26-07-2012, 01:13 PM
PREFABRICATION:QUALITY CONTROL & SAFETY
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Prefabrication-
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure 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. It is frequently used when fabrication of a section of a machine or any movable structure is shifted from the main manufacturing site to another location, and the section is supplied assembled and ready to fit. It is not generally used to refer to electrical or electronic components of a machine, or mechanical parts such as pumps, gearboxes and compressors which are usually supplied as separate items, but to sections of the body of the machine which in the past were fabricated with the whole machine. Prefabricated parts of the body of the machine may be called 'sub-assemblies' to distinguish them from the other components.
The process and theory of prefabrication
An example from house-building illustrates the process of prefabrication. The conventional method of building a house is to transport bricks, timber, cement, sand, steel and construction aggregate, etc. to the site, and to construct the house on site from these materials. In prefabricated construction, only the foundations are constructed in this way, while sections of walls, floors and roof are prefabricated (assembled) in a factory (possibly with window and door frames included), transported to the site, lifted into place by a crane and bolted together.
Manufactured homes
Manufactured housing
Constructing manufactured homes typically involves connecting plumbing and electrical lines across the sections, and sealing the sections together. Manufactured homes can be single, double, or triple-wide, describing how many sections wide it is. Many manufactured home companies manufacture a variety of different designs, and many of the floorplans are available online. Manufactured homes can be built onto a permanent foundation, and if designed correctly, can be difficult to distinguish from a stick-built home to the untrained eye
Manufactured homes are typically purchased from a retail sales company, initially assembled by a local contracting company, and follow-up repairs performed by the manufactured home company under warranty.
A manufactured home, once assembled, goes through a "settling-in" period, where the home will settle into its location. During this period, some drywall cracking may appear, and any incorrectly installed appliances, wiring, or plumbing should be repaired, hopefully under warranty. If not covered under warranty, the costs will be borne by the consumer. For this reason, it is important that the consumer ensure that a reputable and honest contractor is used for the initial set-up. If any repairs are not completed by the initial set-up crew, the manufacturer will send repair crews to repair anything covered by the warranty.
Methods of Prefabrication:
1. Hand-laying Pre-fabrication Method
o In this method, bricks are laid in mortar using conventional masonry bricklaying tools or automated equipment. Automated equipment uses devices to establish the shape of the panel, scaffolding (which usually is adjustable to increase mason productivity), delivers the required materials to the brick layers and aids in the application of mortar. Mortar spreaders, are used to spread the mortar especially at the joints. This method can be used at the construction site or elsewhere. It is commonly used for brick and block masonry panels.
2.Casting Pre-fabrication Method
o This method involves the use of unskilled labor to combine masonry units, mortar and grout. Automated equipment, such as jigs, are used to place units, reinforce, align and introduce mortar and grout. This method is used at a place far from the designated construction site and it involves the application of atmospheric pressure to the form that is full of grout. The pressure is applied at the surface of the front part of the brick by placing a heavy load on the brick.
3.Special Mortar Equipment Pre-fabrication Method
o There are various tools that are used for this method. However, regardless of the tools that are used, the process must meet the standards set by the American Society for Testing and Materials (ASTM). During the preparation of pre-fabricated panels, pressure is applied using high bond mortars which provide the required amount of bonding strength.
Conclusion:
Advantages of prefabrication
1. Self-supporting ready-made components are used, so the need for formwork, shuttering and scaffolding is greatly reduced.
2. Construction time is reduced and buildings are completed sooner, allowing an earlier return of the capital invested.
3. On-site construction and congestion is minimized.
4. Quality control can be easier in a factory assembly line setting than a construction site setting.
5. Prefabrication can be located where skilled labour is more readily available and costs of labour, power, materials, space and overheads are lower.
6. Time spent in bad weather or hazardous environments at the construction site is minimized.
7. Less waste may be generated and in a factory setting it may be easier to recycle it back into the manufacturing process, for instance it is less costly to recycle scrap metal generated in a metal fabrication shop than on the construction site.
8. Moulds can be used several times.
Disadvantages
1. Careful handling of prefabricated components such as concrete panels or steel and glass panels is required.
2. Attention has to be paid to the strength and corrosion-resistance of the joining of prefabricated sections to avoid failure of the joint.
3. Similarly, leaks can form at joints in prefabricated components.
4. Transportation costs may be higher for voluminous prefabricated sections than for the materials of which they are made, which can often be packed more efficiently.
5. Large prefabricated sections require heavy-duty cranes and precision measurement and handling to place in position.
6. Larger groups of buildings from the same type of prefabricated elements tend to look drab and monotonous.
7. Local jobs are lost.