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The Importance of Using Self Compacting Concrete (SCC) in Engineering Applications
2 September 2011
The self-compacting concrete (SCC) was first produced and developed in Japan in 1988 to achieve durable concrete structures. Since it was produced in Japan, self-compacting concrete has been used in Japan, Europe and the United State of America because it has many benefits. This project will firstly be aimed to gain understanding more about SCC. Secondly, its importance uses regarding civil engineering fields and finally, it shows some advantages and possible disadvantages or drawbacks of using SCC. The main conclusions from this study is that using of SCC can reduce the production time period of project effectively, increasing the efficiency of the project production and improving work environment. Based on researches, it can be expected that using SCC will be increased progressively.
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1. Historical reasons for development of SCC 3
2. Ingredients and requirements of SCC mixture 4
3. Mechanism for achieving self-compactability of fresh concrete 6
4. Some applications of SCC 8
5. The advantages of using SCC in engineering implementations 10
6. Disadvantages and obstacles for the implementation of SCC 13
Concrete has become one of the most popular construction materials in worldwide, this because its raw materials can be provided widely in different places around the world and it is considered as the man-made material. Moreover, it has a key role to play in sustainable construction, since it needs less effort in its manufacture. These facts have not only led to many inventions in the field of concrete but have also led to many studies in order to improve its quality, reducing the cost of implementation and make the concrete friendly with the environment; besides, amending its aesthetical appearance when it is used as a structural construction members.
Nowadays, It can be seen that as a result of the research progressions concerned the concrete technology, various types of concrete such as High Performance Concrete, Ultra High strength concrete, Light Weight Concrete, Architectural Concrete and Self compacting concrete (SCC) are commonly heard not only among engineering Society but ordinary people also. In this prospective, the awareness and more knowledge regarding the concrete types should be considered. In fact, civil engineers should have more awareness and information about concrete types so as to be able to keep themselves abreast with the most recent developments, new technological innovations and future prospects.
Regarding the SCC, it can be considered as a one of the recent developing types of contemporary concrete. It is an innovative concrete mixture that can be mold into place without the use of vibrators to form a product free of empty spaces within the formwork. It is commercially known by various names such as self-consolidating concrete, self-compacting concrete, self-leveling concrete, or rheoplastic concrete (Mehta and Monteiro 2006: 476). The Prototype of (SCC) was first developed in 1988 in Japan, by Professor Ozawa in 1989 at the University of Tokyo (Okamura and Ouchi 2003).However; the name of SCC was given to this Prototype by Okamura of the University of Tokyo.
The aim of this project is to review or gain understanding about SCC in terms of its ingredients, mixture requirements and mechanism for achieving self-compatibility depending on previous researches. It also evaluates the results of using this type of concrete in some modern implementations. Finally, it demonstrates the benefits and potential disadvantages of using SCC in terms of safety, economy, and construction quality.
1. Historical reasons for development of SCC
According to Goodier (2003:405), the initial emergence of Self compacting concrete was in Japan in the late 1980s and its subsequent introduction into Europe through Sweden in the mid- to-late 1990s.
In the 1980s, the problem of the durability of concrete structures and acceptable compaction were the main subject that was considered in Japan; meanwhile, the skilled workers were required in order to achieve durable concrete and acceptable compaction. It seems to be the lack of the numbers of skilled workers in Japan's construction industry at this time was the main reason for decrease in the quality of construction work. One solution for achievement of durable concrete structures was the using of self-compacting concrete, which can be compacted into every corner of a formwork, completely by means of its own weight and without the need for vibrating compaction. This means that it can be gained sustainable concrete structures and acceptable compaction by using minimum number of workers and equipment (Fig.1).Moreover, the use of SCC was also implemented to provide economic, social and environmental benefits over conventional vibrated concrete construction.
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(Okamura and Ouchi, 2003:5)The conception of this type of concrete was proposed by Professor Hajime Okamura in 1986 who Studied the development of self-compacting concrete and comprised a fundamental study on the workability of concrete (Okamura and Ouchi 2003).
2. Ingredients and requirements of SCC mixture
With respect to its composition, SCC includes the same components as conventionally vibrated normal concrete, which are cementitious materials, different sizes of aggregates, and water, additives and admixtures. On the other hand, in the SCC, high volume of super plasticizer for reduction of the liquid limit and for better workability, the high powder content as "lubricant" for the coarse aggregates, as well as the use of viscosity-agents to increase the viscosity of the concrete have to be taken into account (Dehn et al. 2000).
Regarding to the requirements of SCC mixture, a concrete mix can only be classified as SCC if the requirements for all the following three workability properties are fulfilled which are filling ability, Passing ability, and Segregation resistance (Abdul Hameed 2005).
Filling ability or flow ability means that, the SCC must have the ability of to fill a formwork entirely under its own weight. According to the European Guidelines for Self Compacting Concrete (2005: 20), the tests that are used for assessing filling ability of the fresh SCC are Slump-flow, and Kajima box.
Passing ability means that the SCC must have the ability to flow through restricted places, such as spaces between steel reinforcing bars, sharp corners, and small openings, without using any means of vibrators (to flow by its own weight), also without produce any blocking or hindrance during its use. According to the European Guidelines for Self Compacting Concrete (2005: 20), the tests that can be used to determine passing ability are U-box, L-box, Fill-box, and J-ring test methods.
Segregation resistance means that the SCC must meet the filling ability and passing ability with Homogeneous composition during and after the process of transport and placing.
It is generally accepted that the three main properties of fresh self-compacting concrete must then be maintained for required period of time after mixing.
3. Mechanism for achieving self-compact ability of fresh concrete
Okamura and Ouchi (2003:5 ) suggested that In order to achieve self -compact ability of fresh concrete, deformability of past ( mortar) and resistance to segregation between aggregate particles and mortar when the concrete flows through the restricted zone of reinforcing bars, have to be considered.
Okamura and Ozawa (1995) have utilized the following methods to achieve self-compact ability of fresh concrete. Firstly, they limited the amount of aggregate content (coarse aggregate Â 50% of the concrete volume and sandÂ 40% of the mortar volume).Secondly, they used low water/powder ratio, with higher dosage of superplasticizer admixture. According to SHETTY (2005:573), using superplasticizer which is used with concrete components as an essential chemical admixture, leads to increase the workability of fresh concrete without regard to reducing water content in the concrete mixture.
When concrete is placed in to the formwork, the relative distance between the concrete particles will decrease. As a result, the frequency of collision and contact can increase and consequently, internal frictions between concrete particles also increase, especially near hindrances. Research has found that the energy which is required for flowing the concrete during the casting is consumed by the increased internal friction between particles, and resulting in blockage of aggregate particles to flow through confined places. For this reason, limiting amount of coarse aggregate, whose consume more energy, to a level lower than normal will increase the efficiency of concrete flow ability and avoid concrete particles from this kind of blockage.
To prevent the interlocking (blockage) incidence of coarse aggregate when the concrete is poured through obstacles, highly viscous past is required. Since the concrete which has a high viscosity prevents localized increases in internal stress as a result of the approach of coarse aggregate particles (Okamura and Ouchi 2003:6).
4. Some applications of SCC
Based on the researches which have been performed on SCC over the last two decades, it can be said that the use of SCC has been increased dramatically. It is commonly used in place of traditional concrete not only to reduce time of construction projects but also to reduce the cost of construction projects. According to Okamura, "Whatever conventional concrete can do, SCC can do better, faster, and cheaper, especially for concrete elements with special textures, complex shapes, and congested reinforcements" (Mamaghani et al. 2010:5). It can be seen that there are a range of using of SCC around the world. Many researches show that this type of concrete is commonly used within cast-in-place (in field) and precast construction. Furthermore, it is also used in the structural and architectural concrete sections where the tightness of steel reinforcement and /or surface quality is required. However, other implementations of SCC include drilled piers, caissons, bridge abutments and walls.
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In Japan, the first use of SCC was in 1991 for the construction of bridge towers (Daczko and Vachon, 2006:638). While the first large-scale project using SCC was the Akashi-Kaikyo Bridge, which completed in 1998. It is estimated that the construction time of this project was reduced by 20%, from 2.5 to 2 years due to the use of SCC (ibid.). A second implementation was performed, in 1998, for the building of the walls of a natural gas tank for the Osaka Gas Company. The estimated amount of SCC that was been using during the construction is approximately 12000 m3. It was observed that the construction time of the project was also shortened from 22 to18 months meanwhile; the number of labours worked in the project was reduced from 150 to 50 (ibid.).
In Sweden, the Sodra Lanken Project (SL) was one of the largest infrastructure projects that were implemented by using SCC. The total cost of this project was estimated at approximately 800 million USD. Ouchi et al. (2003) point out that the SL project included seven major junctions, with bridges, earth retention walls, tunnel entrances and concrete box tunnels; the overall length of the rock tunnels were 16.6 kilometers. Also, the volume of concrete used in the project estimated at 225,000 cubic meters. It was observed that SCC was used in the project because it included structural sections that were required high demands on aesthetics, and at the same time they were difficult to compact by normal vibration.
Recently, the New York State Department of Transportation (NYSDOT) has used significant amounts of SCC. Mamaghani et al. (2010: 5) report that "the current projects in the New York State include the use of SCC for prestressed, high performance concrete bridge beams on the Brooklyn-Queens Expressway from 61st Street to Broadway in New York City". According to a NYSDOT official, "The performance of SCC has been excellent. NYSDOT is achieving very good quality with a minimum of defects. There has been a slightly higher cost for admixtures, but NYSDOT saves on labor".
It can be anticipated that, in the future, concrete elements or buildings will be designed considering SCC from the start with shapes, textures, and structures that would be impossible to achieve with normal concrete.
6. The advantages of using SCC in engineering implementations
Improved concrete construction environment
The SCC is considered an environmentally friendly material. Firstly, regarding the construction site, the SCC can eliminate the need for vibration to compact the concrete; since SCC has ability to fill the form work completely under its own weight. The compaction of fresh concrete by vibration is generally recognized as a heavy physical job and an unpleasant activity in the concrete construction process. Moreover, using the vibration can also cause high noise levels which are not good for public health; especially the health of the operators. Secondly, the acceleration generated by vibrators can reach 0.70 to 4m/s2 and has potential to injure the vibrator operator. Hence, eliminating the vibration significantly improves health and the environment on a concrete construction site (Li 2011). It can be said that some skills and experience are needed for vibrator operators in compacting fresh concrete in order to gain a satisfactory concrete. Lack of experienced workers in many regions may lead to decline the quality of concrete. It is generally accepted that the application of SCC can solve this problem and ensure consistent high quality for concrete structures.
With respect to the required number of labours on the construction project compared with conventional (normal) concrete types, the SCC needs less number of workers and shorter times for placing (casting). It is generally believed that by using SCC, the process of production can be accelerated, and the quality of concrete structures can be improved as well. According to Peterson (2008:66), the elimination of vibration means "rationalised casting technique with less need of personnel and/or more rapid production cycles and thereby presumptively reduced production costs."
New opportunities with respect to architectural and structural applications
SCC benefits structural and architectural applications. New types of structural elements, which were not possible to be produced by traditional concrete, can be produced by using SCC because these types of structural elements include different types of steel or concrete structural elements with more complex shapes, which are thinner with a much heavier reinforced cross section.
With regard to the construction of heavy structure (mega structure) fields such as multistory building, high rise building or ultra-high rise building, since the SCC does not need any vibrators and finishing surface tools, it means that it reduces lifting of heavy equipment and reduces handling of fresh concrete by labours. Also, there is no need of repairs to hardened concrete as there are no chances for producing any remarkable voids.
With respect to construction consideration, Ouchi et al. (2003:18) found that "When placing a new layer of SCC on old SCC, the bond between the old and new SCC is equal to or better than in the case of conventional vibrated concrete". On this basis it may be inferred that the normal vibrated concrete needs vibration to provide well compacted concrete, while SCC will not need the vibration and consequently it will not destroy the concrete.
Regarding the durability of SCC, researches have shown that ingredients of SCC and the avoidance of using vibration have a significant role in improving the microstructure of the concrete (Holton 2004). The ingredients and avoidance of using vibration acquires the concrete many benefits. First, they gain the concrete lower surface permeability and absorption against harmful chemical materials such as chlorides and carbon dioxide which have a negative effect on the durability of concrete. Second, they improve the resistance of the concrete against freezing- thawing which the concrete may be subjected during different seasons. Finally, they play a key role in producing a good bond between the concrete materials and the steel reinforcement after the concrete hardening.
Another advantage of the SCC is that, it can be used when the normal placement of concrete is difficult e.g. tunnel linings, heavily reinforced structures, prestress concrete structures and other inaccessible places. One example is the design of the Millennium Tower in Vienna, which is described as it had been impossible to build without using SCC (Peterson 2008:66). Moreover, Using SCC for the construction of tall building, bridge tower, and tunnels leads to increase the use of concrete pumping. Nowadays, by using special high- pressure pumps, SCC can be pumped distance as far as 1400m horizontally or as high as 420m vertically (Li 2011:130).
7. Disadvantages and obstacles for the implementation of SCC
Absence of acceptance SCC
Despite the fact that the SCC has a great numbers of stunning and successful applications; it was only hesitantly accepted by the industry. This could be attributed to a number of reasons (Holton 2004).First of all, the standard specification tests that were used for testing and evaluation conventional concrete was no longer useful for SCC. As a consequence, it was very difficult to smoothly put SCC into practice; also, before using SCC, the intensive preparation efforts were required in the laboratory in order to achieve an appropriate quality. Second, the sensitivity of SCC is increased proportionally with change of its mix proportions (particularly the water content) or with change of the environment such as temperature variations. This increased sensitivity produces an extra hindrance to the wider use of SCC as it puts a prices working at the production field (ibid.).
Potential disadvantages and obstacles of using SCC
Based on the recent studies, despite the possible benefits of SCC compared to normal concrete, it can be seen that the implementation of the SCC is still noticeably limited. According to Peterson (2008:67), the obstacles for increased application of this type of concrete can be attributed to technical and non- technical aspects. In the SCC, the quality of mix design and the conditions of casting concrete should be considered; slight differences in mix proportions or during its casting may cause a number of technical quality problems that may pose as hindrances for future use of SCC.
Despite of the intensive and extensive studies on SCC, it can be seen that there are still some unsolved technical problems and a great number of technical difficulties related to SCC. Peterson (2008:67) described these problems in many aspects; such as the problems related to the quality production of the ready-mix design and the Problems related to the hardened SCC especially when there is a low surface quality which leads to reduce fire resistance due to spalling, increase cracking owing to early shrinkage. It is widely accepted that SCC tends to dry faster than traditional concrete, since there is slight or no bleeding water at the surface before it is hardened. For this reason, SCC should be cured when it is practicable after placement to avoid the incidence of surface shrinkage cracking. Moreover, Peterson (2008:71) indicates that although the SCC has been considered as one of the main important invented technology for a more rational way of building with cast in-situ concrete; there are some projects where SCC has led to create technical problems such as concrete segregation with non-acceptable surfaces, concrete cracking owing to plastic shrinkage and formwork failure due to the high form pressure that may be produced as a result of the flow ability feature of SCC. It would appear that further studies are required to achieve a durable and fully satisfying concrete product.
It can be concluded that SCC helps to improve the environment of the construction locations (reducing noise produced in the plants and construction fields) and reducing the exposure of labours (Reduction manpower) where concrete is being casted. In the other word, the SCC is an ideal type of concrete that can be used for narrow spaced of reinforcement and architectural demanding sections, or, in more general , for all structural applications where require higher efforts in order to gain sufficient compaction. Furthermore, it can be seen that SCC offers many other benefits for the precast, prestressed concrete sector and for cast-in-place construction such as eliminated problems related to vibration, faster construction, gaining higher strength and better quality for the concrete after hardening.
On the other hand, Although SCC can have many advantages and significant effects on the engineering applications such as its importance role in reducing the project construction time period, it can be said that special attention should be concentrated on particularly, in terms of selection the suitable ingredients of SCC mixture before the production (mix design of SCC).
It can still be expected that, in the future, using SCC will help engineering designer (architects and structural engineers) to design concrete sections or structural elements which would be impossible to achieve with conventional concrete. Based on these facts it can be said that SCC will have a bright future.