To ensure the requirements of strength, impermeability, and durability of the hardened concrete in the actual structure, the operation of placing concrete is most important. Before placing concrete it should be determined that forms are tight, clean, properly braced and moistened or oiled.
As far as placing concerned, the main objective is to deposit the concrete as close as possible to its final position so that segregation is avoided and the concrete can fully be compacted. To achieve this objective, the following rules should be followed:
1. The concrete should be placed in uniform layers, not in large heap or slopping layers.
2. Hand shoveling and moving concrete immersion or poker vibrator should be avoided.
3. The thickness of a layer should be compatible with the method of vibration so that entrapped air can be removed from the bottom of each layer.
4. The rates of placing and of compaction should be equal.
5. Where good finish and uniform color are required on columns and walls, the form should be filled at a rate of at least 6 ft per hour, avoiding delays.
6. Each layer should be fully compacted before placing the next one, and each subsequent layer should be placed whilst the underlying layer is still plastic so that monolithic construction is achieved.
Collision between concrete and formwork or reinforcement should be avoided. For deep sections, a long down pipe or tremie ensures accuracy of location of the concrete and minimum segregation. Concrete should be placed in a vertical plane. When placing in horizontal or sloping forms, the concrete should be placed vertically against and not a way from, the previously placed concrete.
Placing concrete by tremie is particularly suited for deep forms, where compaction by the usual methods is not possible, and for underwater concreting. In the tremie method, high workability concrete is fed by gravity through a vertical pipe which is gradually raised. The mixed must be cohesive, without segregation or bleeding, and usually has high cement content, a high proportion of fines and contains a workability aids.