You can level an existing concrete floor with a levelling layer of new concrete, but you must prepare the old concrete floor first. Neglecting to prepare the old surface will prevent the new concrete from adhering correctly, resulting in a weak bond between the two layers. When done correctly, a levelling layer of concrete increases the longevity and strength of an old concrete floor.
Mark the intended height for the new concrete. If you're working inside, snap a chalk line around the walls. If you're working outside, dig a trench around the perimeter of the floor 4 inches deep. Place wood form boards on their sides in the trench and nail them together at the ends. The surface of the new concrete will be level with the top of the form boards. For instance, if you're raising the level 2 inches, use 2-by-6 inch form boards.
Pour concrete etching acid over the surface and spread it with a shop broom. Scrub the acid into the floor, and wait half an hour before rinsing it off with a hose.
Go over the surface with a wire brush to rough it up to help the new concrete adhere to the old. Water the old concrete to prevent it from absorbing moisture from the new concrete.
Prepare enough concrete to pour a 1- to 2-inch layer over the old floor. In a concrete mixer, add cement, aggregate, sand and water as directed on the concrete package instructions.
Pour the concrete over the old floor and spread it over the surface with a rake. Continue pouring the concrete until you reach the height of the chalk line or the top of the form boards.
Settle the concrete with a clean straightedge long enough to drag across the surface. Smooth the surface with a float beginning with the first section of poured concrete and working across. Stop manipulating the concrete once water begins to appear on the surface.
Cover the floor with moist burlap and allow it to dry for four days, keeping the burlap moist so the floor cures slowly.