Cracks in Flat Plate Slab 6

[Den32]

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.

 

[Den32]

Photo

 

[Den32]

Another Photo

 

[qcjr]

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.

 

[slickdeals]

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

 

[ron9876]

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.

 

[BAretired]

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

 

[jike]

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

 

[BAretired]

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

 

[jike]

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.

 

[chrede]

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

 

[BAretired]

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

 

[chrede]

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

 

[hokie66]

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.

 

[beton1]

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?

 

[kslee1000]

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.

 

[dik]

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

 

[kslee1000]

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.

 

[BAretired]

Den32 said:

- 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

 

[ali07]

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.