8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
1/29
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
2/29
102
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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
3/29
103
Transporting,
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
4/29
104
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
5/29
105
Transporting,
Deposition, Curing and
Finishing of Concrete
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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
6/29
106
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
7/29
107
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
8/29
108
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
9/29
109
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
10/29
110
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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
11/29
111
Transporting,
Deposition, Curing and
Finishing of Concrete
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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
12/29
112
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
13/29
113
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
14/29
114
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
15/29
115
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
16/29
116
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
17/29
117
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
18/29
118
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
19/29
119
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
20/29
120
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
21/29
121
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
22/29
122
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
23/29
123
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
24/29
124
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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
25/29
125
Transporting,
Deposition, Curing and
Finishing of Concrete
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
26/29
126
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
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
27/29
127
Transporting,
Deposition, Curing and
Finishing of Concrete
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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
28/29
128
(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.
8/13/2019 Transporting, Deposition, Curing and Finishing of Concrete
29/29
Transporting,
Deposition, Curing and
Finishing of Concrete
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.
Top Related