When are concrete control joints put into the moist concrete?

• A. After the sheen disappears
• B. Immediately after pour
• C. After 24 hours
• D. Before finishing

Answer: (A)

Controlling cracking in concrete hinges on placing joints at the right moment as the slab cures. Control joints are meant to create a planned weak plane so cracks from shrinkage form along the joint, not randomly across the surface. That timing is when the surface has stiffened enough to accept a clean cut but hasn’t dried out completely—specifically, after the bleed water sheen disappears. Cutting at this point yields a clean, deep joint without tearing or flaking the surface. If you cut too soon, the plastic concrete would be damaged and the finish could be ruined; waiting too long increases the chance of uncontrolled cracks that won’t line up with the joint. Cutting before finishing isn’t practical because the joint should be formed after the surface has stiffened but before final finishing is completed.