What describes forms that are placed by extrusion?

The description of forms that are placed by extrusion is best represented by slip forms. Slip forms are a type of construction formwork that allows for continuous pouring of concrete as the form moves or "slips" along with the setting material. This method is especially useful for creating tall structures like silos and high-rise buildings, where the formwork can be raised steadily while concrete is poured. This continuous pouring process results in a uniform and dense concrete structure, which can be vital for the structural integrity and aesthetic of the final product.

Stay in place forms are designed to be left in their position after the concrete cures; they do not move or change. Slab forms are typically used to create flat surfaces, such as floors, but they do not fit the characteristics of forms that are extruded. Load-bearing forms refer to temporary structures designed to support loads during construction but do not relate to the continuous nature of slip forms. Therefore, slip forms accurately encapsulate the concept of forms being placed by extrusion.