If construction joints are designed to perform as contraction joints, specifications commonly require construction joints to be sawcut similar to sawcut contraction joints. For these types of joints, finishers should edge along the edge form so joints are easily visible for accurate sawing. Saw cutting construction joints encourages the formation of clean, straight cracks along the joints. Also, consider applying a curing compound or bond breaker to the face of construction joints to discourage bonding of the adjacent concrete to the existing joint faces. This also helps ensure a straight crack forms along the construction joint. Typically, sawcut construction joints are filled with semi-rigid joint fillers, especially when joints are exposed to hard-wheeled traffic.
Stoning or lightly grinding is an alternative to edging construction joints. After stripping edge forms or bulkheads, a carbide stone can be used to round the edges of the existing slab before placing the adjacent concrete. If edges are not ground sufficiently, joints may be difficult to follow while sawing resulting in misaligned saw cuts. Misaligned saw cuts are unsightly and difficult to repair or conceal.
For edging isolation or construction joints, use an edger with a 18-inch radius edge. A larger edge radius will create surface widths that are too wide for sawing and hard-wheeled traffic. Before edging, use a pointed mason's or margin trowel to clean off the edge form and to loosen or cut the concrete from the inside face of the edge form using a vertical sawing motion. During the edging process, hand-trowel adjacent surfaces to remove edger markers and to minimize differences in floor color, texture and gloss.
Addressing the details does take extra time, effort and perhaps more finishers, but the overall appearance and quality of the floor will be enhanced.
Kim Basham is president of KB Engineering LLC, which provides engineering and scientific services to the concrete industry. Basham also teaches seminars and workshops dealing with all aspects of concrete technology, construction and troubleshooting. He can be reached via email at KBasham@KBEngLLC.com.