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.

 

[dik]

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.

 

[kslee1000]

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

 

[JAE]

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

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.

 

[ali07]

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.

 

[BAretired]

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

 

[Den32]

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.

 

[Den32]

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.

 

[Tomfh]

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...

 

[msquared48]

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

 

[aayjaber]

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.

 

[csd72]

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.

 

[hokie66]

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.

 

[ron9876]

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.

 

[kslee1000]

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.

 

[ali07]

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

 

[ron9876]

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.

 

[dik]

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

 

[TXStructural]

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.)

 

[hokie66]

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?

 

[AggieYank]

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

I would assume that would be preferable.