EXPERIENCE WITH SELF-COMPACTING CONCRETE TECHNOLOGY IN CZECH REPUBLIC RUDOLF HELA, LENKA BODNÁROVÁ

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1 CIB World Building Congress, April 2001, Wellington, New Zealand Page 1 of 9 EXPERIENCE WITH SELF-COMPACTING CONCRETE TECHNOLOGY IN CZECH REPUBLIC RUDOLF HELA, LENKA BODNÁROVÁ Brno University of Technology, Faculty of Civil Engineering, Veveri 95, Brno, Czech Republic ABSTRACT The paper discusses a new technology of concrete manufacture in which the concrete during depositing into the formwork needs not any vibrating or other compacting. The concrete has high compactness thanks to its high mobility under the action of its mass. This concrete is called selfcompacting-concrete (SCC). In the paper the basic demands concerning the properties of fresh concrete and experience obtained during the development in the Czech Republic during the last 2 years are presented. KEYWORDS Self-compacting concrete; superplasticizer; rheological properties; admixtures. INTRODUCTION The continuously-rising demands concerning the quality of concrete and the utility and technological properties of concrete result in a steady search for new possibilities to improve the utility properties of this material. Nowadays, concrete constructions are designed and built with parameters which were formerly not attainable. As one focus for a new stage of development in respect to the improvement of concrete properties we can consider the recognition, that the only way to get qualitatively higher parameters of concrete is to reduce its porosity. The American T. C. Powers proved (Powers 1958) in the middle of the century that not only the strength characteristics of concrete but even its durability, frost resistance and water permeability are functions of porosity and of the structure of the hardened cement binder. The problem is, that with a normal cement water ratio w/c = 0,5 we get about % of pore volume in the structure of hardened cement putty and this has a negative influence on the properties of hardened concrete. The development was for these reasons concentrated to the reduction of water content and to the methods of processing (esp. of compacting) the fresh concrete mixture. A significant possibility to eliminate the quantity of batch water and to reach good compactness of concrete is the technology of so called self compacting concrete i.e. of a concrete which as the result of its specially treated rheological properties without an excess of batch water doesn t need further compacting during imbedding into the formwork. The self-compacting concrete was known already in the past, before the introduction of plasticizing agents. They were used only in special structures, where the necessary compacting couldn t be done by vibration - for instance, during concreting under the water, when concreting underground walls or bored piles. This concrete had always a high cement content (more than 450 kg.m 3 ) a high water/cement ratio that caused the segregation of deposited concrete and the separation of the batch water excess to the surface (bleeding of concrete) These deficiencies were substantially still not removed even after the introduction of plasticizers. Towards the end of the 80 s concrete appeared with higher resistance against separation in contact with water. These resistance was achieved by the application of admixtures which increased the cohesion of fresh concrete.

2 CIB World Building Congress, April 2001, Wellington, New Zealand Page 2 of 9 PROPERTIES OF FRESH SELF-COMPACTING-CONCRETE Rheological properties of fresh self-compacting-concrete (SCC) are substantially different from normal concrete. The required properties of SCC we can define by the following rheological characteristics: high filling ability (flow) so that the fresh concrete mixture will fill all parts of formwork even with complicated shape, without any formation of cavities and large pores. The mixture will fill the given volume only by the pressure of its own mass low blocking (passing ability), the mixture has to be capable to flow between the individual reinforcing bars without causing a segregation or change in the composition of the mixture. segregation resistance - the mixture needs to have a sufficient internal coherence and stability to maintain the uniform inner composition during conveying and during depositing into the formwork. To these basic properties other demands are connected, such as the resistance against floating, pumpability, minimization of plastic sedimentation, of bleeding of concrete etc. The composition of a concrete mixture which meets these demands was for the first time published by a team of Japanese researchers in the year 1989 in the paper Self Compacting Concrete. The selfcompacting-concrete can be defined as a multicomponent composite silicate system containing as main components Portland cement, sand and aggregates with high strength values and with a suitable grain size composition. The modifying components are superplasticizers, fine fillers with high specific surface of particles, agents which correct the viscosity and the water separation of the cement mass after a longer period and antifoaming agents. DEMANDS CONCERNING THE COMPOSITION OF SCC The fresh mixture of SCC contains a considerable portion of fine powder components. The total sum of the combination of cement, limestone powder, fly ash and silica fume respectively or stone fillers which have at a mesh 0,25 mm minus mesh is 30 % higher than was used for normal concrete. The addition of effective superplasticizers of the fourth generation and of substances which correct the viscosity in a way which prevents the segregation of batch water and has a positive influence on the rheological properties of concrete mixtures, in spite of the considerably higher content of the volume of modified cement paste, which is the decisive factor of volume changes during the setting of concrete and which takes significantly part as far as the character and distribution of the cement stone are concerned. The basic condition for the formation of very good quality concrete with high utility properties is the securing of most convenient technological parameters in manufacture, transport and imbedding of the fresh concrete mixture. This concerns especially the optimization of rheological characteristics viscosity and thixotropic behaviour, the removal of tendency of excessive batch water to segregation for a period of at least 2 hours after mixing the concrete. This has to be secured by selected types of admixtures in a way that no physical and chemical interactions between these admixtures may cause negative influences in hardened concrete. The demand for compatibility of materials in the complicated system of SCC is necessary. TESTING METHODS FOR VERIFICATION OF THE PROPERTIES OF FRESH SCC The process of verification of properties of the fresh concrete mixture has to be easily practicable under operating conditions in a short time and it has to be at the same time sufficiently reliable. The values which represent the fresh mixture of self compacting concrete with optimum workability are a yield stress between Pa and a plastic viscosity between 6 12 Pa.s determined by help of a rheometer. The fresh SCC mixture should have a minimum flow of 550 mm, and the time to empty

3 CIB World Building Congress, April 2001, Wellington, New Zealand Page 3 of 9 the funnel should be between 2 6 s. This methods are however more suitable for testing fine grain size concrete. Basically, there is no unified testing method - it is always a combination of different methods without explicit criteria for the evaluation. It is absolutely necessary to elaborate an unified methodology, for the following reasons: To get a high quality and safe utilization of the properties of SCC it is necessary not only to determine explicitly the rheological properties i.e. the fluidity and mobility of the mixture, its resistance to blocking during the passage through bars of closely reinforced construction and a resistance against segregation and demixing of components in the mixture. Without explicit and reproducible verification of these properties it is not possible to accept the SCC concrete for usual building practice because of the danger of many serious failures or even to following destruction of buildings. For the elaboration of suitable test methods it is necessary to realize a research of the behaviour of cement mortars modified by the addition of fine fillers and of improved special superplasticizers. It is necessary to verify the rheological behaviour and the reciprocal regularities during the parallel study of the influence of these factors to the character of porous structure of the cement stone. THE PROPERTIES OF HARDENED SCC Compression strength, from our and foreign experience, is usually higher than the strength values of normal cements with analogical water/cement ratio. An increase is caused by the densification of the microstructure by the effect of fine fillers or by the action of for instance finely ground slag or fly ash. The course of strength increase is analogical to the same in the case of normal concrete. Positive influence has the smaller sensitivity to concrete treatment during the maturing of concrete. Bond strength of concrete with reinforcement determined by RILEM method shows higher values with SCC than with normal concrete. Durability - for the time being we don t have sufficient sets of tests for a correct evaluation. Shortterm tests of frost resistance show higher durability than in the case of normal concrete. This could be connected with the porous structure of SCC. Surface quality - the quality of concrete surface was in most cases significantly better than in the case of vibrating concrete. With suitable composition the SCC perfectly fills the formwork, it forms perfect edges and surfaces without defects. EXPERIENCE WITH THE DEVELOPMENT OF SCC IN CZECH REPUBLIC In the laboratories of the Faculty of Civil Engineering in Brno, the first verification tests of SCC, which were concentrated to the production of SCC from domestic raw materials (except the superplasticizer of foreign production) were realized. It was confirmed that cements of higher classes CEM I (class 42,5 and 52,5 respectively) manufactured in the Czech Republic are suitable for the production of SCC. Similar is the situation concerning the use of fine aggregates. There is no significant difference between pitsand and riversand; more important is the content of fine particles under 0,25 mm. Coarse aggregates which are substantially more suitable because of the spherical shape of grains and because of the smooth surface of the grains of extracted aggregates in comparison with crushed aggregates are limited by the maximum size of grain. As a matter of fact, mixtures with the maximum grain up to 16 mm were examined. It is of course necessary to pay elevated attention to the selection of microfillers. Due to its higher price, silica fume is not acceptable. Good results we have received with fly ash from power plants, finely ground blast furnace slag and in particular with microfillers based on stone powder, fine natural sand or microground lime stone. The attained rheological properties of fresh concrete are comparable with results reached in Holland or Great Britain, when we use identical testing methods. At the present time there is no practical experience

4 CIB World Building Congress, April 2001, Wellington, New Zealand Page 4 of 9 esp. a statistically significant extent of results of physical mechanical tests. Very reliable results are reached when determining the compression strength of concrete because we have relatively comprehensive sets of results at our disposal. The effects to volume changes are not yet sufficiently confirmed, neither the creep of concrete under load, the elasticity module and the durability for instance under cyclic frost effects. In the time being in the Czech Republic an unified comprehensive testing process for the verification of rheological properties of fresh self-compacting-concrete (SCC) is in preparation. This process is based on a combination of following tests: 1. The slump test + L box 2. The slump test + Orimet in combination with J Ring 3. A separate L Box test or a separate Orimet + J Ring test This combination of above mentioned tests is sufficient for both, the evaluation of mobility and liquidity of fresh SCC mixtures in time and the evaluation of the demixing of aggregate fractions in the fresh self-compacting mixture. Ad 1) The slump test evaluates informatively the mobility and the liquidity of the fresh SCC mixture in dependence on time, and provides at the same time information about the occurrence of aggregate demixing in the batch. The L - Box test records again the mobility and the liquidity of the fresh SCC mixture and additionally with this test we can plainly determine whether the fresh mixture is resistant to blocking between the reinforcement. Ad 2) The slump test shows, as in the first case, the mobility and the liquidity; the combination of Orimet + J-Ring tests gives us more detailed results, concerning the blocking and segregation of the fresh SCC mixture. The J-Ring test is, because of the close spacing of bars of aggregates the most strict test for the determination of mobility and liquidity of the fresh mixture. Ad 3) The separate L-Box or Orimet + J-Ring tests are suitable for the determination of these properties too. They have an excellent expressiveness and they are not so often affected by errors from the not quite identical ways of measurement, as often happens in the course of cone elevation during the slump test. The separate application of the slump test, which is probably the most simple and the least exacting test, is open to question. For the rough determination of fresh SCC mixture the slump test itself may be sufficient, but for the determination of all detailed properties of the fresh SCC it is inconvenient. Because of its simpleness this test will probably become the most practicable test in the field at building sites for the quick verification of the liquidity of fresh SCC. For the laboratory verification of a proposed recipe of SCC we suggest the following testing method: 1. The slump test + the evaluation of all criteria, presented by this test of fresh concrete. 2. The L-Box test + the evaluation of all properties presented by this test for fresh SCC. 3. The Orimet and J-Ring test + the evaluation of all observed criteria. 4. The maturing of the fresh concrete mixture and its following remix after about 90 minutes from the moment of the first addition of batch water. This will be followed by the whole above mentioned system of tests, because without the verification of the repeatability of properties the checkup of the recipes of fresh SCC cannot be complete. The recommended values of individual tests are shown in Table 1.

5 CIB World Building Congress, April 2001, Wellington, New Zealand Page 5 of 9 Table 1 Recommended measured values: SLUMP TEST L-BOX ORIMET + J-RING T50 = 2 4 sec Maximum spreading = cm T40 = 2 3 sec, T60 = 3 4 sec H1 / H2 (blocking ratio) 0,8 1,0 Outflow time from ORIMET Ot = 2-5 sec. Flow through J-Ring the surface of the mixture has to be leveled When proposing the composition of SCC, in the Czech Republic most frequently we start at grain-size curve according to the standard SIA 162 in the zone marked off by curves SIA A and SIA C (see fig. No. 1): 40,0 35,0 34,0 36,5 Concrete strength [MPa] 30,0 25,0 20,0 15,0 10,0 5,0 14,0 29,0 0,0 1 day 3 days 7 days 14 days Time Fig. 1. Ideal grain size-curve for SCC An example of a suitable recipe of SCC is in table No. 2. Table No. 2: Recipe of SCC Kg/m 3 % m 3 Cement ,32 Water 200 8,76 Admixture 6 0,25 Filler 150 6,57 Fraction 0-4 mm ,20 Fraction 4-8 mm ,97 Fraction 8-16 mm ,94 Total

6 CIB World Building Congress, April 2001, Wellington, New Zealand Page 6 of 9 The actual resulting grain-size curve of the proposed mixture is shown in Figure 2. Grain size curve for SCC 0-16 mm Total undersizes [%] 100,00 90,00 80,00 70,00 60,00 50,00 40,00 30,00 20,00 10,00 0,00 0,125 0,25 0, Mesh size [mm] Actual resulting grain-size curve Ideal grain- size curve SIA2 Figure 2 Grain size curve The results of some laboratory tests of fresh concrete are in table No. 3. Slump test L-Box test Table No..3 Test results of the recipe tests Time (min) Time period (sec) Final slump (spreading) (cm) St 1 0 5,5 73 x 72 St ,5 64 x 65 Time period of high of the level flowing out h 1 S h 2 (sec) h 1 /h 2 0,8 40 cm End End 40 cm Orifice LB 1 0 2,7 3,9 8,9 9,6 10,0 0,89 LB ,0 3,0 9,1 9,2 10,5 0,86 The development of compression strength values of concrete tested on cubes with the edge of 50 cm in dependence on time is graphically illustrated in Figure 3.

7 CIB World Building Congress, April 2001, Wellington, New Zealand Page 7 of 9 Concrete strength [MPa] 40,0 35,0 30,0 25,0 20,0 15,0 10,0 5,0 14,0 29,0 34,0 36,5 0,0 1 day 3 days 7 days 14 days Time Fig. No. 3 Course of concrete strength values in time dependence. The course of shrinkage of SCC and of a reference concrete of the same consistency (slump test) and analogous final strength values, which was proposed following the current practice is shown in Figure 4. The measurement of shrinkage was performed by means of a dilatometer which is accurate to 0,001 mm. The test pieces had the dimensions 100 x 100 x 400 mm. Shrinkage [%] 0,00-0,05-0,10-0,15-0,20-0,25-0,30-0,35-0,40 Time [days] SCC concrete reference concrete Fig. No. 4 The course of shrinkage of SCC and the reference concrete. THE DIRECTIONS FOR FURTHER DEVELOPMENT In the next research work the influence of mineralogical composition of cement, of the specific surface of cement and of the cement dose to the rheological properties of fresh SCC should be examined. In a similar way the influence of admixtures (fly ash, limestone, ground blast furnace slag, fillers) will be examined. These tests will allow the formulation of the optimum recipes, which will be verified from the point of view of selected physical mechanical properties. During the tests of rheological properties of fresh concrete the regularity during the behaviour of strongly liquefied

8 CIB World Building Congress, April 2001, Wellington, New Zealand Page 8 of 9 cement pastes will be verified. Afterwards the fluidity, mobility, blocking, and resistance against segregation and separation of water will be tested by the newly suggested testing methods. The objective is to elaborate an unified testing method with high reproducibility for routine application in practice. Besides strength parameters, the elastic modulus and volume changes, the frost resistance of the resulting concrete will be tested, and the resistance against combined influence of frost and chlorides. In the final phase corrosion tests of selected self-compacting concrete, esp. sulphateresistance and resistance against strongly acidic and strongly basic media will be performed. In SCC concrete even the course of the hydration process is influenced, especially in the initial period of hardening, when volume change takes place during the change of the quasi-liquid to solid form of the mass. Therefore it is necessary during the further research do pay particular attention to the study of the hydration process, to the reaction rate and especially. to measure the volume changes after the initial autogenous shrinkage. The problem of volume changes of the self-compacting-concrete is not yet cleared up. The current approximate doubling of the initial shrinkage doesn t involve the phase of autogenous shrinkage, which represents the chemical changes during the change of the system from the quasi-liquid to the stable state. ACKNOWLEDGEMENTS This paper was written within the framework of the research project carried out at the Institute of Technology of Building Materials and Components, University of Technology Brno, Faculty of Civil Engineering, Brno, No. VVZ CEZ MSM and also with backing of GACR, within the research project No. 103/01/0814, Microstructural features and related properties of cement based highly liquefied composites. REFERENCES Bartoš, P.J.M Self-compacting-concrete, technology into the New Millennium. In Proceedings CONCON ČBZ, Praha. Chai, H Design and testing of self-compacting concrete. Ph. D. thesis, University of London. Person, B Creep, shrinkage and elastic modulus of self-compacting concrete. In Proceedings, 1999, 1 st International Symp. On Self-Compacting Concrete. Stockholm. Hela, R Influence of the fine fraction of aggregates to the properties of transport concrete. In Proceedings, 1999, Betonářské dny. ČZB, Pardubice. Hela, R Testing of the influence of fly ash addition to the frost-resistance of concrete. In Proceedings, 1999, Construmat 99, Conference about structural materials. Ostrava. Hela, R., Bodnárová, L Increasing the strength of concrete with vibration-action technology. In Proceedings, 1999 International conference Creating with Concrete. Dundee, Scotland, UK. Hela, R., Dufka, A Self-compacting- concrete in the Czech Republic. Course CONCON 2000, New types of concrete. ČBZ, Praha. Hela, R., Frýbort D Experience with the use of plasticizing admixtures of new generation. In Proceedings, 1999, XI. Mezinárodní vědecká konference VUT v Brně. Brno. Hela, R., Frýbort, D., Bodnárová, L Concrete with higher utility properties utilising microsilica. In Proceedings, 1999, Betonářské dny. ČZB, Pardubice. Hošek, J., Kolář,K Self-compacting-concrete. Beton a zdivo (2). Powers, T.C Structure and physical properties of hardened cement paste. J.Amer. Ceram. Soc., 41 (1), 1 6.

9 CIB World Building Congress, April 2001, Wellington, New Zealand Page 9 of 9 Kendal, K., Howard, A.J., Birchal, J.D Relation between porosity, microstructure and strength and the approach of advanced cement-based materials. Phil. Trans. R. Soc., 310, Moranville, Regourd, M Microstructure of high performance concrete. High Performance Concrete. E & FN Spon., London, 542 s. Ozawa, K., Maekawa, K., Kunishima, M., Okamura, H High performance concrete based on the durability design of concrete structures. In Proceedings. of 2nd East-Asia and Pacific Conference on Structure Engineering and Constructions.