PREFABRICATION

In the United States, a construction worker's wage can be 4 times higher than that of Hong Kong, but the production cost of high rise residential flat is more or less the same as that in Hong Kong

Reasons:

• Extensive use of prefabrication; e.g. wall, staircases, external facades, door sets, etc.
• Use of labour saving fixing technology.

A steel mould for precast facade is lifted up from horizontal to vertical position by portal frame machines

The movement of a precast facade by portal frame

The Advantages of Prefabrication are:

Mass production of units

• Automation of the manufacturing process can save labour and reduce price
• Designers can get used to the standards units and have ready access to details

Reduction of costs and construction time on site

• Less work to be done on site
• Saving in the use of formwork on site
• Precast units can be erected in bad weather

Effective use of formwork

• Steel formwork is normally used and increases the number of use to 200 times
• Precast units can be shaped so that they are self-stripping and this means the reduction in labour and wear on moulds

Improved quality of units

• Factory production under strict quality control
• Precast units can be closely checked after manufacture

Special shapes and surface finishes

• Units can be cast in any position, such as upside down, on their sides, etc
• Coloured concrete can be produced by using white cement and a colour pigment

Casting under cover

• Protection from hot or drying winds

Demountable structures

• Bolted connections can be easily dismantled and re-erected in other places

Construction over and under water

• No or little formwork is required
• Falsework is not required
• Minimal disruption to traffic
• Precast bridge can be constructed without falsework

Casting of units before the site becomes available

• Units can be casted and stocked up before the site becomes available which can shorten the construction time

Built-in services and insulation

• Services and insulation can be built into precast units accurately in the factory

Use of semi-skilled labour

Accelerated curing techniques

• Higher turnover per mould and plant
• Controlled curing results in more durable units

Solution to the problem of lack of local resources and labour

• Units can be produced thousands of kilometres away from the site

The Limitations  of Prefabrication are:

• A small number of units required may prove to be uneconomical
• Special connections, such as special bearings to transmit the vertical and horizontal loads, can add cost to the system
• Waterproofing at joints
• Transportation difficulties
• Need for cranes

Components and Jointing

1. Components are manufactured units, made to pre-determined sizes, to be used in building. Dimensional co-ordination governs their design and use and forms the necessary discipline for industrialised, "system" or "component" building. For example a prefabricated standard staircase component to rise 2660 mm will govern the floor to floor height of the building in which it is to be fixed.

The installation of a precast staircase resting on the staircase landing

A precast staircase

2. Prefabricated wall cladding panels will closely govern the storey heights and the length of the building, or part of a building where they are used. The structural frame is usually erected by site work methods, e.g. insitu reinforced concrete, and the prefabricated units, whether mass produced to standard sizes or specially made for the particular building project, are fitted to it. An accurate tie-up between the respective dimensions of structure and cladding units is essential, and only a certain degree of tolerance my subsequently be allowed for either.

A Precast facade panel

A precast facade panel held in position with temporary support

3. Standardised components are joined together to form building elements. The dimensional co-ordination between all the components concerned is essential, and for this method of building it is necessary that this co-ordination shall be based upon a suitable module. That is a certain unit length which shall form the increment of change of size, and so the overall dimensions of the building will be a multiple of these modules or units of length. The dimension of the individual component need not be that of the module, it may extend over a number of modules, or possibly be a sub-multiple of a module, but must (with allowance for joints) be directly related to it.

Prefabricated wall panels being installed in public housing

4. It is necessary, when intending to use extensive prefabrication of components, to design the building from the start on a reference grid related to the intended module.

5. A module of 300 mm is generally satisfactory when dealing with the plan of a building where small increments of dimension are not necessary. However, in vertical heights and in the dimensions of main building elements, such as column sizes, thickness of internal partitions, an increment of 300 mm is excessive and clumsy, so a smaller module (related to the standard module) is required.

A module could, of course, be any suitable arbitrary length, but for convenience and to take full advantage of this method of building it is necessary to have an agreed length which is generally 100 mm, termed the basic module.

6. Actual manufactured components will vary slightly when made from the specified dimensions and may be subject to further movement subsequently during storage, transport and handling, and possibly during use.

   Two sorts of variation or deviation must be taken into account:

   1. Variation in size
   2. Variation in shape (distortion)

   Both variation in size and shape are to be allowed within acceptable limits. For any component there will be a maximum and a minimum limit of size, between which the actual size (i.e. the size as ascertained by measuring the actual component) must lie. The difference between these two limits represents the tolerance allowed.

   The tolerance will vary according to the size and nature of the component and upon the degree of accuracy called for - working within fine limits of accuracy may be expensive in relation to the cost of the article as normally produced.

   The variations in size and shape, and space necessary to move the component into position and fix it, are allowed for in the width of the joint between components.

   The joint width will vary with the work size (size specified for its manufacture) + deviation (variation in size and shape) for each component.

   The actual size of one component must be contained within the co-ordinating space and must not encroach upon the basic space of the next. Thus the actual size must always be less than the basic size.

7. The Joint

   The greater the accuracy of the components to be joined, the less width will the joint require, subject to a minimum gap to allow for movement of the components into position and for filling material (e.g. mastic) if required.

   The joint between components is the major problem in industrialised system building. If external, its problems are those of weather exclusion, thermal insulation, and fire resistance. All joints have a need for adequate strength and security. The aesthetic problem of large joints exposed is a serious one.

   Weather exclusion is accomplished either by designing the joint to be self draining (i.e. water which gets in will run out without penetrating to the inside surface) or rely upon some form of sealant, usually bitumen, rubber or plastic based.

8. Joints in Precast Concrete Wall Panels

   The design of the joints should be as simple and economical as possible, so long as the following basic requirements are taken into account:
   
   

   • Allowance for all dimensional changes
   • In-situ concrete connections should be kept to a minimum
   • All joints should be able to transfer loads form one unit to another or to other parts of a structure
   • All joints must have the same degree of fire resistance as the structure
   • All external panel joints should be insulated so that cold bridges will not be formed
   • External panel joints should be weather-proofed to resist moisture penetration and draughts.
   • Ease of maintenance