Cracks in new concrete?

[OutbackL130]

I added 3 new buildings onto my shop this year and am planning a fourth as our business is expanding. Two of the concrete slabs were poured last winter and look great, not even a single crack in either. Now I expect all concrete to crack, however, I am concerned about my 3rd slab which was poured a couple months ago. It immediately developed about 20 of these cracks.

How acceptable is this in a new slab? What was the possible cause of this new slab cracking so badly and what is the remedy? I had the same guy pour all the concrete and he is well known for quality work. Like I said the other 2 slabs are nothing like this one.

 

[grsthegreat]

I added 3 new buildings onto my shop this year and am planning a fourth as our business is expanding. Two of the concrete slabs were poured last winter and look great, not even a single crack in either. Now I expect all concrete to crack, however, I am concerned about my 3rd slab which was poured a couple months ago. It immediately developed about 20 of these cracks.

How acceptable is this in a new slab? What was the possible cause of this new slab cracking so badly and what is the remedy? I had the same guy pour all the concrete and he is well known for quality work. Like I said the other 2 slabs are nothing like this one.

honestly, those cracks dont look bad at all. you should see how my slabs all tend to crack in a few years. I dont think i have a single slab...even my 5-1/2" thick slab, that is without cracks.

 

[BeezFun]

Are there control lines cut about every 10-15'? If so I would not expect to see those kinds of cracks. If the contractor is reputable ask him to come back and take a look. It's hard to tell from a photo, but was the surface sprayed with a curing agent when it was poured? None of the photos quite have the sheen I'd expect to see if they were. Could also be a problem with settling, was the base compacted, or did it have time to settle before they poured?

 

[Gary Fowler]

Those are stress cracks from curing at high ambient temperatures. Anytime concrete is placed at temps above 60-70F then the heat of hydration will cause surface cracking in large slabs. Using a retarder will help a bit along with adding the nylon or plastic particles in the mix.
I don't see any problem with your slab as far as endurance.
I prefer to pour slabs in the winter time when temps are just above freezing. This slows the curing and helps prevent those surface stress cracks.

 

[Clint S]

Yea, my pole barn is the same as that, poured in the summer. My garage, poured in the late fall, no cracks

 

[OutbackL130]

Ok I really dont mind the cracks, was just curious and making sure its normal. I saw a guys garage that he built a few years ago which developed massive cracks in the concrete spreading out from his car lift.

Should you seal them up or just leave it?

 

[murphy1244]

Yea, my pole barn is the same as that, poured in the summer. My garage, poured in the late fall, no cracks

Mine was poured in July, Temp was 85 plus and I don't have any cracks.

 

[crazyal]

How well was the ground prepared before the cement was poured?

 

[GManBart]

Mine was poured in July, Temp was 85 plus and I don't have any cracks.

You didn't use too much water, which 95% of the people do.

I drove mixer trucks in the summers through college...concrete poured at the right slump, for state highway and airport runway jobs don't crack, because they won't let you add much water, and they count barrel revs. Less water, with more revs (slow RPM) can produce workable concrete that won't crack. If you want it easy to work with, and are in a hurry, dump a bunch of water into it, and it's going to crack. My dad did the slab on his outbuilding when I was 7-8yrs old....it's still not cracked 35 years later.

 

[sdkubota]

Srinkage cracks and all concrete will get them eventually. You can not prevent them but if you have contraction joints sawed into the concrete right after hardening you can make the cracks straight and easy to manage. A 4" slab needs to be sawed every 8' and a 6" slab needs to be sawed every 12' both ways. Saw depth is 1/4 the concrete depth.