Making Expansion Joints in Concrete Slabs

Expansion joints are installed in a concrete slab where expansion and contraction caused by temperature cycles and moisture are likely to occur. Expansion joints can also alleviate the pressures caused by frost heave, foundation settlement, and the curing process.

The function of the joints is to relieve stress concentrations and to control or minimize the size and locations of potential cracks in the slab. Larger slabs, in particular, require expansion joints at regular intervals.

Expansion joints provide an added advantage when a slab is too large to finish in one pouring: the individual sections created by the expansion joints can be poured one at a time.

Below is some helpful information about installing expansion joints in a concrete slab.

Step 1 - Where to Install Expansion Joints

A prime location for expansion joints is along the perimeter of a slab in a basement floor. The edge of a slab in a full basement rests on the foundation footing and butts against the side of the foundation wall. The expansion joint is installed along the edge of the slab where it meets the foundation wall. A thin layer of sand placed between the slab and the top of the footing allows for some additional expansion. Joints can be installed in the interior of the slab with maximum spacings of 25 feet.

Step 2 - How to Install Expansion Joints in a Typical Residential Foundation

An expansion joint is installed in the slab in one of several ways. The most common method creates a space for the joint as part of the formwork. A board 1-inch thick will leave sufficient space for the insertion of the expansion joint. The board should be coated with a form-release compound or even ordinary motor oil so that it is easily removed when the concrete has set.

Step 3 - Other Types of Expansion Joints

Other types of expansion joints function in much the same way but take on a different form. A saw joint is made by sawing to a maximum depth equal to ¼ the thickness of the slab with a concrete saw. The width of the cut should be no greater than 1/8-inch. The saw cut is then filled with a sealant or with a pre-molded metal strip that is finished flush with the surface of the slab.

Joints that extend through the entire depth of the slab can be butt joined or keyed (tongue and groove) with a preformed metal or plastic joint material. The type and location of expansion joints needed for the construction of an unconventional foundation must be determined by a structural engineer.