DISCUSSION ON MIXTURE RATIO DESIGN METHODS OF SELF-COMPACTING CONCRETE

Transcription

1 DISCUSSION ON MIXTURE RATIO DESIGN METHODS OF SELF-COMPACTING CONCRETE Zeng Li (1),Jiang Rui (1) and Jinxiang Zhuang (1) (1)State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, , China Abstract: Now there are two common ratio design methods of self-compacting concrete (SCC). One is the fixed aggregate volume calculation method, and the other is the overall calculation method. To design the SCC with the same design index, big differences exist in the two methods. Then improved overall calculation method is proposed, but the mixture ratio got through it is almost the same as the fixed aggregate volume method. Furthermore, when to design concrete mixture ratio, with the designed strength grade of concrete improved (water to cement ratio reduced), paste volume keeps invariant and the loose piled volume of gravel increases (sand ratio decreases) by the overall calculation method; while the loose piled volume of gravel and the sand ratio keep invariant by the fixed aggregate volume calculation method (including improved overall calculation method), and the above don t match the concrete design theory. Thus, the problems on the design method and the value of design parameters are discussed, and then a more reasonable design concept called fixed gravel volume method is proposed. Keywords: SCC; mixture ratio design; strength grade of concrete 1. INTRODUCTION Self-compacting concrete (SCC) is a high-performance concrete, also known as high flowing concrete, self-leveling concrete, vibration-free concrete and so on. Through the choices of cementing materials, aggregates and admixtures and the mixture ratio design, concrete mixture gets enough plastic viscosity and the yield value reduces. The coarse and fine aggregates can be suspended in the water, neither disintegrate nor secrete water, and can fill all the gaps to form a dense and homogeneous concrete structure without any vibration. Now there are many researches on the ratio design methods of SCC [1, 2, 3, 7, 8], among which the fixed aggregate volume calculation method and the overall calculation method [6] are two common ones. To design the SCC with the same design index, big differences exist in the two methods. Then improved overall calculation method is proposed, but the mixture ratio got through it is almost the same as the fixed aggregate volume method [9]. Furthermore, when to design concrete mixture ratio, with the designed strength grade of concrete improved (water to cement ratio reduced), paste volume keeps invariant and the loose piled volume of gravel 145

2 increases (sand ratio decreases) by the overall calculation method; while the loose piled volume of gravel and sand ratio keep invariant by the fixed aggregate volume calculation method (including improved overall calculation method), and the above don t match the concrete design theory. Thus, the problems on the design method and the value of design parameters are discussed, and a more reasonable design concept is proposed. In this paper, fixed gravel volume method is given, and its design principles are: with a fixed gravel volume (or an unchanged gravel amount) and a smaller W/C ratio, the sand amount decreases and the paste volume increases correspondingly to meet the density and mobility. 2. MIXTURE RATIO DESIGN METHODS OF SCC At present, mixture ratio design methods of SCC mainly are the fixed aggregate volume calculation method, the overall calculation method and the improved overall calculation method. 2.1 Fixed aggregate volume calculation method The fixed aggregate volume calculation method (or can be called fixed aggregate volume method) is based on the balance between the high-flow fluidity and anti-segregation of SCC and mixture ratio factors, and is a mixture ratio calculation method according to experiment study, which is better suited to the characteristics and requirements of high mobility SCC. This method has ever been introduced by Japan Ready-mixed Concrete Federation [4] and Academician Wu Zhongwei [5]. Its design principles are as follows: (1) The loose piled volume of gravel is assumed to be 0.50 ~ 0.60 m 3 in 1m 3 concrete to calculate the amount of gravel and mortar. (2) The sand volume is assumed to be 0.42 ~ 0.44 of mortar to calculate the amount of sand and paste. (3) According to the W/C ratio and the proportion of mixture in cementing materials, the water consumption and the total amounts of cementing materials are obtained. (4) Finally, the amounts of cement and admixture are calculated by the total amount of cementing materials respectively. 2.2 Overall calculation method The design principles of the overall calculation method are as follows: from the mathematical deduction of the assumed concrete volume model, draw the water consumption in 1m 3 concrete (Eq. 1) and the sand ratio (Eq. 2), and combine them with the traditional W/C ratio rule to get a comprehensive amount of each component in the concrete quantitatively, then the mixture ratio design method of SCC from semi quantitative to whole quantitative is achieved. Ve Va Vw = 1 m( c + f ) 1+ [ ρ (1 ϕ) + ϕρ ] m( w) c f where Vw = water consumption (kg); Ve = volume of paste (m 3 ); Va = volume of air (m 3 ); ρc = density of cement (kg/ m 3 ); ρf = density of fly ash (kg/ m 3 ); φ= volume percentage; m(c+f) /m(w)= cement to water ratio. (1) 146

3 Sp ( Ves Ve + Vw) ρ s = (2) ( Ves Ve + Vw) ρ + (1 Ves Vw) ρ s g where Sp = sand ratio (%); Ves = volume of dry mortar (m 3 ); V W = volume of water consumption (m 3 ); ρ S = apparent density of sand (kg/ m 3 ); ρ g = apparent density of gravel (kg/ m 3 ). 2.3 Improved overall calculation method Using the fixed aggregate volume method and the overall calculation method to design mixture ratio respectively, the results show that the plasma of the former is significantly higher than the later. The sand ratio obtained by the overall calculation method was lower with the reduction of water consumption, and the loose piled volume of gravel is a little too large, leading to a less paste volume. Therefore, an improved overall calculation method is put forward, characterized by: the equation for water consumption in the overall calculation method is reserved, but fixed gravel volume method is used to calculate the aggregate volume. 2.4 Comparisons of calculation examples (1) Raw materials The main raw materials are P.O42.5 (density 3.06g/cm 3 ),Ⅰ-class fly ash (density 2.32g/cm 3 ), river sand (fineness modulus 2.7, apparent density 2.72g/cm 3, loose piled apparent density 1440kg/m 3 ), gravel (diameter 5~20mm, apparent density 2.70 g/cm 3, loose piled apparent density 1430kg/m 3 ) and water-reducer, etc. The cement strength and the chemical composition of cement and fly ash are shown in Table 1 and Table 2, respectively. Table 1:Cement strength compressive strength(mpa) flexural strength(mpa) sample 3d 28d 3d 28d cement Table 2:Chemical composition of cement and fly ash sample SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 K 2 O Na 2 O Loss cement fly ash (2) Mixture ratio calculation of SCC The fixed aggregate volume method, the overall calculation method and the improved overall calculation method are used to calculate the mixture ratio of C30 ~ C60 SCC, and the results are shown in Table 3.Ⅰis the fixed aggregate volume method; Ⅱis the overall calculation method; Ⅲ is the improved overall calculation method; Ⅳ is the fixed gravel 147

4 volume method. Table 3: Mixture ratio of C30 ~ C60 SCC by different calculation methods concrete calculation C S G W F V S/S+G G past grade method /kg /kg /kg /kg /kg /m 3 volume/m 3 Ⅰ C30 Ⅱ Ⅲ Ⅳ Ⅰ C40 Ⅱ Ⅲ Ⅳ Ⅰ C50 Ⅱ Ⅲ Ⅳ Ⅰ C60 Ⅱ Ⅲ Ⅳ Note: V G loose piled volume of gravel/m 3. W/C+F (3) Results and analysis According to the calculation results from Table 3, we can see: there are big differences between the material amounts in the fixed aggregate volume method and the overall calculation method, and both of them have the problems that don t match the concrete design theory. 1) With the design strength grade improved (W/C ratio decreased), the loose piled volume of gravel obtained through the fixed aggregate volume calculation method (including the improved overall calculation method) remains unchanged, and so does the sand ratio (which it gradually reduces in ordinary concrete mixture ratio design), as the changes are from the amounts of plastic materials and water. Under the condition that paste volume is invariant, the gradual decrease in the W/C ratio will lead to the concrete workability changes inevitably (mobility reduction in particular) and affect the construction of concrete. 2) The loose piled volume of gravel obtained by the overall calculation method is obviously too large, while the paste volume is a little small, which is only 0.35 (usually 35%-45%); and with the design strength grade improved (W/C ratio gradually decreased), the loose piled volume of gravel obtained by the overall calculation method increases gradually, the sand ratio decreases, and the paste volume keeps invariant (while the paste volume gradually increases in ordinary mixture ratio design), which affect not only the concrete workability but also the performance. 3) The mixture ratio got from the improved overall calculation method is almost the same as that by the fixed aggregate volume method actually, so it also has the same problems and no real improvements. 148

5 3 FIXED GRAVEL VOLUME METHOD 3.1 Mixture ratio design method The mixture ratio design of ordinary concrete should follow the dense filling theory that sand fills the gravel gaps (Eq. 3) and paste fills the sand gaps. When other conditions are the same, with the decrease of W/C ratio, sand ratio should be gradually reduced in order to get the same fluidity. Then the paste supplement is used to fill densely, so the cementing materials amount increases. During this process, the gravel change is in a small amount relatively. S KG = P (3) γ s γ g where S = sand amount(kg); G = gravel amount (kg); γ S = loose piled apparent density of sand (kg/ m 3 ); γ g = loose piled apparent density of gravel (kg/ m 3 ); P = gravel porosity (%); K = poking coefficient, usually 1.1~1.4. The mixture ratio design of SCC should also comply with the dense filling theory. Therefore, a fixed gravel volume method based on the discussion of present SCC mixture ratio design methods is proposed in this paper, and its design principles are: with a fixed gravel volume (or an unchanged gravel amount) and a smaller W/C ratio, the sand amount decreases and the paste volume increases correspondingly to meet the density and mobility. For the purpose of comparison, the same raw materials and concrete design grade are still used. The specific methods and steps are as follows: (1) Concrete strength (guaranteed rate 95%) (Eq. 4) fh = fd+1.645σ (4) where fh = concrete strength (MPa); fd = design compressive strength of concrete (MPa); σ = standard deviation of concrete strength (MPa). (2) W/C ratio fh = Afce(C/W-B) (5) where fce = 28d actual compressive strength of cement (MPa); C/W = cement to water ratio of concrete; A, B= experience coefficients, usually in rubble concrete A = 0.48, B = 0.52, while in pebble concrete A = 0.50, B = (3) Gravel amount The loose piled volume of gravel is assumed to be 0.50 ~ 0.60m 3 in 1m 3 concrete, and the specific value depends on the loose piled apparent density. It is 0.55 in this article for the purpose of comparison, and the gravel amount is calculated by Eq. 6. G = V G γ G (6) where G = gravel amount (kg); V G =loose piled volume of gravel (m 3 ); γ G =loose piled density of gravel (kg/m 3 ). (4) Sand amount Sand ratio of SCC is usually in 45% ~ 52% and sand amount is affected by many factors, such as gravel gradation, fineness modulus of sand, W/C ratio and mixture cohesion and so on. 149

6 Therefore, it is suitable to use the experience chart to estimate preliminary sand ratio (see table 4), and then test it through the concrete mixture workability. In this paper, W/C ratio is the main basis for the values of sand ratio, which is usually in the range of 0.28 ~ 0.45 in SCC. According to gravel amount and sand ratio, sand amount is determined. Table 4:Reference table of sand ratio W/C ratio sand ratio(%) 0.45~ ~ ~ ~ ~0.52 (5) Paste amount Gravel gaps are filled with paste. In the fixed volume method, with the W/C decreased, sand amount is in a corresponding reduction in order to meet the requirements of concrete workability. Then the gaps because of the sand reduction are filled with paste. Eq. 7 is for paste amount. Ve=1-S/ρ S -G/ρ G (7) where Ve = paste volume (m 3 ); S = sand amount (kg); G = gravel amount (kg); ρ S = apparent density of sand (kg/m 3 ); ρ G = apparent density of gravel (kg/m 3 ). (6) Water volume(vw) Water volume is calculated by the water volume equation in the overall calculation, as Eq. 8. The symbols in Eq. 8 are the same with Eq. 1. Ve Vn Vw = (8) 1 m( c + f ) 1+ [ ρ (1 ϕ) + ϕρ ] m( w) c (7) Cementing material content(b) f B=W/(W/C) (9) (8) Admixture (fly ash) amount F=B(F/B) (10) (9) Cement amount C=B-F (11) (10) Amount of water reducer The amount of water reducer is determined by the fluidity of mixture. 3.2 Results and analysis In the SCC mixture ratio calculation by the fixed gravel volume method, the loose piled volume of gravel is fixed to be 0.55 m 3 for the comparisons with the other three methods, and the calculation results are in Table 3. Results and analysis are as follows: 150

7 (1) When the maximum aggregate size is determined, with the concrete strength grade increased (W/C ratio decreased), changes in the amount of gravel is usually very small. Therefore, the fixed gravel volume method is reasonable. (2) With the concrete strength grade increased (W/C ratio decreased), in order to meet the requirements of concrete workability, sand ratio gradually decreases and the paste volume increases. (3) The fixed gravel volume method is more reasonable for the mixture ratio design of SCC. 4 CONCLUSIONS In the mixture ratio design of SCC, with the design strength grade increased (W/C ratio decreased), the loose piled volume of gravel calculated by the overall calculation method gradually increases (sand ratio decreases), but the paste volume is unchanged; while the loose piled volume of gravel by the fixed aggregate volume calculation method is invariant (including the improved overall calculation method), and so is the sand ratio. The above don t match the concrete design theory. Therefore, the design methods and design parameters on above problems are discussed in this paper, and a more rational design method named fixed gravel volume method is proposed. Its design principles are: with a fixed gravel volume (or an unchanged gravel amount) and a smaller W/C ratio, the sand amount decreases and the paste volume increases correspondingly to meet the density and mobility. It is more suitable for the mixture ratio design of SCC. REFERENCES [1] Hajime Okamura, Masahiro Ouchi, Self-Compacting Concrete, Journal of Advanced Concrete Technology, 2003, 1. [2] Zhang Qing, Lian Huizhen, Research and Design of Mixing Ratio for Manufacture of a Self-condensed High Performance Concrete, Architecture Technology, Vol. 30, No.1 pp , [3] H.W. Chai. Design and Testing of Self-Compacting Concrete, Department of Civil and Environmental Engineering, University College London, London, [4] 全国生工業組合連合會. 高流動 ( 自己充填 ) ユンヮリート製造マンユヲル [5] Wu Zhongwei, Lian Huizhen, High Performance Concrete. ISBN: , China Railway Publishing House, [6] Chen Jiankui, Wang Dongmin, New Mix Design Method for HPC Overall Calculation Method, Journal of the Chinese Ceramic Society, Vol. 28, No.2 pp , [7] C. L. Hwang, S. L. Lee, F. Y. Lin, J. C. Liu. Densified Mix Design Algorithm and Early Properties of HPC [J].J. Chin. Inst. Civil Hydraul.Eng.8, [8] Japanese Ready-Mixed Concrete Association, Manual of Producing High Fluidity (Self-Compacting) Concrete. Japanese Ready-Mixed Concrete Association, Tokyo, [9] Yu Zhiwu, Pan Zhihong, Xie You-jun, Liu Baoju, Comments on Mix Calculation Method of Self-Compacting High Performance Concrete, Concrete, No. 1 pp.54-57,2004. [10] Zheng Jianlan, Huang Pengfei, Testing on the Mixture Ratio of Self-compacted High Performance Concrete. Journal of Fuzhou University (Natural Science), Vol.29, No.5 pp.72-76,