Numerous hairline cracks in poured concrete foundation

[Lizzy Fisher]

Client's foundation was poured 2 years ago, perimeter is square, each foundation wall is about 30 feet long, 4 feet high. Recently client noticed hairline cracks in foundation all over the places (every wall has about 20 cracks, evenly distributed in the middle 2/3 section).
Inspected the cracks, almost all of them are extremely thin (same size of hair), nearly vertical, only two of them are obviously bigger, but they are still <1/16" (one at the middle of north wall and another at the similar location of south wall). Same amount of cracks can be observed from inside of crawlspace. All cracks start from top, the biggest two successfully reached to somewhere right above the footing (wider at the top, getting thinner and thinner until disappearing above the footing), most of others stopped around the middle.
The foundation was poured in Summer, not sure about mix and w/c ratio, I was thinking of shrinkage cracks, although my client said they are new, I believe they might have been there for a long time and he didn't notice due to the small size. But I'm not sure if it's common to see shrinkage cracks every one or two feet. Another theory in my mind is settlement, but in order to generate cracks like this pattern, all four corners need to sink down while the middle sections of the four walls are not.
What do you think?

 

[dik]

First you need the mix design spec'd... strength, slump, w/c, aggregate, admixtures... Then you need details of any reinforcing... how much, where and what type... Then you need a history of the placement... was it freezing, hot, humid and windy... then you need an idea if the concrete was cured? Wet burlap? or sprayed curing agent, or maybe nothing...

Cracking doesn't sound too bad... hairline cracks in the middle of the walls, away from corners that may have offered some restraint... good luck in getting your answers... let us know what the answers are...

They stop at the footing because the footing may offer restraint... what sort of soil?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lizzy Fisher]

Spec is almost impossible to get, not to mention how much it could be changed during the implementation.
The foundation is rebar reinforced, vertical for sure, horizontal maybe. Silty soil. Concrete was cured in an environment of temperature 70F daytime and 50F at night.
My understanding is shrinkage crack does not impact the integrity of the foundation, so my current focus is to eliminate the possibility of settlement.
Question 1: Is it unusual to have so many shrinkage cracks?
Question 2: Under what condition settlement can generate cracking pattern like this?

Thanks.

 

[jimstructures]

How can anybody offer an opinion on what caused the cracking without seeing at least a picture of what the cracks look like?

Please put some object in the picture to give a sense of scale. Maybe a ruler or a quarter coin.

Jim


If the spec is almost impossible to get, then that means with some effort you could get it. You should also explain how and why it was changed during implementation.

 

[HTURKAK]

Some pictures could be helpful.. I think also these are shrinkage cracks and i suspect, the ftg was casted before the walls moreover, the wall casted without CJ. IMO, these cracks were old and developed just after the construction..
Sometimes, this type of hair cracks become visible with dust penetration into the cracks.

So many shrinkage cracks implies the amount of horizontal rebars should be questioned.
The settlement ( uniform or differential ) could not be the reason for the subject cracks ..

 

[Settingsun]

The tension is at the top from your description of the cracks. Some possibilities are:

- Restraint of wall shrinkage due to something at the top of the wall. Slab?

- Something on top of the wall expanding. Upper structure exposed to sun?

- Load is applied at the corners causing them to sink relative to the middle of the wall?

Try to find out where any movement joints are, reinforcement details, and sequence of construction.

Numerous but fine cracks implies that the width of cracks was controlled, eg by adequate reinforcement or just the other structure due to the low height of the wall.

 

[dik]

Are the cracks a problem... visually, you may cause more of a problem if you try to repair them.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

The wall was poured two years ago, so it has felt a range of temperatures consistent with climate in the area for the last two years. Hairline cracks are not usually deemed a structural concern, so there does not seem to be any action which needs to be taken at this time.

BA

 

[dik]

agree... but I've seen some terrible 'patch' jobs...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lizzy Fisher]

Thank you everyone for the great suggestions.
Most of you believe these are shrinkage cracks and not a structural issue.
But if the four corner areas are sinking downward just a little bit and created some tension to the top edge, does it also generate this cracking pattern?

 

[hokie66]

Yes, that could be it. With non conhesive soils without adequate containment at the edges, gradual loss of soil can cause that type settlement. Or in another scenario, if swelling soils are present, the centre can lift due to increased moisture.

Sounds like an issue for a geotechnical engineer.

 

[Lizzy Fisher]

Soil containment should not be a big issue, this is a typical crawl space foundation, footing was poured in a trench, thus the soil under the footing should not be able to move horizontally easily. And there is not expansive soil detected when I inspected the soil type in the crawl space.
And the footing is exposed in the crawl space so it's very easy to inspect, at least I didn't see any voids under the corners of this foundation. Laser tested the length of the four walls, at least at this moment the top edges are still deadly straight.
I think one thing I'm very sure is, it's not caused by soil expansion in the middle sections, this is summer time where the rain in the area is almost 0, the cracks should be closed by now, plus I don't see any reason why the soil could only expand in the middle sections of the four walls.
If the four corners are sinking down, let's say due to some weird underground moisture pattern that cause more soil compression around the corner, this is where I'm not so sure about(Although it's still a mystery why only the corners sink), will the biggest cracks appear near the corners first?

 

[hokie66]

Yes, if the corners are settling, you would expect some cracking near the corners, and they would be diagonally dowm from the top.

So that leaves shrinkage and temperature change as the most likely culprits. Without control joints in a 30' long x 4' wide concrete element, shrinkage cracking is inevitable, and as the cracks initiated at the top, they likely started due to thermal variation.

 

[dik]

Is there any damage to the partitions the wall is supporting? Check the corners of doors and windows to see if the interior finishes have 'cracked'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lizzy Fisher]

Answer Dik's question, no, no crack in any interior drywall, all door and window frames are perfectly square, window sills are perfectly level. I even laser tested a long crown molding that runs along the exterior wall, perfectly straight and level. This result is aligned with the test I ran in the crawl space, because the top edge of the foundation walls are perfectly straight and level at this moment.

 

[dik]

I wouldn't worry too much the... any corrosive environment?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lizzy Fisher]

Good question Dik. This area is a basically in a forest, extremely good natural environment, tested the soil PH value, around 7. Found some efflorescence at the bottom of the foundation where the exterior footing drain locates, but no efflorescence around the cracks. But I'm very concerned by the pesticide pest control company applied on the foundation wall. I know this is common practice to spray pesticide on foundations, but the PH value of these chemicals are as low as 4, thus might cause some chemical reactions to the surface of concrete. But usually it'll take decades for the vertical concrete to deteriorate (it'll be a different situation if it's applied on a horizontal surface where the chemicals stay and soaking deep down), and usually it only damages the surface instead of generating penetrated cracks like these.
BTW, my client is asking if the shrinkage cracks will get wider in the future, my answer would be no, will they?

 

[dik]

They might... shrinkage occurs over time... if after two years they are not significant, it is likely that this will bode well for the future.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lizzy Fisher]

I did some further research and found this article:
Link
It mentioned "If horizontal steel is present, you are more likely to get several very small cracks in lieu of one or two much wider cracks"
I guess this is the reason why the cracks are numerous but small in size. Shrinkage cracks only happens when restraint is present, Rebar is acting as restraint that caused shrinkage cracks in the first place, but at the same time, it's also controlling the size of cracks.

 

[dik]

Rebar didn't cause the cracking... the natural shrinkage of concrete did... the rebar helps contain the size of the cracking and distributes it more... Rather than have a 1" crack without rebar, you have sixteen 1/16" cracks with it... of that ilk.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik