Large Deflections in Concrete Slab Without Cracking 6

[Rdar43]

I have an unusual issue on a project where there is a concrete slab that is deflecting 2.5" in the worst location only under construction loading. The two-way slab was designed using RAM concept and from the results, there should only be 0.6" of deflection occurring under construction. The area in question is a 33' span, the slab is 10" thick with an f'c of 5,000psi.

I've gone to the site and confirmed the deflections but strange part is there was no visible cracking on the top or bottom of the slab. The slab was poured all in 1 day and there were no visible construction joints in the slab either. I also noticed that there were larger than expected deflections throughout the slab, not just at the worst case condition. At 1st we thought that the contractor may have pulled the forms off to early as they pulled them 15 days and didn't reshore but the concrete break tests indicate roughly 4,000 psi at just 7 days (High-range ware reducers were used in the mix). It's also worth noting, the slab was poured 6 months ago and then there was a pause on the job over the summer so the 2nd story slab has yet to be constructed. The contractor stated that there were no materials that were stored on the slab over the pause and they noticed the deflections when water would begin to pool when it rained. Based on the inspection reports everything seemed to be correct regarding the rebar and pour. We have asked the contractor to reshore the entire slab until the source of the problem can be found.

Due to the lack of cracking my current theory is that there may have been an issue with the formwork while the slab was being poured that resulted in the slab deflecting before it ever started to cure. The issues is this doesn't fully explain why the deflection can be seen on the top and bottom of the slab or why the larger deflections are consistent throughout. If we can't determine the source our next step is to try and have testing done on the slab to see if there is a discrepancy between the slab and the concrete core results. Does anyone have any suggestions on what else could be the cause?

 

[rapt]

That span arrangement will change things.

We still do not know transverse span length.

Column stiffnesses will not have much effect with that span arrangement so no need to worry about how they work.

Is the reinforcement continuous top and bottom or confined to the tension areas on each face?

I assume 125/20 is LL and SDL.

 

[GC_Hopi]

I think 125/20 is DL/LL since 150 pcf times 10 inches is 125 psf and OP said the slab in 10 inches.

 

[Tomfh]

At 6 months load history I land on 2.66" of deflection:

I just checked it and get similar. Around 65mm / 2.5" depending on configuration.

 

[Rdar43]

The columns were detailed to not hook into the slab but similar to what rapt said, after modifying the fixity the deflections only slightly changed. In the transverse spans are 23',7',23' all supported on columns. GC Hopi is correct, it was 125DL/20LL.

In regards to the thermal loading I've never come across it until now but made a simple model similar to the image shown and was getting around 2.6 deflection as well but had it modeled as a 200' building. When I reduced it down to the building size I was getting around 0.8" deflection

 

[Celt83]

Are you able/willing to share a screenshot of the actual floor plan, some things are getting lost in the piecemeal text descriptions?

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[bookowski]

Rdar - In your original post you say that you were getting 0.6" deflection "under construction". I'm thinking that maybe this is where you are getting off track. When you say "under construction" that sounds to me like the day after the forms are stripped, i.e. with some construction loading, cracked but maybe not fully, and no long term effects. At this point, 6+ months out, you should basically be looking at total long term deflection - at 6 months you'll have a good portion of the lt effects baked in there and the slab weight is most of the total load anyway so I'd look there first.

In your analysis what was your total long term deflection? There's no way it would be 0.6" (that would imply L/660 for a 10" slab at 33ft, no way). 10" is very thin for a 33' span for any kind of typical deflection limits, at 33' even a 12" slab would be marginal. This is assuming 5ksi and minimal reinforcement as you stated (33' span with high strength/high mod concrete and jammed with rebar and you could do a bit better).

I'd take a step back - forget all the load histories, thermal, etc. and do some big picture thinking here. I'd say 1) Go grab any flat plate span/depth table such as this one, and you'll see that you're outside the typical recommendations. Granted those are ranges but that should be enough to at least make you suspicious that if you guys didn't do something special here then there's no reason to think your slab will outperform these ranges by much. 2) look at what your total long term deflection was supposed to be in your design. Not the "construction" step you are referring to, I mean total long term (5 yrs from now), does that jive with span/depth tables? It sounds to me like you're looking at cracked but immediate deflections, so expect a factor of say 2.5 to 3.5x that for long term, which at 6 months you are close enough to to not get bogged down in exact time history. The self weight of a 10" slab is the majority of the loading so expect to be near your total now.

What about strength? If you've got the capacity then this isn't the end of the world. You can probably muddy the waters enough by blaming delays, thermal etc. to have them level it (have to deal with that extra load), learn a lesson for next time, and move on.

 

[calvinandhobbes10]

For deflection, one item in your favor is that ACI requires L/480 for the portion of load applied AFTER attachment of the crack-prone elements....presumably drywall and finishes, in your case. It’s not total deflection. Meaning, one way to look at it is the clock hasn't started yet on your accrued problematic deflection (and your delay actually helps you more, letting creep accrue). Code also requires L/360 for live, which you will likely be fine for. That's it. Doesn't mean your slab isn't too thin, or that the 2.5" shouldn't be filled for basic serviceability and optics, but that's what the Code says.

 

[human909]

When I reduced it down to the building size I was getting around 0.8" deflection

Some far more experienced people than you or me have already done some quick calcs or models and have confirmed the 2.5" deflection. I'd stop persisting for the time being with you current approach and try another. I suggest you try some hand calculations or referring to span tables.

I'm a mostly a steel guy so I'm fairly inexperienced in calculating long term deflection of concrete slabs. But I can quite readily look up tables like bookowski, my favourite is Guide to Long-Span Concrete Floors It is easy to see your slab lies well outside of normal ranges.

(I'm sticking around in this thread because I'm aware that I DO need to learn more about long term deflection of concrete!)

 

[Tomfh]

around 2.6 deflection as well but had it modeled as a 200' building. When I reduced it down to the building size I was getting around 0.8" deflection

Post a sketch of the floor plan so we can understand what you’re talking about.

 

[Rdar43]

Below is a simplified floor plan of the slab

 

[Celt83]

With that arrangement of perimeter concrete walls your shrinkage restraint should definitely be set to severe.

I’d still expect the long term defl to be in the neighborhood of 2”+.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Tomfh]

I’m not sure how that arrangement improves things from 2.5” to less than 1”

 

[Celt83]

I land on 2" after load history considering temperature effects:
From the manual load history does not consider load patterning so the 20 psf construction live load would be considered over the whole slab, if they stacked material in the central bay only that would increase the deflection. Concepts method iteratively cracks the design cross sections if your number of design cross section divisions is too large not enough areas of increase curvature will be picked up it then averages these stiffness over each finite element, the analysis is also still linear elastic so any p-delta effects aren't captured. The mesh size itself will also impact the results a larger mesh will be inherently stiffer than a finer mesh.

I get 0.8" with just self-weight after a single 10 day load history (3ft mesh, 2.5 ft max section division, per the manual though results less than 90 days contain a lot of variability not captured by the analysis so the validity of the results is questionable):

Long term (250 days +/-, 10 days initial, 60-days of construction activity, 180-days of inactivity) with a 1 ft mesh and 1 ft max spacing for the latitude strip divisions, 2.27" total deflection:
again this wouldn't capture any patterning of the construction load. the 3' mesh size was a good estimate, this tighter mesh is an even better estimate but we still haven't captured everything that impacts the deflection.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[haynewp]

What potential problems have been brought up with the current 2.5” over 33 ft? What type of partition walls will be on the slab etc.. And are you confident in the strength of the slab?

 

[Tomfh]

made a simple model similar to the image shown and was getting around 2.6 deflection as well but had it modeled as a 200' building. When I reduced it down to the building size I was getting around 0.8" deflection

Did you make any other changes in the model? As Celt83's results indicate, the reduction in transverse span only slightly improves the overall deflection. It doesn't make sense that deflection would reduce from 2.6" to 0.8" by reducing the transverse span.

 

[cliff234]

Rdapt13 - I have the same question as Tomfl. Assuming that the span in question is an interior span, why would building length (in either the longitudinal or transverse direction) significantly affect slab deflection? (i.e., 2.6" versus 0.80"?)

 

[Brad805]

Did you have the construction team pressure wash the slab or anything to look for cracks? I would expect cracks in this slab design shortly after the forms were removed and by now I do not see how they would not be visible. The slab image posted is basically symmetrical about the vertical line, so I modeled 1/2 the slab with some arbitrary supports. The image below show the estimated cracking with only the dead loads.

I am not sure where the OP is located, but based on the 20LL, it seems it is not NA/AU/NZ. In NA I would expect a Peer review would start now. I am not sure how much more the OP will share. I am curious how others might approach a repair for this assuming they were engaged to look at this.

Ideas that come to mind:
1. Demolish and start over.
2. Hire Ingenuity to come up with a PT repair.
3. New beams??
4. Do nothing.

 

[rowingengineer]

Those are fairly expensive solutions to a deflection issue, assuming there is a deflection issue, while the eng-tips feels like it is too much, Generally deflection issues are engineering judgement on use of floor, if it was a carpark use would we do anything? If it was offices would we do anything? Deflections are normally around finishes and looks. deflections don't constitute a failure and unless the client had a specific contract around deflections, a long argument is ahead as to what is a failure.

There were a number of buildings in the early 1990's that had deflections in the order if 3 inches. In most of these cases the engineer had enough strength for the finishes to take up the deflection, with a self leveling grout or similar.

 

[Brad805]

2 1/2" deflection during construction is terrible and exceeds l/180. I am curious what Celt predicts the total long term creep would be. Adding mass would level the surface, but it will add to the problem as well. If one is going to play with span/depth like this one must be far more careful.

 

[Tomfh]

It’s an interesting discussion as to whether it really is a problem, but OP’s question wasn’t “is this a problem?”, it was “why do I have this problem?”.