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.
- 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
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
I would gladly provide more information but haven't seen this a lot.
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
BA
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate 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
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
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
BA
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
If the reinforcing is too low, you now have both a flexural and a punching shear issue.
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
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
Similar arguments/questions for claim of punching shear failue.
RE: Cracks in Flat Plate Slab
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
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
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
RE: Cracks in Flat Plate Slab
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
- 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
These cracks are hairline (like shrinkage cracks) and the slab is only supporting its self weight.
RE: Cracks in Flat Plate Slab
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
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
RE: Cracks in Flat Plate Slab
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
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 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
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
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
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
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
I would assume that would be preferable.
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
BA
RE: Cracks in Flat Plate Slab
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
RE: Cracks in Flat Plate Slab
RE: Cracks in Flat Plate Slab
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
Dik
RE: Cracks in Flat Plate Slab
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
I got it. Because "d" becomes small? Interesting point.
RE: Cracks in Flat Plate Slab
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
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
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
- 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
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
Dik