Introduction.
Making concrete structure without vibration, have been done in past. For example placements of concrete under water is done by the use of tremie without vibration. Mass concrete and shaft concrete can be successfully placed without vibration. Modern application of self compacting concrete is focused on high performance , better and more reliable and uniform quality. Self compacting concrete has been described as the most revolutionary development in concrete construction for several decades.It has been proved to be beneficial from the following points.
Making concrete structure without vibration, have been done in past. For example placements of concrete under water is done by the use of tremie without vibration. Mass concrete and shaft concrete can be successfully placed without vibration. Modern application of self compacting concrete is focused on high performance , better and more reliable and uniform quality. Self compacting concrete has been described as the most revolutionary development in concrete construction for several decades.It has been proved to be beneficial from the following points.
- Faster construction
- Reduction in site manpower
- Better surface finish
- Easier placing
- Improved durability
- Greater freedom in design
- Thinner concrete section
- Reduced noise level
- Safer working environment.
Materials for self compacting concrete.
1. Cement.
Ordinary Portland cement of 43 and 53 grade can be used
2. Aggregates.
Thr max.size of aggregates is generally limited to 20mm. Aggregates of size 10 to 12 mm is desirable for structure having congested reinforcement. Wherever possible aggregates size greater than 200mm is also used. Well graded rounded or cubical aggregates are desirable. Aggregates should be uniform in size shape and textures.3. Mixing of water.
Must be established on the same line as that for using reinforced or prestressed concrete.4. Chemical admixtures.
Superplasticizers are an essential component of self compacting concrete to provide necessary workability. New generation superplasticizers termed ss poly-carboxylated ethers (PCE) is particularly used for SSC.
Some other superplasticizers are visocity modifying agents VMA for stability.
Air entraining agents to improve freeze-thaw resistance, and retarders for control of setting.
Minerals admixtures.
1. Fly ash. Appropriate quantity should be added to improve the quality and durability of SSC.
2. Silica fumes. Added to improve the mechanical properties of SSC.
3. Stone powder. Finely crused lime stone, dolomite or granite may be added to increase the powder content. Fraction should be less than 125 microns.
4. Fibres. Used to enhanced the properties of self compacted concrete in the same way as for normal concrete.
Requirement for self compacting concrete
The main characteristics of SSC are the properties in the fresh state. The mixed designed is focussed on the ability to follow under its own weight without vibration, the ability of flow through heavily congested reinforcement under its own weight and the ability to retain homogeneity with out segeragation. The workability of SSC is higher than the workability mention in IS 456:2000.
A concrete mix can only be classified as self compacting if it hs following characteristics
1. Filling ability
2. Passing ability
3. Segregation resistance
Several test methods have been developed in attempts to characteristics the properties of self compactingcomposition
Initial mix composition
In the design of mix the relative proportions of the key components may be considered by volume rather than by mass. Indicative proportions of materials shown below for self compacting .
- Water/powder ratio by volume is to be 0. 80 to 100.
- Total powder content to be 160 to 240 litres per cubic meter.
- The sand content may be more than 38% of the mortar volume.
- Coarse aggregatecontent should normally be 28 to 35% by volume of the mix.
- Water/cement ratio is selected based on strength. In any case water/cement ratio should not be exceed 200 litres per cubic meter.
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