Self Compacting Concrete is concrete which flows under its own weight without any segregation and maintaining its homogeneity. it is also called Self Leveling Concrete or Free Flow Concrete due to the property of self spread and level itself during pouring.
SCC full form is self-compacting concrete was first invented in Japan around the year 1980. Professor H. Okamura from the University of Tokyo, Japan is mainly responsible for initiating the evolution of such concrete. The need for these types of concrete arises from the scarcity of skilled manpower in Japan during this period.
It was developed to overcome the deficiency of skilled manpower and problems of placing and compacting congested civil engineering structures. Subsequently, it has been observed that SCC not only reduces the requirement of manpower, both skilled and unskilled but also results in more durable concrete.
Self-compacting concrete can be considered as the greatest technical development in concrete technology over the years, at least from 1980 till today. This is the concrete of the futures, as it will be replacing the normal concrete because of its many advantages.
One report of 2003 reported that approximately 50% of the total concrete executed in Japan was SCC. The growth of SCC in Japan is depicted in the below figure. Subsequently, SCC was used in many developed and developing countries such as Canada, Sweden, Netherlands, Taiwan, Thailand, the USA, Austria, Korea, France, U.K, Germany, etc.
In India, it was used for the first time in the year 2003 and the works were executed by Gammon India Ltd.
SCC is sometimes also referred to as Self-Consolidating Concrete or Silent Concrete though widely it is referred to as Self-Compacting Concrete (SCC).
Self Compacting Concrete
Self-Compacting Concrete is a concrete which flows under its own weight without any segregation and maintains its homogeneity, also flows through all shapes and obstructions and needs no vibrations for compaction (self-compacting).”
Thus the main requirements of SCC are flowability, filling ability, and self-compacting without segregation i.e. maintaining homogeneity.
The above can be achieved by proper mix design, which can be more appropriately stated as the proportioning of various components of the concrete.
To achieve a highly mobile concrete, the low yield stress is required and for high resistance to segregation, a highly viscous material is required. The addition of water decreases the yield stress but lowers the viscosity. The addition of a superplasticizer also lowers the yield stress but will decrease the viscosity slightly.
On the other hand, the viscosity of a mix can be improved by changes in mixed constituents or by adding a viscosity modifier but this will increase the yield stress of the paste. Thus a balance between the two requirements needs to be implemented.
Mix Design of Self-Compaction Concrete
In general, SCC has got less coarse aggregates, more fine aggregates as noted in figure No. 3.1 and Figure No. 3.2 below and needs superplasticizer and viscosity modifying agent (VMA) as admixtures.
- Figure No. 3.1 Constituents of Normal Vs Self compacting concrete (By Volume) (Design & Control of Concrete Mixes by Steven H Kosmatka, Beatrix Kerkhoff, and William C. Panarese).
- Some broad guidelines for proportioning of various components of SCC are given below:
Materials for Self-Compacting Concrete
1. Coarse Aggregate:
- Generally, coarse aggregate has a standard size of 20mm in concrete, sometimes to 15mm though Self Compacting Concrete has been used even with 40mm aggregate where there is no congestion of steel reinforcement.
- Aggregates bigger than 4.75 mm (4 mm in some European Countries) are considered as coarse aggregate.
- Regarding the characteristics of different types of aggregates, crushed aggregates tend to improve the strength because of the interlocking angular particles.
- But at the same time reduce the flowability, while rounded aggregates improve the flow due to the lower internal friction and lesser surface area per mass.
- Gap graded aggregates are generally better than continuously graded aggregates as well-graded aggregates experience greater internal friction and give reduced flowability.
- The amount of coarse aggregates is generally less than 50% by the weight of total aggregates (Coarse and fine aggregate combined). The normal range of coarse aggregates is generally 700kg to 800kg per cum. of concrete.
- By volume, coarse aggregate content is 50% to 60% of the total aggregate volume. The actual volume of the coarse aggregate depends upon the characteristics and maximum size of the coarse aggregate.
- The lower the maximum aggregate size, the higher is the proportion of coarse aggregate and vice-versa. Similarly, with rounded aggregate, a higher percentage of coarse aggregate can be used than with crushed coarse aggregate
2. Fine Aggregate (Sand)
- Aggregates smaller than 4.75mm (4mm in some European Countries) and up to 0.075mm (0.125mm in some European Countries) are considered fine aggregates (sand).
- Sand in Self Compacting Concrete (SCC) is generally finer than in normal concrete. The ratio of the weight of sand to that of coarse aggregates could be in the range of 1.1 to 1.6. The ratio is on the lower side of this range if filler dust is used in SCC.
- The volume of sand content should be in the range of 40% to 50% of the mortar/paste volume. Both natural and crushed sand particle size ranges can be used.
- Particles that are smaller than 0.075mm (0.125mm in some European Countries) are considered dust/fines. The amount of dust to be used depends on various factors including whether secondary cementing materials like ground granulated blast furnace slag (GGBS), fly ash, and/or silica fume is used or not.
- If such secondary cementing materials are used, the requisite of fines reduces. However, the need for fine aggregate is completely eliminated by using secondary cementing materials. Increased fines help in preventing segregation.
- In the absence of secondary cementing material, the amount of fine is generally 160 liters to 240 liters or 400 kg to 600 kg per cum of concrete.
4. SCC Cement
- The content of cement generally used in SCC is the same as normal concrete. Sometimes little more cement is preferred, probably due to the fact that SCC is still in its initial stage of application in our country.
- The addition of secondary cementing material (SCM) like fly ash, GGBS, and silica fume is beneficial.
- From the durability consideration, the water-cement ratio should be restricted as in normal concrete and it should preferably be less than 0.4.
- Generally, free water should be less than 200 liters per cum. of concrete. The ratio of Water/powder by volume is generally 0.8 to 1.10.
6. Chemical Admixtures:
- Advances in admixture technology have played a major role in the development of SCC. Modern superplasticizer (based on polycarboxylic ethers) promotes good workability and can be added at any stage of the batching cycle.
- They achieve this with a mechanism of electrostatic repulsion in combination with steric hindrance.
- Viscosity modifiers enhance resistance to segregation while still maintaining high fluidity, allowing concrete to flow through narrow spaces.
- The below table gives the mix proportioning of typical Self Compacting Concrete used in the 1970s and 1980s.
Typical Mix Proportioning Of SCC used In 1970’s and 1980’s
|Ordinary Portland Cement||400 Kg/m3|
|Very Fine Sand (0.075mm-0.6mm)||180 Kg/m3|
|Sand (0mm-5mm) (Fine Aggregate)||990 Kg/m3|
|Gravel (5mm-15mm) (Coarse Aggregate)||630 Kg/m3|
|Note: Ratio of F.A/C.A = 1.86|
- Considerable modifications to improve SCC have taken place subsequently. The above table provides a mix of proportions and properties of fresh concrete for Normal Concrete (NC), High-Performance Concrete (HPC), and Self Compacting Concrete (SCC).
- This table is a good indication of the variation of composition particularly with respect to coarse and fine aggregates, slump, etc.
Mix Proportions and Properties of Fresh Concrete for Normal Concrete (NC),High Performance Concrete (HPC) & Self Compacting Concrete (SCC)
|Material||Normal Concrete||High-Performance Concrete||Self Compacting Concrete|
|Ordinary Portland Cement||340 kg/m3||300 kg/m3||220 kg/m3|
|Slag||–||60 kg/m3||80 kg/m3|
|Fly Ash||–||–||120 kg/m3|
|Free Water||192 kg/m3||192 kg/m3||182 kg/m3|
|Sand||863 kg/m3||835 kg/m3||1030 kg/m3|
|Gravel||938 kg/m3||945 kg/m3||724 kg/m3|
|High-range water-reducing Admixture/Water Reducer||1.31 kg/m3||3.46 kg/m3||5.04 kg/m3|
|Water/Cementitious Material Ratio||0.556||0.533||0.433|
|Ratio of F.A./C.A.||0.92||0.88||1.42|
|Slump flow, mm||–||–||540*|
|V-Funnel test, sec||–||–||2.97*|
|Air Content %||1.5||1.1||5 (High)|
|Unit Weights, kg/m3||2338||2332||2354|
|*Lower than presently prescribed NC- Normal Concrete HPC- High Performance Concrete SCC- Self-Compacting Concrete|
- It reduces the permeability of the concrete structure
- SCC gives free hand in designing concrete structures
- It makes construction work faster
- Need of vibration to concrete while placing is eliminated
- Allows easy placing of concrete, which results in cost saving
- Construction quality considerably improved
- The concrete structure durability is high compared to normal concrete structures
- The noise produced due to the use of a vibrator is reduced
- There is no standard method for SCC mix design concrete
- The ultimate construction cost of concrete is high compared to normal concrete
- The SCC mix design essential required more trial mix and lab test
- Measurement and quality checks required more attention
- SCC used for the construction of concrete structure which has more complex reinforcement
- It is used for restoration repairs and renewal construction
- Highly stable and durable retaining walls can be constructed with the help of SCC
- It is used in the construction of raft and pile foundations
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