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 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 (i.e. shallow forms) the techniques shown in Figure 1.1 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 (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.
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.
- Where works is below level of truck tray
- Ideal for strip footings, house floor slabs, road pavements etc.
- 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
|Barrows & 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
|Crane & Bucket
- Suitable for mass concrete structures and heavyweight concretes
- Can be used when concrete is unsuitable for pumping
- Adequate crane time must be available
- Limitations dependent on bucket size, crane capacity and reach
|Pump & Pumplines
- Versatile and flexible - can distribute concrete both vertical and horizontally
- 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
Table 1. Summary of Placing Methods