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 formwork.
Following precautions should be taken during placing of concrete.
The formwork or the surface which is to receive the fresh concrete should be properly cleaned, prepared and well watered.
It is desirable to deposit concrete as near as practicable to its final position.
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.
The concrete should be dropped vertically from a reasonable height. For vertical laying of concrete, care should be taken to use stiff mix. Otherwise the 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.
The concrete should be deposited in horizontal layers of about 150 mm height. For mass concrete, the layers may be of 400 mm to 500 mm height. The accumulation of excess water in upper layers is known as the laitance and it should be prevented by using shallow layers with stiff mix or by putting dry batches of concrete to absorb the excess water.
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 bond between successive layers.
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 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.
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.
During placing, it should be seen that all edges and corners of concrete surface remain unbroken, sharp and straight in line.
The placing of concrete should be carries out uninterrupted between predetermined construction joints.