When you have a gap in your concrete, along a wall or in the middle of your sidewalk that you want to close up. The best way to do this is with some backer rod before installing sealant.
Use it to fill most of the gap. When applying you will want the backer rod to be slightly larger then the gap and press it into the gap until you have about 1/4″ space above the rod. Then fill the top of the gap with either a Urethane or Elastomeric sealant.
Watch this great video on installing backer rod and sealant.
Home Depot’s HDX Elastomeric Acrylic Caulk is a premium quality elastomeric acrylic caulk designed for all purpose sealing and weatherproofing for interior or exterior applications. Use for caulking areas that may experience movement and may or may not be painted over. Provides a permanently flexible, crack-proof seal that is ideal for sealing out water and air leaks.