Control joints must be cut within 6-12 hours of finishing or cracks form randomly

Concrete shrinks as it cures — control joints create intentional weak points so cracking occurs at those lines instead of randomly across the slab. Per ACI 360, joints should be cut to 1/4 the slab depth within 6-12 hours (or 4-6 hours in hot weather). Spacing: no more than 2-3x the slab thickness in feet (4-inch slab = 8-12 foot spacing). Late cutting is the most common cause of random mid-panel cracking.