Figure 7-32.-Typical isolation and control joints.

transfer devices built into them. Examples of these are shown in figures 7-32, 7-33, and 7-34.

Control Joints

Movement in the plane of a concrete slab is caused by drying shrinkage and thermal contraction. Some shrinkage is expected and can be tolerated, depending on the design and exposure of the particular structural elements. In a slab, shrinkage occurs more rapidly at the exposed surfaces and causes upward curling at the edges. If the slab is

Figure 7-33.-Isolation joints at columns and walls.

Figure 7-34.-Expansion/contraction joint for a bridge.

restrained from curling, cracking will occur wherever the restraint imposes stress greater than the tensile strength. Control joints (figure 7-35) are cut into the concrete slab to create a plane of weakness, which forces cracking (if it happens) to occur at a designated place rather than randomly. These joints run in both directions at right angles to each other. Control joints in interior slabs are typically cut 1/3 to 1/4 of the slab thickness and then filled with joint filler. See table 7-1 for suggested control joint spacings. Temperature steel (welded wire fabric) can be used to restrict crack width. For sidewalks and driveways, tooled joints spaced at intervals equal to the width of the slab, but not more than 20 feet (6 meters) apart, should be used. The joint should be 3/4 to 1 inch deep. Surface irregularities along the plane of the

Figure 7-35.-Control joints.