Which joint type is used to control cracking in slabs?

• A. Isolation Joint
• B. Control Joint
• C. Expansion Joint
• D. Contraction Joint

Answer: (B)

Cracking in slabs happens as concrete dries and cures and is restrained by surrounding edges. If you don’t create a planned weak point, cracks tend to form randomly and can run across the slab, which is undesirable. A control joint provides that planned weak plane—it's a groove or formed joint cut into the slab to encourage cracks to form along that line. By spacing these joints at regular intervals, the slab is divided into smaller panels and cracking is predictable and easier to repair, improving both performance and appearance. The depth of the joint is typically a fraction of the slab thickness to ensure the crack follows the joint rather than propagating across the surface. That's why control joints are used to manage cracking.

Isolation joints separate slabs from other structures to allow independent movement and do not manage cracking within the slab itself. Expansion joints accommodate temperature and moisture-induced expansion, not cracking control. Contraction joint is a term sometimes used synonymously with a control joint in some contexts, but the function remains to provide a planned cracking line in the slab.