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.

 

[sdkubota]

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?

Leave them alone.

 

[EddieWalker]

Nothing you can do to fix them. Hopefully you used rebar. The rebar will hold the concrete together and level.

They are caused from drying to fast and probably too wet a mix.

It's too soon for it to be related to compaction. If there are compaction issues, that will happen over time. The most obvious clue is that the slab will change heights at the crack.

Cutting expansion joints into concrete just makes it weaker where it's cut and just about guarantees that it will crack. The crack is usually less obvious because it's in the cut. It's not something I recommend, but for some reason, a lot of people seem to like them.

Concrete expands and contracts when it's hot or cold. This movement also causes cracking. This is why on big slabs, you pour it in sections with something between each section to absorb that movement. This is also why you pour a house slab and all your porches separately. Some places also pour garages separate from the house due to the difference in temperature inside the house to outside. You never need to worry about expansion inside a cooled and heated house because the concrete temperature is constant enough to not move to a noticeable degree. It will still crack over time, but if everything is done right, it wont cause any issues.

All concrete cracks.

Eddie

 

[KWentling]

"All concrete cracks" And that's why you do the control joints, so you don't have random cracks. I can't see it that it makes in any weaker. I'm a big believer in rebar, too.

Kim

 

[OutbackL130]

The slab that cracked I know has rebar. The other two slabs are fiber reinforced and I'm not sure they used rebar in them.

 

[dave1949]

The cracks are likely more cosmetic than any real problem, especially since you know there is rebar installed.

If this is not going to be a heated building, and there is a possibility of freezing along with moisture/drips on the floor, I would seal the cracks with caulking. Chickamauga does seem warm for that possibility, just a thought.

 

[OutbackL130]

Just curious would the cracks have formed due to not keeping the concrete wet while it cured? I asked about the need to water it before they poured but they said nobody does that anymore with the new chemicals.

 

[don45640]

The fiber prevents the shrinkage cracking pretty well,which is nice from a contractor's point of view,who doesn't want the customer to have a cracked,brand new slab before he gets his saw-joints cut. I used to use a half-dose of fiber in hot weather,and I conditioned the slab with a trowel machine by using float blades and NOT just combination blades.
A coupla passes with float blades levels the floor,and really helps diminish the likelihood of precracking,even in hot weather.
Something noone has mentioned specifically is that concrete cannot be old and give good results. don-ohio ^)

 

[slow wrench]

The cracks in the pictures look very wide for a concrete slab with rebar in it.Rebar needs to be held up off the ground with dobbies or pulled up into the center of the slab, with a hook, as they pour,otherwise its worthless.

 

[OutbackL130]

Is it important for slabs my size to have expansion joints cut in? None of mine do. The original building is a 24 x 30 built about 30 years ago. The concrete is in fairly nice shape with minor cracking. This year we did a 18 x 20 addition to the front of the building, and then a 20 x 25 addition out the back(which is the slab that has cracked). Then there is a seperate 18 x 20 building as well.

The concrete people are supposed to come this week to pour another slab 20 x 25 adding onto the other 20 x 25 so that I can put up a 20 x 50 building.

 

[SPIKER]

Looks to me like the slab dried (water left it) too fast, this can also cause weak concrete. Keeping it moist for proper hydration would have helped it back when. Nothing can be done to fix it short of jackhammer out the cracks and use a patch or total replacement. Will it weaken the slab? maybe some is it worth replacing or fixing, that is your call but probably not required in a pole building.. On those wider cracks almost look like settling cracks to me due to the width fo them, is there any offset up/down on the slab?

As for Cutting it also not required it just makes the concrete crack in straight line in the crack that is usually wider and more of an eyesore than a natural crooked fracture in uncut concrete. They (cut lines) also are harder to roll a creeper over them and are more likely to trap larger amounts of dirt & bugs...


Mark