cracks in corner of building slab

[tw]

Recently poured slab for a 2 story metal building has developed diagonal cracks in each corners. The cracks are in the slab only, the grade beam had poured several weeks ago. Each corner had embedded anchor bolts (in grade beam/foundation). Not a crack pattern that I have seen that I remember. Possibly due to restraint from embedded anchor bolts. There is 4x4 4 gauge mesh in slab and cracks are tight but owner is worried. I have to pull out my form letter that says concrete cracks but am I missing something?

 

[Ron]

What is the orientation of the cracks relative to the corner? Can you post a photo or sketch?

 

[JacksPanic]

Is it a slab on grade that is cracking?

 

[XR250]

Probably due to slab curling as the corner is restrained from uplift by the anchor bolts.
May also be due to shrinkage from the same restraint.

 

[dcarr82775]

The image of the crack I have in my head is shrinkage due to restraint by the gradebeams. Like Ron said, a picture or sketch would help.

 

[tw]

Here's a picture. The slab is grade supported, 5" typical but thickens to 8" on edges over grade beam.

 

[dcarr82775]

Shrinkage crack.

 

[a2mfk]

Agree with dcarr, and points to Excel for guessing without a picture it was foundation restraint.

 

[dik]

What was the concrete mix design? aggregate, strength, water:cement ratio, and slump?

Dik

 

[msquared48]

Do you have hairpins at the corners in addition to the WWF?

Mike McCann
MMC Engineering

 

[tw]

No hairpins. Anchor bolts are embedded in grade beam/pedestal (with rebar).

Slab is roughly 80x90 and so far no other cracks in slab, though I expect there to be some. These showed up quickly.

The metal building has a second floor and there is not a lot of lateral force at column bases.

 

[xyyy]

It is shrinkage crack. Something at the corner restrained the concrete shrinkage and caused the crack.

 

[TDIengineer]

I concur about the comment on the mix design. That really looks like a pasty mix. I don't like the color or the texture at the top.

They added water to the mix, while it was in the truck would be my guess.. It started to kick some, and the added water for workability.

Concrete contractor will never admit to it though

Anyway, just my assumptions based on the photo... And yes, stress reversal at the corners of a square pad due to shrinkage.

If we made our slabs on grade oval, this would be so bad..

 

[PSUengineer1]

shrinkage