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Example: Use a 3/4"-7/8" rod to fill a 1/2" gap. Compress the rod into the crack leaving it about 1/4‚Äù-3/8" below the surface then fill in over top of it using the caulking.
If you use self leveling polyurethane caulking, it will flow out flat filling the crack.
You do not want to fill the entire depth of the crack with caulking. Without using the backer rod first the caulk will not adhere well to the wall and the concrete slab through periods of movement caused by expansion and contraction.