How to seal crack in concrete floor?

If the crack is large, you'll have to get some more concrete and use it to fill the crack.

If the crack is small, you may be able to simply patch it. If you plan on coating the floor with some type of epoxy floor coating, you may be able to simply leave the cracks alone before application, simply allowing the epoxy material to fill the cracks. The work will still come out evenly and the epoxy will actually hide the spot where the crack was once located.