I am buying a new home. The foundation was poured in Sept 2019. The home is nearing completion but the floors have not yet been installed. I see cracks in the foundation. Most concerning are two separate cracks that run the width of the home. In most, of not all, places I believe they are 1/2 cm (1/4” I think) or less. Do I have a potential structural problem? Do these need inspection by a structural engineer? Home inspector? Or can they just be filled? I’m including a sample photo.
It's hard to say if it's a structural problem without more information. The crack in the photo is not necessarily a structural problem. Most likely it's related to less than ideal subgrade, concrete mix, control joint spacing, or concrete curing conditions. Concrete is always going to crack. That crack is excessive, but most likely not a structural concern. If you notice something translating up the structure -- cracks in the walls, or sloping floors -- then it would be a greater concern.
I do work in the midwest where temp ranges between -10 and 110F. All concrete cracks is just a dumb myth.
There are hairline cracks that are more typical. Mainly with suboptimal mix or not properly adding joints.
However I have many pours over the past 10 years that have zero cracks because I basically use the best mix in the area and pay a good 30% more for it.
Just eyeballing your photo - and it isn't a very good one... I would say your mix looks a bit "white" and "powdery". I also think that the crack was caused by heaving/sinking. I would get an local engineer who specializes in concrete foundations to take a look and basically warranty it for you. I might be able to help out more but I would need a lot of pictures taken. (writing this answer from a room in house on a slab that I just redid the floors and there wasn't a blemish on the concrete)