工程管理外文翻译(原文+译文)

Concrete Construction matter

T. Pauly, M. J. N. Priestley

Abstract

Viewed in terms of accepted practices, concrete construction operations leave much to be desired

with respect to the quality, serviceability, and safety of completed structures. The shortcomings of

these

operations

became

abundantly

clear

when

a

magnitude

7.6

earthquake

struck

northern

Paki-stan on October 8, 2005, destroying thousands of buildings, damaging bridges, and killing an

esti-mated

79,000

people.

The

unusually

low

quality

of

construction

operations

prevalent

was

a

major cause of the immense devastation.

Keywords: Concrete

Placing

Curing

Construction Technology

Placing Concrete

If concrete is placed in the surface, the sur-face should be filled with water sufficiently to prevent

it from absorbing the concrete of its water. If fresh concrete is to be placed on or nearby to concrete

that has solidified, the surface of the placed concrete should be cleaned absolutely, preferably with a

high-pressure air or water jet or steel-wire brushes. The surface should be wet, but there should be no

much

water.

A

little

quantity

of

cement

grout

should

be

brushed

over

the

whole

area,

and

then

followed immediately with the application of a 1/2-in Layer of mortar. The fresh concrete should be

placed on or against the mortar.

In order to decrease the disintegration re-sulting from carriage after it is placed. The con-crete

should be placed as nearly as probably in its

final point. It should be placed in layers to permit uniform compaction. The time interval between the

placing of layers should be limited to assure perfect bond between the fresh and previously placed

concrete.

In placing concrete in deeper patters, a ves-sel should be used to limit the free fall to not over 3

or 4 ft, in order to prevent concrete disintegra-tion. The vessel is a pipe made of lightweight metal,

having adjustable lengths and attached to the bottom of a hopper into which the concrete is deposited.

As the patters are filled, sections of the pipe may be removed.

Immediately

after

the

concrete

is

placed,

it

should

be

compacted

by

hand

pudding

or

a

me-chanical vibrator to eliminate voids. The vibrator should be left in one position only long enough

to reduce the concrete around it to a plastic mass; then the vibrator should be moved, or disintegra-

tion of the aggregate will occur. In general, the vibrator should not be permitted to penetrate concrete

in the prior lift.

The mainly advantage of vibrating is that it permits the use of a drier concrete, which has a higher

strength because of the reduced water content. Among the advantages of vibrating con-crete are the

following:

1.

The decreased water permits a reduction in the cement and fine aggregate because less cement

paste is needed.

2.

The lower water content decreases shrinkage and voids.

3.

The drier concrete decreases the cost of finishing the surface.

4.

Mechanical vibration may replace three to eight hand puddles.

5.

The lower water content increases the strength of the concrete.

6.

The drier mixture permits theremoval of some patters more quickly, which may reduce the

cost of patters.

Curing Concrete

If concrete is to gain its maximum strength and other desirable properties, it should be cured with

adequate moisture and at a favorable tem-perature. Failure to provide these conditions may result in

an inferior concrete.

The initial moisture in concrete is adequate to hydrate all the cement, provided it is not should

replace

the

moisture

that

does

evaporate.

This

may

be

accomplished

by

many

methods,

such

as

leaving the patters in

place, keeping the surface wet,

or covering the surface with

a liquid

curing

compound, which comes being to a water- tight membrane that prevents the escape of the initial water.

Curing compounds may be applied by brushes or pressure sprayers. A gallon will cover 200 to 300 sq

ft.

Concrete

should

be

placed

at

a

temperature

not

less

than

40

or

more

than

80°

F.A

lower

tem-perature will decrease the rate of setting, while a

higher temperature will decrease the ultimate strength.

Placing Concrete in Cold Weather

When the concrete is placed during cold weather, it is usually necessary to preheat the water, the

aggregate,

or

both

in

order

that

the

ini-tial

temperature

will

assure

an

initial

set

and

gain

in

strength .Preheating the water is the most ef-fective method of providing the necessary tem-perature.

For this purpose a water reservoir should be equipped with pipe coils through which steam can be

passed, or steam may bedischarged directly into the water, several outlets being used to given better

distribution of the heat.

When

the

temperatures

of

the

mixtures

are

known,

some

specific

charts

may

be

used

to

cal-culate the temperature of concrete. A straight line pass all three scales, passing through every two

known temperatures, will assure the determina-tion of the third temperature. If the surface of sand is

dry, the fact lines of the scales giving the temperature of concrete should be used. However, if the sand

contains about 3 percent moisture, the dotted lines should be used.

Specifications usually demand that freshly placed concrete shall be kept at a temperature of not less

than 70°

F for 3 days or 50°

F for 5 days after it is placed. Some proper method must be provided to

keep the demanded temperature when the cold weather is estimated.

Reinforcing steels for concrete

Compared with concrete, steel is a high strength material. The useful strength of ordinary reinforcing

steels in tension as well as compres-sion, i.e., the yield strength, is about 15 times the compressive

strength of common structural con-crete, and well over 100 times its tensile strength. On the other

hand, steel is a high-cost material compared with concrete. It follow that the two materials are the best

used in combination if theconcrete is made to resist the compressive stresses and the compressive

force, longitudinal steel reinforcing bars are located close to the ten-sion face to resist the tension

force., and usually additional steel bars are so disposed that they re-sist the inclined tension stresses

that are caused by the shear force in the beams. However, rein-forcement is also used for resisting

compressive

forces

primarily

where

it

is

desired

to

reduce

the

cross-sectional

dimensions

of

compression

members,

as

in

the

lower-floor

columns

of

multi-story

buildings.

Even

if

no

such

necessity exits , a minimum

amount of

reinforce-

ment is

placed in

all

compression members to

safeguard them against the effects of small accidental bending moments that might crack and even

fail an unre-inforced member.

For most effective reinforcing action, it is essential that steel and concrete deform together, i. e.,

that

there

be

a

sufficiently

strong

bond

be- tween

the

two

materials

to

ensure

that

no

relative

movements

of

the

steel

bars

and

the

surrounding

concrete

occur.

This

bond

is

provided

by

the

rela-tively

large

chemical

adhesion

which

develops

at

the

steel-concrete

interface,

by

the

natural

roughness of the mill scale of hot- rolled rein-forcing bars , and by the closely spaced rib-shap-ed

surface deformations with which reinforcing bars are furnished in order to provide a high de-gree of

interlocking of the two materials.

Steel is used in two different ways in con-crete structures: as reinforcing steel and as prestressing

steel .reinforcing steel is placed in the forms prior to casting of the concrete. Stresses in the steel, as in the

hardened concrete, are caused only by the loads on the structure, except for possible parasitic stresses

from shrinkage or similar causes. In contrast, in priestesses concrete structures large tension forces are

applied to the reinforcement prior to letting it act jointly with the concrete in resistingexternal.

The most common type of reinforcing steel is in the form of round bars, sometimes called rebars,

available in a large range of diameters,from 10 to 35 mm for ordinary applications and in two heavy

bar sizes off 44 and 57 mm these bars are furnished with surface deformations for the purpose of

increasing

resistance

to

slip

be-tween

steel

and

concrete

minimum

requirements

for

these

deformations have been developed in experimental research. Different bar producers use different

patterns, all of which satisfy these requirements.

Welding of rebars in making splices, or for convenience in fabricating reinforcing cages for

placement in the forms, may result in metal-lurgical changes that reduce both strength and ductility,

and special restrictions must be placed both strength and ductility, and special restric-tions must be

placed both on the type of steel used and the welding procedures the provisions of ASTM A706 relate

specifically to welding.

In reinforced concrete a long-time trend is evident toward the use of higher strength materi-als,

both steel and concrete.

Reinforcing

bars

with

40ksi

yield

stress

,

almost

standard

20

years

ago

,

have

largely

been

replaced by bars with 60ksi yield stress , both because they are more economical and because their use

tends to reduce congestion of steel in the forms .

The ACI Code permits reinforcing steels up to Fy=80ksi. Such high strength steels usually yield

gradually

but

have

no

yield

plateau

in

this

situation

the

ACI

Code

requires

that

at

the

speci-fied

minimum yield strength the total strain shall not exceed 0.0035 this is necessary to make cur-rent

design methods, which were developed for sharp-yielding steels with a yield plateau, appli-cable to

such higher strength steels. there is no ASTM specification for deformed bars may be used , according

to the ACI Code , providing they meet the requirements stated under special circumstances steel in

this higher strength range has its place, e.g., in lower-story columns of high-rise buildings.

In order to minimize corrosion of rein-forcement and consequent spelling of concrete under sever

exposure

conditions

such

as

in

bridge

decks

subjected

to

deicing

chemicals

,

galvanized

or

epoxy-coated rebars may be specified.

Repair of Concrete Structures

Reinforced concrete is generally a very du-rable structural material and very little repair work is

usually needed. However, its

durability can be affected by a variety of causes, including those of

design and construction faults, use of inferior materials and exposure to aggressive en- vironment. The

need for a repair is primarily dic-tated by the severity of the deterioration as de-termined from the

diagnosis. Good workmanship is essential if any thing more than just a cosmetic treatment to the

creation is required.

1. performance requirements of repair system

Having established the causes of the defect by

carefully diagnosing the distress, the next step

should be to consider the requirements of the re-pair method that will offer an effective solution to the

problem (see fig.).

①

Durability

It is important to select repair materials that provide adequate durability. Materials used for the

repair job should be at least as durable as the substrate concrete to which it is applied.

②

Protection of steel

The mechanism of protection provided to the reinforcing depends on the type of repair ma- terials

used.

For

example,

cementations

materials

can

protect

the

steel

from

further

corrosion

by

their

inhibitive effect of increasing the alkalinity of the concrete, whereas epoxy resin mortars can give

protection against the ingress of oxygen,moisture and other harmful agents.

③

Bond with substrate

The

bond

with

the

substrate

must

produce

an

integral

repair

to

prevent

entry

of

moisture

and

atmospheric gases at the interface. With most re-pair materials, the bond is greatly enhanced with the

use of a suitable bonding aid such as an un-filled epoxy resin systems and slurry of Portland cement,

plus any latex additives for a Portland cement-based repair system. Precautions should also be taken

to remove all loose and friable ma-terials from the surfaces to be bonded.

④

Dimensional Stability

Shrinkage

of

materials during curing should be kept to a minimum. Subsequent dimensional

change should be very close in the substrate in order to prevent failure

⑤

Initial Resistance to Environmentally In-duced Damage

Some

initial

exposure

conditions

may

lead

to

premature

damage

lo

repairs.

For

example,

partially cured Portland cement repairs can dete-riorate from hot weather preventing full hydration of

the cement. To prevent this from happening extra protection during curing time may be nec-essary.

⑥

Ease of Application

Materials should be easily mixed and ap-plied so that they can be worked readily into small

crevices and voids. Ideally, the material should not stick to tools, and should not shear while being

trowel led nor slump after placement.

⑦

Appearance

The degree to which the repair material should match the existing concrete will depend on the

use

of

the

structure

and

the

client'

s

re-quirements.

A

surface

coating

may

be

required

when

appearance is important or when cover to reinforcement is small.

2. Selection of Repair Methods

A suitable repair counteracts all the defi-ciencies which are relevant to the use of the structure.

The selection of tile correct method

and

material for a particular, application requires careful

consideration,

whether

to

meet

special

requirements

for

placing

strength,

durability

or

other

short-or long-term properties. These con-siderations include:1.

Nature of the Distress

If alive crack is filled with a rigid

material,

then either the repair material will eventually fail or

some new cracking will occur adjacent to the original crack. Repairs to live cracks must either use

flexible materials

to

accommodate move-ments or else steps must be taken prior to

the re-pair to

eliminate the movement.

2.

Position of the Crack

Techniques which rely on gravity to intro-duce the material into the crack are more suc-cessfully

carried out on horizontal surfaces but are rarely effective on vertical ones.

3.

Environment

If moisture, water or contaminants are found in the crack, then it is necessary to rectify the leaks

Repair to slop leaks may be further com-plicated by the need to make the repairs while the structure

is in service and the environment is damp.

4.

Workmanship

The skill the operatives available to carry put the repairs is another relevant factors. Some-times

this can mean the difference between a permanent repair and premature failure of the re-pair material.

5.

Cost

The

cost

of

repair

materials

is

usually

small

compared

with

the

costs

of

providing

access,

preparation and actual labor.

6. Appearance

The repair surface may be unsightly, par-ticularly when it appears on a prominent part of the

building. In this case, the repair system will include some form of treatment over the entire surface.

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