Acceptable Flexural Crack Width in Two-way Slab?
Acceptable Flexural Crack Width in Two-way Slab?
(OP)
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?






RE: Acceptable Flexural Crack Width in Two-way Slab?
Dik
RE: Acceptable Flexural Crack Width in Two-way Slab?
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
RE: Acceptable Flexural Crack Width in Two-way Slab?
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.
RE: Acceptable Flexural Crack Width in Two-way Slab?
Dik
RE: Acceptable Flexural Crack Width in Two-way Slab?
The maximum slab span is 25'.
I also have a call into the original design engineer to get his opinion on the findings.
gmf
RE: Acceptable Flexural Crack Width in Two-way Slab?
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.
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RE: Acceptable Flexural Crack Width in Two-way Slab?
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.
RE: Acceptable Flexural Crack Width in Two-way Slab?
RE: Acceptable Flexural Crack Width in Two-way Slab?
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.
RE: Acceptable Flexural Crack Width in Two-way Slab?
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".
RE: Acceptable Flexural Crack Width in Two-way Slab?
RE: Acceptable Flexural Crack Width in Two-way Slab?
RE: Acceptable Flexural Crack Width in Two-way Slab?
RE: Acceptable Flexural Crack Width in Two-way Slab?