malformed existing flat slab

[kipfoot]

As part of a Property Condition Assessment (under ASTM E2018) I looked at an 1980's office building (eastern US) with significant/noticeable deformation in the floors.

The building is a 5 story concrete structure with a flat slab with drop panels. The column bays are 25'-0" x 25'-0" and the slab is 9" thick. In one representative area with no live load, I measured slab elevation at the middle of the bay to be 2" (+/- 1/4") lower than near the column....slightly higher in the end bay.

I did a quick direct design method review and found that the reinforcing (as shown on the structural drawings) is what I'd expect it to be. I was able to look at the underside of this area and found no cracking, visible distress, or evidence of poor concrete.

After mulling this over, I believe that this building was built this way, likely cast on deflected formwork/shoring.

So, my question is: Can you share experience with something similar in an existing building that would support or refute my conclusion? Can you direct me to articles on flat plate shoring that discuss the issue of deflection or case studies?

 

[dhengr]

Kipfoot:
They may have removed the form work to quickly, without any reshoring. Although, I think you would then see at least some tensile and shrinkage cracking on the underside of the slabs. Does the top of that slab show the same 2" settlement, or is it fairly level? Is there any top cracking around the drop panel areas? Can the owner live with this floor condition, maybe some light wt. leveling topping? Does the hurt the ultimate strength of the slab and structure.

 

[kipfoot]

dhengr,
I took the measurements from the top surface which is covered with a thin carpet. The owner and tenants have lived with the perceptible slope in the floor, but they're now trying to sell the building. They're not trying to fix it

I just want to correctly identify the issue as 'just' a serviceability issue.

I'm also looking into the possibility of creep being a factor.

 

[msquared48]

At L/33 +-, I assume this is an unbonded PT slab and not standard CIP? If it is PT, make sure there is no evidence of water penetration into the tendon runs.

More than likely, the problem is what dhengr says, and long term creep could also be an issue. I agree.

Mike McCann, PE, SE (WA)

 

[kipfoot]

msquared48,
It's regular old cast in place. f'c = 4000 psi. As you suggest, L/33 puts it at the ACI table 9.5(c) minimum.

I just noticed this note which I find interesting. (I don't know how to embed the image):
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1493234952/tips/note_skina1.bmp[/url]

 

[JAE]

I had a 12 story building where the main lower floors were two-way CIP non-prestressed slabs with drop panels.
The floors had deflected about 3 to 4 inches (1970's to about early 2000's - a 30 year time span).

The case here was that the slab depth (10") was not up to the amount required by Table 9.5(c)...with almost 40 ft. spans. So the creep deflections had definitely kicked in.

In your case - not sure why 2" of deflection (L/150) would have occurred other than formwork sag or perhaps the particular concrete mix was more prone to creep?
I know that smaller aggregate sizes add to shrinkage and cracking - perhaps if you have a mix with small aggregate you'd have more long term creep?



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[BAretired]

It is also possible that the top bars were either not placed correctly or were trampled down while placing the concrete. I have seen one case where top bars were found to be three inches below specified depth in an eight inch thick flat slab, resulting in excessive deflections.

BA

 

[hokie66]

I agree with BA. This is a common construction defect. If you lift some of that "thin carpet", you may get a better idea of what has happened. And a depth meter should confirm if this is an issue.

 

[kipfoot]

Thanks for the replies.

JAE: I did take note of small aggregate (1/2" maybe) at a chipped location.

BA: Thanks. I'll ask to have some carpet pulled up to have a look at the top of the slab. I would think that I'd see some cracking there if we had a moment distribution event. It's a typical condition throughout the building, so they would have had to have been pretty consistent in stepping on the rebar.

That said, I've seen consistency myself with mis-placed wall reinforcing. Once they start wrong, they just keep going.

hokie: I'd have to expand the scope to include non destructive testing, so I'll give some thought to how to best lay out the next steps.

 

[rapt]

I just did a quick calculation on an RC flat slab with drops of those dimensions and get a mid-span deflection in an end span with no LL ever applied of 49.5m so basically 2". Corner panel would be closer to 60mm, so nearly 2.5".

 

[Ingenuity]

and get a mid-span deflection in an end span with no LL ever applied of 49.5m so basically 2".

Long-term (including creep) deflection under SW loading with some SDL?

 

[hokie66]

I understood the OP to say the 2" deflection was in an interior bay, with more deflection in the end bay.

 

[kipfoot]

I understood the OP to say the 2" deflection was in an interior bay, with more deflection in the end bay.

Yes, that's correct. It's a less at the floor above, 1 1/2" at the end bay.

 

[BAretired]

If the slabs were originally cambered, deflection cannot be accurately measured years later.

BA

 

[rapt]

Missed the internal bit. Depending on concrete strength (assume 3000psi back in the 1980's), drop panel dimensions etc, I can easily get 1.5" deflection for an internal bay with no LL.

Just because the slab currently has no live load, does not mean that it never had any. And it would have been stressed significantly during construction acting as a back-propping floor for the floors being poured above. That could increase my estimates even more.

So 2" +- 1/4" is not far out!

 

[JAE]

rapt, most all of my concrete structures in the 1980's were 4,000 psi concrete. Columns were generally 5,500 psi - 6,000 psi.

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[kipfoot]

rapt: thanks for the sanity check. Though the L/150 or so deformation seems out of whack with a thickness that's at the ACI guideline of L/33. I think there's some creep involved, too. If your calc accounted for creep, then, yeah, that's it.

JAE: yes, drawings called for 4,000 psi. (columns also 4,000 psi)

As I mentioned, I was brought in as part of a general assessment that was part of a potential buyer's due diligence and I didn't start with the scope (fee) to do a soup-to-nuts investigation. However, once I started asking about pulling up carpet and digging deeper I was awarded with a structural report that was done a year ago.

That engineer did some gpr investigation and concluded that the rebar is in the right place. They also performed rebound hammer testing (ASTM C805) to confirm that the concrete exceeds the specified strength.

I found that my yellowed Nilson Winter text was a helpful refresher in this process. Anyone who stumbles on this thread in the future might want to look at ACI 435.8R-85: Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections. I didn't buy it but even the abstract supports other stuff I've found about how applying load to early stage concrete in the process of shoring and reshoring can lead to more creep in the long term.

 

[JAE]

"applying load to early stage concrete in the process of shoring and reshoring can lead to more creep in the long term."

That is interesting.

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[rapt]

JAE,

Yes, you get increased SW deflection due to the low concrete modulus at time of reshoring if it is not done properly, and early creep effects due to early applied compression stresses, but from my calculations it is not a huge amount.