Placed where a slab meets columns, walls, footings, drains or an existing slab, it is filled with a compressible preformed strip such as asphalt-impregnated fiberboard or closed-cell foam. The joint lets the slab shrink, curl and settle on its own instead of locking against the structure and cracking at the corner. Around interior columns it is usually formed as a diamond or circle so cracks do not start at the column corners.
Wrap every column with isolation joint foam before the pour, I'm not chasing corner cracks at the diamonds again.
Most crews call it an expansion joint, but in a slab its real job is letting the concrete shrink and move away from fixed elements.
Also heard as
- expansion joint
Related terms
Control joint
A planned groove or sawcut that makes a slab crack in a straight line exactly where the crew wants it.
Construction joint
A planned stopping point between two placements, prepared so the next pour bonds to it or transfers load across it.
Dowel
A smooth steel bar across a slab joint that transfers load while letting the joint open and close.
Wild crack
An uncontrolled crack that wanders across a slab panel instead of forming under a control joint.