Adding Control Joints and Edging

Control joints
Control joints are special joints that control shrinkage cracks in concrete, and are either hand-tooled into the surface, cut with a circular saw, or created with a fixed divider strip. Maximum spacing of these joints will depend on thickness of the pour, the slump, and the maximum aggregate size.

The table below shows the recommended maximum control joint spacing for concrete slabs. Remember that concrete slump is an indication of how much water is in the mix. Low-slump mixes have less water and will shrink less, so control joints can be spaced farther apart, but these mixes are stiff and can be hard to work. With the maximum spacing recommendations from the table in mind, it is best to subdivide concrete into panels that are square rather than elongated. Rectangular areas that are more than one and a half times as long as they are wide are prone to cracking in the middle across the width.

For a slab that is 4 inches thick and is constructed with concrete that has a 5-inch slump and 1-inch maximum aggregate size, the table recommends control joints spaced every 10 feet. For a 10-foot-wide driveway, these control joints would create square panels; but for a 3-foot-wide sidewalk, these control joints would create elongated rectangular panels. Thus the spacing of the joints in a sidewalk should be much closer; the concrete will be less likely to crack if the joints are spaced 3 feet apart to form square panels.