Why are there gaps in the side walk?

Answer 1

The gaps that are put in place at the time of construction are called, "expansion joints". When concrete, and other materials, heat up, they expand. If concrete expands too much it will press against other sections of concrete and can break or "buckle". When sections of concrete press against each other too hard, sometimes two adjacent sections will press up, creating a dangerous condition for anyone walking on the sidewalk.

Expansion joints give a little space for the concrete to expand before anything bad happens. Even with expansion joints, it's sometimes not enough.