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Cracks in Flat Plate Slab
6

Cracks in Flat Plate Slab

Cracks in Flat Plate Slab

(OP)
Two-way (Flat Plate) slab poured back in middle of December is just now showing cracks in last 2-3 weeks (according to GC).

- 10" thick Flat Plate Roof Slab
- Column bays are 24' x 26' (c-c)
- Cracks have occured on Top Surface only at every concrete column location.  Top Cracks follow outline of concrete column below, then radiate from corners, 45 degrees out.  Will try to upload photo.  
- Cracks are very small, appear to be shrinkage-cracks.  Basically no cracks on bottom surface, and no visible deflection/sagging
- We went back and checked our design and did not find any issues.  
- Cracking has occurred at Top surface of Roof Slab (Exposed Parking) - we have the SAME slab design (thickness, span, reinforcing) at the Fourth Floor Parking below with NO cracking/issues.
- Concrete Contractor says they left forms on for 4-7 days, and they placed reinforcing and slab thickness correct
- Design used 5000 psi concrete, 28 day breaks came in at 9000-10,000 psi, 7 day at 5000+ psi.

Anyone have any ideas or seen something similar?  Thanks.

RE: Cracks in Flat Plate Slab

I may be wrong but it looks like punching shear cracking. As to why this happens on the roof only (and not the other floor), maybe the exposed slab has suffered significant temperature shrinkage..?

I would gladly provide more information but haven't seen this a lot.   

RE: Cracks in Flat Plate Slab

Since the column stops at the roof, the slab moments are higher and this usually increases the unbalanced shear.

RE: Cracks in Flat Plate Slab

The steel may be low which would lead to both flexural problems and punching shear problems. Try to check the depth and quantity of reinforcement. Maybe pulled the shores too soon.  

RE: Cracks in Flat Plate Slab

2
Could be curing conditions were not ideal or the top mats were placed too low.  Hard to say.  Are the slab deflections larger than those of the floors below?


BA

RE: Cracks in Flat Plate Slab

I agree with qcjr. I have only seen this one other time, 37 years ago, and I think that was a punching shear problem.

RE: Cracks in Flat Plate Slab

The cracks do not suggest punching shear.  They suggest tensile stresses in both directions causing radial and circumferential strain in the top fibers of the slab.  

Punching shear failures are rapid and do not show telltale cracking around the column before failure takes place.  The column reaction is only 24*26*125 = 78,000#.  What is the size of the column?  Punching shear stress must be pretty low.  


BA

RE: Cracks in Flat Plate Slab

BA:

Your reasoning seems sound....
Load sounds low enough.
cracking indicates tensile stresses at right angles
punching failure is sudden with no warning

but the crack pattern sure looks to me like something that I would expect if the load (column) was trying to punch thru. Could this occur, if the top layer of bars were set too low?

If it was flexural cracking only, I would expect somewhat concentric circles.

 

RE: Cracks in Flat Plate Slab

I agree with BAretired. The cracks are negative radial lines created by tension in top of the slab. You have to check the area of the reinforcment in top of the plate.(columns)

Do it by the yieldline-theory and check the tension in the reinforcement. If the stress is greater than the yieldstress for the reinforcement you have a bearing problem. If the stress is greater than 250 Mpa you will have a crack-problem.

Chrede  

RE: Cracks in Flat Plate Slab

It would be prudent to locate the top steel by non-destructive testing to confirm that it ended up in the right place after the pour.   


BA

RE: Cracks in Flat Plate Slab

A thought... Maybe the contractor has placed the reinforcement to low?

RE: Cracks in Flat Plate Slab

Just another vote for flexural cracking.  Punching shear cracking on the top surface would be further out from the column below, and would be catastrophic.

If the reinforcing is too low, you now have both a flexural and a punching shear issue.

RE: Cracks in Flat Plate Slab

Tend to agree with BA. A few other points: Slab poured in mid-December; where and what kind of temperatures. Could cold weather have reduced strength development? Forms removed a little earlier to get home for the holidays?  

RE: Cracks in Flat Plate Slab

I would say this is a restraining crack, if no deflection beyond what is anticipated. Sharp temperature drop may have occurred during, or shortly after, the concrete pour. The temperature drop at the column (thick) is at a slower rate than the surrounding slab (thiner), provides this is an area with high negative stress in both direction. This is not a good news, because you have concrete failed even before it has cured - a permanent defect.

 

RE: Cracks in Flat Plate Slab

Often the stiffness of the columns at an intermediate level will lead to a higher moment at the slab with the moment in the columns distributed by stiffness.  Slab moment, however, can be higher.  Also design loads can be higher.

Cracking looks flexural, with the column strips in both directions.  Even flexural cracking can initiate at d/2 from face of column.  I'd do a quick check on the area of steel at the column face.

Dik

RE: Cracks in Flat Plate Slab

My difficulty to accept it as "flextural crack", is because for that to occur, the bars have to be stretched pretty much. What causes the over stretching while roof has not yet loaded, construction weight. If it is the case, either the roof is far under designed, or the contractor didn't provide bars accordingly.

Similar arguments/questions for claim of punching shear failue.  

RE: Cracks in Flat Plate Slab

Den32 said:

Quote:

- Concrete Contractor says they left forms on for 4-7 days, and they placed reinforcing and slab thickness correct
- Design used 5000 psi concrete, 28 day breaks came in at 9000-10,000 psi, 7 day at 5000+ psi.

Stripping forms after four to seven days may be okay if reshores are immediately installed, but if reshores are removed before two weeks, even in ideal curing conditions, it is too early.  Also, reshores should be maintained in the next two slabs down until the upper slab is cured properly.  In this way, the weight of the roof slab can be distributed amongst at least two other slabs.

Removing slab support early is a possible, even probable cause for the observed cracking.

The 28 day strength seems high relative to the 7 day strength.  Get an opinion from the testing company as to why this would be the case.


BA

RE: Cracks in Flat Plate Slab

If they have stripped form at the same time interval, why there are no cracks at lower levels. As this is poured in December and others floors might have poured at some higher temperatures, freeze/thaw action cause the cracks. Removing forms 4 to 7 days are fine, but the cylinder strenght is laboratory strenght not achieved at site at particular time when forms were being removed.

RE: Cracks in Flat Plate Slab

If the roof level, then may be subjected to more drying, etc. It might be that the floor design load is only a fraction of the roof and that since no early time loading has occurred, there is added reinforcing to accommodate the higher live load.  Could be all sorts of things... time to review the flexural design and check shear stresses.  cracking also critical for rectangular columns that are thin and long.  Flexural stresses are dramatically increased at the narrow edge of the column  I often add double 25M bars in the bottom over the column, parallel to the long face.

RE: Cracks in Flat Plate Slab

Shear failure is non-ductile, such crack generally is wide and deep.
 

RE: Cracks in Flat Plate Slab

I had this happen to me some time ago - the cracks I observed were much worse than what you posted.  Here's the thread link:

thread507-82388: Cracks in Two-Way Flat Slab

I don't think we came to any specific conclusion.  The garage has been performing well since that time - even with a topping over it.

 

RE: Cracks in Flat Plate Slab

Take cores in slab at those cracked locations and verify the strenght. Your cylider strenght might not be at strcuture, that are laboratory cured specimens.
 

RE: Cracks in Flat Plate Slab

There are several possible reasons for the cracks:

1)   Shoring was removed too early.
2)   Curing conditions were not ideal.
3)   Water-cement ratio was too high.
4)   Top mats are too low.

It is unlikely that anything useful can be accomplished by taking strength tests at this time.  The major concern is that the reinforcement was placed and maintained in its correct position during the pour.  This should be checked using non-destructive methods.  If the steel is correctly placed and the design is sound, I would be inclined to forget about the problem.  The structure will continue to perform well over many years.

If the top mats are depressed, the design should be reviewed with that in mind and remedial measures taken if warranted.

Cracks in reinforced concrete structures are not unusual and do not spell disaster.  In fact, cracks are commonplace in concrete structures.  As a matter of fact, reinforcement does not start to work until after the concrete has cracked.


BA

RE: Cracks in Flat Plate Slab

(OP)
Thanks for all the responses.  Will try to respond/clarify as much as possible here:

- This cracking has occurred at ALL columns (not just a few)
- Concrete columns are typically 20" x 20" square
- Don't believe it is a punching shear issue b/c of reasons stated above, plus there is absolutely NO cracking on bottom of slab around the columns
- Slab was at design strength (or better) at 3 days per the Cylinder Breaks (Field Box Cured)
- Temperature at time of pour was about 50 degrees with 8 mph wind +/-.  Contractor is claiming cracks have just appeared in the past 3 weeks or so.
- In process of re-checking our structural calcs. for this Flat Plate Design.  Typical Column Strip Top Reinforcing is 18-#7 in short direction and 13-#7 in long direction, 1-1/2" clear from top.  
- It is a Parking Garage Roof, so we had 1-1/2" clear to reinf, epoxy bars, corrosion inhibitor and A/E
- Roof Slab is same design as Fourth Floor Below with the exception that the Roof Slab is SLOPED.  Neither Roof nor Fourth Floor slab exhibits any deflection, sagging, etc.

 

RE: Cracks in Flat Plate Slab

(OP)
JAE and BARetired - yes - currently suspecting the slab thickness and/or the location of the Top Reinforcing.  May have to start with some non-destructive testing to see what is out there, then go from there.

These cracks are hairline (like shrinkage cracks) and the slab is only supporting its self weight.

RE: Cracks in Flat Plate Slab

The pattern matches fan mechanism (see below), suggesting flexural stresses are a primary factor. Shrinkage stresses will contribute too.



If the cracks are only hairline, then why is it such as issue? Pretty much all slabs have hairline cracks at peak moment regions...


 

RE: Cracks in Flat Plate Slab

1.  Pressure epoxy into the cracks to seal them from freeze thaw action and return the matix to a visually uncracked condition.
2.  Monitor the cracks to see if they are expanding or stable.  This would be under live load too.
3.  If they are expanding, you have real problems.  If nogt, no worries.

Mike McCann
MMC Engineering

RE: Cracks in Flat Plate Slab

The cracking patterns show the columns are pushing up this is why you do not see crack at the bottom, may be a close visual or some magnification will show the lower side has micro cracks. If the Columns were sinking (pulling down you will see more cracks at the lower part. You can see clearly the projection of the top of the columns with the cracks around it and no cracks on the 20"x20" part. Most likely forms removed earlier than it should have been.

 

RE: Cracks in Flat Plate Slab

I would suspect that they are caused by a combination of flexure and shrinkage/thermal movement, although for the effects of shrinkage/thermal I would expect the external columns to crack first as these are weaker than the slab as a whole.

The circle in the middle looks too small for a shear crack. A shear crack would normally go all the way through and show some signs of vertical movement.

RE: Cracks in Flat Plate Slab

One other thing to look at closely is whether the columns were poured above the slab soffit.  I have arrived on site for a slab pour only to find the columns projecting 2 or 3 inches into the slab.  The contractor didn't see a problem with that, thought it made for a neater job.

If you do determine that the cracking is flexural, but the slab is adequate as is, I think it needs some type of membrane to protect the reinforcing.  After all, this is a roof, and cars will be parked under it.  When leaks start to damage Mercs and Beamers, you will hear about it.

RE: Cracks in Flat Plate Slab

I certainly wouldn't move on without finding out what caused the cracks. This isn't a normal cracking pattern and these aren't the micro cracks that always form with reinforced concrete.

I feel that coring to verify in-place concrete strength is prudent.

You said that this slab was sloped and that you used the same design as lower floors. Unless you sloped the bottom to match the top slope the slab design likely shouldn't be the same. Also the placement of top reinforcement in a sloped slab often ends up not what is assumed in design. Often the contractor will chair for the low point of the slab and use the same chairs throughout.

RE: Cracks in Flat Plate Slab

It's already cracked, remove the concrete cover at one location would tell some story. Try light hand chipping, to preserve the concrete below, the do another observation.

RE: Cracks in Flat Plate Slab

Contractor normally placed extra concrete compomising the cover. It can be checked that cover might have exceed at all sloping locations.

RE: Cracks in Flat Plate Slab

aliO7 that isn't my experience. The CD's would tell them what to do. For instance 10"/8" slab or 12"/10" thickness neither of which would match the lower floor.

RE: Cracks in Flat Plate Slab

One of the first projects I was involved with as a graduate engineer was to 'supervise' a load test for a concrete slab that had cracked and had been repaired using a bonded topping with reinforcing steel anchored to it.  The failure was caused by the top mat supports failing and the mat was on the bottom reinforcing.  It was common, back then, to have bottom reinforcing with top mats only and no middle band top reinforcing.

The failure had produced a 'classic' yield line pattern of cracking.  The contractor was less impressed when I marked the failure pattern out with a marker to emphasise the pattern.  The facility was a technical school and the 'teachers' would bring their classes down to see the failure.

The pattern was better defined than that shown, but somewhat similar.

You should get a cover meter and determine the size and depth of the reinforcing to see that placement is OK.  Too much cover for the top reinfoccing reduces the flexural capacity but has little effect on the punching shear.  I haven't done any sums on the problem, but a 10" slab for the spans noted (off the top, wouldn't run up any flags).  A good cover meter can determine both the cover and bar size, and the design can be re-evaluated.  Repair may be a simple matter of epoxy injection of the cracks.

Dik

RE: Cracks in Flat Plate Slab

Your concrete was too stiff due to the high early strength.  Cracking is very common in high strength concrete subjected to flexure like this.  It looks like the very stiff concrete simply cracked under dead load flexure.  The column acts like a point load and the cracks focus at the inflection point it creates.

Frequently, there are minimums AND maximums for highway bridge concrete strength to reduce this type of early cracking (which will lead to early deterioration in freeze-thaw and chloride-exposed surfaces.)

RE: Cracks in Flat Plate Slab

Just a few more thoughts/disagreements:

1.  Don't give too much credence to the contractor's report that the cracks only occurred recently.  Contractors don't look for cracks.
2.  Somebody said depth to reinforcing doesn't have much affect on punching shear capacity.  Codewise, shear capacity is directly proportional to depth.  But I still think it can't be punching shear.
3.  One more crazy thought.  Your contractor wouldn't have been stupid enough to pour the top lift of columns monolithically with the slab?

RE: Cracks in Flat Plate Slab

hokie, what is the problem with pouring the top lift of the column monolithically with the slab.

I would assume that would be preferable.

RE: Cracks in Flat Plate Slab

Differential shringage if cares not taken.

RE: Cracks in Flat Plate Slab

Den32 please let us know when you verify the depth to top reinforcement. I would like to know what the outcome is.

RE: Cracks in Flat Plate Slab

If the columns is poured monolithic with the slab, column has not achieved required strength to carry the weight of the slab.

RE: Cracks in Flat Plate Slab

Well, as long as the formwork is left in place is that really a problem?

RE: Cracks in Flat Plate Slab

The mothod is desirable. For this thread, at issue is its restaining effect and differential shrinkage rate may have contributed to the cracks.

RE: Cracks in Flat Plate Slab

As far as I know the only problem would be the column concrete will settle as it dries and that will leave voids in the column and possibly just under the slab. Don't see how that would have anything to do with the cracking pattern in the photos.

RE: Cracks in Flat Plate Slab

Pouring the column monolithic with the slab would be poor practice because of consolidation and gaps in the concrete.  But there is no indication that this contractor poured the columns and slab together.

BA

RE: Cracks in Flat Plate Slab

The slab dispate heat faster than the thicker column. Also, at column region, the congestion in steel helps to limit the shrinkage as opposed to more free room for the slab. So it is not a big surprise to see cracks around this location if quick/sharp temperature drops were allowed to occur without protective measures.

Also, for the voids you have mentioned, doesn't it creates weakness prone to crack if close to the surface?   

RE: Cracks in Flat Plate Slab

I don't think that will be the cause of cracking in this condition. The cracks extend well beyond the column and almost have to be at least related to flexure. The concerns that you are discussing may occur but the effect is likely very limited.

RE: Cracks in Flat Plate Slab

all act like potential suspects (contributors), unless proven otherwise.

RE: Cracks in Flat Plate Slab

Sorry for taking a while getting back to answer AggieYank's question myself, but I was sleeping.  As I was the one who said the practice of pouring a column and the supported slab monolithically is stupid, I should explain my reasoning.

When you do this, the column concrete consolidates under its own weight a lot more than the slab concrete.  Therefore, the bit of concrete directly above the column can drop with the column, leaving the slab concrete supported on the formwork.

This is plastic settlement cracking, the same type cracking sometimes seen reflecting above top reinforcement in thick footings, pile caps, etc. which have not been revibrated to prevent this secondary consolidation from leaving plastic cracks.

Note that I didn't say this happened in Den32's flat plate, but it was one possibility, however unlikely.  If these plastic cracks existed, they could have formed stress risers to aid in formation of the radiating flexural cracks.

RE: Cracks in Flat Plate Slab

hokie66... it's my understanding that although the depth is used to calculate the punching shear capacity, that it has only a marginal effect on it... and I use it when I calculate punching shear to be code compliant.  My earlier post where I noted that the top steel ended up on the bottom, would indicate a punching shear capacity of perhaps 10% of that required.  This would have caused a 'punching' collapse of the floor plate and this did not happen.

Dik

RE: Cracks in Flat Plate Slab

dik:

Are you using S&T method in analyzing punching shear capacity? Not sure how could shear capacity drops to 10% of that required when the top steel ends up on the bottom.

RE: Cracks in Flat Plate Slab

dik:

I got it. Because "d" becomes small? Interesting point.

RE: Cracks in Flat Plate Slab

The failure surface in punching shear is a truncated pyramid around the column.  I doubt that the effective depth of the top reinforcement plays much of a role.  One feature that would play a significant role, however, is the practice mentioned by hokie66 where the contractor casts the column a couple of inches higher than the underside of the slab.  This means that the area of the failure surface is substantially reduced.

This has happened to me only once in the past.  I stopped the pour and ordered the contractor to chisel away the excess concrete around the periphery of each column so that the full depth of the slab was available for punching shear.  

Sometimes scary things happen on job sites.  It is particularly scary when the slab is post-tensioned and punching shear becomes much more critical.

BA

RE: Cracks in Flat Plate Slab

Shear is a diagonal tension phenomenon, without reinforcing, the shear capacity is highly questionable, though the equation does not consider it.

However, as stated before, I don't think punching shear has much to do with these cracks, although couldn't eliminate the possibility completely.   

RE: Cracks in Flat Plate Slab

Den32,

Yes, it looks like flexural cracking.

How did you distribute your top reinforcement over the width of the panel?

RE: Cracks in Flat Plate Slab

(OP)
Sorry haven't been on this lately.

- We suspected many of the same items described above (top of concrete not flush with bottom form, slab poured too thick or too thin, etc.)

To answer some of the above questions:  
- Yes, the sloping roof slab was specified as 10" deep, so top AND bottom of slab was supposed to slope
- Concrete Contractor and Inspector both say the top of column did NOT protrude above forms, top and bottom reinforcing was in correctly
- Top portion of columns NOT poured monolithic with slab
- Evenly distributed top and bottom reinforcing in the column and middle strips (at Column strips: approx. #7 at 12" o/c top reinf. and #6 at 12" o/c bottom)

- We are trying to verify the actual slab thickness.  Being too thick seems like it may cause this cracking b/c "set height" standees were probably used for the top reinf. in this 10" slab - if slab poured to say 13", that would give 4-1/2" clear to top reinforcing - which wouldn't be effective in holding the top cracks together at highest flexure
 - Will defintely post results when we figure it out

Thanks for continual input.

RE: Cracks in Flat Plate Slab

Den32,

That explains part of the problem. The column strip reinforcement should not be equally spaced over the column strip width. There should be a concentration over the column and D either side. Clause 13.5.3.4 suggests this but leaves it to you to decide how much. Other codes specify specific ratios to be concentrated in the column head area for 2 reasons, moment transfer to the column and also because a lot of the moment is concentrated in this area.

EG British code sayys that 2/3 of the column strip reinforcement should be in the middle half of the column strip over the column  (ie the spacing over the column is half the spacing of the rest)

EG Australian code requires 25% of the total reinforcement (column + middle strip) within D either side of the column.

RE: Cracks in Flat Plate Slab

and you can go higher depending on the width of the column.  If a column were 12" wide, you could use 30% and if you have drop panels you can increase the area of steel over 1/2 the width of the drop panels.  One of the early firms I worked with had modelled the moments using deflection on a plastic model to determine the concentrations.  Per Christofferson developed the distributions.

Dik

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