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.
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.