Blowups are usually caused by compressive forces produced when concrete expands during hot weather. If the concrete is prevented from moving, the expansive forces cause vertical displacement of the slab at each side of the crack or joint.

Blowups are most likely to occur in concrete slabs with contraction joints that have partially or completely filled with soil, sand, or other incompressible materials. Incompressible material in the joints prevents them from absorbing the expansion that occurs when the ambient temperature rises. Pavements or sidewalks constructed while ambient temperatures are below 40° F are more likely to experience blowups because the larger the difference between placing and service temperature, the greater the temperature-induced expansion.

Prevention: Good design, construction, and maintenance of highway and street contraction joints should prevent most blowups and avoid problems associated with expansion joints. For golf-cart paths, sidewalks, and other lightly loaded pavements, full-depth expansion joints may help prevent blowups. One contractor solved a blowup problem on golf-cart paths by installing 1-inch-thick expansion-joint material at 250-foot intervals. To provide load transfer at the expansion joint, he used three dowels, one in the center and the other two about 6 inches from each edge of the path. Tooled contraction joints were spaced 6 to 8 feet apart.