80 cm cast oven in the Netherlands

[Muls]

This summer, I am building a pizza oven with my three oldest kids. Goal: to have a functional pizza oven, a nice decorative element in the garden, and a fun summer (and maybe autumn) project.

So far, we have build the base. First we digged out the soil until we hit yellow sand. Then we filled it up with sand until 10 cm below surface. Here, we poured a concrete slab with rebar, 10 cm thick, and after curing we built a stand of cellenbeton, 'aereated concrete', lightweight yet strong bricks that need to be glued rather than mortared. On top we poured a concrete slab with rebar again, 10 cm thick, at least where the oven should come. Final size of the oven part is 94cmx114 cm.




We were enthousiast but didn't correctly plan the size of the concrete slab, so initially it seemed that my oven would become too small. Luckily I discovered this forum and with the help of some of you, managed to readjust my plans. First I decided to cast rather than use bricks. Secondly, I decided to only let the cast itself rest above the concrete; the other layers of the dome (isolation and finish) will extend beyond the slab. Assuming a (homebrew) cast of 5 cm thickness, my dome's inside diameter will be 84 cm.

I would like to offer the next steps as I have planned them. Please comment on them if I do something wrong.

Next days I'm going to :
- drill two holes in the slab for drainage and plug them with some loose steelwool to avoid critters
- put leftover bathroom tiles on the slab, 2 mm distance, for drainage (maybe I'll break the tiles in smaller pieces so there will be more 'canals', they are 12x12 cm now)
- put 2 layers of casi board on top of it, total 5 cm
- put a herringbone pattern of chamotte bricks on top
- cut the casiboard and bricks in the shape of the dome
- Then start the cast on the bricks

I will try to give updates and pictures on this forum.

Tomorrow, I am going to get the casi board and bricks.

I had some trouble figuring out the homebrew recipe, because it's ratio of 3:1:1:1 is by volume. Luckily, I discovered the excellent spreadsheet on https://community.fornobravo.com/for...l-to-buy/page3 (why isn't this a sticky?). With this spreadsheet, I came up with the following recipe:
- 3 parts sand: 104 kg
- 1 part lime (gebluste kalk): 14.1 kg
- 1 part powdered clay (from a pottery store) (17 kg)
- 1 part (25.2 kg) portland cement

As I couldn't find SS needles, I'll use 5 kg of glassfiber ('Witzand')
As burnout fibers, I found an online seller that can deliver 300 grams of polypropylene fibres (SikaCem Fibres) with diameter: 31 μm, length 6 mm

With regard to the fibers, are these ok?

Thanks in advance for reading and maybe offering your valuable advice!
I'll keep you posted.

[Muls]

Dear all,

Today, we removed the outer boards. As you can see on the pictures below, the concrete has some gaps in it. At the left part (the countertop) the concrete seems to have been poured better and more dense than at the right.

It has been the first pour in my life. I tried to shake and press it to remove bubbles, but apparently this didn't work out well. What do you think? Is this a total failure or can I move on? Is it too weak or do I run the risk of the rebar inside to rust? I followed the guidelines and had the rebar not too close to the edge, but I'm worried now.

I plan to finish this edge with decorative bricks. Maybe that helps to close it off?
Any thoughts?

[Muls]

As you can see in the picture, I put tiles (leftovers from our bathroom) in the shape of the oven, and made two drainholes of 1 cm through the concrete. I'll put casiboard and then bricks on top.

I wonder whether they will change place over time? The outer tiles are kept in place by the homebrew mix, but the middle ones might creep due to movement/temperature changes maybe?
Do others lay them loose, or is it advisory to glue them? Or put some course grid between them to keep them in place?

[Muls]

With regard to my question about the gaps in the concrete, I already have found my answer here https://community.fornobravo.com/for...-to-start-over

In short, it doesn't seem to be a problem.

With regard to my other question, I'll crack some tiles and put the shards between them, for extra stability.

[Muls]

See pictures for our progress. Next step: get a flue pipe, build the mould for the dome with sand and get our last ingredients (fibers & lime).

[Muls]

In the coming few days I want to make the mould with sand and then cast the oven. I'm going to work with homebrew mix, fortified with basalt fibers that can stand up to 800 degrees celcius, and pp fibers added as burnout fibers.

Now I'm stuck, and wonder...

How should I mix everything? Mix everything, and then add water at once? Or work in batches? How fast should I work before this homebrew mix cannot be used any longer?

I can hire a cement mixer but I prefer using my drill with mixer blades... what would be your advice?

[david s]

The mix should be fairly stiff so it will stand up vertically against the mould. It does get easier the higher you go and the walls lean in. If you make it too fluid you'll get slumping, too dry and you'll get more voids. You'll get at least 1/2 hr after thorough mixing before the mix begins to go off.The consistency is far easier judged by mixing in a barrow with a spade. Be careful to mix really thoroughly as the fine pp fibres tend to clump. When you think the mix has been turned sufficiently and looks ok, keep going and give it the same amount of time mixing. Wriggle handfuls of the mix against the mould to try to eliminate voids. Suggest you try a reasonably small batch to start with. Start at the bottom going all the way around the base and make a flat ledge at the top of the row so there's something to sit the next row on, this also makes judging the wall thickness easier.

You didn't leave yourself a lot of room at the sides.

[Muls]

Thanks, David! So mixing in batches it is.
This is easy for the homebrew itself, being 3:1:1:1. But is there a trick to mix the needles and fibers evenly across the various batches? Do you use a scale or just eyeball it?

You are right about the sides. I try to build the oven as large as possible on this stand, and am going to let the insulation and stucco overhang for now.
(See https://community.fornobravo.com/for...351-dimensions for my initial problems with the stand)

[david s]

The smaller and finer the fibres are, the more difficult it is to get them to disperse. This means it’s only the really fine polypropylene fibres that require the extra mixing. A surprisingly small amount is needed. About a small handful for every 10 litres of wet mix is sufficient. After the casting is laid up and set you will notice the tiny hairs sticking up from the surface. Apart from their function of burning out and providing a tiny network of mini pipes to help moisture escape safely, they also hold the wet mix together nicely making placement easier as well as reducing shrinkage and slump cracks.

I’m keen to hear about what you think of the basalt fibres. I’ve not tried them, but have priced them, they’re expensive, but seem ideal for our application. I use a combination of stainless needles.,AR glass and pp fibres.

[Muls]

Today we've cast the oven. See pics.
It was very fun to do for the kids and myself, and not so difficult (assuming it turns out ok). I think that our mix was a bit too wet, because sometimes it slumped a bit but this was easily fixed by pushing it upward again with a trowel. We ended up using almost exactly the amount of homebrew that was calculated.

I premixed the basalt fibers with the dry cement, so that the fibers would be evenly distributed. Then we would make batches of roughly one bucket (10 liters), one at a time. First we would mix the water, sand, lime, clay, and fibers. Finally we would add the cement and mix it through with a mixer on a drill.

Plan now is to let it dry in the open for 48 hrs.
Thursday, we'll remove the sand, fill holes in the inside of the dome, and cast the gallery.
Immediately afterwards wrap everything in wet blankets and cellophane and let it cure for one week.
Is this the way?


(NB About the PP fibers: it said 'water soluble' on the label in the shop. Indeed when added to water it immediately dispersed throug the water, so I assume that some kind of surfactant has been added. They didn't clump together and dispersed easily throughout the mixture. Maybe, in general, regular PP fibers could benefit from a bit of soap mixed in with the PP fibers, before mixing it with water?

NB2 About the basalt fibers: it was the most cost efficient option for me (and fastest delivery). Althoug it was a bit more expensive (20 euro for a kg of basalt fibers, vs 15 euro for a kg of AR fibers), the website of the vendor claimed that I needed less basalt fibers compared to AR glass fibers. I used 1 % of total weight, that meant 1.7 kg of basalt fibers (19 mm long).)

[Muls]

Last saturday, I cast the gallery. It came out pretty nice when I removed the mould yesterday. Some minor gaps but nothing that I couldn't fill.

I improvised the mould (too much improvisation in my oven design actually). But I think that it will suffice. As you can see on the picture, the flue pipe sits on the dome a little bit. It is a bit loose now (I put some cardboard in between) but I plan to tighten it up by wedging three screws or something around the pipe. That way, the pipe will be fixed but still be able to expand. My idea is that the heat expansion will deform the pipe due to the three screws, but not break the gallery. Am I right?

The pictures show the mould for the gallery. At this moment, everything is curing. When can I start putting the blanket on the dome, and the vcrete on the blanket? Three days left before the summer holidays are over...Also: the dome is connected to the gallery, but the gallery will be finally connected to the stucco. Right? I did not plan any heatbreak. Should I have? And must the blanket remain visible in the gallery , only to be hidden by a decorative arch in front? Or does the vcrete play a role here...?

[david s]

Hi Muls,
The oven is coming on well. Any formwork for a one off mould does not need to be too fancy or time consuming, it only needs to stay in place until the cast has set somewhat.
Both the dome and gallery castings should be kept damp for a week at least to allow decent hydration for the chemical reaction. After that, depending on the weather, at least a week of drying in the sun and wind before applying insulation layers. I usually cover the back half of the gallery with blanket while the front half is just covered with vermicrete. This results in a thinner insulation cover, but I figure that that part of the gallery is cooler anyway so it should be sufficient. I hope the pics explain what I mean. An outer decorative arch is not mandatory, but I do have an expansion joint between it and the flue gallery, mainly to reduce the pressure of the inner parts of the oven expanding and placing undue stress on the cooler arch (I've seen plenty of cracked outer arches and I'm sure this is the cause). The expansion joint gap is filled with blanket and sealed off with 6:1 vermicrete. This reduces heat flow by eliminating heat travel by conduction which makes the outer arch much cooler and prevents decorative tiles falling off.

This is only my solution, there are plenty of alternative methods.

[Muls]

Thanks David, for your valuable advice. The pictures are clear and I now better understand how to connect the two layers.

Question: in this scenario, am I right to conclude that the decorative arch could be made from regular concrete? Or would you still advise homebrew mix for this? I have seen some conflicting comments on the forum, but it seems that the decorative arch would never reach a temperature above 300 degrees celsius..


Based on your time scheme, I'll wait till the next holidays to put on the insulation, and spend the remainder of the time making the stand a bit more robust. I am not happy with the wall thickness (7cm) of the aerated concrete blocks and plan to add a second layer. As it is now, it seems that one could easily knock over the walls. In fact, I initially planned to used the thicker blocks (15cm) instead of the thinner blocks (7 cm) from the start, but somehow forgot about this. Luckily it's neither difficult nor expensive to place a second layer. Also gives me more room to round off the corners of the stand so to make it fancier.

[david s]

I agree about the use of standard concrete for the decorative arch as it doesn’t get too hot. Although as a further measure I use a different mix for the inner half. I replace 2/3 of the aggregate with perlite so the inner half is more insulating. The perlite addition is pretty conservative because it drastically reduces strength. Because I cast the arch outside face down I make a standard mix for the outer half and use 6mm steel bar as well as AR glass reinforcing fibres to a depth of 35mm then immediately use the more insulating mix to cast the inner 35mm half.

[Muls]

See the pictures for the final result of the cast.
This afternoon, I put a vcrete layer on the blanket. I already had plenty of insulation (two layers of blanket, three on top), so the main reason is to restore the shape.

However, I underestimated the amount I needed, so in the end some places ended up with 5 cm of vcrete, whereas other spots are less than 1/2 cm, and some blanket is even visible! The shape is definitely more round again compared to the picture with the blanket, althoug it is far from a perfect dome. Next year, I plan to finish it with a layer of stucco.

Will it be a problem that the vcrete layer is so thin or even absent on some places? Should I add a second layer of vcrete?