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“Prefabrication” is the manufacture of an entire building or components cast in a factory or on site before being placed in position, assembling the structural units so that they can be easily and rapidly erected. Prefabricated buildings are pre-cut, pre-drilled, and pre-engineered before the actual building is constructed. Prefabricated Structures (PFS) are useful for sites, which are not suitable for normal construction methods such as hilly regions, and also when normal construction materials are not easily available. Structures which are used repeatedly and can be standardized such as mass housing, storage sheds, godowns, shelters, bus stands, security cabins, site offices, foot over bridges, road bridges, concrete building blocks etc., are prefabricated structures. This paper discussing about the mix proportions f the materials to manufacture the prefab components and the construction procedure for an ordinary single story residential building.
Criteria for selection of prefab in India
In India, the technology adopted for housing components should be of the order that, the production and erection technology be adjusted to suit the level of skills and handling Facilities available under metropolitan, urban and rural conditions. In other words, the structural systems and components selected should ensure minimum material Utilization with maximum structural advantage. However, the component and systems so designed are to be manufactured and erected by manual means in villages, semi mechanical techniques in towns and more or less fully mechanical operations in the metropolitan cities. In urban areas, the concentration of construction activity does not justify prefabrication. A wide variety of roofing methods to suit these requirements have been developed and used on mass scale in many housing projects such as funicular shell, cored slabs, RCC channel units, precast cellular units, precast RC planks and joists, prefab brick panels and joists, RCC joists and hollow concrete blocks.
Material used for Prefabrication
Prefabricated components can be manufactured using many materials, depends up on the type of structure. Generally materials used are Wood, Steel, concrete, plastic and composite materials .If the structure is wooden house then prefabricated wooden panels, columns, posts are used. If the structure is prefabricated steel truss bridge then steel members like angles, I, L-sections and rives, bolts are used. If the structure is houses in developed countries then the prefabricated plastic internal partition walls, doors, panels and toilets are used. The man made rock concrete is also widely used for the manufacturing of the prefabricated members. Instead of using complete cement concrete we are replacing the some quantity of cement with an industrial waste product like flyash. Because of adding the flyash we can get many benefits for the environment as well as prefabricated structure. Before going for the advantages of it, first know the properties of the flyash.
ADVANTAGES OF PRECAST/PREFABRICATED CONSTRUCTION
1) In prefabricated construction as the components is readymade, selfsupporting, shuttering and scaffolding is eliminated with a saving in shuttering cost.
2) In traditional construction, the repetitive use of shuttering is limited, as it gets damaged due to frequent cutting, nailing etc. On the other hand, the mould for the precast components can be used for large number of repetitions thereby reducing, the cost of the mould per unit.
3) In prefabricated housing system, there is saving of time as the elements can be casted before hand during the course of foundations being laid and even after laying slab, the finishes and services can be done below the slab immediately. While in the conventional in-situ RCC slabs, due to props and shuttering, the work cannot be done, till they are removed. Saving of time means saving of money.
4) In prefabricated construction, there is better quality control, shape and size of precast elements. Therefore, in structural design, full advantage of properties of cement and steel can be exploited. There is disciplined use of scarce materials like cement, steel and timber.
5) In precast construction, the construction is not affected due to weather, rain, wind etc.
6) We can save Labour cost of mixing, placing of concrete.
7) As the components are light weight they can be transport easily and place and construct ever is needed especially during natural calamities like floods, earth quakes, major accidents are takes place.
8) Because of easy of placing, these are mainly used for construction of temporary bridges during wars.
9) We can construct structures which are located in congested places like where the lack of place for storage of materials and machinery.