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If discontinuous joints cannot be avoided, insert two or three #4 x 3 feet reinforcing bars in the next slab to intercept the crack that will grow from the discontinuous joint (Fig. Due to stress concentrations that occur where joints (i.e., cracks) terminate, cracking will continue into the unjointed concrete. Rule #2: Contraction joints should be continuous, not staggered or offset. For better crack control, limit the length of the long side to 1-1/4 times the short side (Fig.2).įigure 2: Keep jointed panels as square as possible and limit length of long side to about 1.25 x short side for better crack control but absolutely no more than 1.5 x short side. The long side should never be larger than 1-1/2 times the short side.
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Avoid long and narrow panels, L-shaped and T-shape panels. The joint layout should divide a large slab into relatively small, square shaped panels. Rule #1: Panels formed by contraction joints should be as square as possible. For a specified project, the engineer or architect is responsible for designing the joint layout but for unspecified work, the concrete contractor typically becomes the designer. Joint design including the layout is the responsibility of the slab designer. For either method, follow these jointing rules to minimize the risk of random or out-of-joint cracking.
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AC Business MediaĬontraction joints are typically installed with jointing tools while the concrete is still plastic or by sawing after the concrete has been finished with either a wet-cut saw or more commonly, an early-entry dry-cut saw.
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Kim BashamĬlick here to download a free infographic on these rules at /21415569. 1, below).įigure 1: Sawcut creates weakened plane inducing crack below sawcut. As slabs shrink due to cooling and drying, shrinkage or tensile stresses start building and cracks form at contraction joints because the concrete is weaker or thinner at these locations. Contraction joints (sometimes called control joints) are used in unreinforced and lightly reinforced slabs-on-ground to minimize random cracking.īy creating straight-line weakened-planes in concrete, contraction joints “control” the cracking location by inducing cracks at predetermined locations.
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