|ASTM Puts Self-Consolidating Concrete to the Test
by Martin Vachon
ASTM Committee C09 on Concrete and Concrete Aggregates has begun work toward developing
standards for self-consolidating concrete. Martin Vachon describes
how a special type of concrete self-consolidates without the use
of vibration, its market share around the world, and the work
of C09 in writing standards for it.
A Little Bit of History
Self-consolidating concrete (SCC) or self-compacting concrete,
as its sometimes known, arrived as a revolution in the field
of concrete technology. The concept was proposed by Professor
Hajime Okamura of Kochi University of Technology, Japan, in 1986
as a solution to the growing durability concerns of the Japanese
government. During his research, Okamura found that the main cause
of the poor durability performances of Japanese concrete in structures
was the inadequate consolidation of the concrete in the casting
operations. By developing concrete that self-consolidates, he
eliminated the main cause for the poor durability performance
of their concrete. By 1988, the concept was developed and ready
for the first real-scale tests.
The first paper on SCC was presented at the second East-Asia and
Pacific Conference on Structural Engineering and Construction
(EASEC-2) in 1989, followed by another presentation at an Energy
Diversification Research Laboratories (CANMET)/American Concrete
Institute (ACI) meeting in 1992. (1) In 1997, a RILEM committee
(TC 174) on SCC was founded. Today, SCC is studied worldwide with
papers presented in almost every concrete-related conference.
How Does It Work?
Some people may say that concrete that self-consolidates on its
own weight without the use of vibration has been used for years
in the construction field. But at that time fluidity was obtained
by adding water, superplasticizer, or a combination of both. In
addition, that earlier type of concrete was not used in civil
engineering structures with normal durability requirements because
they either contained too much water or were not stable and led
to unacceptable heterogeneity in the structure.
Then what is different with SCC? Even if there is still no official
definition of SCC in the United States, the concept could be defined
A self-consolidating concrete must:
Have a fluidity that allows self-consolidation without external
Remain homogeneous in a form during and after the placing process,
Flow easily through reinforcement.
To achieve these performances, Okamura redesigned the concrete
mix design process. His mix design procedure focused on three
Reduction of the coarse aggregate content in order to reduce
the friction, or the frequency of collisions between them, increasing
the overall concrete fluidity;
Increasing the paste content to further increase fluidity;
Managing the paste viscosity to reduce the risk of aggregate
blocking when the concrete flows through obstacles.
Because conventional concrete is placed using external energy,
there is no need for specific rheological characteristics. As
a matter of fact, the intensity of energy applied in the consolidation
process is adjusted to compensate most plastic property variation.
That is probably why, up until now, a simple slump window requirement
was enough, in conjunction with good consolidation practices,
to lead to dense concrete elements in most applications. With
SCC, other rheological requirements are needed to obtain good
consolidation since no additional placing operations will compensate
for any lack of rheological performances.
(In rheological terms, even though a significant amount of research
tends to show that SCCs viscosity varies with the shear rate
and acts as a pseudoplastic material, SCC is often described as
a Bingham fluid (viscoelastic) where the stress/shear rate ratio
is linear and characterized by two constantsviscosity and yield
stress (Table 1). This latter model will be used in the following paragraphs
because of its simplicity.)
Back to the performance-based definition of SCC, the self-consolidation
is mainly governed by yield stress, while the viscosity will affect
the homogeneity and the ability to flow through reinforcement
(Table 2). As the SCC viscosity can be adjusted depending on the application,
the yield stress must remain significantly lower than other types
of concrete in order to achieve self-consolidation.
At this time, various mix design methods have been proposed in
order to obtain the unique rheological performances of SCC and
may lead to different mix designs for a given application. However,
the performance criterion remains the only objective means to
evaluate the adequacy of an SCC mix. Since criterion relates to
measurements, test procedures need to be developed. ASTM Committee
C09 on Concrete and Concrete Aggregates undertook this task in June
2001 in its Subcommittee C09.47 on Self-Consolidating Concrete.
Exploratory work has looked at existing SCC characterization procedures
around the world. Most of the work still has to be completed,
but with over 40 participants, this team should be able to do
it in a timely fashion
Several tests have already been developed worldwide to characterize
the performance of a fresh self-consolidating concrete. A few
of them are briefly presented in the following paragraphs. The
dimensions may vary from one country to the other.
A rheometer is a device that applies a range of shear rates and
monitors the force needed to maintain these shear rates in a plastic
material. The force is then converted to stress. A few concrete
and mortar rheometers are available on the market and have been
and are still used for measuring the yield stress, viscosity and
other rheological characteristics of SCC. They are of tremendous
help in understanding SCCs behavior.
However, this type of equipment is fairly expensive and not easy
to use on a job site. Therefore, numerous test methods have been
developed for SCC with ease of use in mind. None of these test
methods measures yield stress or viscosity, but they all simulate
more or less real-scale casting environments.
This procedure, which is readily used today, relies on the use
of the Abrams cone (Figure 1). The cone is filled in one layer without rodding and the diameter,
instead of the slump of the concrete sample, is measured after
the cone has been lifted . This test is mostly used for evaluating
the SCCs self-compactibility as it mainly relates to its yield
Monitoring the time it takes for the concrete to reach a spread
of 500 mm can also be used as an evaluation of SCC viscosity.
These two tests (Figure 2) simulate the casting process by forcing an SCC sample to flow
through obstacles under a static pressure. The final height H
and H2/H1 for the U-box and the L-box respectively are recorded.
They provide indication on the static and dynamic segregation
resistance of an SCC as well as its ability to flow through reinforcements.
They are frequently used in the field as acceptance test methods.
V-Funnel and Orimet
By monitoring the time it takes for the SCC to flow through an
orifice under its own weight, these two test methods give an indication
of its viscosity. Both tests are used in the field and are sometimes
used as acceptance tests. The V-funnel is shown in Figure 3.
This apparatus is used to force the SCC to flow through reinforcement
(Figure 4). It must be used in conjunction with an Abrams cone or the Orimet
setup. The concrete flows from the inside to the outside of the
ring. The size and the spacing between the bars can be adjusted
to simulate any reinforcement configuration. The differences between
the spread with and without the ring or the height difference
between the concrete inside and outside the ring is measured.
German studies showed that with a bar spacing equivalent to 2.5
times the maximum aggregate size, the spread difference with and
without the J-ring must be smaller than 50 mm.
This procedure is used to evaluate the resistance to static segregation
of an SCC. A sample of concrete is poured over a 5-mm sieve and
the amount of mortar passing through the sieve in a two-minute
period is measured. The French Civil Engineering Association published
a complete procedure (in French and English) in July 2000. (2)
SCC in the Field
SCC has already been used in several countries. In Japan, major
construction projects included the use of SCC in the late 90s.
Today, in Japan, efforts are being made to free SCC of the special
concrete label and integrate it into day-to-day concrete industry
production. Nevertheless, the SCC market share is still under
one percent in ready-mixed concrete (RMC) as well as precast concrete
(PC). In Sweden, the market share was at three percent in RMC
and PC in 2000, and was expected to double in 2001. Housing and
tunneling as well as bridge construction for the Swedish National
Road Administration were the main areas of use for SCC. In the
Netherlands and Germany, the precast industry is mainly driving
the development of SCC, with an expected six percent of market
share in 2001 in Netherlands.
In the United States, the precast industry is also leading SCC
technology implementation. The Precast/Prestressed Concrete Institute
(PCI) is very active, with the creation in 2002 of a Fast Team.
This teams task is to draw recommendations on the use of SCC
in precast/prestressed operations by October 2002. Meanwhile,
the author estimates that the daily production of SCC in the precast/prestressed
industry in the United States will be 5000 m3 in the first quarter of 2002. Furthermore, several state departments
of transportation in the United States (23 according to a recent
survey) are already involved in the study of SCC.
With such a high level of interest from the construction industry,
as well as manufacturers of this new concrete, the use of SCC
should grow at a tremendous rate in the next few years in the
United States. However, even if it is made from the same constituents
the industry has used for years, the whole process, from mix design
to placing practices, including quality control procedures, needs
to be reviewed and adapted to make the most of this new technology
and prevent growing pains. //
(1) Ouchi M (1998). History of Development and Applications of
Self-Compacting Concrete in Japan. Proceedings of the First International
Workshop on Self-Compacting Concrete.
(2) Cussigh F. et al (2000). Betons autoplacants Recommandations
provisoires. Documents scientifiques et techniques de lAFGC
Copyright 2002, ASTM