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Reinforced concrete

From

《

English on Civil Engineering

》

Concrete and reinforced concrete are used as building materials in every country.

In many, including the United States and Canada, reinforced concrete is a dominant

structural

material

in

engineered

construction.

The

universal

nature

of

reinforced

concrete

construction

stems

from

the

wide

availability

of

reinforcing

bars

and

the

constituents of concrete, gravel, sand, and cement, the relatively simple skills required

in concrete construction, and the economy of reinforced concrete compared to other

forms of construction. Concrete and reinforced concrete are used in bridges, buildings

of

all

sorts

underground

structures,

water

tanks,

television

towers,

offshore

oil

exploration and production structures, dams, and even in ships.

Reinforced concrete structures may be cast-in-place concrete, constructed in their

final location, or they may be precast concrete produced in a factory and erected at the

construction site. Concrete structures may be severe and functional in design, or the

shape and layout and be whimsical and artistic. Few other building materials off the

architect and engineer such versatility and scope.

Concrete

is

strong

in

compression

but

weak

in

tension.

As

a

result,

cracks

develop whenever loads, or restrained shrinkage of temperature changes, give rise to

tensile

stresses

in

excess

of

the

tensile

strength

of

the

concrete.

In

a

plain

concrete

beam,

the

moments

about

the

neutral

axis

due

to

applied

loads

are

resisted

by

an

internal tension-compression couple involving tension in the concrete. Such a beam

fails

very

suddenly

and

completely

when

the

first

crack

forms.

In

a

reinforced

concrete beam, steel bars are embedded in the concrete in such a way that the tension

forces needed for moment equilibrium after the concrete cracks can be developed in

the bars.

The construction of a

reinforced concrete member involves

building

a from

of

mold

in

the

shape

of

the

member

being

built.

The

form

must

be

strong

enough

to

support both the weight and hydrostatic pressure of the wet concrete, and any forces

applied

to

it

by

workers,

concrete

buggies,

wind,

and

so

on.

The

reinforcement

is

placed

in

this

form

and

held

in

place

during

the

concreting

operation.

After

the

concrete has hardened, the forms

are removed. As the forms

are removed, props of

shores are installed to support the weight of the concrete until it has reached sufficient

strength to support the loads by itself.

The designer must proportion a concrete member for adequate strength to resist

the

loads

and

adequate

stiffness

to

prevent

excessive

deflections.

In

beam

must

be

proportioned

so

that

it

can

be

constructed.

For

example,

the

reinforcement

must

be

detailed so that it can be assembled in the field, and since the concrete is placed in the

form

after

the

reinforcement

is

in

place,

the

concrete

must

be

able

to

flow

around,

between, and past the reinforcement to fill all parts of the form completely.

The choice of whether a structure should be built of concrete, steel, masonry, or

timber depends on the availability of materials and on a number of value decisions.

The

choice

of

structural

system

is

made

by

the

architect

of

engineer

early

in

the

design, based on the following considerations:

1.

Economy.

Frequently,

the

foremost

consideration

is

the

overall

const

of

the

structure.

This

is,

of

course,

a

function

of

the

costs

of

the

materials

and

the

labor

necessary to erect them. Frequently, however, the overall cost is affected as much or

more by the overall construction time since the contractor and owner must borrow or

otherwise allocate money to carry out the construction and will not receive a return on

this investment until the building is ready for occupancy. In a typical large apartment

of commercial project, the cost of construction financing will be a significant fraction

of

the

total

cost.

As

a

result,

financial

savings

due

to

rapid

construction

may

more

than offset

increased material costs. For this reason, any measures the designer can

take to standardize the design and forming will generally pay off in reduced overall

costs.

In many cases

the long-term

economy of the structure may be more important

than the first cost. As a result, maintenance and durability are important consideration.

2.

Suitability

of

material

for

architectural

and

structural

function.

A

reinforced

concrete

system

frequently

allows

the

designer

to

combine

the

architectural

and

structural functions. Concrete has the advantage that it is placed in a plastic condition

and is

given the desired shape and texture by means

of the forms

and the finishing

techniques. This allows such elements ad flat plates or other types of slabs to serve as

load-bearing

elements

while

providing

the

finished

floor

and

/

or

ceiling

surfaces.

Similarly, reinforced concrete walls can provide architecturally attractive surfaces in

addition

to

having

the

ability

to

resist

gravity,

wind,

or

seismic

loads.

Finally,

the

choice

of

size

of

shape

is

governed

by

the

designer

and

not

by

the

availability

of

standard manufactured members.

3. Fire resistance. The structure in a building must withstand the effects of a fire

and remain standing while the building is

evacuated and the fire is

extinguished. A

concrete building inherently has a 1- to 3-hour fire rating without special fireproofing

or

other

details.

Structural

steel

or

timber

buildings

must

be

fireproofed

to

attain

similar fire ratings.

4. Low maintenance. Concrete members inherently require less maintenance than

do structural steel or timber members. This is particularly true if dense, air-entrained

concrete has been used for surfaces exposed to the atmosphere, and if care has been

taken

in

the

design

to

provide

adequate

drainage

off

and

away

from

the

structure.

Special

precautions

must

be

taken

for

concrete

exposed

to

salts

such

as

deicing

chemicals.

5. Availability of materials. Sand, gravel, cement, and concrete mixing facilities

are very widely available, and reinforcing steel

can be transported to most job sites

more

easily

than

can

structural

steel.

As

a

result,

reinforced

concrete

is

frequently

used in remote areas.

On the other hand, there are a number of factors that may cause one to select a

material other than reinforced concrete. These include:

1.

Low

tensile

strength.

The

tensile

strength

concrete

is

much

lower

than

its

compressive

strength

(

about

1/10

),

and

hence

concrete

is

subject

to

cracking.

In

structural

uses

this

is

overcome

by

using

reinforcement

to

carry

tensile

forces

and

limit

crack

widths

to

within

acceptable

values.

Unless

care

is

taken

in

design

and

construction,

however,

these

cracks

may

be

unsightly

or

may

allow

penetration

of

water.

When

this

occurs,

water

or

chemicals

such

as

road

deicing

salts

may

cause

deterioration or staining

of the

concrete. Special

design details

are

required in

such

cases. In the case of water-retaining structures, special details and / ofprestressing are

required to prevent leakage.

2. Forms and shoring. The construction of a cast-in-place structure involves three

steps not encountered in the construction of steel or timber structures. These are ( a )

the construction of the forms, ( b ) the removal of these forms, and (c) propping or

shoring the new concrete to support its weight until its strength is adequate. Each of

these steps involves labor and / or materials, which are not necessary with other forms

of construction.

3.

Relatively

low

strength

per

unit

of

weight

for

volume.

The

compressive

strength of concrete is roughly 5 to 10% that of steel, while its unit density is roughly

30%

that

of

steel.

As

a

result,

a

concrete

structure

requires

a

larger

volume

and

a

greater

weight

of

material

than

does

a

comparable

steel

structure.

As

a

result,

long-span structures are often built from steel.

4.

Time- dependent

volume

changes.

Both

concrete

and

steel

undergo- approximately

the

same

amount

of

thermal

expansion

and

contraction.

Because there is less mass of steel to be heated or cooled, and because steel is a better

concrete,

a

steel

structure

is

generally

affected

by

temperature

changes

to

a

greater

extent

than

is

a

concrete

structure.

On

the

other

hand,

concrete

undergoes

frying

shrinkage,

which,

if

restrained,

may

cause

deflections

or

cracking.

Furthermore,

deflections

will

tend

to

increase

with

time,

possibly

doubling,

due

to

creep

of

the

concrete under sustained loads.

In

almost

every

branch

of

civil

engineering

and

architecture

extensive

use

is

made of reinforced concrete for structures and foundations. Engineers and architects

requires basic knowledge of reinforced concrete design throughout their professional

careers. Much of this text is directly concerned with the behavior and proportioning of

components that make up typical reinforced concrete structures-beams, columns, and

slabs. Once the behavior of these individual elements is understood, the designer will

have the background to analyze and design a wide range of complex structures, such

as foundations, buildings, and bridges, composed of these elements.

Since reinforced concrete is a no homogeneous material that creeps, shrinks, and

cracks, its stresses cannot be accurately predicted by the traditional equations derived

in

a

course

in

strength

of

materials

for

homogeneous

elastic

materials.

Much

of

reinforced

concrete

design

in

therefore

empirical,

i.e.,

design

equations

and

design

methods are based on experimental and time-proved results instead of being derived

exclusively from theoretical formulations.

A thorough understanding of the behavior of reinforced concrete will allow the

designer to convert an otherwise brittle material into tough ductile structural elements

and

thereby

take

advantage

of

concrete’s

desirable

characteristics,

its

high

compressive strength, its fire resistance, and its durability.

Concrete, a stone like material, is made by mixing cement, water, fine aggregate

(

often

sand

),

coarse

aggregate,

and

frequently

other

additives

(

that

modify

properties ) into a workable mixture. In its unhardened or plastic state, concrete can be

placed

in

forms

to

produce

a

large

variety

of

structural

elements.

Although

the

hardened concrete by itself, i.e., without any reinforcement, is strong in compression,

it lacks tensile strength and therefore cracks easily. Because unreinforced concrete is

brittle,

it

cannot

undergo

large

deformations

under

load

and

fails

suddenly-without

warning.

The

addition

fo

steel

reinforcement

to

the

concrete

reduces

the

negative

effects of its two principal inherent weaknesses, its susceptibility to cracking and its

brittleness. When the reinforcement is strongly bonded to the concrete, a strong, stiff,

and

ductile

construction

material

is

produced.

This

material,

called

reinforced

concrete, is used extensively to construct foundations, structural frames, storage takes,

shell

roofs,

highways,

walls,

dams,

canals,

and

innumerable

other

structures

and

building products.

Two

other characteristics of

concrete that

are present

even when

concrete

is

reinforced

are

shrinkage

and

creep,

but

the

negative

effects

of

these

properties can be mitigated by careful design.

A

code

is

a

set

technical

specifications

and

standards

that

control

important

details of design and construction. The purpose of codes it produce structures so that

the public will be protected from poor of inadequate and construction.

Two

types

f

coeds

exist.

One

type,

called

a

structural

code,

is

originated

and

controlled by specialists who are concerned with the proper use of a specific material

or who are involved with the safe design of a particular class of structures.

The

second

type

of

code,

called

a

building

code,

is

established

to

cover

construction in a given region, often a city or a state. The objective of a building code

is also to protect the public by accounting for the influence of the local environmental

conditions

on

construction.

For

example,

local

authorities

may

specify

additional

provisions

to

account

for

such

regional

conditions

as

earthquake,

heavy

snow,

or

tornados. National structural codes genrally are incorporated into local building codes.