Homebrew cast oven by the sea

[CoastalPizza]

I've started the build of my 32" home-brew cast oven. I've removed the sod from the area where I'll be pouring a 4" thick slab. I'm planning for the slab to be level with the slab under the house so will be adding a bit of fill dirt (sand) to the area before I place the form(the line in the photo is level with the under-house slab). A few questions:

1. The stand will be 3x3 CBUs - so 46.5" on a side. I was thinking about making the slab +3" all around making it a 52.5"x52.5" slab - is this big enough?
2. I was figuring on 8 pieces of 1/2" rebar in a grid pattern - I seem to recall that the rebar shouldn't be closer than ~3" from the edge - is this correct?
3. How long should the slab cure before stacking the CBUs for the stand?

Thanks!

[david s]

It is always better to leave yourself a little more room than you need. I'm not sure how you plan for insulation etc. but if you are planning a 32" internal oven diam, and a 2" inner casting with two layers of 1" blanket, then a couple of inches of vermicrete and a 1" outer shell (assuming igloo rather than dog kennel style) it all adds up to 46". However the entry at the front will take up extra room so you need to account for this too. The supporting slab needs to be deeper than it is wide to accomodate this. If you build the supporting slab counterlevered over the stand you can make it bigger than the foundation slab, but it requires more form work to build it this way. The foundation slab should really be the same size as the supporting slab for stability.

Yes, keep the steel a couple of inches inside the edge of the concrete slab.
Put plastic down before pouring the slab and cover it for a week. This will retain moisture to enhance strength.

[CoastalPizza]

Well its been a bit over a year since this project started. I got the stand finished early last September and have done nothing since then - at least the concrete is well cured! I've started working on it again and hope to finish this fall. I have the FoamGlas for the hearth, SS needles and PVA fibers for the home-brew castable. I have CalSil board that I was going to use over the FoamGlas but it just soaks up too much water so I'm going to pour a 2" layer of Perlcrete over the FoamGlas before laying down the firebrick - I don't want to put the bricks directly on the FoamGlas since it only has a 900°F service temperature. I'm picking up Perlite, WG firebricks, lime, fireclay, Portland cement, type S lime, mason sand, and some more cut off wheels for my grinder (brick cutting). The form for the ring is almost done and will get some sand inside to support it before I pour the castable. I have a Harbor Freight concrete vibrator that I'll use to get the castable really settled into the form - it really helped with getting the hearth slab nice and even. Here are some pictures to get caught up:

[CoastalPizza]

I managed to get the hearth finished last week. I placed 3.5" of FoamGlas as a base then 'poured' perlcrete around and over to encapsulate the FoamGlas and provide a 2" perlcrete layer between the fire brick hearth and the FoamGlas. I added some more perlcrete around the fire brick to lock it in place (see photo). I also finished the ring mold that will be placed over the hearth and filled with home brew mix.

[CoastalPizza]

This last weekend I dry mixed a bunch of home-brew by tumbling the mix in a covered cement mixer for 15 minutes or so (photo 1). The resulting mix was way too much to mix in one batch so I pulled out about 2/3rds into 5 gallon buckets (photo 2). I added water and mixed the remaining 1/3 for about 15 minutes to get a nice smooth and pourable mix (photo 3). I then filled the ring mold and used a concrete vibrator tool to settle it into the mold (photo 4). It had 4 days to cure before Florence hit and is now wet curing (~34" of rain so far!).

[david s]

I’m not sure you have the correct fibres. You need polypropylene fibres not PVA. The polypropylene ones are designed for increasing compressive strength in concrete, but in our case will not provide any strength because they melt at 160 C. They should be quite short (approx 15 mm) and finer than human hair. I suggest you test a small amount in your kitchen oven to check their melting point. Once melted they then provide a network of tiny pipes that will assist in safer water removal without reducing the strength or density of the castable. Because they are so fine they need more mixing time for good dispersal as they do tend to clump together

[CoastalPizza]

David, you are correct - I used two types of fibers: about 3% by weight of stainless steel (melt extract) needles and a couple of cups of plastic fibers (in ~12 gallons of mix) that will burn out later. The SS needles were indeed about 15 mm in length. I had heard that it takes a long time to mix both fiber types uniformly which is why I did the initial dry mixing. I'm not positive that the plastic ones are PVA but even if they are, PVA has a 200°C melting point - plenty low enough to burn out.

[CoastalPizza]

We are back home after Florence and the oven survived - the banana trees are looking a bit ragged and some of the sand washed out from under the corners of the base slab, but otherwise everything looks good. Now to remove the ring mold and fill the core with sand so that I can pour the dome.

[CoastalPizza]

The ring has been unmolded and it looks pretty good. You can see the 3/8" holes from the spacers I used to hold the inside and outside mold forms in place - should I fill these? I also pulled off the form I used to pour the perlcrete over and around the Foamglas and around the firebrick. I put a back bevel on the firebricks along the front of the arch - there will be another set of beveled firebrick (or other material) to form a front counter with the wedge of perlcrete acting as a heat stop. Next up is to fill the core with sand and pour the dome. Then comes a poured outer arch and chimney.

[CoastalPizza]

Mid-October I filled the center with sand and cast the dome. The pivoting arch over the oven swings around and acts as a guide - first for the cast dome (photos #1 & 2) and then it was adjusted to act as a guide for the perlcrete that will go over the fiber insulation. I added a row of fire bricks along the front that are beveled to match those on the floor of the oven - this should give a pretty good thermal break (photo #4). Here are the pictures:

[CoastalPizza]

Today I wrapped the casting in Rockwool insulation (~3.5" thick) and built the form for the outer arch casting. The form also has side 'wings' that will support the vent assembly that will be cast later.

I'll pour the outer arch and start building up the perlcrete over the Rockwool tomorrow. How thick a perlcrete layer can I stack against the fiber insulation before I have issues with the perlcrete slumping? How long should I wait before adding another layer?

[david s]

I used Rockwool insulation on two ovens, but now only use ceramic fibre. Although Rockwool is slightly cheaper and almost as good an insulator, it’s draw back is that it is much softer (at least the product I used was, yours could be different) than ceramic fibre blanket. This made working the vermicrete against it, particularly low down where it has to stand up vertically by itself, way more difficult. I’d suggest you work all the way around the bottom to a height no greater than 6” then allow this to set for 24 hrs. This will give you a firmer base to work on as you go higher. Another problem with Rockwool is that it is also more water absorbent than ceramic fibre blanket. You can make the layer as thick as you want, but it’s primary purpose is to provide a firm surface to render against as well as restoring a nice rounded form and surface. Around 1.5” should be sufficient.

[CoastalPizza]

I used Rockwool insulation on two ovens...it's draw back is that it is much softer (at least the product I used was, yours could be different) than ceramic fibre blanket.

The Rockwool insulation came in 48"x16" sheets that are 3.5" thick. They aren't too soft (firmer than typical glass wool insulation) but did require the use of cotton string to compress it a bit and to hold it in place (see the first picture). I used cotton so that wouldn't be any noxious fumes when it charred or burned off.

I'd suggest you work all the way around the bottom to a height no greater than 6" then allow this to set for 24 hrs.

I first poured the outer arch / vent support and then I did as you suggested: I built up the perlcrete over the Rockwool to about 6" in height. I also applied a thin layer of perlcrete to the center 2/3 of the dome. It now all covered with plastic and I let it cure overnight before adding more perlcrete.

[CoastalPizza]

Continued to build up the perlcrete - I'm about 80% there so tomorrow might see it done. I also built the mold for the vent arch and will pour that tomorrow as well. I added a vent at the apex (see pictures) and another internal to the vent arch.

Also, here are a few pictures of the Rockwool: the bats are 3" thick and compress to about 2.5" when loaded with a 3/4 gallon jug. The cost was about $40 for 12 batts - I only used 5 so I'm not sure what I'll do with the leftover.

I have two section of clay flue liner (8.5" O.D. - 7.25" I.D.) that I'm going to use as a chimney - do I need to insulate this with perlcrete or can I just cover it with stucco?

[david s]

If you don’t insulate it then it will crack. A 10:1 vermicrete mix around it will work well. You can then stucco over the vermicrete.