Flexural cracks or punching cracks?

[MegKCB]

Hi All. We have been asked to write a report on a flat slab which has lots of cracks on the top surface, mainly at the column positions. I have uploaded a photo - do you agree that this is flexural cracking? This photo is taken on the top surface of the slab, at the column position. It is a square 450x450 reinforced concrete column. I am trying to confirm that this isn't punching shear cracking that we are seeing?

 

[LittleInch]

It's always best to also load this as an image.

Zoomed in a bit.

Got a drawing to show the re-inforcement?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tomfh]

Looks more like flexural/shrinkage cracking than it does punching shear. The steel is quite visible as crack pattern. Like a plastic settlement crack pattern. Maybe the steel is too high, or maybe the mix wasn’t ideal.

 

[lexpatrie]

I'm not sure I've seen anything about the sequence of cracking in punching shear. You mentioned this is the top surface. What's the bottom look like?

 

[JoshPlumSE]

Can't tell from the picture.

What I would do is mark the locations of the column on the top surface of the slab. Then you will be able to see where the cracking is relative to the columns.

I was on a job (relatively high profile building that had to get demolished post Northridge). The engineer hired an agency to "laser scan" the top surface of slab to plot all the cracking. It was pretty neat..... But, when we examined the mapped cracks we realized there were definitely some VERY MINOR cracking right about where the punching shear cracks would be expected. Without that specialized crack map, we wouldn't have seen it.

 

[Compositepro]

It look to me like the concrete flowed up through the reinforcement causing aggregate to separate out of the mix above the bars.

 

[MegKCB]

Thank you all for your responses! I should have given more context. The slab is situated in a warehouse and is being used to store product. The tenant reported that when he loaded the slab with his product, cracking noises were heard. Windows which are situated below the slab, around the perimeter of the slab, cracked, so there was obviously excessive deflection of the slab. The tenant reported cracking noises but it is unclear if this was just from the windows, or also from the slab itself. It is also unclear if these cracks above the column were there before the slab was loaded.

There are no obvious cracks on the soffit of the slab, however it is pretty high up so we aren't able to have a close look.

We have hired a company to scan the slab to determine reinforcement positions as well as a surveyor to determine deflections.

The slab is large and contains many equally spaced bays. The cracking in the photo is evident at many of the column positions.

 

[MegKCB]

Another example

 

[hokie66]

All you have told us is the column size. If you want better informed responses, give us a description of the slab, span, thickness, how it is reinforced, design loading if you know. Flat slab or flat plate? Dimensions of the rectangular crack pattern may also help.

 

[bugbus]

I think punching shear normally produces a much more radial crack pattern than what's shown here. Even with a square or rectangular column below, the failure surface is usually also more of a circle/cone.

These cracks look like they started as plastic settlement cracks given the very regular orthogonal pattern, which have maybe widened under flexure.

The audible cracking while loading the slabs may just be normal flexural cracking occurring elsewhere. I used to work in a lab testing full-scale concrete elements, and the cracking is surprisingly loud.

 

[HTURKAK]

My points ,

- Apparently these are flexural cracks for excessive hogging moments . The crack patterns implies the top reinf. mesh
spacing is 150 mm . Moreover , one can estimate the 450 mmX 450 mm size column location .
- The cracks developed in both directions so the reinf. and span should be similar in bothe directions,
-The cracks developed due to overloading as you have stated ,

Pls provide more info . regarding the deflection, cracks at midspan, reinf . picture of the bottom ..to get better ideas if repair could be an option.
...


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[MegKCB]

Thank you for all your comments.

@HTURKAK The slab thickness is 300mm. It is a flat slab. The column spacing is 7.5m in both directions. I am waiting for the scan report but from the small bit of reinforcement which was exposed, it appears to be Y20 T1 and Y16 T2 at centres of 150-200mm.

Thanks @bugbus, those were my thoughts.

 

[lexpatrie]

Damage Evolution in the Event of Impact-Punching Shear of RC Flat Slabs, D. Z. Yankelevsky, F.ASCE, Y. S. Karinski, and V. R. Feldgun, Journal of Performance of Constructed Facilities, Volume 37, Issue 4, August 2023

I would really hesitate with this narrative, and comparing the loud cracking from what is probably a reaction floor in a research facility versus an in-place concrete structure of unknown provenance, curing, pour quality, W/CM (or W/C if olde enough) ratio, or intended capacity versus potential overloading, well, when the loud noises coincide with the loading, something is cracking. The question, as yet unresolved here, if is that cracking is indicative of damage and imminent collapse, cracking indicative of structural damage that is not in danger of imminent collapse, or cracking that does not affect structural strength, well, that's on somebody besides this internet forum.

The cracking you observe on the top side might be non-structural, but that narrative suggests something significant happened here in addition to, or as well as, the cracking you note of the top side.

The design approach on punching shear is to look at a distance d or d/2 (rebar depth) from the face of the support, that does NOT mean, however, that that is where the cracks will appear, that is a design convenience (code provision) that is relatively easy to calculate to provide a safe design based on research.

If the structure is not within those researched parameters, the shear provisions won't yield reliable results. On taller structures the column concrete strength is typically larger than the strength of the floor concrete if the difference is too large, special attention is to be paid to the construction of that joint (e.g. ACI 318-95 10.15 it's probably moved since then). What is sometimes done, and is potentially permitted in the U.S. code is the concrete surrounding the column is made with the higher strength concrete that is being used for the columns, (provided the floor strength is too low). This is how it is approached in the U.S. Design codes.

Your reference to Y20 T1 doesn't mean anything to me, and the use of millimeters suggests you're not in the U.S., either.

You need to do your own research on this subject.

 

[hokie66]

You say it is a flat slab rather than a flat plate. If so, how thick and what size are the drop panels? Are T1 and T2 the two different directions of top bars, or what do they designate?

 

[MegKCB]

@lexpatrie No I am not in the US. T1 refers to the top layer of reinforcement, T2 refers to the layer directly below that, in the orthogonal direction.
I am doing my own research, thank you. I am not intending to base my report on the views of strangers on an internet forum, but thought it may be helpful to get the views of others. I think that's what the forum is for.

@hokie66 We refer to slab with no beams or drop panels as a flat slab. So potentially called a flat plate where you are?

 

[Tomfh]

What did they put on the slab?

 

[MegKCB]

@Tomfh Paper. They store documents.

 

[Tomfh]

Do you know the load they applied? Tonnes? Kpa?

 

[hokie66]

Yes, a flat slab has drop panels. A flat plate has none.

 

[bookowski]

@Meg - There's a good webinar for free on youtube by ACI (American Concrete Institute), search for: Identifying, Evaluating, and Correcting Punching Shear Deficiencies in Flat Plate Construction. It's a slimmed down version of a more extensive paid course they have. It will be helpful for your situation.