Cracks in a fairly new slab

[mattlikesbikes]

Slab was poured late summer 2013, garage/apt. completed in February 2014.

I noticed the other day after it rained that a small hairline crack had developed starting at the bottom corner of our apron and moving toward the garage. A few days later I noticed (not sure if it was there before or not, I had just cleaned) almost in line with the crack on the driveway was a crack interior to the garage. The section of apron (6') did not have a crack along the same line, though I noticed this weekend the faintest of cracks generally near the long apron crack.

<a href="https://www.flickr.com/photos/mattlikesbikes/15581569828" title="New Slab cracks by Matt, on Flickr"><img src="https://farm8.staticflickr.com/7576/15581569828_1f4bf4ba58_c.jpg" width="412" height="800" alt="New Slab cracks"></a>

The garage slab is 6 inches thick I think and poured with a dug out footer and with 11 piers, each 12' bellbottom. So is this normal settling? Is there a problem? We had piers put in on the house this summer, but at 18' distance, I cannot imagine they would have impacted the garage.

At this point do I call the guys that did our foundation at the house and have them come map the floor of the garage with a water level and just keep that in the records, checking every 6-12mo?

 

[warren57]

Stress cracks are normal in concrete. Concrete like all materials expands and contracts. Or course that all depends on the compaction of the soil before it was poured. In the past I have called a structural engineer out to inspect. Each time they had no issue with hairline cracks. Seems it's pretty normal in concrete. I recently poured a 6" slab with 6,000# mix and it seemed to get a lot more hairline cracks than a standard 4,000# mix.
The structural engineer stated the only time for structural concern is if cracks open up and keep moving or heaving.
Keep in mind, expansion joints are put in the concrete to control the direction of the cracking. Sounds like you could have used a few more expansion joints to "hide" the cracks. Not to late, on job sites they concrete saw (score) the concrete for expansion joints. Doesn't have to be deep. A 1/4" to a 1/2" works fine. Look at the expansion joints in any old pour and you will see the cracks.

 

[mattlikesbikes]

Stress cracks are normal in concrete. Concrete like all materials expands and contracts. Or course that all depends on the compaction of the soil before it was poured. In the past I have called a structural engineer out to inspect. Each time they had no issue with hairline cracks. Seems it's pretty normal in concrete. I recently poured a 6" slab with 6,000# mix and it seemed to get a lot more hairline cracks than a standard 4,000# mix.
The structural engineer stated the only time for structural concern is if cracks open up and keep moving or heaving.
Keep in mind, expansion joints are put in the concrete to control the direction of the cracking. Sounds like you could have used a few more expansion joints to "hide" the cracks. Not to late, on job sites they concrete saw (score) the concrete for expansion joints. Doesn't have to be deep. A 1/4" to a 1/2" works fine. Look at the expansion joints in any old pour and you will see the cracks.

So I'll have our foundation guy spend 10 minutes taking measurements and I'll have the piece of mind and if I notice spread or heave I'll worry about it

 

[Falcon67]

The 40x60 bare slab next door has lots of small stress cracks, been on the ground since 2006. My slab has some, developed in the first week. Slab temp not wet after it set up ran 130F. 12x12 footers, 3 short ways and one long ways 12x12 beam. Cracks have not changed in 3 years. Neighbor's house slab took 92 yards, cracks formed in the first 30 days. they stained their concrete, The cracks give the floor "character". Looks really nice. As far as I know, those haven't changed in 3 years either.

 

[warren57]

So I'll have our foundation guy spend 10 minutes taking measurements and I'll have the piece of mind and if I notice spread or heave I'll worry about it

Yep, and add saw cut expansion joints about every 8-10' on centers and a any corners to control the direction on the cracks...

Did they put wire mesh in the pour? Some people use the fiberglass reinforced concrete, for me that cracks the worst....

 

[Will McRay]

I have a 30x57 slab about 5 months old and I too have cracks. At this point it nothing in my opinion structure, but the floor is everything in a garage and the cracks are unsightly. I want to tend to the cracks with an epoxy repair by Radon Seal, but not sure how long I should wait. We are going into winter and I want to see how the slab will handle the expansion and contraction first.

 

[Ed ke6bnl]

My concrete is not new but 30 years and the crack are getting pretty good size any thing to slow it down or do to it we are in So. California so we do not get much freezing just about 20 some times a year below freezing temps.

 

[NZ0J]

Stress cracks are normal in concrete. Concrete like all materials expands and contracts. Or course that all depends on the compaction of the soil before it was poured. In the past I have called a structural engineer out to inspect. Each time they had no issue with hairline cracks. Seems it's pretty normal in concrete. I recently poured a 6" slab with 6,000# mix and it seemed to get a lot more hairline cracks than a standard 4,000# mix.
The structural engineer stated the only time for structural concern is if cracks open up and keep moving or heaving.
Keep in mind, expansion joints are put in the concrete to control the direction of the cracking. Sounds like you could have used a few more expansion joints to "hide" the cracks. Not to late, on job sites they concrete saw (score) the concrete for expansion joints. Doesn't have to be deep. A 1/4" to a 1/2" works fine. Look at the expansion joints in any old pour and you will see the cracks.

You're confusing some terminology here. A saw joint and an expansion joint are two different things. Also, general rule of thumb on a saw joint is that it should be 1/4 the thickness of the slab at a minimum. The purpose of the saw joint is to intentionally cause a weak spot in the concrete, causing the cracks to form in the joint instead of randomly going where they want.

Also, your 6000 psi would most definitely be more prone to cracking over a 4000. The higher the cementitious content, the more shrinkage you will have.

As to the O.P., I'd like to see pictures of how everything was sawed. Also, if they reinforced the concrete, I wouldn't worry too much about the cracks, the rebar or mesh should hold everything together.

 

[BlackTalon]

My concrete is not new but 30 years and the crack are getting pretty good size any thing to slow it down or do to it we are in So. California so we do not get much freezing just about 20 some times a year below freezing temps.

If the crack is only widening, and not cycling from narrower to wider and back, then there is instability in the substrate. It could be from water getting in there somehow. Is this a crack in exposed paving, or in a slab inside a garage?

 

[rancherbill]

It looks like there is settling, perhaps around the house. It's time to get a laser level and find out which part is going down. How old is the house?

 

[Slednut]

When I had the excavation done on my garage addition I had them remove 2 feet of dirt, fill it with gravel and they used a compactor on the end of the back hoe on it. It’s been over a year and no cracks. I do see some between the saw cuts and the foundation (4 inches) so maybe the saw cuts are doing their job

 

[Fyrme]

Concrete has two guarantees.
1. It will not burn
2. It will crack

Just ask any concrete guy.

If you have a good amount of rebar in the slab, it shouldn't get much worse than am 1/8" or so. And if you don't have any relief cuts, you need to add them.

 

[BlackTalon]

The 'relief cuts' (contol joints) are what are intended to control shrinkage cracking early in the curing process. Cutting them now will not provide any benefit. If you need an expansion joint or two, those need to be cut through the full slab thickness, and from end-to-end of the slab. But the cracks that are illustrated strike me as either shrinkage cracks or possibly cracks due to settlement. The cracks may have been there since the slab was poured, but mainly became visible after it rained because the slab surface was dry and moisture was still in the crack.

 

[Zeke]

Tennis courts don't have cracks. If that's what you want you need 8" of concrete with no. 4 rebar on 12" centers over a compacted base of at least 12" over compacted soil down to undisturbed grade. That could be 6" or 36".

It's expensive to have perfection.

 

[Riverside]

I don't know a lot about concrete, but it is hard for me to believe that the alignment of the cracks is a coincidence. It looks like there is some degree of settling. Hopefully, it won't get worse.

 

[Fyrme]

I don't know a lot about concrete, but it is hard for me to believe that the alignment of the cracks is a coincidence. It looks like there is some degree of settling. Hopefully, it won't get worse.

After going back and looking at the pic again, I agree. That is if the drawing is accurate.

 

[Shredwagon]

It would help if you included some details on your existing control joint locations and spacing.

For those who say random "cracking" is normal - it's not. You live in Houston (no freeze/thaw). Inadequate compaction or wrong material coupled with drainage issues would seem to make the most sense. Ensure the existing ground drains away from the slab to prevent future erosion (if that's the issue).

Random info for anyone else: after compaction, concrete is all about water. Anyone pouring should be watching the mixer truck operator and finishers to ensure NO additional water is added at site. This will prevent almost all surface hairline cracking (with correct control joint spacing). That and a good hardener/curing compound with initial moist curing = 0 cracks.

Pretty simple really.

 

[wssix99]

It would help if you included some details on your existing control joint locations and spacing.

+1

If those dark lines are expansion joints and there is nothing else - then these cracks would be totally consistent with what would happen (due to shrinkage stresses) if no saw cut joints were made.

^ If that is the case, the concrete guy should be smacked across the face with a wet leather glove. That's Concrete 101. Nothing to worry about structurally, (until global warming upsets the polar vortex to the point where outside water freezes in Houston) but unsightly.