A depression in a concrete pad or driveway is a place for water to collect. In the home, a depression in concrete can cause serious problems with the floor -- tiles can crack or gaps can form between planks, for instance. Whether indoors or out, the best way to level a depression in concrete is to fill it in level. The main difference between doing this indoors or out is the type of leveling material you use.
Clean the driveway thoroughly by scrubbing with a solution of all-purpose detergent and water to remove dirt and grease. If a significant amount of oil has collected in the depression, wick up oil with cat litter or oil absorbent material. Clean the area inside and around it with a power washer. Use a chemical degreaser to get rid of more stubborn deposits. You don't have to remove all the discoloration -- just the oily surface residue that could prevent adhesion of the patching material. Let the concrete dry thoroughly before proceeding.
Chip away any loose concrete from around the area you need to repair, using a cold chisel and hammer. Don't break up any concrete that's in good condition, but make sure to remove all the loose concrete, or the patch won't adhere. Sweep the concrete thoroughly with a broom.
Use a straightedge to get an idea of how much leveling compound you need to fill the depression. A straight two-by-four works well. Lay it on edge over the depression and visually inspect the distance between the two-by-four and the bottom of the depression.
Mix enough mortar to fill the patch, using Portland cement, sand and water. The mixture should be stiff enough to remain on your trowel without running off. Trowel it into the depression and spread it with the trowel.
Allow the mortar to stiffen, then trowel more fresh material on top of it. Screed the patch with the straightedge to bring the level of the patch flush with the surface of the pad.
Feather the edges of the patch into the surrounding concrete. Let the patch set for 48 hours before walking or driving over it.
Clean the area around the depression by scrubbing it with detergent and water. Let it dry, then vacuum or wipe the floor clean.
Gauge the depth of the depression using a straightedge, such as a straight two-by-four. If the floor has more than one low spot, move the straightedge around the floor and mark the spots with a pencil.
Fill the depressions with self-leveling concrete filler. Many types are available, typically consisting of epoxy, latex or a combination of these materials. They come premixed.
Pour the self-leveling compound into the depression and screed it with the straightedge to flatten it and bring it level with the rest of the pad. One application of leveling compound should be all you need.
Let the leveling compound set before painting or covering the floor. Check the container for the setting time -- it's usually between 12 and 24 hours.