more - Minimum Concrete Cover Tolerance

[boffintech]

Am inspecting on a job where the elevated slabs are 3' thick with #6@6 bot. short-way, #6@12 bot. long-way, #6@12 top long-way and #8@6 top short-way.

top mat cover reqd. = 3/4"

With the tolerance in reduction in cover can be 1/2".

In the completed slabs, LOTS surface cracks in the short-way direction, spaced curiously at multiples of 6".

Is the full 3/4" of concrete cover enough to provide sufficient aggregate interlock over that big of a bar to prevent these cracks? Or am I just barking up the wrong tree altogether?

 

[DWHA]

Did you take a look at the concrete mix design? Often when there is not a lot of cover, a mix with smaller aggregate will be used. This is the first place I would look.

 

[Ron]

DWHA is correct about looking at the mix design. For a 3/4" cover on the slab, the maximum size of the aggregate would be about 3/16"....that's pretty small for concrete aggregate. You could also interpret the code to mean the aggregate in this application should be no more than 1/2". In either case, you have to look at the mix design to check the nominal coarse aggregate size.

If the mix design was for No. 57 stone, with 3/4" cover, that could cause aggregate segregation and the cracking you noted.

 

[hokie66]

I think what you are seeing is plastic settlement cracking. In a thick slab, revibration is required if these cracks over the bars are to be prevented. This type cracking can occur even with much larger cover if the consolidation is inadequate. Often seen in the top of footings which have top bars.

 

[BAretired]

Settlement cracking is likely the correct explanation, but 3/4" cover is not enough for a 36" thick slab. It should be at least 1.25" in my opinion.

BA

 

[boffintech]

It's a #7 stone mix and the cover was increased to 2".

Possible formwork issue?

 

[hokie66]

I doubt that it is formwork related. If the cracks follow the #8 top bars, it is almost certainly plastic settlement cracking due to inadequate consolidation. Another possibility would be restraint shrinkage cracks, but as you said there are LOTS of cracks, that doesn't sound like planar shrinkage. No cracks on the bottom surface?