#11  
Old 09-20-2010, 04:43 PM
Laborer
 
Join Date: Aug 2009
Location: Columbus OH
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Default Re: arch crack created stucco crack

So here is what I have done with my mighty angle grinder:
damaged stucco 003 | Flickr - Photo Sharing!
damaged stucco 001 | Flickr - Photo Sharing!

I cut that little groove around the whole arch.

Now I need to fill it with something that I can later paint.
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  #12  
Old 09-22-2010, 05:17 AM
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Location: Columbus OH
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Default Re: arch crack created stucco crack

I fired up the oven last night to see if the stucco cracks open up even though they are not connected to the cracked arch.

Yes-they do. Looks like there is enough force to push the insulation against the stucco layer and lift the stucco up a little, so the cracks open up. It is not much, but I am afraid that if I only patch the cracks it will open up again.

Now I have a dilemma what to do.
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Old 09-22-2010, 05:57 AM
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Location: Maryland, USA
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Default Re: arch crack created stucco crack

Greg - how thick is your blanket insulation? When your oven is fired, does the stucco feel warm/hot?

Looking at your pictures, I'm guessing that you put the stucco layer on top of the blanket layer. I know many here have done just that without any issues. However, if you're seeing cracking, you might consider either adding more blanket insulation or putting vermicrete on top of the blanket and then the stucco on after that.

S

Last edited by sjmeff; 09-22-2010 at 05:57 AM. Reason: Typo
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  #14  
Old 09-22-2010, 06:09 AM
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Default Re: arch crack created stucco crack

I have 1-2 inches of blanket plus about 6 inches of vermicrete. The oven remains cold, the temps goes up only about 10 degrees above ambient.

Can I apply another layer of stucco over the whole dome? I am thinking to use that surface bonding stuff, I found one home depot in columbus that sells that.
greg
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Old 09-22-2010, 06:41 AM
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Default Re: arch crack created stucco crack

Your insulation sounds comparable to my oven (I think I had more blanket, less vermicrete).

I will let a more experienced member respond regarding SBC over stucco. My novice inclination is that it would probably work well. The acrylic fortifier was highly recommended and so I used that with the SBC. It's expensive but worth the benefits (improved water repellency, helps prevent cracks during curing, etc.)

S
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  #16  
Old 09-29-2010, 07:20 AM
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Default Re: arch crack created stucco crack

I shimmed all stucco cracks open with small nails while the oven was hot. After it cooled down, I filled them with a very fine, sieved stucco mix and covered with a think stucco. Next fire, the same cracks opened up.
I will do it all once more, but this time I will add maybe 1/4 inch of the surface bonding concrete, maybe this will hold the stucco from cracking.
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Old 09-29-2010, 06:20 PM
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Default Re: arch crack created stucco crack

There are 2 methods for preventing the mirroring of cracks through stucco:

The first is to provide a slip plane between the cracked layer and the new layer. The size of the slip plane depends upon the movement of the crack, but in general will be around 6 inches for a hairline up to 1/16th inch crack. The slip plane is normally composed of 20 to 40 mill poly, but this may not withstand the heat. Teflon sheeting or may work. Basically, you are providing enough surface area that the new layer is able to absorb the amount of movement without cracking. For relatively small cracks, say 1/8", this is rather large, on the scale of a 12-18" slip plane.

The second is to either fill the crack or cover the crack with a material that has a high enough modulus of elasticity to bridge the crack and also to shrink back to it's original size. Elastomeric coatings are one way, forming a proper joint and using a sealant is another. For small cracks, elastomeric coatings are best, for anything over 1/4" a properly formed sealant joint is the best answer.
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Old 09-30-2010, 05:16 AM
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Default Re: arch crack created stucco crack

nice! I was just thinking last night about sealing this crack with high temp silicone and spreading the silicone about one inch on either side of the crack. Looks like I was on the right track.

I think I will do the silicone and put a 3 inch wide aluminum tape over it. Slip plane, I like it, thanks!
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