Concrete shrinks as it dries and masonry moves with moisture and temperature, so builders weaken the section at set spacings with saw cuts, tooled grooves or vertical joints in block walls. The crack then forms below the groove, where it can be sealed neatly. Control joints see smaller movement than expansion joints but still need an elastomeric sealant with proper depth control.
Saw-cut control joints every 12 feet on the slab, rod them and seal them with the self-leveling urethane.
Saw-cut slab joints that are sealed before the concrete has finished shrinking often fail adhesively in the first winter; seal as late as the schedule allows.
Also heard as
- contraction joint
- CJ
Related terms
Expansion joint
A designed gap between building elements that lets them move with temperature, settlement and load without cracking.
Cold joint
The seam where one concrete pour meets a later one, a classic leak path in below-grade walls and slabs.
Backer rod
A round foam cord pushed into a joint to set sealant depth and keep the sealant from bonding to the back.
Polyurethane sealant
A tough, paintable elastomeric sealant used widely on concrete, masonry and traffic joints.