Concrete with soft spots very strange

[NealinNC]

I have a concrete slab that was poured about 6 months ago. It was cold the day the concrete was poured and Calcium Chloride was added. The slab looks good except there are about 10 bad spots. I tested the concrete with a schmidt hammer and have compressive strengths of 3500 plus psi except for these bad spots where it is basically zero. There are no cracks or spalling or anything else other than the bad spots.

The spots are about an inch or so in diameter. At first they kind of raise up a bit like a blister, then they fail. You can stick your finger down in the hole and it is like wet clay. It has a slight chlorine odor. Some have spots have been open for a week or better and it is still wet even with temperatures above 90 degrees F. The "clay" is whitish gray similar to portland cement. My first guess would be cement clumps that were not mixed or possible lumps of Calcium Chloride, but they are wet and not drying or curing. The concrete supplier said it was mud on the workers shoes, but there is no mud on site that would match this mud.

Has anyone ever seen anything like this?

 

[Ron]

What you've described is classic reactive aggregate popouts. Where are you located? Do you have any chalcedony chert in the concrete coarse aggregates in your area? It could be clumps of CC that have reacted, but less likely. You should see a grayish, greenish, whitish residue at the apex of a cone of the popout. If this is the case, probably reactive aggregate.

 

[NealinNC]

Thanks Ron, I'm located in NC we don't have any chert that I know of. Most of our rock quarries are granite. It is gray/green/white. I'll see if I can clean one of the holes out and look at the apex of the cone.