Damages to concrete generally are repaired by filling them with a patch product. However, this won't work when the surface is vertical. Gravity will pull the patch down. To repair vertical concrete surfaces, a special product is used that starts as a liquid or paste, dries quickly and won't shift during repair. This product is epoxy, and because of its multiple uses, it should be a staple in your home repair kit.
Things You'll Need
Epoxy repair kit with injection ports, adhesive and epoxy
Prepare the damaged area by removing any loose concrete particles. Wash dirt from the area and let the concrete dry. Widen the damage slightly using a router if it is too small for the injection ports.
Remove caps from the injection ports by cutting along the tab attachments. Push one port into an end of the damaged area. Insert each of the remaining ports between two anchors, spacing each by half an inch.
Apply epoxy adhesive thinly into and around the damage with a small paintbrush. Work the adhesive close to the edges of the ports, but not over them. Cover edges of the damage and some of the undamaged concrete around it with the adhesive, too.
Wait about three minutes for the adhesive to become sticky. Fill each injection port with the prepared epoxy. Insert the tip of the tube into the lowest port. Squeeze until you see epoxy in the opening of the next tube, and work up.
Cap each injection port once it is full of epoxy. Wait 48 hours for the epoxy to dry. Pry each port free with pliers. Lightly sand dried stray epoxy with fine-grit sandpaper or scrape it with a paint scraper.