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 in Fig-1 should 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

To aid subsequent compaction of the concrete, care should be taken to place concrete in layers which are of a suitable depth for the compaction equipment. Layers that are too deep make it virtually impossible to adequately compact the concrete, leaving entrapped air which will create voids and blow holes in the surface of the concrete, and prevent it achieving its potential durability and strength.

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

The common sizes found in normal concrete construction work have a radius of action between 200 and 350 mm. This means, in practice, concrete should be placed in uniform layers ranging from 250 to 400 mm, depending on the vibrator used. To ensure each layer is properly melded together, 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).