Slip forming

Slip forming, continuous poured, continuously formed, or slipform construction is a construction method in which concrete is placed into a form that may be in continuous motion horizontally, or incrementally raised vertically.

In horizontal construction, such as roadways and curbs, the weight of the concrete, forms, and any associated machinery is borne by the ground. In vertical construction, such as bridges, towers, buildings, and dams, forms are raised hydraulically in increments, no faster than the most recently poured concrete can set and support the combined weight of the concrete, forms, and machinery, and the pressure of concrete consolidation.

Slipforming enables continuous, non-interrupted, cast-in-place, cold joint- and seam-free concrete structures that have performance characteristics superior to those of piecewise construction using discrete form elements.^{[citation needed]}

Slip forming relies on the quick-setting properties of concrete, and requires a balance between workability and quick-setting capacity. Concrete needs to be workable enough to be placed into the form and consolidated (via vibration), yet quick-setting enough to emerge from the form with strength. This strength is needed because the freshly set concrete must not only permit the form to "slip" by the concrete without disturbing it, but also support the pressure of the new concrete and resist collapse caused by the vibration of the compaction machinery.

In horizontal slip forming for pavement, curbs, and traffic separation walls, concrete is laid down, vibrated, worked, and settled in place while the form itself slowly moves ahead. This method was initially devised and utilized in Interstate Highway construction initiated by the Eisenhower administration during the 1950s.

In vertical slip forming the concrete form may be surrounded by a platform on which workers stand, placing steel reinforcing rods ahead of the concrete and ensuring a smooth pour. Together, the concrete form and working platform are raised by means of hydraulic jacks. The slipform can only rise at a rate which permits the concrete to harden by the time it emerges from the bottom of the form.

The slip forming technique was in use by the early 20th century for building silos and grain elevators. James MacDonald, of MacDonald Engineering of Chicago was the pioneer in utilizing slip form concrete for construction. His concept of placing circular bins in clusters was patented, with photographs and illustrations, contained in a 1907 book, "The Design Of Walls, Bins, And Grain Elevators".

In 1910, MacDonald published a paper "Moving Forms for Reinforced Concrete Storage Bins," describing the use of molds for moving forms, using jacks and concrete to form a continuous structure without joints or seams. This paper details the concept and procedure for creating slip form concrete structures. On May 24, 1917, a patent was issued to James MacDonald of Chicago, "for a device to move and elevate a concrete form in a vertical plane".