Micro cracks, also known as hairline cracks, can appear in cement for a number of reasons, most often from the natural settling of the concrete slab. These are minor flaws that rarely pose a threat to the foundation of your cement slab. Cracks that reopen after sealing or widen to larger openings should be seen by a professional. You can repair concrete micro cracks with commercial concrete filler, available at most hardware stores.
Clean the area around the micro crack with water to remove any silt or grime. Run a stiff brush in and around the area three inches to each side of the crack. If there is grease, apply a small amount of concrete cleanser and water to remove residual oils from the area, and rinse with clean water.
Allow the area to thoroughly dry. Shake the tube of concrete sealer well, or according to manufacturer’s directions. Make an angled cut on the tube tip that is sized to the width of the crack.
Position the tip of the tube over the crack and slide the tube slowly and evenly, beginning at one end of the crack and continuing on to the other end. Move slowly enough to fill the crack while not over-saturating the area.
Wait ten minutes to allow the concrete filler to set up inside of the crack. It may sink into some areas, requiring an additional application. Go over areas of indentation again with the filler until it is flush with the surrounding surfaces.
Use a trowel to lightly smooth the filler over the area and even out the line. Let the filler set up and dry overnight.
Examine the crack daily for the next few days to see if there is any additional seeping of material into the crack. Apply additional filler as needed and smooth over with the trowel. Allow the area to dry for 72 hours after the last coat before sealing.
Apply a polyurethane sealant to the area of repair. This will help waterproof the crack and keep the area from staining. Apply the sealant with a roller brush according to manufacturer’s instructions. Apply three coats of sealer, allowing the manufacture-recommended dry time in between coats.