Concrete Slab Delaminating?

[BSVBD]

8" THICK CONCRETE SLAB
w/ 6"x6"-#6/#6 WWM
OVER 8" OF COMPACTED CRUSHER RUN GRAVEL
OVER COMPACTED DRAINAGE FILL

The general contractor and project manager say they hear "hollow" sounds from tapping on the concrete slab with a hammer.

The first thought was that either the gravel or drainage fill was not compacted enough and may have sunk or fallen. The second thought was that the sound sounded higher than 8" deep, and thus, perhaps that concrete delaminated.

Has anyone had experience with delaminated concrete? Does this make sense? Any thoughts?

Thank you!

 

[TehMightyEngineer]

My father poured a garage foundation slab with limited manpower (two guys) and no vibrator, and he had a similar problem in a few spots. Likely you're hearing voids created by poor consolidation of the concrete, when "sounded" with a hammer it will make a dull hollow sound. These can be repaired relatively easily but if there's a lot of large areas that has issues it might be easier to just hydro-demolish the entire top surface of the slab to expose the aggregate and pour a topping coat.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Archie264]

In case it's of interest large areas can be "sounded" using a metal chain attached to a pipe. Drag the chain across the slab and it reverberates through the pipe. Here's a little youtube clip of someone doing it. He doesn't have the metal pipe attached so I guess it's possible with just the chain attached, though I do think the pipe will amplify it.

https://www.youtube.com/watch?v=aoYDp1oa00M

As for your specific situation, I'll leave that to some of my betters to diagnose. It sounds [heh, heh] like you're on the right track, though.

 

[Ron]

A chain drag has been used for years to check delamination. We use a device specifically for that called a Delam 2000....a device with two sprockets on an axle that is rolled across the surface to detect delamination. It works well.

Lack of subgrade compaction would not create this issue. Further, finding delaminations is tricky, particulary in slabs on grade. Don't believe the findings unless you can corroborate.

 

[oldestguy]

Lacking a chain, you can take a typical carpenter's hammer and bounce it(4 or5 inch high)in circular arc sweeps. Works on concrete walls also with a little difficulty.