Crack in the hearth slab

[Frances]

Originally Posted by christo

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]

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]

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]

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]

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.

[Frances]

Christo, that's an interesting site you found there. It'd be nice to know more about concrete strength, necessary reinforcement ect ect, but its difficult to come up with relevant info. Do you happen to know how heat resistant concrete is? Could you light a fire on a concrete slab, or would it crack beyond recognition?

George, no problem, go ahead and second guess (or even third and fourth guess) my work . I'm always glad of input. I got most of my info off this site anyway, and just tried to follow the guidelins to the best of my ability.

[Neil2]

"I have one in mine as well... actually two."

All reinforced concrete slabs over a certain size will crack. The trick is to make it crack where you want it. If you cast an expantion type joint it will crack at that location. The attached picture shows one of four joints I put in the circumference of a round structural slab. It cracked at each of the joints.

If the crack dosen't widen over time don't worry too much about it. I would not worry about sealing it either unless you are worried about the cosmetics of it.

[gjbingham]

Actually, My comment was suggesting that you shouldn't question your work. I would never question it. I question mine all the time and more often than you can imagine, ususally with good reason.
I followed the same plans as you.

Sounds like Neil knows what he's talking about. I'd follow his lead.
G.