Is this a slab or a basement type addition , where do you live . I have never seen expansion cuts in concrete floors that are going to be inside a home but I have no idea about too many things to make a guess

wasn't rebar or steel reinforcements used in the mix?

found this:

Control joints are usually cut into all engineered slab floors such as the malls, low rise industrial, etc. Page 14 of the Portland Cement Association's book Design and Control of Concrete Mixtures states "Drying shrinkage is an inherent, unavoidable property of concrete, therefore, properly positioned reinforcing steel is used to reduce crack widths, or control joints are used to predetermine and control the location of cracks."

They are not cut into basement slabs (1) to save money, (2) the contractor doesn't know the difference, (3) the homeowner doesn't know the difference, and (4) codes don't require them (since codes are minimum standards, not necessarily good standards)

They do not compromise the strength of the slab which by the way is nothing more than a heavy concrete carpet. The slab should have no bearing on the integrity of the structure unless it was designed with footing strips or post bearing pads under the support points. The slab does not prevent the exterior walls from being pushed inward by soil/moisture pressures. The properly designed/executed footings take care of that.

The cracks do not allow soil moisture upward as there should be a moisture barrier under the slab. They are usually cut only 1/3 or 1/4 the thickness into the slab to make a weak point so that the shrinkage crack will occur where you plan it to.

Basically, he's using the cuts to control the cracks that normally occur.

Read this if you want to see how the cuts should be made

https://www.concreteconstruction.net/how-to/construction/three-myths-about-contraction-joints-in-residential-slabs_o

Thanks! This a poured basement addition, and I live in Wisconsin (so plenty of cold :))

I know rebar was used in the walls, but I don't think it was used in the floor at all.

I have garage slabs cut in a + configuration so that the inevitable cracking looks neater. It's cosmetic only. I've never bothered on basement floors as they've always been covered with another surface--carpet, stone or ceramic tiles, etc.