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Shrinkage in existing slab?

Shrinkage in existing slab?

(OP)
I got called from the contractor on an adaptive re-use project we have under way to review some supposedly new cracks found in an existing elevated slab. The cracks were discovered when someone on the upper (2nd) floor knocked over a bucket of water, which subsequently poured through the cracks to the ground level below.
We have the original building drawings from 1935, which show a 4" concrete slab over 18" wide flange steel beams, which are encased in concrete. 4 approximately equal continuous spans (~8') with 1/2" bars continuous along the bottom and 4' bars at the top centered on the steel beams. 3/8" bars transverse at 18" o.c. for T&S. (3) 18WF70 beams spanning 31'-6" between 33WF200 girders, also encased in concrete.
The "new" cracks run parallel to the span of the slab, at approximately the 1/3 points of the 31'-6" main beam span. Almost identical in each of the 4 bays, and all but one of the cracks run roughly perpendicular to the support beams. The cracks propogate down and around the encasement on the 18WF's. Standard shrinkage crack, no? There are also small cracks approximately mid-span of the 33WF girders at the bottom, which could be explained by beam deflection (the drawings do not even mention the beam encasement let along any T&S control).
My issue is that the cracks were not noted prior, which doesn't mean they were there, just that they didn't stand out as an issue. However the underside of the slab was recently painted and the cracks appear to be through the paint. I just want to make sure I can't be missing something before I tell them to fill in the cracks and don't worry about it any more. Could the recent enclosure / finishing / HVAC installation of the renovated space (which had been originally used as a partially enclosed loading dock area and had been vacant for several years) bring about "new" or at least expanded shrinkage cracking? Last week was also the first "cold" overnight temps of the year (<40 counts as cold around here).
Thanks for any opinions.

RE: Shrinkage in existing slab?

Cracks across slab are almost certainly classic shrinkage cracks. Though shrinkage of concrete continues for long time, it approaches zero after several years (at most)......PCA, ACI have various graphs depicting shrinkage over time.

"New" cracks are quite doubtful unless some new heavy loading condition has occurred.......or corrosion of steel beams or rebar has been occurring.

Bending of steel beams might contribute to cracks at bottom of slab acting as "T-beam" type assembly......though sounds like just classic shrinkage.

John F Mann, PE
www.structural101.com

RE: Shrinkage in existing slab?

I agree that these are drying shrinkage cracks, which have been there for a long time. In the situation as described, these cracks are inevitable. And just because there is also a crack in the paint doesn't mean that the concrete is moving.

RE: Shrinkage in existing slab?

This classic question is answered difinitively with an Otoscope. Go look *into* the cracks where they were painted... If you see paint inside the crack, even the least little bit inside, they are old. Where the cracks are new, it is extremely obvious.

This recent paint job is your best friend...

RE: Shrinkage in existing slab?

(OP)
thank you all for your input.

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