Concrete garage floors crack and become unlevel for many reasons, including an incorrect original application or settling of the earth below the concrete floor. If you have large cracks in your concrete, shifting and/or settling could be active, and the root problem needs to be corrected or the crack will reappear. Small and large concrete cracks are repaired differently and must be repaired before a floor can be levelled. Repairing and then levelling a concrete garage's floor is a project that will span a few days to allow everything to set properly and might require a few helping hands.
Remove all debris from the crack with a wire brush.
Use a non-acidic concrete cleaner to clean the crack and surrounding area. Use the same brand of non-acidic concrete cleaner and crack sealer. Let the area dry.
Fill cracks that are deeper than 1 inch with a layer of sand ¼ to ½ deep. Then pour a ¼-inch layer of crack sealer over the sand in the crack. Leave the crack sealer to dry overnight. Apply a second ¼-inch layer of sealer and let dry. Continue until the crack is flush with the surrounding area.
Widen the bottom of the crack with a hammer drill to make the bottom wider than the top.
Apply a vinyl concrete patch with a trowel directly to cracks a ½ inch deep or shallower. Apply a vinyl concrete patch in ¼-inch layers to the crack with a trowel. Let dry for a few days then add more layers if needed until flush with the surrounding area.
Paint a bonding adhesive into cracks that are deeper than ½ inch. Use the same brand of bonding adhesive as the vinyl concrete patch. Mix sand into the vinyl concrete patch for deep cracks. Apply a vinyl concrete patch in ¼-inch layers to the crack with a trowel then let dry for a few days. Add in layers until the patched area is flush with the surrounding area.
Allow the patch to dry and become shiny. Smooth with a wooden float for textured concrete surfaces. Smooth with a metal float for smooth concrete surfaces.
Clean the floor and patch all holes and cracks before beginning. Prevent levelling compound from seeping into other areas by nailing strips of wood to the door frames of the doors at the edge of your work area.
Mix levelling compound following the manufacturer's directions. Brush on a thin layer of levelling compound mix with a wide push broom to prime the floor. Apply one layer for newer floors and two layers for older floors. Let the levelling compound dry one to two hours.
Pour a thick layer of mixed levelling compound onto the floor in a far corner of the wall opposite to the exit of the garage. Empty a second layer on to the floor next to the first along the wall opposite to the exit. Pour more layers along the wall opposite to the exit, then move across the room until the floor is covered. Work toward the door while pouring. Let the levelling compound dry to the manufacturer's recommendations.
Use caution when using a hammer drill and wear eye and hand protection.