These are called stress joints. Because of the weight and how brittle concrete is it is expected to crack, these joints are usually cut into the concrete to give the concrete a weak point so that when I’d does crack it cracks in an expected place. Sometimes these spaces can also be what is called an expansion joint to sometimes when a piece of new conrect is poured up against an old piece of concrete an expansion joint will be installed this is usually just a small piece of material set between the two slabs. These are the thicker black lines you see.
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