Cracking in undesirable places! Help!

[jgd915]

Hey guys,

So I just finished building my first pizza oven, and I am thrilled with it. As you can see in this photo there is a 2 inch thick concrete slab sitting on top of cinder blocks. On top of that is a fire brick arch dome. I did about 5 curing fires, of gradually increasing heat. I have used the oven twice for cooking; first time made only 2 pizzas and then the next time making 5. After making 5 pizzas, there was a noticable crack leading down a fire brick in the front into the slab. This is very worrisome as I thought the fire brick, with heat stop beneath it would prevent the slab from getting too hot. The slab is not made to withstand extreme heat. I patched it up with some repair cement, but I am afraid to use my oven again! And I was planning to have a party this weekend! Will a regular cement slab crack if it gets too hot?? I attached some photos of the crack as reference. It is hard to tell how deep the crack goes. Short of building a third cinder block leg for support, any suggestions for reinforcement? I do not want my slab to break in half and destroy the oven.

[Tscarborough]

Re: Cracking in undesirable places! Help!

You have a lot of issues to deal with with this fireplace.

First, 2" is not thick enough for normal concrete slabs. 3" is pushing it and only doable with correct reinforcement and additives. 4" is considered normal minimum thickness for a suspended concrete slab.

Second, without insulation, both under and over, it is not really an oven. There are several products that are suitable, vermiculite/portland cement, perlite/portland cement, cal-sil board and foam glass. Normal with the perlite/vermiculite, it will be 4" over and under at a ratio 0f 1 portland to 6 or 8 perlite/vermiculite. Cal-sil and foam glass are normally 2-3" thick.

Third, the chimney needs to be outside of the oven, normally at the front, and there needs to be a reduction in the front opening size to approximately 64% of the ceiling height.

Good luck!

[Tscarborough]

Re: Cracking in undesirable places! Help!

So that is all the bad news. Temporarily, to use the oven for your party, drystack block in the center, and wedge the slab up. Buy a lot of wood and throw your party. You will probably want to burn a decent fire for at least 3 or 4 hours before you are ready to cook, assuming you want 6-700F to cook in.

edit-Oh yeah, block up the chimney and let it vent out the front opening.

[jgd915]

Re: Cracking in undesirable places! Help!

Hi,

So I have been able to heat my oven far north of 1000 degrees. Even without bottom insulation, the oven seems to work. I have thought about reducing the front opening size. But I have found that cooking with a door on seals in most of the heat pretty well. Do you think it will crack because it is only 2 inches?

[Tscarborough]

Re: Cracking in undesirable places! Help!

I can heat my fireplace to 1000+ too, but it won't hold that temp very long.

[jgd915]

Re: Cracking in undesirable places! Help!

Why would we block up the chimney and let it vent out the front?? Also is the issue with the slab bearing weight? Or extreme heat? Thanks.

[Tscarborough]

Re: Cracking in undesirable places! Help!

Because with the flue in the oven dome, you lose most of the heat immediately.

Heat causes gradual deterioration (but sometimes explosive decomposition depending upon the aggregate used). Your cracking issue is structural, the slab is too thin.

[jgd915]

Re: Cracking in undesirable places! Help!

The slab is 2.3 inches thick. It is reinforced with 2 pieces of rebar. If the top were too heavy for it, wouldn't the slab have cracked already? I was thinking it was a heat issue, not a weight issue (which you seem to be saying). Thanks for your help. With the rebar, could it hold? Or is it a combination of heat and weight?

[Tscarborough]

Re: Cracking in undesirable places! Help!

I am sure it will last for a while, but with a center support and it will last longer.

[jgd915]

Re: Cracking in undesirable places! Help!

Is heat or weight the issue?

[david s]

Re: Cracking in undesirable places! Help!

Is heat or weight the issue?

Both. If you had insulated between the supporting slab and the oven floor it would have protected the supporting slab from the excessive heat as well as retaining the heat in the floor. It would be neigh on impossible to insulate between them now or replace the supporting slab without a total rebuild. It may be simpler to just keep using the oven as is until it becomes unusable, then you can knock it down and reuse your bricks. In the meantime you'll learn plenty by firing and using the oven while taking a good look at the free plans on this site. That will help you understand how to build a better oven.

[jgd915]

Re: Cracking in undesirable places! Help!

How exactly do you think it will break? The crack healed over night and are now pretty much gone. Eventually will the slab just split down the middle and crumble?

[Tscarborough]

Re: Cracking in undesirable places! Help!

Portland cement does not exhibit autogenous healing. The crack is structural, that means it affects the structural integrity of the slab. It has already failed because concrete does not have the flexural strength to support a suspended slab at 2 or 3 inches thick unless you have an engineered mix design and reinforcement schedule.

It may completely fail tomorrow or never, it doesn't matter, because you would enjoy your oven more if built correctly. Use it until you can rebuild it, then start over and either follow some plans or research it some more.

[Gulf]

Re: Cracking in undesirable places! Help!

How exactly do you think it will break? The crack healed over night and are now pretty much gone. Eventually will the slab just split down the middle and crumble?

The "soft iron" rebar and concrete are designed to expand and shrink at the same rate. But, that is at normal ambient temperatures for most parts of the world. Heat cycling for a "stove" is a different animal all together. It is too fast, and too different extremes, for both to accommodate. When you heat it up again the crack will get just as wide, or more. It will Introduce more moisture and O2 to the equation. Over time, that will also equal, rust. Rust expands, putting even more pressure on the concrete. Concrete has great strength in compression. But it doesn't have much tensil strength, unless it is installed to specs.

just sayin'

[david s]

Re: Cracking in undesirable places! Help!

You can get away with a slab less than 4", provided the oven is small and not too heavy, by cantilevering the slab over supporting piers. This reduces the required span and places the support right under the heaviest parts of the oven.