72 The Masterbuilder - September 2013 • www.masterbuilder.co.in

Self Compacting Concrete: Challenge for Designer and Researcher

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

Joost WalravenDelft University of Technology, The Netherlands

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.

Fig. 1. Excess paste layers around aggregate particles

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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.

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.

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

Fig. 2. Relation between thickness of excess paste layer and water to powder ratio for various particle grading curves [2]

Fig. 3. Modulus of elasticity of self-compacting concrete [3].

Fig. 4. Formwork pressure for different rising speeds for SCC

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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.

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.

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.

Fig. 5. Failures due to unsuitable application of SCC: at the left side air entrap-ments 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 Slump Flow

Class Class mm Class mm

C0 1.46

C1 1.45-1.26 S1 10-40 F1 340

C2 1.25-1.11 S2 50-90 F2 350-410

C3 1.10-1.04 S3 100-150 F3 420-480

S4 160-210 F4 490-550

S5 220 F5 560-620

F6 630

Table I: Consistency classes according to EN 206

Fig. 6. Japanese tools to measure the rheological properties of SCC in the fresh state: the cone (left) and the funnel (right).

Fig. 7. Extension of conventional consistency classes with SCC, according to a Dutch proposal.

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

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measuring methods. Reports on the results of this research will be given elsewhere at this conference.

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. 8. Areas of application of SCC in relation to optimum rheological proper-ties, defined using the criteria funnel time and flow diameter.

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.

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Concrete: 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 an SCC with relatively high flowability was used (flow diameter 730mm) and a low viscosity (funnel time 8-9 seconds). Fig. 14

Fig. 11. Foundation piles of SCC

Fig. 12. Concrete arches made of SCC

Fig. 13. SCC façade in The Hague, The Netherlands

Fig. 14. City and County Museum, Lincoln, UK.

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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.

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.

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.

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

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 ten-dons, aiming at increasing the bearing capacity (Ketelbridge in The Nether-lands, 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.

Special Self Compacting Concretes

A remarkable development occurred with regard to the workability of fiber reinforced concrete. For a very long period

Fig. 16. Casting the end wall of an element for a submerged tunnel in SCC

Fig. 17. Maximum fiber content for SCC in dependence of fiber type and mixture composition

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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.

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

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

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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 (tailor-made) 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.

References

1. Midorikawa, T., Maruyama, K., Shimomura, T. and Momonoi, K., “Application of the water layer model to mortar and concrete with various powders”, Proceedings of the Japan Society of Civil Engineers, No. 578/V-37, pp. 89-98, 1997 (in Japanese).

2. Midorikawa, T., Pelova, G.I., Walraven, J.C., “Application of the water layer to self compacting concrete with different size distribution of fine aggregate”, Proceedings of the Second International Symposium on Self-Compacting Concrete”, Tokyo, Japan, 23-25 October 2001, pp. 237-246.

3. Holschemacher, K., “Design relevant properties of self compacting concrete”, Symposium “Self Compacting Concrete”, Leipzig, Nov. 2001, Proceedings, pp. 237-246 (in German).

4. Billberg, P., “Form pressure generated by self-compacting concrete”, 3rd International Rilem Symposium “Self-compacting concrete”, 17-20 August 2003, Reykjavic, Iceland, Proceedings, pp 271-280.

5. Den Uijl, J.A., “Properties of self-compacting concrete”, Cement 6, 2002, pp. 88-94 (in Dutch).

6. www.concretecentre.com.

7. Grünewald, S., “Performance based design of self-compacting reinforced concrete” Dissertation, TU Delft, 4. June 2004.

8. Grünewald, S., Walraven, J.C., “Optimization of the mixture composition of self-compacting fiber reinforced concrete”, Conference SCC 2005, Chicago, USA, Oct. 30 – November 2, 2005.

9. Müller, H.S., Guse, U.,“Concrete Technology Development: important research results and outlook in the new millennium”, Concrete Plant + Precast Technology, 2000, Nr. 1, pp. 32-45

10. Haist, M., Mechtcherine, V., Beitzel, H., Müller, H.S., „Retrofitting of building structures using pumpable self-compacting lightweight concrete”, Proceedings of the 3rd International RILEM Symposium on “Self-Compacting Concrete”, pp. 776-795.

11. Grünewald, S., Walraven, J.C., “The effect of viscosity agents on the characteristics of self-compacting concrete”, Conference SCC 2005, Chicago, USA, Oct. 30th – Nov. 2nd, 2005.

12. Takada, K., “Influence of admixtures and mixing efficiency on the properties of self compacting concrete”, PhD-Thesis, TU Delft, May 11th, 2004.

Fig. 20. Lightweight aggregate particle with skin of with cement paste [9].

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