Self Compacting Concrete free essay sample

One solution for the achievement of durable concrete structures is the employment of SCC which can be compacted into every corner of a formwork, purely by means of its own weight and the without the need for vibrating compaction. SCC was first developed in 1988 to achieve durable concrete structure. Investigations for establishing a rational mix-design method and Self-compact ability testing methods have been carried out from the view point of making SCC a standard concrete. Modern application of SCC is focused on high performance, better and more reliable and uniform quality. SCC has been described as â€Å"the most revolutionary development in concrete construction for several decades†. It has proved to be beneficial because it allows faster construction, improved durability and greater freedom in design etc. EFNARC, making use of practical experiences of its members with SCC, has drawn up specification and guidelines to provide a framework for design and use of high quality SCC during 2001. We will write a custom essay sample on Self Compacting Concrete or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page Generally SCC is used in situations for concrete requiring high strength say over 40MPa to 100MPa or more. It is being blindly believed that SCC is much costlier than normal or high strength concrete. Though the material cost for SCC is high, if other components of costs such as cost of compaction, finishing etc are considered, one can realize that SCC is certainly is not a costly concrete for comparable strength. Contents Page no * Introduction 3 * Development3 * Requirements4 * Material requirements 4 * Fresh properties6 * Hardened properties6 * Benefits8 * Complexities9 * Indian scenario of SCC 10 * How economical is SCC? 11 Conclusion11 REFERENCES12 BIBILOGRAPHY12 Introduction SCC is defined as a concrete which can be placed and compacted into every corner of a formwork, purely by means of its self-weight by eliminating the need of either external energy input from vibration or any type of compacting effort. Portland cement concrete mixes are being used over 150 years. They are mixed, placed into the form and then compacted. It is essential to compact the concrete so that it completely covers the reinforcement bars and fill the space within the forms, for meeting strength and durability requirement. If the compaction is not complete, it will lead to loss in strength and also effect the performance of the structure. The compaction becomes difficult when percentage of reinforcement is high and rebars get congested without allowing for insertion of the vibrator at some places. In such places the concrete should be flowable in nature and should be self compacted. SCC has more favorable characteristics such as high fluidity, good segregation resistance and the self compacting ability without any need of vibration during the placing process. Development of SCC For several years beginning in 1983, the problem of the durability of concrete structures was a major topic of interest in Japan. The creation of durable concrete structures requires adequate compaction by skilled workers. However, the gradual reduction in the number of skilled workers in Japan’s construction industry has led to a similar reduction in the quality of construction work. Recognizing the lack of uniformity and complete compaction of conventional concrete by vibration, researchers at the University of Tokyo, Japan, started in 1980’s to develop SCC. By the early 1990’s, Japan has developed and used SCC that does not require vibration to achieve full compaction. The necessity and workability of SCC were proposed by Okamura, Ozawa, and Maekawa. By the year 2000, the SCC has become popular in Japan for prefabricated products and ready mixed concrete. Several European countries recognized the significance and potentials of SCC developed in Japan. During 1989, they founded European federation of natural trade associations representing producers and applicators of specialist building products (EFNARC). EFNARC, making use of practical experiences of its members with SCC, has drawn up specification and guidelines to provide a framework for design and use of high quality SCC during 2001. Requirements of SCC Material requirements Cement: All types of cement can be used. But for better SCC mix high grade cement is used. Aggregates Size: The maximum size of aggregate is generally limited to 20mm. For congested reinforcement 10-20mm size aggregate is desirable. Shape: Well graded cubical or rounded aggregates are desirable. Grading must be uniform throughout the work. Moisture content: Must be closely monitored as quality of SCC is sensitive to such changes Mixing Water: High quality must be established on the same line as that for using reinforced concrete or prestressed concrete. Admixtures Chemical admixtures Superplasticizers (New generation superplasticizers): An essential component of SCC to provide necessary workability. Poly-Carboxylated Ethers (PCE) is particularly used for SCC. Viscosity Modifying Agents (VMA): It is used to increase the flowabilty or stability of SCC. Air Entraining Agents (AEA): Used to improve Freeze-thaw resistance Retarders: To control Setting. Mineral admixtures Fly ash: Fly ash in appropriate quantity may be added to improve the Quality and durability of SCC. Ground Granulated Blast Furnace Slag (GGBFS): GGBFS which is both cementitious and pozzolonic material may be added to improve rheological properties. Silica Fume: Added to improve mechanical properties of SCC. Stone powder: Finely crushed lime stone, dolomite may be added to increase the powder content. Fibres: To enhance the properties of SCC in the same as for normal concrete. SCC Mixes SCC is often classified as one of three types, powder, VMA or combined type, depending on the method of providing viscosity. Powder-type SCC is characterized by a low W/P ratio and a high powder content, which are required to limit the free water content and increase the plastic viscosity. This was the first prototype of SCC generated. The key to success is to increase the powder content while decreasing the W/P ratio and use a superplasticizer to provide consistence. Usually additions are used to replace cement to control strength and heat of hydration. Due to the low W/P ratio, such concretes are anticipated to have a high strength and shrinkage, and low permeability. VMA-type SCC is characterized by a high viscosity modifying agent (VMA) dosage, which is added primarily for increasing the plastic viscosity. Compared with powder-type SCC, VMA-type is higher in superplasticizer dosage or W/P ratio to obtain the required filling ability. Powder content is less because viscosity is controlled by the addition of VMA. Combined-type SCC is developed to improve the robustness of powder type SCC by adding a small amount of VMA. In such mixes, the VMA contents are less than those in the VMA-type SCC. The powder content and W/P ratio are less than those in the powder-type SCC. Viscosity is provided by the VMA along with powder. This type of SCC was reported to have high filling ability, high segregation resistance and improved robustness. Fresh Properties Filling ability (confined flowability) The ability of SCC to flow under its own weight (without vibration) into and fill completely all spaces within intricate formwork, containing obstacles, such as reinforcement. Passing bility The ability of SCC to flow through openings approaching the size of the mix coarse aggregate, such as the spaces between steel reinforcing bars, without segregation or aggregate blocking (This property is of concern only in those applications that involve placement in complex shapes or sections with closely spaced reinforcement. ) Stability (segregation resistance) The ability of SCC to remain homogeneous during transportation, p lacing, and after placement. Hardened Properties Hydration The same hydration mechanism governs SCC as that of Normally Vibrated Concrete (NVC). However a higher content of admixtures and powder materials may exert some influence on hydration development. For example, incorporation of limestone powder in SCC led to a shorter induction period, an increase in hydration reaction and the appearance of a third hydration peak. Fine powder particles acted like heterogeneous nucleation sites to accelerate hydration. The setting time of SCC was reported to be twice as long as that of NVC due to the superplasticiser and fly ash used. Durability Durability is a general analysis of the service life and the performance of concrete in an aggressive environment. Physical damage to concrete includes wetting/drying, freeze/thaw or heating/cooling cycles. Chemical damage consists of sulphate attack, acid attack, chloride attack and alkali-silica reaction. This difference between SCC and NVC might be due to the difference in constituent materials: the higher cement content in NVC contributed more calcium hydroxide than in SCC; the lower W/P ratio used and the incorporation of limestone, which is finer than cement, both led to a denser matrix of SCC than NVC. SCC exhibited lower resistance to freeze-thaw than NVC. Shrinkage and Creep Volume change, e. g. shrinkage, is important for concrete because it produces tensile stress within the concrete leading to adverse cracks which makes it possible for gas, water and harmful chemicals to penetrate into the concrete and cause further durability problems. Shrinkage was important for prestressed concrete because it relaxed the prestressing force, thus reducing structural capacity. The use of a higher content of paste, powder and superplasticiser in SCC all may contribute to higher shrinkage and creep than in NVC. The drying shrinkage of SCC was found to be 10~50% higher than that of NVC Application of limestone powder in SCC was found to reduce shrinkage Creep is defined as the gradual increase in strain for a constant applied stress. It is also a time-dependent deformation. Creep takes place in the cement paste and is influenced by porosity which relates to the W/C ratio. As cement hydrates and porosity decreases, creep decreases. In addition, aggregates restrain the creep of paste. For this reason, a higher amount of aggregates and a higher elastic modulus of aggregates will lead to a reduced creep. Creep was influenced by cement paste porosity and reduces with an increase in strength in the same way for both SCC and NVC. The creep of SCC is anticipated to be higher than NVC due to its higher cement paste Strength Strength is one of the most important properties specified for concrete because it is a direct reflection of the capacity of the structure to resist forces and it is a reasonable indicator of other properties. Compressive strength: Where the W/P ratios are similar, the compressive strength and the strength development of SCC are not significantly different from NVC. The strength development of SCC and NVC over a period of time is also similar. Tensile strength: Where the W/P ratios are similar, the splitting tensile strength of SCC was higher than that of NVC (Holschemacher and Klug, 2002; Zhu et al. , 2004); the tensile to compressive strength ratio of SCC was 10~30% higher than that of NVC (Gibbs and Zhu, 1999; Gram and Pentti, 1999). This probably results from the better microstructure of SCC. Benefits of SCC Better progress: It allows more amount free fall and also ma\ore horizontal distance upto which concrete can flow without segregation. Better quality: Better Surface finish When SCC is placed in a form, its motion may be a creeping movement or a rapid flow. Because of this style of flow, the surface finish between the form and the concrete can be exceptionally smooth, creating a much-improved form finish over conventional concrete. * Better Durability – Durability increases as dense packing leads to relatively impermeable concret e. Safe working environment: Safety hazards are also reduced in the plant, as use of SCC minimizes the need for workers to walk on the top of the form, and eliminates the cords and hoses associated with concrete vibrators. It has been reported that worker absenteeism and accidents have both seen significant reductions when SCC has been introduced into precast production activities. Concrete forms also benefit from lack of vibration with increased life cycle. Typically, form vibration is one of the elements that lead to form damage, associated repair requirements, and ultimately to form replacement Easier placing: It is easier to place SCC than ordinary concrete as it has the versatile quality of flowing. Reduction in site manpower:   Ã‚  Ã‚   SCC is a high performance concrete in the plastic state. It takes less energy to move the material (lower shear stress) (viscosity) and should not separate or segregate. A material that takes less energy to move will require fewer workers or finishers to produce a quality precast/prestressed concrete unit. SCC has the potential to allow reallocation of manpower and increased production with existing resources. Environmental-friendly: Noise level at construction site is reduced considerably. Industrial waste like Fly ash are used which otherwise have to be disposed off safely. Better Mouldability: More innovative structures can be designed because SCC can flow through any shape of formwork. Complexities in making SCC Normal strength concrete itself is a complex material. High strength and high performance concrete with low water /binder ratio adds to the complexity. Making self compacting concrete, particularly of high strength, adds further to the complexity. For production of high strength concrete would the use of relatively low water/binder ratio. Binder generally include silica fume while increasing the strength reduces the workability needed for SCC. A dose of superplasticizer is needed. Very high dosage of superplasticizers leads to two major problems. Firstly, all the superplasticizers available in the market are not suitable for application at high dosage. One which does not have any adverse side effect such as excessive retardation, and one that could retain the slump for sufficiently long time. Also the superplasticizers based on Naphthalene or Melamine is generally not suitable SCC requiring very high strength. From various studies of production of SC it was found better to use pol-carboxylate based superplasticizers (hyperplasticizers) and is more efficient than naphthalene or melamine based superplasticizer wrt plasticizing property and slump retention property. Indian scenario of SCC In India, during the last few years, attempts were made in the laboratories and in the field to develop and use SCC. However large scale users have been rare. Some pioneering efforts have been made in Delhi Metro Projects in association with Lamp;T and MBT. Nuclear power corporation, gammon India, Hindustan Construction Company have made large scale laboratory trials and on the ground Moch up trials. Laboratory studies conducted at SERC Chennai, Indian Institute of Technologies at Madras, Roorkee and other places have given enough inputs and confidence to adopt SCC in India. Delhi Metro Project Of all the places Delhi Metro Project has used SCC in large scale for dome construction, tunnel lining, column casting etc. About 10,000 m3 of SCC has been used in as many as 40 locations during the year 2004. This is the by far the biggest use of SCC in India. Hindustan Construction company have also carried out considerable studies on the use of high volume Fly ash self compacting concrete for domes, walls in turbine building in Rajasthan Power Project and concrete for piers in Bandra-Worli sea link projects. Based on their extensive trails they have used high volume fly ash self compacting concrete in the above projects and in many other works. | Method | | Property| Unit| | minimum| maximum| Trial result| | Slump flow| | Filling ability| mm| | 650| 800| 680| | V-funnel| | Filling ability| sec| | 8| 12| 8| | L-Box | | Passing ability| mm| | 8| 1| 0. 91| U-Box| | Passing ability| %| | 0| 30| 15| | V-funnel at 5 min| | Segregation| sec| | 0| 3| 2| How Economical is SCC? There is a feeling that cost of SCC is much higher than that of corresponding normal strength or high strength concrete. It is seen that cost of material of SCC is about 10-15% higher. If one takes the other components of cost such as cost of compaction, finishing etc then one would realize that SCC is certainly not a costly concrete for comparable strength. Conclusion SCC is versatile concrete which has the key properties like filling ability, passing ability and segregation resistance. In today’s world it is gaining its own importance in the field of construction because of its serviceability, maintenance and durability. The contribution of SCC towards economy is tremendously increasing. SCC is being used in many construction activities, civil projects etc. Its efficiency and effectiveness on site reduces the construction time and also improves the work place environment by reducing noise pollution and eliminating the health problems. SCC is therefore called ‘the quite revolution in concrete construction. ’ REFERENCES Okamura H, self compacting high performance concrete, concrete international 19(7)1997. * Manu Santhanam and Subramanian S, current development in SCC June 2004. * EFNARC specification and guidelines for SCC, European federation of producers and applicators of specialist products for structures 2002. * PCI Committee Summary Report. Summary appearing in PCI Journal, May-June 2003, pp 14-18. BIBILOGRAPHY * M S SHETTY, â€Å"Concrete technology, theory and practice ’’. New Delhi, S Chand amp; Co. Ltd. * Concrete Technology, 2/E A. M. Neville and J. J. Brooks.