Concrete
Control Joint
A planned, tooled or sawed groove in a concrete slab that creates a weak point so that shrinkage cracking occurs in a straight, controlled line.
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In the Field
Saw-cut joints must be made within 4–12 hours of finishing, before random shrinkage cracks form. Cut too early and you ravel the edges; cut too late and the crack has already gone where it wanted. Timing depends on mix, weather, and set time.
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Rule of Thumb
Space control joints no farther apart than 2–3 times the slab thickness in feet. A 4-inch slab gets joints every 8–12 feet maximum.