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12 precast material& ITS COMPONENT.pdf
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Transcript of 12 precast material& ITS COMPONENT.pdf
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S. V. National Institute of Technology, Surat
2.1 Introduction
Precast concrete is a construction product produced by casting concrete in a reusable mould
or "form" which is then cured in a controlled environment, transported to the construction site and
lifted into place. In contrast, cast-in-site concrete is poured into site specific forms and cured on site.
FIGURE 2.1
By producing precast concrete in a controlled environment which is called precast plant,
the precast concrete is afforded the opportunity to properly cure and be closely monitored. Utilizing a
precast concrete system offers many potential advantages over site casting of concrete, one of themthe production process for precast concrete is performed on ground. There is a greater control of the
quality of materials and workmanship in a precast plant rather than on a construction site.
Chapter
02 PRECAST MATERIAL & ITS COMPONENTS
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Financially, the forms used in a precast plant may be reused hundreds of times before they have to be
replaced, which allow cost of formwork per unit to be lower than for site-cast production.
2.2 Historical Brief
Ancient Roman builders made use of concrete and soon poured the material into moulds to buildtheir complex network of aqueducts, culverts, and tunnels. Modern uses for pre-cast technology
include a variety of architectural and structural applications featuring parts of or an entire building
system.
FIGURE 2.2
In the modern world, precast panelled buildings were pioneered in Liverpool, England in
1905, when the process was invented by city engineer John Alexander Brodie who designed the tramstables at Walton in Liverpool in 1906.
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2.3 Advantages of Precast Concrete
Because precasting is done at the ground level, the cost of formwork and shoring is considerably
reduced. Formwork cost reduction is also achieved through the use of standard-size elements cast in
permanent forms, which are reused several times. Precasting also allows greater quality control over
the strength of concrete and surface finishes, and most surface finishes are obtained more easily in aprecast plant.
FIGURE 2.3
Factory-made products:
The only way to industrialize the construction business is to shift the work from the site to modern
permanent factories. It means rational and efficient manufacturing processes, skilled workers,
repetition of actions, quality surveillance, etc.
Prefabrication has a much greater potential for economy, structural performance and
durability: Optimal use of materials is obtained through modern manufacturing equipment and
carefully studied working procedures.
As a consequence, precast products are much more slender and present a high durability and
longevity. Shorter construction time - less than half of conventional cast in-situ construction: Today
the demand for a speedy return on investment is becoming increasingly important: the initiallyagreed construction terms have to be met.
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Adverse weather conditions don't influence the construction process: Prefabrication is
independent of adverse weather conditions and production continues normally in wintertime when
the temperature falls below200 C. Site work stops when the temperature falls below - 5C..
2.4 Disadvantages of Precast Concrete
Precast concrete has many disadvantages, its main disadvantage is the cost of transportation,
although precast members are generally lighter than corresponding site-cast members, they are still
fairly heavy, transportation also limits the length and width of precast members.
FIGURE 2.4
Another disadvantage of precasting is the need for heavier hoisting equipment at the
construction site and additional safety measures that must be observed during construction.
Installation and assembly at the site also introduce the need for more skilled workers compared with
site-cast concrete construction. Architecturally, the most limiting factor in the use of precast concrete
is the difficulty in sculpting concrete at a large scale, which is more easily realized with site-cast
concrete, this is because precast elements are generally straight, with standard profiles.
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TABLE 2.1 Precast Concrete Cast-in-Place Concrete
1. Machinery & Labour
Requires more machinery like trucks
and cranes, in addition skilled workers
and qualified contractors.
Requires less machinery, and it is
easier to find suitable labours and
contractors.
2. Cost
Lower initial cost, especially for large
projects, also the formworks cost lessbecause they may be used for
hundreds of times.
Higher initial cost, but as cast-in-
place structures requires lessmaintenance, its life cycle cost may
be less than precast structures, so this
factor is relative.
3. Time
Shorter site Construction time, precast
concrete structures can be created in
advance and held until the time we
need them.
Longer construction time due to
formwork installation and removing
and concrete curing time.
4. Concrete Quality
We can achieve better concrete quality
control in plant conditions. Thereforestronger and lighter concrete units.
A number of uncontrollable factors
can decrease the strength and qualityof cast-in-place concrete including
inaccurate mixing, weather changes,
etc
5.Weather Conditions
Pre-cast concrete structures are poured
in a controlled environment, so
weather is not an influencing factor
for delaying the construction progress.
Cast-in-place concrete is affected by
cold temperatures and extremely wet
conditions, this can delay concrete
casting. Also sever hot and an arid
environment requires strict treatments
and control for concrete curing.
6. Maintenance
Precast structures need more
maintenance, they require sealant
joints to prevent leakage, and they
may need to be replaced every decade.
As cast-in-place structure have fewer
joints, so they need less maintenance.
7. Recyclability
Can be unfastened and removed and
reused in another location with
minimal damage and loss.
Cast-in-place concrete structures are
very difficult to be reused; steel bars
only can be recycled.
8. Durability
Because precast structures have many
joints, they dont afford lateral forcesand they require more shear walls to
resist those lateral forces like wind
pressure or seismic forces.
Monolithic structures with fewer
joints, so they are more resistant tolateral and seismic forces.
9. SustainabilityLess materials consumption, more
sustainable.
More materials consumption, so less
sustainable.
10. Flexibility
Modular designs offer less flexibility
in forms but more flexibility in last
minute changes and extensions. Less
head rooms and more projections anddepths for connections and layered
Cast-in-place structures offer more
flexibility in design and ability to
create plastic shapes. Neat structures,
as the structural elements are casttogether and intersected.
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2.5 Precast Concrete Production
Concrete elements, cast and cured in a manufacturing plant, then transported to the
construction site, plant casting allows increased efficiency and higher quality control. Durable,
permanent steel forms are reused many times, reducing formwork costs compared to cast-in-place
concrete.
FIGURE 2.5
The use of high early strength cement and steam curing allow concrete members to be cast and cured
in only 24 hours, controlled casting conditions and high quality forms allow for greater control of
surface finishes.
2.6 Transportation & Hoisting of Precast Concrete
Precast members are often more difficult and costly to ship than the bulk cement and gravel
ingredients used for mixing concrete on the job site. Giant cranes are needed to hoist these heavyhunks of concrete.
FIGURE 2.6
Special care must be taken, during hoisting and placing, not to put stresses on them that are
different from the ones they are designed to take in their final position in the structure. The erection
procedure, and in turn the design of the structure is very much affected by the weight and size of the
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individual components. The transport of the components is typically by truck and as such, it is
imperative to be aware of allowable component size and weight.
.
The lifting of any precast product requires the use of special equipment and substantial
planning and knowledge. The selection of crane type and size is an important ingredient to the
viability of the precast structure, usually; mobile cranes are the most economical solution.
FIGURE 2.7
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2.7 Precast Concrete Elements
FIGURE 2.8
2.8 Precast Concrete Slabs
Precast concrete slabs are used for floor and roof decks, deeper elements span further than those that
are shallower.
FIGURE 2.9
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2.9 Precast Concrete Beams
Structural beams, including rectangular beams, L-shaped beams, and inverted tee beams,
AASHTO are used to support horizontal deck components such as double tees and hollow core slabs.
FIGURE 2.11
FIGURE 2.10
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2.10 Precast Concrete Columns
Precast concrete columns are typically used with precast beams to form a structural frame.
Because rigid joints are difficult to fabricate in a precast structural frame, shear walls or diagonal
bracing are normally relied upon to stabilize the structure against lateral forces.
`
FIGURE 2.12
FIGURE 2.13
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2.11 Precast Concrete Walls & panels
Precast concrete wall panels may serve as bearing walls capable of supporting site cast
concrete or steel floor and roof systems. Together with precast concrete columns, beams, and slabs,
the wall panels form an entirely precast structural system that is inherently modular and fire-resistive.
The lateral stability of a precast concrete structure requires that those floors and roofs that serve ashorizontal diaphragms be able to transfer their lateral forces to shear-resisting wall panels. The wall
panels, in turn, must be stabilized by columns or cross walls as they transfer the lateral forces to the
ground foundation. All forces are transferred by a combination of grouted joints, shear keys,
mechanical connectors, steel reinforcement, and reinforced concrete toppings.
Precast concrete wall panels are cast and steam-cured in a plant off site, transported to the
construction site, and set in place with cranes as rigid components. Fabrication in a factory
environment enables the units to have a consistent quality of strength, durability, and finish, and
eliminates the need for on-site formwork. The precast wall panels may be conventionally reinforced
or prestressed for greater structural efficiency, reduced panel thicknesses, and longer spans. In
addition to the required tensile, shrinkage, and temperature reinforcement, extra reinforcement may
be necessary to resist the stresses of transportation and erection. Precast wall panels may be of solid,
composite, or rifted construction, window and door openings, corbels, and anchoring devices are cast
into the wall panels. A variety of quality-controlled surface textures and patterns are available.
FIGURE 2.14
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2.12 Precast Special Elements
FIGURE 2.15
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Summary
Precast construction includes those buildings, where the majority of structural components are
standardized and produced in plants away from the building, and then transported to the site for
assembly.
Because precasting is done at the ground level, the cost of formwork and shoring is considerably
reduced.
Precast concrete has disadvantages; most of them are the transportation and hoisting requirements.
Concrete elements, cast and cured in a manufacturing plant, plant casting allows increased
efficiency and higher quality control.
Precast concrete slabs are used for floor and roof decks, deeper elements span further than those
that are shallower.
Because precast structures lack rigid joints, shear walls or diagonal bracing are normally relied
upon to stabilize the structure against lateral forces.