Crack in structural slab. Bummer.

[Yannick]

This morning I took a little tour around the oven and found a crack on the structural slab of the base, under the vermicrete.
Slab is 3.5" and rebar reinforced. The Vermicrete is 4".
I have a crack on the left and right side of the slab (see picture)
Picture1: crack on left side of slab
Picture2: crack on right side of slab
picture3: crack on the right of the oven opening. I keep adding stucco but the crack still shows up.
Picture4: crack on stucco side.

My course of action for the slab is to build up a wall inside the base, in the middle, to support the slab. (maybe going from the back wall of the base towards the front for half the length of the inside .

[jeeppiper]

Re: Crack in structural slab. Bummer.

Puzzling....

Unless there are cracks in your foundation, I am thinking that your front wall/brick arch might be weak. Rather than the arch, a concrete lintel might have been better.

Not sure if your support wall is best solution. The cracks may just be minor stress cracks and as long as they don't get worse, you might be ok. However, am curious why stucco keeps cracking, which would indicate that it is still moving.

Maybe a concrete expert will chime in here.....

[Toomulla]

Re: Crack in structural slab. Bummer.

This morning I took a little tour around the oven and found a crack on the structural slab of the base, under the vermicrete.
Slab is 3.5" and rebar reinforced. The Vermicrete is 4".
I have a crack on the left and right side of the slab (see picture)
Picture1: crack on left side of slab[ATTACH]44464[/ATTACH]
Picture2: crack on right side of slab[ATTACH]44465[/ATTACH]
picture3: crack on the right of the oven opening. I keep adding stucco but the crack still shows up.[ATTACH]44466[/ATTACH]
Picture4: crack on stucco side.[ATTACH]44467[/ATTACH]

My course of action for the slab is to build up a wall inside the base, in the middle, to support the slab. (maybe going from the back wall of the base towards the front for half the length of the inside .[ATTACH]44468[/ATTACH]

Firstly I do not believe the cracks are structural but most likely shrinkage cracks if the slab was sagging the top would be in compression and the cracks in the underside of the slab. I would not be concerned I suggest that you just cut them out with an angle grinder and fill with a good sealant or do nothing

[Yannick]

Re: Crack in structural slab. Bummer.

The foundation is not cracked.
Just the slab beneath the oven.
I cannot see if it is cracked underneath because i still have the backer board.
I haven't noticed the slab cracks before the fire started.
I might grind them and patch them estheticaly but i think I will still add those blocks underneath the slab.
I thought about doing it during the contribution but I thought all those rebars should hold up. Better safe then sorry.

[NCMan]

Re: Crack in structural slab. Bummer.

Those are not shrinkage cracks in the slab.

[deejayoh]

Re: Crack in structural slab. Bummer.

Those are not shrinkage cracks in the slab.

Wondering how you are so sure about that?

Looking at a pic of his full oven, if the cracks are on the right and left side of the oven, and there are no cracks in the foundation, I don't see how it would be any sign the concrete is failing. I also don't see how a wall at the center of the oven would help.

[kbartman]

Re: Crack in structural slab. Bummer.

Yannick,
Not sure what is going on with your support slab but here is a post I posted a while back. It may help you understand why your getting cracks in the dome stucco. A very informative article on the stucco process.

Glad to see your enjoying your oven. I've been enjoying mine also.

Did you finish your stucco yet? Hope to start my stucco process soon. I been doing some research on the process. Attached is article on best practices if you are interested it was quite a eye opener for me glad I read it before I attempted anything. I hope your still in the planning process.

Too big to post the pdf file here's a link http://c.ymcdn.com/sites/www.nibs.or..._011_WB4-2.pdf

[NCMan]

Re: Crack in structural slab. Bummer.

Wondering how you are so sure about that?

Looking at a pic of his full oven, if the cracks are on the right and left side of the oven, and there are no cracks in the foundation, I don't see how it would be any sign the concrete is failing. I also don't see how a wall at the center of the oven would help.

I have about 40 years of hands on experience w/concrete and masonry and know what a shrinkage crack looks like. And no, a wall placed there now will be of little help, except to keep it from collapsing. With that span, in my opinion, a block support wall should have been built there from the start. My one question I have is.....how long did he leave the forms on supporting the slab underneath after it was poured? They should have been left on for a minimum of 28 days underneath. I'd have left them on for the whole build, if they were not in my way. A crack, simply by the location, size, direction and appearance can tell you an awful lot about what is going on. As I said, the slab cracks are not from shrinkage. For one thing, shrinkage cracks appear right away, not later on. I'm not there and cannot offer much more, other than seeing the pictures that I've seen.

[stonecutter]

Re: Crack in structural slab. Bummer.

Does this crack run from side to side or front to back?

[Yannick]

Re: Crack in structural slab. Bummer.

The crack appears to run from side to side only.
The support might have stayed underneath the slab for maybe a week or so.
My idea to put blocks underneath is indeed to prevent collapsing if it comes to that.

[stonecutter]

Re: Crack in structural slab. Bummer.

Alright... I think I know what the problem is. The crack is directly above your Durock seam. I'm pretty sure that the lack of support on that line during the cure is why the slab cracked along that plane. The crack had nothing to do with your arch, that is solid and a lot stronger than a lintel. A center support will help keep the slab from opening up further.

[NCMan]

Re: Crack in structural slab. Bummer.

I feel a center support added now won't do a whole lot, except, obviously, to keep it from opening further, as you say. The slab would have to settle even more to really bear on it good, though. It will be pretty tough to get the block right up against the bottom of the slab, where it would need to be to do much. Likely, he will get it close, then pack mortar in the rest of the gap. It will help, sure. Perhaps an adjustable lolly column or screw jack w/a steel I beam spanning the crack area would be more effective. Even doing that puts alot of direct load on one spot in the bottom slab, though. If that's what he does, I'd consider pouring another smaller slab or find something to spread out the dead load on his bottom slab. He needs to get something up tight against it......in my opinion.

[stonecutter]

Re: Crack in structural slab. Bummer.

I feel a center support added now won't do a whole lot, except, obviously, to keep it from opening further, as you say. The slab would have to settle even more to really bear on it good, though. It will be pretty tough to get the block right up against the bottom of the slab, where it would need to be to do much. Likely, he will get it close, then pack mortar in the rest of the gap. It will help, sure. Perhaps an adjustable lolly column or screw jack w/a steel I beam spanning the crack area would be more effective. Even doing that puts alot of direct load on one spot in the bottom slab, though. If that's what he does, I'd consider pouring another smaller slab or find something to spread out the dead load on his bottom slab. He needs to get something up tight against it......in my opinion.

I like the screw jack and steel idea better than block. Good one, Carl.

[Toomulla]

Re: Crack in structural slab. Bummer.

Suspended concrete slabs often crack cracks are usually in areas of tension the centre of a slab or beam span is in compression on the top and tension on the bottom. Accepted cracking is usually 0.01 of an inch yours do not appear in excess of that.

The load of a dome is around the base which is located very close or over the walls. The weight on the slab is only the insulation a and hearth bricks.

Before you get carried away google cracking in suspended concrete slabs
50 years of building including major projects leads me to discount the cracking as significant

[NCMan]

Re: Crack in structural slab. Bummer.

Oh, I personally don't think his slab is going to collapse and fall down. Nor do I think anyone implied that. I sure didn't. I looked at his prep photos and he has plenty of rebar in there and it appears to be an excellent prep job, to me. He asked about what the possible causes and remedies were. He got that. Also, to say the weight on a slab is only the insulation and some hearth bricks is not entirely accurate. Maybe "on" it, but what about the weight of the slab itself? Is it even a factor? In addition, his dome does not appear, to me, to be sitting on the walls. Perhaps near them. So, is he to just sit by and see what happens, or try to stop the possible worsening of the problem? His choice. Either way, I'm sure the food doesn't mind the cracks and is still delicious.