- This article is about pouring concrete in moving forms. See Slipform stonemasonry for another type of slip forming.
Slip forming, continuous poured, continuously formed, or slipform construction is a construction method in which concrete is poured into a continuously moving form. Slip forming is used for tall structures (such as bridges, towers, buildings, and dams), as well as horizontal structures, such as roadways. Slipforming enables continuous, non-interrupted, cast-in-place "flawless" (i.e. no joints) concrete structures which have superior performance characteristics to piecewise construction using discrete form elements. Slip forming relies on the quick-setting properties of concrete, and requires a balance between quick-setting capacity and workability. 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" upwards but also support the freshly poured concrete above it.
In vertical slip forming the concrete form may be surrounded by a platform on which workers stand, placing steel reinforcing rods into the concrete and ensuring a smooth pour. Together, the concrete form and working platform are raised by means of hydraulic jacks. Generally, the slipform rises at a rate which permits the concrete to harden by the time it emerges from the bottom of the form.
In horizontal slip forming for pavement 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.
The slip forming technique was in use by the early 20th century for building silos and grain elevators. In the 1947-1950 period, AT&T Long Lines built 49 concrete towers up to 191 ft tall for microwave relay stations; most using the slip-form method.
The technique was introduced to residential and commercial buildings in the late 1960s. The first notable use of the method in a residential/retail business was the Skylon Tower in Niagara Falls, Ontario, which was completed in 1965. One of Its first uses in high-rise buildings the United States was on the shear wall supported apartment building at Turk & Eddy Streets in San Francisco, CA, in 1962. Another unusual structure was the tapered buttress structures for the Sheraton Waikiki Hotel in Honolulu, Hawaii, in 1969. Another shear wall supported structure was the Casa Del Mar Condominium on Key Biscayne, Miami, FL in 1970.
From the 1960s, the vertical technique was adapted to mining head frames, ventilation structures, below grade shaft lining, and coal train loading silos; theme and communication tower construction; high rise office building cores; shear wall supported apartment buildings; tapered stacks and hydro intake structures, etc. It is used for structures which would otherwise not be possible, such as the separate legs of the Troll A deep sea oil drilling platform which stands on the sea floor in water about 1000 feet (300 m) deep, has an overall height of 472 meters (1,549 ft), weighs 656,000 tons, and has the distinction of being the tallest structure ever moved (towed) by mankind.
In addition to the typical silos and shear walls and cores in buildings, the system is used for lining underground shafts and surge tanks in hydroelectric generating facilities. The technique was utilized to build the Inco Superstack in Sudbury, Ontario, and the CN Tower in Toronto. In 2010, the technique was used to build the core of the supertall Shard London Bridge tower in London, England. It is the most common method for construction of tall buildings in Australia.
- Nawy, Concrete Construction Engineering Handbook, 2008, p. 10—33.
- "'Slip Forming' Technique Introduced in Baltimore." Washington Post. May 1, 1971.
- Nawy, Concrete Construction Engineering Handbook, 2008, p. 10—34.
-  The Concrete Microwave Towers of AT&T's First Transcontinental Radio Route>
- Nawy, Edward G. Concrete Construction Engineering Handbook. New York: CRC Press, 2008.