Crack in hearth slab after removing forms

[cheyne]

I just took out the supporting forms underneath the slab a few days ago, and I just noticed three separate cracks in the earth slab. In addition to 2x4 wood framing underneath, I had square blocks stacked underneath the center of the slab which i had intended on leaving in there. The blocks seemed loose and weren't supporting much weight at all so I decided to take them out to make more room for wood storage. I did have to use a hammer to knock out the wooden supports... maybe I was ignorant in thinking that a few thousand pounds over my head would never move? I used a mix of 3/8" and 1/2" rebar every 6-8" when I poured the hearth 3 months ago, and I completed my dome 6 weeks ago.

I have read all of the different posts about cracked hearth, but my situation seems to be unique in that the cracks formed right after I removed the supports while at the same time I'm almost done curing my oven and it's the first time the hearth has potentially experienced heat with cool evenings right at freezing temp. I have not been able to find any cracks in the foundation slab, and cannot see if the cracks underneath hearth because cement board is covering it.

3 cracks:
-Front center that continues into the quickwall layer underneath the archway firebrick
-Back center that continues both 3" into the dome and 3" into cement block below
-Right center (not pictured) same as back center
-Left side center crack

Should I put a support back in the center under the slab?? Maybe a basement jack post? Huge thanks for any advice!

[Gulf]

The jack sounds like something to do for the time being. But do it from the door. Don't go under it! Place a wide board on top of the jack to spread the load out. Just, use the jack as support. Don't put it in too much of a bind. Once the jack is in place you, can later think about a more permanent fix.

What type of concrete did you use?
Did you fill any of the cmu cores with concrete?
Are there any cracks in the foundation slab/footings?

[cheyne]

Thanks Gulf,

I used standard quikrete from the hardware store
I filled more than every other cmu core with concrete & rebar.
No cracks in foundation slab/footings

Do you think it could just be settling from taking the forms out? Any suggestions for a permanent fix??

Thanks!

[Toomulla]

they look like standard shrinkage cracks to me Quickcrete is not recommended for structural concrete due to excessive shrinkage cracking as a result of the accelerant used in the mix. I do not think they are significant but inspection by a qualified person would be prudent

[david s]

I just took out the supporting forms underneath the slab a few days ago, and I just noticed three separate cracks in the earth slab. In addition to 2x4 wood framing underneath, I had square blocks stacked underneath the center of the slab which i had intended on leaving in there. The blocks seemed loose and weren't supporting much weight at all so I decided to take them out to make more room for wood storage. I did have to use a hammer to knock out the wooden supports... maybe I was ignorant in thinking that a few thousand pounds over my head would never move? I used a mix of 3/8" and 1/2" rebar every 6-8" when I poured the hearth 3 months ago, and I completed my dome 6 weeks ago.

I have read all of the different posts about cracked hearth, but my situation seems to be unique in that the cracks formed right after I removed the supports while at the same time I'm almost done curing my oven and it's the first time the hearth has potentially experienced heat with cool evenings right at freezing temp. I have not been able to find any cracks in the foundation slab, and cannot see if the cracks underneath hearth because cement board is covering it.

3 cracks:
-Front center that continues into the quickwall layer underneath the archway firebrick
-Back center that continues both 3" into the dome and 3" into cement block below
-Right center (not pictured) same as back center
-Left side center crack

Should I put a support back in the center under the slab?? Maybe a basement jack post? Huge thanks for any advice!

One possible cause is failure to place wedges under the supporting uprights, which results in pressure on the slab during their removal. If you have wedges then when they’re removed the uprights can drop vertically.
assuming you have reinforced the slab with steel then it should be ok, but if you are concerned you should support it in the middle, preferably with masonry.

[cheyne]

they look like standard shrinkage cracks to me Quickcrete is not recommended for structural concrete due to excessive shrinkage cracking as a result of the accelerant used in the mix. I do not think they are significant but inspection by a qualified person would be prudent

Could they be shrinkage cracks that just showed up after 3 months of no cracking?
I with I would have known not to use different cement when I started...

[cheyne]

One possible cause is failure to place wedges under the supporting uprights, which results in pressure on the slab during their removal. If you have wedges then when they’re removed the uprights can drop vertically.
assuming you have reinforced the slab with steel then it should be ok, but if you are concerned you should support it in the middle, preferably with masonry.

Ah that was my fear, when I was taking out the forms I figured the upward force I might put on it was so minimal in comparison to how heavy the whole thing is..
I have plenty of rebar in the slab.. do you think the slab and dome will crack and settle, and then stop cracking at some point?? Is there any way I can keep the outer quikwall layer on the dome from cracking more?

[Gulf]

Quikrete is a good product. It comes in different grades. I'm pretty sure that the Fast Setting type that is used for fence posts has some accelerators in it. The standard high strength Quikrete concrete is just fine for our purposes. That said, I have turned around and walked away from a pallet of Quikcrete that had been sitting too long in the store. If it is lumpy I will pass on it and go somewhere else that has a faster turn over. Do you happen to have a bag left over from the pour? I would like to see a pic of it if you do.

[cheyne]

Quikrete is a good product. It comes in different grades. I'm pretty sure that the Fast Setting type that is used for fence posts has some accelerators in it. The standard high strength Quikrete concrete is just fine for our purposes. That said, I have turned around and walked away from a pallet of Quikcrete that had been sitting too long in the store. If it is lumpy I will pass on it and go somewhere else that has a faster turn over. Do you happen to have a bag left over from the pour? I would like to see a pic of it if you do.

Gulf, this is the stuff I used:

[Gulf]

That concrete should be fine for a hearth provided that there were no great amount of lumps in it. When I say lumps, I mean lumps that don't easily fall apart when crushed in the palm of the hand.

I'm still not so sure that those are only shrinkage cracks. Shrinkage cracks are usually only surface cracks. From your pics, they look like they penetrate through the entire slab. Are they visible from underneath the hearth slab? If so, are the cracks wider when viewed from underneath?

I am with David on the permanent solution for shoring to be masonry. If you are going to use the area underneath for wood storage, the back half is almost useless when you get up in age.

[cheyne]

From where I can see, they definitely penetrate the thickness of the slab. I can't see underneath the slab because it's covered with cement board...

The best permanent solution I can think to support the slab would be to stack blocks inside and then tap in a few wedges of granite that i've cut with a tile saw to it's tight up against the underside of the slab.. then maybe mud over that to prevent the wedges from coming out over time with the freeze/thaw. any other suggestions?

I'm 6 days into curing my oven, and heat has penetrated through the bottom of the slab and reads 180 degrees.. I've been trying to keep the oven at temperature 8 hours each day. I wish I would have known to insulate more than 3.5 in of perlcrete on the bottom.. Do you think the heat transfer is adding to the cracking of the slab? ?

[Toomulla]

Gulf would have the best opinion on the material used, from the description above it looks fine. you have not told us how you mixed and poured the slab the cracks could be cold joints, shrinkage or stress from stripping regardless the slab should be fine the concrete works only in compression so the steel is doing all the work, and the load on the slab is only low in relative terms. The dome spreads the load to the sides close to the supports so the only load is the heart bricks I am sure it will be fine

[Tscarborough]

If there is reinforcing in the slab, no worries. There is no real "fix" other than remove and replace(and maybe leave that jack in place).