The When and Why of Concrete Expansion Joints

by Kevin Smith Author

A concrete expansion joint is a useful tool for when you need to ensure that pressure does not build up between two slabs of concrete or as opposed to using one solitary slab. Knowing when to use a concrete expansion joint, however, is a bit trickier. Here is a brief rundown on the most appropriate setting in which to use this type of joint.

It's Purpose

As mentioned, a concrete expansion joint is used to prevent a pressure buildup between two concrete slabs. That pressure is generally created when hot weather warms up the concrete to the point it expands. The expansion causes two joints to compress against each other. The result is chipping and cracking.

An easy way to think of this is to picture our earth’s tectonic plates at a fault. When a plate shifts into another plate, immense pressure is built up that eventually leads to a fracturing of the earth’s crust. If the pressure and shift are large enough, earthquakes occur. An easy way to envision the role of a concrete expansion joint is that it provides a bridge over two “plates” of concrete and prevents them from colliding, building pressure and fracturing.

It's Uses

Concrete expansion joints are used in a variety of ways. They play a role with concrete based roadways to allow for expansion on highways or bridges when summer heat causes everything to expand. They have multiple domestic purposes, including, but not limited creating a bridge between separating slaps in a concrete driveway, patio, garage or sidewalk. They can also be used to bridge two concrete surfaces, such as a patio to a staircase or a driveway to a sidewalk, etc.

How it Works

Expansion joints generally come in two forms:

Preset Connection Points
Sawed Connection Points

Preset connection points are marked before any slab is poured or positioned. They are based on the perimeters of each slab. The positioning is uniform throughout the concrete work. The joint creates a buffer zone between slabs that allows for expansion and contraction.

Sawed Connection Points are installed after a slab has been laid and hardened. They are generally positioned in areas where the pressure in the slab could lead to cracking. The benefit of a sawed connection point expansion joint is that they can be placed wherever they are needed and not just where the perimeter of a slab ends.

When to Use

The answer of when to use a concrete expansion joint is where pressure can build up and lead to fracturing. The type of joint to use and its placement has to be assessed as accurately as possible or the intended purpose will not be achieved.