Cracked concrete slabs are a hazardous eye sore, but most of them are simple fixes. Concrete slabs crack due to frost heaves, lack of expansions joints, a heavy object dropped on the surface and other reasons. You can repair the cracks and pour new concrete over the existing the slab in a few days. Plan to give the slab at least four days to cure slowly to increase the strength of the new surface.
Scrub the surface with mild laundry detergent and a scrub brush to remove debris from the pits and crevices. Wash the surface with concrete stain remover and a power washer, if necessary. Rinse off detergent and stain remover with a hose.
Use a putty knife to fill any pits and cracks that are ½-inch deep or less with vinyl reinforced patching compound. Even the compound with the surrounding surface by spreading a thin layer around both sides of the crack with a trowel, a technique referred to as "feathering."
Widen any cracks deeper than ½-inch with a chisel and hammer to create a V-shaped crack about 2 inches wide at the top. Remove any debris from the crack and pour sand into the crack until it is ½-inch from the surface. Fill the remaining expanse with the patching compound using a trowel and feather it over the area.
Secure expansion joints to the old surface with duct tape. The distance between joints should be no more than three times the thickness of the new slab. If you're pouring a 2-inch thick slab, tape the joints no more than 6 square feet apart.
Install wood forms around the exterior walls of the existing slab. The top side of the forms will determine the level of the new concrete surface. Use 2-by-4-inch boards if your existing surface is 2 inches from ground level and you want pour a 2-inch thick slab. Nail the boards together at the ends and brace them by inserting 1-by-4-inch wood posts into the ground every 3 feet.
Pour concrete over the existing surface from one side to the other. Push a squeegee hard over the concrete to spread it evenly and fill low areas. Scrape across the top of the forms with a long 2-by-4 to level the concrete.
Use a float or clean straightedge to smooth the top until water bleeds through the surface.
Lay wet canvas over the new concrete and keep the material moist to dry the slab slowly for four days.