Keyed construction joints are not recommended for slabs less than how thick?

• A. 6 inches
• B. 4 inches
• C. 8 inches
• D. 12 inches

Answer: (A)

Keyed construction joints rely on a groove or interlock to transfer shear between concrete pours. For the joint to function properly, there must be enough concrete around the key to carry those stresses and to keep the slab’s cross-section intact after the joint is formed. If the slab is thinner than about six inches, there isn’t enough thickness to form a durable key and still maintain adequate strength and integrity at the joint. The key can become a weak point, risking spalling, cracking, or movement across the joint under load or shrinkage, which defeats the purpose of the joint. Therefore, keyed joints are not recommended when slabs are thinner than six inches. In thicker slabs, the key can be properly formed and the slab retains sufficient cross-section to handle loads.