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What is a Key Joint?

By B. Turner
Updated May 17, 2024
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A key joint is a pre-manufactured device used to create control joints in concrete structures. As freshly-poured concrete begins to dry, it tends to shrink or contract. This contraction can lead to cracks or breaks in the surface of the concrete, or in tiles and other floor finishes installed over the slab. To minimize cracks associated with concrete shrinkage, builders place control joints at specific intervals along the concrete. These control joints represent a planned break in the concrete, eliminating the need for unplanned cracks at other locations.

To form a traditional control joint, installers must form and pour the concrete in a checkerboard pattern. Different sections are poured and allowed to dry before other adjacent sections can be poured. This process is time-consuming, and involves extensive form work and planning.

Using a key joint, installers can pour an entire concrete structure as one single unit. The key joint device is placed into the form work at all locations where control joints are required. Concrete is poured into the forms, and the key joint automatically creates a control joint in the structure.

A standard concrete key joint consists of a U-shaped steel channel with flanges at either end. The device is inserted into the forms so that the flanges are perpendicular to the ground. A metal dowel is hammered through the U-shaped portion of the joint to hold the device in place until the concrete is poured. The top flange usually has a rounded or square shape to create an aesthetically pleasing finish, while the remaining flange simply tapers to a point. As the concrete dries, the U-shaped portion of the joint maintains space between various sections of the concrete, which allows them to shift and contract with minimal cracking.

Concrete key joints offer a number of benefits to builders and developers. These devices speed up the construction process, and can reduce both labor and equipment costs during concrete installation. Because the key joint is pre-manufactured, it allows installers to create a uniform, even joint along the entire length of the structure. Steel key joints are also easy to work with, and often fit together using a simple tongue-and-groove connection.

Key joints may require some maintenance once the concrete is completely cured. If the concrete shrinks too much as it dries, some extra space may be left between the joint and the edge of the slab. These spaces should be filled with caulk or epoxy to prevent wear, and to keep dirt and debris from collecting in the joint. These devices also tend to cost more than traditional concrete forms, though material costs may be offset by savings on labor and equipment.

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