A key joint is a precast device used to create control joints in concrete structures, reducing cracking caused by shrinkage. It consists of a U-shaped steel channel with flanges at either end, inserted into the forms where control joints are required. Key joints speed up construction, reduce labor and equipment costs, and create a smooth, even joint. They may require maintenance once the concrete has fully cured.
A key joint is a precast device used to create control joints in concrete structures. As freshly poured concrete begins to dry, it tends to shrink or contract. This shrinkage can lead to cracks or breaks in the concrete surface, or in tile and other floor finishes installed above the slab. To minimize cracking associated with concrete shrinkage, builders place control joints at specific intervals along the concrete. These control joints represent a planned crack in the concrete, eliminating the need for unplanned cracks elsewhere.
To form a traditional control joint, installers must form and pour concrete in a checkerboard pattern. Several 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 a single unit. The key joint device is placed in the formwork at all points 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 fixture 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 fixture in place until the concrete is poured. The upper flange is usually rounded or square in 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 the various sections of the concrete, which allows them to move and contract with minimal cracking.
Keystone joints in concrete offer a number of benefits to builders and developers. These devices speed up the construction process and can reduce both labor and equipment costs when installing concrete. Because the key joint is prefabricated, it allows installers to create a smooth, even joint along the entire length of the structure. Steel key joints are also easy to work with and often interlock using a simple male and female connection.
Key joints may require maintenance once the concrete has fully cured. If the concrete shrinks too much when it dries, there may be some extra space left between the joint and the edge of the slab. These spaces should be filled with mastic or epoxy to prevent wear and to keep dirt and debris from collecting in the joint. These fixtures also tend to cost more than traditional concrete forms, although material costs may be offset by savings on labor and equipment.
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