When building the basement, the separate installation of basement floor and walls creates a small joint between the surfaces. Occasionally, as the concrete settles, the joint may widen, causing leaks or serving as an entrance for insect infestations. You can seal these joints, but you must ensure that the seal does not interfere with joint movement; otherwise, you may end up with cracks radiating outward from the newly patched area. A closed cell foam backer rod and polyurethane sealant can create the seal you desire, closing the joint while continuing to flex as the concrete moves in place.
Remove any dirt or debris from the joint between the floor and the wall, using a wire brush, and then sweep along the base of the wall with a stiff-bristle broom to remove any remaining loose dirt and debris. Clean the joint and about a foot of adjacent wall and floor space with a concrete degreaser, using a scrub brush to remove oil deposits. Rinse thoroughly with water, then vacuum up any moisture with a wet vacuum. Allow the concrete to dry overnight.
Measure the length and width of the joint, using a measuring tape. Cut a piece of foam backer rod slightly larger in diameter than the width of the joint to the measured length, using a utility knife. Stuff the backer rod into the joint, packing it firmly while leaving a space of about ¼ inch along the top of the rod to the surface of the floor.
Place strips of masking tape along the edges of the joint to prevent the sealer from overflowing onto the floor or wall.
Pour the self-leveling polyurethane sealant into the joint above the backer rod, filling it to the top. Start at the end of the joint and pour along the entire length, moving in a single direction. The sealant will level itself out at the top of the joint. Wait two hours for the sealer to begin to cure.
Remove the masking tape to create a clean line in the sealer at the joint edges. Allow the sealer to cure completely for seven days.