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Cracks in Two-Way Flat Slab

Cracks in Two-Way Flat Slab

Cracks in Two-Way Flat Slab

I've got a parking garage repair project going and have a condition that I'd like to get some opinions on.

The garage floor is a two-way reinforced concrete flat slab (10" thick slab with drop panels at the columns).  The columns are steel W14's that are connected to the floors with steel channel shearheads that extend about 3 feet either side of the column (4 directions in plan).

The slab was originally covered with a waterproof membrane and a 3" concrete topping slab.  The floor below was a storage area and the garage floor was leaking from snow-melt.

Our project called for the topping slab to be removed, the membrane scraped off, a new topping slab to be placed integrally bonded with the structural slab, and a traffic bearing waterproofing membrane added to the top where the building owner could properly maintain it without tearing out concrete every time there was a leak.

My issue is that after removing the membrane, the structural slab shows numerous cracks extending radially out from the columns, like spider legs in all directions.  However, the cracks are primarily running down the length of the column lines, perhaps 4 or 5 parallel cracks spaced out within 3 feet of the column centerlines.

The cracks are very obvious as the scraping equipment has rounded off all the edges, making them look worse than they really are.  I think most are hairlline tension cracks but some do show some openness.

Also, the main concern is that there are some cracks that circle the columns, approximately at the point where the shearhead channels would end...about 3' out from the column.

The concrete, as viewed from below, looks beautiful, no cracks near the columns (compression zone) or in midspan regions.

Would there be a shear problem here?  I initially don't think so as the tension area around each column is assumed cracked  in flexure (basic reinforced concrete design assumption) and the shear capacity at a column should be based on the fact that some tension cracking will occur.

Any thoughts on this?  

RE: Cracks in Two-Way Flat Slab

I am also concerned about the crack pattern! Are you absolutely sure that there is no shear problem?

Did you compare the cracking moment of the slab to the design moments at the column and at midspan (where you observed cracking and no cracking)?

I would suggest taking some cylinders and doing selective exploratory work. Is there any evidence of structural deterioration? Were the top bars placed at the correct elevation in the slab. What is the concrete strength? Can you measure the various crack widths?

RE: Cracks in Two-Way Flat Slab

Appreciate the responses.

The concrete strength is not an issue.  The building has been in service, with constant garage use since 1971.  The cracks are only now revealed by the removal of the topping slab, which will soon be replaced.

There is no seismic issue (midwest USA).  The geometry of the drop panels and the reinforcing layout is all consistent with ACI chapter 13 (Two-way design).

What appears to have happened is that the top column-strip bars were placed very close to the top concrete surface (very little cover and in some cases no cover).  

As I mentioned, cracks along the column lines appear to reflect the top layer of the top rebar along column lines.  Its just that the spider-leg radial cracks do seem to go in all directions (from 1 o'clock to 12 o'clock) for some distance and there are the circular cracks around the columns - but these circular cracks are few.

All cracks appear "wide" but I know that in most cases this is an inaccurate perception due to the grinding that rounded all crack edges.  I also firmly believe that these cracks do not go all the way through the slab as the underside looks wonderful.

I just really wanted some commentary on the fact that at columns, tension rules in the top layers, and that the ACI shear capacities take this into account.  I mean, they have to don't they as the design in ACI is based on using zero tension in the tension portion of the cross section.  Ultimate design assumes a cracked section. So phi x Mn is based on cracked sections as well as phi x Vc, right?

RE: Cracks in Two-Way Flat Slab

as long as the top reinforcing is intact... I've encountered a few parking garages where it isn't <Sigh>...

I like to proportion drop panels so that the depth is approximately the thickness of the slab... rather than use the minimum's that ACI (and CSA) stipulate...

RE: Cracks in Two-Way Flat Slab

Page 126 in Reinforced Concrete Design (4th Edition) by Wang and Salmon has a diagram showing that shear capacity consists of shear resistance of the uncracked portion of the section (the bottom part of the slab in your case), aggregate interlock in the cracks, and dowel action at the reinforcing bars.  So, yes, phiVc assumes the section is cracked.


RE: Cracks in Two-Way Flat Slab

I think to check it thoroughly, visual inspection is best. From what you describe, it sounds as if the slab is serviceable, and performing well.

The maintenance is being done to prevent salt water migration into the minor cracking in the slab at top of columns. I should think sealing the concrete cracks before replacing the membrane would also be good.

RE: Cracks in Two-Way Flat Slab

I would not be so quick to dismiss structural problems. Just because the garage is been in service for thirty plus years does not mean you don't have a problem.
Did you verify the shear and moment capacity of your slab section at the column? I have a similar two way slab with cracking as you described. When I checked the capacities I found them to be marginally acceptable under prescribed gravity loads, however this didn't take into account outside factors such as thermal effects.
You don't want to repair cracks and then have them reappear after one season. The garage I am working on had previous repairs that have failed because they had not determined the original problem that caused the cracking.
I would suggest further analysis prior to any repairs be undertaken

RE: Cracks in Two-Way Flat Slab

Just a follow-up on this thread - after some top-slab repairs had been undertaken, we basically found out that the structural slab was built in such a way as to develop a 3" to 4" slope for drainage.  The topping slab was set at 3" and it is currently removed.  

The cracks appear to be due to excessive cover over the top mat of reinforcing.  The bottom surface was formed flat, as was the reinforcing in the slab.  So to get the slope, the thickness of the concrete was varied above and beyond the 10" base thickness.  This adds some weight to the slab at the columns (flexure is unaffected but shear is not but I have extra depth).  The top reinforcing mat is almost 4" to 5" below the top of the concrete so ANY flex in the slab would open up cracks.  This is what I was observing.

Thanks for all the replies.

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