CHAPTER caused by lack of complete and uniform
CHAPTER 1INTRODUCTION1.1 Self Compacting Concrete (SCC)Self compacting concrete is defined as a Concrete that is able to flow under its own weight and completely fill the formwork, even n the presence of dense reinforcement ,without any need of vibration, while maintaining homogeneity. It was originally first developed in Japan, to overcome the problems caused by lack of complete and uniform compaction through vibrators. Self-compacting concrete is not affected by the skills of workers, the shape and amount of reinforcing bars or the arrangement of a structure and, due to its high-fluidity and resistance to segregation it can be long distances (Bartos, 2000).The concept of self-compacting concrete was proposed in 1986 by professor Hajime Okamura (1997), but the prototype was first developed in 1988 in Japan, by professor Ozawa (1989) at the University of Tokyo.
Self compacting concrete was developed at that time to improve the durability of concrete structures.However, the Bureau of Indian Standards (BIS) has not brought out a standard mix procedure although number of agencies and researchers carried out extensive investigations to establish rational mix design procedures and self compactibility testing methods. Since then various investigations have been carried out and SCC has been used in practical structures in Japan, mainly by large companies. Investigations for establishing a rational-mix design method and self compactibility testing methods have been carried out from the viewpoint of making a standard concrete. When the Self Compacting Concrete is cast, so there is no additional inner or outer vibration is necessary for the compaction. It flows like a “honey” and as a very smooth surface level after placing of the SCC.
With this regard to its composition, self compacting concrete consists of the same materials as conventionally vibrated concrete, which are cement ,fine aggregate, coarse aggregate and water ,with the addition of chemical and mineral admixtures in different proportions. Usually, the chemical admixtures are used for the high-range of water reducers (Super Plasticizer) and Viscosity Modifying Agents, which change the rheological properties of concrete. Mineral admixtures are used as an extra fine material besides cement, and some cases, they replace cement.
In this study cement content was partially replaced with mineral admixture is used, i.e., fly ash. Admixtures that are improves the flowing and strengthening properties of concrete.1.2 Advantages and Disadvantages of Self Compacting ConcreteThe advantages of SCC over the normal concrete includes increased a productivity , more uniform and cohesive material with few or no honeycomb structures ,improves a strength and durability characteristics, and a very good finish effect shown in figure 1.
A pure cement SCC is placed in a steel mould and after that demoulded 48 hours of the casting. The surface is smooth and dense, adoptability in congested reinforced sections, then reduction in the size if the structural members and so on.The workplace at site environment is improved by SCC, by eliminating the noise pollution and creating the safe environment to formwork due to elimination of compaction through vibrators to leading a pleasant work atmosphere. For much concrete construction, the structural performance is improved by increasing reinforcement volumes, limiting of the cracks by using smaller bar diameters and complex formwork, all of above, which increase the difficulty of compaction (Okamura and Ouchi, 2003a; RILEM TC 174 SCC, 2000). SCC meets the above developments by making casting homogeneous concrete in congested structures possible; it also improves efficiency and effectiveness on site by reducing the construction time and labour cost.
SCC requires the higher powder and admixture ( particularly Super Plasticizers) contents than Normal Vibrated Concrete (NVC) and also the material cost is higher (The Concrete Society and BRE, 2005).It was reported that in most cases, the cost increased from 20% to 60% compare to similar grade Normal Vibrated Concrete (Nehdi et al., 2004; Ozawa, 2001). However in large structures increased material cost by using SCC was outweighed by savings in labour cost and construction time (Billberg, 1999).
Increased content of powder and admixture also leads to higher sensitivity (i.e., reduced robustness) of SCC to material variation than that of NVC; thus greater care with quality control is required (Walrven, 1998).1.
3 Properties of Self Compacting ConcreteSCC has three essential properties: filling ability, passing ability and segregation resistance (Testing-SCC, 2005; The Concrete Society and BRE, 2005).Filling ability is the characteristic of SCC to flow under its own weight and to completely fill the formwork.Passing ability is the characteristic of SCC to flow through and around the obstacles such as reinforcement and narrow spaces without blocking.Segregation resistance is the characteristic of SCC to remain homogeneous during and after placing.
It is passing ability that distinguishes SCC from other high consistence concrete (Domone, 2000).Additional properties such as robustness and consistence retention, are also important applications of SCC. Robustness refers to the ability of SCC to retain its fresh properties, when the quality and quantity of the constituent materials and the environmental condition change. Consistence retention to the period of duration of the fresh properties.A number of commonly used tests are subsequently described for evaluating the fresh properties. There no difference between test methods for harden properties (strength, stiffness and durability etc.) between SCC and NVC.
Both fresh and harden properties are key to the successful operation of SCC. SCC, therefore, it is to be designed based on the fresh or harden requirements.1.4 Historical Development of Self Compacting ConcreteSelf compacting concrete is a principle, and it is not a new concrete.
Special applications such as underwater concreting is always required concrete. This concrete is placed without the need of compaction (Bartos, 2000). In some critical positions, vibration is impossible. Early self compacting concretes relied on very high contents of cement paste and, once Super Plasticizers are available, they were added in the concrete mixes. The mixes required specialized and well-controlled placing methods in order to avoid a segregation, and the high contents of cement paste made them to prone to shrinkage.
The overall costs are very high and applications are also remained very limited.Introduction of “modern” self levelling concrete or self compacting concrete (SCC) is associated with the drive towards better quality concrete is developed in Japan around 1983, where the lack of uniform and complete compaction has been identified as the primary factor responsible for poor performance of concrete structures (Dehn et al., 2000). Due to the fact that there is no practical compaction means by which full compaction of concrete on a site was ever to be fully guaranteed, the focus turned the eliminate need to compact by vibration or any other means.
This leads to development of the first practicable SCC by researchers Okamura and Okawa, around 1986, at the university of Tokyo and the large Japanese contractors (e.g., Kajima Co., Maeda Co., Taisei Group Co., etc) quickly took up the idea.
The contractors used their large in-house research and development facilities to develop their own SCC technologies. Each company developed their own mix designs and trained their own staffs to act as technicians for testing on sites their SCC mixes. A very important aspect was that each of the large contractors are also developed their own testing devices and test methods(Bartos, 2000). In the early 1990’s there was only a limited public knowledge about SCC, mainly in the Japanese language. The fundamental and practical know-how was kept secret by the large corporations to maintain commercial advantage.
The SCCs were used under trade names, such as the NVC (Normal Vibrated Concrete) of Kajima Co., SQC (Super Quality Concrete) of Maeda Co. or the Biocrete (Taisei Co.
). Simultaneously with the developments in the SCC area, research and development continued in mix-design and placing of underwater concrete where new admixtures were producing SCC mixes with the performance of matching that of the Japanese concrete (e.g. University of Paisley / Scotland, University of Sherbrooke / Canada) (Ferraris, 1999).1.5 Applications of Self Compacting ConcreteSince the development of the prototype of self-compacting concrete in 1983, the use of self compacting concrete in actual structures has gradually increased. The main reasons for the employment of self-compacting concrete can be summarized as follows:To shorten the construction speedTo assure compaction in the structure: especially in confined zones where vibration Compaction is difficult.
To eliminate noise due to vibration: effective especially at concrete product plants.That means the current condition of self-compacting concrete is a “special concrete” rather than standard concrete. Currently, the percentage of self-compacting concrete in actual product of ready-mixed concrete in Japan is around is 0.
1% (Fig 1.2.).Fig 1.2. Annual production of SCC in Japan Total production of ready-mixed concrete in Japan in 1997 is 67,620 m x 1,000 mA 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.
1.3). The volume of the cast concrete in the two anchorages amounted to 2,90,000 m3. A new construction system, which makes full use of the performance of self-compacting concrete, was introduced for this. The was mixed at the batcher plant beside the site, and was pumped out of the plant. It was transported 200 meters through pipes were arranged on row 3 to 5 meters apart.
The concrete was cast from the gate valves located at 5 meter intervals along the pipes. These valves are 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. Fig.1.3.
Anchorage 4A of Akashi-Kaikyo BridgeSelf-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. The volume of the self-compacting concrete used in the tank amounted to 12,000 m3. The adoption of self-compacting concrete means thatThe number of lots decreases from 14 to 10, as the height of one lot of concrete casting was increased.The number of concrete workers reduced from 150 to 50.The construction period of the structure decreased from 22 months to 18 months.Self-compacting concrete is often employed in concrete products to eliminate the noise of vibration. This improves the working environment at plants and make it possible for concrete product plants to be located in the urban area.
The annual production of concrete products using self-compacting concrete exceeded 2,00,000 tons in 1996. 1.6 Tests for Fresh SCC MixesFresh SCC must possess its required levels the following key properties:1.6.1 Filling abilityThis is the ability of the SCC to flow into all spaces within the formwork under its own weight.
1.6.2 Passing abilityThis is the ability of the SCC to flow through tight openings, such as spaces between the steel reinforcing bars, under its own weight.
1.6.3 Segregation resistanceThe must meet the required levels of properties A ; B whilst its composition remains uniform throughout the process of the transporting and placing. Many tests have been used in successful applications of SCC. However, in all the projects the SCC was produced and placed by an experience contractor whose staff has been trained and acquired experience with interpretation of a different group of tests. In other cases, the construction was preceded by full-scale trails in which a number, often excessive specific tests used (Ouchi et al.
, 1996). The same tests are used under the construction site.1.7 Fibre Reinforced Self Compacting ConcreteConcrete technology has undergo revolutionary changes in the recent past years with the availability of various grades of cements and admixtures. Tough there are notable improvements , some problems still remained. When compared to materials like steel, these problems can be regarded as the drawback for the cementatious materials.
Concrete, which is a “quasi-fragile material” has the drawback of very low tensile strength and its almost zero. The solution to these two problems is to extent some addition of fibres. This is found to increase the energy absorption apart from bridging cracks retarding their propagation. Fibre Reinforced Concrete (FRC) has been used since 1960s and much investigation has been reported in this regard. Some important applications of FRC are bridge decks, pre-cast elements, pavements, industrial floors, some critical zones in RCC elements.The development of FRC has covered the entire range of concrete types using different varieties of fibres in Plain and RCC.
Further development of latest generation ‘concretes’ in the recent past needs to update the knowledge of behaviour of such concrete with the addition of fibres to make them more efficient and effective. One such latest generation of concrete is Self Compacting Concrete (SCC).Incorporation of fibres enhances the benefits of this special concrete at fresh state and the harden state, by this keeping states of viewing, concrete technologies have concentrated on investigating the mechanical and durability aspects of SCC introducing different types of fibres they are:Steel FibresNylon FibresPolypropylene FibresGlass FibresCarbon FibresAsbestos Fibres1.7.
1 Anti Crack Highly Dispersion (HD) Glass Fibres These fibres are designed chopped strand glass fibres prepared for uniform in the concrete to preventing the cracks and improve other characteristics of concrete. These fibres are used in the conventional concrete mixes at low dosages of 0.6 kg/m3. The high aspect ratio of these fibres has an effect on setting period while the large number of fibres gives a close distance between the fibres. Some important characteristics of these fibres are raid dispersion, easy and safe handling, facilitating homogeneous mix, control of cracks at fresh and harden states, and enhancing the durability.
1.8 Objectives and Scope of Present Investigation The main objective is the poor quality of vibration of concrete is in suggested locations, has often being a shortcoming of new traditional concrete. In such cases, SCC, which flows under in its own weight and does not require any need of vibration for compaction has revolutionized concrete placement.
SCC does not show any segregation and bleeding. Self-Compacted ability is largely affected by the characteristics of the materials and mix proportions.In the present study, it was require to achieve a properties of Self-compacting concrete with available materials in the laboratory.
Coarse aggregate with nominal size 10 mm was used. Viscosity Modifying Admixtures (VMA) is used to increase the suspension of power of aggregates and also to eliminate possible segregation. Fine powdered materials like fly ash is also used for eliminating the possible segregation.In the present study, Anti-crack highly dispersible glass fibres are used.
Durability studies like Acid attack factors, Sulphate attack factors, Sorptivity, and Acid-Durability factors for the Plain SCC and Glass Fibre SCC.