The strength and durability of virtually all concrete relies critically on its full compaction during placing. Usual methods for verifying strength rely on separately cast specimens which are much easier to compact. These cannot reflect the reality of substandard, poorly
compacted concrete placed insitu.
Inadequate compaction dramatically lowers the performance of mature concrete no matter how well it has been produced and how good is its mix design. Concrete is normally compacted by vibrators, often operated by untrained labour, and the supervision of the process is inherently difficult.
Consequences of poor compaction are well known: entirely satisfactory repairs are invariably difficult and often very costly. For operators working with concrete vibrators, the long term consequence are no less damaging. Vibrations can lead to white finger syndrome and there is a significant noise loading of both the workplace and the environment around the site. A compelling case for removing compaction from general purpose concrete construction process has been with us for a very long time.
In principle, self compacting concrete is not new. Even before the advent of modern admixtures, concrete had to be placed in conditions where compaction was impractical. Concreting underwater, insitu concrete piling and filling of inaccessible spaces were situations where fresh mixes which performed without having to be compacted were required.
Such mixes were successfully placed but reinforcement was often absent, and more demanding, expensive placing procedures had to be used. Cement contents were often substantially above 450kg/m3 and their side effects combined with higher costs greatly restricted scope for applications of such early SCCs.
The introduction of superplasticisers permitted production of flowing fresh mixes attaining high strength without excessive cement contents. However, the very high workability was achieved at a cost of a much reduced resistance to segregation and bleeding.
Developments in underwater concrete technology from the mid-1980s in the UK (for example at the University of Paisley) and elsewhere led to fresh mixes having a high degree of resistance to washout when passing through water; a great advantage offering a safer and less expensive underwater placing. This was achieved by addition of a viscosity agent which increased the internal cohesion of the mix without impairing workability too much. The mix remained self-compacting!
Such concrete was taken beyond underwater applications first in Japan in the early 1990s. Following research at the University of Tokyo, major contractors such as Taisei, Maeda, Kajima developed in-house mix designs supported by a variety of test methods and applied this 'modern' SCC in an increasing number of projects and larger volumes. Examples are anchorages for the record-holding Akashi-Kaikyo bridge which contain 512,000m3 and 250,000m3 of SCC respectively, cast at rates of 1,900m3 per day.
Producing a practical SCC is not simply a matter of adding suitable admixtures. Compared with ordinary concrete, the SCC mix design must lead to a fresh mix with a greater resistance to segregation and no blocking when it flows through and around reinforcement and fills the formwork. In many cases, the quality of the surface is also important.
Achieving the necessary compromise between high flow and cohesion with a very low blocking reduces room for manoeuvre in the mix design calling for greater supervision and site control. The latter is due to the fact that the behaviour of fresh SCC puts it outside the scope of current standard tests for concrete (BS1881, EN206).
Standard tests such as the slump or the flow/spread are inappropriate, and there are no widely used, reliable tests which can assess quantitatively segregation resistance (stability) and blocking. Current SCC practice therefore relies on a greater than usual number of tests, which can be confusing, and on a substantial degree of experience in interpretation of their results.
SCC mixes often contain higher than normal proportions of fine materials, such as PFA, GGBFS or limestone powder. Total fines content is balanced against size and grading and water content, assisted by admixtures. Latest developments try to limit admixtures to one for the production of a general purpose SCC by using new types and combinations of polymers.
Eliminating the compaction process opens, at last, the way to an effective introduction of automation into the concrete construction processes, both insitu and in precasting. The positive environmental effects on health and safety on the site and on the surrounding environment are also very important.
In Japan, the commercial success of SCC has been such that it has broken out of the confines of large corporations. The Japanese government now supports an overall plan for SCC to achieve a more than 50% share of all concrete placed there by 2003.
Europe is not lagging behind, with a very large EC-funded project led by major contractors NCC of Sweden and GTM of France, with the Advanced Concrete & Masonry Centre of the University of Paisley responsible for two key tasks. The 38month project, now in its mid-term is on target to overtake the Japanese technology.
Independently, the Dutch construction industry has launched a major SCC development programme and there is additional R&D carried out in France and Sweden.
The UK concrete industry has also woken up to the challenge. Major companies representing ready-mixed concrete suppliers, contractors, consultants and relevant R&D are bidding for DETR support using the Partners in Innovation programme.
The aim is to develop practical UK-focused SCC guidelines which will allow the construction industry to adopt SCC without the need for additional investment and risk of experimentation on a significant project. A working group on SCC has been set up by the Concrete Society and a one-day seminar on practical SCC is planned for spring 1999.
Practical concretes that need no compaction are already here, and their technical and cost benefits are being exploited in special applications. The spread of SCC into general concrete construction is on the nearest horizon, its introduction largely dependent on standardisation of appropriate site tests and the development of reliable guidelines for the industry.
Professor Peter Bartos is director of the Advanced Concrete & Masonry Centre at the University of Paisley