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

 

[phamENG]

How much load is on the wood wall above? It's probably loading the basement wall with some eccentricity, so you'll end up with a moment applied to the top of the wall. It will be additive with the retaining wall resisting moment. The weight of the wall would begin to counteract the moment due to eccentricity the further down the wall you go, so it makes sense that its failure (if it is the cause) would be a bit higher up on the wall, and the form ties give the cracks a decent place to start propagating.

This doesn't account for the lower crack, though. My first thought was some inherent stiffness at the base but then realized the cracks would be on the other side...

Nevermind...I thought the wall above was aligned to the other face. Thanks, Celt....

 

[Celt83]

Both could be shear cracking, being a home wall is likely per IRC and 8" with bars at 48"o.c.

Bottom crack is about "d" away from the support created by the slab on grade

Top crack is a bit further than "d" but in a location with potentially compromised section at the form snaps or maybe poor consolidation being the top of the wall.

Eccentricy from the wall above would tend to place the face with the crack in compression, wouldn't rule out flexural cracking from the soil loading though.

Edit: Is there a joint between the slab on grade in the garage and the wall and is there any exposure to water/salt on the garage face of the wall?

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Robbiee]

phamENG,
The loads are standard loads for small houses. The joists of the ground floor are parallel to the wall and I am assuming the same is for the second floor. That said, the studs are located on the inside face of the wall as shown in the sketch, which means any moment created by the studs will help counteracting moment from the soil pressure.
Celt83:
The slab is cast against the wall,but there is a small gap developed that can allow water to infiltrate. The only water/salt that can go in is the any water that the car brings in the garage.
I was thinking that salty water could have corroded the form ties and caused the top crack. But the bottom crack is puzzling

 

[DaveAtkins]

Does the concrete above the horizontal crack look different than the concrete below (e.g., different color)? If so, the crack could be a cold joint, with insufficient bond between the two pours.

DaveAtkins

 

[Robbiee]

DaveAtkins,
I looked at that and no there is no cold joint.

 

[STrctPono]

Robbiee,
Did you try hammer sounding the wall to check for corrosion/debonding of the rebar?

How old is the townhouse?

The significant efflorescence on top of the foundation wall is peculiar. Why are they getting efflorescence on the inside of the garage? What kind of joint is there between the garage SOG and the foundation wall? Joint Filler? Any sealant?

 

[phamENG]

The efflorescence makes sense if the humidity in the basement is significantly higher than in the garage. Where I am, the houses old enough to have basements (they're no longer allowed by local ordinances) can claim an indoor swimming pool about 8 months out of the year. Even with high relative humidity outside (and in the garage), these cesspools generate enough humidity in the basement to make it plausible for water to migrate through the wall - especially if there's a crack helping it along.

 

[Dvdt18]

Robbiee,
Like you said in your previous post it could be attributed to the egress of water within a crack/formwork ties eventually making it to the horizontal reinforcement. If at the time of construction a horizontal reinforcement bar is touching the outer formwork and has insufficient embedment within the wall and water is present it could easily get to the horizontal reinforcement and cause it to corrode. Given the presence of "significant" efflorescence there must be water close by to cause that to occur. I'm located in Canada where they use an abundance of road salt and we commonly see this occur to foundation walls at formwork ties and just below exterior grade and within garages. Typically when looking in the crack you should be able to see the corroding rebar and once the repair is carried out the contractor typically chases the reinforcement to remove all corroded portions and sure enough there is always a portion severely corroded at the location the water enters the crack(s). If needed, I could dig up some photos from some of the horizontal crack involving steel reinforcement. I'd be curious to know if the width of the crack on the exterior is the same width as on the interior or if they are fairly uniform. As for the lower crack location it is in a strange location.. did your client mention problems with a high water table or constant running sump pump (if one is present)?

I'm unsure if your located in a cold environment, but in Canada we commonly see horizontal and vertical cracking due to adheasion and heaving of any frost susceptible backfill on the exterior portion of the wall. The history of the crack helps with determining the cause if it is an older home and this is just beginning to occur.

Lastly, i've seen some fairly strange location of horizontal cracks which were attributed to excess backfill heights, surcharge from car loading, and the wall not having anchor bolts fastening the sill plate to the foundation.

Let me know how your project goes.

Thanks.

 

[RPMG]

The rebar is corroded due to chlorides in the concrete mix or deicers.

 

[r13]

What was the earth pressure used in design, active or at rest? Also, there should be a surcharge load at the garage side.

 

[HouseBoy]

Agree with RPMG
Typical residential wall reinforcement is 1 or 2 bars top and bottom. 12" is a good place for the top and bottom wall ties.
Probably no way to know exactly when the concrete was placed but... chloride accelerator is the likely culprit.
A couple of pics might help too.

 

[PEinc]

The original question was about the horizontal wall cracks at the top and bottom of the wall. Adding horizontal rebars at top and bottom will not help prevent the horizontal cracks. Usually, when I see horizontal wall cracks, they are on an exposed, north, block wall of a house. Poor drainage and freezing ground push and horizontally crack the wall near the top few feet. Since this is basically an interior wall (between low basement and high garage), I don't think freezing is the cause. If the basement does not have water seepage, then excessive water pressure probably isn't the cause of the horizontal cracks. When houses are built, compaction of foundation walls is rarely done properly, if at all. Poorly placed backfill and a surcharge from the car in the garage are more likely the cause for this probably-unreinforced, ungrouted, block, or minimally-reinforced, concrete wall to crack horizontally. The wall wants to move but is somewhat restrained at the top (by joists and wall framing) and at the bottom (by basement slab). I wonder how plumb the wall is between the top and bottom cracks.

http://www.PeirceEngineering.com

 

[JLNJ]

retired13 asks what soil pressure was used in the design. I would guess that there was no site-specific design other than the empirical "that's the way we always do it" type.

 

[HouseBoy]

PEinc
Just to be clear:
My understanding is that this is a cast-in-place concrete wall.
My vote is that the existing rebar is rusting due to chloride in the concrete and the existing rebar is very likely to be in the two locations where the cracking is happening.
I agree that plumb condition would be helpful to know and (naturally) might shed light on things.
Also, if the horizontal crack is due to soil pressure, I'd expect it to be located at or near mid height.

 

[Robbiee]

Thanks to all for the input. Here are some answers to questions raised so far:
- I don't think the wall has rebars at all, as we are, until now here in Ontario Canada, allowed to build houses with unreinforced foundation walls within certain limits.
- I have seen so many cracks resulting from corroded rebars where the crack aligns with the rebar and rust stains are visible. These cracks do not fall in this type of cracks.
- The house is about 40 years old, and what earth pressure was used in the design at that time is unknown.
- The wall is plumb with about 1mm of shifting towards the basement at each crack- An indication of pressure exerted from the garage side
- The same issue was noted at another unit
- The garage slab is not settled. Compaction under the garage slab doesn't appear to be the suspect.
- The slab of the garage is poured against the wall with no sealant or movement joint or any filler. A small gap (about 1mm) has developed between the slab and the wall, where water could seep.
- The garage is not heated, so freezing is possible, but that is the condition of almost all garages, yet we don't see similar cracks everywhere.

 

[PEinc]

HouseBoy, you are correct. The wall is concrete, but that does not significantly change my response. I will edit my response. Thank you.

http://www.PeirceEngineering.com

 

[hoshang]

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.

The width of first crack increased by about 1mm. This widening induced a compressive stress (greater than the nominal strength of the section) on the bottom of the wall, so a new crack developed (not a tension or shear crack).

 

[r13]

I highly suspect the wall has suffered higher soil pressure than designed. I am not a geotechnical engineer, so I couldn't tell why the soil pressure might have increased without noticeable settlement. All I can suggest is to take good pictures and measurements on each crack, seal it, then observe. If new crack develops, or the old one cracks again, it is probably the time to call for expert helps, to identify the root cause, and options on retrofit.

 

[HouseBoy]

While there is a lot that I still don't know about the existing conditions, I am not feeling "very high soil pressure" as the cause.
Not saying that's not the cause, just saying I don't see that as the likely cause for these reasons:
One way shear failure seems incredibly unlikely.
IF the soil pressure is sufficient to crack the concrete at the top and bottom, I would not expect the horizontal anchorage of the wall to be adequate to restrain the top of wall against movement ESPECIALLY considering the joists are parallel to the wall. Even if the joists were perpendicular, the sill plate anchorage would still be the weakest link.



Hoping OP can post some pics!