External crack

[Frances]

Re: External crack

Doesn't vemiculcrete have a bit of give in it? Maybe if you filled the gap with that, and it would insulate at the same time?

[talisker63]

Re: External crack

Well my first impression was that the flue (sheet metal) expanded on the first firing and caused the colored render shell to crack. The crack started at the flue and has worked it's way down to the arch. It is getting very slightly wider each time I fire the oven. It closes a little when the oven cools.

My plan is to keep firing the oven for another month or so (once per weekend) and then repair after that time. Repair will be;

1. cut a gap around the flue to allow for expansion. Suggestions on how to fill this gap so as to reduce heat loss??

2. re-render the entire oven with another 1" of sand/cement/oxide render.

I have quite a few hairline cracks on the sides so the re-coating will cover these also.

Any other suggestions?

cheers

[cvdukes]

Re: External crack

FRC......but not the same issue with chickenwire? How about the steel reinforcing wire mesh?

I guess the chicken wire doesn't have enough mass to push anything around...each indiviual cell would expand slightly and might would crete just a slight internal pressure but not high enough to counteract the adhesive pressure of the cement...would also guess the stucco mesh is about the same but can't say from experience

[cvdukes]

Re: External crack

The fiberglass reinforced concrete is a commercially available product a.k.a. surface bonding cement. Brand I use most often is Bonsal "Surewall" although lately Lowes has been carrying a Quickcrete product instead. Both products come in grey or white and runs about $16 per 50 lb. The stuff is made for quick construction of concrete block walls. You dry stack the blocks and then smear this on both sides of the wall. The Bonsal product will pass most building codes making a wall up to two stories high. I assume the same for the Quickcrete but haven't read the specs on it.

[gjbingham]

Re: External crack

XJ - good information on the kiln bricks heating properties and need for space. It agrees with what I've been guessing about stresses/cracking of ovens when they get heated. I wish I knew how to apply this to stopping cracks from forming.
G.

[david s]

Re: External crack

cvdukes,
apparently the recomendation for steel reinforcing for refractories is stainless steel needles, but they say they are a bitch to work with. Although I hate working with fiberglass I did throw some into a mix once after my dog got into some chopped strand matting and made it into chopped loose birds nest matting. I thought I better research it a little and found out you should use AR (alkaline resistant fiberglass) apparently the stuff reacts with the alkalinity of your cement mix.

[Xabia Jim]

Re: External crack

The problem beomes magnified as the size of the pieces increase in size as I found out earlier this year.

...Have any of you brickwork domers ever tried a final coating of fiberglass reinforced concrete? I use it on all my exteriors now and don't see any problems from thermal expansion so It must have enough flex in it.

FRC...what's your mix for this? sounds like a great stucco coating.

and I agree Craig on using the solid steel materials....this should then be a problem with rebar...but not the same issue with chickenwire? How about the steel reinforcing wire mesh?

[cvdukes]

Re: External crack

I just found this reference "The differential in thermal expansion between steel and concrete is so small that it almost never has to be taken into account. Much more
important, for a kiln or a chimney, is the differential in expansion
between the inside and outside resulting from a hot interior and cold
exterior. It is so serious a problem that kilns are made of bricks
and chimneys typically lined with space between layers to accomodate
thermal movement. That's why you leave a space between the ceramic
liners and the supporting brickwork in a conventional masonry
chimney."

That first statement above surprised me...I always thought that using steel in a thermal masonry device was going to be an issue due to differential expansion of steel and masonry....I'm still learning!

Have to respectfully disagree with the first sentence. There is a fairly significant difference in the coefficients of expansion between masonry and steel. The problem beomes magnified as the size of the pieces increase in size as I found out earlier this year. I had built the oyster cooker side of the oven complex with a metal angle-iron framework surrounded by brickwork and forgot to leave any expansion space. The longest dimension of the metal was about 45 inches so that seemingly little difference between the two materials gets porpotionally larger. The oyster cooker only gets up into the low 600's degrees F, yet the metal expansion was enough to open up a 3/4 inch crack in the brickwork.
Also, think a lot of the reason for the airspace between the inner and outer flue is to shield the inner flue from exposure to colder air, which causes thermal cracking...the air in between reachs some happy medium temp and acts as an insulator. Last year's dry-stack version of the oyster cooker had a chimney made from a couple of flue tiles stacked up and held in place by a little bit of wire and luck (testing the design)...The flues were exposed directly to the air temp and every one of the flues cracked big time.
Have any of you brickwork domers ever tried a final coating of fiberglass reinforced concrete? I use it on all my exteriors now and don't see any problems from thermal expansion so It must have enough flex in it.

[Xabia Jim]

Re: External crack

This also tells me that having a thermal blanket and / or slip plane between the inner dome and outer dome cover is a very good idea. Almost like two separate structures! (makes me think of those russian dolls!)

[Xabia Jim]

Re: External crack

I just found this reference "The differential in thermal expansion between steel and concrete is so small that it almost never has to be taken into account. Much more
important, for a kiln or a chimney, is the differential in expansion
between the inside and outside resulting from a hot interior and cold
exterior. It is so serious a problem that kilns are made of bricks
and chimneys typically lined with space between layers to accomodate
thermal movement. That's why you leave a space between the ceramic
liners and the supporting brickwork in a conventional masonry
chimney."

That first statement above surprised me...I always thought that using steel in a thermal masonry device was going to be an issue due to differential expansion of steel and masonry....I'm still learning!

[edschmidt]

Re: External crack

Did you check the temp on the outside of the exterior? My guess is that since you have the exterior extends to the chimney which doesnt have the same amount of insulation, that the chimney expanded and caused pressure on the cooler section. If my theory holds true I would suggest somehow cutting a channel on the exterior around the chimney to allow expansion. If you have an IR thermometer it would be quite simple to measure the temperature differential.
Good luck.

[david s]

Re: External crack

make sure you use plenty of lime in that outer shell. Lime is much more elastic than cement. Did you use chicken wire mesh ?
For ferro cement to make it stronger you add more mesh not more cement. Ferro cement boats are incredibly strong but not very thick. I could be wrong but I think a thinner shell is better. I made mine only 10-15 mm thick

[Xabia Jim]

Re: External crack

What about silicone sealers? I think they can handle high temps right?

[carioca]

Re: External crack

Good to hear others DO have cracks in their render/stucco - I thought I was singularly inept in my cover-up :-)

I've noticed a crack about 5cm (2in) long starting in the two-coat render/stucco at the 'naked' arch near the top of the dome and pointing toward the apex.

I measured the outside temperature at that point when the oven was hot, and it registers 50-odd decrees Celsius (sorry about the missing F, I'm lousy at instant conversions). Perhaps I didn't put down the insulation properly at that point close to the masonry?

Whatever: it hasn't grown in subsequent fires but I agree that it will be best to push some more cement render into that hairline and paint over again.

BTW, the two render coats are covered with two acrylic paint undercoats and two acrylic top coats - I'd have though that stuff is elastic enough to accomodate minuscule movements?!

Cheers,

LMH

[Dutchoven]

Re: External crack

There is a material used to caulk the joints in log homes...my brother usedit is his...supposed to stretch up to 4 inches I think he said...if it isn't hot it should work because I don't know how it holds up to heat
Dutch