transporting, deposition, curing and finishing of concrete

8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete

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Concrete Technology appreciate the importance of placing of concrete, explain the steps taken for preparation of surface before placing

concrete,

discuss the different types of vibrators, appreciate the importance and objectives of curing, describe the methods of curing, duration of curing and effects of

delayed curing,

explain the finishing operations, and explain the purpose of providing the joints in concrete construction.

6.2 TRANSPORTATION OF CONCRETE

The process of carrying the concrete mix from the place of its mixing to final

position of deposition is called as transportation of concrete.

The time factor is very important in case of transportation of concrete. Theconcrete mix should be transported as quickly as possible.

6.2.1 Importance of Transportation

The concrete mix should carry from its mixing place to final position of

placement in a very short time. It will minimize the loss of water by evaporation.

It will also not allow the concrete to become stiff. The mix can be transported

either manually or mechanically. In case of manual labour, the concrete is

transported in iron-pans, wheel barrows, etc. Pumps or trucks or belt conveyors

are used to transport the concrete mechanically. The concrete mix required at

lower levels is transported by chutes and at upper levels is hoisted by means of

barrow lifts attached to the scaffolding.

6.2.2 Precautions in Transporting of Concrete

The following precautions should be observed during transporting of concrete

from the mixing place.

(a) When water is added to cement, the process of hydration starts andwith the passage of time, the cement-water paste starts solidifying

thus making concrete stiffer. Concrete should be transported as

quickly as possible to the formwork within the initial setting time of

cement.

(b) The process of mixing, transporting, placing and compacting concreteshould not take more than 90 minutes in any case.

(c) No water should be lost from the mix during transportation.(d) The concrete mix should be protected from drying in hot weather and

from rain during transport from the place of mixing to the position of

placing.

(e) The cost of transportation should be as low as possible.(f) Segregation of concrete should be prevented under all circumstances.

Ready Mixed Concrete in which the segregation is observed, should

not be placed in any case.


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Deposition, Curing and

Finishing of Concrete

(g) The concrete should be kept agitated in truck mixer in order toprevent it from becoming stiff if more time is likely to be spent during

transportation.

(h) The permissible duration of transport of concrete should bedetermined in the laboratory.

6.2.3 Transporting OperationsThe following are the transporting operations used for concrete in order to

discharge it directly into the position where it is required.

(a) Transport of concrete by pans,(b) Transport of concrete by wheel barrows,(c) Transport of concrete by tipping lorries,(d) Transport of concrete in containers,(e) Transport of concrete by belt conveyors, and(f)

Transport of concrete by pumps.

Transport of Concrete by Pans

Iron pans are used for transportation of concrete. This method is adopted

where the quantity of mix is small and access to the point of placing is

restricted. This method is slow as well as costly.

Transport of Concrete by Wheelbarrows

The capacity of wheelbarrows varies from 70 liters to 80 liters. Steel

wheelbarrows with pneumatic tyres are used for moderate distances.

Transport of Concrete by Tipping Lorries

This method is widely used for the transport of concrete mix and

discharging it directly at the position of concreting such as in the

construction of air field pavements, dams, concrete floors, canal lining and

roads, etc. Tipping lorries are used for transporting ready mixed concrete

and for arrangements for receiving, loading and reloading it. The height of

mix in the lorries should not be less than 40 cm in order to avoid

segregation. The mix delivered in tipping lorries is discharged directly into

the structure. If it becomes difficult, then the mix is discharged into skips or

tubs, which are then moved to the position of placing by crane.

Transport of Concrete in Containers

This method exclude the loss of mortar and cement slurry during

transportation and also ensure the gradual emptying which is necessary for

concreting reinforced structures. This is one of the main methods of

delivering ready-mixed concrete from the mixing place to the point of

placing without the need for unloading and reloading. For this purpose

hoppers, skips and wagon bodies are used. Lorries transport hoppers and

skips. Containers are delivered from the transport vehicle to the position of

placing the concrete mix by cranes of different types and desired lifting

capacity. This method reduces the risk of segregation of concrete and also

protects it from atmospheric effects.

Transport of Concrete by Belt Conveyors

In this method, the belt used has a covering of rubber of width 60 cm and

speed not exceeding 1 m/s. The slump of concrete mix should not be more


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Concrete Technology than 6 cm in any case. The line of the conveyors should not have any

marked breaks in plan. The angle of inclination of the conveyor must not

exceed 18for 4 cm slump and 15for 4 to 6 cm slump. Flat belts are used

only for a distance of 20 m and if the distance is large, the working part of

the belt should be trough shaped. When the concrete is unloaded from the

conveyor or when it is transferred from one part of the conveyor to another,

guide boards should be used to prevent segregation of concrete. Boards

should be arranged properly so as to avoid segregation (Figure 6.1). Free

falling of concrete from the roller of the conveyor is also not allowed.

For getting continuous supply of concrete this method is used. This method

is most suitable under summer conditions for concreting structures at a

distance of 250 to 500 m from the place of mixing and also in those cases

where it is difficult for lorries and other types of transport to reach the

structure.

Figure 6.1 : Methods of Discharging Concrete from Belt

Transport of Concrete by Pumps

Concrete pumps are used to deliver concrete for concreting densely

reinforced structures, internal structural elements of buildings and large

mass concrete structures. These are also used for concreting of tunnel

linings. Such pumps cover a horizontal distance of 300 to 400 m and a

vertical distance of 40 to 50 m. Pumping is a process of transporting

concrete without unloading and reloading in a vertical or horizontal

direction and without a harmful effect on its quality.

Figure 6.2 : Transportation of Concrete by Pumps


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In this method, the concrete is charged into the receiving hopper and kept

mixed continuously by the paddles in order to retain uniformity of mix and

to prevent segregation. The impeller blades through the open suction valve

push the concrete into the barrel of the pump. The suction valve closes

when the plunger moves to the left. The pressure valve opens which moves

the concrete from the barrel into the pipeline. The diameter of the pipeline

depends upon the size of the aggregate used but it does not exceed 30 cm in

any case. The slump should not be less than 5 cm and more than 8 cm. The

water-cement ratio should remain between 0.5 to 0.65. Stiff mixes are not

suitable to be delivered by pumps because their pumping properties are not

good. Sharp turns and bends should be avoided in the pipeline. The number

of bends in a pipeline should be as small as possible.

6.2.4 Safety during Transportation of Concrete

The following safety measures must be observed during transport of concrete.

(a) The distance between wagons being moved by hand simultaneouslyin one direction should not exceed 20 m in any case and 30 m on

slopes.

(b) The distance from the bottom of the skip to the surface on which it isdischarged should not exceed 1 m in any case if concrete is

discharged from the skips.

(c) Places where lorries are discharged should be equipped with strongsupports for tipping lorries.

(d) No person should be present on the structure being concreted at theposition of discharge of the tipping lorry.

(e) The whole system of the pipe line should be checked up by usinghydraulic pressure 1.5 times the operating pressure before pumping

concrete. Generally, water is used for this purpose.

(f) The driver of the hoist should be able to see the positions of chargingand discharging the hoist when the mix is to be raised by shaft hoists.

(g) The barrow run should be cleaned for the removal of dirt and concretewhen concrete is transported by wheelbarrow. If the barrow run is

located at a height of more than 1 m, the boarded surface should not

be less than 1.2 m wide and fencing must be 1 m high. The thickness

of the board for fencing should not be less than 15 cm.

SAQ 1

(a) What do you mean by transportation of concrete?(b) What precautions should be taken during transportation of concrete?(c) Enlist the methods of transportation and explain any one.(d) State the safety measures to be taken during transportation of

concrete.


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Concrete Technology6.3 PLACEMENT OR DEPOSITION OF CONCRETE

The process of depositing the concrete in its required position is called placement

of concrete.

It is very essential to place concrete properly and carefully in order to obtain good

quality of surface. Areas which will be in contact with concrete must be prepared

carefully before placing concrete.

6.3.1 Importance of Placing of Concrete

The quality of concrete depends on the method of placing it during concreting. If

it is not placed properly, segregation will result. The formation of irregular and

unsightly lines will also take place. Therefore, in order to remove the possibility

of occurrence of segregation and irregular lines, it is very important to place

concrete uniformly and properly.

6.3.2 Precautions while Placing Concrete

The following precautions must be observed during placing of concrete in orderto get required strength of the concrete.

(a) The concrete should not be thrown from a height of more than 1 m toprevent segregation.

(b) The concrete should be deposited in horizontal layers of uniformthickness not exceeding 45 cm for mass concrete and 30 cm for

reinforced concrete.

(c) No person should be allowed to walk over freshly laid concrete.(d) The old surface should be made rough, cleaned and cement grouted

before placing any new layer of concrete.(e) The alignment of reinforcement and formwork should not be

disturbed in any case when concrete is placed in RCC members.

(f) Placement of concrete should be discontinued during rainy periods.(g) The laitance should be removed, i.e. squeezed out, before placing new

concrete.

(h) Concrete should be laid continuously in order to prevent theformation of irregular and unsightly lines.

(i) The internal surfaces of the formwork should be oiled in order toprevent concrete from sticking to sides.

(j) The placing of concrete should start widthwise in reinforced cementconcrete slabs.

6.3.3 Preparation of Surface before Placing Concrete

It is very essential to prepare a proper base or place before placing the concrete

mix in order to develop proper bond between the base and fresh concrete. Before

placing concrete, the different types of bases should be prepared as below.

In the Case of Hardened Concrete Base

The coarse aggregates should be exposed by making the base rough. It ismade rough by striking sand under high pressure (i.e. hand blasting). After

this the surface should be washed with water under pressure in order to


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remove the dust particles from it. After the surface is cleaned, a thin coat of

cement paste is applied before placing concrete over the hard surface.

In the Case of Specially Prepared Sub-bases

In case of brick soling and water bound macadam, base should be made

rough with the help of steel brush. Dust and loose particles should be

removed from the sub-base before placing the concrete over it. Before

placing concrete the water should be sprinkled over the surface.

In the Case of Natural Soil

Before placing any layer of concrete over natural soil, the soil should be

compacted uniformly and its moisture deficiency should be removed

initially in order to prevent it from absorbing moisture from beneath the

concrete layer.

In the Case of Rocky Base

In the case of rocky base the sides should be cut in vertical direction and not

in sloping direction. All the loose particles should be removed off the rocky

base. Before placing concrete the water should be sprinkled over thesurface.

SAQ 2

(a) What is meant by placement of concrete?(b) What are the precautions to be taken while placing the concrete?(c) How surface should be prepared before placing of the concrete?

6.4 COMPACTION OF CONCRETE

The process of consolidating concrete mix after placing it in position is called as

compaction of concrete.

The object of compaction is to remove air from the concrete and to give

maximum density to the concrete. Presence of more air voids will reduce thestrength. It also ensures an intimate contact between the concrete and the surfaces

of reinforcing steel and other embedded parts of the structure. During the process

of compaction it is important to note that the reinforcement should not be

disturbed and the forms should not be damaged or displaced. If the compaction is

not uniform, the concrete becomes porous, non-homogeneous and attains less

strength. The mix to be used should have adequate workability for placing

without any difficulty and in order to obtain maximum density. The mix should

also not be too wet, as it would otherwise cause segregation, lower density, and

excessive laitance at the top.

6.4.1 Importance of CompactionA considerable amount of air is entrapped in concrete along with the partial

segregation of aggregates during the manufacture of concrete. It lowers the

quality of concrete by making it porous and non-homogeneous. The importance


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of compacting operations is to remove the entrapped air and arrange the

ingredients uniformly to obtain better quality of concrete. The strength of

concrete may reduce by 30% by the presence of only 5% voids. The density,

durability and strength of concrete are the factors, which affect the quality of

compaction.

Concrete Technology

6.4.2 Compaction Methods

It is very important to decide whether to use a workable mix with hand or a stiffermix with vibration before considering the method of compaction. It has been

determined that a better surface with less blow holes is obtained for workable

concrete.

The compaction methods are classified as follows :

Hand Compaction

Hand compaction method is adopted for pavements, narrow and deep

members. Compaction must be uniform and concrete must reach to the

corners of the formwork. Excessive compaction is not good because it will

try to push the aggregates at the bottom thus bringing the mortar at the

surface. Iron rods and rammers are used for the hand compaction. Mass

concrete is compacted in successive layers of thickness not exceeding 30

cm by tamping with light rammers or templates. Iron rods are used for

compacting reinforced concrete work in layers not exceeding 15 cm in

thickness.

Hand compaction is further classified as follows :

Rodding

Rodding is adopted in case of unimportant concrete work of small

magnitude. When rodding is used, the consistency of concrete is

maintained at a higher level. The thickness of the layer is limited

about 15 to 20 cm. Rodding is nothing but poking the concrete with

about 2 m long, 16mm diameter rod to pack the concrete between the

reinforcement and sharp corners and edges. Rodding is done

continuously over the complete area to effectively pack the concrete

and drive away the entrapped air.

Instead of iron rod, bamboos or cane may also be used for rodding

purpose.

Ramming

Ramming should be done with care as it may disturb the position of

reinforcement or the formwork may fail if steel rammer is used. Lightramming can be permitted in un-reinforced foundation concrete or in

ground floor construction. It should not be permitted in case of

reinforced concrete or in the upper floor construction, where concrete

is placed in the formwork supported on struts.

Tamping

Tamping is one of the usual methods adopted in compacting roof or

floor slab or road pavements where the thickness of concrete is

comparatively less and the surface to be finished smooth and level.

Tamping consists of beating the top surface by wooden crossbeam of

section about 10

10 cm.Mechanical Compaction


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In case of mechanical compaction concrete is compacted by vibration

during which the vibrator communicates rapid vibrations to the particles

thus increasing the mobility of concrete. The particles occupy a more stable

position under the force of gravity. The concrete mix fills all the spaces thus

forcing air to the surface and making concrete denser. The frequency and

duration of vibration, its amplitude depends on the conditions of vibration

for compacting the mix. The maximum frequency of vibration depends on

the size of the particles and on the mobility of concrete. Following are the

advantages of vibratory compaction.

(a) A better finish and better quality of concrete is obtained ascompared to hand compaction.

(b) The concrete produced is more dense and impermeable.(c) Bond between steel and concrete is improved.(d) Laitance is partially reduced.(e) Speed of placing concrete is increased by the ready flow of

vibrated concrete into difficult positions.

(f) Creep and shrinkage are also reduced due to the possibility ofhigher aggregate-cement ratio.

(g) Concrete with even lower w/c ratio and with lower cement canbe compacted effectively.

(h) Better compaction can be achieved in heavily reinforcedconcrete members.

The following difficulties are encountered in vibratory compaction.

(a) Finishing of horizontal surface.(b) Discharging very stiff concrete from mixers.(c) The lack of sufficient vibration at points not immediately in

contact with the vibrating equipment thus resulting in

segregation.

(d) Discharging stiff concrete from lorries and other means oftransport.

Compaction by Pressure and Jolting

This is one of the effective methods of compacting very dry concrete. This

method is used for compacting hollow blocks, cavity blocks and solid

concrete blocks. The stiff concrete is vibrated, pressed and also given joltsand due to this the stiff concrete gets compacted to dense form to give good

strength. By applying great pressure, a concrete of very low water cement

ratio could be compacted to yield very high strength. Generally this method

is used in the laboratory.

Compaction by Spinning

Spinning is one of the recent methods of compaction of concrete and is used

for the fabrication of concrete pipes. The plastic concrete when spun at a

very high speed, gets well compacted by centrifugal force.

6.4.3 Types of VibratorsThe vibrators commonly used are classified as follows :

(a) Internal vibrator


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Concrete Technology (b) Screed vibrator(c) Form vibrator(d) Table vibrator(e) Platform vibrator

Internal Vibrator

Internal vibrator is also known as needle or immersion or poker vibrator. Itconsists of a power unit and a long flexible tube at the end of which fitted a

vibrating head. These vibrators have higher efficiency since all the energy

is directly transmitted to the concrete. As they are portable, therefore, can

be readily used in difficult positions. The vibrating element is immersed in

the fresh concrete, which transmits vibrations through the vibrator body. It

is very important to keep the vibrating head in the concrete while running in

order to keep the bearings cool and avoid breakdowns. It should be inserted

vertically or nearly so at points 45 cm to 75 cm apart and should be

withdrawn slowly at the rate of 7.5 cm/sec. These should not be used for

pushing concrete laterally in the formworks because it will cause

segregation. The frequency of vibration is about 7000 cycles/mt. Suchvibrators are used for compacting large sections of mass concrete in

structures, for concrete of beams, columns, foundations, etc.

Figure 6.3 : Internal Vibrator

Screed Vibrator

Screed vibrator is also known as surface vibrator. These vibrators are used

for compacting plain concrete or one-way reinforced concrete floors, road

surfaces, the thickness of which does not exceed 20 cm. This vibrator is setup on the concrete surface after placing the concrete and vibrations are

transmitted to concrete through a working platform.


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Figure 6.4 : Section of a Typical Metal Form for Road Slab Construction

Figure 6.5 : Compaction of Cement Concrete Road Slab by a Screed Vibrator

Form Vibrator

Form vibrators are also known as external vibrator. It is fastened to the

formwork by a clamping device and transmits vibrations to the concrete.

External vibrator is clamped at a distance up to 25 cm in depth from the

formwork. The vibrator must be firmly clamped to the form as otherwise its

efficiency will be reduced. The formwork should be sufficiently strong and

rigid to resist the oscillatory action. The time of vibration is from 45 to

90 seconds. The vibrator is switched off for 5 minutes after its continuous

working for half an hour in order to allow cooling of motor. If the motor of

the vibrator heats up more rapidly, the work should be immediately

stopped. The electrician should be called in order to check the motor

winding. In this case, more power is required in comparison to internal

vibrators. Therefore, emergency vibrators must be kept near the concreting

work as replacement for vibrators, which go out of order. Form vibrators

are used in concreting of thin components of monolithic structures, heavily

reinforced arches and tunnel and tunnel lining, etc. They are also used in the

production of different pre-cast reinforced concrete components.

Table Vibrator

This method is mostly adopted in the laboratories. This is the special case

of a formwork vibrator, where the vibrator is clamped to the table. This

method is used in making small but precise prefabricated reinforced cement

concrete members. In this method, any member kept on the table gets

vibrated.

Platform Vibrator

It is nothing but a table vibrator larger in size. Sometimes, the platform

vibrator is also coupled with jerking or shock giving arrangements so that a

thorough compaction is given to the concrete. Platform vibrator is used in

the manufacture of large prefabricated concrete elements such as railway

sleepers, prefabricated roofing elements, electric poles, etc.

6.4.4 Handling of Vibrators

Needle vibrators should be penetrated in vertical direction and vibrations should

be transmitted till the concrete flattens and no entrapped air appears on the

surface. The vibrator should be taken out of concrete slowly and carefully. Skilled

workmen are essentially required for producing a uniform and well-compacted

concrete. The correct placing and tamping of concrete mainly depends on the skill

of the workmen and their experience. Vibration by unskilled persons is probably

more dangerous than hand tamping, as there is always a risk of either under-compaction or over-compaction of concrete. The equipment to be used for

vibration must be kept clean and free from coatings of set concrete. Thus, the

vibrators should be used very carefully in order to compact the concrete layer

uniformly.


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Concrete Technology The following steps should be observed in order to obtain good results by

compacting concrete with vibrators.

(a) Vibrators should be controlled carefully by immersing the internalvibrator for 5 to 15 seconds at points 45 to 75 cm apart.

(b) The formworks should be as tight as possible in order to prevent anyleakage of mortar.

(c) The vibrator should be inserted vertically otherwise it will not bepossible to regulate the degree of compaction in all portions of

concrete.

(d) The lift should not be less than 15 cm in order to avoid air beingtrapped.

(e) The vibrator should be immersed through the full depth of the freshlylaid concrete. It should also be immersed into the lower layer if the

concrete in that layer is still plastic.

(f) The vibrator should be withdrawn very slowly and should be allowedto penetrate of its own accord.

(g) The vibrator should not touch the form surface otherwise a sandstreak is likely to occur. The surface of the formwork can also be

damaged.

(h) The vibrator should only be used for compaction purposes. It shouldnot be used for pushing the concrete laterally in the formworks, which

would otherwise cause segregation.

6.4.5 Suitability of Mix for Compaction with Vibrator

The consistency of concrete depends upon the placing conditions, type of mix and

the efficiency of the vibrator. The slump should not exceed 5 cm when

compacting concrete with vibrators. If the slump of the concrete mix is more, the

segregation will take place. Segregation should be prevented under all

circumstances.

6.4.6 Selection of Vibrators for Various Situations

Following table indicates the use of various vibrators depending upon the

situation.

Table 6.1 : Selection of Vibrators

Sl. No. Type of Vibrator Places where Used

1. Internal For large sections of mass concrete in

structures, for concrete of foundations,

columns, beams, etc.

2. Screed For plain concrete or one-way

reinforced concrete floors, road

surfaces.

3. Form For concreting thin components ofmonolithic structures, arches and

tunnel lining, etc; for production of

pre-cast reinforced concrete


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components.

SAQ 3

(a) Why concrete is required to be compacted?(b) State the methods of compaction and explain in brief.(c) How you will judge that compaction is proper or not?(d) What should be the duration of compaction? State the factors

affecting it.

(e) State the different types of vibrator and their usefulness. Explain anyone of them in brief.

SAQ 4

(a) What is vibrator? What is the objective of using vibrators inconcreting operation?

(b) State the places where different types of vibrator are used.(c) State the precautions to be taken during the use of vibrators.(d)

Tick the correct answer.(i) Vibrators should be penetrated in (horizontal/vertical) direction.

(ii) Which type of vibrator is generally used for compaction of

concrete?

(1) Needle vibrator

(2) Form vibrator

(iii) For compacting thin reinforced concrete slabs following

vibrator is recommended.

(1) Immersion vibrator

(2) Surface vibrator

(iv) Surface vibrator is effective only when thickness of concrete

member does not exceed

(1) 200 mm

(2) 500 mm

(v) A surface vibrator for compaction of concrete is preferred for

(1) Raft footings

(2) Columns


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Concrete Technology

6.5 CURING OF CONCRETE

The process of hardening the concrete mixes by keeping its surface moist for a

certain period after compaction and finishing is called curing of concrete.

Curing is one of the important factors for obtaining better strength. The concrete

hardens because of the chemical reaction between water and cement, i.e.

hydration. The chemical action that accompanies the setting of concrete is

dependent on the presence of water. Although there is sufficient water at the time

of mixing yet it is necessary to ensure that the water is retained to enable the

chemical action to continue till the concrete is fully hardened. Properties of

concrete such as strength, durability, wear resistance, water-tightness and volume

stability improve with the passage of time. Three gallons of water are required

approximately to hydrate one bag of cement. If the loss due to evaporation is

more from newly placed concrete, the hydration process will stop and concrete

will shrink thus creating tensile stresses at the drying surface. The development ofthese stresses will result into the formation of plastic shrinkage cracks.

6.5.1 Importance of Curing

The importance of curing of concrete is to improve its properties such as water-

tightness, wear resistance, strength, volume stability and durability.

6.5.2 Objects of CuringFollowing are the objects of curing.

(a) Maintaining the process of hydration by preventing the loss of waterby evaporation.

(b) To reduce the shrinkage of concrete.(c) To preserve the properties of concrete.

6.5.3 Methods of CuringDifferent methods of curing are used for maintaining the concrete in a moist

condition over a period of several days till it hardens and attains full strength. The

method of curing depends upon the nature of work and atmospheric conditions.

Following methods are generally used for curing.

(a) Sprinkling water,(b) Ponding method,(c) Membrane curing,(d) Covering concrete surfaces with gunny bags,(e) Shading concrete works,(f) Chemical curing,(g) Steam curing,(h) Curing of concrete by infrared radiation, and(i) Electrical curing of concrete.

Sprinkling Water


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Excellent method of curing is the continuous sprinkling of water. The

quantity of water required by this method to cure concrete is much more

than other methods. If water is sprinkled in intervals, then the concrete must

not be allowed to dry between applications of water. A constant supply of

water helps in preventing the formation of cracks caused by alternate

wetting and drying.

Ponding MethodsPonding method is mainly adopted for surfaces members such as

pavements, floors, roofs, slabs and sidewalks, etc. In this method, a small

dam of earth or water retaining material is placed around the perimeter of

the surface. The surface is firstly covered for 24 hours after placing and

compacting concrete. The enclosed area is further divided into a number of

rectangles and is kept flooded with water. Water is filled in the rectangles

two to three times per day depending upon the climatic conditions.

Membrane Curing

Membrane curing is used at the places where there is acute shortage of

water. In membrane curing a newly laid concrete surface is covered byusing chemical or liquid membranes in order to prevent evaporation of

moisture from concrete. This method has been found good in maintaining a

satisfactory state of wetness in the body of concrete to promote continuous

hydration when original w/c ratio used is not less than 0.5. The membranes

should be applied immediately after the concrete has been finished. The

concrete surface should be kept moist till the membrane is applied. Such

compounds should neither be applied when there is free water on the

concrete surface nor after the concrete has dried out. The correct time of

applying membranes that when water film disappears from the surface of

the finished concrete. Curing membranes are plastic films, bitumen

emulsions, wax emulsions and waterproof papers, etc. Curing compounds

should not be applied in between two courses in order to obtain perfect

bonding. The disadvantage of this method is that concrete loses its strength

due to less rate of hydration. No supervision is required in this method.

Covering Concrete Surfaces with Gunny Bags

The exposed surface of concrete is covered with old empty cement bags or

hessian, which are kept moist by spraying water frequently. This method is

widely used for structural concrete. This method can also be applied for

horizontal and vertical members. The surface should not be allowed to dry

even for small duration during the period of curing.Shading Concrete Works

The object of shading is to avoid the evaporation of water from the surface

of concrete after it is placed and compacted. This method is used to protect

the newly laid concrete from wind, heat and direct sunrays. In cold weather,

it prevents the freezing of concrete under frost condition. This process is

mainly suitable for large surfaces such as road slabs. Using canvas and

starching them on frames achieve shading over the newly laid concrete.

Chemical Curing

Chemical curing is accomplished by spraying the sodium silicate solution.

About 500 gm sodium silicate mixed with water can cover 1 m2of surface

and forms a hard and insoluble calcium silicate film. It actually acts as case

hardener and curing agent. The application of sodium silicate results in a


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Concrete Technology thin varnish like film, which also fill pores and surface voids, thus sealing

the surface and preventing the evaporation of water.

Steam Curing

The hydration of cement accelerates with increase in temperature, which

leads to faster development and strength.

To steam curing, the concrete is subjected to higher temperature bymaintaining the required wetness.

For concrete mixes with water-cement ratio ranging from 0.3 to 0.7, the

increased rate of strength development can be achieved by resorting to

steam curing. The mixes with low water-cement ratio respond more

favorably to steam curing than mixes with higher water-cement ratio. In

steam curing, the heating of the concrete products is caused by steam either

at low pressure or high pressure. Steam curing is preferred for precast

concrete products. These precast members are passed through the steam

chambers. It is also used, when early removal of formwork is required to

put the structure in sense without delay.

Curing of Concrete by Infrared Radiation

It is claimed that a much more rapid gain of strength can be obtained in this

method than even with steam curing. The rapid initial rise of temperature

does not result in a decrease in the ultimate strength as it does in the case of

steam curing. The system is described as particularly applicable to the

manufacture of hollow concrete products in which case the heaters are

placed in the hollow spaces of the product. The normal operating

temperature is 90C. The curing of concrete by infrared radiation has been

used in Russia.

Electrical Curing of Concrete

This method is not used in India. It is more expensive. It is mostly used in

very cold climatic regions. Passing alternating current of low voltage can

cure concrete products and high amperage through electrodes in the form of

plates covering the entire area of two opposite faces of concrete. The

potential difference generally adopted is between 30 and 60 V. Evaporation

is prevented by using an impermeable rubber membrane on the top surface

of the concrete. Initially up to 3 hours, the resistance of concrete to flow of

current decreases due to rise in temperature. There is rise in resistance

afterwards, due to decrease in the quantity of free water available in the

concrete due to hydration and evaporation. This period of rise intemperature should be about 12 hours. The duration of electrical-curing

should be about 48 hours at the temperature of 50C or 36 hours at the

temperature of 70C. The concrete products are cooled gradually in heat-

insulated chambers for a minimum period of 24 hours. By electrical-curing

concrete can attain the normal 28 days strength in a period of 3 days.

6.5.4 Effects of Delayed Curing

The concrete specimens placed in laboratory air for varying periods after casting,

before being moist-cured, have indicated that the strength at 7 to 28 days

decreases progressively as the period of air curing is increased. An exposure for

3 days to air at a temperature of 23C and having a relative humidity of

approximately 60% before being moist-cured at 23C has been found to reduce

the 7 days strength by 12% and the 28 days strength by about 10%. The


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specimens left in air at 23C for the entire curing period have shown a reduction

of 25% in the strength at 7 and 28 days as compared with standard moist-curing.

The reduction under field conditions would probably have been greater. Similar

adverse curing causes greater relative reduction in strength when Portland blast

furnace slag cement and the cements blended with fly ash are used.

6.5.5 Duration for Curing

As per IS 456 : 1964, the concrete should be cured for at least seven days. The

strength of concrete increases by 50% than that which is exposed to dry air for the

entire period. The duration for which concrete should be protected against loss of

water depends upon cement mass, weather and future exposure conditions. The

period of curing may be a month or even more for lean concrete mixtures used in

massive structures such as dams. The curing may extend for few days for richer

mixes. As all the properties of concrete are improved by curing, therefore, it must

be as long as practicable in all cases. Concrete must be kept at a temperature that

is favorable for hydration.

Table 6.2 : Strength of Cement Concrete with Different Periods of Curing

Sl. No. Period of Curing Strength in Percentage

1 1 day 16

2 3 days 40

3 7 days 67

4 28 days 100

5 3 months 122

6 year 146

7 1 year 155

SAQ 5SAQ 5

(a) What is curing? State its importance.(b) What are the objectives of curing?(c) Enlist different methods of curing.(d) Write short note on water curing.(e) What is membrane curing?

SAQ 6

(a) What is steam curing?(b) How are the following structural elements cured?

(i) Test block

(ii) Columns

(iii) Hume pipe

(iv) Bridge slab

(v) Pre-cast products

(vi) Wall


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Concrete Technology (c) State the effects of delayed curing.(d) Tick the correct answer.

(i) The following method may be used for the curing of concrete

(1) Electrical curing

(2) Mechanical curing

(ii) The following sealing compounds can be used for the

membrane curing

(1) Rubber latex emulsions

(2) Sodium silicate solution

6.6 FINISHING OF CONCRETE

The operations adopted for obtaining a true, uniform concrete surface are called

as finishing operations.

Concrete mix should be spread in such a way that no segregation takes place.

Only designing the mix properly can ensure this. The results of finishing are good

if the slump is about 5 cm. The choice of concrete finish depends upon the

ultimate use of the completed job and the desired effect.

6.6.1 Importance of Finishing

Finishing is very important from engineering point of view. The importance of

finishing is to keep the concrete surfaces free from undulations. Many concrete

structures have an unsatisfactory appearance after exposure for some time. Some

of the surfaces, which were quite pleasing when new, have weathered badly. The

surface of concrete cannot be made pleasing to the eye as many unsightly features

result from cracks, carelessly constructed and badly placed construction joints,

patching or honey-combed or damaged areas, poor formwork and lack of

sufficient cover to reinforcement.

6.6.2 Finishing OperationsFollowing are the operations adopted for finishing of concrete surface.

(a) Screeding,(b) Floating, and(c) Trowelling.

Screeding

The leveling operation that removes humps and hollows and gives a true,

uniform concrete surface is called screeding.

The process of screeding is also known as striking off. Straight edge is used

for screeding. Straight edge is specially prepared and is slightly longer thanthe section being finished. The surface is struck off by moving the straight

edge back and forth with a saw like motion across the top of the forms.

The straight edge is advanced forward a short distance with each


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movement. A small quantity of concrete mix should always be kept ahead

of the straight edge to fill the voids and maintain a plane surface. Screeds

may be of vibrator type or roller type. Vibrators can also be fitted on

screeds.

Figure 6.6 : Screeding

Floating

The process of removing the irregularities from the surface of concrete left

after screeding is called floating.

The process of floating is done with the help of wooden float. Floating

helps in leveling the surface and compacting concrete. The wooden float is

1.5 m long and 20 cm wide. A handle is fixed in the center. Moving the

wooden float backward and forward performs finishing. Bull float is used if

the area of concrete surface is large. It is moved with the help of a handle

without the operations getting on the concrete surface.

Figure 6.7 : Floating

TrowellingThe final operation of finishing is called trowelling.

Trowelling is performed where a smooth and dense surface is desired.

Trowelling should be delayed as far as possible. In most of the cases,

trowelling is performed while the concrete is too soft and plastic. At this

stage, excessive trowelling will cause crazing and result in a surface having

less wear resistance. The important point is the period at which the

trowelling is done and the pressure used by person responsible for the final

operation of finishing. Finishing is always carried out with the trowel and

the float. After floating surface should be allowed to dry and stiffen till all

the excess water has disappeared.

Trowelling should be done after water has evaporated from the concrete

surface. Spreading dry cement on a wet concrete surface to absorb excess


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water is not a good practice. The occurrence of such wet surfaces should be

avoided. The finishing process should be delayed if such spots occur.

Concrete Technology

A gap of some time should be left between successive trowellings in order

to permit concrete to increase its set. Initially, an older trowel is preferred to

a new trowel. Trowel blade should be kept as flat against the surface as

possible.

Power float or power trowels are generally used in finishing large surfacearea such as aircraft hangars or factory floors and is not economical for

small areas such as flats or dwelling houses. During trowelling, cement

should not be spread, as it is dangerous and liable to produce a neat cement

skin, which peels off after some time. Trowelling with power trowels

should be delayed till the concrete becomes firm.

Trowelling should be finished on the same day as the laying operation.

Artificial drying can be very effective by laying cotton sheets on the floor

and covering them with 2.5 cm to 5 cm of dry cement. The water is

absorbed by the sheets which can be lifted afterwards say 10 or 15 minutes

and after this trowelling can proceed for finishing the concrete surface.

SAQ 7

(a) What is finishing? How a good finish can be obtained?(b) Explain the necessity of finishing.(c) State the different methods of finishing of concrete.(d) Explain screeding in brief.(e) What is floating? Explain in brief.(f) What is trowelling? What are the precautions to be taken while

trowelling?

(g) Tick the correct answer.The final operation of finishing is called ___________________

(floating/screeding/trowelling ).

6.7 JOINTS IN CONCRETE CONSTRUCTIONWhen the large concrete members such as factory floors, workshop floors, long

columns, residential floors, road pavements, air-field pavements, etc. are

constructed, it is not practicable and advisable to place the concrete continuously

in one stretch. A joint is left between subsequent concreting stretches and it iscalled as construction joint. Concrete may undergo expansion and contraction due

to thermal changes, moisture movement, drying shrinkage and due to structural

reasons. Therefore to make up for the impending expansion or contraction of


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concrete a proper provision is made in the form of a joint. Such joints are called

contraction or expansion joints.

The concrete joints can be classified as follows :

(a) Construction joints,(b) Expansion joints, and(c) Contraction joints.

6.7.1 Construction Joints

Construction joints are the temporary joints provided between subsequent

concreting operations. The positions of construction joints should be

predetermined before starting of concreting operations. The joint should be

horizontal in walls and columns and should be arranged at such a level to

coincide with the general architectural features so that it does not look odd. In

columns, the concrete should be filled up to a few inches below the junction of

beams. The joint should be made at the point of minimum shear in beams and

slabs. Concrete should be poured in one stretch till construction joint is reached.

The construction joint must not be provided at the point of maximum bending

moment in any case. Badly made and poorly finished construction joint will give

an ugly appearance to the construction therefore it must be properly finished. The

joints must be made at such places that the concrete comes under least maximum

bending moment and shear force.

6.7.2 Expansion Joints

Concrete is subjected to volume change due to temperature variations, shrinkage,

etc. Therefore, the provision must be made to cater for the volume change by way

of joint to relieve the stresses produced. In small buildings, no expansion joint is

needed in the floor or in the roof because expansion is very small as it is afunction of length. A long building undergoes large expansion. It is estimated that

for the worst conditions, a long building may undergo an expansion of as much as

2.5 cm. Therefore, buildings longer than 45 m are generally provided with one or

more expansion joints. Roof of a long building is affected by maximum

temperature variations. The roof is subjected to expansion and contraction during

day and night and causes pushing or pulling to the load bearing walls. Serious

cracks have been found in the masonry wall supporting the slab. Hence, attempts

have been made to create a condition for a slab to slide over the wall when it is

under expansion or contraction. The details such as the length of structure where

expansion joint is to be provided can be determined after taking into

consideration many factors such as temperature, exposure to weather, time andseason of the laying of the concrete, etc. Under no circumstances shall a structure

of 45 m or more be without an expansion joint.

In the past, expansion joints were provided at closer intervals in the floors and

pavements. These days from experience, it is seen that concrete does not actually

expand to the extent indicated by the simple analytical calculations, because of

the frictional resistance offered by the sub grade. It is, therefore, possible to

provide expansion joint at a much farther interval than in the past. IS 456 : 1978

recommends as under.

In view of the large number of factors involved in deciding the location, spacing

and nature of expansion joints, the provision of expansion joint in reinforced

cement concrete structures should be left to the discretion of the reinforced

cement concrete designer. For purposes of general guidance, however, it is


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recommended that structures exceeding 45 m in length shall be divided by one or

more expansion joints.

Concrete Technology

6.7.3 Contraction Joints

Concrete is subjected to plastic shrinkage and drying shrinkage due to which

concrete shrinks. Stresses are developed and cracks are formed when shrinkage is

restrained. Contraction joints are provided to avoid these cracks. Normally these

joints are provided at 5 m to 10 m interval. Contraction joints are also called

dummy joints or control joints. These joints will not be required if a provision is

made to take up the shrinkage stresses by reinforcement. Hence, contraction

joints are provided in un-reinforced floors and pavements. Contraction joints are

made at the time of laying concrete by imbedding a timber plank or batten of

sufficient depth and required thickness. This is afterwards removed when the

concrete is hardened. Sometimes, steel plates of required thickness and width are

forced down into the fresh concrete and then removed when the concrete is

hardened. Sometimes contraction joints of required width and depth are cut by

using a joint sawing machine. It is necessary that groove cut should be filled up

with some joint sealing compound to protect the edges of concrete and also toprevent water from being held. The depth of joint should be about 2 cm. In

residential flooring, the conventional contraction joint is omitted by casting the

slab in alternate bays, to allow for the complete plastic shrinkage and also for

maximum extent of drying shrinkage. It is usual practice to place glass-strip or

aluminium strip in between the bays to create discontinuity between adjacent

bays to prevent the development of continuous cracks.

SAQ 8

(a) Why joints are essential in concrete construction?(b) What are types of joints in concrete?(c) What is construction joint? Explain in brief.(d) Explain the expansion joint in brief.(e) Explain the purposes of providing contraction joint.(f) What is the method of joining the old concrete to new concrete?

(Hint :Answer is in Sub-section 6.3.3.)

6.8 FORMWORK

The temporary construction used as a mould for the structure, in which the

concrete is placed and in which it hardens is called as formwork.

Formwork must be strong enough to withstand the hydrostatic pressure of wet

concrete. The method of compaction, size and shape of the formwork also affects

the shutter pressure. Formwork should be tight in order to prevent concrete from

leaking out. Formwork should be easy to handle so that no time is wasted inassembling and disassembling. Formworks vary both in type and in the method of

construction. Another type is the sliding type. In this type, an arrangement is

more to rise the form with the placement of concrete. It is generally used where a


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good surface finish is required. It has two advantages. Firstly, it enables a high

speed of construction and secondly, it eliminates the horizontal construction joint.

6.8.1 Requirements for Formwork

The following are the requirements for formwork :

(a) The forms should have sufficient strength to support the loads.(b) The sheeting should be durable and rigid.(c) The sheeting should be thick enough to withstand the pressures of wet

concrete.

(d) The components of forms should be large enough to carry the loadsand forces without buckling.

(e) The formwork should be of reasonable size and weight for easyhandling, transportation and erection.

(f) The formwork should be easily and speedily erected.(g) The form surface should give a smooth finishing.(h) The joints of the form should be tight enough to prevent any material

leakage.

6.8.2 Checking of FormworkThe following points must be checked before placing concrete in the formwork :

(a) The geometric dimensions of the formwork should be checked.(b) The accuracy of the horizontal surfaces should be controlled by levels

and vertical surfaces by plum-bobs.

(c) The sawdust, nails, mud particles, etc. should be removed beforeplacing concrete.

(d) The alignment of the formwork should not be disturbed duringconcreting operations.

(e) Oiling should be done in order to prevent sticking of concrete withformwork and to ensure smooth surface of concrete.

(f) The joints in the formwork should be checked to avoid the leakage ofmaterial.

(g) The old surface should be wetted before placing the concrete.6.8.3 Materials for FormworkThe following are the materials used for formwork :

Timber

The partially seasoned timber is most satisfactory for use of formwork.

Green timber dries out and shrinks. It causes fins and ridges on the

concrete. Kiln-dried timber has a tendency to swell when soaked with water

from concrete. It causes bulging and distortion if the boards are tightly

joined. Provision for slight swelling should always be made in case of

seasoned timber.

SheetingBy using boards 12.5 cm, 15 cm, 17.5 cm, 20 cm, 22.5 cm and 25 cm

wide, various heights of centering can usually be obtained without

excessive sawing. Thin sheeting (2.5 cm) is used for the sides of the


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Concrete Technology beams and column boxes. The sheetings of 3.75 cm to 5 cm are used

for the soffits of beams.

Joists

Joists are commonly 15 cm 5 cm but may vary from 10 cm 5 cm

to 22.5 cm 10 cm according to type of work.

PostsPosts may be 7.5 cm 10 cm to 12.5 cm 12.5 cm.

Studs, Wales, Headtrees, Subsidiary Bearer and Transoms

These may vary from 10 cm 5 cm to about 12.5 cm 12.5 cm.

Sections about 7.5 cm 5 cm are useful for general bracing and light

shuttering.

Steel

Steel forms can be used for as many number of times as desired and as such

are economical. Better finish can be achieved by the use of steel forms, as

they are factory made and fit exactly. Steel forms have the followingadvantages over other types of forms :

(a) Quality of work is assured.(b) The wastage of material is eliminated.(c) Handling and transportation costs are reduced.(d) It reduces finishing costs.(e) It reduces friction losses.(f) Smaller number of tie rods is required.(g) These are safer than other material.(h) In order to obtain additional usage of the steel forms,

adjustment of various sizes can easily be incorporated in the

design of steel forms.

(i) Collapsing and stripping is a smooth and gradual operationbecause it is usually controlled by jacks, etc.

6.8.4 Formwork for Walls

A common arrangement for wall formwork is shown in Figure 6.8. It is held

together with bolts fitted with sleeve pieces. In this case, the sheathing is placed

horizontally. It spans between vertical studs under the horizontal pressure due to

wet concrete. The pressure on either side of the form is self-balanced.


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Figure 6.8 : Formwork for Walls

6.8.5 Formwork for Columns

A common arrangement for column formwork is shown in Figure 6.9. The two

sides are held together by bolts and the two opposite sides by hardwood wedges

between the bolt and the form. The sheeting runs in vertical direction. It is madeup into panel units. At the head, provision is made for linking with the formwork

for horizontal beam.

Figure 6.9 : Formworks for Columns

6.8.6 Formwork for Beams and Slabs

The formwork in case of beams is constructed in such a way that the sides may be

stripped before the soffit. It is so because the soffit must be left in place till the


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Concrete Technology beam can safely support its own weight. The slab formwork is supported by the

centering to the beam.

Figure 6.10 : Formwork Supporting a Heavy Beam and Slab

Figure 6.11 : Posts for Beam Centering

Figure 6.12 : Beam Centering with Slab Centering


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Figure 6.13 : Centering for Heavy Beams

Figure 6.14 : Beam Centering (Alternative Method)

6.8.7 Faults in Erection of Formwork

The following are the faults in the erection of the formwork :

(a) Sheeting is too thin.(b) Bad sheeting joints, which will lead to leakage of concrete.(c) No distance piece is provided.(d) Strut is small and buckles.(e) No wailings are provided.(f) Warped sheeting is used.(g) Cleat is too small. It is cut wrongly.(h) Sole plate is not provided which will cause uneven distribution of

load to ground.

(i) Stake section is small and driven to insufficient depth.(j) No strutting at the base of the shutters is provided.

6.8.8 Removal of FormworkThe suitability of the materials of the formwork for re-use depends in many

respects on the way removal is done. The shuttering boards, frameworks are

broken, the smooth surface is damaged and the bracing is bent, if the forms are

not removed carefully. Therefore, removal of formworks should be carried out ina careful and orderly way. Formwork should not be removed without specific

instructions from the engineer-in-charge. The engineer-in-charge before issuing

instructions should make sure that the concrete has attained sufficient strength to

carry its own load and any other load, which it carries at the top. The sides of the

formworks, which are not loaded by weight of concrete member, should not be

removed before the concrete gains strength. The lateral shuttering is usually

removed in summer within 2 to 3 days. Posts supporting the formworks of load

bearing structures should be removed only after stripping the sides of the form.

Load bearing formworks should be removed only after concrete has gained

strength.The following are the steps for striking the formworks :

(a) Loosening of the tie wires.


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(b) Removing the supports, which support the formworks.Concrete Technology(c) Removing the shuttering boards or panels.

Under normal circumstances when the temperature is 20oC and the cement used is

ordinary, the formwork may be stripped after the expiry to the following periods :

(a) Vertical sides of columns, walls, beams and slabs 1 to 2 days(b) Beam soffits 7 days(c) Bottom of slabs

(i) Span of 4.5 m 7 days

(ii) Span of 4.6 m and above, bottom of beams up to

span of 6 m, bottom of arch ribs up to a span of 6 m. 14 days

(d) Bottom of beams and arch ribs over 6 m spans 21days

SAQ 9

(a) Fill in the blanks

(i) __________________ are used in heavy wall construction.

(ii) __________________ dries out and shrinks.

(iii) ___________________ should be done in order to make the

surface of formwork smooth.

(iv) Forms require ___________________ study as they account for

reasonable percentage of the final cost of concrete.

(v) In case of sliding type of formwork, it ____________________

as the concrete is placed.

(b) State True or False

(i) Steel supports minimize the finishing costs.

(ii) Hard woods are difficult and costly to work.(iii) Partially seasoned timber is most satisfactory to use for

formworks.

(iv) Sliding type of formwork enables high speed of construction.(v) The forms should be adequately braced.

(c) What is formwork?

(d) What are the requirements of formwork?(e) What are the precautions to be taken before placing concrete in

formwork?

(f) Enlist the faults in erection of formwork.(g) Discuss the procedure of removal of formwork.(h) Enlist the advantages of steel formwork.


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6.9 SUMMARY

In this unit, you have studied concreting operations like transportation, placement

or deposition, compaction, curing and finishing of concrete. You also studied

different joints in concrete construction and formwork. In the next unit, you willstudy special types of concrete and concreting methods under extreme

environmental conditions.

6.10 ANSWERS TO SAQs

Refer the relevant preceding text in the unit or other useful books on the topic

listed in the section Further Reading given at the end of the booklet to get the

answers of SAQs.

transporting, deposition, curing and finishing of concrete

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