Exterior door sills, also known as thresholds, are the bridge between the door frame and the inside and outside of a residential home or commercial building. Door sills may be constructed of wood or metal and are generally fitted above a concrete foundation. Over time, foot traffic and weather elements may weaken and corrode the concrete to the point it begins to crack and break up. If this happens, a gap may appear between the concrete and the sill. The old concrete must be removed and replaced to close the gap.
Put on eye protection, along with work gloves and a dust mask.
Use a chipping hammer with a chisel bit to break up the concrete under the exterior door sill then pull out the rubble by hand. Vacuum out any remaining pieces with a shop vac for a clean surface.
Combine one part Portland cement with three parts masonry sand inside a wheelbarrow then mix up the dry ingredients with a paddle mixer. Wet the cement/sand mixture with a garden hose lightly then mix it again until it becomes the consistency of mud.
Pour concrete bonding agent into the exterior door sill area, allowing it to spread over the remaining concrete foundation. Immediately, tilt the wheelbarrow up, allowing the concrete to pour down onto the bonding agent before it dries.
Even and smooth out the wet concrete with a trowel then let it dry and harden for 24 hours before walking on the sill or using the door.