Typically, concrete walks, driveways and floor slabs are fitted with incrementally spaced expansion joints made of an oiled fibre material. These joints help prevent concrete cracking caused by the minor geologic movement of the natural terrain supporting the slab. When these fibre joints age and decompose, silicone's natural properties refill the joints to act as a sealant and flexible expansion material between the concrete sections.
Remove the old expansion joint material, using a screwdriver tip or narrow wood chisel. Use a broom or vacuum to clean the loosened material from the expansion joint space.
Apply a thick bead of silicone caulk into the cleaned-out expansion joint space until the silicone sealant is almost flush with the concrete surface. Use a narrow paint stick or cloth rag to smooth the silicone's surface slightly depressed from the slab surface. Continue filling the expansion joint space until the full length is filled and the surface smoothed.
Clean any silicone sealant that smears onto the concrete surface, using a cloth rag saturated with white spirit. Do not allow the silicone to dry before removing.
Repeat Steps 1 through 3 to fill any additional expansion joints, as necessary.
Using a black or grey silicone will help obscure the expansion joint appearance in the slab.