Two Different Purposes
Contraction joints encourage shrinkage cracks to occur at planned locations. Isolation joints accommodate movement between the concrete and adjoining structures. Their details depend on the work.
Layout and Timing Matter
Joint locations and installation timing should be planned. A joint cut too late cannot undo cracking that has already occurred. Do not assume all slabs need the same spacing or depth.
When a Crack Needs a Closer Look
A photograph can help start a conversation, but it cannot by itself establish the cause or significance. Note when the crack appeared, whether it is changing, and whether the sides have moved vertically. Ask for a project-specific review.
Plan joints early. Review cracks in the context of the actual project.
Further Reading
NRMCA — CIP 4: Cracking Concrete SurfacesGeneral education for project planning. Your site's conditions and project requirements determine the construction details.