DEVELOPMENT AND EXPERIMENTAL STUDY ON THE PROPERTIES OF LIGHTWEIGHT SELF COMPACTING CONCRETE

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1 DEVELOPMENT AND EXPERIMENTAL STUDY ON THE PROPERTIES OF LIGHTWEIGHT SELF COMPACTING CONCRETE M. Hubertova (1) and R. Hela (1,2) (1) Brno University of Technology, Institute of Technology of Building Materials and Components, Czech Republic (2) Lias Vintirov LSM k.s. (Liapor), Czech Republic Abstract The paper describes a new field of development and use of self-compacting concrete (SCC) using artificial light weight aggregate (LWA) based on burnt clay for coarse fraction of aggregate. There is experience of using common SCC for monolithic structures and prefabrication. The ability to produce light weight structural self compacting concrete of volume weight between 1600 and 1800 kg/m 3 and strength around 40 N/mm 2 shows new ways of development and use of SCC. The paper describes mixtures and methods for testing rheological properties of fresh concretes to fulfill parameters of SCC including description of testing apparatus and parameters observed. Different versions of mixtures are mentioned with sufficient physico-mechanical properties of hardened concrete - compression strength, tensile bending strength, static elastic modules and freeze resistance using artificial LWA and active admixtures based on secondary raw materials (fly ash and other active admixtures - metakaolin, silica fume). 1. INTRODUCTION The development of concrete has been heading towards so called high performance concrete recently. In particular we can see application of self-compacting concrete, increase of application of high-strength concrete and application of light weight concrete using porous aggregate. This paper deals with uniting these three directions, in particular developing light weight self-compacting concrete (LWSCC). This topic is not new, many scientists have dealt with development of lightweight high strength concrete, lightweight high performance concrete and also LWSCC [1, 2, 3] and their results lead to a conclusion that type and properties of LWA used is a basic factor influencing characteristics of fresh concrete, in particular physico-mechanical properties of hardened concrete. There are differences in characteristics of not only LWA based on different material (for example expanded clay vs. fly ash aggregate or natural pumice), but also LWA based on identical material, but produced in different factories. The properties of porous aggregate with highest influence are bulk or 851

2 volume weight, strength of grains and water-absorbing capacity under normal (or high) air pressure. These properties are closely related to the structure of the aggregate as such /inner porosity, porous or closed grain surface).this paper focuses on the properties of LWSCC with lightweight expanded clay aggregate Liapor produced in Czech Republic. No current directive for self compacting concrete (SCC) mentions LWSCC. Therefore the intention to develop LWSCC was based on the assumption of necessity to fulfill the requirements of both SCC and light-weight concrete. Following questions were the most important, when solving our problem. Is there a way of making LWSCC using LWA, in particular considering its high absorbing capacity? Will this concrete be able to fill up shuttering only by means of its own weight and reach homogeneity, i.e. will the concrete be self-compacting? Can fresh LWSCC be tested using the same methods as for SCC? Can LWSCC have strength of at least 30 N/mm 2 and volume weight of 1800 kg/m 3? SCC use for movability and fluidity of fresh concrete its own weight. At the beginning of experimental work there was an assumption that LWSCC will not have enough inner kinetic energy due to low volume weight (i.e. LWSCC will probably be slightly slower in comparison with normal weight concrete with natural aggregate). Therefore it was necessary to find out if current methods of testing rheological properties of fresh SCC are also suitable for testing LWSCC. Methods used for testing are described in the Directive EFNARC [4]. 2. EXPERIMENTAL OUTLINE 2.1 Material Cement used in the study was the ordinary portland cement CEM I 42,5 R with Blaine fineness of 3250 cm 2 /g and density of 3.1 g/cm 3. As an aggregate, light weight expanded clay aggregate Liapor with 8 mm of Dmax was used (bulk weight kg/m 3, cylinder strength N/mm 2, absorption capacity from 1 to 45 % ; void space of bulk % ; heat conductivity coefficient λ from 0.09 W/m.K) and river sand with bulk density of 2.5 g/cm 3 was used as natural fine aggregate. Furthermore water reducing agent (polycarboxylate) and stabilizing agent was used. Table 1: Basic characteristics of used fly ash and metakaolin % mass SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO K 2 O TiO 2 MnO Na 2 O S fly ash (brown coal) fly ash (black coal) metakaolin powder microsilica For manufacturing of LWSCC, these admixtures was used: micronized limestone (97.7% CaCO 3 ), fly ash, reactive metakaolin and powder or suspension (50% solid content) silica fume (see Table 1). To obtain an adequate sample of metakaolin, an X-ray analysis was executed, which identified following mineralogical composition: kaolinite 852

3 (Al 2 O 3.2SiO 2.2H 2 O), β-silica (SiO 2 ). The value of metakaolin is 644 mg Ca(OH) 2 to 1g of metakaolin according to Chapell test. The value represents medium level of reactivity. 2.2 Mixture proportion of concrete As a part of the project, more than 50 mixtures were designed with different strengths and volume weights. First, mixtures LWSCC solely with LWA Liapor were designed, then combination of LWA and natural normal weight aggregate (NWA) were used. Table 2 shows some mixture proportion (selected). Table 2: Example of mixture proportions of concrete Mix No. C W/C LWA [m 3 ] NWA W / (LWA + NWA) [%] stabilizing agent PCE [%] fly ash lime stone metakaolin silica fume suspense silica fume powder < 0,25 mm All mixtures were designed with maximally 8 mm gradation of aggregate since LWA Liapor is not produced in fractions bigger than 8 mm in sufficient quality. It is possible to use either dry or water pre-moisturized LWA Liapor. When using dry LWA, the mixing water consists of effective water and additional water. Certain amount of effective water is necessary for required workability; this part of water contributes to cementing compound and counts in W/C. Additional water is necessary for soaking LWA during mixing, but does not contribute to cementing compound. The amount of additional water depends on absorbing capacity of LWA which technically depends on initial moisture content of LWA, its size of fraction. After extensive comparing different mixtures using dry and water pre-moisturized LWA, we came to unambiguous conclusion, that it is much more useful to use pre-moisturized LWA. There are two reasons: It is difficult to determine real amount of additional water since individual fractions of LWA have different absorbing capacity and in practice also different initial degree of wetness, which has considerable effect on absorbing capacity. Using premoisturized LWA also eliminates problems with consistency and workability. It has also been 853

4 proven that using additional water can improve physico-mechanical properties in particular strength and durability of LWSCC (e.g values of strength of mixtures with dry LWA were 5 N/mm 2 lower). The best way of pre-moisturizing Liapor aggregate for laboratory prepared LWSCC was soaking dry aggregate in water for 24 hours. In practice, values of pre-moisturized aggregate are more difficult to determine. The easiest way is to water the deposit of aggregate for at least two days, which provides average soak above 20%. Examining suitability of proportioning by volume or by weight showed that individual mixtures can not be reproduced and meet the requirements of once verified properties including workability of individual mixtures if proportioning by weight is used. It is caused by variable volume weight of artificial LWA, which can vary within the range of ± 15%. 3. RESULT AND DISCUSSION 3.1 Properties of fresh concrete For examination of suitability of fresh SCC test methods for LWSCC we chose most frequently used methods: Slump flow, T500 Slump flow, Orimet, J Ring and L Box. Recommended range of values for these test methods were taken from Directive [4]. Table 3 states all measured values, statistical values of file of more than 40 mixtures of LWSCC. All of them were tested straight after mixing, after 60 and 90 minutes to ensure long-time workability, which is important in the field of transported ready mixed concretes. Table 3: Statistical values of rheological properties, file of 50 LWSCC mixtures Method Entity Target values Consistency Min. Max. After mixing After 60 min. After 90 min. Slump flow mm T 500 Slump flow s J-Ring mm Orimet s L-Box h2/h On the basis of the results we can say that methods used are in principle suitable for determining consistency of LWSCC. It is only necessary to adjust criteria of tests in cases of flow time intervals. Since values of volume weight of LWSCC are low, their inner kinetic energy is not high enough and compared to normal concretes, LWSCC is slower. It is necessary to say that it does not affect final results of fresh LWSCC and mixed concretes met the requirements of homogeneity and even compaction across whole cross-section. This phenomenon is easy to understand on the basis of Slump flow test and T 500 Slump flow test of LWSCC. The concrete needed more time but final slump flow was sufficient (see Figure 1). As a result of measurements, we propose adjustment of some of the criteria of methods used - in particular the flow time. For example by Orimet test is target value from 2 to 5 s 854

5 our proposed value is from 2 to 10 s. The same proposed values are by Slump flow test. It has to be said that it is proved by not only these results but also by practical application, that for reasons stated above it is impossible to use this concrete (without vibration) for structural elements with high density of reinforcement and low thickness, where concreting height is more than 1 m. However, planar structural elements can be realized with slightly slower LWSCC with no problems at all. Slump flow (mm) Flow time (s) target range Mix No. after 0 min. after 60 min. after 90 min target range Mix No. after 0 min. after 60 min. after 90 min. Figure 1: Slump flow and flow time 3.2 Mechanical properties In the course of the experiment, LWSCC mixtures were proportioned with values of strength between 13 and 50 N/mm 2 and volume weight between 1100 and 1800 kg/m 3. Figure 2 shows the relationship of compressive strength and volume weight. LWSCC has good freeze resistance (coefficient after 100 cycles is from 90% to 98%). LWSCC of higher strength (using ultra-fine admixtures) is also resistant to water and chemical anti-freezing agents. Using LWA Liapor results in good thermal characteristics (λ = 0.29 W/m.K). These properties get worse when increasing proportion of NWA (λ = from 0.33 to 0.69 W/m.K). Light weight concretes in general have low value of static elastic modulus. Values of LWSCC with LWA Liapor only are around N/mm 2, LWSCC with NWA and ultra-fine admixtures reached values of N/mm

6 Compressive strength (N/mm2) y = 60,614Ln(x) - 415,87 R 2 = 0,8245 R = 0, Volume weight (kg/m3) Figure 2: Relationship of compressive strength and volume weight (after 28 days) 4. CONCLUSION Using porous aggregate for high strength concretes might be surprising considering importance of strength of aggregate for strength of high-strength concrete. LWA is porous and not very strong. Nevertheless, drop of volume weight of concrete with strength of N/mm 2 below 1800 kg/m 3 can represent certain cost saving due to reduction of total construction weight. Thanks to favorable physical properties, low volume weight and relatively high strength combined with good workability, low noise emission and reduction of consumed work in the course of placing, there is a wide range of application for LWSCC, in particular in the field of prefabricated elements and reconstruction of old buildings, where extra load would be undesirable. ACKNOWLEDGEMENTS This outcome has been achieved with the financial support of the Ministry of Education, Youth and Sports of the Czech Republic, project No. 1M0579, within activities of the CIDEAS research centre and with the financial support of project GA 103/07/076. REFERENCES [1] Y.W. Choi et al., 'An experimental research on the fluidity and mechanical properties of highstrength lightweight self-compacting concrete', Cement and Concrete Research 36 (2006) [2] Spiratos Haist, M. et al., 'Retrofitting of Building Structures using Pumpable Self-compacting lightweight concrete', In 3rd International Symposium on Self-Compacting Concrete, Reyjkjavik, Iceland p [3] Haque, M.N., Al-Khaiat, H, Kayali, O., 'Strenght and durability of lightweight concrete', Cement and Concrete Composites 26 (2004) [4] EFNARC, 'Specification and Guidelines for Self-Compacting Concrete', Surrey United Kindom 2002, ISBN , 856