SCC 2009-China, June 5-7 2009, Beijing,China A MULTI- SCALE INVESTIGATION ON THE EFFECT OF VISCOSITY MODIFYING ADMIXTURES ON THE FLOWABILITY AND STABILITY OF SELF COMPACTING CONCRETE Mohammad Shekarchi (1), Nicolas Ali Libre (1), Rahil Khoshnazar (1), Solmaz Dehghan Marvasty (1) And Reza Rashet Nia (1) (1) Construction Materials Institute, Department of civil Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran Abstract Self compacting concrete, due to its high flowability, may suffer from different signs of instability such as bleeding, blocking and segregation, during transportation and placement. Viscosity modifying admixtures (VMAs), also called segregation reducing agents or anti-washout admixtures, have been extensively used in self compacting concrete for their effects on the stabilizing rheological properties of fresh concrete. The applied VMAs are in various types and their effects are widely varied according to their chemical structures. In this study, the effect of two types of VMAs on the fluidity, viscosity and stability of self compacting concrete was investigated. The experimental investigation was carried out in two phases: cement paste and mortar originated from self compacting concrete and the same trend was observed in all the cases. The obtained results show that some types of VMAs mainly affect the stability of self compacting concrete while having no effect on the viscosity. It was also concluded that using VMAs may decrease the fluidity of self compacting concrete which might be compensated with addition of superplasticizer content or W/C ratios. Keywords: Self compacting concrete, Viscosity modifying admixture, Flowability, Viscosity, Stability 1. INTRODUCTION Self-compacting concrete (SCC), initially developed by Okumura in Japan, is considerably a new concrete technology [1, 2] which mainly characterized by its extraordinary workability and superior flowability [3]. In the last years, there has been a growing interest on SCC technology among constructors and construction industry in several countries. The principal reasons for this interest are concerned with its unique fresh state properties [4, 5]. Self-compacting concrete is able to flow and consolidate under its own weight, completely fill the formwork even in the presence of dense reinforcement and can be pumped easily through long distances [1, 5]. Along with these advantages, this technology also tends to a significan 303
reduction of the construction period and reducedeffort in accomplishing some of the casting tasks and enables a considerable reduction of the acoustic noise levels [4]. An increase in the flowability of concrete is known to increase the risk of segregation. Therefore, it is essential to have proper mix design. Various investigations have been carried out in order to obtain rational SCC mix-design methods which impede from segregation, honeycombing, laitance, settlement and plastic cracking over the top bar [4, 5]. For enhancing homogeneity properties of SCC increasing the fine aggregate content; limiting the maximum aggregate size; increasing the powder content; or utilizing viscosity modifying admixtures (VMAs) is suggested [1]. This study focuses on the use of VMAs in self-compacting concrete as an alternative for enhancing stability and eliminating heterogeneity. VMAs are water-soluble polymers that increase the viscosity of mixing water and enhance the ability of paste to retain its constituents in suspension. The mode of action of a VMA depends on the type and concentration of the polymer in use [6]. Although use of VMAs is considered as an effective way of increasing the stability of self-compacting concrete, these admixtures are not a substitute for poor quality constituents or mix design. For example, aggregates with a good grading curve should always be used for self-compacting concrete. A lack of fines in aggregates will affect the rheology and may contribute to segregation and settlement. However, where suitable aggregates are not economically available the required rheology of the mix can often be achieved by utilizing a VMA to provide a more homogenous and cohesive concrete [7]. In spite of great effect of VMAs in stabilizing concrete, these admixtures may increase the yield stress to some extent. Therefore, optimizing the amount of VMA seems to be unavoidable. The main objective of this study is to investigate the effect of VMAs on the flowability and stability of self compacting concrete. Two different VMAs were studied herein. The experimental program was carried out in two phases. The first phase concerned with the investigation of cement paste because admixtures mainly affect the flow behavior of the cement paste without altering the composition or behavior of the aggregates. Moreover, determining workability properties by testing concrete is not always practical. Expensive concrete testing requires a large amount of materials and labor, which is expensive [8]. The fluidity and viscosity of cement pastes were determined using mini-slump tests and marsh-cone test, respectively. In the second phase, the rheological behavior and stability properties of cementitious mortars were evaluated using mini-slump flow test, mini- V-funnel test and mini-column segregation test. Finally, the results of cement paste and mortar experiments compared to establish a relationship between these two phases. 2. EXPERIMENTAL PROGRAM The experimental program consisted of two phases. The first phase concerned the investigation of the effect of different dosages of two types of VMAs on the fluidity and viscosity of cement pastes. In the second phase, the effect of VMAs on the rheological properties and also stability of cementitious mortars originated from self compacting concrete was evaluated. 2.1 Materials ASTM type Ι ordinary Portland cement was used in all the mixtures. The fine aggregate was well-graded river sand with a specific gravity of 2.7 and water absorption of 2.81%. The 304
maximum size of aggregates was 2.36mm and its fineness modulus was 2.67. The superplasticizer was a polycarboxylic acid-based with a specific gravity and solid content of 1.07gr/cm 3 and 36%, respectively. There were also two types of VMAs used in this investigation. 2.2 Mixture proportions and testing methods All the cement pastes were prepared with a fixed W/C ratio of 0.45 and SP content of 0.2% by cement mass. For each type of VMA, there were a control mixture and four mixtures with different dosages of VMA. The VMA dosages were 0.5%, 1%, 1.5% and 2% by cement mass. In this phase, the testing methods included mini-slump test and marsh cone test (according to ASTM C939-94) which their results corresponds to yield stress and viscosity of cement pastes, respectively. In phase two, a total of 10 mortar mixtures were proportioned with a fixed cement content of 700 kg/m 3 and a W/C ratio of 0.45. SP content of mortar mixtures was fixed at 1% and VMA dosages were 0%, 0.5%, 1%, 1.5% and 2% (by cement mass). The flowability of cementitious mortars was evaluated by measuring the mini-slump flow and mini V-funnel flow time in conformity with the standard procedures given by EFNARC [9]. It has been shown previously that slump flow is related mainly to yield stress [10-13]. The flow time is also related to plastic viscosity [12,13]. In addition, a visual assessment of stability was made in the slump flow test and the visual stability index (VSI) was evaluated. The VSI ranged from 0 for excellent; 1 acceptable; 2 needs improvement; to 3 unacceptable [14]. To evaluate static segregation of the mixtures, the column segregation test was performed. The apparatus used was similar to the one described in ASTM C1610 but in a smaller size. It consisted of a 75 mm diameter, 210 mm tall PVC pipe split into three 70mm tall sections. Each section was clamped together to form a water-tight seal. Mortar was placed into the pipe and left undisturbed for 15 minutes. Each section of the pipe was then removed and the mortar inside was collected. Each mortar sample was washed over a 300 μm (#50) sieve. The retained aggregates were then dried and weighed. The percent static segregation index calculated using the following equation: ( CAB CAT) SI = 2[ ] 100 ( CA + CA ) B T where: SI = static segregation index, percent CA T = mass of coarse aggregate in the top section of the column CA B = mass of coarse aggregate in the bottom section of the column 3. RESULTS & DISCUSSION 3.1 Rheological properties of cement paste The spread diameter of the cement pastes is illustrated in Fig. 1-a. The results show that using VMA1 does not significantly affect the fluidity of cement pastes while VMA2 has a great influence on the fluidity. The effect of VMA2 on the fluidity is not linear. Increasing VMA2 content from 0% to 0.5% results in 30% reduction in the fluidity while increasing this ( 1 ) 305
admixture from 0.5% to 1% decreases the fluidity about 4%. However, increasing VMA2 beyond 1.5% does not have any significant effect on the fluidity. The results of marsh cone test are presented in Fig. 1-b. The figure shows that increasing VMA1 content slightly increases the viscosity. The use of VMA1 up to 2% increases the viscosity about 5.3% compared to control mixture. On the other hand, addition of VMA2 increases the viscosity of cement pastes and every 1% of VMA2 increases the viscosity by an average of 23%. Fig. 1: Effect of VMA on the rheological properties of cement pastes The relationship between fluidity and viscosity of cement pastes is plotted in Fig. 2. The figure shows that all the cement pastes incorporating VMA1 has the same level of fluidity and increasing this type of VMA does not influence the fluidity while slightly increases the viscosity. On the other hand, VMA2 has a great effect on both fluidity and viscosity of cement pastes and improves viscosity while suffering fluidity. The obtained results indicate that as the viscosity of the cement pastes incorporating VMA2 increases, the fluidity decreases considerably. Power response can be observed between viscosity and fluidity of cement pastes incorporating VMA2 with R 2 values of 0.89. Fig. 2: Relationship between fluidity and viscosity of cement pastes 306
3.2 Rheological properties and stability of mortar In the second phase the effect of two types of VMAs on the fluidity, viscosity and also stability of self-compacting mortars was investigated. The obtained results indicate that addition of VMA1 does not have any significant effect on the fluidity of mortar mixtures i.e. increasing this type of VMA does not suffer the fluidity of cementitious mortars. On the other hand, VMA2 has a significant influence on the fluidity of mortar mixtures. As the amount of VMA2 increases, the fluidity of mortar mixtures decreases to a great extent. For example, increasing the VMA2 content from 0% to 0.5% causes an 18% reduction in the fluidity [Fig. 3-a]. Fig. 3-b shows the viscosity of mortar mixtures. As shown in the figure, VMA1 does not influence the viscosity of mortar mixtures. The obtained results also indicate that addition of VMA2 up to 1% does not affect the viscosity of mortar mixtures while increasing VMA2 content from 1% to 1.5% increases the viscosity by about 36%. Fig. 3: Effect of VMA on the rheological properties of mortar mixtures The stability of mortar mixtures was also investigated herein. Table 1 summarizes the visual stability index (VSI) evaluated by mini-slump flow test. The results indicate that both types of VMA seem to be effective in stabilizing self compacting mortar. As shown in Table 1, control mixture exhibits a highly noticeable bleeding and a slight segregation while performing mini-slump flow test. On the other hand, by use of VMA the heterogeneity of may be reduced. Based on the obtained results, addition of only 0.5% VMA1 reduces bleeding and segregation. Bleeding and segregation are completely disappeared in mixtures incorporating 0.5% or more dosages of VMA2 and the VSI of all these mixtures are ranked 0 that is an indication of excellent stability. However, VSI is an approximate indication of the stability of cementitious mixtures and an acceptable VSI does not ensure adequate stability. To evaluate the stability of mortar mixtures more accurately the mini-column segregation test was also performed and the segregation index (SI) was determined. Fig. 4 shows the SI of mortar mixtures. For VMA dosages of 0% to 0.5%, both types of VMA seems to have a similar effect on the SI of mortar mixtures. Addition of each type of VMA up to 0.5% results in a 56% decrease in the SI of mortar mixtures. However, the relationship between VMA content and SI is not linear. As VMA content is increased, the SI is 307
first reduced drastically and then reduced gradually. For example, the incensement of VMA1 beyond 0.7% does not seem to have an improving effect on the stability of mortar mixtures. Mortar mixtures incorporating VMA2 beyond 1% seems to be more stable compared to mixtures with the same dosages of VMA1. However, this type of VMA may decrease the fluidity of mortar mixtures to a large extent which might be compensated with addition of higher SP% or higher W/C ratios. Table 4: VSI of mortar mixtures VMA% VMA1 VMA2 0 2 2 0.5 1 0 1 1 0 1.5 0 0 2 1 0 Fig. 4: Effect of VMA on the SI of mortar mixtures The relationship between fluidity and viscosity of mortar mixtures is plotted in Fig. 5-a. The figure shows that increasing VMA1 does not influence the fluidity of mortar mixtures while slightly increases the viscosity. On the other hand, VMA2 has a great influence on the fluidity and viscosity of mortar mixtures. The higher VMA2 content the lower the fluidity and the higher the viscosity. The relationship between fluidity and viscosity of mortar mixture is similar to that of cement pastes. Power trend is observed between viscosity and fluidity of mortar mixtures incorporating VMA2 with R 2 of 0.84. Fig. 5-b summarizes the relationship between the fluidity and stability of mortar mixtures. The obtained results indicate that VMA1 may be useful in reducing the risk of aggregate segregation without any negative effect on the fluidity. On the other hand, the effect of VMA2 on stabilizing mortar mixtures seems to be more dominant, but this type of VMA reduces the fluidity of mortar mixtures to a great extent. In a constant fluidity, mortar mixtures incorporating VMA1 seems to be more stable than mixtures incorporating VMA2. 308
Fig. 5: Relationship between fluidity, viscosity and stability of mortar mixtures 3.3 Relationship between cement paste and Mortar The results of cement paste and mortar compared to establish a relationship between these two phases. As VMA mainly acts on paste, understanding the rheological behavior of cement pastes incorporating VMA may be useful for selecting the optimum dosage of VMA for mortar and concrete. The relationship between fluidity of cement pastes and fluidity of mortar mixtures is plotted in Fig. 6-a. As stated before, VMA1 does not have a significant effect on the fluidity of cement pastes and mortars. Therefore, the figure shows a slight variation for cementitious mixtures incorporating VMA1 and a relationship may not be established between fluidity and viscosity of cement pastes and mortar. On the other hand, VMA2 has a great influence on the fluidity and viscosity of cement pastes and mortar mixtures. The obtained results indicate that there is a direct relationship between fluidity of cement pastes and mortars. The viscosity of cement pastes incorporating VMA2 is also directly related to viscosity of mortars [Fig. 6-b]. The correlation between cement paste and mortar fresh properties indicates that simple tests performed on cement pastes can be used to evaluate materials and proportions. Fig. 6: Relationship between rheological properties of cement pastes and mortars 309
4. CONCLUSIONS There are several types of VMAs used in self-compacting concrete which their effects are widely varied according to their chemical structures. In this study the effect of two types of VMAs on the fresh properties of cement paste and mortar originated from self compacting concrete was investigated. Based on the obtained results, the following conclusions can be made: VMA1 does not have a significant effect on the fluidity and viscosity of cement pastes and mortars. This type of VMA mainly affects the stability of mortar mixtures and reduces the risk of segregation. VMA2 increases the viscosity of cement pastes and mortars while have a negative effect on the fluidity. In addition, this type of VMA seems to be very effective in stabilizing cementitious mortars and increasing the homogeneity. Mortar mixtures with VMA2 content beyond 1% seems to be more stable than those with the similar content of VMA1. On the other hand, mortar mixtures incorporating VMA2 may suffer from poor fluidity. Proper type and dosage of VMA should be applied due to different materials and proportions and various applications of self compacting concrete. When concrete exhibits sever segregation and fluidity is not a concern incorporating VMA2 may be effective for eliminating the heterogeneity. On the other hand, VMA1 seems to be more applicable to provide a homogeneous mixture without suffering the fluidity. There is a direct relationship between the rheological behavior of cement pastes and mortar mixtures incorporating VMA. Therefore, understanding the rheological behavior of cement pastes incorporating VMA may be useful for selecting the optimum dosage of VMA for mortar and concrete. ACKNOWLEDGEMENTS The authors are grateful for the financial support provided by Construction Materials Institute (CMI) at university of Tehran. 310
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