After you solve your basement's moisture problems, your main concern becomes the floor: Is it sound and level enough to finish with your floor-covering choice? If your floor shows no signs of moisture problems, is level (no high spots of more than 1/8 inch in 10 feet), and is not severely cracked, it may need only surface repairs before you install underlayment (in areas you intend to cover with vinyl flooring) or a finished floor. If the floor is badly cracked, broken, or damaged, you'll have to resurface it, either with self-leveling compound or new concrete.
First check the floor's surface for evenness by rotating a 6-foot level on the floor in sections. Mark dips or high spots with a carpenter's pencil. Repair those areas as follows:
Lows and Highs. Fill depressions with patching compound, troweling them smooth and feathering them to the surrounding floor. Rent a concrete grinder to level high spots. Check the surface of your repairs with a straightedge, continuing to fill or grind until the floor is flat and level.
Cracks and Holes. Use hydraulic cement to repair them.
Salt Deposits. White or yellow alkaline deposits impair adhesive bonding on glued-down floor coverings. To remove the deposits, mop the floor with a solution of four parts water and one part muriatic acid, then rinse the slab with clean water. Muriatic acid is extremely caustic, so follow package directions carefully.
If your floor is extensively damaged and can't be repaired using these techniques—or if such repairs would be too time-consuming—you still have two more options: self-leveling compound or a new concrete slab. Before you choose either, though, consult a structural engineer to determine the cause of your floor's damage and to ensure that its condition is stable enough that it won't sustain more damage.
Self-leveling concrete is a liquid mortar that you pour and spread onto a sloped, rough, uneven, but structurally sound floor. Most compounds require that you first coat the floor with a primer. After the primer has cured, mix the compound and spread it onto the floor—up to 1/2 inch thick—with a floor squeegee. The compound levels itself and dries hard and smooth. If you need a thickness greater than 1/2 inch, add aggregate to the mix.
If your floor is not structurally sound, you don't have to break up the old slab and start again. You can pour a new slab right over the old one, as long as the increase in floor height leaves you enough headroom. (Consult your local building codes for minimum ceiling-height requirements.) First install any new plumbing. Then lay a waterproof membrane—such as 6-mil polyethylene plastic sheeting—over the old slab as a moisture barrier, overlapping edges by at least 4 inches. Then lay 1/2-inch rigid foam around the perimeter of the floor as an expansion barrier, and suspend 6x6-inch #10 wire mesh on brick or blocks to center the wire in the concrete when it's poured. Pour at least 4 inches of concrete and finish it with a float.