13-07-2012, 04:21 PM
Self-compacting concrete (SCC)
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INTRODUCTION
Self-compacting concrete (SCC) is an innovative concrete that does not require vibration for placing and compaction. It is able to flow under its own weight, completely filling formwork and achieving full compaction, even in the presence of congested reinforcement. The hardened concrete is dense, homogeneous and has the same engineering properties and durability as traditional vibrated concrete.
Concrete that requires little vibration or compaction has been used in Europe since the early 1970s but self-compacting concrete was not developed until the late 1980’s in Japan. In Europe it was probably first used in civil works for transportation networks in Sweden in the mid1990’s. The EC funded a multi-national, industry lead project “SCC” 1997-2000 and since then SCC has found increasing use in all European countries.
Self-compacting concrete offers a rapid rate of concrete placement, with faster construction times and ease of flow around congested reinforcement. The fluidity and segregation resistance of SCC ensures a high level of homogeneity, minimal concrete voids and uniform concrete strength, providing the potential for a superior level of finish and durability to the structure. SCC is often produced with low water-cement ratio providing the potential for high early strength, earlier demoulding and faster use of elements and structures.
The elimination of vibrating equipment improves the environment on and near construction and precast sites where concrete is being placed, reducing the exposure of workers to noise and vibration.
The improved construction practice and performance, combined with the health and safety benefits, make SCC a very attractive solution for both precast concrete and civil engineering construction.
In 2002 EFNARC published their “Specification & Guidelines for Self-Compacting concrete” which, at that time, provided state of the art information for producers and users. Since then, much additional technical information on SCC has been published but European design, product and construction standards do not yet specifically refer to SCC and for site applications this has limited its wider acceptance, especially by specifies and purchasers.
In 1994 five European organizations BIBM, CEMBUREAU, ERMCO, EFCA and EFNARC, all dedicated to the promotion of advanced materials and systems for the supply and use of concrete, created a “European Project Group” to review current best practice and produce a new document covering all aspects of SCC.
DEVELOPMENT OF SCC
The SCC concept was introduced into scientific world in Japan in 1986 by Professor Hajime Okamura from Tokyo University. The first prototype was developed in 1988 by K. Ozawa from Tokyo University as a response to the growing problems associated with concrete durability and the high demand for skilled workers.
In Europe it was probably first used in civil works for transportation networks in Sweden, in the middle of 1990’s. The EC funded a multi-national, industry lead project SCC 1997-2000 and since then SCC has found increasing use in all European countries.
SCC was first developed so that durability of concrete structures can be improved. Since then, various investigations have been carried out and the Concrete has been used in practical structures in Japan and Europe, mainly by large construction companies. Investigations for establishing a rational mix design method and self-compact ability testing methods have been carried out from the viewpoint of making it a standard concrete. Recommendations and manuals for self-compacting concrete were also written.
To make durable concrete structures, sufficient compaction by skilled workers is required. However, the gradual reduction in the number of skilled workers in 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 use of the SCC, which can be compacted into every corner of a formwork, purely by means of its own weight and without the need of vibrating compaction.
Addition
Finely-divided inorganic material used in concrete in order to improve certain properties or to achieve special properties.
Admixture
Material added during the mixing process of concrete in small quantities related to the mass of cementitous binder to modify the properties of fresh or hardened concrete
Binder
The combined cement and Type all addition
Filling ability
The ability of fresh concrete to flow into and fill all spaces within the formwork, under its own weight
Fines
See Powder Flow ability the case of flow of fresh concrete when unconfined by formwork and/or reinforcement
Fluidity
The ease of flow of fresh concrete
Mortar
The fraction of the concrete comprising paste plus those aggregates less than 4 mm
Paste
The fraction of the concrete comprising powder, water and air, plus admixture, if applicable
Passing ability
The ability of fresh concrete to flow through tight openings such as spaces between steel reinforcing bars without segregation or blocking