Cast dome with 2nd hand arch. Queensland.

[david s]

There is negligible clay in your sand so no need to reduce clay in the recipe.
When cutting firebricks the inhalation of brick dust is a dangerous activity, you really need to use a wet saw. Even after soaking the bricks for ages it still makes a large amount of dust as you no doubt found. However it seems like it's too late now. Also a cheap dust mask is inadequate as you've probably also already found out when you blew your nose.

[Nicholasd]

Laid the fire bricks down and cut the the pieces with an angle grinder (125mm). Soaked the bricks for a couple of hours while I exchanged the correct cutting disc, the inner diameter was incorrect. One disc cut about eight bricks before slowing down. Very happy the bricks are very flat with no edges catching on a trowel to test it.

[Nicholasd]

Now I'm back from camping, time to get into the oven again.

Here is a pic of the sand in water after letting it sit over night. I can see a very narrow band of clay on top (about 1-2mm) Do I need to adjust the homebew mix to account for this?.

[david s]

Because of the hot weather, I suggest you time your application of the dome cast for the late afternoon, say around 4 pm. That way the casting will be out of the sun and will be slower to go off. Also the use of chilled water will help. In summer I have to always do this to avoid wet casting material going off too fast. We’re in Townsville so the heat is an even greater problem. Two people applying the wet mix helps get the job done quickly, but make sure you wear rubber gloves or you’ll regret it. Cover the casting the next morning, but don’t use black plastic which will also increase temperature if the sun hits it.
The addition of the fine polypropylene fibres, apart from their use as burn out fibres to assist water elimination, also assist greatly in reducing early shrinkage cracking which is their primary function for standard concretes.

[david s]

Basically any sand will work ok and actually a mix of grain size is better than sand that has the coarser grains sifted out. Some brickies loam does naturally contain a little clay, but probably not enough to cause you any problems. Too much clay can impart more shrinkage than you want, leading to shrinkage cracks. You could test the clay content of your sand by simply filling a glass jar 1/2 full and topping it up to 3/4 full with water. Give it a decent shake and let it stand for 24-48 hrs. The sand will separate from the water with a small layer of clay in between, if there's any there at all.
The homebrew already has a high clay proportion and extra could impart the shrinkage cracking. However, as it really helps to bind all the other ingredients together to make it easy to apply, apart from imparting refractory qualities, it is an important ingredient.

[Nicholasd]

Very hot here today (for the gold coast), not going outside until some shade is over my work area.

My question is about the sand part of the homebrew.

I got some brickies loam for the cast before xmas thinking I would get some casting done and because the shops shut over the holiday period. Now I am not sure if it is correct after reading some FB posts about brickies loam/sand for homebrew. One post said that it already had clay in it. Is this correct? Do I have to adjust the homebrew recipe or look for different sand?

[david s]

That’s fine. Either side up, it doesn’t matter. It is the gaps between the tiles that you want to create.

[Nicholasd]

I managed to drill some weep holes in the slab.
I found some tiles in the garage that I can use under the CS board. They are fairly large 300mmx300mm so I was going to break them up in to pieces like in the pic. How does the size of the pieces and spacing look? I may have to get more as I may not have enough, but at this spacing of the pieces I may be able to get away with it.

I have seen pics where people put the glazed side down? Not sure on this though.

Cutting the CS today as in pic 3.

[david s]

How far out the CS board should be makes little difference as long as the whole oven is insulated under and over. Both CS board and blanket are super water absorbent so the parts in contact with the supporting slab also makes little difference. The 3 pics explain it better.(my own preference is 3). The oven can sit on top of the floor bricks or on the CS board.Sitting on top of the bricks is my preference because it does not require precise brick cutting at the perimeter. The downside is that you will never be able to replace a floor brick that is sitting under the dome. Any that ever get such a beating will be in the centre though, so that's not an issue.

If drilling the weep holes do about 3 near the centre and use a small pilot drill first in case you strike the reo. Enlarge the holes with around a 1/2" bit. glue some insect screen over the top of the holes to prevent creepy crawlies.
Also a good idea to place some ceramic tiles on the slab before placing down the This will allow water vapour a pathway between the tiles and out through the weep holes as well as elevating the CS board from getting wet if water enters.

[Nicholasd]

Main slab is poured with normal aggregate mix, mixed strong with more cement.

The perlite platform is just for the arch to get to the height of the CS (60mm) and firebrick (75mm) so its the same as the floor of the oven. I will have a 8mm(?) thermal break before the CS. I ended up using the very strong mix 3:2:1 so it is really strong but not very high insulation properties.

I planned weep holes but forgot! Should I drill them while the concrete is still a bit green? How many weep holes for a 85cm oven? Also do they need to line up with holes in the cal sil board (one piece) ?

Next step. Cutting the CS board to shape of the dome. 50mm casting on the firebricks.

• Am I correct in thinking that the CS board should be the same diameter as the external size of the dome? ie an 850mm/33" +50mm cast, will have the CS board cut to about 90cm. The Ceramic blanket goes all the way to the slab.

Thanks and have a good new year!

Nicholas

[david s]

If you are looking for strength in your supporting slab, don’t replace any of the heavy aggregate with either perlite or vermiculite. As your slab is only 75 mm thick you’ll need it as strong as you can make it. The strength of a slab is proportional to the square of its thickness. So a 75mm thick slab is only 1/2 as strong as a 100mm one. Replacing aggregate with perlite or vermiculite will further weaken it.
However, if you mean a cast slab on top of the 75mm one, yes you could make it weaker and lighter. A 5:1 vermicrete is considered strong enough (but don’t go weaker), as an insulating slab, on top of which an oven can sit.

Did you cast in some weep holes in the supporting slab to assist water removal from under the floor? If not they can be drilled after the slab has been cast.

Regarding the cal sil board, a minimum thickness of 50 mm is usually recommended. It depends on what thickness you can get. I have some that is 50mm, but more recently have only found 40mm available, so I do 2 layers. To obtain the equivalent insulating value of a 50mm cal sil board using a 5:1 vermicrete, you need to cast it 100mm thick.

[Nicholasd]

Just found this about making strong perlcrete.

From David s "You can make up a standard 3,2,1 aggregate, sand, cement concrete mix and use the perlite as the aggregate. This produces a fairly strong result, but reduces the insulating capacity. If you want it even stronger then make the aggregate portion of the mix 50% perlite 50% standard aggregate. Be aware that perlite takes up a large amount of water that you need to eliminate later."

The reason I am wanting strong concrete is to build a 100mm platform for the arch/gallery/flue to sit on. (photo at the beginning of the post).

[Nicholasd]

Here is an update to my oven.
Eventually went with a slab as it was difficult and expensive to get hebel board.

Slab is 75mm thick and has 5mm mesh in with thicker bars crossing the whole slab.

Next is to make a little platform to sit the arch on so it is the correct height to the finished floor of the oven. I was going to use permacrete for this in a 5:1 mix for strength. How does this sound?

I also have some questions about the CS board and how big to cut it for cast dome on brick (85cm/33"). I just have to measure and make a template to help with that.

[david s]

I think you’d be better to get a decent bond between the bottom blocks and the concrete under them, so plastic there maybe not the best solution. I use an additive in my supporting slab that makes it waterproof, or at least reduces porosity, to prevent wicking. Another method is to use a sealer on top of the filled blocks prior to Hebel or concrete slab on top. Some sealer between the under floor insulation and supporting slab is also prudent.

[Nicholasd]

Thanks for that info.
The oven sits on a concrete slab with a painted pattern on it.
The retaining wall sits on foundations but I'm not sure of the quality of this work as some of the walls are not filled with gravel.
"drill a hole in your concrete into which you can hammer a steel bar that will sit in the middle of the block cores up to the top block., mortar the first block, glue subsequent blocks on then fill the voids with concrete."
I was wondering about reinforcing the blocks to the slab, this looks like a good solution. Would this wick moisture up through the blocks? I was going to lay plastic under the blocks to help prevent this or is this not an issue with a core filled block column this high?
Our outdoor area is long, narrow and terraced so we don't want to take up too much room with an oven hence putting it in the corner hanging over the retaining wall. We do have some area up some stairs to a terrace above this but the ground is absolutely full of roots of now dead palm trees and isn't stable. I might think about that one though.
I'm also thinking about pouring my own slab instead of Hebel...