What is the primary purpose of control joints in concrete slabs?

• A. To allow thermal expansion and contraction freely without cracking
• B. To create planned cracking at specific locations to control crack widths and distribution
• C. To anchor reinforcing bars at midspan
• D. To provide decorative joints along the surface

Answer: (B)

Cracking of concrete mainly happens because it shrinks as it dries and experiences temperature changes, which creates tensile stresses. Control joints give the concrete a planned weak plane where cracks can form, so cracking occurs at designated locations rather than randomly across the slab. This keeps cracks narrow and evenly distributed, improving appearance and durability. The joints are placed at intervals appropriate for the slab size and are cut to a depth about a quarter of the slab thickness so the crack follows the joint. They don’t prevent cracking entirely or serve as anchors for reinforcing bars, and they aren’t decorative—their purpose is to control crack location and width.