Concrete crack evaluation / questions

[Coyote556]

I was hoping some concrete wizards could tell me if this is going to be a problem. I have a 48x75 floor that was poured in late October of 2013. It has developed several cracks across the 48' length. It was sawed a couple days after the floor was poured, but I do not think the saw cuts were anywhere close to deep enough.

The specs on the floor are 5" of 4000psi with half shot of fiber and 1/2" rebar on 18" centers. The rebar was set on 2.5" chairs so it should be right in the middle. I have attached photos of the cracks and the first one is the biggest and the one concerning me the most.

Also, do you all think I should seal the crack so water cannot get into it come freeze time? It goes right out the overhead door opening.

Are these cracks anything to worry about?

Thank you much.

 

[ddawg16]

There are two kinds of concrete.

Concrete that has cracked.

Concrete that will crack.

That crack is just like what mine did. If I had to guess, it's just a typical shrinkage crack.

When it comes to joints, I was told concrete likes to expand/contract the same in both directions. Hence, cuts should be the same distance in both directions. If you have a 10' wide drive...the expansion joint should be at 10'.

The experts will chime in shortly, but I think you're ok

 

[Rookie2]

That's a shame but I think there is no worries with all that rebar. Probably settled. I would seal (caulk) it to keep the dirt out. After I saw cut mine I never filled in the cuts and it irritates me when I sweep it .

 

[Rookie2]

Is it enclosed or has it been exposed for 10 months ?

 

[Falcon67]

Those don't look out of place. If they are growing, then that would signal a problem I think. I had several but after about 60 days everything was static. Neighbor same thing, has some good cracks in a 92 yard house foundation. They stained the floor instead of carpet and the cracks give it a nice textured look.

 

[Coyote556]

The building went up about a month after the pour. The biggest one in the first picture Has grown a little since the pour. That's kinda what worries me.

 

[ACDNate]

Nothing there that would be a big concern to me. Like others have said, all concrete cracks, no way around it. If that crack starts to gap out or heave, then I might be concerned but otherwise I wouldn't put much thought to it.

Oh and yes, i'd seal it or go over with whatever flooring you're doing.

 

[hoho98925]

What was the spacing of the saw cuts?

 

[ConCretin]

As has been mentioned, it's just a typical shrinkage crack. Your rebar was properly positioned and well supported and will keep the crack from opening up. It's pretty much just an aesthetic issue.

Think of it this way; If the control joints had done their job, you'd have the same cracks - you just wouldn't see them because they would occur in the bottom of the joints instead of thru the middle of your floor.

You got random shrinkage cracks because your sawcut control joints were done too late. Shrinkage cracks form early so ideally control joints should be cut immediately after finishing using an early entry saw but no later than 24 hours. These cracks probably occured before your cuts were made even if you couldn't or didn't see them.

As an aside for those who believe that rebar prevents cracks, this is a pretty good example where it didn't.

 

[Coyote556]

I think they are about 15 feet apart and only an inch or so deep. Really, I don't think the cuts did anything at all.

 

[Coyote556]

When I watched the pour I felt they wet the concrete up quite a bit too much. That would probably help explain the shrinkage I guess. I did all the form work myself but hired a crew to do the flat pour

 

[A**holes Garage]

not a concern unless the crack is wider than the thickness of a nickel

 

[LX-Markham]

When I watched the pour I felt they wet the concrete up quite a bit too much.

Quite possible. The more water used, the more it will shrink.

Like LLWillysfan said: those cracks probably formed before the slab was cut.

Reinforced concrete isn't doing it's job until it cracks.

 

[Coyote556]

A dime will not fit in the crack now

 

[Daniel Dudley]

A dime will not fit in the crack now

Cracks don't mean a lot when you have that much rebar. That is a GOOD slab.

Just think of your cracks as unplanned expansion joints, and you will be OK.

 

[Coyote556]

I appreciate the complement on the slab, I guess I am just a perfectionist and I was hoping for no cracks. Here is a shot of the day of the pour

 

[Falcon67]

It is nicely done, lots of bar and nice footers. FWIW, there is a 40x60 slab on the next lot from me that has 18" x 12" deep footers and is at least 5" thick, 18" OC rebar plus 1/2 bars in the footer, poured in 2006. It's got small to medium shrinkage cracks all over it. Some places it looks like little spider webs. The guy that did it spared no expense. I have a few, I could keep the slab watered for 3 days then had to go back to work. That was a hot summer and the slab would be 135F in the afternoon. Can't escape them cracks. I have 12"x12" footers, 24" OC 3/8, two 1/2" in the footers and beams, 3 cross beams and on longways beam all 12" x 12" and still got them. BUT - my old shop was cheated by the concrete guy, after 5 years had a diagonal crack that would eat a pencil.

 

[Coyote556]

There is a 12" wide by 30" deep footing under the entire edge of my slab too. I really did not want it to move with frost. I poured the footing first.

 

[hoho98925]

I think they are about 15 feet apart and only an inch or so deep. Really, I don't think the cuts did anything at all.

I personally want to see control cuts for this type of construction on a grid no larger then 10'x10' and minimum 1/4 depth of the slab.

As has been mentioned, it's just a typical shrinkage crack. Your rebar was properly positioned and well supported and will keep the crack from opening up. It's pretty much just an aesthetic issue.

Think of it this way; If the control joints had done their job, you'd have the same cracks - you just wouldn't see them because they would occur in the bottom of the joints instead of thru the middle of your floor.


You got random shrinkage cracks because your sawcut control joints were done too late. Shrinkage cracks form early so ideally control joints should be cut immediately after finishing using an early entry saw but no later than 24 hours. These cracks probably occured before your cuts were made even if you couldn't or didn't see them.

Exactly the cut has to be made same day the concrete is placed, there are also chemicals available through the concrete suppliers which they add to the mix to limit shrinkage cracks.

As an aside for those who believe that rebar prevents cracks, this is a pretty good example where it didn't.

Rebar in slabs only holds broken concrete together, it does add strength but does nothing as far as limiting cracks.

When I watched the pour I felt they wet the concrete up quite a bit too much. That would probably help explain the shrinkage I guess. I did all the form work myself but hired a crew to do the flat pour

Finishers always add as much water as they can get away with. It makes it easier on them. The less water added the stronger the concrete is, and less shrinkage. That is the main reason on commercial structures there is a private inspector to test the slump of the concrete and make sure the concrete is not compromised with too much water. Anyone of us pouring a slab should meet with the concrete supplier and discuss options, agree on a mix design, and max allowable water per design. Most batch plant tickets list the amount of water that can be added to the mix, most finishers completely ignore this, in favor of wetter mud=less work for them. Also Concrete suppliers have admixes available to limit hydration shrinkage. There are also sealers that you put down as soon as the mud is finished to seal in the moisture, thus slowing the shrinkage, and strengthening the slab. Many of these sealers have no effect on applying later finishes.

 

[zporta]

as everyone has stated it looks to be a shrinkage crack. the concrete was cut too late and could have also cracked on a cold joint. due to the size of the slab it takes multiple trucks to pour and isnt always worked properly

 

[Rookie2]

Why are they finishing one truck load of concrete ? where are the next 6 trucks ? That would make me nervous !

 

[Coyote556]

They did use a "cure and seal" spray on it right after they finished. I requested it because I knew there was no way I was going to be able to keep it properly watered till cured.

I fully agree they wet it up to make it easier to pull. If I had it to do over again I would have mandated a slump on each load out of the chute. Real close to 70yds in the slab and about 27yds in the footing.

No way for me to do the flat work myself. Had to hire it out. All I could do was all the prep, forms, rebar, footing ect.

 

[hoho98925]

They did use a "cure and seal" spray on it right after they finished. I requested it because I knew there was no way I was going to be able to keep it properly watered till cured.

I fully agree they wet it up to make it easier to pull. If I had it to do over again I would have mandated a slump on each load out of the chute. Real close to 70yds in the slab and about 27yds in the footing.

No way for me to do the flat work myself. Had to hire it out. All I could do was all the prep, forms, rebar, footing ect.

Well it's set in concrete now. You can Mediate the cracks with a floor covering, caulk them with a concrete caulk, or just ignore them. Crack Happens.

 

[pauls340]

That darn w/cm ratio causes so many problems in concrete....plastic/ dry shrinkage cracking, slab curl, efflorescence and ASR to name a few. Someone should invent an admixture to prevent all that, Oh wait, someone has. It's called Vapor Lock 20/20. It converts all that water to additional C-S-H gel. We switched 100% of our concrete to "Premium Concrete" using that stuff. And we upcharge for it. The manufacturer calls it Vapor Lock Enhanced Concrete. If you're about ready to pour concrete, you owe it to yourself to track it down.