Figure 7-22.-Use of a contraction Joint.incident to shrinkage of the concrete. A typical dummycontraction joint (fig. 7-22) is usually formed by cuttinga depth of one third to one fourth the thickness of thesection. Some contracting joints are made with no filleror with a thin coat of paraffii or asphalt and/or othermaterials to break the bond. Depending on the extentof local temperature, joints in reinforced concrete slabsmay be placed at 15-to 25-ft intervals in each direction.Expansion JointsWherever expansion might cause a concrete slab tobuckle because of temperature change, expansion joints(also called isolation joints) are required. An expansionjoint is used with a pre-molded cork or mastic filler toseparate sections from each other, thus allowing roomfor expansion if elongation or closing of the joint isanticipated. Figures 7-23, 7-24, and 7-25 showFigure 7-23.-Expansion joint for a wall.Figure 7-24.-Expansion joint for a bridge.Figure 7-25.-Expansion joint for a floor slab.expansion joints for a variety of locations. Expansionjoints may be installed every 20 ft.CONCRETE FORMSMost structural concrete is made by placing(also called CASTING) plastic concrete into7-17
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