My first WFO design, any comments before I build?

[Mark1986]

3 days should see it about as hard as it's going to get, but it'll need plenty more to dry out, see drying experiment.

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Following earlier your earlier advice I will wait at least a week before I will start laying the fire bricks on the perlite concrete. But what is the significants of the water content after the three first days when it is not going to get harder? Should you try to get more of the water out before you lay bricks on it? should you cure it with small fires etc? Or can I just start putting fire bricks on to after 7 days?

Cheers!

[david s]

This graph shows strength development vs time. Also read the attached experiment on my last post for an explanation to your question.

[Mark1986]

This graph shows strength development vs time. Also read the attached experiment on my last post for an explanation to your question.

david s

Ah ok so wet curing can also be done with perlite concrete. Didn't know that!

Today I did a compression test of the perlite concrete. First my girlfriend (about 60kg) stood on it and then I did. I weigh about 105kg. The concrete was hard enough to carry us.

One thing that stood out is that there are quite some loose top grains of perlite. I uploaded a video to imgur showing this: brittle perlite

Should I fix this before I start laying fire bricks on it? Some fixes I thought of:
- Patch with perlite concrete with more cement
- Patch with homebrew

Any ideas?

[Petter]

Use homebrew. I did too. Works fine. Only patch where the floor bricks (but don't lay the bricks) will be and/or the dome. Use a slighty wetter than normal mix to enable troweling.

[Mark1986]

Use homebrew. I did too. Works fine. Only patch where the floor bricks (but don't lay the bricks) will be and/or the dome. Use a slighty wetter than normal mix to enable troweling.

Thanks for the tip! Btw why only where the fire bricks will be and not on the other spots?

Any idea why the top layer is brittle? Is this normal or did I do something wrong?

[david s]

Anything made with OPC (ordinary portland cement) can have its strength enhanced by damp curing. As perlcrete takes up more than double the water requirement for the hydration process it doesn't really need extended damp curing, but keeping the sun and wind off the surface for a few days is wise so the surface doesn't dry off too quickly. If it were mine I wouldn't be worrying about resurfacing, but use a dry 50/50 mix of sand and powdered clay which acts as a leveller for uneven floor bricks. Using the mix dry and laying the bricks loose enables each individual brick to expand freely, given the differing temperatures the floor will experience. This method also has the advantage of making any brick replacement (years later) far easier.

A 5:1 perlcrete mix is not particularly strong. If you make it strong it won't insulate. See table attached.

[Petter]

Or in other words, do not patch where you will put more insulation. Under the bricks and dome is fine.

it feels fragile since the top grains have the least number of neighbours. Actually, even without cement - the perlite alone will support the weight if five ovens on top of eachother. Don't worry, it is all in order.

[NCMan]

No need to do any patching, as was said. Actually, patching it w/homebrew is not advised, as it would be adding mass and taking away the insulating property of it. Do what david has said, for sure. In the end, there is no real need to patch it.

[Mark1986]

Thank you all for the good advice! I'm going to go for the 50/50 mix of powdered clay and sand.

Today I went ahead and started casting the pizza dome with homebrew. I went for casting over an exercise ball wrapped in foil. I did this even though it was advised not to do so, but I suffered from escalation of commitment and went for it anyway .

How I performed the casting:
- worked in double batches of 3:1:1:1 sand, lime, fireclay, portland cement
- Measured dry ingredients on scale and measuring cups
- Mixed the dry ingredients
- Added water, which was about 1 liter per 3:1:1:! batch
- Scooped the homebrew onto the cast and troweled with each go
- After each couple of layers, I vibrated the homebrew with a sander. Here is a video of this step: vibrating homebrew
- At the and I troweled the hole cast

I am quite happy with the result. The homebrew wasn't to saggy and it was quite easy to do the casting. I won't cast over an exercise ball again, it was unstable and the result won't be straight enough to put on the firebricks. I will fix this by filling up the gaps with homebrew wants I place it on the wooden stand. I hope the vibrating I did was enough to eliminate big gaps in the cast.

At the end I didn't have enough propylene fibers, so I had to stop before It was finished. Somewhere in the coming days I will buy more and finish the rest.

Any remarks before I continue?

[david s]

Good job. Waiting to complete the casting in a asecond go will work, but a single pour is preferable as you don’t have a problem with the second pour not bonding properly to the first. The casting is going to be really heavy and difficult to relocate into position. This is one of the reasons why the cast in situ method is preferable.The other reason, as you have found, is that the base doesn’t end up truly flat, which results in extra work filling gaps.

[Mark1986]

Good job. Waiting to complete the casting in a asecond go will work, but a single pour is preferable as you don’t have a problem with the second pour not bonding properly to the first. The casting is going to be really heavy and difficult to relocate into position. This is one of the reasons why the cast in situ method is preferable.The other reason, as you have found, is that the base doesn’t end up truly flat, which results in extra work filling gaps.

Thanks!

Ah well, that was what I was afraid of. Next time I'll make sure I buy enough materials.. Is there any way to remedy the two pours not bonding together?

In the coming days I'm also going to start laying the firebricks. Two questions on that:
- Can you use homebrew as a mortar for the firebricks?
- How long should you let the firebrick floor set/dry before you put on the dome?

Cheers!

[david s]

" Is there any way to remedy the two pours not bonding together?"
Don't over worry it. It will bond but if there's any structural weakness, and the heat becomes a test for this, the weakness is likely to occur where the two pours come together.
Yes use the same brew for mortar, if you have polypropylene fibres left they can also be added to the mortar mix. The floor bricks are better laid loose IMO, to allow for thermal expansion of individual bricks as well as easier replacement down the track if needed. The gaps will simply fill with ash.

[Mark1986]

" Is there any way to remedy the two pours not bonding together?"
Don't over worry it. It will bond but if there's any structural weakness, and the heat becomes a test for this, the weakness is likely to occur where the two pours come together.
Yes use the same brew for mortar, if you have polypropylene fibres left they can also be added to the mortar mix. The floor bricks are better laid loose IMO, to allow for thermal expansion of individual bricks as well as easier replacement down the track if needed. The gaps will simply fill with ash.

Ok perfect I won't worry about the cracks then!

About the spacing, would you advise about a 2mm gap? I'm asking because I want to use spacers

And the setting/drying time? Can I put to the dome on it after three days?

Cheers!

[david s]

As my drying experiment shows, the vermicrete requires weeks of drying. You can set the floor bricks over it sooner, but it makes getting the moisture out by fire a lot longer. The floor bricks should be laid butted up to one another without any gap. As the graph shows, anything made using OPC will benefit from holding the moisture in, this includes a homebrew casting. Any voids on the inside of the casting should be filled while the casting is still moist.

[NCMan]

Ok perfect I won't worry about the cracks then!

About the spacing, would you advise about a 2mm gap? I'm asking because I want to use spacers

And the setting/drying time? Can I put to the dome on it after three days?

Cheers!

No gaps. Lay them tight.