Contraction joints, also called control or shrinkage joints, allow for contraction or shrinkage of the slab that is inevitable during the concrete curing process. They comprise two types: sawed and pre-molded insert. Sawed joints cut a ridge in the concrete with an induced crack beneath. Pre-molded insert contraction joints place a hardwood board or plastic preformed strip in the sawed-out ridge.
Concrete construction joints are used at stoppage points during the construction process. Three types of construction joints are butt-type joints with dowels, butt-type joints with tie bars, and butt-type. Butt-type joints with dowels require edging each side with 3 mm (1/8 inch) radius and a specially-coated dowel to prevent bonding. Butt-type concrete joints with tie bars require edging on each side with a 1/8-inch radius. It is not a contraction joint. Like the other construction joints, butt-type joints require edging on each side with a 1/8-inch radius. It does not use dowels or tie bars.
Isolation and Expansion Joints
Isolation joints are used to control flexing of concrete due to vertical movement of concrete applications attached to fixed structures, such as buildings and columns. Their primary function is to prevent stress buildup due to confinement of a slab, hence isolating the structure. An expansion joint is used to alleviate stress when a slab is placed next to structures on more than one face and provides protection of both slab expansion and contraction.
Crack Control Joints
Concrete crack control joints are used to control the natural cracking of concrete over time. These joints are similar to contraction joints but are specially formed to isolate cracks at an exact location and in a predictable way.