Self-compacting concrete

Self-compacting concrete (SCC) flows under its own weight, fills formwork around dense reinforcement and levels itself, without vibration and without segregating. It suits congested elements such as beam-column joints in earthquake-resistant frames, thin walls, heavily reinforced rafts, precast units with fine finishes and repair work where vibrators cannot reach.

Its behaviour comes from balancing two properties that normally conflict: high fluidity and high resistance to segregation. A high-range water-reducing admixture gives fluidity at a modest water content; a higher powder content, using fly ash, slag, limestone powder or silica fume, and sometimes a viscosity-modifying admixture give cohesion. The coarse aggregate content is lower and its maximum size smaller than in conventional concrete, so that particles do not lock together and block between bars.

Because the slump cone cannot describe it, SCC has its own fresh tests. Slump flow measures the spread of the concrete when the cone is lifted, and the time to reach a 500 mm spread indicates viscosity. The V-funnel time measures how fast it flows through a narrow outlet. The L-box and J-ring measure passing ability through bars; the L-box blocking ratio compares the height of concrete at the far end with that at the gate. A sieve segregation test checks stability. In India these tests are in IS 1199 (Part 6) and mix proportioning guidance in IS 10262.

Formwork must be designed for full fluid pressure, because SCC can act almost like a liquid during placing. Placing rate, drop height and finishing still need control, and curing is just as important as for normal concrete.