What is the primary purpose of a control joint in a concrete slab?

• A. To crack along a predetermined line.
• B. To prevent cracking altogether.
• C. To create expansion joints.
• D. To improve surface finish.

Answer: (A)

Control joints provide a planned weak line for cracking. As concrete dries and cools, it tends to shrink and develop tensile stresses. Without a designated crack path, cracks can form randomly across the surface, which is undesirable. A control joint creates a notch or recess that concentrates those stresses along a specific line, guiding most cracks to occur there and producing a predictable, neater crack pattern.

They do not stop cracking altogether, and they are not the same as expansion joints, which allow movement between slabs or structures. They also don’t improve surface finish.