What Is Self-Compacting Concrete?

What Is Self-Compacting Concrete? . Self-Compacting Concrete (SCC) is a cutting-edge material revolutionizing the construction industry with its unique ability to flow and settle into place without external vibration. Introduced in the late 1980s in Japan, SCC was developed to overcome challenges in congested reinforcement areas, ensuring uniform quality and eliminating voids. Today, it is widely used for its superior workability and finish, especially in complex architectural designs.

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Introduction

Self -compacting concrete (SCC), also known as self –consolidating concrete, is a highly flowable and non-segregating concrete that can spread into place, fill the formwork and encapsulate the reinforcement without any mechanical compacting .

Growth and development of SCC

• Self- compacting concrete (SCC) was first developed in Japan in the year 1980, by Professor Hajime Okamura of Kochi University of Technology, Japan .
• During his research, he found that the main cause of the poor durability performances of Japanese concrete in structures was the inadequate compacting of the concrete in the casting operations.
• By developing concrete that self- compacting, he eliminated the main cause for the poor durability performance of their concrete.
• By 1988, the concept was developed and ready for the first realscale tests

Though SCC was first developed to overcome the deficiency of the skilled man power, subsequently it is observed that SCC not only reduces the requirement of man power, but it also results in more durable concrete with the excellent user friendly characteristics.

• It is now used in many countries such as Canada, Sweden, USA, Austria, Korea etc.

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Problems with Conventional Concrete

• Requirement of skilled worker for compaction in conventional concrete.
• Difficult to use mechanical compaction for (Underwater concreting, cast in-situ foundation and structures with heavy reinforcement, as shown in Fig.(2)

Mechanism for achieving Self compacted concrete

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Comparison between conventional concrete and SCC

Table (1) The ingredients for normal concrete and SCC

Ingredients	Normal concrete	Self -compacted concrete
Fines	12%	18%
Sand	24%	34%
Coarse aggregate	46%	28%
Water	18%	20%
Admixture	–	0.01%

Materials used for  SCC

• Cement: OPC can be used.
• Aggregates:

Aggregate of size ( 1 to 20) mm.

Congested Reinforcement (10 to 12) mm

Well graded cubical or rounded aggregates.

FA can be either natural or manufactured and of uniform grade. Particle sizes <0.125mm are considered as fines.

• Water: It must be of the same quality used for that of RC or Pre stressed concrete.
• Chemical Admixtures: The new generation super plasticizers termed as poly carboxylated ethers is particularly used.
  • VMA : For stability
  • Air Entraining Agents : To improve Freeze-Thaw resistance
  • Retarders : To control setting time

• Mineral Admixtures:
  • GGBS : To improve Rheological Properties.
  • Fly ash : To improve the fresh properties and durability.
  • Silica Fumes: To improve Mechanical Properties.

Stone Powder: To increase the powder content.

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Requirement of SCC

The main characteristics of SCC are the properties in the Fresh state.A concrete mix can only be classified as SCC if it has the following characteristics in fresh state:

• Filling ability (excellent deformability) : the ability to flow under its own weight without vibration and flow easily at suitable speed into formwork
• Passing ability (ability to pass reinforcement without blocking) : passes through reinforcements and retain homogeneity without blocking
• High resistance to segregation: the distribution of aggregate particles remains homogeneous in both vertical* and horizontal** directions without segregation

Deformability (flow and filling ability)

• “Excess Paste Theory” explains the mechanism governing the workability of concrete.
• Enough paste to cover the surface area of the aggregates, and that the excess paste serves to minimize the friction among the aggregates and give better flow-ability.

Without the paste layer, too much friction would be generated between the aggregates resulting in extremely limited workability.

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vertical direction, segregation may occur due to gravity.

horizontal direction, segregation may occur due to flow.

Passing ability

• The probability of blocking increases when the volume fraction of large aggregates increases.
• The size of aggregates, their shapes and their volume fraction influence the passing ability of SCC
• The contacts between particles increases as the distance between particles decreases; which therefore results in an increase in the internal stresses when concrete is deformed and causing blockage.

Segregation resistance

• Segregation resistance is largely controlled by viscosity
• ensuring a high viscosity can prevent a concrete mix from segregation and/or bleeding.
• Bleeding is a special case of segregation in which water moves upwards by capillary action and separates from the mix.
• Some bleeding is normal for concrete, but excessive bleeding can lead to a decrease in strength, high porosity, and poor durability particularly at the surface
• Two basic methods can ensure adequate stability:
• The first approach uses a super-plasticizer (SP), low water/cement ratio, high powder content, mineral admixtures, and low aggregate content.

The second approach is based on incorporating a viscosity modifying admixture (VMA), low or moderate powder content and super-plasticizer

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The tests on  self-compacting concrete and its limits

Table (2) Workability properties and test methods for SCC

Filling Ability Tests	Passing ability Tests	Segregation resistance Test
Slump Flow Test	J-Ring Test	V-funnel At T5minutes
T50cm Slump Flow	L-Box Test	GTM Screen stability Test
V-Funnel Test	U-Box Test
Orimet	Fill-Box Test

Table (3) The limits of various tests on SCC

The test	The limits
Slump Flow Test	650mm to 800mm
T50 slump flow	2sec to 5sec
J-ring Test	0 to 10mm
V-Funnel Test	8sec to 12sec
V-funnel at T5minutes	+3 seconds, max.
L-box Test	H1/H2=0.80 to 1.0
U- Box Test	H2-H1=30mm max.
Fill Box Test	90% to 100%
GTM Screen Stability Test	0 to 15%
Orimet Test	(0 – 5)sec

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Slump Flow and T50 cm Tests:

• Evaluate the deformability of SCC in the absence of obstacles.
• Two different aspects are measured:
  • The filling ability by measuring the horizontal flow (spread) diameter (SF)
  • The viscosity of mix by measuring the time needed for SCC to reach 500 mm flow (t500).
• The segregation resistance in this test can be detected visually

Slump flow test was done following EFNARC and ASTM C1611_, following the same test procedure

• Moistening the flow plate and inside of slump cone is necessary, with ensuring there is no free water.
• Placing the flow table horizontally on level ground and the cone in the middle of the plate.
• The cone is filled with concrete using scoop; the concrete leveled off with the top of the cone using trowel and rose vertically.
• The timing is started and recorded by stopwatch when lifting of the cone starts till the concrete reaches the 500mm (T₅₀₀).
• The final diameter is measured by measuring tape as the concrete has stopped flowing, by taking the average of two perpendicular diameters.

The difference between d1 and d2 should be less than 50 mm otherwise the test should be repeated, spread diameter (SF) is calculated using the following equation

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SF1 (650 – 750 mm) for normal applications (e.g. walls, columns) • SF2 (760 – 850 mm) is typically produced with a small maximum size of aggregates (less than 16 mm) for vertical applications in very heavy structures or structures with complex shapes.

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Advantages of Self-Compacting Concrete

1. Time-Saving:
   Since SCC eliminates the need for mechanical vibration, it significantly reduces the time required for placement.
2. Labor Efficiency:
   With reduced labor demands for consolidation, SCC minimizes the risk of human error and fatigue.
3. High-Quality Finish:
   SCC delivers smooth surfaces with minimal defects, making it ideal for exposed concrete structures.
4. Improved Durability:
   The dense composition enhances durability, offering better resistance to environmental factors like water and chemical ingress.

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Applications

1. Complex Formworks:
   SCC is perfect for intricate architectural designs, ensuring complete filling of forms without manual intervention.
2. Precast Elements:
   Its high flowability makes it an excellent choice for precast components like beams, columns, and panels.
3. Rehabilitation Projects:
   SCC’s ease of application helps in restoring and strengthening existing structures with minimal disruption.
4. High-Rise Buildings:
   SCC supports the rapid construction of skyscrapers, improving efficiency and quality in challenging environments.

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How Economical Is SCC ?

• There is a feeling that cost of SCC is much higher than that of corresponding normal or high strength concrete.
• It is seen that the materials of SCC is about 1015%higher than NC.
• If one takes the other components of costs such as cost of compaction, finishing, etc. then one would realize that SCC is certainly not a costly concrete for comparable strength.

USES OF SCC

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FAQs

What is the cost of SCC compared to regular concrete?
SCC is generally 20-30% more expensive due to its specialized materials and admixtures.

Can SCC be used in all types of construction?
While versatile, SCC is particularly beneficial in projects requiring high-quality finishes or complex designs.

Is SCC environmentally friendly?
Yes, SCC promotes sustainable practices by reducing energy consumption and incorporating industrial by-products.

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