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]

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]

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]

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

[gjbingham]

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]

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]

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]

Originally Posted by Frances

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]

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]

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....