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.

 

[kslee1000]

Differential shringage if cares not taken.

 

[ron9876]

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

 

[slickdeals]

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

 

[JAE]

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

 

[kslee1000]

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

 

[ron9876]

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.

 

[BAretired]

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

 

[kslee1000]

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?

 

[ron9876]

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.

 

[kslee1000]

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

 

[hokie66]

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.

 

[dik]

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

 

[kslee1000]

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.

 

[kslee1000]

dik:

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

 

[BAretired]

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

 

[kslee1000]

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.

 

[rapt]

Den32,

Yes, it looks like flexural cracking.

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

 

[Den32]

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.

 

[rapt]

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.

 

[dik]

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