Special Concreting Techniques

Syllabus
• Pumped Concrete, Shortcrete, underwater
concrete, pre-placed concrete, Vacuum
dewatered concrete, hot and cold weather
concreting, ready mixed concrete.

Special Concreting Techniques
• Pumped Concrete: A concrete which can be pushed through a
pipeline is called pumpable concrete. The Concrete mix is designed
in such a manner that it does not wedge while flowing and its
friction at the inner wall of the pipe line does not become very high.
• Pumpable concrete emerging from a pipeline flows in the form of a
plug which is separated from the pipe wall by a thin lubricating
layer consisting of cement paste. The water in the paste
hydraulically linked with the interparticle water layer in the plug.
Fig shows the flow of concrete under pressure. For continuous plug
movement, the pressure generated by the flow resistance must not be
greater than pump pressure rating. However, if w/c ratio is high, the
concrete becomes too saturated and water is forced out of the mix,
creating an increase in flow resistance and a possible blockage of
concrete.
• Thus, a very stiff concrete is not pumpable and also a concrete with
w/c ratio is also not pumpable. It is interesting to note that if a
concrete is pumpable, it implied that it is a good concrete.

The Flow of Concrete Under
Pressure

Reason for Blockage of Concrete
• There is two much frictional resistance due to nature
of the ingredients of the mix.
• Water is being forced out of the mix creating bleeding
and blockage by jamming.

Design mix for Pumpable Concrete
• The mix of pumpable concrete is proportioned in such a
way that it is able to bind all the ingredients together under
pressure from the pump avoiding segregation and bleeding.
The mix also facilitates the radial movement of sufficient
grout to maintain the lubrication film on the internal face of
the pipeline wall. The mix should also be able to deform
while flowing through bends.
• To achieve this, the proportions of fines, i.e. cement and
fine particles below 0.25 mm size particles below 300 μ is
of prime importance. The quantities of fine particles
between 350 to 400 kg/ m3 are considered necessary for
pump able concrete.
• The slump of pumpable concrete is kept at 75 mm to
collapse range and the diameter of the pipeline is at least 3
to 4 times the maximum size.

Pumps and Pipeline
• Pumping of concrete through steel pipelines is one of the successful
methods of transporting concrete. Pumped concrete has widely been
used in the construction of multi-storied buildings, bridges and
tunnels.
• The equipment consists of a heavy duty, single acting, horizontal
piston pump of special design. The concrete is fed from the hopper
into the pump cylinder largely by gravity, but is assisted by the
vaccum created on the suction stroke of the piston and forced into
the pipeline on the pressure stroke. The pipeline is completely filled
and concrete moves uniformly cohesive and fatty mix will feed
more readily than harsh and dry mix.
• Concrete pumps are normally available in capacities ranging from
15 m3 / h to 150 m3 / h. The normal distance to which concrete can
be pumped is about 400 m, 80 m vertically. Bends in the pipeline
reduce the effective pumping distance by approximately 10 m from
each 90 0 bend , 5m for a 45 0 bend and 3m for a 22.5 0 bend.

Shortcrete or Guniting
• ‘Shortcrete’ or ‘Gunite’ is a mortar or a fine concrete that is
pneumatically transported through a hose and projected on
to a surface at a high velocity.
• This system is called by different names in different
countries such as Blastcretes, guncrete, Jet-crete, nucrete,
spraycrete etc. though the principle is essentially the same.
This system is very well suited for construction of lightly
reinforced, thin sections, Shortcrete is more economical
than conventional concrete because of less formwork
requirements, requiring a small portable plant for
manufacturing and placement. The force of the jet
impacting on the surface compart the material. Sometimes
set accelerators are used to assist overhead placing. The
newely developed ‘ Redi-set cement’ can also be used for
shortcreting process.

The various applications of
shortcrete are:
• Thin overhead, vertical or horizontal surface.
• Swimming pools and prestressed tanks.
• Canal and tunnel lining.
• Repair of damaged concrete.
• Overlays on concrete roads.
• Refractory lining works.
• At present two different processes are employed
for shortcreting, namely,
• Dry-mix process
• Wet-mix process

Dry-Mix Process
• The various stages involved in the dry-mix process are as
follows:
• The cement and sand are throughly mixed. The mixture of
cement-sand is fed into a special air-pressurized mechanical
feeder termed as ‘Cement gun’.
• The material is carried by compressed air through the
delivery hose to a special nozzle. The nozzle is fitted with a
perforated manifold through which water is introduced
under pressure and intimately mixed with other ingredients.
• The mortar is jetted from the nozzle at high velocity onto
the surface to be shortcreted. Any alteration in the quantity
of water can be easily accomplished by the nozzle man

Advantages of Dry-mix Process
• The dry-mix process is preferred for light
weight aggregates concrete.
• The lower w/c ratio obtained with the dry
process probably accounts for the lesser creep,
higher strength and greater durability of
concrete.
• The dry process equipment can covey the
material to a distance of 300 m to 500 m
horizontally and 45 to 100 m vertically.

Wet Mix Process
• In this process, cement, sand, small sized coarse
aggregate and water are mixed before entering the
chamber of delivery equipment. The Ready
Mixed Concrete (RMC) is received into a feeding
chamber from which the concrete is blown by
compressed air at a pressure of 5.5 to 7
atmosphere through a rubber hose. Equipment's
are available which can place concrete at the rate
of 3 to 9 cubic metre per hour. Additional air is
injected at the nozzle to increase the velocity and
improve the gunning pattern.

Under Water Concrete
• Placing concrete under water:
• Special precautions should be taken whenever concrete
is to be placed under water. Such a concrete should
have cement content 450 kg/m3 of concrete and a slump
of 10 to 17.5 cm.
• The methods used for placing concrete under water are:
• Bagged Concrete
• Bottom dump bucket
• Tromie
• Grouted aggregate
• Concrete pump.

• Bagged Concrete:
• In some situations gunny bags are filled about 2/3
rd. full with dry or semi-dry mixture of cement,
fine and coarse aggregate. They are lowered into
the water and placed carefully in a header and
stretcher fashion like that of brick masonry
construction with the help of divers. This method
does not give satisfactory concrete, as the
concrete mix will be full of voids.

• Bottom Dump Bucket:
• Other method of placing concrete under water or
in a trench filled with the bentonite slurry is by
bottom dump bucket method. In the bottom dump
bucket method concrete is taken through the
water in a water tight bucket or box. On reaching
the final position the bottom of the bucket is made
to open by some mechanism and the whole
concrete is dumped slowly in water. There are
chances of washing away of some quantity of
cement when concrete is dumped from the bucket.

Tremie
• The most satisfactory method of placing concrete
under water is by the use of tremie pipe.
• In this method, a tramie pipe of 200 mm to 250
mm diameter is used. The length of pipe can be
easily increased or decreased by using couplings.
A funnel is provided at the top end pipe to
facilitate pouring of concrete. The bottom end is
closed with a plug or thick polyethylene sheet to
prevent entry of water into the pipe.

Tremie
• The pipe is lowered and made to rest at the point where the
concrete is going to be placed. The concrete having a very
high slump of about 15 to 20 cm is poured into the funnel.
When the whole length of pipe is filled up with the
concrete, the tremie pipe is lifted up and a slight jerk is
given by a winch and pulley arrangement. When the pipe is
raised and given a jerk the plug is forced out and the
concrete gets discharged. After concreting has started the
lower end of the tremie should be kept as deeply submerged
in the previously placed concrete to prevent entry of water
into the pipe from bottom end. The tremie should be lifted
slowly to permit the concrete to flow out, care being taken
not to loose the seal at the bottom. In this way, concrete
work is progressed without stopping till the concrete level
comes above the water level.

Grouted Aggregate
• Another method of placing concrete under water
is the grouting of pre-packed aggregate. Coarse
aggregate is dumped in the forms to assume full
dimension of the concrete mass. Cement mortar
grout is injected through pipes, which extend up
to the bottom of the aggregate bed. The pipes are
slowly withdrawn as grouting proceeds. The grout
forces the water out of the forms and fills the
interstices in the aggregate. This method,
however, has been used very little. For plugging
the well foundation this method is often adopted.

Concrete Pump
• Concrete pumps and pipes can also be used for
placing concrete under water. The pipeline is
plugged at the end and lowered until it rests on
the bottom. Pumping is then started, when the
pipe is completely filled, the plug is forced out,
the concrete surrounding the lower end of the pipe
seal the pipe. The pipe is held in this position until
the pressure becomes too great. Then, the pipe is
withdrawn and the operation is repeated. This
process is repeated until concrete reached the
level above water.

Pre-Packed Concrete
• Prepacked concrete is a special technique of placing
concrete under water. When ‘tremie method’ or bottom
dump bucket method’ are not feasible, this method is
adopted. This technique, also called ‘grouted concrete’
consists of placing the coarse aggregate only in the form
and thoroughly compacting it to form prepacked mass. This
mass is then grouted with the cement mortar of the required
proportions. This process can be employed for both plain or
reinforced cement concrete. This method is employed where
the reinforcement is very complicated or where certain
arrangements like pipes, conduits, openings are required to
be incorporated in the concrete. This technique is employed
in mass concreting, in bridges abutments and piers, well
staining etc.

Vacuum Concrete
• Whenever thin section like slabs and walls have to be concreted, it is
necessary to adopt a fluid mix with water-cement ratio 0.5 to 0.65 to
facilitate the placing and compaction. Such a mix will lead to a
concrete of relatively low strength and poor abrasion resistance. In
such situations vacuum treatment of concrete is a solution.
• A considerable part of excess water and air are removed by suction
through a mat connected to a vaccum pump. An arrangement for
vaccum treatment of concrete using suction through a surface mat
connected to a vacuum pump is shown in fig.
• Generally, higher workability and higher strength or very low
workability and higher strength do not go hand in hand. This can be
achieved in vacuum concrete. In this process, excess water used for
higher workability, not required for hydration, and harmful in many
ways to the hardened concrete. This excess water is withdrawn by
means of vaccum pump immediately after placing of the concrete. The
process when properly applied, produces concrete of good quality.

Vacuum Concrete
• The advantage of vaccum treatment are:
• It permits removal of formwork at an early age to be used in
other repetitive work.
• The vaccum concrete bounds very well with old concrete.
• There is considerable increase in strength and quality of
concrete.
• The resistance to wear and abrasion is increased.
• Application:
• Production of precast plain and reinforced concrete
elements in mass production in factories.
• Construction of thin concrete walls, partition wall and slabs,
• Resurfacing and repair of road prevented.

Cold-Weather Concrete
• The production of concrete in cold weather introduces many problems
such as delay in setting and hardening, damage to concrete in plastic
condition due to the formation of ice lenses. Hence, it is essential to
maintain the temperature of the concrete above 5 0 C. it is generally
accepted that there is little cement hydration and strength gain if concrete
is frozen and kept frozen below 10 0 C. Therefore, fresh concrete must be
protected against disruptive expansion by freezing until adequate strength
has been gained. Without external heat sources, heat of cement hydration
in large and well-insulated concrete members may be adequate to
maintain satisfactory curing temperature. It may be noted that lower
concrete temperatures are permitted for massive sections because the
amount of heat generated is very large.
• The effect of cold weather on concrete are as follows:
• Delay Setting: The rate of hydration of cement depends upon the ambient
temperature. If the ambient temperature is low, hydration process will go
slow and concrete takes a longer time to set and to develop strength. The
setting period necessary before removal of formwork is thus increased.
The rate of progress of work will be slow. Although the initial strength of
concrete is lower, the ultimate strength will not be severely affected.

• Freezing of concrete at early age: When the temperature of
concrete falls below freezing point, the fee water held in the plastic
concrete freezes. Due to freezing of water, concrete expands and
hydration of cement will be stopped. This will result in considerable
loss of strength
• Freezing and Thawing: When concrete is subjected to alternate
cycles of freezing and thawing, its durability is greatly impaired. It
has been found that even one cycle of freezing and thawing during
the pre-hardening period may reduce the compressive strength to 50
% of what would be expected for normal temperature concrete.
• Stresses due to temperature differential: In case of mass
concreting in cold weather there will be a large temperature
differential due to high temperature inside the mass, which may
promote micro cracking and has a harmful effect on durability of
concrete.

• Recommended Practice and Precautions:
• Selection of suitable type of cement:
• Cements contain higher percentage of C3S and comparatively lower
percentage of dicalcium silicate (C2S), hydrates fast and gives out more
heat of hydration and early strength. Rapid hardening cements, extra rapid
hardening cement or high alumina cement are such cement that can be
used.
• Temperature control of ingredients:
• In sub-zero temperature concreting pre-heating of concrete has found to be
very effective. It would be easier to heat the mixing water. The temperature
of the water should not exceed 65 0C as the flash set of cement will occur
when the hot water and cement come in contact in the mixers. Therefore,
the heated water should come in contact with the aggregates, and not the
cement, first, aggregates may be heated either by closed steam coils under
the stock pile or by hot air blowers. Fine aggregate can also be heated on
hot plates. The temperature of ingredients should be so decided that the
resulting concrete sets at a temperature of 10 0C to 20 0 C

• Electrical heating of concrete mass:
• Concrete mass can be heated using AC current, if cheap power is available.
Electricity is conducted through reinforcing bars or mats. Sometimes special
electrode are carefully positioned for uniform heat generation. But, electrical
heating reduces the strength of concrete by about 20 % because of loss of
water and temperature stresses.
• Use of Insulating formwork:
• The hydration of cement generates considerable quantity of heat during first 3
days of hydration. Such heat can be gainfully conserved by using insulating
formwork covers, like timber, clean straw, tarpanlines, blankets, plastic
sheetings etc.
• Admixtures of anti-freezing materials:
• Normally, accelerating admixtures are used to fasten the hydration of cement.
Accelerators also work as anti-freezing agents. The most commonly used
accelerator is calcium chloride. Many specifications restrict the use of CaCl 2
upto 3 % by weight of cement. For fear flash set and loss of long term strength.

• Use of air entraining agents:
• It has been found that the air-entrained concrete has higher durability than that of
ordinary concrete under freezing conditions. Weaker concrete with air-entrainment
is more durable under freezing condition than that of strong concrete without air
entrainment. Air entrainment, modify the pore structure of the concrete and
improves the resistance to frost attack.
• Delayed removal of formwork:
• Because of the slower rate of gain of strength during the cold weather, the
formwork is required to be kept in place for longer time in usual concreting
practice.
• Placing and curing of concrete:
• Before placing the concrete, all ice show frost should be completely removed from
the surface of formwork. During freezing conditions water curing is not applicable.
Low pressure wet steam curing may be useful..
• Covering the concrete surfaces:
• During cold weather, all concrete surfaces shall be covered as soon as the concrete
has been placed in order to prevent the loss of heat and to help prevent freezing.
The concrete surfaces may be covered plastic sheets, tarpaulins, clean straw
blankets etc.

Hot-Weather Concreting
• Concreting in hot weathers specially in tropical
countries and desert areas, where the
temperature above 40 0 C are reached, poses
some problems. High temperature and reduced
relative humidity are the main climatic factors
affecting concrete. In India, most of the areas
are in tropical regions. The procedure of
concreting in hot-weather is set out in IS:
7861: 1975

Hot-Weather Concreting
• The effect of hot-weather are as follows:
• Rapid rate of hydration:
• A higher temperature results in a more rapid hydration leading to
quick setting, thus reducing the handling time of concrete and also
the strength of hardened concrete. With the increase in the
temperature of concrete, the workability of concrete decreases and
water demand increases. The addition of water without proper
adjustment in the mix proportions adversely affects the ultimate
quality of concrete.
• Rapid evaporation of mixing water:
• As mentioned earlier, due to high ambient temperature, the water
mixed with the concrete to give the required workability will be lost
by evaporation. Therefore, workability of concrete will be reduced.
Such concrete cannot be properly compacted and it will result in
reduction in strength. The rate of evaporation depends on the
ambient temperature, relative humidity and wind speed.

The effect of hot-weather are as
follows
• Rapid evaporation during curing:
• Hot weather requires early and a continuous effort for curing,
particularly when 53 grade cement is used. If there is only lapse, the
concrete surface dries up fast interrupts the continuous hydration. The
subsequent wetting does not fully contribute to the development of
full strength
• Air-entrainment:
• At higher temperature it is difficult to control the air content in air –
entrained concrete. For a given amount of air-entraining agent, hot
concrete entrains less air than does concrete at normal temperatures.
• Increased tendency of cracking:
• Rapid evaporation of mixing water leads to plastic shrinkage
cracking and subsequent cooling of hardened concrete introduces
tensile stresses.

Recommended Practice and
Precautions
• Cooling of Aggregates: The temperature of the concrete can be kept
down by controlling the temperature of the aggregates. Aggregates
should be stockpiled in shade. Water can also be sprinkled on the
aggregate before using them in concrete. The precooling of aggregates
can be achieved at the mixing stage by adding calculated quantities of
broken ice pieces as a part of mixing water, provided the ice is
completely melted by the time mixing is completed. Heavy blowing of
cold air over the aggregate just before it is batched is desirable.
• Mixing Water: The temperature of the mixing water has the greatest
effects on temperature of concrete, because the specific heat of water
(1.0) is nearly five times that of common aggregate (0.22). Moreover,
the temperature of water is easier to control than the other ingredients.
Cooled water may be added to reduce the temperature of concrete. If the
ambient temperature is very high, ice pieces are incorporated directly
into the mixer.

Recommended Practice and
Precautions
• Production and Delivery: The temperature of the ingredients of
concrete should be maintained at the lowest practical levels so that
the temperature of concrete is below 40 0 C at the time of placement.
The concrete is mixed to the minimum required time. When ice is
used it must be mixed to such an extent that all the ice gets melted.
• Reinforcement, formwork and subgrade should be sprinkled with
cooled water just prior to placing the concrete. Concrete should be
placed in comparatively thin layer and sufficient time must be
allowed between successive lift.
• Immediately after finishing, the top of concrete must be covered by
plastic sheets, tarpaulins, gunny bags etc. to prevent the loss of
water by evaporation

Ready Mixed Concrete (RMC)
• A Concrete whose constituents are weight-batched at a central
batching plant, mixed either at the plant itself or in truck mixers, and
then transported to the construction site and delivered in a condition
ready to use, is known as ready mixed concrete (RMC).
• This technique is very useful in congested sites or at diverse work
places and saves the consumer from the botheration of procurement,
storage and handling of concrete materials. Due to low cost,
durability and its ability to be customized for different applications,
ready mixed concrete is becoming more and more popular. The
concrete quality in terms of its properties or composition and
quantity or volume required for the particular application is
specified by the customer.
• The use of ready mixed concrete is also advantageous when only
small quantity of concrete are required or when concrete is placed
only at intervals. Usually, the price of ready-mixed concrete is
somewhat higher than of site mixed concrete, but this may often by
saving in the cement content, site organization and supervisory staff.

• There are three principal categories of ready-mixed
concrete.
• The plant mixed concrete (central mixed concrete)
• The transit mixed concrete
• The shrink mixed concrete

• In the plant mixed type, the mixing is done at a central
batching plant and the mixed concrete is then transported, in
special agitator truck. The agitator truck keeps the concrete
moving along a circular path at about 2 to 6 revolutions per
minute, so as to prevent segregation and undue stiffening of the
mix.
• In the transit mixed or truck mixed type, the material are
batched at a central plant but are mixed in a mixer truck either
in transit to the site or immediately prior to the concrete being
discharged. Transit mixing permits a longer haul and is less
vulnerable in case of delay.
• In case of shrink-mixed type, the concrete is partially mixed at
a central plant in order to increase the capacity of the agitator
truck and mixing is completed end route. Truck mixers usually
have a capacity of 6 m3, but 7.5 m3 trucks also exist.

• The main problem in the production of ready-mixed
concrete is maintaining the workability of the mix right up
to the time of placing. Concrete stiffens with time and
handling ready mixed concrete often takes quite a long
while. The stiffening may also be aggregated by prolonged
mixing and by high temperature. The safe time of haul is
limited to 90 minutes or total number of revolutions of 300
revolutions whichever is less. However, agitating up to 6
hrs. need not adversely affect the strength of concrete,
provided the mix remains sufficiently workable for full
compaction.
• Since RMC is a manufactured product, specific control tests
and evaluations are required during the manufacturing
process to produce desired quality concrete.

Questions
• What is RMC ? What are its advantages ?
• What is shotcreting ? Explain dry mix process of
shortcreting.
• List various methods of concreting under water
and explain ‘Termie’ method.
• Cold Weather Concreting
• Hot Weather Concreting
• State the different types of special concreting
techniques and explain ready mixed concrete.
• Write a S.N. on pumped Concrete.

References
• • Concrete Technology by: R.P. Rethaliya
Atul Prakashan
• • Concrete Technology by . M.S. Shetty
• • Internet websites

Special Concreting Techniques

More Related Content

What's hot (20)

Viewers also liked (7)

Similar to Special Concreting Techniques (20)

More from GAURAV. H .TANDON (20)

Recently uploaded (20)

Special Concreting Techniques