Most people know that contraction (a/k/a, control) joints are placed in concrete slabs to force them to separate at controlled places as the concrete dries or the underlying base shifts. Isolation and expansion joints serve a similar purpose, keeping the forces of expansion or shifting of a section of concrete from being transferred to an adjacent structure.
Isolation joints separate a concrete slab from something else like a wall, a post, or a pipe. Since walls and columns are on their own footings, deeper than a slab’s, they don’t move the same as it does when it shrinks or expands, or the subgrade below it compresses.This differential motion can cause the slab to crack or heave if separation isn’t maintained between it and the other structural object. It can also damage pipes and drains.
While isolation joints are recommended both inside and out, expansion joints are designed primarily for outdoor situations where weather extremes can add to slab movement. While contraction control joints will generally suffice for concrete paving, if the pavement is next to a building or other immovable structure, expansion joints can be advisable. Expansion joints are basically extra-wide isolation joints that add extra protection against blowups in extreme conditions or in long unjointed sections.
Making Isolation and Expansion Joints
Both isolation and expansion joints should extend the full depth of the slab with preformed joint material extending down to the subbase and ending flush with the slab. For a cleaner look, the joint filler material can be trimmed slightly below the slab surface and covered with an elastomeric sealer. At columns, contraction joints should extend in all four directions from the isolation joint, which should be in a diamond or circular shape around the column. Isolation joints should not be done until the column is carrying its full dead load.
For more information about isolation and expansion joints, see this article at ConcreteNetwork.com.
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