Acceptable Flexural Crack Width in Two-way Slab? 2

[gmf]

I completed a visual review of the underside of a two-way concrete slab as part of a reserve fund study. The slab supports first floor residential space and the structure for three additional residential floors and roof above. Below the slab is a parking garage. The parking garage is below grade. The slab is supported at the perimeter by concrete foundation walls and at the interior by concrete columns, drop slab, and column capitals. Review found cracking at the underside of slab in regions of positive bending (tension side; which I know can be expected) away from columns, but some of the cracks are larger than expected (1/8" wide). Is this a function of poor crack control? Has anyone had similar experiences?

 

[dik]

It depends on the type and location... I've seen flat slabs (with drops) hinge about the drop and the middle strip had a crack that you could pass a business card through...

Dik

 

[gmf]

In this case, we have a 10" flat slab (6" drop at column; column capitals extend 12" from columns). Cracking appears to be located at the centre of middle strips but does extend through both middle and column strips in the opposite direction.

At this point, I am trying to determine if recommendation for further review is warranted or whether this is something to expect in a flat slab. Should cracks be repaired, or simple monitored? It seems cracking should be repaired to at least reinstate the integrity of the concrete with respect to protecting the reinforcing steel from the outside environment.

gmf

 

[Teguci]

OK, lets talk about protecting the reinforcement.
1. are there any signs of concrete delaminations (take a rod and sound the soffit) or exposed rusted rebar? If no then you are good to go - no repair or monitoring.
2. if 1 = yes. Its not in the splash zone. Is this site in a marine environment? If so, sample concrete for chlorides near cracking. Check for carbonation of the concrete near a crack at the garage entrance. If either of these are high then I'd recommend local cathodic protection and repair and further monitoring/ongoing repair schedule.

Next, check the original design (probably want authorization before going too far on this).
1. You have large cracks why? Does the design meet the requirements of ACI 318 for minimum slab thickness to span? Provide objective statement in report. Are the cracks distributed or just localized? Pick one of the worst bays and map the cracks with spray chalk. Take a picture and analyze.
2. Are the slabs being used to resist lateral loads under frame action? Might be an issue worth thinking about but probably not.

Further review - can you get access to the top side of the slab to see what kind of cracking there is on the column lines?

I would not expect a single large crack in the center of a flat slab span and would want to further the investigation.

 

[dik]

Proportions of the drop and slab thicknesses are different than I use. Drop is usually the same thickness as the slab or just a tad thicker. From the proportions I would thinking the spans were just under 30'. Often find cracking in the middle strip, between drops because the rotation occurs at the bottom of the drop panel and the entire slab between drops can be in tension. The one I mentioned was an actual case and a business card could be dropped through the slab. Matter of doing a quick analysis and if OK, then unless a corrosive environment, it is likely a matter of epoxy/polyurethane injection.

Dik

 

[gmf]

There are no signs of concrete delamination or exposed rusted rebar. Other than the cracking, the slab appears to be in pretty good condition. The underside of slab is not in a splash zone and I do not see signs of carbonation (this is not a marine environment). Unfortunately, there is no easy access to the top side of the slab due to floor finishes within the residential spaces above.

The maximum slab span is 25'.

I also have a call into the original design engineer to get his opinion on the findings.

gmf

 

[JAE]

I would follow Teguci's advice above.

A crack could be a sign of impending delaminations so I'm not sure I would discount that without doing some soundings around the cracks.
Also - if you do find loose delaminated concrete near a crack - perhaps remove some concrete to get a visual on the rebar itself.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[MrHershey]

ACI 224 provides some guidance on acceptable crack sizes. 1/8" seems fairly large. A 'reasonable' crack in dry air according to ACI 224 is 0.016" (1/64th of an inch), though they concede that a portion of cracks could be over this width and potentially be over by a significant amount.

Even if no evidence of issues now and slab is fine structurally, I would tend to want to inject or otherwise try and close a crack that large to try to prevent future issues.

 

[psychedomination]

According to Eurocode 2, crack widths should be equal to or less than 0.3mm if it has been designed correctly.

 

[rapt]

Dik,
They are capitals, not drop panels with those dimensions!

GMF,

is it RC or PT? The L/D appears to be in between but I assume RC.

What are the deflections like?

Have you checked the strength to see if it is adequate. You should not get that crack width in a properly designed and detailed RC slab.

 

[hokie66]

rapt,
The OP did say in his first post that the slab had both drop panels and capitals...at least the way I read it. Then he said the drop panels were 6" deep, and the capitals extend 12" from the columns.

These cracks are probably restraint shrinkage cracks. The perimeter walls restrain the slab, and there is not enough reinforcement to control the cracks to acceptable widths. I think most of us have seen this, more than once, when a design of a restrained slab just uses "minimum T&S reinforcing".

 

[Teguci]

I think hokie's nailed it. For shrinkage, that is restrained by the exterior wall, I'd expect to see the large crack parallel with the retaining walls in the first bay. For the interior bays, I'd expect far less cracking. The corner bays should turn the corner. Does that match up, gmf?

 

[gmf]

The slab is reinforced concrete and yes there are both drop panels and column capitals. There is cracking at each interior bay (roughly middle span), but the worst cracking is located at the first bay. And yes, the cracks start to turn the corner at the corner bays. The overall slab (and building) dimensions are 145' x 56'. The majority of cracks travel in the short direction and are spaced from each other in the long direction.

 

[hokie66]

The cracks are at midspan for two reason: the bending and the lack of top steel. Restraint shrinkage cracking is constrolled by all the steel in the section, so where there is no top steel, you only have the bottom bars for control.

 

[rapt]

A picture/drawing/diagram would be useful, or did I miss that also. Something showing the structure, supports restraints and crack pattern!