Interesting concrete crack pattern - what is the mechanism of failure? 4

[KZSteel]

I'm having an issue on one of my projects, and I'd like to get the community's input. Let me outline the problem:

A 250mm wide cast in place wall appears to be failing under just dead load. It supports a small tributary area on two levels. Two deep beams frame into the wall at its end, but do not carry proportionately high loads for their depth; their depth is to catch some steel framing at a lower elevation. The base of the wall beneath the beams rests on a basement column (see elevation sketch and photo). At first glance, the toe of the wall that rests on the basement column appears to be crushing (Photo 1). However, to me, the crack pattern near the "crushing" zone (Photo 2) to me does not indicate compression at the toe. In some ways, it appears as though the corner has been pulled away from the wall, as though the column was sinking. To support that strange claim, there is actually a 3mm gap at the toe that you can slide a credit card into (Photo 3), and horizontal cracking halfway up the wall (Photo 4), indicating to me a general lack of compression.

I nearly suspect that a site labourer may have driven into the toe with a forklift to cause the damage. I do not see the magnitude of load to cause a failure, and I do not understand the mechanism if it wasn't site damage. You'll notice in Photo 3, there is a strange diagonal crack *across* the section of the wall. I can't really explain that.

Any thoughts?

It's worth noting that supported elements have since been shored up, and the necessary safety precautions have been taken in the event that failure is imminent.

 

[charliealphabravo]

i don't think any of your links are alive.

 

[TXStructural]

Without the photos, I'm guessing, but it sounds a lot like restraint cracking due to excessive shrinkage of the concrete. How long ago was the concrete placed, how long before the cracking appeared, and how is is reinforced?

 

[KZSteel]

I'm not certain why the links didn't work, but here are the URLs.

Elevation sketch:

http://files.engineering.com/getfil...c-a97d-b13f10b3df6d&file=elevation_sketch.pdf

Elevation photo:

http://files.engineering.com/getfil...fc-bbfa-17a69fe4a3cf&file=elevation_photo.JPG

Photo 1:

http://files.engineering.com/getfil...-a393-47c2-b4d5-7af4cf30c352&file=Photo_1.JPG

Photo 2:

http://files.engineering.com/getfil...-d39d-456a-85b8-c83974dd38a8&file=Photo_2.JPG

Photo 3:

http://files.engineering.com/getfil...-6300-418e-938b-522efcd29a42&file=Photo_3.JPG

Photo 4:

http://files.engineering.com/getfil...-c96b-4f4f-8398-6e6134dda837&file=Photo_4.JPG

 

[KZSteel]

TXStructural,

I had considered that, but there is no visible damage to the column below the main floor slab, it's just the toe of the wall that is showing damage. The junction between the walls and the 2nd and 3rd level beams is also in good shape.

 

[charliealphabravo]

I agree that it doesn't look like crushing. The gap at the base of the wall and the open cracks are not consistent with crushing.
I also don't see anything that is clearly a significant point of impact.

Restrained shrinkage is still the most promising mechanism I think. Consider if the reinforcement from the basement column was only stubbed into the lower part of the wall at the end. Shrinkage of the wall toward the center of mass would cause it to pull away from the base and leave a gap and crack like you see in Photo 3. Also I think I see an embedded steel plate in Photo 1. It is possible that it interferes with the wall reinforcement at the base. The corner/edge of the wall would also be subject to more drying and shrinkage I think.

 

[TXStructural]

I do not see how the sketch and the elevation photo match. Are we seeing the back side of the wall at ground level with the column below grade and out of sight? You sketched that the wall is on top of a column. acting as a deep beam of sorts.
Photo 1 appears to show debris in the concrete. I would probably demo the broken stuff to find out wheat else in behind the concrete cover. What is the deformed, rusty looking thing in the spalled area, bottom center?
Photo 2 - cracking appears to be restraint cracks. Could be self-restraint from the wall. I see cold joints at mid-height of the walls, possibly there was a problem which resulted in more shrinkage of the second lift. Is there PT elsewhere in the structure? And you mentioned tying into (existing) steel beams?
Photo 3 appear to show that the wall is hovering above the slab, again, a sign of restraint deformation. Where a column was forced to rotate by the connected structure deforming or shrinking, you can get this. I suppose a very wet (high shrinkage) concrete mix could cause this (the concrete could be "hanging" from the reinforcement where is shrunk but the bars and remaining wall restrained it.) I'm curious about the reinforcement.
If photo 4 is above the area in photos 1 and 3, lacking further information, I think I stand by my assessment of restraint-induced shrinkage cracking.
Or maybe they ran into the wall with a piece of equipment. ;-) (I don't see that as the answer, since the wall has remained deformed and not just broken in a place or two.)

 

[JedClampett]

When I see issues like that and loads can be eliminated, I move next to soil issues. How was the compaction under the column? Is it possible that the soil has settled under the column and the wall is supporting the column instead of the other way around? Or is the soil under the rest of the building some kind of expansive clay and it's pushing up?
Also in the foundation area, are the bearing loads the same under the column footer and the wall footer?

 

[TXStructural]

CAB wrote: "The corner/edge of the wall would also be subject to more drying and shrinkage I think."

And the corner is a likely place to start placing the first load of concrete. We frequently see excessively wet concrete, often with line lubricants mixed in, at the corner where the pour starts. Never let a concrete placer purge the line into the forms, especially near edges, columns, or corners.

 

[Michel60]

Good morning KZSteel,

Restraint seems likely as has been suggested, how long are the wall segments and how much of the wall was placed monolithically? Any construction or relief joints in the wall? What is the time line of the concrete placement of the column and the upper walls/floors vs. the first appearance of the cracks? Are the floors by any chance PT?

Differential settlement also maybe? Do you have any survey data?

If it is shrinkage and restraint caused it should ultimately be repairable.

regards,
Michel

 

[charliealphabravo]

I am with TXS.
I think if you demo the broken concrete corner you will find a train wreck of reinforcement.
I wouldn't suspect foundation issues unless the basement column and basement floor also show signs of distress. The condition is very localized and is located on an end wall that is exposed to the sun. Excessive drying and shrinkage, I would say, along with a reinforcement issue.

 

[hokie66]

Photo 2 tells the story. That is definitive of restraint shrinkage cracking. The big section of wall wants to shrink to the left, and thus upwards. The reinforcement is insufficient to resist that massive force, thus the concrete cracks. It happened to me once.

 

[KZSteel]

All,

The principles and senior people at my company are in agreement with what has been proposed here: Restraint shrinkage cracking. And we agree the issue is repairable. Re-running the numbers on this area showed there should absolutely be no issue with crushing.

As was mentioned, there seems to be no foundation settlement issues anywhere else in the area, so a bearing failure of the basement column footing is unlikely.

TXStructural: Your assessment of the sketch and elevation is correct; there is a sub-grade column beneath the slab. There is no PT in the building. Your comment about more shrinkage in the second lift seems valid. Having more shrinkage in successive lifts would cause the wall to generally curl upward at the ends, which may explain the gap between wall and slab.



A big thank-you to everyone who contributed in this post. This humble engineer thanks you all!

 

[hokie66]

KZSteel,
Those are principals who agree with the principle of restraint cracking.

 

[hetgen]

Hi,

Why isn't the base slab supporting the column on photo 3 not cracking?

 

[TXStructural]

It looks to me like the wall is lifted above the slab, so there is no reason for the slab to crack. This brings into question why the wall could lift and not drag the slab with it; it is likely that there was no reinforcement (or very little) connecting the two, which would not be unusual. I would hope there is adequate connection between the pier/column below and the wall above, but the shrinkage may have yielded that reinforcement.