SELF COMPACTING CONCRETE

Introduction about self compacting concrete

        Self – compacting concrete (SCC) is a fluid mixture, which is suitable for placing difficult conditions and also in congested reinforcement, without vibration. In principle, a self – compacting or self – consolidating concrete must have a fluidity that allows self – compaction without external energy remain homogeneous in a form during and after the placing process and flow easily through reinforcement

    Self-consolidating concrete consists of cement, aggregates, water, and chemical admixture and/or VMA. Most common chemical admixtures used for SCC are HRWR, VMA and AEA. Aggregates, cement, water, and HRWR are the principal materials whereas SP, VMA, AEA, and other chemicals can be used as the optional materials.

         The three properties that characterize a concrete as self-compacting Concrete are Flowing ability—the ability to completely fill all areas and corners of the formwork into which it is placed Passing ability—the ability to pass through congested reinforcement without separation of the constituents or blocking Resistance to segregation— the ability to retain the coarse components of the mix in suspension in order to maintain a homogeneous material.

background of self compacting concrete 

Self-compacting concrete was first developed 1988 in order to achieve durable concrete structures. Since then, various investigations have been carried out and the concrete has been used in practical structures in Japan, mainly by large construction companies. Investigations for establishing a rational mix-design method and self-compact ability testing methods have been carried out to make the concrete the standard one.

For several years beginning in 1983, the problem of the durability of concrete structures was a major topic of interest in Japan. To make durable concrete structures, sufficient compaction by skilled workers is required. However, the gradual reduction in the number of skilled workers in Japan's construction industry has led to a similar reduction in the quality of construction work. One solution for the achievement of durable concrete structures independent of the quality of construction work is the employment of self-compacting concrete, which can be compacted into every corner of a formwork, purely by means of its own weight and without the need for vibrating compaction. The necessity of this type of concrete was proposed by Okamura in 1986. Studies to develop self-compacting concrete, including a fundamental study on the workability of concrete, have been carried out by Ozawa and Maekawa at the University of Tokyo.

The prototype of self-compacting concrete was first completed in 1988 using materials already on the market. The prototype performed satisfactorily with regard to drying and hardening shrinkage, heat of hydration, denseness after hardening, and other properties. This concrete was named ―High Performance Concrete.

At almost the same time, ―High Performance Concrete‖ was defined as a concrete with high durability due to low water-cement ratio by Professor Aitcin ET. Al. Since then, the term high performance concrete has been used around the world to refer to high durability concrete. Therefore, Okamura has changed the term for our proposed concrete to ―Self-compacting concrete.

testing required for S.C.C.

slump flow test

• slump flow test is simplest and most commonly adopted test method for evaluating self compatibility of self compacting concrete.
• This test consists of measurement of the mean diameter in two perpendicular direction of the concrete spread after the concrete had stopped flowing 
• slump flow value is between 650 to 800 mm for self compacting concrete.

v-funnel test

• It consists of a funnel with a rectangular cross section
• The concrete is poured into the funnel with a gate blocking the bottom opening
• When the funnel is completely filled, the bottom gate is opened and the time for the concrete to flow out the funnel is noted
• A flow time less than 12 seconds is recommended for a concrete to qualify for a self-compacting concrete

L- box test

• This test gives an indication of the filling, passing, and segregation ability of the concrete
• The L-box test method uses a test apparatus consisting of a vertical section and a horizontal section
• Reinforcing bars are placed at the intersection of the two areas of the apparatus
• after the opening of the gate from the vertical section, is measured. The heights of concrete at both ends of the apparatus (H1 and H2) are also measured to determine L-box results
• Limit h2/h1=0.8 to 1.0