What is the difference between a control joint and an expansion joint? What is their purpose, and how are they formed?
Control joints are typically used in concrete masonry to reduce the occurrence of shrinkage-related cracking. A control joint is a continuous vertical joint filled with mortar, but with a bond breaker on one side so that tensile stress cannot develop across the joint. If control joints are not provided, a concrete masonry wall may crack as it shrinks over time. Where control joints are provided in such a wall, they widen as the concrete masonry shrinks, preventing it from cracking. Control joints should be provided at regular intervals along the wall's length and near corners, returns and changes in the wall's height, support or stiffness. Control joints will not relieve masonry expansion. Although concrete masonry expands during warm weather, it generally expands less than it shrinks. Control joints are often constructed to transfer lateral loads across the joint. The National Concrete Masonry Association TEK 10-2 shows several construction methods. Expansion joints, on the other hand, are typically used to accommodate thermal and moisture expansion in clay brick masonry. An expansion joint is a continuous vertical or horizontal joint, left completely free of mortar and filled with elastomeric sealant to keep it watertight. Clay brick masonry expands over time. Expansion joints accommodate this expansion as the sealant compresses. Masonry expansion joints for clay or shale brick should be designed using the procedures outlined in the Brick Industry Association's Technical Note 18A. Like control joints, expansion joints should be provided near corners in the masonry, near returns or changes in the planes of the masonry wall, at any significant changes in the wall's height or stiffness, at changes in foundations and at regular intervals along the wall. In particular, horizontal expansion joints should usually be provided underneath the shelf angle of the overlying story in clay masonry veneer. Building expansion joints are different from masonry expansion joints. Building expansion joints are generally joints in the building structure that separate the building into different sections. Often wide, these joints are intended to accommodate movements that exceed those associated with the masonry itself.