What is the difference between a control joint and an isolation joint?

• A. Control joints are planned cracks; isolation joints separate components to permit movement.
• B. Isolation joints are planned cracks; control joints separate components.
• C. They have identical purposes.
• D. Control joints prevent adhesion along the joint.

Answer: (C)

Control joints and isolation joints serve different roles in concrete slabs. A control joint is a deliberately formed weak line designed to control where cracks will occur due to shrinkage and temperature changes. By creating a planned plane of weakness, the concrete cracks in a predictable, neat location rather than in random fashion, helping maintain appearance and structural integrity. An isolation joint, on the other hand, fully separates two components or slabs so they can move independently. This separation prevents movement or cracking from one element transferring to the other, which is important where differing loads, temperatures, or settlement might cause relative movement.

So the best description is that control joints are planned cracks to control cracking, while isolation joints separate components to permit independent movement. They do not have identical purposes, and the other choices misstate either the intended function or the nature of the joints.