14. TYPES OF PRECAST CONSTRUCTION.pdf

Chapter

04

TYPES OF PRECAST CONSTRUCTION

4.1 Introduction to Precast Concrete The concept of precast construction includes those buildings where the majority of structural components are standardized and produced in plants in a location away from the building, and then transported to the site for assembly. This type of construction requires a restructuring of the entire conventional construction process to enable interaction between the design phase and production planning in order to improve and speed up the construction. One of the key premises for achieving that objective is to design buildings with a regular configuration in plan and elevation.

4.2 Precast Concrete in Detail Precast concrete slabs, beams, and structural tees are one-way spanning units that may be supported by site cast concrete, precast concrete, or masonry bearing walls, or by steel, site cast concrete, or precast concrete frames. The precast units are manufactured with normal density or structural lightweight concrete and prestressed for greater structural efficiency, which results in less depth, reduced weight, and longer spans.

FIGURE 4.1

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4.3 Prestressed Concrete Prestressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose those found when in use. Prestressed concrete refers to concrete that has applied stresses induced into the member. Typically, wires or tendons are stretched and then blocked at the ends creating compressive stresses throughout the member's entire cross-section. Most Prestressed concrete is precast in a plant.

4.4 Lift-Slab Method Lift slab construction is a method of constructing concrete buildings by casting the floor or roof slab on top of the previous slab and then raising (jacking) the slab up with hydraulic jacks, so being cheaper and faster as it does not need forms & shores as it is needed for cast-in-place slabs.

FIGURE 4.2

4.5 Slip-form Construction Slip-form construction (slip forming, continuous poured, or continuously formed) is a construction method in which concrete is poured into a continuously moving form. Basically, this method involves the continuous placing of concrete in a shallow mould having the same plan as the building to be constructed. This rigid mould, or "slip-form" as it is called, forms the working deck which is jacked slowly upwards at a controlled rate until the required elevation is reached.

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4.6 Tilt-up Construction Tilt-up construction is a method of casting reinforced concrete wall panels on site in a horizontal position, then tilting them up into their final position. The principal advantage of tilt-up construction is the elimination of the costs associated with constructing and stripping vertical wall forms; this cost saving is offset by the cost of the crane required to lift the completed wall panels into place.

4.7 Tunnel Forms

FIGURE 4.3

Tunnel form is a formwork system that allows the contractor to cast walls and slabs in one operation in a daily cycle. It combines the speed, quality oral accuracy of factory/off-site production with the flexibility and economy of in-site construction and it is recognised as a modern method of construction (MMC).

FIGURE 4.4

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The result is a cellular reinforced structure, the surfaces of which are sufficiently high quality to require only minimal finishing for direct decoration, while the end walls and facades are easily completed with thermally insulated units that can be clad as required.

4.7 Timber Construction Timber construction (wood framing) is the predominant method of building homes and apartments in the United States. Increasingly, wood framing is also being used in commercial and industrial buildings. Wood frame buildings are economical to build, heat and cool, and provide maximum comfort to occupants. Wood construction is readily adaptable to traditional, contemporary and the most futuristic building styles. Its architectural possibilities are limitless.

4.8 Light Gauge Steel Framing Light gauge steel construction is the non-combustible equivalent of wood light frame construction. The external dimensions of the standard sizes of light gauge members correspond closely to the dimensions of the sizes of nominal 2-inch framing lumber, and these steel members are used in framing as closely spaced studs, joists, and rafters in much the same way as wood light frame members are used.

FIGURE 4.5

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4.9 Prefabricated Buildings Prefabrication in construction is the practice of assembling components of a building in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the building is to be located. The term is used to distinguish this process from the more conventional construction practice of transporting the basic materials to the construction site where all assembly is carried out. The term prefabrication also applies to the manufacturing of things other than structures at a fixed site.

FIGURE 4.6

4.10 Portable & Temporary Buildings Portable & temporary buildings are buildings designed and built to be movable rather than permanently located. Portable are often used temporarily and taken away later. They have been used since prehistoric times. The most familiar modern type of portable buildings are designed so that one can be carried to or from site on a large lorry and slung on and off by a crane.

FIGURE 4.7

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4.11 Construction Equipment It is a common fact that we find a wide variety of construction equipment on every construction site, which makes the construction jobs easy, safe and quicker. The selection of the appropriate type and size of construction equipment often affects the required amount of time and effort and thus the jobsite productivity of a project.

FIGURE 4.8

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