Transporting and placing of concrete
The concrete, as it comes out of the mixer or as it is ready for use on the platform, is to be transported and placed on the formworks.
The type of equipment to be used for transport of concrete depends on the nature of work, height above ground level and distance between the points of preparation and placing of concrete.
For ordinary building works, human ladder is formed and the concrete is conveyed in pans from hand to hand. For Important works, the various mechanical devices such as dumpers, truck mixers, buckets, chutes, belt conveyors, pumps, hoist, etc. may be used.
The two Important precautions necessary in transportation of concrete are as follows:
i) The concrete should be transported in such a way that there is no segregation of the aggregates.
ii) Under no circumstances, the water should be added to the concrete during its passage from mixer to the formwork.
The precautions to be taken during the placing of concrete are as followed:
1. The formwork or the surface which is to receive fresh concrete should be properly cleaned, prepared and well-watered.
2. It is desirable to deposit concrete as near as practicable to its final position.
3. The large quantities of concrete should not be deposited at a time. Otherwise, the concrete will start to flow along the formwork and consequently, the resulting concrete will not have uniform composition.
TRANSPORTATION AND PLACING OF CONCRETE || IS 456:2000 SECTION 2 CLAUSE 13
4. The concrete should be dropped vertically from a reasonable height. If concrete is poured from a vertical height exceeding 150 cm, it results into segregation of the aggregates and Increase in the cost of formwork. For vertical laying of concrete, the care should be taken to use stiff mix. Otherwise bleeding of concrete through cracks in forms will take place. The term bleeding is used to mean the diffusion or running of concrete through formwork.
5. The concrete should be deposited in the horizontal layers of about 15 cm height. For mass concrete, the layers may be of 40 cm to 50 cm height. The accumulation of excess water in the upper layers is known as laltance and it should be prevented by using shallow layers with stiff mix or by putting dry batches of concrete to absorb the excess water.
6. As far as possible, the concrete should be placed in single thickness. In case of deep sections, the concrete should be placed in successive horizontal layers and proper care should be taken to develop enough bonds between successive layers.
7. The concrete should be thoroughly worked around the reinforcement and tapped in such a way that no honeycombed surface appears on removal of the formwork. The term honeycomb is used to mean a comb or mass of waxy cells formed by bees, in which they store their honey. Hence, if this precaution is not taken, the concrete surface so formed would have a honeycomb like surface.
8. The concrete should be placed on the formwork as soon as possible. But in no case, it should be placed after 30 minutes of its preparation.
9. During placing, it should be seen that all edges and corners of concrete surface remain unbroken, sharp and straight in line.
10. The placing of concrete should be carried out uninterrupted between pre-determined construction joints.
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