What is a control joint and why is it used?

• A. A joint that connects two structural elements to transfer loads.
• B. A planned joint that concentrates cracking to controlled locations from shrinkage.
• C. A random joint to reduce costs.
• D. A joint that allows movement due to thermal expansion only.

Answer: (B)

A control joint is an intentional weakening plane placed in a concrete member to direct where cracks will form as the material shrinks and changes temperature. As concrete cures it loses moisture and tends to crack in tension. By creating a planned joint, those cracks are concentrated along a defined line rather than appearing randomly across the surface. This helps control the crack pattern, keeps cracks from running across the slab in unwanted places, and preserves both strength and appearance. The joint is typically formed by sawing a groove or forming a groove during placement to a depth about a quarter to a third of the slab thickness, and it’s spaced based on slab size and conditions.

Other options aren’t correct because a joint that transfers loads is a different type of joint and not what a control joint is for; a random joint to save costs would defeat the purpose of controlled cracking; and a joint for thermal expansion only omits the shrinkage cracking control that is the primary function of a control joint.