Concrete expansion joints are absolutely essential for large concrete slabs in order to prevent the expansion and contraction of the concrete from cracking the slab. There are at least three types of expansion joints that can be installed into concrete, which will prevent cracks and protect the concrete. The average do-it-yourselfer can install expansion joints with relative ease.
The first concrete expansion joint can be installed during the pouring of the concrete. Bitumen expansion strips are manufactured in 10 cm (4 inch) wide strips that are 3 m (10 feet) long. The strips are one-half inch thick and composed of tar and fibre. When the strips are inserted between concrete sections, they allow for concrete movement. For example, a 3 m (10 foot) concrete section is poured and allowed to dry, and another ten foot section is poured against the previous one, with the expansion strip being laid after the previous section has dried and before the next is poured; however, you will need to remove your wooden forms from the previous concrete section.
You may also install expansion joints using an expansion joint tool while the concrete is wet. Use a 5 x 10 cm (2 x 4 inch) beam as a guide while you run the tool along the top of finished, wet concrete. The tool will insert a 1.2 cm (1/2 inch) deep joint that will separate the concrete into sections and prevent cracking outside of the joint. Make sure that the concrete is dried enough to tool. If the concrete is too wet, then the tool will cause damage to the concrete. You should be able to touch the concrete without leaving a fingerprint.
You can also place expansion joints into your concrete after it has dried using a concrete saw. You will need to snap chalk lines onto your dried concrete to guide your work. The concrete saw will create large amounts of dust, so use a water hose to wet the cut as you go. This will prevent dust. Make sure that the joint depth is at least one-quarter of the way through the concrete when using the saw method.