Oh, the Humanity!!

[gbohnstedt]

My apologies for being missing for a while (adopted 4th child, lost job, been busy), but I need to share what happened to my beautiful pizza oven:

IT COLLAPSED! It has been wet this year in Utah, but I kept it covered with a tarp. Still, I think the combination of moisture and freezing temps did their damage.





She used to look like this:



I can't be sure exactly what happened until I do a complete post-mortem but my mason went through all the mortar that came with the kit and then I bought at least 3 more 50lb. bags of Sairset. There was a lot of mortar in there. The Sairset never seemed to form a decent bond with the brickwork, either. Anyway, I'll post a "lessons learned" when I figure out what happened.

Greg

[Tscarborough]

I will help you with a few things.

First, your entry arch does not have nearly enough mass to buttress it.

Second, the flue stack has basically no support, see above.

I am somewhat surprised that the entry failing brought down the whole thing though.

[Tscarborough]

Let me add, though, that I really like the entry design with the wedge shaped landing.

[Faith In Virginia]

So sorry for your loss. Oh, the Humanity!! says it all.

Congratulations on #4

You used Sairset. I think I looked at that product and I thought it was for kilns. I remember some property of it that made me go NO not the correct stuff. Perhaps someone else would know more. But I would look at that first.

[Tscarborough]

Just for general information, the purpose of most mortars, and especially refractory mortars, is to keep the bricks apart, not hold them together. That is to say, the mortar is a chinking material used to fill the gaps, not a glue to hold them together. Firebrick are dry pressed solely to allow them to maintain a very tight dimensional tolerance compared to extruded brick. 3/8" joints are normal for masonry work, but 1/8" is desired for fireplaces.

Keeping this in mind, almost any material that can withstand high heat is suitable for the oven mortar. Sairset, and similar mortars are not designed for high build use (thick joints). They are designed for very narrow joints. The aggregate used is very small, and that limits the ability of the mortar to dry without cracking.

For larger joints like those present in hemispherical ovens, a high build mortar is desired. They will have more and larger aggregate, as well as the ability to remain somewhat plastic though the drying cycle. That is the role lime fulfills in the home brew mortar for example.

Another aspect is the bond strength of the material used as mortar. As a rule, the higher the compressive strength (the "harder" or "richer" the mix), the less bond strength it will have. Proper masonry design always favors the weakest possible mortar that can withstand designed loads. Those loads are very small in relationship to normal concrete construction. Mortar is not concrete.

Doing things like soaking the highly absorptive firebrick and mixing the mortar somewhat wet for workability and potlife decrease bond strength and should be avoided.

[gbohnstedt]

You know you're absolutely right. I knew that there needed to be buttressing of the edges, especially with an entry arch that low, but somehow, forgot. Still, I noticed that the Sairset flaked off and didn't cure out as hard as I expected it should have.

While it was cooking the flaired entry was the bomb. People could see in and I had lots of room to support food.

Of course the support for the stack and the sides was going to be provided by the surrounding structure which would have waterproofed it as well.

Buttvision is always 20/20.

Greg

[Les]

Originally Posted by gbohnstedt

Buttvision is always 20/20.

Thats got to go in the WFO dictionary somewhere - I'm still laughing. Sorry for the crash...

[gbohnstedt]

Originally Posted by Tscarborough

Just for general information, the purpose of most mortars, and especially refractory mortars, is to keep the bricks apart, not hold them together. That is to say, the mortar is a chinking material used to fill the gaps, not a glue to hold them together.

Now he tells me. That, of course, is completely counter-intuitive to how we typically use the word mortar. So, if my mason was not cutting tapers on the bricks which created a high build in the outside and not so much on the inside, would this have contributed to the "fall of the house of Pizza"? I am posting a picture of the inside of the oven so you can see the mortar joints.







Greg

[Tscarborough]

It looks like he did a good job to me. The mortar didn't fail, the structure did.

I used a Sairset type mortar on my oven, although the vault was designed to have minimal face joints, and do not anticipate any issues (and spent a couple of days standing on top of it with my considerable bulk plus 20 odd brick and buckets of mortar).

[dmun]

Is this a first? Has an oven (other than the wet clay geodesic at MFA) ever collapsed before?

I don't think you can blame the flared entry. I have one and there seems to be no danger of collapse (so far).

I think I can safely keep warning builders off of wet premix mortars.