Curing, Cracks & Other Kentucky Conundrums

[Ken524]

Uhh-Ohhh...

I'm on about the 5th fire and starting to get things hot. The top 2-3 courses of the dome went white (well actually, clean of soot).

I discovered my first crack. It's not in the best place so I would like to solicit advice from the crowd.

The crack runs through both my front and rear vent arches on the left side and into one course on the dome. I'm not too worried about the dome part; thanks to the staggered courses, the crack stops there and doesn't look like it's going to spread.

The crack runs all the way through the joints. You can see daylight through it. I think my vent box mortared on top of the arch is what's supporting the arch in position (the whole thing is pretty much cantilevering now). Without the vent box, I suspect the other side would crack as well. If this happens, the arch isn't going to fall down. It'll just settle on the cracks.

When I had the oven hot, the crack was just wide enough for a dime to fit in. As it cooled down, the crack closed back to about 1/2 the width of a dime. This tells me that this is now (for better or worse) an expansion joint. I don't want to patch it with mortar because it will just crack again, or worse, crack somewhere worse.

SO... Any advice?? My first inclination is to get the oven hot and try to fill the gap with furnace cement/caulk to seal it and add support, but still give it the ability to expand and contract.

What do all you experts think? If you agree with my furnace cement idea, what product do I use?

The good news is that so far the dome looks good on the inside.

[jahysea]

Re: Curing, Cracks & Other Kentucky Conundrums

Hi Ken.

I had a similar crack when initially curing. I'm a little surprised you have one as your masonry is clearly world class and your arch is beautiful. I think physics is just against you despite perfect brickwork. An arch doesn't just transfer weight downward, it also pushes outward. When I had my problem pops told me why it was what it was, and how to solve it....

(sideline.. You can doubt pops on this, but my father was a teacher, and he taught college Statics, not to be confused with statistics. Wikipedia reveals: "Statics is the branch of applied physics concerned with the analysis of loads (force, torque/moment) on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at rest under the action of external forces of equilibrium. When in static equilibrium, the system is either at rest, or moving at constant velocity through its center of mass.")

Heat will cause expansion and further the effort.

I hadn't yet placed the chimney onto the arches, so I just caulked the inside arch (it supports the oven walls so I couldn't tear it out) and tore the outside arch apart and remortared it. I also poured concrete around the outside of the arch base and built buttress walls. It hasn't spread further since and the remortared outside arch still has no cracks.

I see you have started your walls. I'd recommend simply building some mass on the outside of your arches, inside those walls. No-one will ever see the bracing buttress walls you build.

I'd also recommend re-supportting your arch and chipping some mortar out of the joint that has the crack, then re-mortaring it and letting it cure.

Your masonry is too great to suffer a crack in such a visible place....

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

JC,

Thanks for the response!

...I just caulked the inside arch...

What did you caulk with?

I'd recommend simply building some mass on the outside of your arches, inside those walls. No-one will ever see the bracing buttress walls you build.

I installed some bracing on the outside of the arch supports before starting my enclosure.

I'd also recommend re-supportting your arch and chipping some mortar out of the joint that has the crack, then re-mortaring it and letting it cure.

Good thing I saved my arch supports! Do you think I should do a few more curing fires first or do this right away?

Thanks for the great reply!!

[wlively]

Re: Curing, Cracks & Other Kentucky Conundrums

Ken

Wondering where the gaps in the crack are widest when hot, it may a good indicator of the direction of force. The place where you have the crack also corresponds to the widest part of the dome circle (the opening is centered on that). I ask cause I was wondering if the crack is caused by the force of the expanding dome pushing outward and not the arch pushing outward. The front inside part of the dome will see all of the high temp convection current, as well as the inside half of the arch. The highest concentration of internal stress will be between the areas of highest temp difference.

If the crack remains stable in size over a couple of more firings, I would think that you are not in danger of failure. In that case furnace caulk should be a good flexible solution.

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

Hi Wade... Really good theory! The crack does appear to be widest in the very front, furthest from the heat. I had thought about that heat differential when trying to figure out what was moving. I like your theory of the dome actually expanding and pushing the arch out.

I'll put another fire in the oven tomorrow (rain/snow today) and see what the crack does.

[asudavew]

Re: Curing, Cracks & Other Kentucky Conundrums

Sorry your badwfo cracked Ken.

I feel your pain.

I have a crack in my arch as well.. it bugs me some.... But once the fire gets to raging...I don't notice it.

If I were you, I would try mortaring that crack and see what happens, at least the visible part. It may take a couple of applications, but your oven is to beautiful to not try and fix it.

You might want to wait until you really get it smoking hot a few times. That crack may not be done yet. Mine grew and grew the first few raging fires. I hope yours doesn't do the same thing, but it might.

Anyways... my two cents.

I watched your video on how your oven draws. It does the job very well. I will have to try and build a vent like yours to see if I can get mine to draw better. My temporary vent system is inadequate. i.e. smoke in Dave's face sometimes. So it looks like I have vent to build this weekend.

Are you getting snowed on by that huge storm system in the east?

Dave

[asudavew]

Re: Curing, Cracks & Other Kentucky Conundrums

Another thought.....

I used cardboard around my floor bricks to create a gap between the dome and the floor just like you did.

Do you think that we should of made a bigger gap? I have been wondering if the floor may be pushing against the dome or vice versa. What do you think?

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

You might want to wait until you really get it smoking hot a few times. That crack may not be done yet.

I'm thinking the same thing. I'll give it a few fires to see what happens. I'm not upset about it. I expected some cracks, just have to find the best way to deal with it. Crack Happens!

Good luck with your vent engineering. Let us know how it goes!

Are you getting snowed on by that huge storm system in the east?

Nope. Missed us. The kids are not happy.

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

Do you think that we should of made a bigger gap? I have been wondering if the floor may be pushing against the dome or vice versa. What do you think?

I think we are ok there. I think Wade hit it on the head... The whole dome is expanding outward slightly. It has to crack somewhere.

[christo]

Re: Curing, Cracks & Other Kentucky Conundrums

Ken,

My condolences....

When my vent arch cracked - I hadn't yet put a fire to the arch....

My vent arch was pushing the walls apart. I pushed a little (then a lot with the aid of a rubber mallet) and managed to break loose one of the vent sides. I re-mortared the joints and reassembled the big pieces. That's when I poured the bags of 80lb mortar on each side of the vent to butress it.

If I had thought of it, I would have used the last of my extra fb iso board to insulate the concrete from the vent.....

Even with the concrete on each side of the vent, my oven retains heat pretty well and no more cracks in the vent arch.

Good luck!!

Christo

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

Thanks Christo.

I have the heavy-duty shelf brackets bolted into place and also steel studs for the enclosure pressed hard against the arch walls. Tomorrow I'm going to beef up the studs with some angles bolted to the concrete so they can take some weight/force if necessary without compromising the enclosure.

So far, I like Wade's advice as it's the least invasive solution. I'll do another hot fire or two and get some furnace caulk/sealant and see what happens. The nice thing about the crack is that it won't show when the enclosure is finished. I'm going to build a decorative arch out of brick veneer outside of the firebrick arch.

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

Grrrrrrr

I fired up the oven this morning to see what would become of my crack. Unfortunately, it grew a bit wider and the extra weight of the chimney started causing the arch to sag on the left side. Cracks began to develop on the far right side joint (mother nature attempting equilibrium).

The fire was starting to get really hot, so I pulled the big logs out then spread the coals around. I wanted it to be just a little warm to keep the bricks and mortar above freezing tonight.

I reinstalled my arch form and jacked it up to get the bricks back in alignment (the left side had sagged about 1/16"). Then used my angle grinder to remove as much mortar as I could from the top and front of the exposed joint. Repeated the process on the right side as well.

Wet down the bricks and filled them with fresh HeatStop 50. I also added some splits to the top in an attempt to add some support from above.

I'm praying to the Oven Gods that this will fix the issue. I'm not looking forward to tearing out the ventbox and arch if this fails. And if I do need to tear them out and rebuild, how do I keep this from happening again?

[wlively]

Re: Curing, Cracks & Other Kentucky Conundrums

I feel your pain Ken. You are right, you have to stop the cracking or tear it down.

With such beautiful mason work, who would of thought. I wish I could offer sage advice, but maybe we can learn something by putting our experiences together. I am very lucky enough to not have any structural cracks from the outside (last looked at after 6th or 7th fire) in the beginning and continued inspection have not revealed any cracks visible from the inside to this day. So what is different?

Knowing my masonary skills were not a strong suit, I tried to make the dome and arch as "idiot proof" (me) as possible. The only thing you can 100% count on is that mortar will not fail in compression. To that end I cut the arch bricks so that if the mortar failed completely it would still not move. I tested it by pulling the form as soon as the arch was finished and sponged down, with the mortar still very wet. I aslo kept the mass at the arch interface to a minimum, wanting it to heat up quickly and transfer the heat to the cast vent. At that point I have the cast vent on top and ledging bricks for the dome are only about 1 inch thick. After that I actually have the next 2 dome courses resting on top of the arch and ledging bricks. I also built the arch so that it sits on top of the side supports. I was thinking if it expanded enough to crack then it would float on top. But I have no cracks at all so, who knows there. I also layered a 1/2 to 1 inch layer of cladding over the arch sides when doing the dome. And I have a layer of 1-2 iches of Perlcrete over that, not that may make much difference, but just trying to include them all. My arch is on the small side of the ovens here, and definitely not the prettiest, but it seems to be plenty strong.

Hopefully another data point for you will provide helpful.

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

...I cut the arch bricks so that if the mortar failed completely it would still not move.

Wade,
Makes perfect sense! I'll cut the arch bricks to much closer tolerances.

I also built the arch so that it sits on top of the side supports. I was thinking if it expanded enough to crack then it would float on top.

I like that thinking as well. If I have to rebuild, I also plan on redesigning the vent box to span the entire arch to better distribute the weight and provide more support from above.

Thanks for the input. I'll let the mortar set for a few days (4 days of rain forecast anyway!) and then fire it up and see what happens.

[Frances]

Re: Curing, Cracks & Other Kentucky Conundrums

Good luck Ken! I'm sure everyone here is keeping their fingers crossed for you...


Don't know if this is any help (well, it isn't in this case, but maybe in general) but from a gut feeling I would have thought that a full rounded archway is structurally stronger than one with a flattened rounded top, which a lot of people seem to be building at the moment. Or not?