Concrete Flaking

We are building a new house and had our concrete post-tension slab poured just 3 weeks ago. It was about 60 degrees when they poured and windy out, sunny day.  Well now, after the framers started framing, I noticed places all over the slab that are 'flaking' severely. I took a nail and was able to dig a 1/2 inch deep six inch across hole with little effort. The concrete came apart like layers and seemed to be powdery below.  I guess the framers are knocking the weak spots in the concrete and that is why they are showing up now.  I did watch them finish the slab.  One guy used a power trowel with another guy following him with a bottle of water and a hand trowel to finish. 
Can you please tell me what is going on with my slab? Is it serious? Can it be fixed? How do I know that the interior concrete is not weak? Thanks?
From your description it sounds serious.

When you say "hole", do you mean crack or actually a hole?

Some cracking is normal, but holes are not.  Was the slab "tensioned" already?

The purpose of post-tension is mostly to minimize cracking.

If there is actual flaking - large pieces of concrete spalling and leaving holes behind - I would stop the construction immediately until the concrete is tested and approved by an independent laboratory.  Doing it later will be terribly costly.

I assume you are in Texas. Check with your building department for information about local testing facilities. Have a qualified engineer review the concrete test results so far.  At 3 weeks, the strength should be at least 3000 psi for post-tension slab. (Unless the specific design calls for something else)

The finishing process you describe sounds normal.  However, if the trowling began while there was still free water on the surface of the concrete, it could "lock" water just under the surface - resulting in a weak layer near the surface. This by itself is not dangerous - just ugly and bad finish. It can be easily repaired at this time by using one of the "self leveling" epoxy-based toppings. After the frame is done it will be very difficult to do.

You are lucky to have caught it early.  You may want to hire your own "building inspector" to do regular inspections for you. This can save a lot of trouble down the line.  Also, document everything and keep good records.

 Good luck.