What Are Joint Filler Strips?

Joints in concrete made with filler strips prevent cracking.

Joints in concrete made with filler strips prevent cracking.

Concrete expands and contracts as the temperature changes and when it cures. When the slab expands or contracts, internal stress forces the concrete to crack. Joints placed in the concrete relieve the stress and allow the concrete to expand and contract freely. Different types of joints with joint filler strips are used in different applications.

Joint Filler Strips

Joint filler strips are used to make expansion joints, control joints and construction joints in concrete to create gaps that allow the concrete to move freely or crack in a controlled manner. As the concrete expands and contracts, the filler strip changes in size as the joint narrows or widens. Filler strips are made from materials that change size easily: low-density fiberboard, rubber, plastic and other materials. Joints in concrete slabs prevent uncontrolled, random cracks

Expansion Joints

Expansion joints separate a concrete slab into smaller pieces and provide a gap between new and existing concrete structures. They allow the concrete to expand and contract with temperature changes. A joint filler strip is placed against an existing concrete structure before pouring the new slab to create an expansion joint. Within a new slab, expansion joints are created after the concrete is poured by inserting a filler strip into the freshly poured concrete. Common applications include patios, driveways and sidewalks.

Control Joints

Control joints -- also called contraction joints -- force concrete to crack along a predetermined line to relieve stress as the concrete cures. A filler strip with a height that is one-fourth the depth of the concrete is inserted into fresh concrete and made flush with the concrete surface. The same effect is created by a shallow groove or cut made before the concrete cures. As the concrete shrinks, it cracks in a relatively straight line beneath the filler strip, groove or cut. Control joints are common in basement, patio and driveway slabs.

Construction Joints

When construction stops at the end of a days work or for an extended period of time, a joint between an existing concrete slab and the new concrete is required because they will not complete the curing process at the same time. In some cases it is important to join the two slabs, even though a gap between them is required. Filler strips create the gap as required and dowels or rebar connect the two slabs.

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