Concrete Placing

As with the handling of concrete, certain fundamental considerations govern placing techniques. First and perhaps foremost is the need to avoid segregation of the concrete caused by improper techniques. Second is the need to ensure thorough compaction of the concrete. The manner in which concrete is placed can have a significant influence on its ability to compact under vibration.

Avoiding Segregation

The most important rules for avoiding segregation during the placing of concrete, in any element, are:

• Concrete should be placed vertically and as near as possible to its final position.
• It should not be made to flow into position. Where concrete must be moved it should be shoveled into position.

Other techniques for avoiding segregation during placing depend on the type of element being constructed and on the type of distribution equipment being used.

For flat work and slabs incorporating ribs and beams (i.e. shallow forms) the techniques shown inFig-1should be adopted.

For walls and columns (i.e. deep, narrow forms), problems occur when the concrete is dropped from too great a height and ricochets off the reinforcement and form-faces, resulting in segregation. The means of avoiding this vary with the type of distribution equipment being used (Fig-2).

Aiding Compaction

为了帮助随后的混凝土压实，应注意将混凝土放入适合压实设备深度的层。太深的层几乎无法充分压实混凝土，从而留下了空气，从而在混凝土表面产生空隙和吹孔，并防止其实现其潜在的耐用性和强度。

The two main types of compaction equipment areimmersion (poker) vibratorsandvibrating-beam screeds. The effective radius of action of an immersion vibrator depends on its frequency and amplitude.

The普通尺寸found in normal concrete construction work have a radius of action between200和350毫米. This means, in practice, concrete should be placed in uniform layers ranging from250 to 400 mm，取决于所使用的振动器。为了确保将每一层正确融合在一起，the vibrator should penetrate about 150 mm into the lower layer(Fig-3).

The effective depth of compaction of vibrating beam screeds depends on the beam weight, the amplitude, the frequency and the forward speed. For the common available range of surface vibrators, the maximum effective depth is 200 mm. For slabs between 150 and 200 mm thick, immersion vibrators should be used alongside all construction joints and edges to supplement the surface vibrator in these areas.

For slabs greater than 200 mm thick, immersion vibrators should be employed to compact the concrete and the vibrating-beam screed to finish it (Fig-3).