Introduction
Concrete should be transported to the place of deposition at the earliest without the loss of homogeneity obtained at the time of mixing.
A maximum of 2 hours from the time of mixing is permitted if trucks with agitator and 1 hour if trucks without agitators are used for transporting concrete. Also it should be ensured that segregation does not take place during transportation and placement.
Methods of transporting.
The methods adopted for transporting concrete depend upon the size and importance of the job, the distance of the deposition place from the mixing place, and the nature of the terrain. Some of the methods of transporting concrete are as below:
1. Mortar pan.
This is the most common method of transporting concrete. This is labour intensive method wherein the pans are passed from hand to hand and is slow and expensive method. Since pan conveys small quantity of concrete, more and more concrete area is exposed to atmosphere during transportation. This may lead to evaporation of water from concrete particularly in hot weather and under conditions of low humidity. In this method of transporting concrete can be deposited with equal ease on above or below the ground level without segregation.
2. Wheel barrows.
Wheel barrows are used for transporting concrete to be placed at ground level. These are used for concreting rigid payments. For long hauls due to uneven ground surface segregation may take place. To check this, the barrows are provided with pneumatic wheels instead of steel wheels. Alternatively, wooden plank way is made over which barrows are moved to check vibrations in barrow and thus result in reduced segregation. These are most useful for small jobs, small hauls, and over muddy ground. The average capacity of a wheel barrow is 35 litres (80 kg).
3. Chutes.
Chutes are used to transport concrete below the ground levels. These are made with metal sheets with a slope more than IV: 2.5H to ensure that unloaded concrete slides easily without segregation. This method of transporting concrete should be avoided until unevitable such as lack of space or congested reinforcement.
4. Dumpers.
Dumpers, lorries or, trucks are used economically for hauls up to 5 km. Dumpers are usually of capacity 2 to 3 cu m whereas trucks are of 4 cu m capacity. For long hauls agitators are used to prevent segregation. The concrete should be covered with tarpaulins to prevent evaporation of water from concrete.
5. Bucket and ropeway.
These are used when concreting is to be done in a valley or for construction work of piers, dams etc. The bucket is brought close to the mix site, filled and moved over ropeway to the site of deposition. The bucket may be of tilting type or with a gate at bottom for discharging concrete. Concrete should be discharged from the smallest height.
6. Belt conveyor.
The use of belt conveyors for transporting concrete is very little. The two main objections are segregation and drying, and stiffening of concrete. Concrete segregates due to steep slopes, changes of directions, and the points where belt passes over rollers. The drawback is that the concrete will have to be remixed at the place of deposition which is not recommended.
7. Skip and hoist.
This is the most useful and advantageous method of transporting concrete for multistorey buildings. The mixer feeds the skip which moves up over rails up to the level of deposition. If height of travel is too much, the concrete may require a turning over before deposition in to the place.
8. Pumping.
Pumping of concrete is done for multistorey buildings, tunnels, and bridges. The concrete is fed from the hopper into the pump cylinder largely by gravity, assisted by the vacuum created on the suction stroke of the piston and forced into the pipe line on the pressure stroke.
The pipes are made of steel, aluminium, or plastic of sizes ranging from 80 to 200 mm diameter. The aluminium pipes have a draw back of formation of hydrogen gas and are not recommended. The pumps of capacity 15 to 150 m3/hr are available. These can pump concrete 400 m horizontally and up to 80 m vertically. The pumpable concrete should be cohesive and fatty. It should have a slump of 50 to 100 mm or even more and a compacting factor of 0.9 to 0.95.
Pumping compacts the concrete partially and reduces the slump by about 25 per cent at the delivery end. Sometimes, admixtures are added to offer additional lubrication, reduced bleeding, and segregation. Generally, air entraining admixtures are used giving an air content of 3 to 5 per cent.
The pipes are made of steel, aluminium, or plastic of sizes ranging from 80 to 200 mm diameter. The aluminium pipes have a draw back of formation of hydrogen gas and are not recommended. The pumps of capacity 15 to 150 m3/hr are available. These can pump concrete 400 m horizontally and up to 80 m vertically. The pumpable concrete should be cohesive and fatty. It should have a slump of 50 to 100 mm or even more and a compacting factor of 0.9 to 0.95.
Pumping compacts the concrete partially and reduces the slump by about 25 per cent at the delivery end. Sometimes, admixtures are added to offer additional lubrication, reduced bleeding, and segregation. Generally, air entraining admixtures are used giving an air content of 3 to 5 per cent.
At the end of the work, the pipe line is cleared off concrete by a plunger, or go devil. The pipe line is then cleared by washing of all the cement slurry.
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