Self-compacting concrete (SCC) is an innovative concrete that does not require vibration for placing and compaction. It is able to flow under its own weight, completely filling formwork and achieving full compaction, even in the presence of congested reinforcement. The hardened concrete is dense, homogeneous and has the same engineering properties and durability as traditional vibrated concrete, Self-compacting concrete (SCC) possesses enhanced qualities and improves productivity and working conditions due to elimination of compaction. SCC is suitable for placing in structures with congested reinforcement without vibration and it helps in achieving higher quality of surface finishes, in adequate homogeneity of the cast concrete due to poor compaction or segregation may drastically lower the performance of mature concrete insitu. SCC has been developed to ensure adequate compaction and facilitate placement of concrete in structures with congested reinforcement and in restricted areas. The development of self-compacting concrete (SCC) marks an important milestone in improving the product quality and efficiency of the building industry. SCC homogenously spreads due to its own weight, without any additional compaction energy and does not entrap air. SCC improves the efficiency at the construction sites, enhances the working conditions and the quality and the appearance of concrete. Fibres bridge cracks and retard their propagation. They contribute to an increased energy absorption compared with plain concrete. Self-compacting fibre reinforced concrete (SCFRC) combines the benefits of SCC in the fresh state and shows an improved performance in the hardened state compared with conventional concrete due to the addition of the fibres. Due to its special characteristics new fields of application can be explored.
Keywords: rheology, high strength concrete, fresh concrete, viscometer, self-compacting, viscosity, mix design, durability, consolidation, ready mixed concrete, innovation, research, use, deformation, bond, vibration, dust, economy, yield value, segregation, shear rate.
Самоуплотняющийся бетон (SCC) представляет собой инновационный бетон, который не требует вибрации для размещения и уплотнения. Он способен течь под собственным весом, полностью заполняя опалубку и достигая полного уплотнения даже при наличии переполненного арматуры. Затвердевший бетон является плотным, однородным и имеет те же технические свойства и долговечность, что и традиционный вибробетон. Самоуплотняющийся бетон (SCC) обладает повышенными качествами и улучшает производительность и условия работы благодаря устранению уплотнения. SCC подходит для размещения в конструкциях с перегруженной арматурой без вибрации и помогает в достижении более высокого качества отделки поверхности, при достаточной однородности литого бетона из-за плохого уплотнения или сегрегации может резко снизить производительность зрелого бетона. SCC был разработан для обеспечения адекватного уплотнения и облегчения размещения бетона в конструкциях с усиленным армированием и в зонах ограниченного доступа. Развитие самоуплотняющегося бетона (SCC) знаменует собой важную веху в улучшении качества и эффективности продукции строительной отрасли. SCC однородно распространяется из-за собственного веса, без дополнительной энергии уплотнения и не захватывает воздух. SCC повышает эффективность на строительных площадках, улучшает условия работы, качество и внешний вид бетона. Волокна разрушают трещины и замедляют их распространение. Они способствуют увеличению поглощения энергии по сравнению с обычным бетоном. Самоуплотняющийся волокнистый бетон (SCFRC) сочетает в себе преимущества SCC в свежем состоянии и показывает улучшенные характеристики в закаленном состоянии по сравнению с обычным бетоном благодаря добавлению волокон. Благодаря специальным характеристикам могут быть изучены новые области применения.
Ключевые слова: реология, высокопрочный бетон, свежий бетон, вискозиметр, самоуплотнение, вязкость, дизайн смеси, долговечность, уплотнение, готовый смешанный бетон, инновации, исследования, использование, деформация, связь, вибрация, пыль, экономичность, урожайность, сегрегация, сдвиг ставка.
Introduction and background of SCC: Self-Consolidating Concrete has properties that differ considerably from conventional slump Concrete. SCC is highly workable concrete that can flow through densely reinforced and complex structural elements under its own weight and adequately fill all voids without segregation, excessive bleeding, excessive air migration (air-popping), or other separation of materials, and without the need for vibration or other mechanical consolidation, Self-consolidating concrete is a highly flow able concrete that spreads into the form without the need of mechanical vibration. Self-compacting concrete is a non-segregating concrete that is placed by means of its own weight. The importance of self-compacting concrete is that is maintains all concrete’s durability and characteristics, meeting expected performance requirements. In certain instances the addition of super plasticizers and viscosity modifier are added to the mix, reducing bleeding and segregation. Concrete that segregates loses strength and results in honeycombed areas next to the formwork. A well designed SCC mix does not segregate, has high deformability and excellent stability characteristics, SCC was developed first in Japan in the late 1980s to be mainly used for highly congested reinforced structures inseismic regions (Bouzoubaa and Lachemi, 2001). As thedurability of concrete structures became an important issue in Japan, an adequate compaction by skilled labors was required to obtain durable concrete structures. This Requirement led to the development of SCC and its development was first reported in 1989 (Okamura and Ouchi, 1999).
Self-Compacting Concrete Properties: Self-compacting concrete produces resistance to segregation by using mineral fillers or fines, and using special admixtures. Self-consolidating concrete is required to flow and fill special forms under its own weight, it shall be flow able enough to pass through highly reinforced areas, and must be able to avoid aggregate segregation. This type of concrete must meet special project requirements in terms of placement and flow.
Self-compacting concrete with a similar water cement or cement binder ratio will usually have a slightly higher strength compared with traditional vibrated concrete, due to the lack of vibration giving an improved interface between the aggregate and hardened paste. The concrete mix of SCC must be placed at a relatively higher velocity than that of regular concrete. Self-compacting concrete has been placed from heights taller than 5 meters without aggregate segregation. It can also be used in areas with normal and congested reinforcement, with aggregates as large as 2 inches.
Self-Compacting Concrete Uses: Self-compacting concrete has been used in bridges and even on pre-cast sections. One of the most remarkable projects built using self-compacting concrete is the Akashi-Kaikyo Suspension Bridge. In this project the SCC was mixed on-site and pumped through a piping system to the specified point, located 200 meters away. On this particular project the construction time was reduced from 2.5 years to 2 years.
Self-Compacting Concrete Benefits: Using self-compacting concrete produce several benefits and advantages over regular concrete. Some of those benefits are:
‒ Improved constructability.
‒ Labor reduction.
‒ Bond to reinforcing steel.
‒ Improved structural Integrity.
‒ Accelerates project schedules.
‒ Reduces skilled labor.
‒ Flows into complex forms.
‒ Reduced equipment wear.
‒ Minimizes voids on highly reinforced areas.
‒ Produces superior surface finishes.
‒ Superior strength and durability.
‒ Allows for easier pumping procedure.
‒ Fast placement without vibration or mechanical consolidation.
‒ Lowering noise levels produced by mechanical vibrators.
‒ Produces a uniform surface.
‒ Allows for innovative architectural features.
‒ It is recommended for deep sections or long-span applications.
‒ Produces a wider variety of placement techniques.
Factors Affecting Self Compacting Concrete: Using self-compacting concrete must not be used indiscriminately. These factors can affect the behavior and performance of self-compacting concrete:
‒ Hot weather.
Long haul distances can reduce flow ability of self-compacting concrete.
‒ Delays on jobsite could affect the concrete mix design performance.
‒ Job site water addition to Self-Compacting Concrete may not always yield the expected increase in flow ability and could cause stability problems.
Mix design of SCC: Before any SCC is produced at a concrete plant and used at construction site the mix has to be designed and tested, During this evaluation the equipment and the local Materials used at the plants have to be tested to find new concrete mixes with the right mixing sequences and mixing times valid for that plant and material used and also suitable for the element to be cast, Various kinds of fillers can result in different strength, shrinkage and creep but shrinkage and creep will usually not be higher than for traditional vibrated concrete. A flow-chart describing the procedure for design of SCC mix is shown in Figure below:
Figure: SCC mix design procedure
Casting on site: It is divided into 4 following sections:
1- Planning: The process of casting SCC can be mechanized to a great extent. Increased productivity, lower cost and improved working environment is achieved. A minimum of manual interaction in the process is however necessary. Based on formwork configuration, reinforcement, temperature, casting equipment, casting speed etc., the persons in charge of the concrete supply and the form filling respectively have to plan and jointly agree on SCC workability data, including accuracy, open time, casting speed etc. In more complex industrialized casting operations, the planning of flow of concrete can be computer modeled in order to optimize the rheological material data to the specific formwork, the reinforcement configuration and the sequence and methods of casting. The planning also includes agreement on the quality assurance procedure, test methods, frequency of test as well as of actions taken as results of tests. The planning should also address the corrections of the mix that might be done at the casting site through extra dosage of plasticizer. Even if there will always be options of buying SCC off the shelf as standard products, the strongest benefits and highest profits will come from optimizing the fresh concrete as an integral approach in an industrialized process designed for the specific situation at hand. Even if there is a significant reduction in the needed skill for the actual casting when SCC is applied, the need for skills in planning, preparation and quality assurance is raised.
2-Filling of Formwork: SCC is a liquid suspension following the rules of fluid mechanics while vibrated concrete is a granular mass requiring vibration to be compacted. SCC is well suited for pumping and can be fed through valves under pressure into vertical formwork. This technique is frequently used when casting complex enclosed volumes where release from above is not possible or no limited entrance to the interior of the form work is possible, nor vibrating it by hand tools. Pumping SCC into the form work from underneath has proven to be beneficial when high demands of aesthetics are of importance. The problem with pores and pot-holes also tends to be less when the concrete has been fed from underneath through valves. Experience from pressurized castings of 30+vertical meters exists from practice. If the pipe-based feeding system used includes furcating, the concrete flow chooses the easiest way through the piping system. This may result in parts of the concrete not moving, thereby preventing the concrete to fill the form work uniform and symmetrically Vertical formwork can also be cast by dropping from above using pumps or crane skips. Experience from dropping heights of 8 meters exists but 1–3meters will be more common. Flat and shallow formwork such as slab and decks are most often filled from above even if in certain situations. E.g. in industrial production, casting through valves by pumping might be an attractive option. For flat and shallow structures the dropping height is about0.5–0.8 meters. High dropping heights require a stable mix to counteract the risk of segregation and damage of the air pore system.
3-Finishing: Finishing operations can be more difficult for SCC due to the thixotrophy, sometimes sticky behavior. The absence of bleeding makes it even more difficult and the finishing operations should be related to the setting time of the mix in actual conditions. The characteristics of the SCC mix, and the skill and timing of the finishers during placement affect the quality of the surface of slab cast. The general experience seems to be that conventional tools and ways to finish the upper surface can be used working with SCC but sometimes finishing tools with other surface materials are used. It is wise to expect this operation to take a little longer in comparison with the finishing of conventional vibrated concrete.
4- Curing: SCC mixes are characterized by a moderate to higher amount of fines in the formulation, including various combinations of powders such as Portland cement, limestone filler, fly-ash or ground granulated blast furnace slag. Thus, there might be very little or no bleeding and the concrete will sometimes be more sensitive to plastic shrinkage cracking. The tendency of plastic shrinkage increases with the increase in the volume of fines. This situation is sometimes more complicated if the setting time is delayed because of the admixture effect, and the concrete remains many hours in the fresh state. Curing to counteract longer term shrinkage is to be handled like what is done for vibrated concrete. It should be observed that due to a lower permeability of SCC, the drying rate and following from that also the shrinkage rate might be slower.
Conclusions: Self Compacting Concrete has become an industrial product. It is mass produced in a reliable way using components that, although may vary from place to place, are usually available. Performances in real applications are as good as expected in terms of compressive strength, homogeneity and filling capacity. Pouring SCC needs minor changes in building sites, while gives engineers a largely higher confidence of material performance in the structure. Perfect representatively of specimens help evaluating the final result of the construction process. Further developments are going on, showing good flexibility of SCC to be tailored to suit different needs and to enhance his performances.
References:
- Ozawa K., Kunishima, M., Maekawa, K. and Ozawa, K.: Development of High Performance Concrete Based on the Durability Design of Concrete Structures, Proceedings of the second East-Asia and Pacific Conference on Structural Engineering and Construction (EASEC-2), Vol. 1, pp. 445–450, January 1989.
- Kodama, Y. Current Condition of Self-Compacting Concrete, Cement Shimbun, No. 2304, Dec 1997.
- Ozawa, K. and Ouchi, M.: Proceedings of the International Workshop on Self-Compacting Concrete, Kochi, March 1999.
- Okamura H, and OZAWA, K.: Mix-design for Self-Compacting Concrete, Concrete Library, JSCE, No. 25, pp.107–120, June 1995.
- Walraven J, Structural aspects of Self Compacting Concrete. Proceedings of 3 rd International RILEM Symposium. August 2003, Reykjavik.
- Joe Nasvik, “The ABCs of SCC”, Concrete Construction, January 2002.
- Kamal H. Khayat and Joseph Assaad, “Air-Void Stability in Self-Consolidating Concrete,” ACI Materials Journal, July-August 2002.
- Aaron.w Saak, Hamlin M. Jennings, and Surenda Pl Shah, “New Methodology for Designing Self-Compacting Concrete”, ACI Materials Journal, v.98 no. 6, NovemberDecember 2001.
