Concrete foundations can sustain damage that results in cracks in your basement's floor. While filling the crack is a very simple fix, you should take stock of where the crack is originating to ensure that further damage isn't being done.
Simple settling isn't much of an issue, but tree roots or a faulty foundation may be. The tools you'll need to use are not specialized; however, they may not be part of your toolkit and will need to be purchased at the local home improvement store. For an extremely large crack, the same process can be used except you'll have to pour sand into the crack, followed by concrete and concrete fortifier.
The crack needs to be chiseled out leaving a backward slope. That is the interior recess of the crack. It should be larger than the surface crack to allow the patching compound to penetrate and hold the crack in place and really seal it. By undercutting the interior, the patching compound can properly set, alleviating further damages.
Using the wire brush, scrape away all loose material from the crack and the surrounding area. In order for the compound to seal the remaining walls of the concrete, it must have a solid surface to which it may adhere. A portable drill with a wheel wire wheel attachment can also be used.
Use the paintbrush to apply a thin layer of bonding adhesive to the entire area around the crack. The bonding adhesive should also be applied to the upper interior of the crack. It will hold the smaller particles together so that they do not separate and fall out of the crack. The bonding adhesive also helps to adhere the patching compound to the sides of the crack. Allow the bonding adhesive to completely dry before applying the patching compound.
Use a trowel to fully stuff the crack. The crack should be completely filled with the air bubbles removed. In order to remove the air, stab at the concrete with the edge of the trowel to apply pressure and pop bubbles. After filling, be sure at least 1-inch of vinyl patching compound sticks from the crack. Smooth this inch to a layer around the sides of the crack.
Also called feathering, smoothing the crack completely level with the floor is not always necessary; however, a smoothly feathered edge provides much easier movement in the area and eliminates a safety hazard. Use extra vinyl patching compound to apply around the edges of the crack at least 8-inches on both sides. Gradually smooth the compound from the crack onto the 8-inch sides. This added compound not only removes the bulge from the patched crack but can also provide added support so that the crack doesn't split around itself later.