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.
