Polishing a concrete floor is a way of finishing the surface to a glossy reflective sheen that adds to the attractiveness of the material without reducing its strength. When a crack develops in a polished concrete surface, it's just as dangerous to the integrity of the concrete as any other crack. Allowed to remain, it can grow, increasing in length, width or depth, until it threatens to break through the slab. A simple repair can return the integrity of the concrete, but to truly repair the surface, you'll have to blend the patch into the polish of the rest of the concrete as well.
Things You'll Need
Water-based concrete coating remover
Angle grinder with crack chaser blade
Epoxy gel adhesive
Joint repair compound
Handheld concrete grinder
Diamond grinding pads
Resin grinding pads
Concrete polishing compound
Low-nap paint roller
Place masking tape onto the floor surrounding the crack and an additional 1 foot of floor space around the crack. The masking tape will prevent you from damaging the rest of the floor surface while making your repairs. Place a face mask over your nose and mouth and rubber gloves over your hands. Open the doors or windows to ventilate the area.
Brush two layers of water-based concrete coating remover over the floor within the crack area. Use a thin coating of the remover for the first layer, and let it sit on the sealer covering the concrete for 20 minutes. Place the second thicker coat of remover onto the sealant, building a total remover depth that's twice that of the sealant on the surface of the concrete. Wait 30 minutes for the remover to strip away the sealant and then scrape the remover from the concrete surface using a putty knife.
Rinse the surface of the concrete clean with water. Empty out any dirt or debris from the crack with a wire brush. Wash the inside of the crack and the stripped concrete with a pH-neutral cleanser and a sponge. Rinse the cleanser residue from the concrete with clean water and remove any water from the crack with a sponge.
Remove the masking tape from the surface of the concrete. Replace the rubber gloves with work gloves and place a pair of safety goggles over your eyes.
Use an angle grinder equipped with a crack chaser blade to prepare the crack for repair. Route out the crack with the blade. Cut the walls of the crack vertical, slanting slightly wider towards the bottom of the crack. Level the floor of the crack to a depth of about 1 inch with no sudden rises or falls in the crack floor that can lead to weakened patches.
Remove the cutting debris from the crack with the wire brush and then sweep up the residue. Clean the surface again, making sure to get all areas within the crack. Wait for the concrete to dry.
Brush the sides and bottom of the crack with epoxy gel adhesive to serve as a bonding agent between the concrete and the repair material. Wait for the adhesive to dry to a point where it no longer appears wet but remains sticky when touched. Fill the crack with joint repair compound. Pack the compound in tightly with the putty knife until it slightly overflows the crack. Go over the surface of the crack with the edge of the knife to level the compound with the surrounding floor. Wait two hours for the compound to dry.
Attach a 32-grit diamond pad onto a handheld concrete grinder. Go over the patch with the pad. Work the grinder in small circles over the epoxy to smooth it to the same level as the surrounding concrete. Switch to a 60-grit pad and repeat the process, smoothing the patch further. Change to a 120-grit pad and go over the patch as well as the immediate area surrounding the crack to create a level surface of the same texture, blending the patch in with the surrounding concrete.
Change to diamond resin-based pads to begin polishing. Start with a 50-grit pad and grind the surface of the resin and the surrounding concrete up to the point where you masked it off and removed the sealant. Switch to a 200-grit pad and go over the area again, smoothing the surface further. Continue with the switch and grind cycle, changing the pad after each pas with the grinder until you reach a 1500-grit pad. Each pass makes the concrete progressively smoother, polishing the surface to a matte finish.
Use a squeegee to spread polishing compound over the work area. Change the pad on the grinder to a 300-grit pad and go over the surface, buffing the polishing compound to create a gloss finish to the concrete. Wipe the area clear with a clean lint-free cloth.
Apply a coating of polyurethane finish over the surface using a low-nap roller to protect it from damage. Extend the coating to overlap slightly the coating remaining on the undamaged concrete. Allow the coating to dry for the manufacturer's recommended length of time before stepping onto the concrete.
Color the patching compound with concrete coloring to match the color of stained concrete when needed.