Why NOT control where the CRACKS will happen in your dome?

[Sixto]

So, as a relatively new builder, I had the same rookie hopes that my 36" brick dome would not crack. Well, during Drying Fire #2 or 3, things got a little hotter, and OOPS!!! It took me a while to realize (and accept) that it's part of the process, and that it would have happened sooner or later. There's no way to avoid cracks. In reality, your only choices so far are: 1) Turn a blind eye (includes not seeing any cracks, but they are there, trust me) 2) Accept wherever they happen (sort of disappointing, especially at first) and 3) Make a decision to control where your cracks happen (Since they ARE going to happen whether you want them or not).

As a former Architect, I am familiar with various strategies to deal with expansion, contraction, settling and heaving. They all involve planning ahead, and making conscious decisions on how you want to deal with the inevitable challenges that nature and the environment throw at us.

One only has to look at the various PROFESSIONAL pre-cast ovens to realize that they have learned this lesson too, and many of them cast in sections to allow for the expansion of various sections of the dome at different rates. I am particularly impressed by segmented domes with offset (lapping) joints that also have a separate section for the very top. The idea behind the lap, is that the two sections can slide a millimiter or so past each other, but still are performing as one. This construction technique allows for expansion of the segments with the expected range of a normal pizza oven, while maintaining structural integrity, and enhancing aesthetic the aesthetics of the oven for the owner.

What I now realize is that there is NO REASON why we can't adapt and improve these same building techniques to the DIY Pompeii oven construction made from Firebrick. The illustration below shows how I would do it now if I were starting over... The arch is an inherently weak section, so I would frame it with joints on both sides. I'd also avoid a joint straight at the back, for aesthetic reasons, so I have two more joints at the 10:00 and 2:00 hour positions. I think it's important to have a solid top, so I would recommend at least an 18" diameter top section, which in my dome would have been the top 3 courses.

This approach goes against the general advice to not align the masonry joints from one course to another. While that is true in most brick construction walls are built with running-bond, control-joints are recommended where expansion might happen. Ovens just happen to have relatively small areas where differential heat and expansion might cause cracking, and thus defy conventional masonry construction techniques. I would also find a way to insert either a rope gasket or some other flexible insulator at the controlled lap joints to minimize smoke leaking out, or moisture leaking in- at those places. See blowup sketch of lapped joint below.

I would love to hear feedback on this UNTESTED strategy... it does require more time and craftsmanship during the construction period, but hopefully it results in a better oven in the end.

[fox]

I would think the integrity of the whole dome would suffer and the individual sections would start to break up.
The basic principle of a brick dome is that each brick hold the next in place, I think if you set a gap the other bricks in between will move?
However I like your thinking but feel if you keep going you will go a complete circle and end up with a cast multi part oven!

[david s]

This is certainly the approach taken by cast manufacturers, so perhaps it might work for a brick oven.
My own approach was to use three sections, a front and back with partially half lapped joints and a large top section using a conical joint that can expand if it wants to. The top section I feel, needs to be fairly large as it is the one that will receive the bulk of the heat first and therefore suffer more from thermal expansion.This approach was also confirmed by my home oven that was built as a single one piece dome that has a hairline crack running around the dome in exactly the same place as my ,now, second cast domes. These sections once fitted together with refractory mortar are then cladded with another mix which not only helps hold all the sections together, but adds thermal mass.

[Sixto]

I would think the integrity of the whole dome would suffer and the individual sections would start to break up.
The basic principle of a brick dome is that each brick hold the next in place, I think if you set a gap the other bricks in between will move?
However I like your thinking but feel if you keep going you will go a complete circle and end up with a cast multi part oven!

I don't disagree. This is more of a theoretical inquiry so far... if someone ever tries it, we'd find out for sure. I think the brick movement could be mitigated with some removable shim segments placed between the non-mortared faces perhaps 1mm thick? Similar to how tile is set with spacers?

I also think with 3 or 4 control joints, with lapping joints (both vertical and horizontal) the integrity of the dome would be weakened, but not so much as to cause structural failure. Especially when we know that a crack or two don't really change the structural stability of the dome. But again, it remains to be seen that the dome segments perform as a unit like a cast segment, and don't break down with further cracks or movement within them.

It sort of brings us to the question of Cast vs Brick - is one better than the other? I used brick because I could get materials easily and cheap (used firebrick), and was somewhat familar with the process of cutting and mortaring - even though the learning curve was high and steep. If I were to do it again, I'd probably purchase a ready-made cast kit from Forno Bravo, and save myself the hassle. The only functional benefit I got by doing it myself is the extra mass for baking, and of course the bragging rights of doing it myself, which is kind of a curse if you know what I mean.

[Sixto]

This is certainly the approach taken by cast manufacturers, so perhaps it might work for a brick oven.
My own approach was to use three sections, a front and back with partially half lapped joints and a large top section using a conical joint that can expand if it wants to. The top section I feel, needs to be fairly large as it is the one that will receive the bulk of the heat first and therefore suffer more from thermal expansion.This approach was also confirmed by my home oven that was built as a single one piece dome that has a hairline crack running around the dome in exactly the same place as my ,now, second cast domes. These sections once fitted together with refractory mortar are then cladded with another mix which not only helps hold all the sections together, but adds thermal mass.

Ah, Interesting! - so you did the base of the dome as a kind of cylinder, the top of the dome as a circle, and the flue gallery as a separate unit. I like it.

Did you use any reinforcement? Just wondering if the center of the arch, being the thinnest section of the dome base, and also getting a lot of heat from escaping gasese might be prone to cracking, much like where I got a crack on my brick arch... (which I can't see anymore, by the way, since the top of the dome arch is now all covered with black soot! )

[Kvanbael]

An interesting idea, some first gut reactions:

1. Why all the effort... if you can also let the oven decide on its own few micro-cracks. These will be a tighter fit and better interlocked than any "designed" seam.
2. Not convinced that aligning bricks along a vertical seams will ultimately improve easthetics.
3. The cracks that I find problematic, are the ones in the outer layers (that is, for outdoor ovens that are exposed to rain), as they can enlarge over freeze-cycles and ultimately let water in.
4 If there would be structural reasons to prefer certain locations for cracks over others, then why not just remove some of the mortar from the (invisible) outside of some joints, to guide potential cracks along those weaknesses.

[david s]

A further consideration regarding brick construction is this advice from Daniel Rhodes, Kilns (considered the kiln builders bible). Although we're not building kilns which experience at least double the thermal expansion than the ovens members build, the same principles apply and should be taken into consideration.

"Expansion joints must be provided in the brickwork of kilns. If no expansion space is provided for, the kiln will bulge and swell on heating due to the expansion of the bricks. In practice, a space of about 1/4" is allowed between the ends of every third or fourth brick. This space should not be filled with mortar. More space than this is sometimes allowed just to be sure that the wall will not be too tight. In kiln building it is much better to have a loose structure than a tight one. For this reason the amateur may have a slight advantage over the professional mason, because his bricklaying is apt to be somewhat loose and not tightly locked together. I have seen kilns made by professional bricklayers which in use suffered severe cracking and swelling due to the overly tight and precise workmanship and the lack of expansion joints."

Sixto, we make extensive use of stainless, pp, AR glass and now nanotube fibres in our castings and about to trial basalt fibres as well.

Kvanbael ,"if you can also let the oven decide on its own few micro-cracks." This is exactly what furnace and kiln builders do. They see where cracks develop then place expansion joints in those areas in subsequent builds. It's also exactly what I did. See attached pic of the hairline crack running around my one piece cast dome. It is 15 years old and still fires as good as ever, the hairline crack never getting bigger.

[Sixto]

An interesting idea, some first gut reactions:

1. Why all the effort... if you can also let the oven decide on its own few micro-cracks. These will be a tighter fit and better interlocked than any "designed" seam.
2. Not convinced that aligning bricks along a vertical seams will ultimately improve easthetics.
3. The cracks that I find problematic, are the ones in the outer layers (that is, for outdoor ovens that are exposed to rain), as they can enlarge over freeze-cycles and ultimately let water in.
4 If there would be structural reasons to prefer certain locations for cracks over others, then why not just remove some of the mortar from the (invisible) outside of some joints, to guide potential cracks along those weaknesses.

Hi KVanbeel, thanks for the insights!

Re: comment #1 - Before I heated up the dome, I expected I would get cracks. When I got one smack on top of the arch, I started wondering what could be done if one wanted to avoid an unsightly crack... (some people are more accepting of happy accidents than others?) I happen to be totally comfortable with what I have, but wanted to explore alternatives for future builders that want more control.

Comment #2 is fair. I'm not sure either. It's all personal preference.

Comment #3 Since I'm still curing the dome render, I don't have any visible cracks yet on the exterior, and haven't gone through a winter yet - so I'm still learning a ton. (Render is reinforced with Alkaline Resistant Glass Fiber Mesh) If I get external render cracks large enough to leak, I will seal them somehow, or build a roof over the dome. I also plan to tile over the render/stucco next spring...

Comment # 4 is most intriguing, along with david s quote from Daniel Rhodes' book on Kilns. There's probbly an easier way to encourage cracks to form along certain lines, by omitting or weakening the mortar in some joints. Could that even be done after-the fact, and run a grinder with a diamond blade along the outside face of the dome bricks to make one or more 3" deep score -lines and encourage cracking there? Sort of like sidewalk joints every 5' or so?

[david s]

Mortar for general brick construction, not only ovens, is always meant to be weaker than the brick units. This is to prevent any cracks going through the bricks themselves, thus relying on staggered brick courses for structural strength.

The main reason cracks occur, particularly at the top of the inner arch is that builders don't heed the warning to avoid direct flame impingement especially in the first few drying fires.

[fox]

I made my first cast ovens 55mm thick, they are still holding up fine. My second version, I made 65mm thick, they are also holding up fine but, you could make a cast oven 100 mm if you wanted.
However one of the main benefits of a cast oven is the portability and ease of on site construction so a 100mm thick oven would be best built with more components.
So you could use a 50 mm thick cast oven and add your own extra mass over the outside of the dome.

I know you wanted to have a lot of mass to cook bread but my 65mm thick ovens with 100mm of ceramic fiber insulation will take three days to return to ambient temperature and only take 1-1/4 to 1-1/2 hours to saturate with heat.
Of course there are many variables like size and type of fuel, out side temperature and humidity, gallery design, door opening size, hight of chimney and wind conditions….etc ….

[Sixto]

I know you wanted to have a lot of mass to cook bread but my 65mm thick ovens with 100mm of ceramic fiber insulation will take three days to return to ambient temperature and only take 1-1/4 to 1-1/2 hours to saturate with heat.

I love the amount of detail in the responses! I hope I can match that level of performance with my brick oven (both during the heating and cooling phases) especially after I finish building an insulated door

david s do you think that the cast saucer section you have on top of the dome could be copied in brick construction by placing a wet sheet of newspaper on top of the brick ring below -before adding the mortar bed for the next course? I'm thinking that could be enough to allow the top of the dome to move as a unit without further cracks, while still having mortar in the joint as a cushion between brick courses.

[david s]

Possibly, try it out to see what happens.
Also I’m a fan of homebrew, but there could be an advantage to using the more heat tolerant calcium aluminate cement in the top third of the dome and the top of the inner arch.