Crumbling concrete is not just unsightly, it may also be a sign of serious damage beneath the structure. Address the problem as soon as possible to prevent it from spreading or causing other damage, such as crazing, in which a network of random cracks occurs. To prevent crumbling damage in the future, avoid placing heavy items along the edges of concrete structures and do not use salt to de-ice your concrete in cold weather.
Rub the bristles of a stiff scrub brush over the edges of the crumbling area to remove loose concrete and dirt. Spray these areas with water from a garden hose on high pressure. Allow the concrete to dry completely for 24 hours.
Apply a thin coat of concrete bonding agent over each of the crumbling areas, using a medium to large paintbrush. Wait a few moments for the bonding agent to become slightly tacky.
Apply a layer of prepared concrete 3/8 inch thick with a pointed metal trowel. Smooth the surface of the new concrete with the face of the trowel. Wait two hours.
Trowel on more prepared concrete, filling each of the crumbled areas until they are level once again with the undamaged concrete. Use the trowel to smooth the surface of the concrete again. Allow the concrete to cure in the open for 30 minutes.
Place a plastic tarp over the repaired areas so that they are completely covered. Allow the concrete to cure for one week under the tarp, lightly spraying the new concrete with water from a garden hose once a day. Remove the tarp once the concrete has cured completely.