In this article we will discuss about the methods of transportation and placement of concrete.
Transportation of Concrete:
After mixing, concrete should be transported and placed at site as quickly as possible without segregation, drying, etc. Hence transportation of fresh concrete is very important operation. Depending upon the type of work and equipment available various methods of transporting of concrete can be employed. The method selected should be suitable for use with the consistency of concrete employed.
It is important that the concrete be handled without segregation of the constituent materials. This condition may easily occur if special attention is not given to its prevention. Segregation occurs because concrete is not a homogenous combination, but a mixture of materials differing in size and specific gravity. Thus as soon as the concrete is discharged from the mixer, internal as well as external forces start acting to separate the dissimilar constituents. If over wet concrete is confined in restricting forms, the coarser and heavier particles tend to settle and finer and lighter materials tend to rise.
Thus the equipment for conveying concrete from mixer to form should be such as to result in uniform concrete of the desired consistency at the form. The method of conveying should not require wetter concrete than is required at form for placing nor it should produce segregation or excessive drying or stiffening etc.
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To avoid segregation, when concrete is to be transferred from one conveyance to another, use of hoppers, baffles and short vertical drops should be used through a pipe to the centre of the receiving container as shown in Fig. 10.1. This belief is not correct that segregation produced during conveyance can be rectified by compaction. Segregation should be prevented rather than corrected after its occurrence.
Fig. 10.1 shows correct and incorrect method of filling hoppers or buckets. Fig. 10.2 shows the control of separation of concrete at the end of conveyer belts, while Fig. 10.3 shows control of separation at the end of chutes. In this case a 60 cms long pipe is provided at the end of the chute. This Fig. 10.3 shows the correct and incorrect methods of concrete discharge. Fig. 10.4 shows placing of concrete on a sloping surface. A baffle and drop at the end of chute will avoid separation and concrete remains on slope.
Methods of Conveyance for Transporting Concrete:
For moving concrete from mixer to the form following methods may be employed:
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1. Pan method
2. Wheel barrows
3. Dumpers
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4. Lorries
5. Mono rail transporters
6. Hoists
7. Crane
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8. Skips and buckets
9. Chutes
10. Conveyer belts
11. Pneumatic pressure equipment, and
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12. Pumps.
Ready Mixed Concrete:
Concrete available for placing from a central plant is known as ready mixed concrete or pre mixed concrete.
Advantages of Ready Mixed Concrete:
Following are the advantages of ready mixed concrete:
1. This type of concrete is particularly useful on congested sites.
2. For road construction where little space is available for mixing plant and for stocking aggregate etc.
3. Ready mixed concrete can be made under better control than on site. Actually a factory type control on all operations is possible on the central mixing plant. This is the greatest advantage of ready mixed concrete.
Ready-made concrete can be considered as a factory made product almost comparable with steel. Hence a great deal of uncertainty and variability associated with the production of concrete on many sites is removed.
4. The use of ready mixed concrete is advantageous when only small quantities of concrete is required or concrete is to be placed at intervals.
Disadvantages:
The main disadvantage of ready mixed concrete is its high cost. It is costlier by about 10 to 15% than concrete mixed at site.
Types of Ready Mix Concrete:
Ready mix concrete can be classified into two groups as follows:
1. Central mixed concrete
2. Transit mixed or truck mixed concrete.
1. Central Mixed Concrete:
In this case the mixing is done at a central plant and the mixed concrete is transported in a special type of truck known as an agitator truck. The truck (agitator truck) revolves slowly while moving. The agitating container is conical in shape having smaller blades than mixer. The capacity of the container is 1.25 m3 to 2.0 m3. However the height of this container is more than a mixer. The container is put upright in the truck and to check the evaporation of water from the concrete an air tight stopper is placed on the top of the container.
The segregation and stiffening of concrete is checked by revolving the container. The revolving of the container is also done by the engine of the truck. The rate of revolving of the container depends upon the speed of the lorry varying 2 to 6 revolutions per minutes. However 2 to 3 revolutions per minute are quite sufficient.
Advantages of Agitating Trucks:
1. To revolve the container, very less amount of power is needed.
2. During even in one hour journey, no ill effect on the properties of concrete is developed.
Disadvantage:
The main disadvantage of this method is that these agitating trucks cannot be used for any other purpose. Hence this method is uneconomical.
2. Transit Mixed Concrete:
In this method the materials are batched at a central plant and the dry mix is put in mixers kept in trucks. The mix is mixed either during transit to the site or immediately prior to the concrete being discharged at the site. The speed of rotation is between 4 to 16 revolutions per minute. ASTM has laid down the limit of 300 revolutions for both agitating and mixing, and concrete must be laid within 90 minutes of mixing. The capacity of mixer may vary from 0.75 m to 4.5 m3.
Advantages:
i. Transit mixing permits a longer haul and is less vulnerable in case of delay.
ii. As the water is added to mix near the site of work, dislocation of the work does not take place.
iii. The discharge of concrete is done by reversing the direction of the revolving mixer rather than tipping the truck.
iv. These mixers have small revolving tubing, which can spread the concrete on a sufficient wide area. This operation makes the concrete spreading operation economical.
Disadvantages:
i. Transportation by this method is limited to an area within a radius of 9 to 15 km.
ii. These trucks can be used only for this purpose.
iii. The moist aggregate should not remain in contact with cement for more than 90 minutes.
Note:
The agitating differs from mixing solely by the speed of rotation of the mixer. The agitating speed is between 2 and 6 revolutions per minute, whereas mixing speed varies from 4 to 16 revolutions per minute. It may be noted that the speed of mixing affects the rate of stiffening, while the total number of revolution controls the uniformity of mixing.
Causes of Blockage during Concrete Mix:
Generally blockages take place due to the following causes:
1. Unsuitability of the concrete mix i.e. concrete mix is not designed properly.
2. Deficiencies in pipe line and joints.
3. Careless use of hose end.
4. Error of operator.
Sometimes use of admixtures, particularly accelerating admixture and high grade cement, high temperature may cause blockage. If continuous pumping is not done there are more chances of blockage.
If the pipe line is not cleaned properly after its previous use there are more chances of blockage. The use of un-cleaned, worn out hoses, too many and too sharp bends, use of worn out joints are also other causes of blockages.
Thus to avoid blockages, the hose must be well lubricated. Sufficient quantity of lubricating grout should be used to cover the entire length of pipe line before pumping concrete. The flexible rubber end hose should be handled with care. Careless handling may cause blockage.
Location of the Blockage:
The location of blockage can be determined by tapping the pipe line and observing the sound emitted from the pipe line.
Remedial Measures of Blockage:
1. A minor blockage may be cleared by forward and reverse pumping. However excess pressure should not be applied blindly. It may aggravate the problem.
2. Blockage can be cleared by rodding or using sponge ball pushed by compressed air or water at high pressure.
3. Sometimes shortening the pipe line reduces pressure and on restarting the pumping, blockage is removed.
Safety Precautions during Transportation of Concrete:
Usually following precautions should be adopted during transportation of concrete:
1. Places where Lorries are to be discharged should have strong support for tipping the Lorries.
2. No person should be allowed to stand at the place where the tipping lorry discharges concrete.
3. To avoid segregation, distance between the bottoms of the skip to the top of surface where concrete is to be placed should not be more than one metre.
4. The distance between wagons being moved simultaneously by hand in one direction should not exceed 20 m on level surface and 30 m on slopes.
5. The whole system of pipe line should be checked by using hydraulic pressure 1.5 times the operating pressure before pumping the concrete. Generally water is used for this purpose.
6. The run of wheel barrow should be clean and at least 1.2 m wide.
7. While using hoist, the driver of the hoist should be in a position to see the place of charging and discharging the hoist.
Placement of Concrete:
The process of depositing the concrete in its required position is termed as placement of concrete.
When concrete is to be placed at a place it should be deposited as near as possible to its final position, so that when is it to be compacted it does not have to flow appreciably. Areas which will be in contact with concrete should be prepared well before placing concrete. Hence in order to get good finished surfaces, it is essential that surface where concrete is to be laid should be prepared carefully.
Preparation of Surface before Placing Concrete:
In order to develop a proper and adequate bond between the base and the concrete placed, it is essential that the base is prepared well before placing the concrete.
Before placing concrete different types of bases should be prepared as discussed below:
1. Natural Soil:
Before laying any concrete layer over a natural soil base, the soil should be properly compacted and leveled. After leveling the coarse grained soil should be made moist for a thickness of 30 cm and loamy and fine grained soils upto 15 cm thickness for 24 hours before laying the concrete layer. The moistening of soil is essential, to check absorption of water from concrete by the soil. However water should not be allowed to stand on the prepared surface.
2. In case of Rocky Bases:
In this case the sides should be kept vertical. All loose particles should be removed and water sprinkled over the base and cleaned before placing the concrete. In this case also water should not be allowed to stand on the surface.
3. In case of Brick soling and W.B.M. Bases:
The base should be made rough with the help of wire brush and cleaned. Loose particles and dust removed, water sprinkled and surface compacted before placing the concrete over it.
4. In Case of Hardened Concrete Base:
(a) The difference in two operations is 4 hours. The laitance on the surface of old concrete should be removed by rubbing it with wire brush and cleaned with water. All dust and rubbish should be removed by broom before laying the new layer of concrete.
(b) Upto 48 hours. The surface is rubbed with steel brush and washed with water and cleaned with the help of the broom. Now 1.5 cm thick layer of cement and sand mortar of similar composition as that of concrete is applied to the surface before new concrete is placed.
(c) More than 48 hours old concrete. In this case the hard surface is chiselled or sand blasted and cleaned as above Now a slurry of neat cement is applied on the roughened surface and worked into the interstices after which 1.5 cm thick layer of cement sand mortar of the same composition as that of concrete should be applied before the slurry dries and then new concrete is placed immediately,
Placing of Concrete:
Placing and compacting operations are the most critical field activities. Unless care is exercised in this operation a very poor job may result even though concrete prepared is the best. To avoid segregation is most important if it occurs remedial measures should be taken to eliminate it, so that homogeneous concrete is obtained.
When concrete is placed in deep forms, it is usually allowed to drop regardless of height. These results in segregation damage to form and embedded fixtures.
Reinforcement and forms above the level of placement become coated with mortar which may dry before the concrete comes to that level. This condition should be avoided by dropping the concrete into an outside pocket and allowing it to flow over into the form without segregation. Fig. 10.7 shows correct and incorrect method of pouring concrete in deep forms.
In case of horizontal layers, it should be deposited close to its final location. In case of placing concrete in a slab, it should be dumped into face of previously placed concrete not away from the previously placed concrete. Fig. 10.8 shows a correct and incorrect method of placing concrete in slab with the help of buggies.
When concrete is to be placed on a slight slope, placing should begin at the lower end of the slope, permitting consolidation of the layer by vibration or any other suitable means. If placement is started at the top of the slope vibration tends to shift it down the slope. Fig. 10.9 shows a correct and incorrect method of placing concrete on a slope.
Precautions to be Taken While Placing Concrete:
Following precautions should be taken while placing concrete:
1. In case of reinforced concrete, thickness of concrete may vary from 15 cms to 30 cms. and 45 cms in case of mass concrete. II should be laid in horizontal and uniform layers.
2. Concrete should not be thrown from a height of more than 1 m to prevent segregation.
3. No person should be allowed to walk over freshly laid concrete.
4. Placement of concrete should be discontinued during heavy downpour i.e., during heavy rain fall period.
5. Before placing new concrete, lataince over the old surface should be removed.
6. In order to prevent formation of irregular surface, concrete should be laid continuously.
7. The alignment of reinforcement and form work should not be disturbed.
8. In order to avoid sticking of concrete to surface of the form work, the internal faces of form work should be oiled or greased.
9. In R.C.C. slabs concreting should start from width wise from one end.
10. In order to produce better bond between concrete and reinforcement, grease, oil, dry cement etc. should be removed from the reinforcement.
11. While pouring concrete in deep forma, concrete should not be thrown from a height.