SELF COMPACTING CONCRETE: CHALLENGE FOR
DESIGNER AND RESEARCHER
Joost Walraven
Delft University of Technology, The Netherlands
ABSTRACT
Self compacting, or “self-consolidating” concrete (SCC) was first developed in Japan in
the early nineties. The idea was picked up and further developed in Europe from about
1997. Substantial research was carried out with regard to the properties of SCC. Because
of the well-controlled conditions, the introduction of SCC in the precast concrete industry
was successful. With regard to the application in situ, the development is slower, because
of the sensitivity of the product. In this paper the mechanical properties of SCC in
comparison to conventional concrete are discussed. Examples of applications are shown,
both for prefabricated concrete elements and in-situ structures. The way of measuring the
rheological properties is discussed. Examples are given of special self-compacting
concrete’s. Needs for further research are defined.
INTRODUCTION
Self compacting (or self consolidating) concrete (SCC) was first developed in Japan, in
the early nineties of the previous century, under the stimulating leadership of Prof.
Okamura. The main idea behind self compacting concrete was, that such a concrete is
robust and relatively insensitive to bad workmanship. In Western Europe the idea was
picked up at the end of the last century. The main drive to develop self compacting
concrete’s was the option to improve the labor conditions at the building site and in the
factory (noise, dust, vibrations). During recent years self-compacting concrete developed
to research item nr. 1. A large number of research projects was carried out, followed by
recommendations for potential users. Especially for the precast concrete industry self
compacting concrete was a revolutionary step forward. Contrary to that, casting of SCC
at the construction site was regarded with more reservation. The variable conditions at the
construction site, the more complicated control of the mixture composition and
disagreement with regard to the question how the properties should be measured at the
site were retarding factors. In spite of a number of successful examples, some problems
due to unsuitable use of SCC generated further scepticism. Hence, the major task now is
to develop SCC mixtures, which are less sensitive to deviations in properties of the
components and external conditions.
PROPERTIES OF SELF COMPACTING CONCRETE
The Japanese way of composing the optimum mixture composition of SCC consists of a
number of steps. At first, in a small test, the optimum ratio water to powder is
determined. Then a number of general criteria have to be met, the most important of
which are that the coarse aggregate volume should be 50% of the solid volume of the
concrete without air, and that the fine aggregate volume should be 40% of the mortar
volume, where particles finer than 0.09mm are not considered as aggregate, but as
powder. If the composition of the mixture, obtained in this way, is mathematically
analyzed, it is found that this procedure leads to a concrete composition with a little bit of
“excess paste”. That means that slightly more paste is in the mixture than necessary to fill
all the holes between the particles: this implies that around any particle a very thin
“lubricating” layer exists, by virtue of which the friction between the particles in the fluid
mixture is greatly reduced in comparison to conventional mixtures, Fig. 1. The optimum
thickness of those layers lies between narrow limits. If the thickness is too small, there is
too much friction to achieve self compactability. If the thickness is too large, the coarse
aggregate sink down and segregation occurs. The rheologic properties of the excess paste
layers are determined by the choice of the superplasticizer. Furthermore, in the fresh state
around the cement and powder particles thin layers of water are formed [1]. In this way a
three phase system (coarse particles, fine particles and powder) with intermediate layers
of paste and water is obtained which minimize the internal friction in the fresh state.
Midorikawa [2] carried out tests in order to find the optimum thickness of the excess
paste layer. Fig. 2 shows the optimum thickness of the layer for a varying ratio Vw/Vp
(volume of water to volume of powder) for different grading curves. It is seen, that the
thickness of the paste layer, for which the concrete is still self compacting, increases with
decreasing volume of water. Below Vw/Vp = 0.8 the appropriate thickness increases
overproportionally. For practical application, however, this area is not relevant. The
optimum ratio in this case is in the range Vw/Vp = 0.8 – 0.9. The mean thickness of the
excess paste layer is then in the order of magnitude of 0.05mm.
Vwater/Vpowder
Thickness of excess paste [µm]
Fig. 1. Excess paste layers around
aggregate particles
Fig. 2. Relation between thickness of excess
paste layer and water to powder ratio for various particle grading curves [2]
An important question is, to which extent the lubricant layer influences the properties of
the concrete in the hardened stage. It seems to be obvious, that for instance the modulus
of elasticity of SCC is smaller than that of a conventional concrete of the same strength,
as a results of the effect of the relatively soft lubricant layers. An evaluation by
Holschemacher [3] showed, that the E-modulus of an SCC is indeed somewhat smaller,
Fig. 3. It should, however, not be forgotten, that also the E-moduli of normal concretes
are subjected to scatter, most of all in dependence of the stiffness of the aggregate used.
For practical applications it can therefore be assumed, that the E-modulus of selfcompacting concrete is not outside of the region of scatter of conventional concretes. Of
course it makes sense to carry out suitability tests in the case of special applications, such
as for structures in high speed railway lines. In that case, however, also the creep- and
shrinkage properties should be carefully investigated.
Fig. 3. Modulus of elasticity of self-compacting concrete [3].
Another important aspect for the behavior in the hardened state is the concrete tensile
strength. When the axial tensile strength of a SCC would substantially differ from that of
the tensile strength of conventional concrete, this should have large implications for
design, since the tensile strength is a governing aspect in the design for shear, punching,
anchorage, crack width control and the minimum reinforcement. It is obvious to expect
that the tensile strength of SCC is higher than for a conventional concrete, because of the
more homogeneous interface between the aggregate particles and the cement past (no
direct contact between the aggregate particles). An evaluation of test results [3] confirms
this. However, also here the results are in the range of scatter of conventional concretes,
so that no complicating exception for SCC has to be made.
Another important aspect is the formwork pressure of self compacting concrete. Many
measurements have been carried out, but the results were often conflicting. Often the role
of the rising speed of the concrete in the formwork was disregarded. Fig. 4 shows the
results of a number of Swedish [4] and Dutch [5] tests, collected in one diagram. It is
visible that the rising speed of the concrete in the formwork influences the formwork
pressure. For the concretes tested, from a rising speed of about 2m/hour the distribution
of the pressure corresponds approximately to the hydrostatic pressure. This, however,
does not imply, that for lower rising speeds a reduction of the formwork pressure is a
reliable assumption. According to the rheologic behavior SCC is a Bingham fluid. Such a
fluid is characterized by two parameters: the yield value and the plastic viscosity. The
yield value is a measure for the force, necessary to get the concrete moving. The plastic
viscosity is a measure for the flow rate (toughness) of the mixture. When the yield value
is high and the plastic viscosity is low, it may happen that the formwork pressure is
initially very low, but suddenly increases due to a shock against the formwork. It is
therefore advisable to work always with the hydrostatic formwork pressure.
Height [m]
3
hydrostatic
2
rising speed in m/h
1,0
1
1,6
0,8
0
0
1
0
1,3
1,4
2
0
3
0
10
2
4
0
5
0
6
0
7
0
Pressure [kPa]
Fig. 4. Formwork pressure for different rising speeds for SCC
TAILORING SCC TO APPLICATIONS
It is often assumed, that SCC is the best solution for every difficult case. This may result
in disappointments. Fig. 5 left shows a problem which occurred during casting a tunnel
wall. During casting it was observed that the concrete was too sticky. Therefore it was
decided to change the concrete composition. As a result, however, air enclosures
occurred at the interface between the two concretes. Fig. 5 right shows a case, in which
the lubricating action of the excess paste layers around the aggregate particles was “too
good”, which resulted in sinking down of the coarse aggregate particles. Those two
examples, however, should not stimulate the conclusion that SCC is a risky material. But
they emphasize, that it is important to be well informed about the properties of the SCC
that are required for the application considered.
Fig. 5. Failures due to unsuitable application of SCC: at the left side air entrapments
between two concrete layers, at the right side segregation of the coarse aggregate.
Even in relatively new codes and recommendations, like the new European code for
concrete technology EN 206, no special reference is made to SCC. Table I shows the
concrete consistency classes according to this code. The flowability classes F5 and F6 are
only characterized by one parameter: the flow diameter.
Compaction
Class
C0
1.46
C1
1.45-1.26
C2
1.25-1.11
C3
1.10-1.04
Slump
Class mm
Flow
Class mm
S1
S2
S3
S4
S5
F1
F2
F3
F4
F5
F6
10-40
50-90
100-150
160-210
 220
 340
350-410
420-480
490-550
560-620
 630
Table I: Consistency classes according to EN 206
For a reliable application, however, this is insufficient. As stated before, SCC is a
Bingham fluid, characterized by two parameters. In The Netherlands therefore an
extension of the consistency classes was carried out. For the qualification of the concrete
the Japanese method was used, offering a simple method on the bases of two tools, the
funnel with defined dimensions and the cone, Fig. 6. The flow diameter and the funnel
passing time are again two qualifying parameters, alternative to the yield value and the
plastic viscosity, with which the behavior of SCC at the construction site can be qualified.
The tools are furthermore very suitable to be used at the building site, because they can
be very easily handled. These tools were used as a basis to extend the consistency classes,
Fig. 7. The slump flow is again used as an important characteristic. In addition to that,
however, for any range of the slump flow three intervals for the funnel time are defined.
270
3
0
70
60
flow
cone
r0=100
paste
r1
240
r2
120
60
30
Fig. 6. Japanese tools to measure the rheological properties of SCC in the fresh state: the
cone (left) and the funnel (right).
Funnel time [sec]
9-25
5-9
3-5
Slump flow
1
2
3
Consistency classes
470-570
4
>
540-660
630-800 [mm]
Self Compacting Concrete
Fig. 7. Extension of conventional consistency classes with SCC, according to a Dutch
proposal.
In this way for the “family of SCC’s” nine sub-classes are obtained. For any application a
most appropriate sub-class exists, see fig. 8. If, for instance, self compacting concrete is
specified for a lightly reinforced wide floor, for practical reasons a short funnel time is
required. If, on the contrary, a column with congested reinforcement has to be cast, a
large slump flow in combination with a low funnel time (high viscosity) is most
appropriate. In Fig. 8 also other areas are defined.
Of course there are many other ways to define the rheological properties of a self
compacting concrete, like the L-box. the Orimet, the J-ring and others. In a Brite-Euram
project, with partners from many European countries a thorough evaluation was made
with regard to the effectivity and the value of those measuring methods. Reports on the
results of this research will be given elsewhere at this conference.
Funnel time (sec)
9-25
High &
Ramps
Slender
Walls
5-9
Floors
3-5
5
6
7
Consistency class
470-570
540-660
630-800
Slump flow (mm)
Fig. 8. Areas of application of SCC in relation to optimum rheological properties, defined
using the criteria funnel time and flow diameter.
APPLICATIONS IN THE PRECAST CONCRETE INDUSTRY
Previously, it was pointed out that self compacting concrete mixtures are sensitive to
variations in composition and environmental influences. For the precast concrete industry
this is not a considerable difficulty, since the processes at the plant can be very well
controlled. The advantages for using SCC in precast concrete plants are very considerable
like,
- the substantial reduction of the noise level
- the absence of vibration
- the reduction of dust (quartzite!) in the air due to vibration
- the energy saving
- the omission of the expensive mechanical vibrators
- the reduction of wear to the formwork
- the use of less robust formwork with simpler connections
- the reduction of absence for illness
- the possibility to produce elements with high architectural quality
For the production of SCC successful production of SCC it is essential that the basic
constituents, like sand, gravel, fillers and the third generation of superplasticizers, have a
constant quality . This is not always the case. Moreover, not all cement producers supply
a constant quality. So, there should be good agreements between the concrete producers
and the suppliers of the constituents on the quality control. The step from a traditional
concrete production to the production of SCC is not a big one. Installations with an age of
say 5-10 years are generally suitable. Further to the traditional equipment a high intensity
mixing machine and an installation to dose the fillers are needed.
As a result of the introduction of SCC the formwork is hardly loaded anymore: it has only
a retaining function. So, the wall can be made of other materials than timber, like
polystyrene. Also steel formwork with magnetic couplers is possible. The time for
demoulding and re-installing the formwork has been reduced by 50%. There is no need
for the installation of vibration isolators anymore. Rubber joint sealings can be omitted,
since by virtue of SCC no leakage through the joints occurs anymore.
Fig. 9. Architectural element of SCC
Fig. 10. Large prestressed SCC girder for
metro station in Amsterdam.
Fig. 9 shows an example of an architectural balcony element of SCC. The element does
not only show a beautiful shape with very sharp profiles, it has also a homogeneous white
color. Fig. 10 shows the assembly of a precast prestressed concrete girder of SCC for the
new metro station at the Amsterdam Arena, the stadium of soccer club Ajax. The girder
has a length of 22,5m. The concrete strength class is C55 (characteristic cylinder
compressive strength of 55 MPa (7850 psi)). The metro station has a length of 350m with
4 tracks. This means that 60 girders had to be produced with a total length of 1.4 km. If
the girder would have been compacted in the traditional way, heavy vibrating machines
would have been necessary. Due to that, the formwork would have had to be replaced
after a relatively small number of casts. By virtue of the use of SCC the life of the
formwork was very long. Another important reason to choose for SCC was the
improvement of the labor conditions in the factory.
Fig. 11. Foundation piles of SCC
Fig. 12. Concrete arches made of SCC
Fig. 11 shows a set of foundation piles. The production of such type of piles in the firm
was 70 000 piles a year. For an average length of 15m a total production length of 1000
km a year is obtained. Until recently the piles were produced with the so-called shock
procedure. This means that the formwork was forced to repeatedly fall down from a
height of 50mm (2 inches), which created a shock effect. By virtue of the change to SCC
the necessary casting time was reduced from 7.5 minutes to 1.5 minutes. Since
mechanical compaction was not necessary anymore 12 further minutes were gained.
Taking also into account the advantages with regard to the reduction of noise and dust,
energy consumption and wear, it is clear that SCC yields considerable advantages. Fig.
12 shows a number of concrete arches. Every arch has a length of 65 meter and is
composed of 5 pieces of 13m. The cross section has a box-shape, with a foam core.
Producing such an element with conventional concrete does not make sense, since the
foam core would move due to vibration. A production in parts could be an alternative but
is by far too time consuming, and therefore too costly. With SCC perfect elements could
be made.
Meanwhile many precast concrete firms have changed their production to SCC, some
even for 100%.
APPLICATIONS OF SCC IN SITU
The introduction of SCC for in-situ applications was slower than in the precast concrete
industry. There are a number of reasons for this:
- in case of failure the consequences for an in-situ application are much more
severe than in the precast concrete industry. In the latter case the unsuitable
elements can be simply rejected, whereas in the first case demolition might be the
ultimate consequence.
- There was often no agreement on the way in which the properties at the building
site have to be controlled.
- Self compacting properties can be more easily reached with higher strength than
with lower concrete strength. In a number of practical applications the concrete
strength was therefore higher than actually necessary, which has cost
consequences. For many applications a concrete strength class C25 is sufficient.
However, especially for the lower strengths classes it is more difficult to obtain
robust and reliable self compacting concrete’s.
Meanwhile, however, a lot of barriers have, or are being, removed. There is now a better
insight into the required properties of SCC for particular applications, like previously
shown in Fig. 8. Furthermore qualifying test methods have been evaluated. Finally a new
generation of superplasticizers has been introduced.
Nevertheless, a number of convincing examples exist, which proof that SCC, if applied in
an appropriate way, can give excellent results. The first application of SCC in The
Netherlands according to modern principles was such an example, Fig. 13. In 1998 a
large façade was made for the National Theatre in The Hague, which, for architectural
reasons, was provided with fine triangular ribs with a side length of 8mm. In this case
Fig. 13. SCC façade in The
Hague, The Netherlands
Fig. 14. City and County Museum, Lincoln, UK.
an SCC with relatively high flowability was used (flow diameter 730mm) and a low
viscosity (funnel time 8-9 seconds). Fig. 14 shows the interior of the City and County
Museum in Lincoln, UK, where SCC proved to be the best solution for the sloping roof
slabs. The architect required a formed finish for the top surface and specified SCC which
not only reached those parts where other concretes could not come, but gave as well a
consistent high quality finish to both sides of the slab, in spite of the complicated and
congested reinforcement [6].
There are many practical problems where SCC gives a suitable solution. An example is
the retrofitting of the Ketelbridge, a glued segmental bridge in The Netherlands. At the
time of retrofitting in the year 2002 the bridge was 45 years old. During the years the
bridge deck was renovated several times, but the old deck was often not totally removed.
So, finally the bridge deck was 180mm thick in stead of 50mm. Since as well the traffic
load had increased, the joints between the segments opened. Therefore it was decided to
increase the load bearing capacity by external prestressing. A difficulty was the provision
of the deviators inside the box girder. Because the lower flange of the girder had not been
designed for the transport of heavy materials, casting concrete inside the girder was no
realistic option, even regardless of the technological difficulties involved. As a solution
therefore SCC was used. The formwork with the reinforcement was built up in the
interior of the girder (Fig. 15), and the SCC was cast from the outside through a little
window in the upper flange. The concrete strength class was C35. By a suitable use of
the rheological properties an excellent result was obtained.
Fig. 15 Remote casting of a wall with openings in the interior of a box girder bridge for
creating deviation points for additional external prestressing tendons, aiming at
increasing the bearing capacity (Ketelbridge in The Netherlands, 2002).
Another interesting case for which SCC gave a solution was the provision of the end
walls in elements for a submerged tunnel. Those end walls had a temporary character and
served only for enabling floating transport and submerging. After the elements had been
coupled under water, the walls were demolished. In order to facilitate easy demolishing,
SCC in a strength class C20 was used. For casting the concrete between the tunnel walls
through small windows in the formwork SCC appeared to the most appropriate solution.
Fig. 16. Casting the end wall of an element for a submerged tunnel in SCC.
SPECIAL SELF COMPACTING CONCRETES
A remarkable development occurred with regard to the workability of fiber reinforced
concrete. For a very long period it was noted that the addition of fibers to concrete
decreased the workability. However, in his PhD-thesis Grünewald [7] showed that this is
not necessary at all. He proved that self compacting fiber concretes are very well
possible, even up to fiber contents of 140 kg/m3, if the right combination of fibers and
mixture composition is chosen. Fig. 17 shows the maximum possible fiber content for
which mixtures are still self-compacting (defined as having a flow circle with a diameter
of at least 600mm, a round shape and a homogeneous fiber distribution). At the vertical
axis the fiber content in kg/m3 is given. At the horizontal axes the fiber type (aspect
ratio/length) and the mixture type (with the sand/gravel vol. ratio) are given.
140
120
100
Fibre
content
80
[kg/m3]
60
Reference
mixture
40
20
Mix 4 (68/39.0)
Mix 3 (68/36.5)
Mix 2 (57/39.0)
Mix 1 (57/36.5)
0
45/30
Fibre type
65/40
80/30
80/60
Fig. 17. Maximum fiber content for SCC
in dependence of fiber type and mixture
composition
Fig. 18. Self-compacting concrete, with 125 kg/m3 fibers,
in strength class C115.
Fig. 19. Testing the flowability of a high performance fiber reinforced concrete C200
Fig. 18 gives an impression of the excellent flowing properties during casting of a
concrete with 125 kg/m3 fibers. Fig. 19 shows the measurement of the flowability of an
ultra high performance fiber reinforced concrete in a U shaped formwork. The concrete
had an average cube strength of about 180 MPa (25000 psi). It contained 235 kg/m3 steel
fibers 20/0.3mm. It was used in a factory to produce prestressed beams for a bridge.
In another paper at this conference the topic of optimizing the self-compacting properties
of fiber concrete is treated more in detail [8].
Another interesting option is self-compacting lightweight concrete. High performance
lightweight concrete could allow significant savings in reinforcing- and prestressing steel
and foundations. Self compacting properties would even increase the attractiveness of
such a material. With regard to the production technology, there is a major difficulty. The
lightweight aggregate particles are porous and therefore influence the mixture
composition by sucking water from the mixture in the fresh state. As self compacting
concrete is sensitive to the right composition this causes a major difficulty. A solution
was developed by Müller [9]. He developed a technology which consists of enveloping
the sucking aggregates with a thin cement bonded surface coating. The composition of
the cement pastes used for the enveloping is optimized in such a way as to make it
economically possible to apply a thin layer to the agglomerate in the fresh state. After
that, the storage of the freshly enveloped aggregates preventing them from sticking
together is assured and finally after a rapid setting a high density and strength of the
formed envelope is guaranteed. Fig. 20 shows an enveloped lightweight aggregate
particle of the type Liapor F5, where the thickness of the cement bonded layer, which in
the section is distinctly recognizable by its lighter color, is equal on the average to
approximately 0.25-0.35 mm.
Fig. 20. Lightweight aggregate particle with skin of with cement paste [9].
Compared to non-enveloped aggregates, water absorption by dry materials is drastically
reduced if the dry materials are stored for 30 minutes in water and under a pressure of 50
bars and if a low or high performance and thus denser lightweight aggregate is used. The
result is that concrete mixtures with enveloped lightweight aggregate behave with respect
to the characteristics and processing of fresh concrete exactly like mixtures with dense,
normal weight additive material.
Further information on this topic is found in [10].
NEEDS FOR FURTHER DEVELOPMENT
The sensitivity of SCC mixtures for minor variations in the mixture composition should
be decreased. This can be done by adding appropriate types and amount of fillers.
Another, not yet fully explored possibility is the use of viscosity agents. Experiments on
mixtures with viscosity agents show that the sensitivity of for instance variations of the
water content on the viscosity can be strongly reduced by applying an appropriate
viscosity agent, see f.i. Grünewald [11]. Especially the potential of viscosity agents for
improving the stability of mixtures with low and medium strength, suitable for large scale
in-situ applications, deserves further attention.
Also the further development of suitable superplasticizers for SCC is worthwhile,
possibly in combination with viscosity agents. Takada [12] showed in his PhD-thesis that
there is a strong influence of the type of superplasticizer on the necessary mixing time
and mixing intensity. In this area there is still a need for further research.
A very important aspect to be regarded is the durability of SCC. There is a tendency that
in the near future structures should not only be designed for safety (ULS) and
serviceability (SLS), but as well – and with the same importance - for service life. This
means that increased demands will be raised on the resistance of SCC with regard to
chloride ingress, carbonation and frost-thaw cycles. It was shown by many research
projects that SCC is approximately equivalent to conventional concretes with regard to
the majority of its mechanical properties in the hardened state. However, with regard to
the microstructure of hardened SCC and its significance to durability there are still quite a
number of open questions. In this respect the interface between matrix and aggregates
plays an important role. Furthermore the role of (combinations of) additives
(superplasticizers, air entraining agents, viscosity agents) on the microstructure, including
porosity and permeability should get due attention. In this respect special attention has to
be devoted to the average and low strength concrete’s used in in-situ structures, if
exposed to more severe environmental conditions.
CONCLUSIONS
1. In spite of its short history, self compacting (or – consolidating) concrete has
confirmed itself as a revolutionary step forward in concrete technology.
2. For the application of SCC in situ, it is necessary that SCC’s are designed (tailormade) for any particular case. General rules are available on the basis of experience.
3. It can be shown by cost analysis, that SCC in precast concrete plants can be more
economically produced than conventional concretes, in spite of the slightly higher
material price. Cost comparisons should always be made on the basis of integral costs.
4. There is a considerable future for self compacting fiber reinforced concretes
5. The most important task for research is to develop SCC’s with decreased sensitivity to
variations in constituents and environmental influences. This holds particularly true for
in-situ concrete’s, with medium and low strengths.
6. Further research into the potential role of viscosity agents and their interaction with
superplasticizers is worthwhile
7. Since in the near future service life design (SLD) of concrete structures will be as
important as design for safety and serviceability, increased attention should be given to
the role of the microstructure of the various types of available SCC’s and its role for
durability.
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2.
3.
4.
5.
6.
7.
8.
9.
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