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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.
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
shovelled 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 flatwork and slabs incorporating
ribs and beams (ie shallow forms) the techniques shown
in Figure 1.1 should be adopted.
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Figure 1.1 Placing Techniques for flatwork |
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 (see Figure 1.2).
Figure 1.2 Placing techniques
for walls and columns
This is not always possible in New Zealand due to the significant
amounts of reinforcement steel used in earthquake resistant
design. To help reduce the risk of segregation, quality concrete
mixes having a high degree of cohesiveness in the wet state
must be used.
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.
This is discussed further here.
Placing Methods
| Method |
Application |
Comment |
| Chute |
- Where works is below level of truck tray
- Ideal for strip footings, house floor slabs, road
pavements etc
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- May be direct form transit mixer if works is
withing radius of its chute
- Free fall of concrete should not exceed 2m without
additional end controls
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| Barrows and hand carts |
- Suitable for small projects such as domestic
construction
|
- Labour intensive
- Low placing rate (typically 1 to 1.5 m³/h)
- Maximum distance about 50m for continuous
work
- Requires relatively level, smooth access
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| Crane and bucket |
- Suitable for mass concrete structures and
heavyweight concretes
- Can be used when concrete is unsuitable for
pumping
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- Adequate crane time must be available
- limitations dependend on bucket size, crane
capacity and reach
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| Pumps and pipelines |
- Versatile and flexible - can distribute concrete
both verticall and horizontally
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- Require little space
- High output
- Continuous distribution
- Short set-up time
- Low labour required
- Not suitable for all concretes
- Likelihood of increased concrete shrinkage
- Downhill pumping is difficult
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Table 1. Summary of placing methods for
concrete
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