Crack in the hearth slab

[Frances]

Hmmm, yesterday I discovered a crack in my hearth...

Just above the entrance. What do you think, can I do anything about it at this stage? Do I need to? It's reinforced with rebar etc.

[christo]

Re: Crack in the hearth slab

Sorry to hear that.

How thick is your support slab and where did you position the rebar within it?

How much do you use your under oven storage? Would a center support column interfere much with how you use it? I would go ahead and shim it up with a 2x6 and a wedge underneath (just to snug it up by hand - don't drive it home with a hammer) while you wait for the experts chime in....

Christo

[dmun]

Re: Crack in the hearth slab

Almost all concrete cracks, because it shrinks when it dries, and successful slab pours depend upon introducing artificial cracks, in the form of expansion strips. These shouldn't be necessary on such a small slab, and indeed it looks like your hairline crack is not structural at all, just the surface drying slightly faster than the underlying layers.

Here's a site that talks about the various forms of cracking in concrete.

[Neil2]

Re: Crack in the hearth slab

It looks like a shrinkage crack. But keep monitoring it.

[gjbingham]

Re: Crack in the hearth slab

Frances,
Your crack doesn't look to serious to me either, but Christo's idea of a support column below that area of the hearth sounds very reasonable, unless it really destroys the look of the oven. If nothing else, you might consider using concrete repair material or sealer in an attempt to keep moisture out of it.

[Frances]

Re: Crack in the hearth slab

Thank you for all the good advice!

I'm glad you don't think its structural - the thing is though, I didn't expect shrinkage cracks four months after the hearth pour. Maybe there was some additional drying because of the several large fires I lit over the last few days? Who knows, maybe the vermiculite layer is even finally drying out

I'll certainly keep and eye on it, and I'll be adding a sealing layer before the final finish in any case. But I'd rather not put a permanent column in the middle of the entry, because at the moment its really easy to get the wood in and out of the storage area. But if the crack gets any worse, I'll wham one in right away...

[Acoma]

Re: Crack in the hearth slab

Frances, how about high temp refactory mortar sealent to keep the crack from potentially becoming larger? I know DMUN, Christo and George will correct me if wrong. Even though it is not structural, couldn't it spread? Just like cracks in dome with mortar, you want to address them at times. I could be wrong, but I would at least address cracks such as those to keep my mind at ease. DMUN, Christo, George, what do you think?

[asudavew]

Re: Crack in the hearth slab

I'll certainly keep and eye on it, and I'll be adding a sealing layer before the final finish in any case. But I'd rather not put a permanent column in the middle of the entry, because at the moment its really easy to get the wood in and out of the storage area. But if the crack gets any worse, I'll wham one in right away...

I have one in mine as well... actually two. They follow the same line as the crack in the dome.
I believe my bottom slab sank a bit leading to my problem.
I have yet to remove the hearth forms and supports inside my stand, and I don't think I ever will.
But I never found it to handy for wood storage anyways, although I can still fit wood in just fine when I want to.

Anyways.. just thought I would let you know.

I will post some pics soon. I'll try to take some this weekend.

Dave

[asudavew]

Re: Crack in the hearth slab

By the way.

Does the crack run under the slab as well?
It sort of looks to me that you may have had some settling like I did. Just not as bad.

How is the dome holding up? Any cracks there?

[Frances]

Re: Crack in the hearth slab

Ah yes of course, I didn't think of settling. We've also had some fairly cold weather recently (10 below freezing), that could well be it.

I don't know if the crack runs under the hearth, because (for reasons that seemed quite logical at the time) I left the support board in. Its securely cemented in place... But the crack doesn't seem to run any further back.

The dome has no cracks as such It does have places on the inside where the mortar has shrunk back into the joints. And on the outside the mortar crackled in places, but nothing that ran for a whole brick length, or as far as the inside of the dome. From that point of view I was really lucky with the mortar I used! Now we'll just have to see how it holds up over time....

[Frances]

Re: Crack in the hearth slab

How thick is your support slab and where did you position the rebar within it?

Christo, the support slab is 3.5 inches thick (as per oven plans). The rebar is 12 mm thick (0.5 inches). I can't actually remember how close the rebar lengths were to each other, but I did follow the plans pretty closely, and added two extra lengths accross the opening. Maybe in hidsight a third or fourth would have been a good idea.... I can't really tell you where the rebar is in the slab, but I tried to get it as close to the middle as possible.

The concrete was on the wet side of oatmeal, but certainly not soup-like. It was quite warm when we poured the hearth (about 25C or 80F), but I did keep it damp for a week.

So basically, the slab should be sound. I think. What do you think?

[stuart]

Re: Crack in the hearth slab

I agree that this crack appears not to be of any concern other than aesthetic. It sounds as if you have the slab well reinforced. If we are wrong here and it continues to expand rather than inserting a post under the opening I would install a lintel piece on angle iron. IMHO

[Frances]

Re: Crack in the hearth slab

I was thinking something like that, too. Or maybe insert a sort of metal doorframe with butressed corners.

Well, with any luck it won't come to that.

[christo]

Re: Crack in the hearth slab

Sounds like you mixed and cured properly and have enough reinforcing steel.

I just downloaded the Pompeii oven plans and see it covers a slab using angle iron and lintel to span the opening with slab on top. It does not yet include instructions for omitting the lintel and spanning the opening with the slab only (an option I really prefer). I'm not sure one would do it any differently - other than adding some additonal rebar (which you did )

I was really interested in this topic so I have done some digging and came up dry. I ended up searching for concrete home construction to try to find span tables, etc. Ran across this link which makes me feel better and basically says the same thing as most of the group.

Concrete Floors, Part 2. Behavior of Concrete Floors, The Concrete Network - The Concrete Network

from above link

2.4 Cracking. Explain that both shrinkage and curling induce tensile stresses in the slab. Concrete is weak in tension so when the stress exceeds the tensile strength of concrete, the concrete cracks. To minimize such cracking, joints are incorporated in the floor to create planes of weakness where the cracks will occur. Nevertheless, some cracking should be expected, otherwise no reinforcement would be needed. Reinforcing steel is incorporated in slabs on grade to minimize crack widths and vertical displacement at cracks.

Happy Cooking!!!!

[gjbingham]

Re: Crack in the hearth slab

Acoma,
I'm no engineer. I've forgotten all my chemistry. I just go with my gut, which is often not the best way to proceed. I'd just be afraid of moisture freezing in there and making the crack worse. That would be my only reason for filling it. I like the idea of retrofitting an angle iron though.

Frances, I wouldn't second guess your work. I've got cracks in my driveway and sidewalks, and they were poured by professionals. As long as it's not getting worse, I'd try to turn a blind eye to it (and keep cooking).
G.