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  #81  
Old 03-28-2013, 05:54 AM
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Default Re: Share your crack stories

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Originally Posted by wotavidone View Post
Speaking of comparing apples with oranges, I'm not sure your barrel vault is a reasonable comparison to a dome with a sailor course when discussing crack resistance.
This topic is definitely thread worthy.....

I can't speak for Tscar, but a comparison between a hemispherical dome and a barrel vault is reasonable IMO. The point of failure in a vault or dome would be at the same point...where the line of thrust exits the structure.
And that will vary depending on the weight of the dome, addition of cladding, buttressing or banding on the base course or springer of the arch. That's why sailor courses are inherently weak...there isn't enough mass for the thrust line to stay within limits of structural integrity.
But, that doesn't mean it can't work, as you have experienced. The reinforced cladding is acting to counter outward forces that would have otherwise pushed the sailor course outward...if not right away, it would over time with all the thermal cycling. The same with a barrel vault. If the side walls have disproportionate mass needed to contain the thrust of the arch, then you must buttress or band them...something that keeps the forces within the center line of the structure.
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  #82  
Old 03-28-2013, 07:48 AM
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Default Re: Share your crack stories

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Originally Posted by wotavidone View Post
I was discussing whether my inherently unstable sailor course might have been made more resistant to cracking by being mortared to a substrate that is not brittle and friable.

Within the context of that discussion, whether or not Tscar's running bond barrel vault has cracked is simply irrelevant.
But that is the point...the joint between the substrate and the first course (mortared or dry) will not factor into the failure of, or cause the serious cracking of, any type of arch, whether it's a dome or barrel vault. That is proven by vault arches and domes ( and structural stands that are dry stacked cmu ) that have no mortar between the base course and either the brick floor or slab. Weight and gravity are the bonding factor in both...but if lateral forces are too great relative to the mass of the walls, then you start seeing cracks and failure.

I'm assuming we are not talking about the usual small expansion cracking associated with normal oven operation.
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  #83  
Old 03-28-2013, 08:14 AM
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Default Re: Share your crack stories

Unless there is a tension member tying the bottom of the arches together it makes no difference what the substrate is so long as it has enough compressive strength to hold the weight of the dome. An arch can be designed such that a tension member is used but that is a whole other animal.
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  #84  
Old 03-29-2013, 05:02 AM
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Default Re: Share your crack stories

A crack, especially a vertical crack, is a failure of the arch, even if it does not cause the structure to fail. There are three causes of cracking: Shrinkage, thermal expansion, and excess load/material failure.

Shrinkage is no big deal, and is addressed by keeping the joints small and as even as possible, as well as coursing the brick.

Thermal expansion is also addressed by keeping the joints small and as even as possible, as well as coursing the brick.

Excess load/material failure is addressed making sure that the mortar is softer than the brick and arches are buttressed.
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  #85  
Old 03-29-2013, 05:23 AM
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Default Re: Share your crack stories

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Originally Posted by wotavidone View Post
You are writing about arches failing. We are discussing two different things, but you can't seem to see that.
I give up.
Tscar beat me to it....

Sorry you are confused, but if you read back, all of the replies relate to various points you have made, and none of them are speculation, but proven facts.
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  #86  
Old 03-29-2013, 05:32 AM
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Default Re: Share your crack stories

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Originally Posted by wotavidone View Post
"Higher permeability of SC may be harmful" not "is harmful"
Since we all go to great lengths to keep our insulation layers dry, it shouldn't be a problem.
Right, my mistake....I typed the original reply from memory.

However, I brought that up because you mentioned using SC as the structural slab, and a lot of them are exposed in the wood storage area. There are other cases where the edges of the slab could be in direct contact with a wall or natural structure, like a rock ledge (sharkey) were moisture could permeate the slab. And since this forum is used as a reference, it is worth considering the use of corrosion resistant reinforcement.
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  #87  
Old 03-29-2013, 06:15 AM
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Default Re: Share your crack stories

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Originally Posted by wotavidone View Post
No, your replies relate to not understanding the point I was trying to make.
I'm trying to make the point that, given I have apparently used a brick layout that is unstable, the fact that I have mortared it to a substrate that isn't friable, may be helping to hold it in place, by helping to counter the outward forces you speak of.. If the same course of bricks was mortared to a layer of vermicrete, and these outward forces were applied, I reckon the vermicrete would fail where the mortar joins it, since it is brittle and friable and has little flexural strength, according to Tscar.
Perhaps the high strength of the substrate consititutes the "tension member tying the bottom of the arches together" that Tscar also mentions.
Anyway, I was merely offering an alternative, and I am beginning to suspect you blokes are being deliberately obtuse, so I'll say no more on the subject.
Obtuse, not at all....practicing the art of futility more like. The explanations are trying to make you understand that.....


1) Mortaring the base coarse to substrate wont stop cracking no matter what it is made of...mentioned before, and explaining the way forces work in an arch should have cleared that up.

2) A slab is not a tension member. Banding(with straps), buttressing, and Cladding(what you said you have done) IS. Mentioned before, several times.

It seems you never got what you are looking for, which is validation of the method you constructed your oven.
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  #88  
Old 03-29-2013, 06:41 AM
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Default Re: Share your crack stories

Plus a tension member is used at the spring line. The vertical brick (sailor/soldier) has the line of force through the top of the brick, not the base. A sailor/soldier with cladding works just fine, but is only as strong as the cladding.

The classic Neapolitan design uses the insulation to buttress the sailor course against the reinforced stucco shell and no cladding. The downside to this is that the insulation used (tufa) is not that good of an insulator. A better method is a cladding that is reinforced by some method to allow it to act as a tension ring. The difficulty here is that it is not a closed ring because of the entry. Here is how I solved it:



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  #89  
Old 03-29-2013, 08:04 AM
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Default Re: Share your crack stories

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Originally Posted by wotavidone View Post
I am not looking for validation of my construction method. The validation of my method is that it is staying up and not cracking. I'm not even advocating the use of my method to anyone else.

I was just offering the point that every little bit helps, and I do know how an arch works without your endless repetition of something every half decent school kid knows.

You guys seem so rigid. Not at all open to ideas.
For example, with regard to Tscars pictures below, might it not have been just as simple to clad that ring of bricks with some scoriacrete which has insulating properties and is as strong as the proverbial brick outhouse? Probably wouldn't need to drill re-bar in. and the bottom section of that dome wouldn't have layer of extra thermal mass that Tscar's method undoubtedly added.

You can help me with something though. I've seen so many poor newbies discouraged and confused by all the so-called experts, that I've decided that people like you make this forum too irritating, but I can't seem to find out how to remove myself from it.
Can you assist? How does one de-register?
Sorry you interpret it that way. The repetition was there because it doesn't appear that you do how forces in an arch work. I never got inflammatory, never said I wasn't open to new ideas..but apparently feelings are hurt when reasoning was used to show how something won't work to prevent cracking.
BTW, sarcasm doesn't really encourage new builders to ask questions either.
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  #90  
Old 03-29-2013, 09:22 AM
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Default Re: Share your crack stories

The problem with using un-reinforced cladding is that when it cracks (because it WILL crack), then the cladding is no longer working as a tension ring, and is thus no longer supporting the dome. That is why the Italians use the reinforced shell to buttress the non-reinforced cladding of tufa.

Edit-Also note that with a sailor course, you only have 2.5" of refractory mass, so the cladding is refractory concrete to build the mass up to a total of 5".

Last edited by Tscarborough; 03-29-2013 at 09:27 AM.
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