Share your crack stories

[david s]

Re: Share your crack stories

Whatever works, works. To a large extent the variations we make and their outcomes are educated guesses.

[stonecutter]

Re: Share your crack stories

can't seem to add to my last post. here is what I wanted to say.
I made a structural concrete slab using scoria for the large aggregate. Contrary to what most people are saying here on the forum, I found an academic paper where a guy made up various mixes using scoria for the large aggregate instead of gravel, and some time fine scoria for instead of sand. The mix I used had, according to this paper, an insulation rating of .25 w per metre per kelvin, or whatever the metric units are. Not as good as the 4:1 vermiculite cement at 0.16, but good enough for me. The insulation value of my slab has been a point of contention on other threads.
The research paper I found simply doesn't seem to pry open some minds.
I had the handicap, or perhaps advantage, of knowing nothing about the subject before I started. Not knowing the "rules" I was able to ignore them. I did take out the insurance step of putting a layer of vermicrete under my floor bricks though.
I think the fact I used a rigid substrate to support my dome, rather than something flexible like vermicrete or one of the insulating fibreboards, and actually mortared my bricks to it, might have also contributed to the lack of cracking. For the record, I've used it once a week on average since August 2012. It seems to be holding up well. I'm confident enough that I'd do the same again if I built another.

I haven't heard of the term scoria before seeing this. Interesting use of the material too. Personally, I wouldn't have an issue with using it as an aggregate in a reinforced structural slab. Do you have a link to the paper you read? I would like to see it. As far as the lack of cracking in your dome, I think it is because of your reinforced cladding...

[Tscarborough]

Re: Share your crack stories

Vermicrete/perlcrete is not flexible in the least, is extremely brittle and friable. Using scoria in your slab is a great idea, but comparing it to a 4 to 1 perlcrete is apples to oranges. 4-1 perlcrete is only good for 400-500 PSI compressive with much less than that in flexural strength, whereas using scoria will be similar, if not better than standard concrete with the added benefit of a decent insulation factor.

Where I differ is using scoria as an insulation under the hearth and over the top, BUT if it is available and free/cheap, I would certainly do so.

As for cracking, the only cracks I have are the ones I engineered into the oven, i.e. where the vault meets the front and back walls.

[Bacterium]

Re: Share your crack stories

Scoria should be plentiful.
It used to be popular in gardens about 20-30yrs ago......so a lot of people remove it now.
I had a mate who did this at his house as he hated it.......Should have kept a handful to sneak some in his letter box on his birthday .....

Funny thing is other stone is now popular.......more of a mulch person myself.

[stonecutter]

Re: Share your crack stories

For stonecutter I attach the research paper one last time. (I've already done so on at least one other thread.)
Page 14, the B series mixes.
It was published in the International Journal of Physical Sciences, so you'd have to think it has undergone at least some peer review.

I would have been happy with a link to the thread....but thank you.

After reading the paper, I feel using Scoria to make a light weight insulating concrete with structural properties has great design potential in oven builds....especially in the structure surrounding the oven.

It was noteworthy that it "is harmful to the long term durability and can enhance corrosion of reinforcement needing special protection." So it would be worth using coated rebar and wire or SS reinforcement if making concrete like this. Thanks again...looks like I have another material to experiment with.

[stonecutter]

Re: Share your crack stories

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.

[stonecutter]

Re: Share your crack stories

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.

[Tscarborough]

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.

[Tscarborough]

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.

[stonecutter]

Re: Share your crack stories

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.

[stonecutter]

Re: Share your crack stories

"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.

[stonecutter]

Re: Share your crack stories

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.

[Tscarborough]

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:

[stonecutter]

Re: Share your crack stories

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.

[Tscarborough]

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".