Which statement describes the main purpose of joints in concrete slabs?

• A. To improve color consistency
• B. To control cracking and accommodate movement
• C. To seal moisture completely
• D. To increase load transfer between slabs without constraints

Answer: (B)

Joints in concrete slabs are designed to control cracking and allow movement. Concrete shrinks as it cures and expands or contracts with temperature changes; without joints, stresses would cause cracks to form randomly and the slab could warp or split. Contraction or control joints provide predetermined weak lines where cracks will occur, keeping them in serviceable locations, while expansion joints accommodate larger movements where the slab meets other structures or changes temperature. This purpose isn’t about color or creating a moisture-proof seal—joints aren’t guaranteed to seal moisture completely—and while there are methods to transfer load across a joint, that isn’t the primary reason joints are included.