Cracking Slab 2 Weeks after pour

[shooon]

2 weeks to the day I had my slab on grade poured, and to my discontent there are at least 4 cracks starting to form from the outer edges working their way in. They appear to be starting close to the middle portions of the slab. I had control cuts placed diagonally from the 4 corners of the slab to the corners of my sump.

I realize it's the nature of concrete to crack as it cures and shrinks... but 2 weeks after the pour? I figured I'd at least get a year or 2 before seeing this.

I checked over my concrete invoice and the contractor had used 30MPA with 20 mm aggregate and 110 mm slump.

Is this likely shrinkage cracking? Could the high slump be a likely cause?
It was poured in ideal conditions. 100% humidity, cool weather with no sunlight and minimal wind. They sealed it and it rained quite a bit 24 hours later.

 

[drg5490]

Maybe post a pic so we can see, and the area around it

 

[Cyberbear]

Sounds too early for cracks, and I often read about cracking happening even with saw cuts.

 

[shooon]

Maybe post a pic so we can see, and the area around it

 

[drg5490]

Photo #4 looks like it originated at the bolt that holds the bottom plate on..are the rest of the cracks streaming out from the other bolts also?

 

[shooon]

Photo #4 looks like it originated at the bolt that holds the bottom plate on..are the rest of the cracks streaming out from the other bolts also?

No, I believe that is the only one. I have the bolts spaced 4' and none of the other cracks appear to be located near any bolts. I'll try to get some better pictures in the daylight- but the one picture there (pic #3 and #6) is no wall, its the sloped front of the slab where the garage door will be. I pulled the 2" rigid foam out of the way and it looks like the crack extends down pretty deep.

 

[drg5490]

Couple questions
when did theycut the control joints?
Are the cracks parallel or perpendicular to the control joints?
And when was the sealer applied in regards to the pour?

 

[shooon]

Couple questions
when did theycut the control joints?
Are the cracks parallel or perpendicular to the control joints?
And when was the sealer applied in regards to the pour?

Control Cuts were done the day after the pour in the afternoon.
Pour was finished 9 AM they spent some time finishing then applied sealer around 4 or 5 PM the same day (was supposed to rain overnight)

The control cuts run in an 'x' pattern and the cracks are forming in somewhat of a '+' pattern. The one crack isn't quite center of the slab, more offset a bit. The square in the middle is my sump.



Also it's a 24 x 31'5" slab. 14" thickened edge.. They also added a curing accelerator to the mix.

 

[ForceFed70]

It's your control cuts. The X may look pretty but it doesn't work well. You should have no more than 12' of space between control joints (10' is recommended). You have 31.5' and 24' between yours. That slab should have had control joints that look like a blank tic-tac-toe game.

 

[Blk88GT]

My slab did the same thing, it was heartbreaking but I'm over it now. All my cracks originated or ended around the sump pit as well. The first one appeared 2 weeks after the pour with nearly identical conditions to yours.

 

[LX-Markham]

Exactly what ForceFed said. The slab is cracking exactly where it would naturally crack. That's where the saw cuts should have been made.

 

[ishiboo]

Exactly what ForceFed said. The slab is cracking exactly where it would naturally crack. That's where the saw cuts should have been made.

Completely agree. Not that you'd be necessarily crackless if they had been done correctly. But it's fairly obvious that that long of a run will have movement that needs to be controlled.

OP mentions the concrete but not the metal in it. Hope that was all done correctly! Bottom plate looks like PT based on the color difference, but the bolts appear to be zinc-plated... those aren't going to last long.

 

[K'ledgeBldr]

They definitely appear to be shrinkage cracks.
My first thought would have been the soil was too dry when the concrete was placed- but then, surely you used a vapor barrier(?). Then, it occurred to me why, if the weather conditions were "ideal" why did they use CC (calcium chloride) in the mix?

The next thought was watering. Construction times keep getting more and more condensed, which doesn't allow enough time for proper processes (curing). Ideally, you would have wanted to water the slab for 4-5days allowing for a proper cure.

Last, but not least; as previously mentioned- why on earth would you have diagonal relief cuts? Any intelligent concrete guy should know that's not an acceptable/working relief.

 

[wssix99]

I realize it's the nature of concrete to crack as it cures and shrinks... but 2 weeks after the pour? I figured I'd at least get a year or 2 before seeing this.

I'm sorry for the bad news. These are shrinkage cracks. The time for these to show up is in the first 4 weeks after the pour. This is when the concrete achieves most of its strength and also builds up most of the shrinkage tension that leads to these cracks. (As the cement cures, it shrinks in volume.)

Your issue is not with the triangles but the size of them. The triangles are unconventional, but they section the slab appropriately. (You only need to make sure you don't have any angles greater than 180 degrees, creating "re-entrant corners.")

The problem are the long legs of the triangles. (This is also where the cracks are forming - at the highest points of stress/tension.) When a slab is cut in to typical squares, those are 10-12 feet in width - sometimes wider if reinforcement is added to the slab. The legs of your triangles are way longer than this span and they are ripping them selves apart.

To help alleviate, this I would saw cut a cross in the garage ASAP right along where the cracks are. This will give them a pathway to the drain in the middle so they continue to form in the new cuts. It may also hide some of what has already cracked. This will give you 8 smaller triangles instead of 4, but it will hopefully keep the cracking from getting worse. (They will continue to grow noticeably over the next two weeks and then a little more over the next year.)

^ That all being said. These control joints are just insurance against cracking. Even with proper control joints, cracks can still form outside of the control joints. If you take my idea and cut the slab in to 8 triangles, your new long legs will be the ones radiating from the center to the corner of the room. You could see some cracks start from those areas - but hopefully, things have cured enough and the cracks that have formed so far have alleviated enough stress and those triangles are small enough to keep that from happening.

 

[drg5490]

Agree with the control joint placement but was wondering is it normal to put down the sealer that soon? Seems that would delay the evaporation and curing of the concrete..I've always been told to wait the 28 days before sealer application

 

[Falcon67]

Had similar within a week. It's been 5 years now, no changes since final cure. And no control cuts here - don't care for them.

 

[morrillm06]

Where I am from (New Hampshire) I have never seen a garage with control cuts? my father has a 28x36 that is ten years old that only has one crack. it is directly above a piece of 1 inch water pipe he threw in for a sleeve right before pour (his fault obviously) and I have a 32x40 that was poured in april of this year with no cracks as of yet. like I said tho I have never seen a garage around her with cuts.

 

[pcmeiners]

"Seems that would delay the evaporation and curing of the concrete."

Just the opposite...The idea of the sealer is to keep the water necessary for curing within the concrete, after 28 days the sealer would have very little benefit in that regard.

 

[toyotadriver]

My shop has control cuts and I still got a couple cracks. It's the nature of concrete. Don't worry about it.

 

[drg5490]

"Seems that would delay the evaporation and curing of the concrete."

Just the opposite...The idea of the sealer is to keep the water necessary for curing within the concrete, after 28 days the sealer would have very little benefit in that regard.

Yeah makes since, but again that's based on using the correct sealer..some require 28 days others say ok as soon as youn can walk on it...depends on the product used I guess

 

[matt_i]

Agree with the control joint placement but was wondering is it normal to put down the sealer that soon? Seems that would delay the evaporation and curing of the concrete..I've always been told to wait the 28 days before sealer application

The curing sealer should be placed as soon as the bleed water is off the surface and one can walk on it without denting the surface. Reason is the curing sealer entraps moisture within the slab, which is later needed to "hydrate" more chemical reactions thus leading to higher strength.

 

[Firebrick43]

"Seems that would delay the evaporation and curing of the concrete."

Just the opposite...The idea of the sealer is to keep the water necessary for curing within the concrete, after 28 days the sealer would have very little benefit in that regard.

However it doesn't work as well as soaking the slab for 7 days. 6 bag and more mix has higher rates of shrinkage cracks as does high slump. Plastisizers are much better than lots of water for workability but will speed in set time so you need to be careful if you are a distance from the mix plant. Better to just demand low slump and supervise. Also if the substrate below the slab is dry or well draining can cause fast drying. Plastic sheet under the concrete help but contractors will avoid it if the customer doesn't demand it as it can slow down finshing time.

Nature of the work means many in the trade are of unsavory character or of low intelligence. Also the construction times and the customers continued demand for the cheapest price has created many situations just like you have.

A smart contractor with a craftsman attitude are rare and worth holding on to if you find one.

 

[drg5490]

Matt i used the exact wording that I was looking for thanks...he referred to it as curing sealer..which is entirely different than just the word sealer...that's why I asked about the sealer application..maybe should of asked what type of sealer was used...

 

[n20junkie]

The control joints that were cut were designed purely for aesthetics and not for their intended job.

You cut so that no section of pad has more than X linear feet. Having full wall sections with not a single joint cut besides the corners was asking for uncontrolled cracking.

 

[matt_i]

I think the good news in all of this, is there is steel reinforcement in there, so it should help to hold those cracks tightly together.

 

[shooon]

Wow!

Thank you all for the expertise.
I reinforced the slab with 15mm rebar in a 12" x 16" grid. 14 x 14" thickened edge with 5" center concrete thickness. Vapor barrier, then 2" foam beneath that. The only reason the control cuts were made were for the (already mentioned) re-entrant corners around the sump. The contractor guaranteed that the slab would crack at the corners of the sump and highly recommended those cuts. At the time, I thought I was being up-sold for something I didn't necessarily need. Mainly through reading these forums a lot of people have had uncut slabs go without any cracking, so I opted to go with bare minimum.

It was definitely an error on my part- I should have made more control cuts as mentioned and actually wanted too after the fact- but the chalk lines were made before the sealer was applied and by the time I had done a bit of research, the contractor didn't want to make the cuts because new chalk lines would wash off at that point.

I am heavily debating getting them back to do another few cuts to split the slab up more. This was not a cheap slab... $17,000 in material and labor which is the going rate where I live (in the middle of nowhere).

-49 yards 3/4 crush gravel
-13 Cubic Meters concrete
-$1500 in rebar

 

[shooon]

Here's a few shots of the setup.

 

[Orionrising]

wow concrete work is expensive in canada

 

[wssix99]

rebar in a 12" x 16" grid.

This is abnormal in my experience. Where did this come from? If the reinforcing is not in squares, the shrinkage tension in the concrete will not build uniformly.

The contractor guaranteed that the slab would crack at the corners of the sump and highly recommended those cuts. At the time, I thought I was being up-sold for something I didn't necessarily need. Mainly through reading these forums a lot of people have had uncut slabs go without any cracking, so I opted to go with bare minimum.

You have a lot to stand on here. They were correct that you would have gotten cracks at those corners and they were correct that control cuts would have been required radiating out from the corners like that. (Typically, we'd see the drain oriented like a diamond and the cuts going square to the building. Had they done that, you would not have gotten any cracks here because the distances across the sectioned areas would have been smaller in that configuration.) However, the triangles are too big. There are endless professional and technical references on both sides of the boarder that would direct the cuts to be closer together. Depending on how far you want to push the contractor with these -> they are your hammer. The contractor totally screwed up here.

Elevating the slab on the broken pieces of block doesn't help either. Those create weak points where cracks can breed. Proper rebar chairs would have been better - but the control joint strategy is your biggest issue.

It is possible to put in enough reinforcement in the slab to bypass the control joints entirely, but you aren't at that level. I have a floor in my house that is reinforced this way and we had to get close to 6 or 8" squares on the rebar spacing. I also sectioned a different part of the house (with different reinforcing) in triangles successfully, but they were also smaller.

 

[wssix99]

I am heavily debating getting them back to do another few cuts to split the slab up more.

I think this would be a good idea and will put you in control of any more cracking that might happen. With the reinforcing you do have, adding a cross pattern to the floor should be enough to get you in to spec for the joints. (Your contractor should be able to go back and do the math...)

Even if you decide to let this pass, I would also recommend on making sure the contractor knows - so they can fill the gap in their understanding. I see a lot of contractors give themselves pats on the back when they do stuff like this because they never have to deal with the owner and the after-effects.

 

[shooon]

This is abnormal in my experience. Where did this come from? If the reinforcing is not in squares, the shrinkage tension in the concrete will not build uniformly

Thank you very much for taking the time to share your knowledge, what you said makes a lot of sense.

My original design and plan was for 12" rebar grid, but under the advisement of my carpenter he suggested the 12x16 would be plenty and would save on time / labor since my pour was schedule in 3 days. my 12" grid was overkill according to anyone I spoke with and they said 16 or 18 would have been enough with 15 mm bar. I figured I was still ok. I ended up hiring the same concrete contractors to get my grid installed so that I still had some time to install my pex.

The concrete contractor recommend a 12x12" thickened edge with 4" thick center slab. I went with 14x14" x5" with more rebar because I dont like building at the bare minimums.

There was a two week waiting period to get the concrete and trying to line up the finishers and trucks was an issue, I did not want to delay the pour any further because we are starting to get into frost season. I now have a pile of extra stirrups and rebar leftover because of this.

Also was planning to use rebar chairs - but was told by my carpenter that these contractors would throw them out and redo it if I had used them because they have had issues with the chairs breaking when they walk on the grid to pour. That's the only reason I went with blocks, in my gut it didn't seem right but I was relying on the experience of the people I had hired.

At this point all I can do is make more cross cuts and learn from the mistakes made so I know better for the next slab I pour later on in life. I will likely just make the 2 more cuts in a + pattern since that is where the cracks are appearing. Hopefully when the cracks continue to spread, they will follow the cuts.

 

[jav]

Shoon-

my .02c

Just this past saturday, I had my 25x42 slab poured. Your prep makes mine look very inadequate. Don't feel bad about following advise because with cracking (or no cracking)- there will always be advice and credit taken for either outcome.

I was amazed when I researched just how much conflicting advise one gets from people "supposedly" in the know. Rebarr- no rebar. Isolation versus tieing in. How much or how little slump. Beefed up areas versus consistent thickness. 3000 PSI vs 4000 psi vs 5000 psi. Additives or no additives... it goes on and on and on.

My slab hasn't cracked "yet" - but like most things in life- no one knows for sure what tomorrow will bring. You job certainly looks impressive and well thought out. Certainly you didn't skimp on effort or costs... but I got to tell you... that's a lot of coin!!! I paid a small fraction of that for my 15 yard pour but I didn't go to the ends you went to.

We can all agree- concrete is prone to cracking and the best we can hope for is to try and control where the cracks go, and how much they separate. Yours seem pretty tight and given your re-enforcement, I can't imagine they'll create any major issues.

I know it *****... but there's way worse things in life. Keep an eye on them,- at this point I'm not sure I'd do anymore cutting since the cracks have already chosen a path and I'm not sure the control cuts will accomplish much since most of the cure process/shrinking has Likely already occurred? Don't let the cracks drain your enthusiasm on the overall project.

 

[outlawz2004]

I've been lucky. Mine is 30x40 fiber reinforced with no rebar and no control cuts. Just one solid slab. I cant believe its been four years and not a single crack yet.

 

[toyotadriver]

That looks well done. I wouldn't worry about it at all.

 

[Majordisorder]

2 weeks to the day I had my slab on grade poured, and to my discontent there are at least 4 cracks starting to form from the outer edges working their way in. They appear to be starting close to the middle portions of the slab. I had control cuts placed diagonally from the 4 corners of the slab to the corners of my sump.

I realize it's the nature of concrete to crack as it cures and shrinks... but 2 weeks after the pour? I figured I'd at least get a year or 2 before seeing this.

I checked over my concrete invoice and the contractor had used 30MPA with 20 mm aggregate and 110 mm slump.

Is this likely shrinkage cracking? Could the high slump be a likely cause?
It was poured in ideal conditions. 100% humidity, cool weather with no sunlight and minimal wind. They sealed it and it rained quite a bit 24 hours later.

The 110mm slump is probably what was ordered but did the invoice or load ticket mention any added water?

Had a job where the concrete was ordered at a 5" slump and it developed a number of cracks. The dispatcher came out to the job with load ticket in hand showing where the finishers added at least 30 gallons of water to each truck load.

 

[wssix99]

At this point all I can do is make more cross cuts and learn from the mistakes made so I know better for the next slab I pour later on in life. I will likely just make the 2 more cuts in a + pattern since that is where the cracks are appearing. Hopefully when the cracks continue to spread, they will follow the cuts.

I would expect that to be the case.

The good thing here is that these wont get any worse than they will be in a few weeks and your reinforcement investment will pay off in keeping thing together and your slab smooth. If you don't like the look of the cracks, you can epoxy over the floor. Since the cracks stop growing, the epoxy fills in small ones nicely. (I had a very small one I couldn't avoid at a reentrant corner on a decorative floor and the epoxy hides it completely.). Larger cracks and your saw cut joints can also be filled in with specialty mortars to make a perfectly smooth surface.

 

[Falcon67]

As above - I didn't sweat the few cracks because everything around here does it. The 92 yard pour for the house next door did it and after they stained the floor it looked really cool - lots of texture and color patterns in the stain. I counted on my beams, footers and bar (24" OC) to control the total end product. Painted the floor and squished some into those few cracks, no changes in the last 5 years between extreme drought to this years 11" over normal rain. And we're on expansive clay soils here too. Plus some subsidence possibly caused by pumping of ground water.

 

[wssix99]

At this point all I can do is make more cross cuts and learn from the mistakes made so I know better for the next slab I pour later on in life. I will likely just make the 2 more cuts in a + pattern since that is where the cracks are appearing. Hopefully when the cracks continue to spread, they will follow the cuts.

Did you decide what you are going to do? I'm back at my books/calculators and took a look at some math relating to your specs.

Your 15mm rebar is really helping you out here and if you had gone with 12"X12" spacing, the slab may not have cracked at all. (It is definitely keeping the cracks narrower than they would be otherwise.)

Cutting a cross in to the floor would relieve the floor in to more "normal" sized sections, except the long runs from the front/back of the floor to the center drain would be just a tad long. (For a 5" slab, you start pushing things when you go over 12 to 15 feet.) So, there would still be a bit of higher-than-normal stress in the middle of those long forward-to-back cuts to the drain, but your rebar should take on those loads. (I would think that the fact that your cracks aren't going all the way to the drain would indicate that the rebar is picking up those stresses.)


So, without the rebar and with the additional cuts, I'd expect you to run a good chance of getting some cracking in the middle of the existing two forward and back triangles. (In addition to the cracking you are currently seeing across the room, which you would also see continuing in the additional cuts.)

The rebar is working for you, but this ^ may be a good reason to do the additional cuts and provide an additional pathway for stress relief in the middle of those triangles.

(I hope this isn't confusing...)

 

[Perrorojo]

My .02 is that concrete is never perfect. I think environment plays a bigger role in determining what works than anything else. Our shop (40 x 70) was poured over 30 years ago and doesn't have a crack. I did my house 21 years ago and have a couple. We used cattle fence in both shops and rebar spaced 12" x 12" in the house. If you did your prep work well (and it looks like you did it very well) you should be fine.

 

[wssix99]

My .02 is that concrete is never perfect. I think environment plays a bigger role in determining what works than anything else.

ALL concrete (with limited exotic exceptions) is going to crack. The unknown is how well distributed those cracks are and if they can be seen. (Reinforcement helps influence this.)

Our shop (40 x 70) was poured over 30 years ago and doesn't have a crack. I did my house 21 years ago and have a couple. We used cattle fence in both shops and rebar spaced 12" x 12" in the house. If you did your prep work well (and it looks like you did it very well) you should be fine.

Shrinkage cracks like these are determined partly by how the slab is finished but mostly by how the slab is designed. If you have a 40X70 with no visible cracking, yours probably does not have thickened edges (like this one) and is thinner than this one. (If you sweep the floor good and wet it down, I expect you'll also find some cracking.)