Self Compacting Concrete

 When large quantity of heavy reinforcement is to be placed in a reinforced concrete (RC) member, it is difficult to ensure that the formwork gets completely filled with concrete, that is, fully compacted without voids or honeycombs. Compaction by manual or by mechanical vibrators is very difficult in this situation. The typical method of compaction, vibration, generates delays and additional cost in the projects. Underwater concreting always required fresh concrete, which could be placed without the need to compaction; in such circumstances vibration had been simply impossible. This problem can now be solved with self-compacting concrete.This type of concrete flows easily around the reinforcement and into all corners of the formwork. Self-compacting concrete (SCC) describes a concrete with the ability to compact itself only by means of its own weight without the requirement of vibration. Self-compacting concrete also known as Self-consolidating concrete or self levelling concrete.Fig.1 shows the flow of Self-compacting concrete.

            Self-compacting concrete is placed or poured in the same way as ordinary concrete but without vibration. It is very fluid and can pass around obstructions and fill all the nooks and corners without the risk of either mortar or other ingredients of concrete separating out, at the same time there are no entrapped air or rock pockets. This type of concrete mixture does not require any compaction and is saves time, labour and energy. The surface finish produced by self-compacting concrete is exceptionally good and patching will not be necessary.

            Self-compacting concrete has been successfully used in France, Denmark, the Netherlands and UK, apart from Japan. It is gaining wide acceptability because no vibration is needed and noise pollution is eliminated. The construction process is safer and more productive. 

               

 

 

 

 

               

 

 

                              Fig.1-Flow of SCC

MATERIALS

The Materials used in SCC are the same as in conventional concrete except that an excess of fine material and chemical admixtures are used. Also, a viscosity-modifying agent (VMA) will be required because slight variations in the amount of water or in the proportions of aggregate and sand will make the SCC unstable, that is, water or slurry may separate from the remaining material.  The powdered materials are fly ash, silica fume, lime stone powder, glass filler and quartzite filler. The use of pozzolanic materials helps the SCC to flow better. The pozzolanic reaction in SCC, as well as in Conventional Slump Concrete (CSC), provides more durable concrete to permeability and chemical attacks.

To achieve a high workability and avoid obstruction by closely spaced reinforcing, SCC is designed with limits on the nominal maximum size (NMS) of the aggregate, the amount of aggregate and aggregate grading. However, when the workability is high, the potential for segregation and loss of entrained air voids increases. These problems can be alleviated by designing a concrete with a high fine-to-coarse-aggregate ratio, a low water–cementitious material ratio (w/cm), good aggregate grading, and a high-range water-reducing admixture (HRWRA).

Following are bases which are commonly used as superplasticizers.

• Modified Lignosulfonates (MLS).
• Sulfonated Melamine Formaldehyde (SMF)
• Sulfonated Naphthalene Formaldehyde(SNF)
• Acrylic Polymer based(AP)

v  Copolymer of  Carboxylic Acrylic

v  Acid with Acrylic Ester(CAE)

v  Cross Linked  Acrylic  Ploymer(CLAP)

v  Polycarboxylatethers(PCE)

v  Multicarboxylatethers(MCE)

v  Polyacrylates

• Combination of above

   Different bases of New Generation super Plasticizers or High Water reducing agents(HRWRA) have different water reduction capacities. The advantage of this water reduction can be taken either to increase the strength as in high strength concrete or to obtain  a better flowability as in case of self compacting concrete.

PRODUCTION OF SCC

Based on the original conception of Okamura and Ozawa, in general three types of SCC can be distinguished:

a) Powder type self compacting concrete: This is proportion ed to give the required self compatibility by reducing by reducing the water-powder (material < 0.1mm) ratio and provide adequate segregation resistance. Superplasticizers and air entraining admixtures give the required deformability.

b) Viscosity agent type self compacting concrete:  This type is proportioned to provide self compaction by the use of a viscosity modifying admixture to provide segregation resistance.Superplasticizers and air entrainment admixtures are used for obtaining the desired deformability.

c)combination type  self compacting concrete: This type is proportioned so as to obtain self  compatibility mainly  by reducing the water powder ratio, as in the powder type ,and a viscosity modifying admixture is added to reduce the quality of fluctuation of the fresh concrete due to  the variation of  the surface moisture content of the aggregates and their gradations during  the production .This facilitates the production control of the  concrete.

Test Methods for Self Compatibility

          Conventional workability tests, devised for normal ranges of concrete mixtures are not adequate for self-compacting concrete, because they are not sensitive enough to detect the tendency to segregation.  For example, a slump test may show collapse, ( a slump of say 280 mm) and yet in one case the mixture may be stable and in other cases either the aggregate may settle down or the slurry may tend to “run”.  Therefore test equipment was fabricated for judging the following characteristics.

(1)   Self-compatibility: The U-tube test gives an indication of the resistance of the mixture to flow round obstructions in a U-type mould, Fig 2.  This test also detects the tendency of the coarse aggregate particles to stay back or settle down, when the mixture flows through closely-spaced reinforcements.

(2)   Deformability: The slump flow test as specified by the Japan Society of Civil Engineers (JSCE) judges the ability of concrete to deform under its own weight against the friction of the base, Fig 3.  This test, however, cannot evaluate whether the concrete will pass through the space between the reinforcement bars.  This test is useful also as a routine control test, to detect the tendency for slurry to separate from the mixture.

(3)   Viscosity: Viscosity of the mortar phase is obtained by a V-funnel apparatus,    Fig 4.This is useful for adjusting the powder content, water content and admixture dosage.

(4)   Filling ability test: It is also used to determine the ability of the concrete to deform readily through closely spaced obstacles.(fig.5)

Many different methods have been developed to characterise the properties of SCC.  No single method has been found till date which characterises all the relevant workability aspects and hence, each mixed has been tested by more than one test method for the different workability parameters.

                                                                             

                                                                               Fig.3.slump flow apparatus

                                                                  

Fig.4.V-funnel                                                                        Fig.5.Filling ability test apparatus      

PROPERTIES OF SCC

Hardened properties of SCC

        Development of concrete strength with time: The compressive strength, as one of the most important properties of hardened concrete, in general is the characteristic material value for the classification of concrete in national and international codes. For this reason, it is of interest whether the differences in the mixture composition and positive dissimilarities in the microstructure, as mentioned before, affect the short and long term load-bearing behaviour. Accordingly, clarification is still necessary to determine whether the hardening process and the ultimate strengths of SCC and conven-tional concrete differ. After 28 days the reached compressive strength of SCC and normal vibrated concrete of similar composition does not differ significantly in the majority of the published test results. Isolated cases, however, showed that at the same water cement ratios slightly higher compressive strengths were reached for SCC. At the current time there is insufficient research to result in generalized conclusions with this fact.  The comparison of hardening processes shows that the strength development of SCC and conventional concrete is similar, Fig. [6]. Some of the published test results show that an increase of the cement content and a reduction of filler con-tent at the same time increases the initial concrete strength and the ultimate concrete strength. For young SCC aged up to 7 days the relative compressive strength spreads to a greater extend as given in the CEB-FIB Model Code 90, whereas higher values as well as lower ones are reached. Especially if limestone powder is used higher compressive strengths are noticeable at the beginning of the hardening process.

Splitting tensile strength :All parameters which influence the characteristics of the microstructure of the cement matrix and of the interfacial transition zone (ITZ) are of decisive impor-tance in respect of the tensile load bearing behaviour. By evaluating the created database it could be shown, that most results of the measured splitting tensile strength values are in the range of valid regulations for normal vibrated concrete with the same compressive strength. However, in about 30% of all data points a higher splitting tensile strength was stated, Fig. [7].

Fig.6 Development of concrete strength with time acc. to CEB-FIB Model Code 90                               

                 Fig. 7: Splitting tensile strength at SCC in comparison to CEB/FIB Model Code9

Hence it appears the tendency of a higher splitting tensile strength of SCC. Likely as not, the reason for this fact is given by the better microstructure, especially the smaller total porosity and the more even pore size distribution within the interfa-cial transition zone of SCC. Further on a denser cement matrix is present due to the higher content of ultrafines. The time development of tensile strength of SCC and normal vibrated concrete are subjected to a similar dependence. Only few publications about SCC refer to a more rapidly increase of the tensile strength opposite to the compressive strength.

Modulus of elasticity:As it is known, the modulus of elasticity of concrete depends on the proportion of the Young´s moduli of the individual components and their percentages by vol-ume. Thus, the modulus of elastisity of concrete increases for high contents of aggregates of high rigidity, whereas it decreases with increasing hardened cement paste content and increasing porosity. A relative small modulus of elasticity can be expected, because of the high content of ultrafines and additives as dominating factors and, accordingly, minor occurrence of coarse and stiff aggregates at SCC. Indeed, it was shown by analysing the database that the modulus of elasticity of SCC can be up to 20 % lower compared with normal vibrated concrete having the same compressive strength and made of the same aggregates. Nevertheless, it is mainly still in the range of the CEB-FIB Model Code 90, Fig.[8]

    

	Fig. 8: Modulus of elasticity of SCC in comparison to CEB-FIB Model Code 90

TECHNICAL ADVANTAGES OF SELF-COMPACTING CONCRETE

     Simple inclusion even in complicated formwork and tight reinforcement

·            Higher installation performance since no compaction work is necessary which leads to reduced construction times, especially at large construction sites

·            Reduced noise pollution since vibrators are not necessary

·            Higher and more homogenous concrete quality across the entire concrete cross-section, especially around the reinforcement

·             Improved concrete surfaces (visible concrete quality)

·            Typically higher early strength of the concrete so that formwork removal can be performed more quickly.

APPLICATION

Current condition on application of self-compacting concrete in Japan:

After the development of the prototype of self-compacting concrete at the University of Tokyo, intensive research was begun in many places, especially in the research institutes of large construction companies.  As a result, self-compacting concrete has been used in many practical structures.  The first application of self-compacting concrete was in a building in June 1990.  Self-compacting concrete was then used in the towers of a prestressed concrete cable-stayed Shin-Kiba Ohashi bridge in 1991.  Lightweight self-compacting concrete was used in the main girder of a cable-stayed bridge in 1992.  Since then, the use of self-compacting concrete in actual structures has gradually increased. Self-compacting concrete has been successfully used in France, Denmark, the Netherlands,Germany,USA and UK, apart from Japan.

·               A typical application example of Self-compacting concrete is the two anchorages of Akashi-Kaikyo (Straits) Bridge opened in April 1998, a suspension bridge with the longest span in the world (1,991 meters) (Fig. 9). The volume of the cast concrete in the two ahchorages amounted to 290,000 m3. A new construction system, which makes full use of the performance of selfcompacting concrete, was introduced for this. The concrete was mixed at the batcher plant beside the site, and was the pumped out of the plant. It was transported 200 meters through pipes to the casting site, where the pipes were arranged in rows 3 to 5 meters apart. The concrete was cast from gate valves located at 5 meter intervals along the pipes. These valves were automatically controlled so that a surface level of the cast concrete could be maintained. In the final analysis, the use of self-compacting concrete shortened the  anchorage construction period by 20%, from 2.5 to 2 years.

·                  Self-compacting concrete was used for the wall of a large LNG tank belonging to the Osaka Gas Company, whose concrete casting was completed in June 1998. (Fig.10) The volume of the selfcompacting concrete used in the tank amounted to 12,000 m3. The adoption of self-compacting concrete means that 

(1) the number of lots decreases from 14 to 10, as the height of one lot of concrete casting was increased.

(2) the number of concrete workers was reduced from 150 to 50.

(3) the construction period of the structure decreased from 22 months to18 months.

Self-compacting concrete is often employed in concrete products to eliminate the noise of vibration. This improves the working environment at plants and makes it possible for concrete product plants to be located in the urban area. The annual production of concrete products using self-compacting concrete exceeded 200,000 tons in 1996 .

·        Application in under water construction

40000 m3 of concrete placed under water (Fig. 11-13) by using the Tremie method for the construction of a dry dock

• Massive structures such as reinforced foundations for skyscrapers (Fig.14-16)

• Vertical Walls & Columns Congested Re-Bar(Fig.17)

• Stripped SCC Wall (Fig.18)

• SCC Pumped into Column(Fig.19)  

• SCC Used In Block fill(Fig.20)
• SCC Horizontal Application(fig.21-22)

• RMC application(fig.23)

• Precast concrete element plants.(Fig.24)

                                                                                          

 

 

Fig.10 LNG Tank                                               Fig.9 Anchorage 4A of AkashiKaikyo

                                                                                                Bridge

Fig.11 Tremie method to place self-compacting         Fig.12 Congestion of reinforcements of a

concrete under water                                                 slab foundation where concrete is placed  without vibration

                                                                                    

Fig.13 Aerial view of the dry dock after                                Fig.14 Placement of

removing sea water                                                     SCCin a reinforced slab foundation of a skyscraper

(Commercial Center) in New York

                                                                                   

	Fig.16 Placement of self-compacting concrete only chutes for the slab through foundation of the Trump Tower New York

Fig.15 View of the reinforced slab foundation

Placed under water without Vibration

                                                                                                                       

Fig.17 Vertical Walls & Columns Congested Re-Bar                        Fig.18 Stripped SCC Wall

    

Fig.19 SCC Pumped into Column                        Fig.20 SCC Used In Block fill

Fig.23 Placement of SCC                                               Fig.21SCC Used in roof

                                                                                       

   

 Fig.24 Finished precast unit

Conclusions

• SCC is made from the ingredients, which are almost same used in producing in conventional concrete. Thorough understanding of role played by each of the ingredient of SCC is essential.

• Properties of fresh and hardened SCC should be established in the laboratory before their use in the field. Even though the initial cost of SCC is comparatively higher than the conventional concrete. Considering the long service of the structure, minimum maintenance, labour cost, cost due to the vibrators required, benefit cost ratio is very much in favour in case of SCC.

• Self Consolidating Concrete, as well as Conventional Slump Concrete, requires proper mixt proportion to become a durable concrete.
• The uses of pozzolanic materials, such as slag, fly ash, silica fume, etc., will help SCC more durable, otherwise these are waste products demanding with no practical applications and which are costly to dispose of.

• The use of proper super plasticizing admixture in combination with proper air entraining admixture is the absolute key to durable concrete due to freeze-thaw and scaling resistance.
• Advantage with respect to sound pollution.
• Considerable improvements in exposed surface (Fair Faced Concrete)

·        Self compacting concrete is ideal for concrete parts with complicated shapes and elements with high quality visible concrete.

·        Vibrating concrete in congested locations may cause some risk to labour in addition to noise stress. There are always doubts about the strength and durability placed in such locations. So it is worthwhile to eliminate vibration in practice, if possible.

·         In countries like Japan, Sweden, Thailand, U.K and U.S.A, etc., the knowledge of SCC has moved from domain of research to application. But in India, this knowledge is to be widespread.