control joint
A groove which is formed, sawn, or tooled in a concrete or masonry structure to regulate the location and amount of cracking and separation resulting from the dimensional change of different parts of the structure, thereby avoiding the development of high stresses.
control joint: where concrete is told to crack
A control joint is a deliberate groove cut or formed into concrete or masonry, typically 1/4 inch to 3/8 inch deep and running the full thickness of the slab or wall. It works by concentrating stress at a weak point you choose, so when the material expands or contracts with temperature and moisture changes, it cracks there instead of randomly tearing the structure. Without control joints, concrete develops map-like crazing or sudden fractures that compromise both appearance and integrity.
Concrete shrinks as it cures and expands when heated, moving anywhere from 0.5 to 2 percent depending on the mix, aggregate, and environmental conditions. A 100-foot pour can shift several inches. Masonry and stone do the same. Control joints relieve this stress by creating a plane of weakness where fractures will propagate predictably. The joint is formed by sawing the surface after the concrete sets but before it fully cures, usually within 24 to 48 hours, or by using a jointing tool or insert during finishing.
Spacing depends on the material and conditions. For interior slabs on grade, joints typically run every 4 to 6 times the slab thickness, so a 4-inch slab might have joints every 20 to 24 feet. Exterior slabs, which experience wider temperature swings, need closer spacing: sometimes every 4 to 8 feet. In masonry walls, control joints run vertically and horizontally, often every 20 to 40 feet and every 20 feet in height. The joint should be fully sealed with a flexible sealant to prevent water infiltration, which accelerates deterioration.
Joints are sometimes filled with compressible foam backer material topped with polyurethane or silicone caulk to allow movement while remaining watertight. Saw cuts can be created with a wet saw fitted with a diamond blade, ideally when the concrete has enough strength to avoid excessive dust and damage but remains soft enough not to cause additional cracking elsewhere. Poor timing, inadequate depth, or wrong spacing leaves the structure vulnerable to random, expensive failure.
Control joints are distinct from expansion joints, which typically accommodate larger movements in bridge decks and building facades, and from isolation joints, which separate concrete from adjacent structures like columns or walls. The terminology sometimes overlaps in the field, but control joints are the most common type in standard concrete work.