31"/800mm in Eindhoven, the Netherlands: Design critique and build topic

[Toiletman]

What happened at the top of the cast?

As mentioned and shown in posts #71/#72: I mixed the concrete too wet and it was slumping down, so flowing out of the top part of the cast. I now have more thickness at the bottom than planned, and had some gaps at the top. I filled them in again, and they're now damp curing... my feeling is that it's good enough to hold up and provide sufficient surface for the chimney connector to sit on.

I am a bit perplexed when going back over this thread. Post#65 shows pics of the top of your flue gallery arch. With your casting sitting on top of it, the slot in the top of the arch becomes redundant and a restriction for smoke flow. I was under the impression that your casting would replace the brick arch. I think it will still work ok as you propose, but not as efficient as it could be.

I did consider a fully cast gallery arch initially, but since the gallery is so wide to accommodate the barbecue area and the arch has such a shallow curvature I thought it would be very challenging. Having a small thickness would probably not have enough strength, and making the cast as high as the brick gallery would make it very unwieldy to manufacture and transport. I also like the look of the brick arch face a lot.

You are right about the shape of the bricks becoming a restriction. Not fully a restriction, as the the smallest surface area is always the chimney pipe, anything upstream is wider. However, the widening of the brick arch will cause a negative pressure gradient which might cause some recirculation. Overall though, from the cross section I think it's still smooth enough. Especially if you consider the lengthwise cross-section: the vertical path in the middle of oven is very straight and smooth, it's only at the sides that the recirculation might cause a decrease in draft.

Attached some pictures of the cross sections, as well as an alternative design I considered fully covering the brick arch with a casting --> in the end I felt it would only add to the complexity of construction and make the casted part less robust --> the only material that's really needed is the material to sit around the smoke opening. My main worry was about the brick arch being able to support the weight of the casted part, that's why I thought of extending it to be supported on the gallery walls. In the end my feeling is the gallery arch should be able to support the weight, as well as that once the vermicrete plaster is in, it will provide a nice stiff surrounding shell, with a lot of height giving it also a lot of stiffness, probably eliminating the problem completely. Feel free to chime in with your thoughts...

[david s]

What happened at the top of the cast?

I am a bit perplexed when going back over this thread. Post#65 shows pics of the top of your flue gallery arch. With your casting sitting on top of it, the slot in the top of the arch becomes redundant and a restriction for smoke flow. I was under the impression that your casting would replace the brick arch. I think it will still work ok as you propose, but not as efficient as it could be.

[Toiletman]

Demolded, inspected and filled the cracks and holes:

[david s]

That looks good. I’m in the habit of finishing castings with a sponge, but you are correct. It doesn’t really matter because it will be covered. You should damp cure it for a minimum of a week to enhance strength. Demould after 48 hrs. The form you have created will produce a very good smoke flow which will result in superior smoke extraction.

[Toiletman]

I edited the previous post, but on my screen it didn't go through, so once again here:

BTW david s you mentioned troweling and using a wet spunge after it sets a little bit. Could you comment on the why of doing that? As it's going to be under insulation and vermicrete I don't very much care about how it looks? Does it serve another purpose?

For 'normal' concrete casting I know floating/troweling and condensing the surface layer helps in getting the surface more 'closed' instead of having this brittle powder on top? Does it also work like that if I have the surface exposed on a curved surface like this?


Here's some pictures of the progress and the slump issue:

[Toiletman]

Well, after a week of being ill today I finally covered the mold in homebrew castable.... I mixed it too wet, so I'm now dealing with a little slump. Had to take it out and mix in some more homebrew. I'm now waiting for it to set a little bit, I can probably fix it up after it hardens sufficiently....pictures will follow later.

BTW david s you mentioned troweling and using a wet spunge after it sets a little bit. Could you comment on the why of doing that? As it's going to be under insulation and vermicrete I don't very much care about how it looks? Does it serve another purpose?

For 'normal' concrete casting I know floating/troweling and condensing the surface layer helps in getting the surface more 'closed' instead of having this brittle powder on top? Does it also work like that if I have the surface exposed on a curved surface like this?

[david s]

SS or AR fibre addition should be around 2% by volume of the wet mix.
The burn out PP fibres about 1%, because they are so fine, but make sure they are well dispersed. The consistency of the mix is important. Too dry and you'll get more voids, too wet and it will slump. I just apply it by hand, wriggling each handful against the mould, then trowel the outer surface and finish off with a wet sponge when it begins to set a bit.

[Toiletman]

That's looking fabulous. Here are a couple of pics showing AR glass fibres and pp fibres as well as the dtainless collar I use to form the top of my gallery castings. I use a fibreglass mould, so I can make more castings, biut it's a lot of work. Your sacrificial mould is far more suitable for a single casting.

Your fibres all sound like the correct ones. Because the gallery sees way less temperature than the dome (the carbon doesn't even burn off in mine), it would be far easier to skip the stainless needles , or unravelled rigging wires because they make application more difficult, especially if you haven't had a lot of experience working with this mix. W hereas the AR glass fibres being softer present no problems. Maybe throw in some more of those to compensate.

That mold looks really sweet. I used to repair surfboards, so had my fair share of glassfibre work.. wonderful stuff. My shape currently has an undercut, so it has to be a sacrificial or multi-body mold.

With respect to the SS fibers: what would their regular amount be in the mix? And with how many AR fibres should I replace them?



I've added some filler to my mold, added a top ring at the right dimensions as well as some support on the vertical area's... waiting for some of the kit to set now and then I'll do the final round of grease. My reasoning is that with vertical support I can do the rest of the shapework by hand.. I"ll probably add some clingwrap on the vertical sections as well, so that I can force the final shape to stay close to the mold instead of peeling off.
With regards to denstiy: I don't think there's much vibration needed as there's no coarse agregates, so probably some manual slapping/hammering will lead to sufficient density, without real liquification, which would really a closed mold.

See below some pictures. Do you guys have any final remarks/tips/warnings about this way of working? I'm guessing I'm going to take a stab at it tomorrow or wednesday at the latest.

[david s]

That's looking fabulous. Here are a couple of pics showing AR glass fibres and pp fibres as well as the dtainless collar I use to form the top of my gallery castings. I use a fibreglass mould, so I can make more castings, biut it's a lot of work. Your sacrificial mould is far more suitable for a single casting.

Your fibres all sound like the correct ones. Because the gallery sees way less temperature than the dome (the carbon doesn't even burn off in mine), it would be far easier to skip the stainless needles , or unravelled rigging wires because they make application more difficult, especially if you haven't had a lot of experience working with this mix. W hereas the AR glass fibres being softer present no problems. Maybe throw in some more of those to compensate.

[Toiletman]

Happy New Year fellow builders! Hopefully 2025 will bring a lot of great wood-fired food!

Small update from my side: I used the xmas holiday to come up with a 'plug' for the chimney casting (see pictures). I made it of vermicrete, as I had the ingredients around and I guessed it would still be relatively easy to destroy it after casting. Typically it seems you are using casting plaster for it instead. Any worries about that?

Steps to still do to get the chimney connection:

• Order/buy the fiber materials: DONE
• Smoothen rough areas with putty: DONE
• Improvise a mold for the vertical/overhanging sections of the plug
• Apply release agent (vaseline?)
• Create the cast
• Cover with plastic sheet and keep moist for at least a week
• Demold

After I finish the connection I'll have to fit it to the gallery arch and then create the actual chimney pipe connection according david s 's sketches. So that'll be when the outside temperatures and moisture levels are such that I can work outside again.


From reading up on castable it seems that 'homebrew castable' is basically just the homebrew mortar recipe with the addition of fibers: AR glassfibers, SS fibres and
PP fibres. From various posts it seems this is the current 'standard':

• 3 parts sand (2 sharp 1 soft depending on preference)
• 1 part lime
• 0.5-1 part clay (less to reduce shrinkage)
• 1 part portland cement
• 250ml AR glass fibers per 10 liter of mix
• half a handfull of PP fibers per 10 liter mix
• SS needles (25x0.5mm) --> cut-up and untwisted SS cable

Questions:

• What's the amount of SS needles needed? Or would they be a replacement of the AR fibres? I'm assuming the function of both is to provide (tensile) strength?
• What size of AR fibers are needed? I've come across 13mm x 18 micron so far.
• I understand that the main function of the PP fibres is to provide microchannels for steam to escape to reduce spalling/cracking. As I will only use this for the chimney entry: would I need it? I guess it also depends on if I insulate the chimney or not: if I don't insulate it it will get less hot and might need this less. If I do insulate it, it will get hotter.
• What's the typical chimney inlet temperature people are achieving? Does your chimney have insulation or not?
• For the domes people typically just apply the homebrew by hand, for chimney inlets I've seen hand application as well as forms being used. I'm slightly worried about how well it will fill the mold? should I tamp it down? or mix it slightly more watery? I might try doing the easily sloping sections by hand and use a form for the more vertical parts tamping down the mortar with a stick or sth like that.

Any feedback/remarks very welcome, the forum has been a great help so far!

[nlinva]

Yes, that Permatex stuff is quite sticky and hard to get on smoothly, especially in a narrow, hard-to-reach crack. I had the same problems you did, and mine looks quite messy; I might try to clean it up a bit, but as you say, it's really not going to matter much. I did learn my lesson and use tape for the silicone gasket between the front landing and the arch landing inside the vent arch. That worked very nicely.

I did actually try to tape where I put the Permatex Red, but the masking tape I had was not sticking to the firebrick surface. If I had to do it again (if I ever build another oven), I would make sure to get tape that sticks well.

[Toiletman]

Forgot the pictures

[Toiletman]

After enjoying a small holiday I did manage to do a little work on the oven: I've added the fire-rope + kit on the back of the gallery and the permatex kit on the inside. The latter was challenging: the small gap doesn't allow the 'spout' to go into it deeply, so the process was rather messy. I should've taped it off beforehand to prevent the mess, as with the granite, but I found out while I had the kit open already, so I figured it might dry out and that I should clean up mechanically afterwards.
This turned out to be only partially possible, but in the end, as already mentioned it's all going to be mostly covered in soot, so I decided not to be too perfectionistic on it .
I took some final measurements of the chimney connection and covered the oven for winter. Although the weather has been really nice the last days I will only be able to do significant work after having the chimney connection on, and I don't think it's realistic I finish that before the weather will turn worse.

You do have a pretty wide opening in the vent. Since the bricks on the left and right edge are not quite vertical, it looks like it becomes even wider beyond the initial opening. I think that's common, but am not sure how it affects air flow. It could be a plus, as a place for smoke to collect rather than coming out the front, but it could also mean that air slows down before speeding up again, which might affect the draw. Or maybe these thoughts just show my complete ignorance in these matters :-)

You are right about this, although I'm not sure how much pressure loss will result from it. It might be in the similar order of magnitude as the improvement by rounding the corners. Anyway, by the time I'm making smoke it might be a fun area to investigate to see if the smoke indeed does 'recirculate' in the expansion chamber.

[nlinva]

Ha, the "before cleaning" picture of the vent arch does make me feel a little better about my brickwork, though sadly my "after cleaning" is still closer to your "before cleaning" :-).

You do have a pretty wide opening in the vent. Since the bricks on the left and right edge are not quite vertical, it looks like it becomes even wider beyond the initial opening. I think that's common, but am not sure how it affects air flow. It could be a plus, as a place for smoke to collect rather than coming out the front, but it could also mean that air slows down before speeding up again, which might affect the draw. Or maybe these thoughts just show my complete ignorance in these matters :-)

[UtahBeehiver]

You are correct on the vent cross sectional should be larger than the chimney cross sectional area. FYI, a rectangular opening is not as efficient as a circular cross section so it is good you vent cross section is larger in area than the chimney.