Self compacting has material requirements same as conventional concrete, but it requires admixtures to enhance the workability. Types of materials required and their properties for self-compacting concrete is discussed.
Following are the materials for self compacting concrete:
Cement for self compacting concrete
All types of cement conforming to EN 197 are suitable. Selection of the type of cement will depend on the overall requirements for the concrete, such as strength, durability etc., C3A content higher than 10% may cause problems of poor workability retention.
The typical content of cement is 350-450 Kg/m3 More than 500 Kg/m3 cement can be dangerous and increase the shrinkage. Less than 350 Kg/m3 may only be suitable with the inclusion of other fine filler, such as fly ash, pozzolana, etc.
Fig: Self Compacting Concrete
Aggregates for self compacting concrete:
1. Sand
All normal concreting sands are suitable for SCC. Either crushed or rounded sands can be used. Siliceous or calcareous sands can be used.
The amount of fines less than 0.125 mm is to be considered as powder and is very important for the rheology of the SCC. A minimum amount of fines (arising from the binders and the sand) must be achieved to avoid segregation.
2. Coarse aggregate
All types of aggregates are suitable. The normal maximum size is generally 16 – 20 mm. however particle size up to 40 mm more have been used in SCC. Consistency of grading is of vital importance. Regarding the characteristics of different types of aggregate, crushed aggregates tend to improve the strength because of the interlocking of the angular particles, whilst rounded aggregates improve the flow because of lower internal friction. Gap graded aggregates are frequently better than those continuously graded, which might experience greater internal friction and give reduced flow.
Admixtures for Self compacting concrete:
The most important admixtures are the super plasticizers (high range water reducers), used with a water reduction greater than 20%. The use of a Viscosity Modifying Agent (VMA) gives more possibilities of controlling segregation when the amount of powder is limited. This admixture helps to provide very good homogeneity and reduces the tendency to segregation.
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