Control joints, or contraction joints, are sometimes confused with expansion joints, but have a different purpose and function. Concrete and asphalt have relatively weak tensile strength, and typically form random cracks as they age, shrink, and are exposed to environmental stresses (including stresses of thermal expansion and contraction). Control joints attempt to attenuate cracking by designating lines for stress relief. They are cut into pavement at regular intervals. Cracks tend to form along the cuts, rather than in random fashion elsewhere. This is primarily an aesthetic issue; the appearance of even, regular cracking, which may be hidden in the joint’s crevice, is often preferred over random cracking.
Thus, expansion joints reduce cracks, including in the overall structure, while control joints manage cracks, primarily along the visual surface.
Roadway control joints may be sealed with hot tar, cold sealant (such as silicone), or compression sealant (such as rubber or polymers based crossed linked foams). Mortar with a breakaway bond may be used to fill some control joints.
Control joints must have adequate depth and not exceed maximum spacing for them to be effective. Typical specifications for a four-inch-thick slab are:
* 25% depth of material
* spacing at 24× to 36× of slab depth (some specification call for a maximum of 30×)
* special care for inside corners