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Correlating dynamic segregation of self-consolidating concrete to the slump-flow test
Construction and Building Materials 28 (2012) 499–505
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Correlating dynamic segregation of self-consolidating concrete to the slump-flow test Nathan Tregger ⇑, Amedeo Gregori 1, Liberato Ferrara 2, Surendra Shah Advanced Cement Based Materials, Northwestern University, 2145 Sheridan Road, Suite A130, Evanston, IL 60202, USA
a r t i c l e
i n f o
Article history: Received 24 July 2010 Received in revised form 13 August 2011 Accepted 16 August 2011
Keywords: Self-consolidating concrete Segregation Slump-flow test T50 Viscosity Yield stress
a b s t r a c t The three key characteristics of self-consolidating concrete are flowability, segregation resistance and passing ability. Quality control of flowability is typically predicted by the final diameter (DF) of the slump-flow test. In this study, the time required to reach the final diameter (TF) of the slump-flow test is correlated to dynamic segregation for mixes with a constant DF and aggregate-to-binder ratio. Segregation was determined by measuring the radial aggregate distribution from the slump-flow test. It was demonstrated that increasing the TF improved dynamic segregation resistance. It was also found that the TF was more indicative of viscosity than the time to reach a diameter of 50 cm (T50). Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The three key characteristics of self-consolidating concrete (SCC) are flowability, segregation resistance and passing ability. Quality control of flowability is typically predicted by the final diameter of a slump-flow test; a larger diameter indicates higher flowability. Segregation resistance concerns the ability to retain homogenous distribution of aggregates; segregation can occur both during and after casting. The ability to keep the homogeneity of the aggregate distribution is governed by the volume fraction, distribution and physical properties of the aggregates, as well as the rheological properties of the suspending matrix [1–3], which can be considered the cement paste or mortar for concrete systems [4]. Cementitious materials such as cement paste or mortar are generally regarded as yield stress fluids which can be characterized by the Bingham, Herschel–Bulkley, Casson or other rheological models [5,6]. For concrete at rest, static segregation occurs when the yield stress of the suspending matrix is insufficient to support the weight of the aggregate minus its buoyancy [7–9]. If an aggregate particle settles within the concrete, the stress in the fluid ⇑ Corresponding author. Present address: W.R. Grace & Co.-Conn., 62 Whittemore Ave., Bldg. 29, Cambridge, MA 02140, USA. Tel.: +1 617 498 4396; fax: +1 617 498 4360. E-mail address: [email protected] (N. Tregger). 1 Present address: Department of Structural Engineering, Università degli Studi dell’Aquila, Nucleo Industriale di Bazzano Sud, 67100 Monticchio, Italy. 2 Present address: Department of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci, 20133 Milano, Italy. 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.052
along the surface of the particle exceeds the yield stress (although Beris et al. [7] clearly demonstrate that certain regions above and below the particle remain rigid due to stagnation points). As the distance from the surface of the particle increases, the stress in the fluid decreases to zero. As such, at a certain point, the stress in the fluid will no longer exceed the yield stress, and the fluid will remain rigid [7]. If the yield stress is not exceeded anywhere along the particle surface, the particle will remain suspended. Stability criteria have been developed which depend on gravity, yield stress of the suspending fluid, particle size, difference in density between the suspending matrix and particle and a particular constant (see for example, Roussel [8]). This constant depends on the size of the yielded region around the particle and has been determined theoretically, experimentally and numerically [8]. In order to avoid static segregation, larger particle sizes require higher yield stresses of the suspending fluid. The viscosity of the fluid can also help mitigate segregation by decreasing the speed at which the aggregates settle before the concrete begins to set [3]. However, for typical SCC mixes, the inherently low viscosity plays a small role compared to the yield stress in mitigating static segregation until concrete sets (see the example given in [8]). Still, for dynamic segregation, where the fluid is in motion, the viscosity may have an important role. During motion, the fluid structure breaks down which may allow aggregates to settle if the yield stress is reduced sufficiently [10]. Thus, dynamic segregation is due to the movement of the fluid. Higher viscosities will help drive aggregates along with the flow and also reduce the rate of settlement until the concrete comes to rest. At this point, the
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fluid structure can rebuild, restoring the yield stress and preventing further static segregation. Several simple field tests have been developed that take advantage of the relationship between segregation and rheology, such as the falling ball viscometer [11], the penetration probe [12], the rapid segregation test [13], the flow trough test [14] and the V-funnel, L-Box, U-Box and J-ring tests [15,16]. The first three tests determine the depth at which an object penetrates concrete at rest, thus predicting static segregation. The last four do not predict segregation directly, but have been correlated to both static and dynamic segregation resistance by considering the rheological properties of the concrete ascertained by these tests. Only the flow trough test measures dynamic segregation directly using a channel device to compare concrete that has flowed through the channel (undergoing potential segregation) to concrete not subjected to flow [14]. This test requires wet sieving on site. For the slump-flow test, it has been shown that the final diameter can be calculated based on the yield stress of the material [17]. Furthermore, for the minislump-flow test, it has been demonstrated for cement pastes that the time it takes for the flow to reach the final diameter is related to the ratio between the viscosity and yield stress [18] as shown in Fig. 1. In the present study, this relationship between time to final spread and viscosity is extended to concrete so that a relation between TF and segregation can be highlighted for SCC. This was demonstrated in a preliminary study by Tregger et al. [19] Two sets of data were obtained for flow diameters of 65 cm and 70 cm, both with a constant water-to-binder ratio (w/b) and aggregate-to-binder ratio (agg/b). Within each set, the viscosity of the matrix was systematically varied with the use of a viscosity-modifying agent (VMA), while the final diameter (and thus yield stress) was kept constant by adjusting the high-range water-reducing (HRWR) dosage. From the slump-flow test, the final diameter, time to 50 cm, time to final diameter and segregation resistance of SCC were recorded. The results show that information about dynamic segregation can be obtained from rheological properties inferred by the slump-flow test. 2. Experimental program 2.1. Materials, proportioning and mixing In this study, an ASTM type I Portland cement was used in addition to pea gravel (maximum size 10 mm, 0.4 in), and river sand (maximum size 5 mm, 0.2 inches). A polycarboxylate-based HRWR was used along with a biopolymer-based VMA. Two sets of compositions were used: Set A was designed to have a final diameter of
Fig. 1. Relationship between viscosity/yield stress and time to final spread of a minislump-flow test for cement paste (adapted from [18]).
Table 1 Mixing protocol for cement paste and concrete compositions. Time (mm:ss)
Action
0:00 1:00
Mix aggregates with 1/3 water on low speed Mix rest of dry ingredients with HRWR and 1/3 water on low speed Stop to scrape sides of mixer Mix at high speed Mix VMA if required and rest of water at high speed Perform tests
3:30 4:30 7:00 12:00
65 cm (25.6 in) while Set B a final diameter of 70 cm (27.6 in). Within each set, mixes were designed with VMA additions of 0–5% by weight of binder while the HRWR was adjusted to keep the final diameter constant. In this manner, mixes with increased viscosity but constant yield stress could be achieved. Mixes with 0% VMA were also designed so that segregation could be visibly evident from the slumpflow test. For both sets, the w/b was held constant at 0.45 while the agg/b was set to 4.4. In particular, a coarse aggregate-to-binder ratio of 2.3 in combination with a fine aggregate-to-binder ratio of 2.1 was employed. Mixing was carried out in a large planetary mixer according to the protocol in Table 1. In order to first justify the assumption that increased viscosity and constant yield stress could be achieved through VMA and HRWR adjustments, cement pastes were tested since the aggregate contents were kept constant and also since the yield stress and viscosity could be conveniently measured with a rheometer. Table 2 shows the mix designs for the cement pastes. The same mixing protocol in Table 1 was performed, but in a small planetary mixer. Cement pastes derived directly from the concrete mixes were not used due to difficulties correlating cement paste and concrete rheologies. Hildago et al. have shown that a higher shear stress for cement paste mixes is necessary to achieve a correlation between the rheological behaviors due to shearing contributed by the aggregates [20]. 2.2. Test methods A concentric-cylinder rheometer was used to determine the yield stress and viscosity of the cement pastes according to a modified Bingham model [21]. This was done to verify that mixes with increased viscosities but constant yield stresses could be achieved with proper VMA and HRWR adjustments. The downward, stepwise, rheological protocol is shown in Fig. 2. Table 2 Cement paste mix designs. w/b
HRWR/b
VMA/b
0.45 0.45 0.45 0.45 0.45
0.0175 0.0225 0.0275 0.0375 0.0425
0.01 0.02 0.03 0.04 0.05
Fig. 2. Rheological protocol to determine yield stress and viscosity for cement pastes derived from concrete mixes for a concentric-cylinder rheometer.
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N. Tregger et al. / Construction and Building Materials 28 (2012) 499–505 The average shear stress value for a particular strain-rate (Fig. 3, for example) was plotted against the strain-rate and fitted to the modified Bingham equation:
s ¼ s0 ½1 expð3c_ =c_ crit Þ þ lc_ ; where s is the shear stress, s0 is the yield stress, l is the plastic viscosity, c_ is the strain-rate and c_ crit , is the critical strain-rate, as shown in Fig. 4. In addition to the rheological performance, the final diameter (DF) and time to final diameter (TF) of the minislump-flow test (see [19], for example), were measured for the cement paste to determine the relationship between minislump properties, rheological properties, VMA, and HRWR dosages. The slump-flow test (ASTM C1611 [22]) was implemented to evaluate the flow and segregation properties of each concrete mix. In addition to the DF, the time to 50 cm (T50), the TF and the aggregate distribution were also measured. For each slump flow test, four diameters were measured to account for any irregular final shapes. The effect of irregular shapes on the results presented in this paper was not investigated as this was not encountered during testing. The aggregate distribution was determined from the slump-flow results by measuring the aggregate content in three concentric areas: directly under the initial location of the slump cone, between a diameter of 50 cm and the edge of the slump cone and between the final spread and a diameter of 50 cm as shown in Fig. 5. The aggregate content was measured by first separating the concrete region by region and then wet-sieving to wash out the cement and sand. The remaining aggregates were then oven-dried and weighed.
Cone
50 cm Final spread
Fig. 5. Concentric areas where aggregate contents were wet-sieved, dried and weighed to calculate the radial aggregate distribution.
3. Test results and discussion 3.1. Studies with cement paste Compositions in each of the two sets were designed to have constant yield stress but increasing viscosity. Increased dosages of VMA were used to increase the viscosity, while HRWR additions were adjusted in order to keep the yield stress constant. In order to verify these trends, rheological and minislump-flow properties for the cement mixes are plotted against the addition of VMA in Fig. 6 and Fig. 7. It can be seen that increases in VMA led to increases in both the viscosity and TF values. On the other hand, the HRWR adjustments kept both the yield stress and DF values constant. Based on these results, it is assumed that for the concrete, a constant DF value but increasing TF value translates to a constant yield stress but increasing viscosity since a constant aggregate content was maintained within sets. Fig. 3. Example shear stress data from rheometer. Averages of shear stresses are calculated at each strain-rate.
3.2. Studies with concrete Concrete mixes with 0% VMA additions were designed to visibly exhibit segregation from the slump-flow test. An example of this is
Fig. 4. Rheometer results showing experimental data (points) and modified Bingham fits (lines).
Fig. 6. Yield stress and viscosity values for cement pastes.
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Fig. 7. Final diameter and time to final diameter values for cement pastes.
shown in Fig. 8. Segregation can be seen with the high concentration of aggregates in the center of the slump, and heavy bleeding at the edges. In Fig. 9, a mix with 5% VMA addition is shown to demonstrate the visible improvement in segregation resistance. There are no concentrations of aggregates, and no visible bleeding around the edges.
Fig. 9. Slump-flow results for a non-visibly segregated mix containing 5% VMA addition and a final diameter of 65 cm (25.6 in).
3.3. T50 versus TF As the final diameter is commonly used to give an indication of the yield stress, the T50 is commonly measured in the field to give an indication of the viscosity of the concrete [15,23]. However for the mixes in this study, the correlation between viscosity and T50 was not reliable as shown in Fig. 10 and Fig. 11, which plots the average of three tests along with error bars representing one standard deviation above and below for each composition. Note that the value of correlation coefficient (R2) was as low as 0.42 for the 70 cm spread. For the 65 cm spread, the correlation coefficient is 0.79, which is much higher than the correlation coefficient for the 70 cm spread. This is most likely because 50 cm is closer to the final diameter of 65 cm than 75 cm. As will be shown later in the paper, correlations with TF are much tighter than with T50. As
Fig. 10. T50 plotted against increasing VMA content for a final diameter of 65 cm (25.6 in).
High aggregate concentration
Bleeding
Fig. 8. Slump-flow results for visibly segregated mix containing 0% VMA addition and a final diameter of 65 cm (25.6 in).
Fig. 11. T50 plotted against increasing VMA content for a final diameter of 70 cm (27.6 in).
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such, since T50 is more similar to the corresponding TF for the 65 cm flow than for the 70 cm flow, it follows that the correlation should be higher for the T50 measurements. When the VMA content is plotted against the TF, a much better correlation was found (R2 of 0.98 for both 65 and 70 cm), shown in Figs. 12 and 13. Again, for each composition, the average of three tests is plotted along with error bars indicating one standard deviation above and below. It should be noted that since time measurements are plotted against VMA dosages, different types of VMAs may result in different relationships. For example, if the relationship between VMA dose and viscosity was nonlinear, the relationships in Fig. 12 or Fig. 13 might be different. Nevertheless, these results suggest that compared to T50, TF is a more accurate indicator of viscosity for mixes with the same final diameter. A one-way analysis of variance (ANOVA) was performed on sets of data from each flow diameter (65 and 70 cm) for each variable (T50, TF) with a confidence level of 95% (four groups). The ANOVA was run to determine if the groups of replicates were statistically different from each other. If the calculated F value is greater than the critical F value, then there is no statistical difference between the groups as the ratio (F) between the within-group variance and between-group variance is larger than a critical value (Fcr). For T50 sets, groups within the 65 cm flow data were statistically
different but not different for the 70 cm flow data. On the other hand, for the TF sets, the groups within both the 65 cm and 70 cm flow data sets were statistically different. Furthermore, the ratios of the F values to the critical F values were much larger for the TF sets. The results from the ANOVA analysis are shown in Table 3. The Fcr value to compare to for each set was 3.11.
Fig. 12. TF plotted against increasing VMA content for a final diameter of 65 cm (25.6 in).
Fig. 14. Relative aggregate density as a function of normalized radial spread for a final diameter of 65 cm (25.6 in).
Fig. 13. TF plotted against increasing VMA content for a final diameter of 70 cm (27.6 in).
Fig. 15. Relative aggregate density as a function of normalized radial spread for a final diameter of 70 cm (27.6 in).
Table 3 ANOVA results for T50 and TF data.
T50 TF T50 TF
Flow (cm)
F value
Statistically different?
65 65 70 70
6.04 438 0.941 473
Yes Yes No Yes
3.4. Aggregate distribution In order to determine if increased viscosity improves dynamic segregation resistance, results in terms of aggregate distributions are shown in Figs. 14 and 15, where the relative aggregate density is plotted against the normalized radial distance. The relative aggregate density is obtained by the following equation:
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Relative aggregate density ¼
N. Tregger et al. / Construction and Building Materials 28 (2012) 499–505
aggregate content of region volume of region = : total aggregate content total volume
The normalized radial distance is the radial midpoint of each region divided by the final diameter. For a particular mix, the slope of the relative aggregate density versus normalized radial distance represents the aggregate distribution gradient, ADG. With increasing VMA, the ADG approaches zero, indicating a more evenly distributed aggregate content. A one-way ANOVA was performed to determine if the slopes are statistically different with a confidence level of 95%. In both cases, the F value was greater than the Fcr value indicating that the slopes indeed are statistically different: for 65 cm the F value was 16.4, while the F value for 70 cm was 41.6. In both cases, the Fcr value was 3.11. This ADG value can be plotted against T50, as shown in Fig. 16 or TF as shown in Fig. 17. In Fig. 17, the error bars represent one standard deviation above and below the average of three tests. This was done for both the ADG as well as TF. A logarithmic fit was calculated for each set using a nonlinear regression analysis. As can be noted, the relationship between T50 and ADG is not strong, however, for TF, there exists an obvious trend. Note the correlation coefficient, R2, for T50 is as low as 0.40 while the lowest R2 value is 0.84 for TF. The curves in Fig. 17 can be used to find specific TF values required to obtain a given ADG value. One important note is that even though slump flows for both
Fig. 16. Aggregate distribution gradient as a function of T50 with logarithmic fits.
Fig. 18. Aggregate distribution gradient boundaries as a function of both the flow diameter and TF.
65 and 70 cm with the highest viscosity appeared to be stable, segregation was still measured by calculating the ADG. This highlights the misleading nature of visual inspections of the slump flow test. For each final diameter’s logarithmic fit, TF values can be calculated for specific ADG values. For the two final diameters in this study, TF values were found for four different ADG values: 2, 1.3, 0.7 and 0 (corresponding to an ideal aggregate distribution). The two TF points for a given ADG value were then connected to estimate ADG boundaries as shown in Fig. 18. In this figure, it is clear that dynamic segregation can be improved upon by either increasing the time to final spread (increasing the viscosity) or if possible, decreasing the flow diameter (increasing the yield stress). Fig. 18 represents a first order analysis; to refine the model, more tests will be required. Nevertheless, even from the present analysis, practical knowledge can be obtained. From Fig. 18 it seems that a perfect aggregate distribution (ADG = 0) is not always practical since a balance between flowability and dynamic segregation must be considered. ADG boundaries, shown in Fig. 18, can be chosen to fit certain applications. As an example, a horizontal pour for a long channel would require a larger viscosity (and an ADG closer to zero) to prevent excessive segregation. On the other hand, a top to bottom pour of a column would not require as large a viscosity so an ADG further away from zero could be chosen. In the column, the fluid at the bottom will eventually come to rest and regain its yield stress, whereas in the long channel, the entire fluid will remain in motion.
4. Conclusions
Fig. 17. Aggregate distribution gradient as a function of TF with lognormal fits.
From this study, the slump-flow test has been shown to be capable of indicating dynamic segregation resistances in addition to flowability. This strengthens the slump-flow test as a more complete quality control test for SCC. It was found that slump-flows with higher TF values result in less dynamic segregation. Higher TF values were achieved by adding VMA, while at the same time keeping the flow diameter constant with adjustments in the HRWR dosage. This increase in TF is due to an increase in viscosity of the suspending matrix. It should be noted that different aggregate contents would shift the value of TF. It was also shown that even though slump flows appeared visually stable, segregation was still detected. The relationship between viscosity and dynamic segregation can be more fundamentally investigated using flows of longer length, such as
N. Tregger et al. / Construction and Building Materials 28 (2012) 499–505
in the flow trough [14]. Results may indicate limits of the slumpflow test due to the relatively short flow length. In addition, mixes with higher final diameters required longer times to achieve segregation resistances similar to those of mixes with smaller final diameters. Based on these two relationships, it is possible that for mixes with specific aggregate content and fixed DF, a critical TF can be determined before hand, for which a value lower would indicate a mix that is more dynamically segregation prone. This information can then be used in the field to predict dynamic segregation using a simple test already widely used in the industry. Acknowledgments The research presented in this paper was funded by the Infrastructure Technology Institute of Northwestern University, the Center for Advanced Cement-Based Materials and the National Science Foundation (Award CMS 0625606). Their financial support is gratefully acknowledged. The first author would also like to recognize the support of Lombardia regional council for research conducted at the Politecnico di Milano (code AZ#2). The third author wishes to acknowledge the financial support of the Fulbright foundation, whose grant made his first stay possible at Northwestern University, during which this study was initiated. Lastly, the authors would like to acknowledge Mark Roberts of W.R. Grace for his assistance with the statistical analyzes. References [1] Khayat KH. Viscosity-enhancing admixtures for cement-based materials: an overview. Cement Concr Compos 1998;20:302–10. [2] Bui VK, Akkaya Y, Shah SP. Rheological model for self-consolidating concrete. ACI Mater J 2002;99:549–59. [3] Saak AW, Jennings HM, Shah SP. New methodology for designing selfcompacting concrete. ACI Mater J 2001;98:429–39.
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[4] TouTou Z, Roussel N. Multi scale experimental study of concrete rheology: from water scale to gravel scale. Mater Struct 2006;39(286):189–99. [5] Atzeni C, Massidda L, Sanna U. Comparison between rheological models for Portland cement pastes. Cem Concr Res 1985;15:511–9. [6] Yahia A, Khayat KH. Analytical models for estimating yield stress of high performance pseudo-plastic grout. Cem Concr Res 2001;31:731–8. [7] Beris AN, Tsamopoulos JA, Armstrong RC, Brown RA. Creeping motion of a sphere through a Bingham plastic. J Fluid Mech 1985;158:219–44. [8] Roussel N. A theoretical frame to study stability of fresh concrete. Mater Struct 2006;39(1):81–91. [9] Petrou MF, Wan B, Gadala-Maria F, Kolli VG, Harries KA. Influence of mortar rheology on aggregate settlement. ACI Mater J 2000;97:479–85. [10] Song T, Chiew YM. Settling characteristics of sediments in moving Bingham fluid. J Hydraul Eng 1997;123. [11] Sun Z, Gregori A, Ferron R, Shah SP. Developing falling-ball viscometer for highly flowable cementitious materials. ACI Mater J 2007;104(2):180–6. [12] Shen L, Struble L, Lange D. New method for measuring static segregation of self-consolidating concrete. J Test Eval 2007;35(3):303–9. [13] Bui VK, Montgomery D, Hinczak I, Turner K. Rapid testing method for segregation resistance of self-compacting concrete. Cem Concr Res 2002;32:1489–96. [14] Shen L, Struble L, Lange D. Measuring dynamic segregation of SCC. In: Proceedings from SCC 2008: challenges and barriers to application, third North American conference on the design and use of self-consolidating concrete, Chicago, IL; 10–12 November 2008. [15] Khayat KH, Assaad J, Daczko J. Comparison of field-oriented test methods to assess dynamic stability of self-consolidating concrete. ACI Mater J 2004;101(2):168–76. [16] Assaad J, Khayat K, Daczko J. Evaluation of static stability of self-consolidating concrete. ACI Mater J 2004;101(3):207–15. [17] Roussel N, Coussot P. Fifty-cent rheometer for yield stress measurements: from slump to spreading flow. J Rheol 2005;49(3):705–18. [18] Tregger N, Ferrara L, Shah S. Identifying viscosity of cement paste from minislump-flow test. ACI Mater J 2008;105(6):558–66. [19] Tregger N, Ferrara L, Shah S. Measuring segregation resistance of selfconsolidating concrete from the slump-flow test. J ASTM Int 2010;7(1):45–54. [20] Hildago J, Chen C-T, Struble LJ. Correlation between paste and concrete flow behavior. ACI Mater J 2008;105(3):281–8. [21] Papanastasiou TC. Flows of materials with yield. J Rheol 1987;31(5):385–404. [22] ASTM Designation C-1611-09b. Standard test method for slump flow of selfconsolidating concrete. In: Annual book of ASTM standards, 04.02. Easton, MD: Am. Soc. Test. Mat.; 1996). [23] Bonen D, Shah S. Fresh and hardened properties of self-consolidating concrete. Prog Struct Mater Eng 2005;7(1):14–26.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Correlating dynamic segregation of self-consolidating concrete to the slump-flow test Nathan Tregger ⇑, Amedeo Gregori 1, Liberato Ferrara 2, Surendra Shah Advanced Cement Based Materials, Northwestern University, 2145 Sheridan Road, Suite A130, Evanston, IL 60202, USA
a r t i c l e
i n f o
Article history: Received 24 July 2010 Received in revised form 13 August 2011 Accepted 16 August 2011
Keywords: Self-consolidating concrete Segregation Slump-flow test T50 Viscosity Yield stress
a b s t r a c t The three key characteristics of self-consolidating concrete are flowability, segregation resistance and passing ability. Quality control of flowability is typically predicted by the final diameter (DF) of the slump-flow test. In this study, the time required to reach the final diameter (TF) of the slump-flow test is correlated to dynamic segregation for mixes with a constant DF and aggregate-to-binder ratio. Segregation was determined by measuring the radial aggregate distribution from the slump-flow test. It was demonstrated that increasing the TF improved dynamic segregation resistance. It was also found that the TF was more indicative of viscosity than the time to reach a diameter of 50 cm (T50). Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The three key characteristics of self-consolidating concrete (SCC) are flowability, segregation resistance and passing ability. Quality control of flowability is typically predicted by the final diameter of a slump-flow test; a larger diameter indicates higher flowability. Segregation resistance concerns the ability to retain homogenous distribution of aggregates; segregation can occur both during and after casting. The ability to keep the homogeneity of the aggregate distribution is governed by the volume fraction, distribution and physical properties of the aggregates, as well as the rheological properties of the suspending matrix [1–3], which can be considered the cement paste or mortar for concrete systems [4]. Cementitious materials such as cement paste or mortar are generally regarded as yield stress fluids which can be characterized by the Bingham, Herschel–Bulkley, Casson or other rheological models [5,6]. For concrete at rest, static segregation occurs when the yield stress of the suspending matrix is insufficient to support the weight of the aggregate minus its buoyancy [7–9]. If an aggregate particle settles within the concrete, the stress in the fluid ⇑ Corresponding author. Present address: W.R. Grace & Co.-Conn., 62 Whittemore Ave., Bldg. 29, Cambridge, MA 02140, USA. Tel.: +1 617 498 4396; fax: +1 617 498 4360. E-mail address: [email protected] (N. Tregger). 1 Present address: Department of Structural Engineering, Università degli Studi dell’Aquila, Nucleo Industriale di Bazzano Sud, 67100 Monticchio, Italy. 2 Present address: Department of Structural Engineering, Politecnico di Milano, Piazza Leonardo da Vinci, 20133 Milano, Italy. 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.052
along the surface of the particle exceeds the yield stress (although Beris et al. [7] clearly demonstrate that certain regions above and below the particle remain rigid due to stagnation points). As the distance from the surface of the particle increases, the stress in the fluid decreases to zero. As such, at a certain point, the stress in the fluid will no longer exceed the yield stress, and the fluid will remain rigid [7]. If the yield stress is not exceeded anywhere along the particle surface, the particle will remain suspended. Stability criteria have been developed which depend on gravity, yield stress of the suspending fluid, particle size, difference in density between the suspending matrix and particle and a particular constant (see for example, Roussel [8]). This constant depends on the size of the yielded region around the particle and has been determined theoretically, experimentally and numerically [8]. In order to avoid static segregation, larger particle sizes require higher yield stresses of the suspending fluid. The viscosity of the fluid can also help mitigate segregation by decreasing the speed at which the aggregates settle before the concrete begins to set [3]. However, for typical SCC mixes, the inherently low viscosity plays a small role compared to the yield stress in mitigating static segregation until concrete sets (see the example given in [8]). Still, for dynamic segregation, where the fluid is in motion, the viscosity may have an important role. During motion, the fluid structure breaks down which may allow aggregates to settle if the yield stress is reduced sufficiently [10]. Thus, dynamic segregation is due to the movement of the fluid. Higher viscosities will help drive aggregates along with the flow and also reduce the rate of settlement until the concrete comes to rest. At this point, the
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fluid structure can rebuild, restoring the yield stress and preventing further static segregation. Several simple field tests have been developed that take advantage of the relationship between segregation and rheology, such as the falling ball viscometer [11], the penetration probe [12], the rapid segregation test [13], the flow trough test [14] and the V-funnel, L-Box, U-Box and J-ring tests [15,16]. The first three tests determine the depth at which an object penetrates concrete at rest, thus predicting static segregation. The last four do not predict segregation directly, but have been correlated to both static and dynamic segregation resistance by considering the rheological properties of the concrete ascertained by these tests. Only the flow trough test measures dynamic segregation directly using a channel device to compare concrete that has flowed through the channel (undergoing potential segregation) to concrete not subjected to flow [14]. This test requires wet sieving on site. For the slump-flow test, it has been shown that the final diameter can be calculated based on the yield stress of the material [17]. Furthermore, for the minislump-flow test, it has been demonstrated for cement pastes that the time it takes for the flow to reach the final diameter is related to the ratio between the viscosity and yield stress [18] as shown in Fig. 1. In the present study, this relationship between time to final spread and viscosity is extended to concrete so that a relation between TF and segregation can be highlighted for SCC. This was demonstrated in a preliminary study by Tregger et al. [19] Two sets of data were obtained for flow diameters of 65 cm and 70 cm, both with a constant water-to-binder ratio (w/b) and aggregate-to-binder ratio (agg/b). Within each set, the viscosity of the matrix was systematically varied with the use of a viscosity-modifying agent (VMA), while the final diameter (and thus yield stress) was kept constant by adjusting the high-range water-reducing (HRWR) dosage. From the slump-flow test, the final diameter, time to 50 cm, time to final diameter and segregation resistance of SCC were recorded. The results show that information about dynamic segregation can be obtained from rheological properties inferred by the slump-flow test. 2. Experimental program 2.1. Materials, proportioning and mixing In this study, an ASTM type I Portland cement was used in addition to pea gravel (maximum size 10 mm, 0.4 in), and river sand (maximum size 5 mm, 0.2 inches). A polycarboxylate-based HRWR was used along with a biopolymer-based VMA. Two sets of compositions were used: Set A was designed to have a final diameter of
Fig. 1. Relationship between viscosity/yield stress and time to final spread of a minislump-flow test for cement paste (adapted from [18]).
Table 1 Mixing protocol for cement paste and concrete compositions. Time (mm:ss)
Action
0:00 1:00
Mix aggregates with 1/3 water on low speed Mix rest of dry ingredients with HRWR and 1/3 water on low speed Stop to scrape sides of mixer Mix at high speed Mix VMA if required and rest of water at high speed Perform tests
3:30 4:30 7:00 12:00
65 cm (25.6 in) while Set B a final diameter of 70 cm (27.6 in). Within each set, mixes were designed with VMA additions of 0–5% by weight of binder while the HRWR was adjusted to keep the final diameter constant. In this manner, mixes with increased viscosity but constant yield stress could be achieved. Mixes with 0% VMA were also designed so that segregation could be visibly evident from the slumpflow test. For both sets, the w/b was held constant at 0.45 while the agg/b was set to 4.4. In particular, a coarse aggregate-to-binder ratio of 2.3 in combination with a fine aggregate-to-binder ratio of 2.1 was employed. Mixing was carried out in a large planetary mixer according to the protocol in Table 1. In order to first justify the assumption that increased viscosity and constant yield stress could be achieved through VMA and HRWR adjustments, cement pastes were tested since the aggregate contents were kept constant and also since the yield stress and viscosity could be conveniently measured with a rheometer. Table 2 shows the mix designs for the cement pastes. The same mixing protocol in Table 1 was performed, but in a small planetary mixer. Cement pastes derived directly from the concrete mixes were not used due to difficulties correlating cement paste and concrete rheologies. Hildago et al. have shown that a higher shear stress for cement paste mixes is necessary to achieve a correlation between the rheological behaviors due to shearing contributed by the aggregates [20]. 2.2. Test methods A concentric-cylinder rheometer was used to determine the yield stress and viscosity of the cement pastes according to a modified Bingham model [21]. This was done to verify that mixes with increased viscosities but constant yield stresses could be achieved with proper VMA and HRWR adjustments. The downward, stepwise, rheological protocol is shown in Fig. 2. Table 2 Cement paste mix designs. w/b
HRWR/b
VMA/b
0.45 0.45 0.45 0.45 0.45
0.0175 0.0225 0.0275 0.0375 0.0425
0.01 0.02 0.03 0.04 0.05
Fig. 2. Rheological protocol to determine yield stress and viscosity for cement pastes derived from concrete mixes for a concentric-cylinder rheometer.
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N. Tregger et al. / Construction and Building Materials 28 (2012) 499–505 The average shear stress value for a particular strain-rate (Fig. 3, for example) was plotted against the strain-rate and fitted to the modified Bingham equation:
s ¼ s0 ½1 expð3c_ =c_ crit Þ þ lc_ ; where s is the shear stress, s0 is the yield stress, l is the plastic viscosity, c_ is the strain-rate and c_ crit , is the critical strain-rate, as shown in Fig. 4. In addition to the rheological performance, the final diameter (DF) and time to final diameter (TF) of the minislump-flow test (see [19], for example), were measured for the cement paste to determine the relationship between minislump properties, rheological properties, VMA, and HRWR dosages. The slump-flow test (ASTM C1611 [22]) was implemented to evaluate the flow and segregation properties of each concrete mix. In addition to the DF, the time to 50 cm (T50), the TF and the aggregate distribution were also measured. For each slump flow test, four diameters were measured to account for any irregular final shapes. The effect of irregular shapes on the results presented in this paper was not investigated as this was not encountered during testing. The aggregate distribution was determined from the slump-flow results by measuring the aggregate content in three concentric areas: directly under the initial location of the slump cone, between a diameter of 50 cm and the edge of the slump cone and between the final spread and a diameter of 50 cm as shown in Fig. 5. The aggregate content was measured by first separating the concrete region by region and then wet-sieving to wash out the cement and sand. The remaining aggregates were then oven-dried and weighed.
Cone
50 cm Final spread
Fig. 5. Concentric areas where aggregate contents were wet-sieved, dried and weighed to calculate the radial aggregate distribution.
3. Test results and discussion 3.1. Studies with cement paste Compositions in each of the two sets were designed to have constant yield stress but increasing viscosity. Increased dosages of VMA were used to increase the viscosity, while HRWR additions were adjusted in order to keep the yield stress constant. In order to verify these trends, rheological and minislump-flow properties for the cement mixes are plotted against the addition of VMA in Fig. 6 and Fig. 7. It can be seen that increases in VMA led to increases in both the viscosity and TF values. On the other hand, the HRWR adjustments kept both the yield stress and DF values constant. Based on these results, it is assumed that for the concrete, a constant DF value but increasing TF value translates to a constant yield stress but increasing viscosity since a constant aggregate content was maintained within sets. Fig. 3. Example shear stress data from rheometer. Averages of shear stresses are calculated at each strain-rate.
3.2. Studies with concrete Concrete mixes with 0% VMA additions were designed to visibly exhibit segregation from the slump-flow test. An example of this is
Fig. 4. Rheometer results showing experimental data (points) and modified Bingham fits (lines).
Fig. 6. Yield stress and viscosity values for cement pastes.
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Fig. 7. Final diameter and time to final diameter values for cement pastes.
shown in Fig. 8. Segregation can be seen with the high concentration of aggregates in the center of the slump, and heavy bleeding at the edges. In Fig. 9, a mix with 5% VMA addition is shown to demonstrate the visible improvement in segregation resistance. There are no concentrations of aggregates, and no visible bleeding around the edges.
Fig. 9. Slump-flow results for a non-visibly segregated mix containing 5% VMA addition and a final diameter of 65 cm (25.6 in).
3.3. T50 versus TF As the final diameter is commonly used to give an indication of the yield stress, the T50 is commonly measured in the field to give an indication of the viscosity of the concrete [15,23]. However for the mixes in this study, the correlation between viscosity and T50 was not reliable as shown in Fig. 10 and Fig. 11, which plots the average of three tests along with error bars representing one standard deviation above and below for each composition. Note that the value of correlation coefficient (R2) was as low as 0.42 for the 70 cm spread. For the 65 cm spread, the correlation coefficient is 0.79, which is much higher than the correlation coefficient for the 70 cm spread. This is most likely because 50 cm is closer to the final diameter of 65 cm than 75 cm. As will be shown later in the paper, correlations with TF are much tighter than with T50. As
Fig. 10. T50 plotted against increasing VMA content for a final diameter of 65 cm (25.6 in).
High aggregate concentration
Bleeding
Fig. 8. Slump-flow results for visibly segregated mix containing 0% VMA addition and a final diameter of 65 cm (25.6 in).
Fig. 11. T50 plotted against increasing VMA content for a final diameter of 70 cm (27.6 in).
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such, since T50 is more similar to the corresponding TF for the 65 cm flow than for the 70 cm flow, it follows that the correlation should be higher for the T50 measurements. When the VMA content is plotted against the TF, a much better correlation was found (R2 of 0.98 for both 65 and 70 cm), shown in Figs. 12 and 13. Again, for each composition, the average of three tests is plotted along with error bars indicating one standard deviation above and below. It should be noted that since time measurements are plotted against VMA dosages, different types of VMAs may result in different relationships. For example, if the relationship between VMA dose and viscosity was nonlinear, the relationships in Fig. 12 or Fig. 13 might be different. Nevertheless, these results suggest that compared to T50, TF is a more accurate indicator of viscosity for mixes with the same final diameter. A one-way analysis of variance (ANOVA) was performed on sets of data from each flow diameter (65 and 70 cm) for each variable (T50, TF) with a confidence level of 95% (four groups). The ANOVA was run to determine if the groups of replicates were statistically different from each other. If the calculated F value is greater than the critical F value, then there is no statistical difference between the groups as the ratio (F) between the within-group variance and between-group variance is larger than a critical value (Fcr). For T50 sets, groups within the 65 cm flow data were statistically
different but not different for the 70 cm flow data. On the other hand, for the TF sets, the groups within both the 65 cm and 70 cm flow data sets were statistically different. Furthermore, the ratios of the F values to the critical F values were much larger for the TF sets. The results from the ANOVA analysis are shown in Table 3. The Fcr value to compare to for each set was 3.11.
Fig. 12. TF plotted against increasing VMA content for a final diameter of 65 cm (25.6 in).
Fig. 14. Relative aggregate density as a function of normalized radial spread for a final diameter of 65 cm (25.6 in).
Fig. 13. TF plotted against increasing VMA content for a final diameter of 70 cm (27.6 in).
Fig. 15. Relative aggregate density as a function of normalized radial spread for a final diameter of 70 cm (27.6 in).
Table 3 ANOVA results for T50 and TF data.
T50 TF T50 TF
Flow (cm)
F value
Statistically different?
65 65 70 70
6.04 438 0.941 473
Yes Yes No Yes
3.4. Aggregate distribution In order to determine if increased viscosity improves dynamic segregation resistance, results in terms of aggregate distributions are shown in Figs. 14 and 15, where the relative aggregate density is plotted against the normalized radial distance. The relative aggregate density is obtained by the following equation:
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Relative aggregate density ¼
N. Tregger et al. / Construction and Building Materials 28 (2012) 499–505
aggregate content of region volume of region = : total aggregate content total volume
The normalized radial distance is the radial midpoint of each region divided by the final diameter. For a particular mix, the slope of the relative aggregate density versus normalized radial distance represents the aggregate distribution gradient, ADG. With increasing VMA, the ADG approaches zero, indicating a more evenly distributed aggregate content. A one-way ANOVA was performed to determine if the slopes are statistically different with a confidence level of 95%. In both cases, the F value was greater than the Fcr value indicating that the slopes indeed are statistically different: for 65 cm the F value was 16.4, while the F value for 70 cm was 41.6. In both cases, the Fcr value was 3.11. This ADG value can be plotted against T50, as shown in Fig. 16 or TF as shown in Fig. 17. In Fig. 17, the error bars represent one standard deviation above and below the average of three tests. This was done for both the ADG as well as TF. A logarithmic fit was calculated for each set using a nonlinear regression analysis. As can be noted, the relationship between T50 and ADG is not strong, however, for TF, there exists an obvious trend. Note the correlation coefficient, R2, for T50 is as low as 0.40 while the lowest R2 value is 0.84 for TF. The curves in Fig. 17 can be used to find specific TF values required to obtain a given ADG value. One important note is that even though slump flows for both
Fig. 16. Aggregate distribution gradient as a function of T50 with logarithmic fits.
Fig. 18. Aggregate distribution gradient boundaries as a function of both the flow diameter and TF.
65 and 70 cm with the highest viscosity appeared to be stable, segregation was still measured by calculating the ADG. This highlights the misleading nature of visual inspections of the slump flow test. For each final diameter’s logarithmic fit, TF values can be calculated for specific ADG values. For the two final diameters in this study, TF values were found for four different ADG values: 2, 1.3, 0.7 and 0 (corresponding to an ideal aggregate distribution). The two TF points for a given ADG value were then connected to estimate ADG boundaries as shown in Fig. 18. In this figure, it is clear that dynamic segregation can be improved upon by either increasing the time to final spread (increasing the viscosity) or if possible, decreasing the flow diameter (increasing the yield stress). Fig. 18 represents a first order analysis; to refine the model, more tests will be required. Nevertheless, even from the present analysis, practical knowledge can be obtained. From Fig. 18 it seems that a perfect aggregate distribution (ADG = 0) is not always practical since a balance between flowability and dynamic segregation must be considered. ADG boundaries, shown in Fig. 18, can be chosen to fit certain applications. As an example, a horizontal pour for a long channel would require a larger viscosity (and an ADG closer to zero) to prevent excessive segregation. On the other hand, a top to bottom pour of a column would not require as large a viscosity so an ADG further away from zero could be chosen. In the column, the fluid at the bottom will eventually come to rest and regain its yield stress, whereas in the long channel, the entire fluid will remain in motion.
4. Conclusions
Fig. 17. Aggregate distribution gradient as a function of TF with lognormal fits.
From this study, the slump-flow test has been shown to be capable of indicating dynamic segregation resistances in addition to flowability. This strengthens the slump-flow test as a more complete quality control test for SCC. It was found that slump-flows with higher TF values result in less dynamic segregation. Higher TF values were achieved by adding VMA, while at the same time keeping the flow diameter constant with adjustments in the HRWR dosage. This increase in TF is due to an increase in viscosity of the suspending matrix. It should be noted that different aggregate contents would shift the value of TF. It was also shown that even though slump flows appeared visually stable, segregation was still detected. The relationship between viscosity and dynamic segregation can be more fundamentally investigated using flows of longer length, such as
N. Tregger et al. / Construction and Building Materials 28 (2012) 499–505
in the flow trough [14]. Results may indicate limits of the slumpflow test due to the relatively short flow length. In addition, mixes with higher final diameters required longer times to achieve segregation resistances similar to those of mixes with smaller final diameters. Based on these two relationships, it is possible that for mixes with specific aggregate content and fixed DF, a critical TF can be determined before hand, for which a value lower would indicate a mix that is more dynamically segregation prone. This information can then be used in the field to predict dynamic segregation using a simple test already widely used in the industry. Acknowledgments The research presented in this paper was funded by the Infrastructure Technology Institute of Northwestern University, the Center for Advanced Cement-Based Materials and the National Science Foundation (Award CMS 0625606). Their financial support is gratefully acknowledged. The first author would also like to recognize the support of Lombardia regional council for research conducted at the Politecnico di Milano (code AZ#2). The third author wishes to acknowledge the financial support of the Fulbright foundation, whose grant made his first stay possible at Northwestern University, during which this study was initiated. Lastly, the authors would like to acknowledge Mark Roberts of W.R. Grace for his assistance with the statistical analyzes. References [1] Khayat KH. Viscosity-enhancing admixtures for cement-based materials: an overview. Cement Concr Compos 1998;20:302–10. [2] Bui VK, Akkaya Y, Shah SP. Rheological model for self-consolidating concrete. ACI Mater J 2002;99:549–59. [3] Saak AW, Jennings HM, Shah SP. New methodology for designing selfcompacting concrete. ACI Mater J 2001;98:429–39.
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