Why Do Sidewalks Have Cracks?
To prevent sidewalks from cracking in random spots and breaking apart, special cracks, or grooves are scraped into the concrete when a sidewalk is constructed. These grooves are called break joints. The technical term for sidewalk lines is contraction joints. The idea is that if the sidewalk cracks, it will do so along these narrow grooves, rather than break where it shouldn’t.
Contraction joints are placed in fresh concrete before the concrete dries and has a chance to create its own joints, which we call cracks. As the concrete dries and shrinks, any cracks that form should follow the path of the contraction joints, since that’s where the concrete is thinnest.
The words “cement” and “concrete” are often misused. You may have heard someone speak of a “cement sidewalk”. Sidewalks, however, are made of concrete. Cement is the grayish powder that is mixed with water and materials, such as sand and crushed rock, to make concrete.
If the cracks follow the contraction joints, the sidewalks won’t look as bad. They also won’t tend to form further cracks and break apart like they might if allowed to crack at random places.
Builders place contraction joints in fresh concrete with saws, special grooving tools or plastic strips called zip-strips. They have to make sure that the joints are deep enough and made before the concrete begins to dry, so that cracks won’t appear in the wrong places.