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Slip forming is a building technique which involves the use of forms which are “slipped” progressively as the structure grows longer, with finished molded material coming out of the bottom of the form, while new material for molding is poured into the top of the form. This technique has been used to build very large structures since the 1950s, and some of the world's most ambitious building projects have been accomplished in part with the assistance of slip forming. People can sometimes see examples of this building method in action at sites such as high rise construction sites.
In slip forming, people start by creating a mold, pouring concrete into it, working it, and then sliding the mold up as the concrete starts to set so that more concrete can be poured into it. When the structure is finished, the concrete will be seamless, because even though it was done in sections, it was done while the concrete was still highly plastic, and the sections blend seamlessly together. This technique can be used in many settings where other types of forms are not possible.
Slip forming can also be done with masonry. In slip form masonry, a mason starts the masonry inside a form and moves the form up as the top of the form is reached to add subsequent layers. The form keeps the masonry even and tidy, allowing the builder to add until the desired height is reached. One drawback to this technique with masonry is that the masonry cement can smear on the forms, necessitating careful cleaning of the masonry so that the face of the wall will be clean.
This technique is used in many settings where casting in place is desired for logistical reasons. Slip forming can be faster, cheaper, and more efficient than other building techniques. In some settings, it may be the only technique which is possible for the application. It also results in structures with a high tensile strength and integrity, which can be useful from an engineering and safety perspective. The strength created with slip forming allows engineers to push structural limits in other ways.
People can use slip forming for small projects as well as high rise buildings. It's a good idea to experiment with small practice projects before delving into a large and complex one to get used to the ins and outs of slip forming on practice materials.
I read somewhere that slip forming is how they made the legs of an enormous deep sea diving platform. They could not have made it any other way, just given the sheer size and the necessary construction parameters. The concrete foundation had to withstand the pressure of the ocean floor, so it had to be strong and seamless.
It weighs over half a million tons! And they towed it out to its present position in the ocean. It's the heaviest thing ever moved by humans, although admittedly the ocean must have taken some of the weight!
If you ever wondered how the concrete chimneys of nuclear power plants and other similar industrial buildings are made, this is how they do it.
Slip form construction is pretty much the easiest way to get such a smooth and even form in a quick and efficient way.
And in structures like these which need to be completely seamless, it is the only way to do it properly.