36" build in heart of Europe, Czech Republic

[mrotter]

I built my outer arch on top of my CF board and it is one of the things I regret doing (regret may be too strong of a term but I'll explain). I did it because even though I have a heat break I didn't want to be bleeding heat into my hearth slab. The reason I kind of wish I didn't is with a heat break the vent structure is not tied into the dome and the entire weight sits on a brick and a half footprint on each side of the opening. I know my dome is virtually bullet proof but always have this uneasy feeling if my vent fails it will be because it is sitting on a compressible surface and is not mortared down to give it lateral stability. I have no way of knowing if my oven stays any warmer because the bottom of the vent structure is isolated from the hearth, but with all that exposed area for radiation and convection I doubt if it would have mattered. I'll be interested to hear what others have to say.

Interesting opinion, thanks.

Why I got to the idea is this: https://community.fornobravo.com/for...805#post175805

And I told myself - "this is very interesting".

The design of outer/vent arch is axed and it is hard to say for me at this point whether buttressing will or will not be needed. I guess it depends on overall weight of my chimney/vent design above it, which is still undecided.

The specs of my CaSi board say its compression value is 2.8 MPa which is about 28 kg/cm2, which means if we take area of single brick - 300 cm2, then the CaSi should support 8400 kg on each side of the arch, which seems to me as crazily high number. Other thing is that if I would need buttresing, I would have to have part of arch sitting on CaSi and "butress" sitting outside of casi - directly on hearth which is the thing making me think that casi might compress a bit and then the whole structure could get some cracking etc.

[JRPizza]

I built my outer arch on top of my CF board and it is one of the things I regret doing (regret may be too strong of a term but I'll explain). I did it because even though I have a heat break I didn't want to be bleeding heat into my hearth slab. The reason I kind of wish I didn't is with a heat break the vent structure is not tied into the dome and the entire weight sits on a brick and a half footprint on each side of the opening. I know my dome is virtually bullet proof but always have this uneasy feeling if my vent fails it will be because it is sitting on a compressible surface and is not mortared down to give it lateral stability. I have no way of knowing if my oven stays any warmer because the bottom of the vent structure is isolated from the hearth, but with all that exposed area for radiation and convection I doubt if it would have mattered. I'll be interested to hear what others have to say.

[mrotter]

Made some progress - laid first half soldier course, laid second standard and part of 3rd course. It was nearly impossible to stagger bricks between first two course, but 3rd is good.

I have question to some more experienced guys - it's the vent (outer) arch. The arch will be separated with fire-resistant rope which will act as heat brake. Also floor will be heat-breaked with steel "jackel" filled with perlite. I have what you call "axed" arch so there will be some horizontal outward forces and arch might need side buttressing. My question is - should I lay outer arch also on CaSi or cut out part of board where outer arch "legs" will stand and lay directly on hearth concrete? See pics.

JRPizza
UtahBeehiver

[mrotter]

Btw, yesterday I had 1 hour time, and decided to lay half of first dome course - I decided to go with half/brick soldier first course as it seems to combine some advantages (steeper shape of lower dome) + does not introduce that much horizontal forces).

The mortar I have is calcium aluminate dry mortar ready-to-use mix. I only had to add water to achieve desired consistency. The mortar is very nicely workable and has wonderful peanut butter like structure, very soft. I noticed several things I would like to share for others:

• mortar comes off very quickly, I noticed I have around 25 minutes before it seems to start getting hard, so make sure you only prepare the amount you are able to process in that time,
• be careful with amount of water you use, it is clearly visible when mixing, if you add just about right amount of water, you right away notice the change of mortar density/structure and you immediately notice that mortar is of butter-like consistency and is VERY adhesive and has almost "glue" like capabilities, it holds two bricks vertically mortared even in completely fresh state
• if you add more water than is needed the mortar will be noticeably less strong and as I've read in technical specs, you should really add rather less water
• also as advised by professional mason, you should almost NEVER make your bricks damp or even wet before applying mortar, only make sure that contact sides of bricks are clean and free of dust.

I laid the course yesterday evening and checked today morning and it seems the bond is there and is strong. Will finish the course today afternoon/evening.

[mrotter]

This picture is very similar to how mine template intersects with ID:

[mrotter]

To follow-up.

For example here: https://community.fornobravo.com/for...780#post415780

it seems that proportions are fairly similar to mine and also, guy's arch seems to "intersect" ID but front line of arch does not line up with ID line. Maybe you really just meant "intersect"?

[mrotter]

In Photo number two (looking down from the top). The inside bottom of the arch form does not appear to be in line with the inner arc (ID) of the dome. It looks like the bottom of the form is about an inch to far left of the dome arc.

No, the front side of template is pushed forward/outward of ID by around 2". But this AFAIK should not really be a problem? The only outcome of that will be that TDC will simply be bit longer - to intersect fully with dome bricks? So the template does "intersect" ("cross" ?) the ID line, but ID line and front of inner arch line do not "line up".

[UtahBeehiver]

In Photo number two (looking down from the top). The inside bottom of the arch form does not appear to be in line with the inner arc (ID) of the dome. It looks like the bottom of the form is about an inch to far left of the dome arc.

[mrotter]

Those are standard light to medium duty firebricks. They are reclaimed, not new. Previously they were used in fireplaces etc.

[Kvanbael]

Maybe a strange comment but those firebricks look quite light and brittle. You might want to double-check the specs that they have decent heat capacity (eg. not made for insulation purposes).

[mrotter]

I tweaked arch template and made it inch lower.

[mrotter]

UtahBeehiver

The it is fixed as per our previous discussion, maybe it's not obvious from the picture.

Based on what you judge the form is too forward? Can you elaborate?

[UtahBeehiver]

The bottom of the arch form should intersect the ID arc of the dome. It appears that the form is too far forward (or outward) by an inch or so. Second, the IT needs to be adjusted. Drawing a line from the pivot point at the floor, it needs to intersect the L bracket where it meets the halfway thickness on the dome brick. This is critical for the dome, if not corrected the errors are cumulative as you go up in courses. Attached is a pic showing each segment of a tapered inner arch brick. They are not the same so you need to cut each one based on the IT and previous brick. The top brick will be the longest.

[mrotter]

Tried to dry-lay first course of dome bricks and also tried to position TDC inner arch brick.

More experienced guys - does the TDC/dome intersection seems about right? I outlined ID+OD on the TDC brick, see pics.

Thanks for hints.

UtahBeehiver
JRPizza

[mrotter]

Started laying floor bricks. These are salvaged, got them nearly for free.