A CRITICISM OF SELF COMPACTING CONCRETE

H Azamirad*, Islamic Azad University of Shabestar, Iran D Beheshti zadeh, Islamic Azad University of Shabestar, Iran

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30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 23 – 24 August 2005, Singapore

A CRITICISM OF SELF COMPACTING CONCRETE

H Azamirad*, Islamic Azad University of Shabestar, Iran D Beheshti zadeh, Islamic Azad University of Shabestar, Iran

Abstract Concrete is a most widely used construction material in the world. As the use of

concrete becomes more widespread the specifications of concrete like durability, quality, compactness and optimization of concrete becomes more important. Self-compacting concrete (S.C.C) is a very fluid concrete and a homogenous mixture that solves most of the problems related to ordinary concrete. Besides, S.C.C gets compacted under its own weight and there is no need for an internal vibrator for the body of the mold. This specification helps the execution of construction components under high compression of reinforcement.

Unlike other papers about S.C.C, which want to introduce this concrete and different test methods of it, this paper investigates the durability and strength of this concrete and studies the role of all important factors in durability of S.C.C by doing some examinations. Another part of this paper studies the obstacles in improvement of S.C.C and tries to offer some solutions as follows: a) Using lightweight aggregates in production of S.C.C in order to decrease the

risk of segregation and specific weight of this concrete. b) Study of production of S.C.C in ready-mixed concrete factories c) Study of using suitable and economic fillers in production of S.C.C considering the resources and talents of each country

The last part of this paper introduces some important projects executed with S.C.C in

U.S.A, France and Japan and gives a brief information about these projects and will provide the readers with reasons that why S.C.C is used in these projects.

Key words:S.C.C, Durability,Segregation

1. Introduction

Self-compacting concrete was proposed in 1988 for the first time in order to access stable concrete structure. Then, different studies were carried out to offer a reasonable mix design and suitable methods to control self-compaction test and introduce the unique characteristics of S.C.C to promote the use of this

concrete like ordinary concrete. Primary studies about S.C.C were carried out by Ozawa (1988) and Okamura [1] (1993) in Tokyo

University. There is no formal definition of S.C.C, but there is a theory saying that S.C.C is a concrete that: (1) Has good fluidity which causes self-compaction and does not need external energy (2) Remains consistent during and after concreting and easily moves within reinforcements. It should be mentioned that the characteristics of fresh and hardened concrete depend mainly on its mix

design. Having access to different sources to introduce S.C.C and different test methods of S.C.C, we have tried to study the advantages and disadvantages of this new concrete and offer a brief and useful criticism for the readers in this paper.

2. Study of Durability of S.C.C Durability of concrete includes its capability to confront the atmospheric conditions, attack of chemicals,

friction and any other destructive process. Sometimes instability of concrete requires very expensive repairs that are higher than the expenses of a good and acceptable design and sometimes the expenses of repairs are so high that destruction of the structure is more economic than its repair.

As we know, destruction factors of concretes can be classified in two groups: chemical and physical factors. Physical damages of concrete are caused mainly by overloading or shocks and we can use different metal fibers to solve problems resulted by physical factors. In order to solve the problems caused by chemical factors we have to study the durability of concrete affected by these factors.

2.1 Theoretical Study of Durability of S.C.C As we know, chemical factors penetrate the concrete and destroy it and in fact the susceptibility of the

concrete is directly related with its penetrability. It is clear that increasing the penetrability of concrete will increase its susceptibility against destructive chemical factors. Different factors can affect the penetrability of concrete the most important of which are improper concreting, insufficient compactness (vibration), lack of curing and improper concrete covering around steel bars.

High compactibility is one of the basic principles of formation of S.C.C and we can theoretically accept that S.C.C has more capabilities in compare with ordinary concrete to confront the penetrability and can be more durable than ordinary concrete. This characteristic of S.C.C is not just because of its high compactibility while the materials used in production of S.C.C have an in important role in durability of S.C.C.

For example, silica fume and slag cement are among the most common materials used in production of S.C.C. The results of studies carried out on pozzolans show that using these pozzolans can increase the durability of all kinds of concrete against attach of sulfates, carbonatation, penetration of chlorine ion, penetration of oxygen and water.

2.2 Practical Study of Durability of S.C.C (Laboratorial) Fortunately, the results of laboratory studies show the priority of durability of S.C.C in compare with

ordinary concrete under the same conditions. To carry out this study, we tried to offer two similar mix designs (one for S.C.C and one for ordinary concrete), by removing different parameters, in order to study the effects of the important factors of durability. We avoid offering the mix design because of the existence of different climates and materials in each country and also different code requirements and, most importantly, lack of a comprehensive and definite mix design for S.C.C. But we mention the specifications and conditions of the tests carried out in this case.

Type of Test Shape of Sample

Maintenance Environment

Ages of Test

Penetrability Cubic

15*15*15 cm Control

1 month, 3month, 6 months

Carbonatation Prismatic

15*10*10 cm Dry 4 months, 6 months

Penetration Percentage of Sulfate

Cubic 10*10*10 cm

Wet 6 months

Penetration Percentage of Chlorine

Cubic 10*10*10 cm

Wet 6 months

Control Environment: Includes a basin with dimensions of (1×3×3 m) containing ordinary water in open air.

Dry Environment: This environment includes open air in a room with temperature of 33 °C and more

than %50 of humidity. This environment is similar to open environment of southern seacoasts of Iran. Wet Environment: This environment is similar to marine environments (lower than tide and sunken) of

southern seacoasts of Iran. In other words, all samples in this environment were kept inside Persian Gulf waters with temperature of

20-22 °C. Generally, the results of these tests show that S.C.C can be replaced by ordinary concrete under difficult

environmental conditions like southern seacoasts of Iran (where the problem of concrete durability has been of concern). It is clear that these studied showed only the results of short-term durability and we need comprehensive studies to obtain more realistic results about durability of S.C.C.

3. Study of S.C.C Problems

Although S.C.C is basically a premium concrete because of having special mix design and suitable supplementary materials and as we told before, it has solved most of the problems of ordinary concrete but there are a lot of obstacles in the way of its ever-increasing development and application.

In fact, when we use it in workshops and bring it out of laboratory or in other words, if we want to produce several m³ of it with the same quality, there will appear the problems of using S.C.C.

3.1 Segregation High fluidity of S.C.C causes some concerns about segregation. The important point about segregation

of S.C.C is that, it seems that segregation of S.C.C is more dangerous than that of ordinary concrete. Now, some of S.C.C tests [2] including VT5min and GTM (directly) and L Box & U Box (indirectly) study about the segregation of this concrete.

3.1.1 Role of Coarse Aggregates in Segregation As we know, segregation of S.C.C has a direct relationship with coarse aggregates used in its

production. When the maximum size of coarse aggregates in concrete is close to smallest distance between the reinforcements, we cannot always forecast about compaction of that concrete in another structure with its own special reinforcement and test results. So the amount and size of coarse aggregates in concrete for a structure are determined considering the special reinforcement of that structure, while the mix design suggested by Okamura [3], which is one of the most acceptable resources for S.C.C production, the amount of coarse aggregates is 50% of concrete’s volume and this shows that we faced a problem in this case. Constant amount of coarse aggregates suggested by Okamura is in order to reach a self-compaction property, high plasticity and confront the segregation between coarse aggregates and mortar. In fact, the amount of energy used in compaction process is equal to the required amount of energy to compensate plasticity of vibration.

Frequency of contacts between the aggregate components at the time of concrete transformation can increase internal stress (friction). The studies show that the required energy for fluidity increases with internal stress and this shows the fact that when the size of coarse aggregates increases, fluidity of concrete decreases.

As a result, we can effectively remove this obstacle by limiting the coarse aggregates that practically increase energy consumption and reaching a level lower than energy consumption in ordinary concrete.

A study is being done in Construction Materials Institute of Shabastar Azad University in order to solve this problem (increasing the size and volume of coarse aggregates according to structural conditions without segregation) and investigates the use of lightweight aggregates with structural strength instead of common aggregates used in S.C.C.

For the purpose of this study, we used the materials obtained from Eskandan mine which contains pumice lightweight aggregates with specific weight of 2.30 – 2.50. The results of primary tests show that increase of internal stress (friction) caused by frequent contacts between lightweight aggregate components is lower in compare with ordinary aggregates and considering that decreasing internal stress will decrease the energy required for fluidity it can be hoped that using lightweight aggregates with structural strength, we

can increase the amount of coarse aggregates in S.C.C up to more than 50% of solid volume of concrete, if required.

On the other hand, using lightweight aggregates with structural strength can be effective in decreasing the specific weight of S.C.C.

As these studies are not completed yet, we avoid offering the detailed statistics and tables but we suggest that the honorable researchers do more researches in this case.

3.1.2 Role of Mortar In Segregation of Fresh Concrete Studying the role of mortar in segregation of S.C.C should be done in two cases. The first case is related

to fluidity of mortar. When the concrete is still wet, the mortar acts as a fluid, which flows the aggregates to let the concrete, get compacted under its own weight.

The mortar brings the coarse aggregates close to each other by its high viscidity and prevents them form being segregated.

Beside the role of the above-mentioned fluidity role, mortar acts as a solid object; in other words it gets strained like other components. When coarse aggregate components get closer to each other and the mortar is under normal stress between coarse aggregate components, the mortar acts as a pressure conveyor.

The decreased amount of shear deformation of mortar depends mainly on physical properties of solid particles in mortar.

3.1.3 Role of admixture materials in Segregation It seems that production of S.C.C with constant properties and characteristics is possible by observing

some rules. Indeed any kind of variability in specification of particles can affect the self-compaction of concrete. For example, one of the important changes is the change of water content of fine aggregates that changes water content of concrete. To solve this problem and prevent changing the properties of concrete as a result of variant consuming materials, new anti-segregation materials are used. These materials are effective in creation of a kind of self-compaction, which is less sensible against changing the content of water. Among these factors we can mention new generation of superplasticizers on the basis of Polycarboxylic ether (PCE) and corrective additives of consistency or in other words, correction factor of viscidity(VMA) and also nanosilica , the grout made from it showed no segregation and bleeding in compare with silica fume.

3.2 Problems of S.C.C Production In Ready-Mixed Concrete Factories It is clear that the main problem of S.C.C production in ready-mixed concrete factories is to prove the fact

that whether the produced concrete is a self-compacting concrete and has the criteria of S.C.C or not? As we know, we can use different tests of S.C.C to study the factors like self-compactness, plasticity,

viscidity and filling properties to determine this factor with laboratory criteria. But we should control the self-compacting property for entire volume of concrete before transferring it to the workshop. It is very difficult to carry out self-compactibility test for entire volume of concrete so a suitable test method was suggested by Oachi [4] for self-compaction. In this method we put the testing machine between agitator truck and concrete pump in the workshop and then all the concrete is poured into testing machine. If the concrete flows through the testing machine, it will get compacted equal with its structure and if it is blocked by the obstacles of the machine, it does not have self-compacting property and the mix ratios should be corrected.

3.3 Economic Problems As we have to use different superplasticizers, fillers and some new and rare materials like nanoslica in

production of S.C.C, it seems that it will be difficult to use it for economic reasons, at least in developing countries. Although using S.C.C can considerably decrease the required manpower but this is not that much important like it is in developed countries because manpower is relatively cheap in developing countries.

So we must explore the ways to economize the production of S.C.C in these countries, while the quality and durability of S.C.C can have long-term economic justifications. On the other hand we can study about the resources of each country to find suitable and economic materials to produce S.C.C.

For example, as coal is used as carbonaceous fuel in Europe, it is an optimum and useful way to use the ashes of fly ash.

In some countries that have Steel Melting Industries, we can use the wastes of those factories. In

Southeast Asia, as there is high production of rice, we can use the ashes of rice bran as an economic and suitable filler. In some cases using stone dust, fine particles of mineral limestone and glass powder is reported.

4. Study of Some Projects Executed By S.C.C It is clear that introducing different projects executed by S.C.C throughout the world can attract the

attention of people to use this concrete. The important point is that the conditions of these projects required the use of S.C.C by the contractors of these projects. And in fact, one of the main reasons for using S.C.C in a project can be the need to use S.C.C.

4.1 National Museum of the American Indian (NMAI),Smithsonian Institution,Washington D.C. Main Projgect Description The NMAI is being built on the washing ton mal and is intended to resemble a solid piece of Rock carved

over time by wind and water and water. Everyting is highly symbolic and non- repetitive. Why S.C.C Was used ? The reason for the use of S.C.C Was the overcrowded reinforcement and the complicated shapes of the

structure. There was little to no room left for vibrators. The use of S.C.C enabled turnaround times to be reduced from five to two days.

Project Requirements Concrete with high flow characteristics that could fill the forms and flow around the dense reinforcement

without vibration.Surface appear ance was a key issue for this high profile project. 4.2 Shark and penguin Aquariums at the Oceanopolis Marine Park in Brest, France Main Project Description An extension of the marin park Oceanopolis near Brest in Brittany.Building of two giant aquariums with

very large openings in the fair faced concrete walls. Why S.C.C Was used ? Very dance network of reinforcing bars.Better placing rates of concrete were desired. Surfaces with very

regular aspect and colour were required. No segregation at the bottom of the walls. Project Requirements Concrete with high flow characteristics to completely fill the formwork, especially around the large

opening for windows etc. 4.3 Basement for a new research and Development Buildingin in Tokyo, Japan Main Project Description Erection of a new R&D building in Tokyo, Japan . For the complex basement, new concrete technologies were necessary. Why S.C.C Was used ? High strength concrete was required for this structure to Improve the earthquake resistance; fore the construction and complex design with extreme

reinforcement easy pumping and placing was required. Project Requirements Flow target 65+/- 5cm for 120 min (25.5+/-2 in.)

Final strength > 60 N/mm² MPa (8760 Psi ) Concrete temperature 28-32° c (82-90°) 4.4 Pipe Screen for a Rail Tunnel at the meinrad Lienert Square, Zurich, Switzerland Main Project Description A pipe screen had to be installed to give additional support to the very thin ground cover over the new rail

tunnel at the Meinrad Lienert Square in the center of Zurich. Ten concrete pipes of 1.55 m diameter ( wall thickness 150mm) 6 in . were pipejacket ed 138-158 meters (460-500 ft.) into the soil from an access trench to form the pipe screen in advance of tunnel boring. The soil from inside the pipes was removed and the soil around the pipes injected to consolidate it. Reinforcing steel cages were then inserted into the pipes and they were concrete d section by section with S.C.C.

Why S.C.C Was used ? The decision was made to use Self Consolidating Concrete, because the concrete had to be pumped at

least 50 m, (160 ft.) then placed with no possibility at additional vibration, ensuring that the complete pipe was filled without segregation and bleeding .Consistent watertight concrete quality had to be maintained .

Project Requirements Concrete quality B 35/36 watertight completely filing of the tubes High efficiency in concreting.

5. Conclusion If we obtain a reasonable mix design and suitable methods of acceptability test in workshops and

overcome the economic problems considering the accessible resources of each country, the main obstacles mentioned here for widespread use of S.C.C will be removed.The main struggle is to let the S.C.C to enter the technology of ready-mixed concrete and prefabricated concrete. Although S.C.C has small share in this technology but as the constructors and mass-producers of buildings are interested in this type of concrete, the rate of demand for this concrete will considerably increase in near future.

6. References

[1] Hajime Okamura,Masahiro Ouchi;self_compacting concrete;Journal of advanced concrete technology;Vol 1,5-15,April 2003 [2]EFNARC;Specification and Guidelines for SELF-COMPACTING CONCRETE;february2002 [3]Okamura.h,”Mix design for self compacting concrete”,library of JSCE,No 25,pp.107-120,June 1995 [4]Masahiro Ouchi;self-compacting concrete development application and investigation