Gaps can form in a sidewalk slab when underground pressure is not adequately absorbed by the foundation, or if the surface is exposed to a heavier weight load than it was built for. Unrepaired gaps and cracks are an eyesore, and a trip hazard. It only takes a few hours and basic materials to fill sidewalk gaps and make the surface safe and attractive.
Widen the bottom of the gap with a chisel and hammer. Place the tip of the chisel at an angle against the inner wall of the gap and hit the flat end with a hammer. Chip down the length of the gap.