The cement in a concrete slab usually holds the surface layer of aggregate in an exposed aggregate slab securely in place. Sometimes, though, this hold fails, and pieces of the aggregate break away. With the loss of the aggregate, the slab develops surface bald patches or depressions. To restore the appearance of the slab, you must patch these damaged areas. An epoxy mix, combined with new aggregate, can provide the patch you need. With the patch, not only will you restore the appearance of your slab, but you'll keep the damage from spreading.
Clean the surface of the damaged area of the concrete with pH-neutral cleanser and a sponge. Remove any dirt present as well as any loose aggregate from the area. Use the sponge to remove any water from the damaged area after cleaning, and then allow the concrete to dry.
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Pour single-component epoxy into a cup, and add concrete sand to the mix, stirring constantly with a mixing stick until you create a smooth, spreadable paste.
Spread the epoxy paste over the damaged area, filling any holes in the surface of the concrete with a putty knife. Cover the surface to the same level as the surrounding concrete aggregate, then smooth with the putty knife.
Spread the same sort of aggregate used with the concrete slab over the epoxy, and firmly press the aggregate into the surface of the paste. Try to achieve the same sort of aggregate coverage as that seen over the rest of the concrete surface. Press the aggregate into the epoxy deep enough to match the height of the surrounding aggregate concrete.
Sweep more of the concrete sand over the epoxy layer to fill in the spaces between the aggregate. Allow the epoxy to cure 24 hours. Sweep the surface again to remove any loose sand or aggregate.
Spread a layer of concrete sealant over the patch. Use a paint roller to press the sealant around the aggregate. Allow the sealant to dry for 48 hours before using the concrete surface.