Concrete control joint spacing — cut joints so the slab cracks where you want it to
Concrete shrinks and will crack — control joints decide where. Space them (in feet) at about 2–3× the slab thickness (in inches): a 4″ slab gets joints every ~8–12 ft, cut ¼ of the depth within 6–12 hours.
What this diagram shows
Two slabs compared. The left slab has no control joints, so as the concrete shrinks while curing it cracks in a random line that wanders across the face. The right slab is divided by sawn control joints into roughly square panels, so the crack forms straight down a joint where it stays hidden. The rule of thumb: control-joint spacing in feet is about 2 to 3 times the slab thickness in inches, so a 4-inch slab gets joints every 8 to 12 feet, and the joints are cut one quarter of the slab depth within 6 to 12 hours of the pour.
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