FLOWABILITY OF FRESH MORTAR IN SELF-COMPACTING CONCRETE USING FLY ASH

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1 FLOWABILITY OF FRESH MORTAR IN SELF-COMPACTING CONCRETE USING FLY ASH Chalermchai Wattanalamlerd, Masahiro Ouchi Kochi University of Technology, Japan ABSTRACT: Self-compacting concrete is concrete that can fill all corners of formwork by mean of its own selfweight and without segregation. In order to achieve that, concrete need to have proper combination of flowability and viscosity, called as self-compactability. It is well known that self-compactability of fresh concrete is mainly affected by the self-compactability of the fresh mortar. Self-compactability of fresh mortar usually depends on the physical properties of solid particles in mortar, and the chemical reaction between cement paste-superplasticizer which known as particle-dispersability of superplasticizer. Fly ash is one kind of pozzolan that widely used in concrete work. Usually, fly ash is not inert material; it can perform the chemical reactions, i.e. pozzolanic reaction and hydration reaction, when used in concrete. Existence of fly ash may influence to the efficiency of superplasticizer and this leads the difficulty to mix proportioning design. Flowability of fresh mortar containing ordinary Portland cement and polycarboxylate based superplasticizer was measured for several combinations of fly ash replacement ratio, superplasticizer dosage and water powder ratio. The influence of fly ash on flowability was evaluated as functions of flow area and funnel speed. The relationships among superplasticizer dosage, water-powder ratio, and fly ash replacement ratio were investigated. Flowability chart (relative flow area versus relative funnel speed) for each type of powder was made by regard the blend of normal Portland cement and fly ash at different fly ash replacement ratio as different powder. Each flowability chart was simplified into two graphs, graph of relationship between water powder ratio and amount of free water, and graph of relationship between superplasticizer dosage and its dispersing effect. Within certain fly ash replacement ratio, the fly ash replacement ratio had linear relationships with: the inclination and interception of both simplified graphs. 1. INTRODUCTION Both superplasticizer dosage and water-powder ratio have to be appropriate so that moderate deformability and viscosity of the mortar or paste phase in self-compacting concrete can be achieved. However, appropriate superplasticizer dosage largely depends on the chemical characteristics of the agent itself and the powder material in use. At present, many kinds of supplementary cementing material are used to replace some volume of cement powder in concrete work in order to improve the properties of concrete and limit the amount of cement powder used. Fly ash, a by-product obtained when burning coal in a power plant, is one of the most widely used supplementary cementing materials in concrete. Since good quality fly ash is in particulate or spherical form, the flowability of concrete is enhanced by its ball-bearing effect (1). This makes it difficult to determine the mix proportioning of selfcompacting concrete. In this paper, the method to estimate the appropriate amount of superplasticizer dosage and water-powder ratio according to the amount of fly ash in use is proposed by using the relationships between fly ash replacement and indices for dispersing effect of superplasticizer and amount of free water. Page 1

2 2. INDICES FOR DISPERSIBILITY OF SUPERPLASTICIZER AND AMOUNT OF FREE WATER (Γm/Rm and Rm/Γm.4 ) Either superplasticizer or water can increase the deformability of fresh mortar. Increasing the water-powder ratio can result in a decrease in the viscosity due to a decrease in the number of solid particles. On the other hand, superplasticizers can not decrease the viscosity as much as water can, because they do not decrease the number of solid particles but instead increase the dispersing force between the solid particles. The difference in the mechanism for increasing deformability between water and superplasticizer enables a wider variety of combinations of deformability and viscosity in fresh mortar. Ouchi et al (2) had already formulated the relationship between the flowability (both deformability and viscosity) of fresh mortar and water-powder ratio or superplasticizer dosage independent of each other. Deformability and viscosity were tested by mortar flow and funnel tests respectively (Figs. 1, 2), and the indices for the flowability of mortar were obtained by combining the relative flow area, Γm, and the relative funnel speed, Rm. It was found that these two indices, Γm/Rm and Rm/ Γm.4 are adequate to be used for evaluating the amount of superplasticizer per powder (SP/P) and volume of water per powder (Vw/Vp), respectively, independent on each other by comparing them with the values for the mortar with proper deformability and viscosity. It has been reported (3) that Γm/Rm and Rm /Γm.4 show a linear correlation with SP/P under constant Vw/Vp and Vw/Vp under constant SP/P respectively. This means that Γm/Rm could be an index of dispersibility of superplasticizer and Rm/ Γm.4 could be index of amount of free water in fresh mortar. 7 mm 3 mm 27 mm 6 mm 24 mm d 1 mm d 6 mm Relative Flow Area Γ m = ( d d ) 1 d 2 2 d 2 Fig. 1 Mortar flow cone test d R 3 mm Relative Funnel Speed m = 1 time [ sec] Fig. 2 Mortar V-funnel test Page 2

3 2.1 Index for amount of free water and the relationship with water-powder ratio (Rm/Γm.4 and Vw/Vp) The ratio Rm/Γm.4 is defined as the amount of free water in fresh mortar in selfcompacting concrete. Only certain values of the amount of free water can provide appropriate flowability of fresh mortar. A larger Rm/Γm.4 corresponds to a larger amount of free water which results as a larger distance between solid particles thus causing the mortar to have a lower viscosity. The simplified relationship between water-powder ratio by volume and amount of free water can be also described as the combination of the intercept and the inclination (Fig. 3). The intercept Retained Water (βwp) can be described as the minimum water-powder ratio for mortar to start to flow. The inclination Unit Free Water (Ewp) can be described as the additional water-powder ratio to enhance the unit increment for viscosityreducing effect. Rm Rm = A Γm VW/VP =1.1.9 =1.5 =1. = =.9 = Γm VW/VP E WP = Inclination =.31 β WP = Interception = Fig. 3. Transformation of Flowability chart to be the simplified relationship between water-powder ratio and amount of free water Ordinary Portland Cement + SP, fine aggregate content 4% Rm/Γm Index for dispersibility of superplasticizer and the relationship with the dosage (Γm/Rm and SP/P) The ratio Γm/Rm is defined as the dispersing effect of powder particles by superplasticizer on the flowability of fresh mortar in self-compacting concrete. A larger Γm/Rm corresponds to a greater effect of the superplasticizer. The simplified relationship between superplasticizer dosage and dispersing effect can be described as the combination of the intercept and the inclination (Fig. 4). The intercept Retained SP dosage (βsp) can be described as the minimum superplasticizer dosage for mortar to disperse the powder particles. The inclination Unit dispersing SP dosage (Esp) can be described as the required dosage of superplasticizer to increase the unit dispersing effect. It is also found that the ratio between unit dispersing SP dosage and retained SP dosage represents the characteristic of type of superplasticizer regardless of type of powder in use (2). Page 3

4 Rm 2.5 SP/P =.8 SP/P Γm = B Rm Γm Ordinary Portland Cement + SP, fine aggregate content 4%.3 E SP = Inclination =.9 β SP = Interception = Fig. 4. Transformation of Flowability chart to be the simplified relationship between superplasticizer dosage and dispersing effect 3. EXPERIMENTAL PROCEDURE 3.1 Materials Γm/Rm Fly ash Class 1 and Class 2 according to Japanese Industrial Standard [JIS A 621] (namely as FA1and FA2) were tested for this experiment. Two batches of fly ash that were from same resource and with the same raw materials were used. Their properties showed that they had the same characteristics except for fineness. Ordinary Portland cement and a polycarboxylate based superplasticizer were treated as the main ingredients. Crushed sand, obtained from crushing the rock, was used as the fine aggregate. All materials were stored and tested in a standard control-temperature room. The temperature of water used in this experiment was also controlled to be 2 ±.5 C. The properties of materials in use and the mixing method are shown in Table 1. Table 1 Properties of the materials in used and mixing method Density in saturated Fineness Surface Fine aggregate Kind Absorption Solid volume Fine aggregate surface-dried Modulus moisture volume ratio condition in mortar Crushed sand 2.58 g/cm % %.9%.4 Cement Kind Density Blaine fineness MgO SiO2 LOI Alkali normal portland cement 3.15 g/cm 3 3 cm 2 /g 1.74% 2.% 1.64%.48% Kind Density Blaine fineness SiO2 Moisture content LOI Fly ash JIS class g/cm cm 2 /g 53.8%.16% 2.% JIS class g/cm cm 2 /g 53.8%.16% 2.% Mixing method Mixer : mortar mixer meeting Japanese Industrial Standard JIS R 521 Mixing volume : 1.5 liters per batch Mixing 3 sec. Mixing 12 sec. Mixing 12 sec. Powder + Sand Water (1) Water (2) + SP END Page 4

5 3.2 Mixture proportioning of mortar The parameters considered in this experiment were fly ash replacement ratio (V FA /Vp), superplasticizer dosage (SP/P), and water-powder ratio (Vw/Vp). The fine aggregate content (S/M) was fixed to be 4% of the total volume of mortar for all mixtures. Then, any change in combination of materials could be treated as a change in the paste properties. The selected values for each parameter are shown as follows: Fly ash replacement ratio (V FA /Vp):,.25,.5,.75, 1 by volume of powder Superplasticizer dosage (SP/P) : %,.8%,1.1%,1.4%,1.7% by weight of powder Water-powder ratio (Vw/Vp) : in the range of by volume of powder For each combination of V FA /Vp and SP/P, trial mixtures for 4 different values of Vw/Vp were evaluated to obtain the test result values of flow area (Γm) between 5 to 7 and flow speed (Rm) of around 1.. These two values are preferable for the production of mortar that has a moderate flowability. The SP dosage was used in range of around.8%-1.7%, because it is the range of SP used in practical work. 4. RESULTS AND DISCUSSION As shown in Figs. 3 and 4, the flowability chart of fresh mortar that use one type of powder material (in these Figures, ordinary Portland cement) can be represented by four indices: retained SP dosage (βsp), unit dispersing SP dosage (Esp), retained water (βwp), and unit free water (Ewp). For the case of one type of powder, the flowability charts of ordinary Portland cement, fly ash Class 1, and fly ash Class 2 were obtained and the four indices for each type of powder were determined. For the case of binary powders, the blended powder of cement and fly ash for each fly ash replacement ratio was treated as one type of powder, and then other flowability charts representing fly ash replacement ratios of.25,.5,.75, and.9 can be achieved. From these flowability charts, those four indices according to each fly ash replacement ratio were determined. 4.1 Influence of Fly Ash on Relationship between Vw/Vp and Rm/Γm.4 The water in mortar can be separated into 2 parts: retained water, which is the water that is retained on the surface of the solid particles in mortar, and free water, which is the water that is excess to the retainability on solid particles and which causes the mortar flow. Considering the flowability chart (Fig. 3), when Rm/Γm.4 is equal to zero, its intercept on Γm-axis means the mortar has a very high flow area but that the flow speed (funnel speed) is zero. This means that mortar can not flow. Increasing the Vw/Vp results in an increase in the value of Rm/Γm.4, so that the flow speed (Rm) is increased. The Vw/Vpaxis intercept then indicates the minimum water-powder ratio that mortar requires in order to flow. Retained water represents not only the amount of water adsorbed on the surface of the solid particles but also includes the entrapped water inside the agglomerated cement particle lumps. The inclination represents the amount of water-powder ratio that is required for an increase of one unit of the Rm/Γm.4 which, in turn, relates to the viscosity-reducing effect of water. Page 5

6 The effect of changing the fly ash replacement ratio on E WP and β WP are shown in Fig. 5. It was found that there is an optimum value for fly ash replacement ratio ( ) for both the retained water (β WP ) and the unit free water (E WP ) That optimum value seems to occur at a fly ash replacement ratio of about.75. For the range of fly ash replacement ratios from. up to.75, the relationship between retained water (β WP ) or unit free water (E WP ) and fly ash replacement ratio ( ) seem to be a linear relationship. Fly ash in Japan has a the high silica content fly ash, which usually results in the fly ash replacement ratio being less than.75 for practical work. Therefore, this linear relationship between retained water (β WP ) or unit free water (E WP ) and fly ash replacement ratio ( ) from until.75 is already sufficient for practical use. β WP 1 E WP Fig. 5. Relationship between fly ash replacement ratio ( ) and retained water (β WP ) and unit free water (E WP ) It is shown that the blended powder of ordinary Portland cement and FA1 (fineness 5, cm 2 /g) provides a lower value of retained water (β WP ) than the blended powder of ordinary Portland cement and FA2 (fineness 3, cm 2 /g). When fly ash is used to replace cement powder, fly ash acts as a lubricant material due to its spherical shape and induces mortar flow easily and prevents the formation of agglomerated cement particle lumps because fly ash particles exist between cement particles. Fly ash itself can not react with superplasticizer and produce a repulsive force the same as cement. Fly ash can only act as a lubricant material. Because it does not produce a repulsive force between the particles as in case of cement, there exists on optimum fly ash replacement ratio. When the finer fly ash FA1 is used at the same volume (same ) as the normal fineness fly ash FA2, the amount of particles of finer fly ash (FA1) is much more than that of the normal fineness fly ash (FA2). This means more lubricant material and thus provides more obstacles that prevent the formation of agglomerated cement particle lumps (4). For unit free water (E WP ), the blend of ordinary Portland cement and FA1 provides a higher value than the blend of ordinary Portland cement and FA2. This may be explained that by increasing the water-powder ratio, the distance between particles is increased, and then the lubrication effect of fly ash is reduced. At high Vw/Vp, the Rm/Γm.4 of each fly ash Page 6

7 replacement ratio become close to each other. So, blended powder that provides a low value of retained water has high value of unit free water. 4.2 Influence of Fly Ash on Relationship between SP/P and Γm/Rm The SP dosage in mortar can be separated into 2 parts: retained SP dosage, which is the minimum superplasticizer dosage for mortar to disperse the powder particles, and dispersing SP dosage, which is the superplasticizer amount that exceeds the minimum requirement to start the mortar flowing. Increasing this excess SP results in an increase in flowability. Considering the flowability chart (Fig. 4), when Γm/Rm is equal to zero, its intercept on the Rm-axis means the mortar has a very high flow speed (relative funnel speed) but the flow area is zero which means that mortar also cannot flow. Increasing the SP/P results in an increasing in the value of Γm/Rm. Then the flow area (Γm) is increased. So the SP/P-axis intercept indicates the minimum SP dosage that mortar requires to flow or it indicates the retained SP dosage in mortar. The inclination represents the amount of SP dosage that is required for an increase of one unit of Γm/Rm or unit dispersing SP dosage. β SP 2 E SP Fig. 6. Relationship between fly ash replacement ratio ( ) and retained SP dosage (β SP ) and unit dispersing SP dosage (E SP ) Fig. 6 shows that both Class 1 and Class 2 are not reactive with this type of superplasticizer, as indicated by very high values of unit dispersing SP dosage (Esp). Fly ash replacement ratios at.9 and 1. have retained SP dosages and dispersing SP dosages quite different from those of other fly ash replacement ratios. It appears that fly ash replacement ratios above.75 are less sensitive to superplasticizer as shown by large values of dispersing SP dosage. From the range of fly ash replacement ratios from. up to.75 (Fig. 7), the relationship between retained SP dosage (β SP ) and fly ash replacement ratio ( ) seems to be a linear relationship. As mentioned above, the ratio between unit dispersing SP dosage and retained SP dosage represents a characteristic of this type of superplasticizer (Fig. 8). Page 7

8 β SP E SP Fig. 7. Relationship between fly ash replacement ratio ( ) up to.75 and retained SP dosage (β SP ) and unit dispersing SP dosage (E SP ) Unit dispersing SP dosage (%) Inclination = Retained SP dosage (%) Fig. 8. Relationship between SP dosage and unit dispersing SP dosage: (the inclination can be used as the characteristic value of SP) As relationship between retained SP dosage and fly ash replacement ratio is linear, then the relationship between unit dispersing SP dosage and fly ash replacement ratio should also be linear. These linear relationships between retained SP dosage (β SP ) or unit dispersing SP dosage (E SP ) and fly ash replacement ratios ( ) from until.75 are thus acceptable. It was shown that blended powder of ordinary Portland cement and FA1 (fineness 5, cm 2 /g) provides higher values of retained SP dosage (β SP ) than blended powder of ordinary Portland cement and FA2 (fineness 3, cm 2 /g). When normal fineness fly ash (almost same fineness as cement powder) is used to replace cement powder, the mortar requires a lower repulsive force to disperse the particles due to the lubricant effect of fly ash. When finer fly ash is used at same volume (same ) as the normal fineness fly ash, the amount of particles of finer fly ash (FA1) is much more than that of the normal fineness fly Page 8

9 ash (FA2). This means that by increasing the surface area, SP can adsorb and reduce the amount of superplasticizer that reacts with cement particles. For a unit dispersing SP dosage (E SP ), the blend of ordinary Portland cement and FA1 provides a higher value than the blend of ordinary Portland cement and FA2. The finer fineness fly ash requires a larger volume of SP in order to increase the thickness of SP on the solid particle so that it is the same as the case of using normal fineness fly ash due to its larger surface area. 5. CONCLUSIONS The flowability of fresh mortar containing ordinary Portland cement and polycarboxylate based superplasticizer was measured for several combinations of fly ash replacement ratio, superplasticizer dosage and water powder ratio. The effect of fly ash replacement ratio and fineness of fly ash on the corresponding proper amount of waterpowder ratio and superplasticizer dosage were found as following: 1. Retained water was an inverse proportion to the fly ash replacement ratio from to.75 for both fly ash Class 1 (FA1) and Class 2 (FA2). 2. Unit free water was increased proportionally with the increasing of fly ash replacement ratio from up to It was found that finer fly ash (fly ash Class 1) provides stronger lubrication effect and agglomerated lumps prevention than the normal fly ash (fly ash Class 2). 4. Effect of fly ash on flowability of mortar with low water-powder ratios is greater than that in high water-powder ratios due to larger distances between the particles. 5. Unit dispersing SP dosage was inversely proportionate to the fly ash replacement ratio from until.75 for both fly ash Class 1 and Class 2 6. Retained SP dosage was inversely proportionate to the fly ash replacement ratio from to.75 for both fly ash Class 1 and Class 2 The authors found that within 75% of the fly ash replacement ratio, the fly ash replacement ratio had linear relationships with the retained water, the unit free water, the retained superplasticizer dosage, and the unit dispersing superplasticizer dosage. From those relationships, the authors proposed the method to predict the flowability of fresh mortar which used any fly ash replacement ratio by using 4 mixes (2 from mixtures of cement only mortar and 2 from mixtures of binary powders mortar) in order to complete the flowability charts that represent all combinations of fly ash replacement ratio, water-powder ratio and superplasticizer dosage. 6. REFERENCES 1. Nawa, T., Izumi, T., and Edamatsu, Y. State-of-the-art Report on Materials and Design of Self-Compacting Concrete ; Proceedings of the International Workshop on Self-Compacting Concrete, Japan, August 1998, pp Page 9

10 2. Ouchi, M., Hibino, M., Sugamata, T., and Okamura, H. A Quantitative Evaluation Method for the Effect of Superplasticizer in Self-Compacting Concrete ; Transactions of the Japan Concrete Institute, Vol.22, 2, pp Ouchi, M., Hibino, M., Ozawa, K., and Okamura, H. A rational mix design method for mortar in self-compacting concrete ; Proceedings of the Sixth East-Asia-Pacific conference on Structural Engineering & Construction (EASEC-6). Taipei, Taiwan, January 1998, pp Sugamata, T., Edamatsu, Y., and Ouchi, M. Distinction between Particle-dispersion and Particle-repulsion effects of Superplasticizers on the Viscosity of Fresh Mortar ; Proceedings of the Second International Symposium on Self-Compacting Concrete. Tokyo, Japan, October 21, pp Page 1

Self-Compacting Concrete

Invited Paper Journal of Advanced Concrete Technology Vol. 1, No. 1, 5-15, April 2003 / Copyright 2003 Japan Concrete Institute 5 Self-Compacting Concrete Hajime Okamura 1 and Masahiro Ouchi 2 Received