cracks in a flat slab

[nidhimenon]

Dear All

I have observed few cracks for the flat slab system. I am attaching the photos herewith. It is a very peculiar crack and is a through crack i.e. propagates through the full depth. To me it looks like a shrinkage crack.

Please advise

Thanks in advance.

 

[TehMightyEngineer]

Hard to tell from the picture. Do you have any other pictures? What's the history of the slab?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[hokie66]

If it is full depth, it is probably a direct tension shrinkage crack.

What is going on with that construction? It appears that a section has been cut out, and you are preparing to cast a new section at a lower level.

The creation of the opening would have caused a stress riser for the preexisting tension in the slab, thus the crack formed at that position.

We certainly can't tell from here the severity of the situation.

 

[rapt]

Bit hard to decide anything from that picture!

 

[nidhimenon]

Hi all

Hereby I am attaching the snapshot of the drawing where the exact location and profile of the crack is marked. Also the surrounding portion is shown for clarity.

Regards

 

[rapt]

Were the spans beside the un-poured area assumed to be continuous interior spans or end spans?

Is the slab RC or PT?

If PT is it banded/distributed distribution? Which directions?

When did the crack happen? Were any large construction loads applied?

Is the crack full depth or only in the bottom?

We need information if you want any sensible answers.

 

[nidhimenon]

Hi

The spans were designed as a continuous span. It is a RC slab. As per the site staff, crack happened 2-3 weeks after the shuttering was removed.
The crack is full depth.

Regards

 

[hokie66]

That snapshot is clear as mud. But I think it is safe to say two things: 1) Because the crack is full depth, it is a restrained shrinkage crack, and 2) Because the crack is wide, there is not much reinforcement across the crack.

 

[Ron]

I think hokie66's first post is right on target....release of tensile stress or residual restrained shrinkage stresses by opening the section of slab for repair.

 

[Ingenuity]

nidhimenon, can you explain to us what the magenta-colored polygon represents?

Is it a slab set down of some magnitude, a full-depth slab opening?

 

[rapt]

Ingenuity,

From the original picture, it is a full depth opening for some reason.

nidhimenon,

If full depth, then it is shrinkage related, but could also be related as well to overstress due to that area being reinforced as continuous but actually being a simply supported end combined with shrinkage. Agree also that it looks short on reinforcement. How wide is the crack? What are the span lengths, how much reinforcement is in there and what is the reinforcement layout?

It would make our life easy if you told us everything and engineer would need to know to comment on something like this.

 

[Ingenuity]

rapt and hokie66,

I think you are correct - probable combination of restraint-to-shrinkage and the end condition created by a dominant opening.

Still cannot understand the slab opening with two columns and an extended drop panel in the middle of the opening, as per the ORANGE clouded area below:

 

[dik]

If those are drop panels, could the slab be rotating about the bottom of the drop panel, putting the entire slab cross-section in tension and the cracking may be flexural tension cracking?

Dik

 

[Jeevan reddy]

May be it was failure due to under reinforcement

 

[TehMightyEngineer]

Jeevan, you realize this thread is from July?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Jeevan reddy]

mostly, we can see the punching failures which leads to formation of cracks in the flat slabs