Expansion Joints in Aluminum Vertical Mullions horizontal Movement

3. Plan of Control Joint Pattern

1. Control Joint in Sidewalk

Control Joint Sidewalk


3. Plan of Control Joint Pattern

A control joint is an intentional line of weakness that is created in the surface of a brittle material that tends to shrink. Its role is to encourage any shrinkage cracking that would occur within itself, in order to avoid random cracking of the surface around it.

1. A sidewalk crack is a control joint that is formed by tooling a deep crack into the wet concrete. When the sidewalk shrinks, cracking is channeled to the tooled crack. The sidewalk remains as a group of large, stable rectangular units, rather than as a weak arrav of irregular concrete fragments.

2. A concrete slab floor on grade should be divided by control joints into smaller rectangles that can be expected to stay crack-free. The joints can be created by tooling the wet concrete or by sawing it during the early stages of its curing. With either method, the depth of the joint should be at least 25% of the depth of the slab. Any reinforcing in the slab should be discontinued across the line of the joint. If it is important to maintain a level surface across the joint, smooth, greased steel rod dowels can be inserted. These allow for in-plane movement while preventing out-of-plane movement.

3. As seen in this plan view, slabs should be divided in a wav that avoids j odd-shaped panels that arc prone to cracking. A rectangular panel whose length is greater than 1.5 times its width is likely to crack across its middle. Control joints around columns and pilasters should be cut on a diagonal, as shown, to avoid inside corners that foster cracks. >

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