Concrete shrinks as it dries and will crack somewhere, so the crew cuts or tools a groove at least one quarter of the slab depth, and the crack forms in the thin section under it. Spacing is commonly held to 24 to 36 times the slab thickness, about 8 to 12 ft for a 4 in slab, with panels kept close to square. Cut too late or too shallow and the slab picks its own crack line.
Control joints at ten feet each way, and get the saws on it tonight, not tomorrow morning.
Sawcut timing is a window: too early and the blade ravels the edges, too late and the slab has already cracked on its own.
Also heard as
- contraction joint
- sawcut
- saw joint
- tooled joint
Do not mix it up with
- Isolation joint: An isolation joint separates the slab from walls or columns through its full depth; a control joint only weakens the slab to steer cracking.
Related terms
Drying shrinkage
The volume loss of hardened concrete as it gives up moisture over months and years.
Early-entry saw
A lightweight saw that cuts control joints within hours of finishing, before shrinkage cracks can start.
Wild crack
An uncontrolled crack that wanders across a slab panel instead of forming under a control joint.
Isolation joint
A full-depth separation between a slab and a fixed element, so each can move without cracking the other.
Dowel
A smooth steel bar across a slab joint that transfers load while letting the joint open and close.