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%.

[ATTACH]n462947[/ATTACH]

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.

[kilian]

Finally got to cast the dome!

For the floor we added some silicate board for extra insulation, potentially overkill, but can't hurt I guess. (Vermicrete slab is still holding some water)

[kilian]

The mould is made from bricks & sand with some wet newspaper to keep it from drying to quickly.

Our homebrew is 3:1:1:0.5(clay) + AR glass fibres + around 1.3–1.4 water. We mixed everything by hand and the glass fibres are super easy to work with – I assume much easier than spiky needles, so thanks david s for the suggestion!

Thanks to bamforp for the idea of creating a newspaper expansion joint at the top.

Everything is now wrapped in damp cloth and cling wrap to damp cure. I was thinking to leave it wrapped up like this for about 72 hrs as we have two really hot days coming up and then take out the mould material and check for voids.

What would you suggest for the curing process afterwards? My rough plan is:
1. leave bowl of water for a few days (dome covered)
2. lamp/diffused heat for a few days (dome uncovered)
3. After the arch is done & fireblanket is in place, start curing fires.

[david s]

Don't be in too much of a rush to dry the casting.



When using OPC Ordinary Portland Cement (calcium silicate) its strength increases rapidly in the first two weeks after casting, and it's generally considered to have reached full strength after 28 days:

• Day 1: Concrete gains 16% of its strength
• Day 3: Concrete gains 40% of its strength
• Week 1: Concrete gains 65% of its strength
• Week 2: Concrete gains 90% of its strength
• 28 days: Concrete gains 99% of its strength

Calcium aluminate cements do not require this extended curing process as they achieve their full strength in around 48 hrs, but the stuff is very expensive, spoils much more easily, has very short working time, but also a higher tolerance for heat. Hence the preference for OPC.


The most important days for curing OPC concretes are the first 14, which account for 90% of its total compressive strength. After that, the rate of strength gain slows down, and it can take at least one to two years for concrete to reach 100% strength.
Concrete's strength is affected by many factors, including:

• Weather: Hot and windy conditions can cause rapid moisture loss, while cold weather can require protection from freezing
• Concrete thickness: Thicker sections take longer to cure because moisture migrates more slowly from the inside
• Structural requirements: Concrete elements with high structural requirements may need longer curing times
• Cement properties: The physical and chemical properties of the cement can affect strength development
• Water to cement ratio: The ratio of water to cement can affect strength development
• Admixtures: Admixtures can affect strength development

Because shrinkage is a problem that can cause cracks when casting over a mould the removal of the mould as soon as the casting has attained sufficient strength is important. A couple of days should be sufficient, but after that the casting shpuld be kept wet for a minimum of a week, lomger for greater strength. So don't force dry it and certainly don't use fire until after you've insulated it.
It is difficult not to get on with the build as it gets exciting, but find another project for the time being. Maybe render the sides of the stand or start forming the mould for the flue gallery.

[kilian]

Thanks so much for the detailed info, David! We will try to cast the arch next week and then let everything dry properly into the new year.

How many milimeter expansion room around the flue pipe would you suggest for the homebrew casting? (we will utilize your "no anchor plate" solution)

[david s]

You can expect around 2% expansion of the stainless flue against the casting. This happens quite fast because the stainless is way more conductive than the casting that surrounds it. As it heats more the casting will also expand relieving the pressure of the pipe against it somewhat. If you create a gap of 3mm all the way around a 6” (150mm) pipe, it will increase the diameter to 156mm which would be 4%. This would be a comfortable fit with enough to ensure the expansion won’t crack your casting.
To do this operation, wrap some 3mm cardboard tightly around the pipe, then also wrap some plastic around the cardboard to ensure easy removal. A light coat of oil over the plastic will also help with this. Cast up against this with your homebrew mix and let it set for 48 hrs before attempting to remove the pipe. The tabs in the pipe can then be cut for the pipe's replacement back in the casting after it has been cured.

The same operation can be used to create a gap between the outer rendered layer and the pipe, with the gap filled with high temp silicone.
The low strength of the lean vermicrete insulation which can be worked directly against the pipe won't bother it.(see pic)