As with the handling of concrete, a few fundamental considerations govern placing technique. First and perhaps most important is the need to avoid the separation of concrete due to improper techniques. The second is the need to ensure thorough compaction of the concrete. The way concrete is laid can have a significant impact on its ability to compact under vibration.
The most important rules to avoid separation when placing concrete in any element are:
- Concrete should be placed vertically and as close to its final position as possible.
- It should not be made to flow in position. Where concrete is to be moved, it must be shoveled into position.
Other techniques to avoid separation during placement depend on the type of element being manufactured and the type of distribution equipment being used.
Techniques for flat work and slabs involving ribs and beams (ie shallow forms) are shown in Figure 1 should be adopted.
For walls and columns (i.e. deep, narrow forms), problems occur when concrete is dropped from too high a height and ricochets from the reinforcement and form-face, resulting in separation. Means to avoid this vary with the type of distribution equipment being used (line drawing number 2,
To aid in the subsequent compaction of concrete, care must be taken to lay the concrete in layers that are of a suitable depth for the compaction equipment. Layers that are too deep make it nearly impossible to compress concrete sufficiently, releasing trapped air that will create voids and blow holes in the surface of the concrete, and prevent it from achieving its potential durability and strength.
There are two main types of compaction equipment: immersion (poker) vibrator And Vibrating-beam screeds, The effective radius of action of an immersion vibrator depends on its frequency and amplitude.
normal size Found in normal concrete construction work, the radius of action is between 200 more 350 mm, This means, in practice, concrete must be laid in uniform layers: 250 to 400 mm, depending on the vibrator used. To make sure each layer is properly thawed together, The vibrator should penetrate about 150mm into the lower layer ,fig-3),
The effective depth of compaction of the vibrating beam screw depends on the beam’s weight, amplitude, frequency and forward speed. For the general 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 along all construction joints and edges to complement surface vibrators in these areas.
For slabs thicker than 200 mm, immersion vibrators should be installed to compact the concrete and vibrating-beams should be screwed in to finish (fig-3),