Cracking in undesirable places! Help!

[Tscarborough]

Re: Cracking in undesirable places! Help!

<shakes head and walks away>

[jgd915]

Re: Cracking in undesirable places! Help!

Thanks for all of your help. Looks like I will need to cancel the party anyway on account of thunderstorms .
Here's another idea. In addition to a third leg, what about filling up the entire underneath part (where the wood is being stored) with cinder blocks? Creating a cube of cinder block underneath supporting the slab so the slab itself is holding barely any weight. Even if the slab broke, it would have nowhere to go. Am I right? I could then cover the cube of cinder block with a stone veneer just to make it look nicer.

[david s]

Re: Cracking in undesirable places! Help!

If you have enough empirical knowledge to engineer it. 2 bars in a couple of sacks of Quickcrete ain't engineering.

Yes you are correct. Empirical knowledge and engineering allows a thin supporting slab as shown containing additives and fibre reinforcing. Not had any failures. The slab is thicker in the middle where the stresses are and thinner outside where there is little stress on it. Almost all the stress is transferred straight down to the piers that support it.
I don't subscribe to building a bomb shelter to house an oven if there is an easier way that allows the use of less materials and labour.

[david s]

Re: Cracking in undesirable places! Help!

As of now the cinder block legs are shaped like a U...would building a third cinder block leg down the center help? Also, is it a bad idea to have my party, a long, hot fire? Would I be better off cancelling the party and only using this oven for personal use (3-4 pizzas, fires only lasting a few hours) or will that not make a difference?

Yes A third leg might help. I'd be just firing the hell out of the thing after that and see how it goes. Don't even contemplate cancelling the party, even with cracks the oven is not likely to collapse and will not alter its performance.

[jgd915]

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.

As of now the cinder block legs are shaped like a U...would building a third cinder block leg down the center help? Also, is it a bad idea to have my party, a long, hot fire? Would I be better off cancelling the party and only using this oven for personal use (3-4 pizzas, fires only lasting a few hours) or will that not make a difference?

[Tscarborough]

Re: Cracking in undesirable places! Help!

Here is a 2" slab that won't crack. To start only an idiot would monolithically pour an "L" shaped section of thin concrete, it is almost guaranteed to crack at the inside corner. That is why there is no inside corner, it is a radius, distributing the stress over enough area that the tensile strength will withstand it. There is also a section of 4" CMU ladder wire across the expected crack as well as the same on the perimeter and around the cutout for the sink, chocked to 3/4" above the bottom of the pour. This is NOT a suspended slab, however, it was poured on top of a left in place hardibacker form.

There is no rebar in it, at 2" even 3/8" rebar does not allow enough coverage over the bar to be effective, and in effect creates a weak plane and probable crack down the road. The mix design is 5500PSI concrete, with 25% of the mix water replaced with latex additive, and a medium loading of poly fibers. Final flexural strength is probably North of 8000PSI.

[Tscarborough]

Re: Cracking in undesirable places! Help!

If you have enough empirical knowledge to engineer it. 2 bars in a couple of sacks of Quickcrete ain't engineering.

[Gulf]

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.

If,............ the slab is insulated

[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.

[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'

[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.

[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?

[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!

Is heat or weight the issue?

[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.