What is control jointin construction?

An expansion joint (or isolation joint) is a full-depth separation with compressible filler that lets two elements move independently under thermal change or differential settlement, typically at building lines or where a slab meets walls, columns, or footings. A control joint only manages drying shrinkage; it is partial depth and gives the slab nothing but a straight place to crack. The cost gap is large: a sawcut is typically a per-lf item in the low single dollars, while a formed expansion joint with filler, sealant, and dowel load transfer runs several times that, so never let one stand in for the other in a takeoff.

Masonry control joints are continuous vertical joints, and the common US rule of thumb is spacing at no more than 1.5 times the wall height and no more than 25 ft, whichever governs, plus joints at openings, intersections, and changes in wall height or thickness. They are typically built with sash block and a preformed rubber gasket, then sealed with backer rod and sealant on both faces. Estimators count them each or in vertical lf, and the sealant often lives in Division 07 rather than the masonry spec, so coordinate the two scopes.

It depends on traffic and exposure. Industrial floors with hard-wheeled forklift traffic typically get a semi-rigid epoxy or polyurea filler installed the full depth of the cut to protect joint edges from spalling, ideally as late in the schedule as possible so more shrinkage has already happened. Exterior and wet-area joints usually get an elastomeric sealant over backer rod to keep water out, while interior slabs going under finishes are often left unfilled. Read the spec either way, because filler is a separate per-lf item that can cost more than the sawcut itself and is easy to miss in a bid.