In concrete constructions such as sidewalks and driveways, expansion joints provide that the concrete slabs, rather than breaking, may move to eliminate stress due to environmental elements such as water freezing under the concrete. For example, a typical sidewalk has expansion joints positioned about every five feet and a typical driveway has expansion joints about every eight feet. Expansion joints typically are made from materials such as cedar wood or asbestos impregnated celloulosic material which withstand weathering and aging. Other useful expansion joint materials include plastic joints and fibrous mesh materials.
In a typical concrete construction, wooden forms, held in place by stakes, confine fresh concrete. Specifically, the wooden forms are placed in desired locations and then expansion joints typically are placed adjacent to one side of the forms. Wooden or steel pegs hold the forms and joints in the desired position. Concrete then is poured in the areas defined by the forms. Since it is impractical to pour concrete a section at a time, it is necessary to pour a large amount of coverage at one time and then work the concrete as it begins to harden. A skillful concrete finisher will know just about when he can enter onto the concrete, usually on some large flat surface such as a section of plywood, and remove the forms. The expansion joints are left in place and a worker then fills interstices left by removing the forms with fresh concrete after which the entire slabs are allowed to set.
The problem of confining freshly poured concrete until it sets is long standing. U.S. Pat. No. 1,495,305, issued in 1924, discloses a system for using side rails to confine concrete and also to function as rails for a creeping concrete mixer used to pour the concrete. This system is still in use today in the fabrication of large concrete projects such as highways.
U.S. Pat. No. 4,198,176, issued in 1980, discloses a concrete expansion joint construction made from bent metallic sheet material formed to include a central trough adapted to receive an expansion joint wherein the entire structure remains permanently imbedded in the concrete. No completely satisfactory system and equipment for retaining fresh concrete has been developed which can be installed and removed quickly with minimum labor.