12 precast material& its component.pdf

7/24/2019 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.

12 precast material& its component.pdf

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