Homebrew Castable 700mm Sydney, AUS

[kilian]

Hi there!

We are a few weeks into building our homebrew dome oven, and just finished the foundation.

Now, the next milestone is casting the homebrew dome.
As it seems crucial to get the mix right, I was hoping to get some feedback on our ingredient choices. Does this seem right to you?

Sand: Does the type of sand matter at all?
Cement: Bastion GP Cement
Lime: Australian Builders Hydrated Lime
Clay: Boral milled fire clay / kaolinic clay

Fibres: Polypropylene Fibres
SS Needles: Those seem hard to come by in small quantities – So we'll probably go with David's stainless steel rigging wire solution

Best wishes,
Kilian

[UtahBeehiver]

Great start. It would be good if you drilled a few weep holes in your hearth while concrete is green. Drill from the bottom since the blow out on top will not be seen and also helps with channeling water egress.

[david s]

The 100 mm wide blocks are not considered structural, but with a pier in the centre I'm sure you'll be fine.They certainly reduce the amount of fill required when pouring in the cores.
Regarding the weep holes, it is easier to cast them in when pouring the slab rather than drilling, but use Utah's advice about drilling from underneath. It should be easy enough. It looks like you are going for a vermicrete mix for the underfloor insulation, so cover the holes with some pieces of insect screen to prevent the mix blocking the holes. (See my experiment on drying a vermicrete slab attached).

Regarding the fibres it may be easier to access some AR (alkaline resistant) fibreglass fibres rather than unravelling a lot of stainless rigging wires. They also, like the melt extract fibres, are a bit difficult to handle because of their spikeyness. The AR fibres are softer, disperse really well and nice to handle in the mix. I suggest you mix by hand in a barrow rather than a mixer as it's far easier to get the consistency required as well as being able to inspect the mix as you go.

Regarding the mix the general consensus for the well tried homebrew is 3:1:1:1 sand, GP cement, hydrated lime, powdered clay, but I've found that at that proportion the clay, while introducing a refractory quality to the mix also has the downside of introducing shrinkage. Consequently I now halve the clay content with better results.For mortars the recommended industry standard is not to exceed 7% clay addition.
Halving the clay in the mix will bring it back to 8%.

Vermicrete insulating slab PDF.pdf

[kilian]

Great start. It would be good if you drilled a few weep holes in your hearth while concrete is green. Drill from the bottom since the blow out on top will not be seen and also helps with channeling water egress.

Hey guys, thanks for bringing up the weep holes idea! Although I haven't added the pictures yet, the vermiculite insulation slab had already been poured a few weeks ago. If I understand correctly, the idea is to drill the holes all the way through the concrete slab, so water from the vermiculite slab can escape? If so, what diameter and what number of holes would you recommend?

[kilian]

The 100 mm wide blocks are not considered structural, but with a pier in the centre I'm sure you'll be fine.They certainly reduce the amount of fill required when pouring in the cores.
Regarding the weep holes, it is easier to cast them in when pouring the slab rather than drilling, but use Utah's advice about drilling from underneath. It should be easy enough. It looks like you are going for a vermicrete mix for the underfloor insulation, so cover the holes with some pieces of insect screen to prevent the mix blocking the holes. (See my experiment on drying a vermicrete slab attached).

Regarding the fibres it may be easier to access some AR (alkaline resistant) fibreglass fibres rather than unravelling a lot of stainless rigging wires. They also, like the melt extract fibres, are a bit difficult to handle because of their spikeyness. The AR fibres are softer, disperse really well and nice to handle in the mix. I suggest you mix by hand in a barrow rather than a mixer as it's far easier to get the consistency required as well as being able to inspect the mix as you go.

Regarding the mix the general consensus for the well tried homebrew is 3:1:1:1 sand, GP cement, hydrated lime, powdered clay, but I've found that at that proportion the clay, while introducing a refractory quality to the mix also has the downside of introducing shrinkage. Consequently I now halve the clay content with better results.For mortars the recommended industry standard is not to exceed 7% clay addition.
Halving the clay in the mix will bring it back to 8%.

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Thank you David for the vermiculite slab PDF, that's helpful.

The AR fibreglass fibres sound great, I had been wondering about potential issues with mixing the spikey stuff! Do you have a suggestion where to get those in small quantities? Also, how much by volume would you add to the mix?

Yes, I've seen you talk about the 3:1:1:0.5 ratio on this forum ans this was the ratio we were intending to go by.

With regards to measuring the volumes of the different ingredients – are these meant to be compacted or non-compacted?

Best regards,
Kilian

[david s]

Yes the AR fibreglass fibres are expensive. I pay around A$200/20kg incl freight. It is the minimum amount my supplier will do. Any other reinforcing fibres are similarly priced. Ask around and you may be able to access them in smaller quantities, but beware. Getting them from an online source runs the risk of obtaining an inferior product. The reason they are dear is that the corrosion resistance is provided by coating them in zirconium which is very expensive. Some low quality manufacturers just give them a whiff of the stuff so they qualify to call them AR.
They are widely used as reinforcing by the concrete countertop manufacturers, which is a good place to source them.
Ingredients proportions are by volume, not weight. I always tap the measuring container a fair bit to get powdered materials to compact somewhat.
For every 10 litres of dry mix I add around 250ml of AR glass fibres. (2.5%)

[kilian]

There is an aussie manufacturer (domcrete) that sells 2.5kg packages of 12mm long fibres. Given your measurement 2 litres of AR fibres should be plenty for our purposes. Assuming they are as light as they look – 2.5kg should be plenty for one dome right?

[david s]

Yes, that’s the crowd I buy from.