28-01-2013, 12:22 PM
Assignment On Slip Form Technology
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INTRODUCTION
In concrete construction, a form designed to move upward slowly (usually by means of hydraulic jacks or screw jacks), supported by the hardened concrete of the wall section which was poured previously is known as Slip form construction.
Slipforming Technique is defined and described in various ways in literatures. According to Risser (1995) “Slipforming differs from conventional concrete forming because of the forming panels move semi continuously in relation to the concrete surface being formed and form ties are not used.” In the opinion of Camellerie (1978) “Slipforming has proved invaluable tools of construction by cutting cost and man hours and at the same time permitting construction to proceed with maximum safety.” American Concrete Pavement Association (ACPA, 1995) defined, “Slipform paving as a process used to consolidate, form into geometric shape and surface finish a Plain Concrete Cement (PCC) mass by pulling the forms continuously through and surrounding the plastic concrete mass.”
Two types of work are mainly carried out by slipform techniques, namely,
1. Vertical Slipforming
2. Horizontal Slipforming
Slip forming, continuous poured, continuously formed, or slipform construction is a construction method in which concrete is poured into a continuously moving form.[1] Slip forming is used for tall structures (such as bridges, towers, buildings, and dams), as well as horizontal structures, such as roadways. Slip forming 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 packed, 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.
1. 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.
2. 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, but was introduced to residential and commercial buildings in the late 1960s.[2] 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.