Crack in structural slab. Bummer.

[cobblerdave]

Re: Crack in structural slab. Bummer.

G'day
I think you have already discovered the cause. Removing the form before the cement has completely cured and having to hammer out the supports did the deed. On an up note the central support and the goodly amount of steel work you used is going to be more than ample.
These stands are more than overbuilt anyway so is strong enough to have a slab in two parts anyway I recon
Regards dave

[Yannick]

Re: Crack in structural slab. Bummer.

Gulf, I looked at the metadata of the picture. The slab was poured on 4/19 and the form was removed on 4/22 (3 days).
Could moisture causes this? if it rained, and if the oven was fired with a damp slab, what if the temp leaking from the vermicrete caused it?

[Yannick]

Re: Crack in structural slab. Bummer.

ok I built the wall inside. There was 1 inch clearance difference between the back and the middle. The last 2 blocks had to be banged in place. So it's a super tight fit under there, I like that. I still had to pack some mortar where i saw a gap but I am confident it is not going anywhere. That could have been like that since the construction. Nothing abnormal was hiappening underneath.
And the crack was not actually right above the backerboard joint. The crack is about 6 inches back of that.
I also noticed that when the oven is cold, I can barely see the crack. The cracks open up when the oven is hot.
Well, it's done. I hope is it all behind me now.
Thank you all for your input.

[Yannick]

Re: Crack in structural slab. Bummer.

I did not think of wedges. I remember that I had to hammer the central posts. I think I left the from more than 3 days but I would still like to check the metadata to make sure.
I haven't measure but the crack seems to be about 8 in away from the backer board joint.

[Gulf]

Re: Crack in structural slab. Bummer.

Gulf,
I will need to check the original time stamp on the picture to let you know. When I checked on my Flickr the dates are wrong.
I will take care of my little center wall this week end if all goes well.

No need to check. You would probalbly remember if it was within a week of the pour. For a suspended slab, (of a short distance in the range of a wfo) it is recomended to wait at least 3 days. I like a week much better. But, my personal advice is 28 days. Anything less than 28 days, the form should be let down easily. Wedges under the supports work good. With them you can release the tension gradually. Placing the upright supports under the slab without them, will cause you to have to remove them with a hammer. That motion jacks up the slab and abruptly drops it. Not good for uncured concrete. After 28 days the concrete is well on it's way to being as about as strong as it is going to be.

[Yannick]

Re: Crack in structural slab. Bummer.

Gulf,
I will need to check the original time stamp on the picture to let you know. When I checked on my Flickr the dates are wrong.
I will take care of my little center wall this week end if all goes well.

[NCMan]

Re: Crack in structural slab. Bummer.

Yannick.....personally, I would recommend against actually trying to jack it back up. That could do even more damage. If you did, it usually doesn't just "go back in place". Concrete usually doesn't work like that. As I was .....trying.....to say, doing something now to stop any more settlement is my advice. Again...it is not going to collapse, but it may continue to get worse. I would make a decision soon, if you do anything at all. I've given you all the advice I can and will wish you all the best.

[Gulf]

Re: Crack in structural slab. Bummer.

Yannick,
I was just wondering something. Back on your post #8, back in march. How much time was there from pic number one which shows your freshly poured hearth slab:

And pic number three which appears to have all the forms removed, including the vertical supports underneath the interior of the hearth slab?

[Toomulla]

Re: Crack in structural slab. Bummer.

It will pay to monitor but remember that all suspended slabs and beams deflect the usual design acceptance being between 180 &240 of the span so on a 48 inch span about a 1/4 of an inch

[Yannick]

Re: Crack in structural slab. Bummer.

I did notice that the oven floor is slightly curving. When i was scraping the ashes I saw that the scraper was touching on both ends but was not touching in the middle. I need to put a level on the oven floor and measure the gap in center of it.
I was certain that the floor was flat. Now that I am aware of the slab crack, I think those two things are linked.
For peace of mind I want to jack up the slab and put
Blocks underneath it, pack mortar between slab and
Blocks and remove the jack.

[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.

[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

[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.

[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.

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.