Advanced Self-Compacting Concrete - Essay Example

Advanced material: Self compacting concrete: Need of this material: Concrete is one of the most fundamental construction materials in the present age. Its use is widespread given the multifarious advantages it offers in comparison to other construction materials as steel or wood. Concrete has been in use for quite a while now but there have always been problems vibrating it in condense reinforcement nets in structural elements. Closely spaced reinforcement bars enclosed in complicated structural elements allow very less room for engineers to run the vibrators between the reinforcement bars in order to ensure that fluid reaches each and every corner of the element being cast. Many times, efforts would go in vain resulting into honeycombed elements upon removal of the formwork. In conventional concreting process, not only is there a chance of segregation and honeycombing in the concrete structural element, but also, chances that concrete would not even approach certain corners are quite fair causing steel to be exposed to open air and facilitating rusting and hence an altogether loss of strength of the structural member. This together with other difficulties in the preparation of conventional concrete and the exaggerated length of time and effort consumed in vibrating it called for a need to have such a model of concrete that would not require vibration to uniformly reach every corner in the element. Also, vibration was a tedious process and required effort of a large crew that would add a lot to the total cost of concreting. In addition to that, vibrated concrete leads to differential compaction and hence, varying durability along the length of the member. In response to these problems, after years of research and experimentation, engineers came up with such a model of concrete that did not require any vibration and was intrinsically self compacting in nature. Today, use of self compacting concrete is widely employed in structures and elements of dense and complicated reinforcement design. Self Compacting Concrete (SCC): Overview: “Self-compacting concrete was first developed in 1988 to achieve durable concrete structures.” (Okamura and Ouchi, 2003). Because of the manifold enhancement in its performance, self compacting concrete (SCC) is also referred to as high performance concrete (HPC). HPC can be defined as, “Concrete that meets special performance and uniformity requirements that may not always be obtained using conventional ingredients, normal mixing procedures and typical curing practices.” (American Concrete Institute, 1997). The requirements that ACI refers to include but are not limited to ease of non-vibrational casting of concrete without segregation, low preparation and concreting cost and enhanced properties in terms of exaggerated strength and stability when exposed to unfavorable conditions. Besides, with the use of SCC in construction, the shortage of skilled labor faced in the conventional concreting process is overcome with an additional benefit of the implementation of an environment friendly alternative given the elimination of all the noise pollution caused by the intense vibrating operation in the traditional concreting. Hence, SCC is a material that flows sufficiently to overcome segregation and undergoes a self compaction process without the aid of vibrators. Moreover, it fully conforms to the high ethical standards of the modern world by causing a reduction in the noise pollution. Discovery of SCC as a useful construction material: The credit of discovering SCC initially goes to the Professor Okamura and his team who worked in the Tokyo University of Japan in the last decade of the 20th century. The concrete they developed was in powder form and after judging its properties, they named it the self compacting concrete (SCC). (Rizwan et al, 20-). Later, other scientists and engineers contributed towards the development of awareness and knowledge about various properties of the SCC. After considerable research and experimentation, scientists reached at the most appropriate flow tests and equipment for testing SCC through mutual consensus. Process of manufacturing SCC: Raw materials: Chief raw materials required to manufacture SCC are superplasticizers, continuous grading of aggregates, secondary raw materials (SRM) that include but are not limited to limestone powder (LSP), rice-husk ash (RHA), fly-ash (FA), ground granulated blast furnace slag (GGBFS) or silica fume (SF) and powder. (Rizwan et al, 20-). Significance of the ingredients: The significance of employing a continuous grading process of aggregates is that the practice serves to minimize the extent of voids in the material thus improving the packing of particles within it. It is always preferred to have minimum voids in the material because of the fact that the lesser the number of voids in it, the lesser amount of water is needed to make the material as workable as required for casting a certain element. This results into a saving in the cost of cement and also minimizes the shrinkage in the element. The superplasticizers function to improve the durability of the concrete structure. It is the desired strength and intended durability not only in the hardened state but also in the fresh state of the final concrete structure that dictate which powder from the range of powders in the SRMs needs to be chosen for making the SCC. SRMs serve the same purpose as that of the superplasticizers and the continuous grading of aggregates i.e. reduce the amount of water in the concrete. Not only this, when added to SCC as ingredients, they actually replace cement in the concrete, thus bringing down the overall cost of preparation of SCC. An added advantage gained by adding SRMs into SCC is the lowered heat of hydration which minimizes shrinkage in the SCC. These advantages brought by the addition of SRMs into the concrete mixture is specially important because cement usually is the most costly, and energy intensive material in the concrete and is also quite unfriendly towards the environment given the large amount of heat it dissipates while undergoing the exothermic reaction as the concrete hardens. SRMs intrinsically acquire cementitious nature and are easily derived as by-products from the industries that need just a minor processing to refine them and render them useful for the SCC. Recently, the utilization of Fly Ash in its low calcium form in SCC has been widely increased because it is one of those materials that have always been readily available in the market. Because of these reasons, environmentalists emphasize upon using SRMs in place of cement. Particulars: The process of manufacturing SCC is much the same as that of conventional concrete, except for the continuous grading of aggregates and a longer mixing time. “SCC normally requires a more efficient mixing, e.g. longer mixing time, to make sure that all constituents have been mixed thoroughly.” (Skarendahl and Billberg, 2006). Properties of SCC: Pouring conditions: In order to prepare SCC for pouring, it is important to keep its slump high. SCC shows minimal segregation despite the large slump. Classification: “In the literature, SCC has been classified as powder type, viscosity agent type and the combination type each differing in the way segregation resistance is achieved.” (Rizwan et al, 20-). The most widely used of all three in the structural construction is the combination type of SCC that is based on a moderate content of powder with an adequate amount of the viscosity agent. Strength: With the use of SCC, structural strength as high as 100 MPA is achievable contrary to the 1940 and 1970 maximum strength limits amounting to 20 MPA and 40 MPA respectively with the use of conventional concrete. (Rizwan et al, 20-). SCC fundamentally derives its high strength properties from the superplasticizers that are used in manufacturing the SCC. It is because of the ease of availability of superplasticizers in the market that it has been possible for engineers to prepare SCC at a large scale locally. Flowability: Workability of conventional concrete is enhanced by adding water to it. However, this is not a reasonable option since the addition of water to concrete reduces its compressive strength. Therefore, instead of using water to improve the workability of SCC, the purpose is achieved by the addition of certain surface active admixtures to the concrete. use of fillers and admixtures increases the surface area of concrete and hence increases the flowability. “The main functional requirements of fresh SCC are described by filling ability, resistance to segregation and passing ability.” (Petersson and Skarendahl, 2000, p. 3). Merits of SCC: SCC offers a number of benefits over the traditional concrete some of which are listed below: Cost reduction: Although the cost of SCC is higher than that of conventional concrete because of greater number of ingredients in it, yet the increase in cost is balanced by the reduction in cost caused by elimination of vibration and repairing of the structural element after removal of the formwork. Repairing the honeycombed concrete, filling the voids and replacing the corroded steel with new portions not only comes as a compromise upon the quality and strength of concrete but also increases the overall cost of concreting operation many times. Hence, it is not rational to think about SCC as an expensive concrete as the effect is balanced in the long run. Time reduction: Time of the overall concreting process is reduced. Accuracy of the results: Results are more accurate because of lack of dependence on humans to vibrate it. Improved finishing: Use of SCC is high recommendable for casting elements of architectural significance specially when they do not have to be plastered. This is so because SCC exhibits very less amount of bleeding when compared with the conventional concrete that allows the SCC to provide a more finished and neat surface to the element being cast. Improved strength: SCC exhibits both high strength and enhanced durability. Improved productivity: Use of SCC results into improvement in productivity and better work environment with elimination of the need to vibrate the concrete. This fundamentally becomes possible due to the avoidance of the vibration activity. Also, workers feel safe because of the simplification of the overall concreting process and hence show better productivity. Ease of filling: SCC exhibits excellent filling ability and maintains homogeneity without segregating. Ease of passing: Passing ability of SCC is improved manifold with the addition of superplasticizers, continuously graded aggregates and SRMs to the concrete mixture. With improved workability, the concrete passes through complicated framework of reinforcement with ease. Segregation avoidance: Unlike conventional concrete, SCC strongly maintains its homogeneity during and after the transportation of concrete from the batching plant to the construction site. Ease of pumping: It takes lesser effort and energy to pump SCC in comparison to conventional concrete. Health and safety considerations: Use of SCC causes avoidance of the disturbance caused to the circulation of blood in the hands of workmen vibrating the concrete. Flow of blood in the fingers of workers responsible for conducting the operation of vibration is hindered as a result of continuous and intense vibration activity. As the time consumed in the concreting process is reduced and also, the level of complexity of the process is lowered, the number of accidents during the operation is reduced accordingly. Improved quality: Use of SCC in place of other concretes increases the quality of workmanship. Best standards of quality assurance and quality control are achievable. Ethical considerations: Use of SCC instead of conventional concrete fully conforms to the high ethical standards in that the working environment is improved manifolds with a reduction in noise pollution. Applications of SCC: SCC has been widely used as a good construction material for the last two decades. Names of some of the structures in which SCC has been used are mentioned below: Akashi-Kaikyo (Straits) Bridge anchorages: This is a very appropriate example of the use of SCC. It is a suspension bridge that is known to have the longest span among all bridges in the world. The length of its span is 1991 meters. The volume of concrete required to cast the two anchorages was as large as 290,000 cubic meter. (Alias, 2010). In order to cast concrete in such a massive volume, a special novel construction mechanism was hired in this case. The concrete was prepared in the batching plant as usual. Once the concrete was ready, it was discharged from the batching plant and was taken to the place of casting with the help of pipes which were two hundred meters long. Pipes were arranged in rows which were three to five meters distant from one another. Casting of concrete was achieved with the help of gate valves which were at a distance of no more than five meters from the pipes containing concrete. The gate valves were monitored in an automatic manner in order to make sure that the surface of the finished concrete comes out leveled. After the completion of process, it was noticed that the use of SCC in place of conventional concrete to cast the anchorages reduced the construction period by 20% by limiting the usual two and a half years of construction of anchorages to just two. (Alias, 2010). Wall of Osaka Gas Company tank: SCC was used instead of conventional concrete for the construction of “the wall of a huge LNG tank of Osaka Gas Company” in June of 1998. (Alias, 2010). In this case, the volume of SCC used for construction summed up to 12,000 cubic meter. Self compacting concrete was used for the construction of this wall because of the following reasons: The choice of SCC served to reduce the number of required workers by 100. There was a decrease in the number of lots required because of the exaggerated height made possible by the use of SCC. The construction duration dropped down to 18 months against the anticipated 22 months of the construction of the structure. Other application areas: SCC is used for the construction of structures in highly sensitive urban areas where the stakeholders are extremely possessive about the maintenance of healthy environment. Because of the minimal pouring noise, SCC is the most appropriate for the construction of concrete structures in such areas. “In 1996, the annual production of concrete products that make use of self compacting concrete was over 200,000 tons.” (Alias, 2010). “It currently makes up about 5% of the Japanese concrete market and around 15% of the Danish and Swedish markets.” (www.sustainableconcrete.org.uk, n.d.). Precautions: SCC requires extremely trained and skilled personnel to manufacture the concrete because its properties are truly dependant on the nature and proportion of the constituent materials. It is imperative that the ingredients are mixed in the right proportion in the right environment. Also, SCC requires several tests to be performed in order for the manufacturer to be sure that the material being synthesized conforms to the specifications. Anticipated future of SCC: The advantages SCC offers in comparison to the conventional concrete make it quite suitable for long-term use in the construction industry. As the modern world places huge emphasis to the health and safety aspects of the work environment, the added safety and reduction in cost and time offered by SCC not only make it environmentalists’ best choice but also make it conducive to maximizing the owners’ profits and enhance their business. References: Alias, JA, 2010. “Self-Compacting Concrete Applications.” viewed 7 Sep 2010. . American Concrete Institute, 363R-92, Re-approved 1997. “State-of-the-Art Report on High-Strength Concrete”, ACI, Detroit, USA. Okamura, H and Ouchi, M, 2003. “Self-Compacting Concrete.” Journal of Advanced Concrete Technology. vol. 1, no. 1, pp. 5-15, 7 Sep 2010, . Petersson, O and Skarendahl, A, (eds.) 2000. Self-compacting concrete. France: RILEM Publications s.a.r.l. Print. Rizwan, SA, Bier, TA, Ahmad, H, 20-. “SELF-COMPACTING CONCRETE -A USEFUL TECHNOLOGY”. Pakistan Engineering Congress, 70th Annual Session Proceedings. Pp. 294 – 319. viewed 7 Sep 2010. . Skarendahl, A and Billberg, P, (eds.) 2006. “Casting of Self Compacting Concrete.” RILEM Publications S.A.R.L. viewed 7 Sep 2010. . www.sustainableconcrete.org.uk, n.d. “Self-compacting concrete.” viewed 7 Sep 2010. . Read More