It seems that just about anyone that has tried to fill gaps in concrete has had the caulk disappear into the crack.

Backer rod will help support the bead and prevents caulk from sinking down as it ages and dries. This technique is also useful for windows, doors and brick walls. Use backer rod for gaps and joints more than 1/4in. wide. You want the rod to fit tight in the join, so buy it one size larger than the gap. Use a putty knife to force the flexible backer into the crack until it sits at least 1/4″ below the surface.

Fill the crack with urethane caulk. Snip the opening of the tube at a 30 ° angle, making the opening the same size as your gap. Use a smooth, even motion, filling the crack flush with the surface. Bevel the caulk if it’s against the house. Smooth the caulk in wide joints with the back of an old spoon. Wipe the spoon clean as needed with a rag and mineral spirits.

A word of advice: Keep the urethane caulk off your bare hands and clothes; it’s the stickiest stuff you’ll ever touch. Wear disposable gloves when you’re tooling the joints. If you get some on your skin, quickly wipe it off with a paint thinner-dampened cloth.

Caulk cracks 1/4in. wide or less without using backer rod. Draw the gun down the crack, smoothing the caulk with the tip as you go.

Once a year, go around your home and fill these gaps and joints with urethane caulk to prevent water from getting into the joints, freezing and making the cracks even larger.

Photos courtesy of HandyMan.com