Control joint
Also called: contraction joint, saw cut, crack control joint
A groove tooled or saw-cut into a concrete slab, at least a quarter of its depth, so that shrinkage cracks form in the joint instead of across the surface. Spaced about 2–3 times the slab thickness in feet: 8–12 feet for a 4-inch slab.
Concrete shrinks as it cures and will crack somewhere. A control joint is a deliberate weak line — a groove 1 inch deep in a 4-inch slab — that tells it where. Joints are laid out so each panel is roughly square, no more than about 1.5 times longer than it is wide, and they run through the slab’s narrowest points and to any inside corner, which is where cracks start.
They’re cut one of two ways. A groover tooled into the surface during finishing, after the bull float and before the broom, which gives a rounded edge. Or a saw cut made after the concrete has hardened enough not to ravel but before it has cracked on its own — usually 6–18 hours after finishing, sooner in hot weather. Miss that window and the slab picks its own line.
Control joints are not expansion joints. An expansion joint is a full-depth strip of fiberboard or foam that separates the slab from a house foundation, a wall, or an existing slab so the two can move independently. A slab needs expansion material wherever it meets something fixed, and control joints across its own surface.