Horizontal Cracks in foundation wall

[Robbiee]

Hello,
any input is appreciated.
Was called to inspect a concrete foundation wall of a townhouse. The wall separates the house from its garage.
The crack first was at about 12" below the top and after monitoring for few months, the width of that crack increased slightly by about 1mm, but a new horizontal crack developed at the bottom.Please see the attached sketch.
I was then told that another unit has similar problem that was repaired by constructing another wall on the garage side.
I had hard time understanding why these cracks happened. If it is the pressure from the soil and the car in the garage, you would expect a flextural crack near the middle.
If the concrete has low compressive strength, these could be shear cracks, but again, would flextural crack be more possible?
Thanks

 

[r13]

One thing seems illogical is the sequence of crack development. I would expect the lower crack to occur first, not the other way around as stated. Are you sure the 1 mm movement is more or less uniform throughout, or there is bulging somewhere below the mid height of the wall?

 

[Robbiee]

HouseBoy,
Agree with your thinking. Here are a couple of images for the top and bottom cracks
retired13: is the 1mm movement uniform along the crack? We don't know as we only were able to see a section of the wall and that is where we installed the monitoring gauges.

 

[HouseBoy]

How does a crack grow in width if the wall is not bowing?
I say there is a bar in there that is corroding (due to chloride).
I can't tell for sure but looking at the bottom pic, it looks like maybe there are two cracks in the lower left corner of the pic. Maybe you can chip that concrete out (it's not doing anything) and see the bar.
Def don't think it's a soil pressure problem (for reasons stated previously).
The BIG QUESTION : What to do about it?
In the past I have taken the following position - Corrosion needs oxygen so, try to cut off the oxygen with painting, epoxy sealant or other masking strategies.
All assumes that the corrosion is the culprit. I'd chip off some concrete and see if there is a bar there with corrosion on it (as I expect).
In a case where I had a rusting bar at mid-height (where bending was greatest) I wanted to instill some bending resistance so I added carbon fiber straps vertically. Still, maybe not a perfect solution (CF reinforcing is not supposed to be used for foundations necessarily) but it was a reasonable way to maintain the existing conditions in my opinion.

 

[r13]

One interesting observation is both cracks are somewhat align with the form ties. The upper crack is right on, the wider crack at the bottom is offset a little from the form tie, but a fine crack has developed to the left of the form tie. The soil behind the wall is doing work, but not as I've suspected before, it is pushing the wall towards the living space. Since the wall looks dry without water marks, and rust stain, I doubt the theory of corrosion. My guess on how the cracks have occurred is,

1. Weak planes were created during concrete pour at the level of form ties.
2. Invisible Un-noticeable fine cracks have developed with the change of soil pressure with time.
3. Soil has reached a new state and pushing the wall out, causing the cracks (weak points) to widen.

I guess there is no bulging of the wall, since it hasn't acted like a beam after the cracks developed. I suggest to simply set up a plumb line, and take measurements along the wall.

 

[STrctPono]

Any chance your firm or some Contractor friend you know that would have a magnetic scanner or ground penetrating radar scanner. We have some in our office, but unfortunately they are pretty expensive. It would be nice if you could verify that the wall is indeed unreinforced so we could rule out the corrosion issue. If no scanner, you could demolish a portion of the wall at the crack line to see how deep the crack goes and if there is reinforcing... and then patch it back up with a repair mortar.

If it's not that, I must admit that I'm a bit stumped. If you set a long straight edge up on the wall, is the wall still plumb? I would expect that if retired 13's last assessment of the reasons were correct you should be able to pick up on some noticeable bulging or rotation at the crack locations.

At last resort, I would think continued monitoring would eventually show the real problem at hand.

 

[BAretired]

The crack pattern is quite unusual. We don't really know the depth of the cracks. Footing settlement is a distinct possibility. Chemical attack on rebar is also possible.

Drilling a hole through the wall at each crack could determine either the presence of rebar or the depth of crack.

BA