PRECAST CONCRETE

PRECAST CONCRETE

Every construction material & system has its own characteristics which to a greater or less extend influence the layout, span length, construction depth, stability system, ect. This is also the case for precast concrete , not only in comparison to steel, wood, & masonry structures, but also with respect to cast in-situ concrete. Theoretically, all joints between the precast units could be made in such a way that the completed precast structure has the same monolithic concept as a in-situ one. However, this is a wrong approach & one, which is very labour intensive & costly. If the full advantages of precast concrete are to be realized, the structure should be conceived according to its specific design philosophy:

Long spans, appropriate stability concept, simple details, ect. Designers should from the very outset of the project consider the possibilities, restrictions & advantages of precast concrete, its detailing, manufacturer, transport, erection & serviceability stages before completing a design in precast concrete.

Primary function of precast concrete is :

1.         Keep water out

2.         Prevent air leakage

3.         Control light

4.         Control radiation of heat

5.         Control conduction of heat

6.         Control sound

Secondary function of precast concrete is :

1.         Resist wind forces

2.         Control water vapour

3.         Adjust to movement

4.         Thermal and moisture expansion or contraction

5.         Structural movement

6.         Resist fire

7.         Weather gracefully

8.         Easy to install

 Architectural precast concrete provides architects with an exciting medium when designing facades for a wide range of buildings, from healthcare facilities to shopping malls, commercial office buildings to sports stadiums.

Precast concrete provides:

•   Complete thermal protection

•   Continuous air/vapour barrier

•   Effective rain screens

•   Superior lifespan

•   Reduced construction schedule and on-site  labour

•   High quality control standards

•   Numerous finish options and colours

There is three kinds of precast building system. That is :

1.      Precast concrete frame

2.      Precast concrete wall

3.      Precast concrete floor

1.         Precast concrete frame

           Precast concrete frames involve an entire structure being fabricated off-site. In addition, structural components can be supplied for incorporation into a structure on-site. Frames can simultaneously achieve both structural and decorative design requirements - a wide variety of mixes, colours and finishes can be accommodated. Architecturally-finished precast structures can be left exposed to exploit concrete's high thermal capacity in a building's "green" energy management system.

           Precast concrete frames are cast in the same way as precast concrete cladding, but as they are designed as structural elements have heavier reinforcement than is required for non-structural cladding. Elegant connections are required between columns and beams to transfer considerable forces without adversely affecting the visual appearance of the frame.

           The benefits of precast concrete frame is :

Ø  Enable faster programme times - not affected by weather or labour shortages.

Ø  Improve buildability - structure is fabricated off-site for rapid erection on-site

Ø  Produce a high standard of workmanship in factory conditions - reduces potential for accidents, addresses on-site skill shortage.

Ø  Have a high quality finish that can be left exposed - concrete's thermal properties can be exploited in low-energy buildings.

2.         Precast concrete wall

           Precast wall are used for internal & external walls, lift shafts, central cores ect. Precast wall system are mostly used in domestic construction, both for individual housing & for apartments. The solution can be considered as the industrialized from of cast in-situ walls or classical brick or block masonry walls.

           The precast walls can be load bearing or only partition walls. The surface of the elements is smooth on both sides & ready for painting or wall papering.

           Precast walls offer the advantage of speed of construction, smooth surface finishing, acoustic insulation & fire resistance.

           

3.         Precast concrete floor

           The principle advantages of precast floors are speed of construction, absence of scaffolding, large variety of types, large span capacity, & economy. Precats floors can also be classified according to their manufacture into totally & partially precast floors.

           Totally precast floors are composed of units, which are totally cast at the plant. After erection, the units are connected to the structure & the longitudinal joints are grouted.In some cases a cast in-situ structural topping screed is added. Partially precast floors are composed of a precast part & a cast in-situ part. Both parts are working together at the final stage to achieve the composite structural capacity. The main totally precast floor & roof types are described here after.

   

                                                                 

Tendencies in precast concrete :

Ø  Structural efficiency

Ø  Flexibility in use

Ø  Optimum use of materials

Ø  Speed of construction

Ø  Quality consciousness

Ø  Adaptability

Ø  Protection of the environment

Advantages of precast concrete :

Ø  Less expensive than real brick.

Ø  Does not require an extensive footing like a real masonry wall. Can be reset should it ever shift.

Ø  More durable than real masonry - no mortar joints to let water in.

Ø  Installs quickly and easily - most can be done in one day.

Ø  Maintenance free - No need to paint or periodically replace slats as you would with wood fences.

Disadvantages of precast concrete :

Ø  System building is less flexible in its design concept than purpose-mode structures

Ø  Most design briefs can be fulfilled without too much modification to the original concept

Ø  Structural connection between the precast concrete units can present both design & contractual problems

        At the conclusion, pre-fabrication has a great potential to respond to new market demands. Possible solution lie not only within the classical advantages related to working conditions, technology & speed of construction, but also in new developments of materials such as high performances & self-compacting concrete, buildings system such as mixed structures, manufacturing technology, automation, service integrated products & others.