Mono Slab Cracks- 1.5 weeks old

spike0137 · Jul 4, 2017

I had a mono slab poured about a week and a half ago. I just noticed some cracks in the slab that run across the floor to a edge and all the way down. Is this normal? I know slabs crack and can get hairline cracks. Probably just me being paranoid but curious what you all think.

About the slab.

-28x50
-has 2" foam and pex for infloor heat ran
-haunches are 12x12 except in back corner where the grade was very low so it is about 24"x12".
-1/2" rebar every 5'
-4000 psi concrete with fiber
-5" thick in middle

Prep work-

-all top soil removed
-3/4" with fines brought in and compacted on 4" lifts
-slab is 5" above natural grade at highest point.

Thanks for the comments

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rburke65 · Jul 4, 2017

Supposedly it all cracks. Was it cut the same day?

walrus · Jul 4, 2017

What does rebar 5 ft on center do?

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Firebrick43 · Jul 4, 2017

There is no reasonable way to keep a residential monolithic slab from not cracking. Concrete shrinks and due to the down turned skirt a mono slab is restrained.

spike0137 · Jul 4, 2017

rburke65 said:
Supposedly it all cracks. Was it cut the same day?

Yea it was cut about 6 hours after pour.

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spike0137 · Jul 4, 2017

walrus said:
What does rebar 5 ft on center do?

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It's better than nothing. Every company I had out said 6x6 I'd did 5x5 to feel better

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ethan.jordon.3 · Jul 4, 2017

I have done residential walls, footings and slabs for years, and we do #4 rebar every two feet in both walls and slabs. 5 feet is way too far apart for the tensile strength of rebar to do much good. Better than nothing, yes, but not correct

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Bighead38 · Jul 4, 2017

Looks like the concrete dried to fast imo. I've poured a good amount of concrete and have only gotten cracks like that I'm very hot days when the concrete dried super quick or didn't get sprayed down enough. I would t worry about it but I would keep an eye on it. I've never heard of rebar being spaced so far apart, that seems wrong.

lakeroadster · Jul 4, 2017

I think Bighead38 identified the issue OP.

Tell us about the temperature and sunshine during and after the pour? Did you keep the slab hydrated? If so, how and for how long?

You haven't driven anything heavy on the slab right?

Any compaction testing done prior to the pour?

The re-bar being spaced to far apart wouldn't cause cracking on a 1.5 week old slab... but 5' spacing is problematic if the slab shifts at the cracks.

readhead · Jul 4, 2017

I would agree with John that the concrete cured to quickly. At this point you are living with it so don't dwell on it. Rebar at five foot on center was a waste of time and material. I'm involved with both residential and commercial construction and I don't think I have ever seen spacing exceed two foot.

Andy

spike0137 · Jul 4, 2017

lakeroadster said:
I think Bighead38 identified the issue OP.

Tell us about the temperature and sunshine during and after the pour? Did you keep the slab hydrated? If so, how and for how long?

You haven't driven anything heavy on the slab right?

Any compaction testing done prior to the pour?

The re-bar being spaced to far apart wouldn't cause cracking on a 1.5 week old slab... but 5' spacing is problematic if the slab shifts at the cracks.

I also have a feeling it is from the heat. I used a few sprinklers on the slab for the first 3 days. Then the company pulled the forms and told me with the sealer they used I shouldn't need to water it anymore. It has been in the 70s the last week with a lot of Sun.

Nothing has been set or driven on the slab.

No compaction tests were done but I will tell you that I had to dig a few spots out after the first day of excavation, I needed to get a pick axe to get throughwhere they compacted it. I believe there is not a compaction issue.

With the rebar spacing. I had 8 estimates done by a lot of very good concrete companies in the area. They all said 6' on center rebar is more than plenty. I questioned them all and asked about doing 2x2 and got the same answer that it would be over kill.

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wssix99 · Jul 5, 2017

spike0137 said:
It's better than nothing. Every company I had out said 6x6 I'd did 5x5 to feel better

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You don't need rebar at all, as long as your saw cut joints are the proper depth and spacing.

A lot of this ^ speculation. Can you give us some more information?
- A layout of the entire slab with a map of your saw cut joints and where the cracks are. Do you have any penetrations in the slab?
- The spacing of your saw cut joints
- The depth of your saw cut joints
- Where you are located

EasyE · Jul 5, 2017

Everyone here is missing the boat...

Its a shrinkage crack, it happens.

The important question is what was your concrete slump at time of pour?
Too much water can exasperate the cracking. This is some of the reason why fluidifiers are used.

n20junkie · Jul 5, 2017

wssix99 said:
You don't need rebar at all, as long as your saw cut joints are the proper depth and spacing.

A lot of this ^ speculation. Can you give us some more information?
- A layout of the entire slab with a map of your saw cut joints and where the cracks are. Do you have any penetrations in the slab?
- The spacing of your saw cut joints
- The depth of your saw cut joints
- Where you are located

I disagree. Rebar is what keeps that crack from becoming a raised edge.

GMCGarage · Jul 5, 2017

walrus said:
What does rebar 5 ft on center do?

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Jacks up the price, lol. OTher than that, no benefit.

GMCGarage · Jul 5, 2017

n20junkie said:
I disagree. Rebar is what keeps that crack from becoming a raised edge.

* properly spaced rebar *

lakeroadster · Jul 5, 2017

Rebar can cause shrinkage cracking

Use Reinforcing to Stop Cracking

joes169 · Jul 5, 2017

Nearly impossible to give an educated guess form online, but from the third picture, I would assume THAT crack developed outside of the joint because they didn't saw deep enough at the thickened edge. Not sure how many more cracks there are.

Can you take a rule or even a knife blade and check the depth inside of the joints at a few places? Should be 2-3" minimum at the edges........

G-ManBart · Jul 5, 2017

I drove a concrete truck for years during high school and college (pre-CDL days at first) and the vast majority of concrete contractors have way too much water added, and that's usually why the slabs crack so soon, and so badly. Hot temps only make it worse.

Those sure look like shrinkage cracks....and having a crack right next to a relief cut reinforces that theory. No way you should be seeing cracks that soon.

We poured the 40x40 section of slab for my barn three years ago, and it still doesn't have a single crack I'm aware of. We kept the mix tight, added steel and fiber, and then kept it wet for about a week.

metaleltr · Jul 5, 2017

"Normal" or not there is no way I would be happy with and brand new slab that developed cracks that quickly. I also would have poured it with rebar spaced closer, wire mesh, and fiber in the mix.

njc41980 · Jul 5, 2017

EasyE said:
Everyone here is missing the boat...

Its a shrinkage crack, it happens.

The important question is what was your concrete slump at time of pour?
Too much water can exasperate the cracking. This is some of the reason why fluidifiers are used.

+1

Looks like nothing to worry about.

njc41980 · Jul 5, 2017

General contractor here,

I can guarantee three things about concrete:
- It IS hard
- It WILL crack
- No one will steal it

Seriously though, small shrinkage cracks, as long as they don't open up and start moving, are often unavoidable and nothing to worry about.

wssix99 · Jul 5, 2017

n20junkie said:
I disagree. Rebar is what keeps that crack from becoming a raised edge.

If a slab is inside and protected against the cracks from getting freezing water in them (causing the cracks to spread) and if the slab isn't exposed to high stresses of fast moving traffic, small shrinkage cracks should be interlocked by the aggregate.

Either way - even with reinforcing bars - one still needs proper control joints and the cracking should still occur in those joints.

EasyE said:
Everyone here is missing the boat...

Its a shrinkage crack, it happens.

The important question is what was your concrete slump at time of pour?
Too much water can exasperate the cracking. This is some of the reason why fluidifiers are used.

It's definitely a shrinkage crack and they definitely (always) happen. I think the bigger question is, "Why didn't it occur in the control joint, right next to the crack, in the very place it was supposed to happen???

cbracer · Jul 6, 2017

Those are normal for 4000 psi concrete. I'm sure someone here on GJ is going to flame me and tell me I'm wrong, but in most cases people who pour drive ways or garages with 4000 psi get those cracks. I did my garage in 3500 psi concrete and it got warm when setting. Kept it wet for two days. No cracks. The guys said 4000 psi gets too warm when setting and it doesn't matter how wet you keep it, the heat is inside and the cool outside together causes those cracks. I wish people wouldn't use 4000 psi and know that 3500 is as high as you can go if you're concerned about those shrinkage cracks. The rebar will be nice to prevent the sides from lifting up as my old original garage floor did.

lakeroadster · Jul 6, 2017

cbracer said:
Those are normal for 4000 psi concrete.

My slab is 4000 psi. It doesn't have the cracks you say are normal.... anywhere at this point.... except in the control joints. Poured over a year ago.

G-ManBart · Jul 6, 2017

cbracer said:
Those are normal for 4000 psi concrete. I'm sure someone here on GJ is going to flame me and tell me I'm wrong, but in most cases people who pour drive ways or garages with 4000 psi get those cracks. I did my garage in 3500 psi concrete and it got warm when setting. Kept it wet for two days. No cracks. The guys said 4000 psi gets too warm when setting and it doesn't matter how wet you keep it, the heat is inside and the cool outside together causes those cracks. I wish people wouldn't use 4000 psi and know that 3500 is as high as you can go if you're concerned about those shrinkage cracks. The rebar will be nice to prevent the sides from lifting up as my old original garage floor did.

I'm not going to flame you, but that's simply not true. I used 4000psi concrete for all my slabs. I have a 40x40 slab with no cracks, a 24x40 slab with no cracks, and a 12x64 slab with no cracks.

People saying that you'll always get cracks on concrete simply don't know what they're talking about. Concrete poured correctly over a good base, with properly spaced control joints shouldn't crack....the expansion joints are the cracks.

EasyE · Jul 6, 2017

G-ManBart said:
I'm not going to flame you, but that's simply not true. I used 4000psi concrete for all my slabs. I have a 40x40 slab with no cracks, a 24x40 slab with no cracks, and a 12x64 slab with no cracks.

People saying that you'll always get cracks on concrete simply don't know what they're talking about. Concrete poured correctly over a good base, with properly spaced expansion joints shouldn't crack....the expansion joints are the cracks.

There is a big difference between expansion joint and control joint.

Expansion joints use black board or redwood or some sort of media that will allow the concrete to move around some. Generally smooth dowels are inserted through the media to keep the concrete from moving differentially on either side of the joint.

Control joints are cut usually 1/3 of the way through the concrete creating a weak area for the cracking to occur. Sometimes things like rebar placement, subgrade uniformity, and mix design and placement can work against you and make the concrete crack else where. Regardless of that, whether it cracks in the joint or not the concrete will still retain the same strength. The cracking in the joint is just a nice aesthetic way of hiding the crack.

Another note worth mentioning, depending on the climate and location of where the concrete is placed will dictate the amount of expansion it can take before cracking. I may be able to pour a 10X12 square in the southeast united states with no expansion or control joints and it probably will not crack. If I was to try the same amount of concrete in a different area or even a different application in the same area (inside vs outside) it may react differently.

wssix99 · Jul 6, 2017

cbracer said:
Those are normal for 4000 psi concrete. I'm sure someone here on GJ is going to flame me and tell me I'm wrong, but in most cases people who pour drive ways or garages with 4000 psi get those cracks.

It's totally normal if the people placing the concrete have no idea what the hell they are doing.

^ So, yes. It's very common.

G-ManBart · Jul 6, 2017

EasyE said:
There is a big difference between expansion joint and control joint.

You're right...I used the wrong term and edited my response. My typing got ahead of my brain...not hard to do

Falcon67 · Jul 6, 2017

spike0137 said:
It's better than nothing. Every company I had out said 6x6 I'd did 5x5 to feel better &#55358;&#56611;

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Code here is 3/8 bar 18" (inch) OC, at least two 1/2 bars in the footers and any grade beams. We have expansive clay type soil so the ground will be pushing any slab around like it or not.

OP -
Looks like the same shrinkage cracks we see in any slab around here. Neighbor's house has 92 yards in the floor. Once they did the dark stained the floor, those cracks became quite stylish. Looks better than tile.

Note the arguments about "control" and "expansion" joints - we are pretty much 100% mono slab here, no frost depth and nobody - nobody - cuts anything in a house slab. Or a garage, or shed, etc. They do in something like a Lowes size slab. I don't have any joints in the 24x40 that we poured with 4000 psi back in 2011 and none of the shrink cracks have changed one bit.

jeepntxj · Jul 6, 2017

We pour 4000 psi slabs daily and like said above, they only crack when the contractor doesn't know what they're doing.

I'm going to agree with shrinkage cracks, but for a different reason... How much rain has your area had lately? If it's been DRY dry, go by the batch plant that the mud came out of and see if they're watering the rock.

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heffneil · Jul 6, 2017

My slab cracked the same day it poured and they drenched it. The crack jumped the expansion joint and I asked my guy how and he told me it did it before the joint was cut. He saw it instantly. I didn't notice it for many days later. He poured my pad for the parking area and it too cracked. Really can't get too upset about it. They say concrete and stucco isn't done until its cracked

The integrity isn't really in question unless it was sizable.

wssix99 · Jul 6, 2017

heffneil said:
They say concrete and stucco isn't done until its cracked

Designers don't (can't) use this excuse.

I keep getting reminded of the saying, Q: "What do you call someone who gets straight "D"'s in medical school?" A: "A Doctor."

We should probably have our own saying, Q: "What do you call someone who takes your money?" A: "A Pro."

Mikeske · Jul 7, 2017

lakeroadster said:
My slab is 4000 psi. It doesn't have the cracks you say are normal.... anywhere at this point.... except in the control joints. Poured over a year ago.

Wait for about three years, my concrete in garage is 4,000 PSI and the cracks started around the three year mark in the control cuts and couple outside of the control cuts. It had rebar every 18" and fiber mesh in the concrete and had absolutely no cracking until about 4-5 months ago which is the three year mark. I am not even bothered by it. The old saying in these parts is there is two forms of concrete "fresh poured concrete and cracked dry concrete"

GMCGarage · Jul 7, 2017

Research the concrete code and calculate the spacing that rebar needs to be to control cracking.

Majordisorder · Jul 7, 2017

Go to the batch plant and ask to see the load tickets. In addition to the specs on what was ordered, the amount of water that was added on site should also be listed. This will more than likely confirm the shrinkage cracks and might indicate a lower than target psi value as well.

n20junkie · Jul 7, 2017

wssix99 said:
If a slab is inside and protected against the cracks from getting freezing water in them (causing the cracks to spread) and if the slab isn't exposed to high stresses of fast moving traffic, small shrinkage cracks should be interlocked by the aggregate.

Either way - even with reinforcing bars - one still needs proper control joints and the cracking should still occur in those joints.

It's definitely a shrinkage crack and they definitely (always) happen. I think the bigger question is, "Why didn't it occur in the control joint, right next to the crack, in the very place it was supposed to happen???

Tell that story to my basement floors. They have a differing opinion.

lakeroadster · Jul 7, 2017

Re: Rebar can cause shrinkage cracking

walrus said:
What does rebar 5 ft on center do?

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spike0137 said:
It's better than nothing. Every company I had out said 6x6 I'd did 5x5 to feel better ��

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After re-reading this thread it seems there was a break down in communication. Those companies saying 6 x 6 were referring to 6" x 6" wire mesh, not reinforcing steel laid out on 6' x 6' spacing.

spike0137 · Jul 7, 2017

Re: Rebar can cause shrinkage cracking

lakeroadster said:
After re-reading this thread it seems there was a break down in communication. Those companies saying 6 x 6 were referring to 6" x 6" wire mesh, not reinforcing steel laid out on 6' x 6' spacing.

No they were talking 6'x6' 1/2" rebar

spike0137 · Jul 7, 2017

I had a guy that has done cement for many years come out to take a look. He said the cracks are just shrinkage. I should have kept a sprinkler going when the sun is directly on it to not have it happen. He said the control joints are doing their job by stopping the cracks from spreading across the floor. Also had the company that did the slab out, he is a great guy and will stand behind their work.

The cracks do look big in the pictures i posted. I will pit a penny down next to one and post later to have a comparison.

I am running a sprinkler on the slab during the sunny hours until i can get to building the garage in August.

I also asked for a second opinion on the rebar spacing and around here in Southern WI 5'x5' spacing is plenty. I even took a trip to look at another guys shop that is a 40x80 mono with 6x6 spacing rebar that was done 10 years ago and it looks great. He even has the small shrinkage cracks like mine, they never raised or got bigger. You can notice them unless you look.

I hope this can help out others that may have the same issue as me and could nt find much info on the web.

Thanks for all the replies guys.

Mono Slab Cracks- 1.5 weeks old

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