1. Characteristics
Self-compacting high-performance concrete is a high performance concrete that can be compacted into every corner of a formwork, purely by means of its own weight and without the need for vibrating compaction (Fig. 1). This concrete is defined as follows at the three stages of concrete: (1) Fresh: self-compactable; (2) Early age: avoidance of initial defects; (3) After hardening: protection against external factors.
2. History of development
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. A solution for the achievement of durable
concrete structures independent of the quality of construction work is
the employment of self-compacting concrete. The necessity of this type
of concrete was proposed by Okamura in 1986. Studies to develop self-compacting
concrete (SCC), including a fundamental study on the workability of concrete,
were 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. At almost the same time, “High Performance Concrete”
was defined as a concrete with high durability due to low water-cement
ratio by Aitcin et. al. Since then, the term high performance concrete
has been used around the world to refer to high durability concrete. Therefore,
Okamura adopted the term “Self-compacting High Performance Concrete” for
Japan’s SCC.
3. Mechanism for the technology
The self-compactability of fresh concrete depends mainly on its ability
to flow through obstacles[1] (Fig. 2). The method for achieving self-compactability
involves not only high deformability of paste or mortar, but also resistance
to segregation between coarse aggregate and mortar when the concrete flows
through the confined zone of reinforcing bars. Okamura and Ozawa have employed
the following methods to achieve self-compactability: (1) Limited aggregate
content; (2) Low water-powder ratio; (3) Use of superplasticizer. Highly
viscous paste is required to avoid the blockage of coarse aggregate when
concrete flows through obstacle. When concrete is deformed, paste with
a high viscosity also prevents localized increases in the internal stress
due to the approach of coarse aggregate particles (Fig. 3). High deformability
can be achieved only by the employment of a superplasticizer, keeping the water-powder
ratio to be very low value. Poly-carboxylate type of superplasticizer is suitable
for SCC (Fig. 4).
4. Practical applications
Self-compacting concrete has been used in many practical structures since
1990. Currently, the main reasons for the employment of self-compacting
concrete can be summarized as follows: (1) To shorten construction period
(Fig. 5); (2) To assure compaction in the structure: especially in confined
zones where vibrating compaction is difficult (Fig.s 6 and 7); (3) To eliminate
noise due to vibration: effective especially at concrete products plants.
Also, SCC is applied to tunnel lining for preventing the cold joint (Fig.
8).
Reference
[1] Ouchi, M. State-of-the-art report on self-compactability evaluation, Proceedings of the international Workshop on Self-Compacting Concrete (CD-ROM), Kochi, Japan, March 1999, Also available from Concrete Engineering Series, No. 30, Japan Society of Civil Engineers, March 1999.