Cured and Dry. Concrete continues to harden for several years after it is poured, but the standard period of initial curing is 28 days, or roughly one month. At this point, the concrete is very close to full strength and is ready to accept most commercially available sealants. However, sealers typically must be applied to dry concrete. You can test the concrete for moisture content by securing a piece of aluminum foil on the sidewalk with duct tape and waiting 24 hours. If the foil is dry when you remove it, the sidewalk is ready for sealing.