Types of joints in concrete

Written by daniel j. gansle
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Types of joints in concrete
Concrete joints are used for different purposes. (fresh concrete image by jimcox40 from Fotolia.com)

Most drivers are familiar with the inevitable "thump, thump" noise while driving on roadways. During the construction process, a number of concrete joints are implemented for specific purposes. Joints help alleviate contraction, expansion, cracking and vertical movement of concrete. They are also used at stoppage points during different phases of construction.

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Contraction Joints

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: sawn and pre-moulded insert. Sawn joints cut a ridge in the concrete with an induced crack beneath. Pre-moulded insert contraction joints place a hardwood board or plastic preformed strip in the sawn-out ridge.

Construction Joints

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 build-up 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.

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