Was there rebar in the slab?

How large is a "fine crack"? A photo would be helpful.

The saw cuts are control joints that do not go all the way through the slab and are intended to encourage the inevitable cracks to form there instead of elsewhere. An expansion joint is a separation between slabs to allow differential movement.

Its impossible to know if the slab was poured properly but I suspect a high water content, poor curing efforts, insufficient reinforcement, or hot/dry/windy weather.

If this is essentially a cosmetic issues, it should only matter if the slab is exposed.

I would definitely get a structural engineer to take a look at it-- an independent one, not someone associated with the foundation crew. A few dollars spent on that consultation will either give you peace of mind or a good solution.

I'm surprised they added dirt and not gravel, so it wouldn't compact.

Inquire with the construction company about your concerns. Get all correspondences with the construction company in writing, and follow BYHYU's advice.

Thanks for all the input and advice. I will email the foundation company with my concerns, and also get a structural engineer to look at it. We did get a soil test done before any construction began to make sure that the soil could hold the structure. But no tests were done after the additional soil was brought in and compacted for the concrete slab.

therein lies the problem. we had a soil compaction test done at each 8" lift. the equipment is very specialized and reads the moisture content of the soil (it has be under 5% I believe) and then uses radiation to test the compaction, etc.

It very much has to be done after each successive amount of fill. In fact - it's the most important reading.

Is this a basement floor? Or a parking pad fpr cars? Or something else? The answer you are going to get is that small cracks are to be expected and will not impact the structural properties of the concrete. Which I would tend to believe.