also to use Hebel blocks as well since they are cheaper than CalSil boards. But I read somewhere that they won't work or fail in this application. I forget the reasons and forgot where I read that but I think I read they would fail with the oven's operating temperatures for sustained periods of time but I'm no sure. Maybe someone can chime in?

[david s]

I'm in the process of demolishing my mobile oven as it is now 9 or 10 years old and has had lots of abuse.It still works perfectly ok but is full of cracks so not a good example of my finished product to show customers. To save on weight I used Hebel Power Panel which is 75 mm thick with 5mm steel bar cast into the centre. I thought I could get away with using it as both underfloor insulation and structural slab. I will not use it again as it cracked badly either from heat or transporting over rough roads including corrugated roads and speed bumps.Pics show cracking from top and underneath.
I'm sure it would be fine if used in a stationary application as Hebei (AAC) is the same as vermicrete, both using OPC and has the advantage of being dry so you don't need to drive the water out of it, but as it is made with standard cement, will not like temps exceeding 300 C. Regarding its strength it is around 1/3 the weight of standard concrete and therefore only 1/3 the strength. I will be using a precast reinforced concrete slab this time.

[dannok]

The Hebel will be supported and not structural in my build, so even if it did crack I'm not too worried. But seeing as though yours has lasted almost a decade in a structural and mobile build im confident it will be fine for my purposes.

[Chicharron]

Well, Hebel Power Panel it is then! they're much cheaper I'm not sure why people keep recommending CalSil Board or Vermicrete as a more economical option every single time. I get that it's probably a lot better. but hey 9 years sound pretty good to me to TBH and they are probably even cheaper than Vermiculite anyway + you get to skip the whole process and drying time.

I'd say it' may have been because of the transportation side of things. it probably wouldn't have cracked if it was sitting on a more solid flat surface and not moved? but who knows?

Did you ever feel if it got hot at the bottom at all?

Also do you see any problems using Hebel blocks instead of panels? there's a 100mm one that easier to find than the power panels plus easier to work as you can easily cut them to fit.

[dakzaag]

Several years ago for a temporary oven, I placed two pieces of Hebel 4" thick 2' X 4' under a 48 inch oven. We set the floor brick on a thin bed of sand and supported the whole thing with Angle Iron and 4" CMU laid on their side. Fired the oven for aprox. 36 hours straight. 150 pizza and various other confections were happily consumed. After demolition of the oven, the Hebel panels were 5-6 pieces instead of 2. I think the Hebel works just fine to lighten the overall weight, but I think it needs a 1-2" buffer of insulation to protect it from the heat of the oven floor.

In the temporary build, it is possible the Hebel panels were not perfectly supported and cracked from uneven distribution of weight, but the pieces were all pretty much evenly cracked leading me to think that the heat created the crack. The Hebel had no rebar in it at all.

[david s]

Dakzaag, I'm inclined to agree with you. I can't be sure to what extent my Hebel cracking is due to heat either, but the pattern of the cracks suggests to me it was the heat. It's interesting that the pattern looks something like a spiders web with the centre coinciding with the centre of the oven floor, where the concentration of heat under the floor would be the greatest. I also agree with you re the 1-2" of a more refractory insulation buffer to protect the Hebel.

i know glass slumping craftspersons use Hebel for forms to slump their glass over because it's so easy to shape and they tell me they get about 3-4 uses out of the Hebel forms before they are no good any longer. Glass melts at around 900 C so plenty hotter than we would give it.

[dakzaag]

The cracks I witnessed were spider web like also, I am really surprised you were able to get 10 years out of the mobile unit. I built an oven on 4 2' X2' Hebel squares with an angle iron support grid beneath and a 2 inch Insblock below the floor. It's about three years old now and no cracks so I am wondering if the grid has a little give to it maybe from heat expansion?