Curing, Cracks & Other Kentucky Conundrums

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

Ken,
I think the theory of the outward force of the dome expansion is correct too. It sounds like the sag of the arch is a secondary failure, occurring after the original problem of the dome's expansion forces causing the mortar debonding.

That's exactly what I'm seeing. I think that's a good analysis.

If you're not seeing any tilt in the vent walls, then I would guess the weight of the arch and vent is probably not the real problem.

The left vent wall must be moving slightly during heat-up/cool-down causing the mortar debonding from the arch. Before I fire back up, I'll start taking measurements of everything to see if I can figure out what's moving and where so I can quantify exactly what's happening.

I'm going to go out and look at mine. I haven't noticed any cracks yet. Maybe I'm just not looking hard enough.

We all will have cracks (some small/insignificant and some big) With that much heat, mass and mortar, something has to give somewhere.

[gjbingham]

Re: Curing, Cracks & Other Kentucky Conundrums

Ken,
Sorry to hear about your setback.

I think the theory of the outward force of the dome expansion is correct too. It sounds like the sag of the arch is a secondary failure, occurring after the original problem of the dome's expansion forces causing the mortar debonding. If you're not seeing any tilt in the vent walls, then I would guess the weight of the arch and vent is probably not the real problem.

The question might be how to minimize the foreward stress on the arches caused by the dome's expansion. Hmmmm......... I'll keep thinking about it.

I'm going to go out and look at mine. I haven't noticed any cracks yet. Maybe I'm just not looking hard enough.

George

[wlively]

Re: Curing, Cracks & Other Kentucky Conundrums

You guys are right about a taller arch being stronger. But, we are not talking about alot of weight to support here. The picture posted is having to support a very tall wall. My arch looks to be pretty close to Ken's as far as arc height and it is fine.

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

Rick,

Thanks for the thoughts. I think you and Frances may be on to something.

I'm going to let my repair sit another day or two before firing again. If it fails, I have an idea for a replacement that incorporates all three observations made by Wade, Frances and Rick.

Thanks for the input!!!

[Ken524]

Re: Curing, Cracks & Other Kentucky Conundrums

...from a gut feeling I would have thought that a full rounded archway is structurally stronger than one with a flattened rounded top

You are correct! It's a lot stronger.

[oventhusiast]

Re: Curing, Cracks & Other Kentucky Conundrums

Hi Ken,
Sorry to see those cracks in the vent opening but I think Frances has a point about the arch. I'm not at all qualified as an engineer but after reading about your problem I went into the photo galleries and looked at photos of other arches. I found that the arches vary in the number of bricks used to span the arch. Maybe some one else here is more qualified in explaining this (I'm sure), but it looks to me that your arch is more shallow than some others and because of that, you needed to use more bricks to make the span, which would transfer the weight to the widest parts of your curve. Anybody out there see what I'm seeing or am I off base here?
Rick

[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?

[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.

[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

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?

[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.

[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

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.

[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.

[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?