What are Isolation Joints in Concrete Structures?

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An isolation joint in concrete structures is a type of separation provided to isolate a given structural member (slab) from the adjacent sections of the concrete structure to allow relative movement in three directions. 

For example, a concrete slab isolated from a wall, column, or a drain pipe.

An isolation joint is a type of expansion joint that allows completely independent movement between adjoining structural components.

This article discusses the features of isolation joints in concrete structures.

Features of Isolation Joints in Concrete Structures

• American  Concrete Insititute (ACI) and Portland Cement Institute (PCI) define isolation joint as a joint that completely separates one structural unit from another.
• Isolation joints are gaps between two concrete structures, like a slab and a wall, that are filled using a preformed joint material for the full depth of the slab. This gap allows the slab to move without disturbing the nearby column or wall.
• Consider a slab surrounding the column footing, separated by an isolation joint, as shown in the figure below. In case of settlement of the column, the slab around is undisturbed or won’t crack. Similarly, the slab is allowed to undergo expansion or contraction due to temperature changes. 

• ACI recommends isolation joints to be located at the junction of slabs and walls, columns, footings, equipment foundation, and other restraint points.

• The joint material used for isolation joints can be foam insulation, plastic, cork, rubber, neoprene, or asphalt-impregnated material. 
• Isolation joints are designed and placed in the desired location before the concrete slab is poured. The reinforcing steel provided stops at the joint and should not continue through the joint.
• Isolation joint fillers are filled in a full-depth in the concrete and must accommodate expansion, contraction and allow horizontal and vertical movement.
•  Isolation joint width ranges between 1/2 to 1 inch. Greater widths may cause excessive movement.
• Isolation joint filler material goes all way through the slab depth, starting from the subbase, but should not extend above the top.

• To get a cleaner look, the top of the preformed joint filler is cut off and the space is filled with an elastomeric sealant as shown in the figure above.
• ACI and PCA recommend that the isolation joints allow complete freedom of vertical and horizontal movement to exhibit high performance. 
• The absence of isolation joints can create cracks in slabs due to the movements of nearby elements. Every crack in the slab is a weak point that can result in maintenance or structural issues.

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