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Method for evaluating segregation in self-consolidating concrete using electrical resistivity measurements
Construction and Building Materials 232 (2020) 117283
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Method for evaluating segregation in self-consolidating concrete using electrical resistivity measurements Hong Jae Yim a, Young Hwan Bae b, Jae Hong Kim c,⇑ a
Department of Civil Engineering, Pusan National University, Busan 46241, Republic of Korea Department of Construction and Disaster Prevention Engineering, Kyungpook National University, Sangju 37224, Republic of Korea c Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea b
h i g h l i g h t s A poor mix proportioning of self-consolidating concrete causes undesirable aggregate settling. A column testing setup is proposed to measure the electrical resistivity of fresh concrete. The variation of electrical resistivity in the top and bottom sections is observed with a mix showing aggregate segregation. As a result, the proposed method can sensitively evaluate the relative degree of static segregation in self-consolidating concrete.
a r t i c l e
i n f o
Article history: Received 6 March 2019 Received in revised form 29 August 2019 Accepted 13 October 2019
Keywords: Segregation Self-consolidating concrete Electrical resistivity Square array electrode method
a b s t r a c t Self-consolidating concrete requires a highly fluid binder with sufficient segregation resistance in the fresh mixture. However, careless mix proportioning with various chemical and mineral admixtures can cause instability among the components in the fresh state, including aggregate segregation. The fully or partially segregated mix expectedly exhibits low durability after it hardens. This study attempted to nondestructively evaluate the degree of static segregation in self-consolidating concrete. A columntype test mold was fabricated, which allowed cross-sectional electrical resistivity to be measured in the top and bottom sections using a total of 24 electrodes. The electrical resistivity can sensitively evaluate the relative degree of segregation in concrete mixtures. The experimental results were also compared and discussed using the coarse aggregate segregation index obtained using the conventional method, that requires wet-sieving of self-consolidating concrete in the fresh state. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Self-consolidating concrete (SCC) has high workability by enhancing its fluidity. Its rheological properties, such as viscosity and yield stress, are controlled by sensitively determined the optimum mix proportioning and chemical and mineral admixtures. However, failure to achieve the proper mix-proportion due to high water content, overdosage of superplasticizer, missed viscositymodifying admixture, and other factors can cause segregation of the SCC mixture. Solid aggregate, with its higher specific gravity, tends to settle down relative to the mortar, and water bleeding occurs simultaneously. This phenomenon leads to inhomogeneity in the concrete mixture, which results in potential damage, including degraded and scattered strength, excessive porosity
⇑ Corresponding author. E-mail address: [email protected] (J.H. Kim). https://doi.org/10.1016/j.conbuildmat.2019.117283 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and resultant high drying shrinkage, and unexpected permeability after curing [1]. To achieve a high resistance to segregation, aggregate particles should be distributed by dimension. They need to be well distributed spatially during mixing and placing. Appropriate methods to evaluate the stability of early-age placed concrete have been proposed with respect to aggregate segregation and bleeding [2–5]. Several methods were developed to evaluate dynamic segregation, including relative prediction methods based on the rheological behavior of the fresh state, such as the V-funnel test and L-Box or J-Ring test. Among them, ASTM C 1611 suggests a direct evaluation method of dynamic segregation, which used a visual stability index determined by slump flow [6]. For static segregation of concrete, the column segregation method has been used to measure the content of settled coarse aggregate in a fresh mixture, via wet-sieving of the column segments: ASTM C 1610 [2]. In addition, the penetration test [3], using a hardened visual stability index [7], measurement of electrical
2
H.J. Yim et al. / Construction and Building Materials 232 (2020) 117283
conductivity [8–10], image analysis of the hardened concrete [11,12], and image processing of lightweight aggregate concrete [13] were also developed to evaluate static segregation. On the other hand, electrical resistivity is determined by ion transport through water-filled pores, and many researchers have used this phenomenon to characterize cement hydration under various mix-proportions [14–18]. Recently, the setting and hardening of cement-based materials were also evaluated by measuring electrical resistivity via four-electrode method [19]. The previous experiment indicated that the electrical resistivity of fresh concrete can be used to distinguish the aggregate content in various concrete mixes. To enable more reliable and rapid evaluation of static segregation, this study proposes a nondestructive test method based on the measurement of the electrical resistivity of fresh concrete. A column testing setup was fabricated that can measure the electrical properties of fresh concrete in the top and bottom sections of a column without wet-sieving of coarse aggregates. Rapid measurements of segregation were performed using SCC mixtures with different degrees of segregation, based on the amount of added superplasticizer. The degree of segregation was also evaluated using a conventional test method [2]. The obtained results were then compared and discussed with the results of the settled coarse aggregate. 2. Experimental details 2.1. Sample preparation A total of 6 concrete samples, labeled C0, C1, C2, C3, C4, and C5, were produced. Their mix proportions were identical except for the dosage of superplasticizer, as reported in Table 1. All of the samples had a water-to-cement weight ratio of 0.6. They were 0%, 1%, 2%, 3%, 4% and 5% per cement mass, respectively. The constituent materials used for the concrete samples were as follows: Portland cement type I with a specific gravity of 3.15, river sand for fine aggregate with a maximum size of 5 mm, and crushed gravel for coarse aggregate with a maximum size of 20 mm. The aggregates used were prepared in a surface-dry saturated condition to minimize the effects of water absorption on the test results. The size distribution of the coarse aggregates was controlled to ensure the same proportions at each grade, as reported in Table 2, which expectedly minimized the effect of aggregate size on the segregation test. Polynaphthalene sulfonate was used as a superplasticizer. The surfactant can lead to a better flow of solid particles in the direction of gravity. The samples were mixed using a planetary mixer for 10 min. All samples were poured into a column-type mold, and before testing, tapping by rubber hammer was applied. 2.2. Column segregation measurement A widely used method to evaluate the static segregation of concrete prior to hardening is the column segregation measurement. ASTM C 1610 defines this method that measures the coarse aggregate content in fresh SCC [2]. A freshly-mixed concrete is placed in
Table 2 Particle size distribution of used coarse aggregate. Particle size range (mm)
Probability density function
5.0–9.5 9.5–16 16–20
0.333 0.333 0.333
a cylindrical mold that is divided into three section in the vertical direction, and after tapping and vibrating, the separated coarse aggregates are measured in each section via wet-sieving. Accordingly, the evaluation of static segregation is accomplished by comparing the coarse aggregate in the top and bottom sections as follow:
Ss ¼ 2
CAb CAt 100 CAb þ CAt
ð1Þ
where, Ss is the static segregation ratio in percent, CAt and CAb are the mass of coarse aggregate in the top section and bottom section of the column, respectively. 2.3. Electrical resistivity measurement The mix proportion of concrete determines the initial microstructure of the concrete, including the solid volume fraction, the spacing of solid particles, and water tortuosity through isolated solid particles. The percolated water network determines current flow between inserted electrodes, and measurement of electrical resistivity can evaluate the initial microstructure. The fourelectrode electrical resistivity method can measure the electrical resistivity of fresh cement-based materials without the polarization error caused by the two-electrode method, where electrodes share current and potential [19]. A fabricated column-type mold was prepared to accommodate the multiple-electrode configuration as shown in Fig. 1. This cylindrical mold was made of polyethylene, a non-conducting material, and its dimension complied with the static segregation test equipment (ASTM C 1610). Its inner diameter and height were 150 mm and 540 mm, respectively. A total of 12 brass-electrodes in each section (90 mm away from the top or bottom of the mold) were placed at 30° intervals. Their diameter was 5 mm and they were inserted into the mold about 10 mm. Fig. 2(a) shows the cross-section diagram of the multiple electrodes. Selecting 4 electrodes at the corners of a square developed a circuit. The generating current electrodes were connected through the red dashed line, and the measuring potential electrodes were connected through the blue dashed line in the figure. The square array has been reported to be sensitive to the anisotropy of a geophysical sample [20]. The apparent resistivity (q) of a sample can be obtained from the geometric condition as follows:
q¼
pffiffiffi 2paR pffiffiffi ¼ paR 2 þ 2 ¼ 10:73 aR 2 2
ð2Þ
where a is the spacing between electrodes (here 107.5 mm), and R is electrical resistance obtained by Ohm’s law (R = V/I) from the
Table 1 Mixing ratios of concrete samples. Label
Water (kg/m3)
Cement (kg/m3)
Sand (kg/m3)
Gravel (kg/m3)
AE admixture (kg/m3)
Superplasticizer (kg/m3)
C0 C1 C2 C3 C4 C5
219 219 219 219 219 219
365 365 365 365 365 365
1025 1025 1025 1025 1025 1025
719 719 719 719 719 719
0.011 0.011 0.011 0.011 0.011 0.011
3.65 7.29 10.94 14.59 18.24
3
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duration of the electric potential was limited to 10 ms, eliminating the effect of electrical charge. The current was measured using an alternating-current input module (National Instrument, 9227), and the potential difference was measured at the potential electrodes using a voltage meter (National Instrument, 9222). 3. Results and discussion
Fig. 1. Schematic illustration of fabricated column-type mold.
current and potential electrodes. The cross-sectional resistivity was then averaged with the twelve electrodes sequentially rotating at 30°. The current flow and equipotential lines are described in Fig. 2(b). To measure the stabilized result of electrical resistivity in the mixture, tests were performed 30 min after placing the mix into the cylindrical mold. For the electrical test setup, a sinusoidal potential was generated through the current electrodes using a waveform generator (National Instrument, 9263), which was beneficial to avoid the polarization of water molecules or one-way movement of ion species during the measurement [19]. The peak amplitude voltage was ±1 V and the frequency was 100 Hz. The
To evaluate static segregation, a conventional segregation measurement was performed using six types of prepared mixtures in accordance with ASTM C 1610 [2]. Based on the coarse aggregates (between 5 mm and 20 mm) obtained through wet-sieving in three duplicated mixtures, the averaged coarse aggregate weight in the top and bottom sections are listed with their increased ratio in Table 3. Here following ASTM C 1610, the top and bottom sections represent the upper half and lower half of the mold, respectively. A higher dosage of superplasticizer in the concrete sample decreased the content of coarse aggregate in the top section and
Table 3 Averaged weights of wet-sieved coarse aggregate at top and bottom section of cylinder-type mold. Sample
Top section (kg)
Bottom section (kg)
Increased ratio
C0 C1 C2 C3 C4 C5
1.663 1.547 1.417 1.138 0.917 0.673
1.776 1.798 2.049 2.169 2.288 2.395
1.068 1.162 1.446 1.906 2.495 3.559
Fig. 2. (a) Cross-sectional diagram for electrodes locations and electrical setup, and (b) current flow and equipotential lines for square array.
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increased it in the bottom section [21]. Superplasticizer molecules are adsorbed onto the cement surface by their interaction with the cement. The adsorbed anionic superplasticizer changes the properties of the cement surface and the interaction between the solution phase and other cement particles [22]. This phenomenon induces repulsive forces between adjacent cement particles, and this affects the electrical resistivity of the fresh mixture. The evaluated static segregation ratios using Eq. (1) are compared in Table 4. The segregation ratio increased with the dosage of superplasticizer as expected. For example, the segregation ratio was 112.3% for the C5 sample incorporating a 5% dosage of superplasticizer. The coarse aggregate in the bottom section was 3.55 times larger than that in the top section. The increased ratio of coarse aggregate in the samples is compared in Fig. 3. Using the C0 sample as a reference, the relative content of coarse aggregate in each section is compared in Fig. 4. Here, the top and bottom sections each took 25% of the volume of the column-type mold, and the middle section held 50% of the volume. As can be seen, the ratio
of coarse aggregate changed. The coarse aggregate settled and increased in the middle section as well as the bottom section. The aforementioned result considers the settling of coarse aggregates and the corresponding migration of binder toward the top. However, it hardly reflects the segregation of fine aggregates and bleed water. After placing the mixture on site, static segregation is accompanied by water bleeding, which is related to the consolidation of the mixture. The phenomenon leads to a change in initial electrical resistivity caused by the dissolution of mobile ions from the cement into the pore solution. Previous studies on self-
Table 4 Averaged results of static segregation ratio in accordance with ASTM C1610. Sample
Segregation ratio (%)
C0 C1 C2 C3 C4 C5
6.59 15.01 36.48 62.35 85.55 112.26
Fig. 3. Increased ratio of coarse aggregate in prepared self-consolidating concrete samples.
Fig. 4. Relative content of coarse aggregate due to segregation.
Fig. 5. Measured electrical resistivity and its variation with rotation at each section of sample (a) C0, (b) C2, and (c) C5.
H.J. Yim et al. / Construction and Building Materials 232 (2020) 117283
5
weight and pressurized bleeding tests have reported that bleeding occurrence converges and stops 30 min after placing [4,5]. To minimize the chemico-physical effects and to get rid of the effect of the hydrating products on the electrodes, this study selected the time of measurement to be 30 min after the mixture’s placing. The electrical resistivity of each section was obtained using the twelve rotating measurements. The values fluctuated slightly due to the cross-sectional inhomogeneity of the concrete mixture. Fig. 5 shows the representative test results (C0, C2, and C5) and their averaged electrical resistivity. The fluctuation due to the inherent inhomogeneity of C0 was within ± 7.65% and ± 5.27% in the top and bottom sections, on average, ± 6.17% and 3.35% in the top and bottom in C2, and ± 2.16% and ± 3.66% in the top and bottom in C5. The range of fluctuation overlapped the difference in the electrical resistivity in the top and bottom section of the C0 sample, and cannot be called segregated. On the other hand, the electrical resistivity in the top and bottom sections of the C5 sample showed a remarkable difference compared to each fluctuation. Obviously it was segregated. Here it should be noted that the electrical resistivity depends on the mix proportion of concrete. Decreasing water content in the mixture increases the tortuosity of the water-filled spaces and consequently electrical resistivity. A larger solid volume in a uniform concrete can break water percolation (cutting electrically conductive wires in the square array), which leads to a higher electrical resistivity. Therefore, the relative electrical resistivity in the top and bottom sections needs to be measured to evaluate the static segregation. The segregation ratio in Eq. (1) is also defined by the relative content of coarse aggregate. Table 5 reports the average cross-sectional electrical resistivity and their increased ratio. Sample C0 (without the superplasticizer) showed similar electrical resistivity as previously discussed. The average was 2.80 X m approximately. The segregation in samples C1 to C5 resulted in an increase in the ratio of electrical resistivity in the bottom section. The measured electrical resistivity increased Table 5 Measured electrical resistivity and increased ratio between top and bottom of cylinder-type mold. Sample
Top section [X m]
Bottom section [X m]
Increased ratio
C0 C1 C2 C3 C4 C5
2.813 2.522 2.485 2.543 2.549 2.423
2.797 2.615 2.699 2.993 3.163 3.232
0.994 1.037 1.088 1.177 1.241 1.334
Fig. 7. (a) Correlation between increased ratios of coarse aggregate and increased electrical resistivity at the bottom section, (b) and correlation between segregation ratio by ASTM C1610 and increased electrical resistivity at bottom section depending on segregation occurrence by superplasticizer.
up to 33.4% with 5% added superplasticizer. Its increased ratio is also comparatively plotted in Fig. 6. The correlation between the conventional test results and the measured electrical resistivity was obvious. Fig. 7(a) shows the correlation between the increased electrical resistivity and the increased coarse aggregate, where their exponential correlation could be found. The segregation ratio defined by ASTM C 1610 was linearly correlated with the increased ratio of electrical resistivity, as shown in Fig. 7(b). Both correlations indicate that the electrical resistivity measurement could possibly be used in place of the conventional wet-sieving technique to evaluate the segregation ratio. The nondestructive test method for measuring electrical resistivity applied here used a column-type mold to verify its accuracy. It can be easily implemented in a real formwork where the concrete mixture is prone to secretion. 4. Conclusion Fig. 6. Increased ratio of electrical resistivity in prepared self-consolidating concrete samples.
This study presents a nondestructive method for rapidly evaluating static segregation in SCC. The proposed column-testing setup
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H.J. Yim et al. / Construction and Building Materials 232 (2020) 117283
is based on multiple measurements of electrical resistivity in the top and bottom sections of the column-type mold. The relative electrical resistivity is immediately determined using 12 electrodes in each section. When the amount of added superplasticizer was increased up to 5%, the coarse aggregate settled in the bottom section was 3.5 times greater than in the top section. The measured electrical resistivity increased up to 33.4%. Based on comparison tests between the proposed electrical method and the conventional wet-sieving method, it can be concluded that the proposed method can effectively evaluate the degree of static segregation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1D1A3B07044605). References [1] T. Soshiroda, Segregation characteristics of concrete containing a high-range water-reducing admixture, ACI Mater. J. 68 (1981) 121–138. [2] ASTM C 1610-06, Standard Test Method for Static Segregation of SelfConsolidating Concrete Using Column Technique, Annual Book of ASTM Standards, 2006. [3] ASTM C 1712-14, Standard Test Method for Rapid Assessment of Static Segregation Resistance of Self-Consolidating Concrete Using Penetration Test, Annual Book of ASTM Standards, 2014. [4] H.J. Yim, J.H. Kim, H.G. Kwak, J.K. Kim, Evaluation of internal bleeding in concrete using a self-weight bleeding test, Cem. Concr. Res. 53 (2013) 18–24. [5] H.J. Yim, J.H. Kim, H.G. Kwak, Experimental simulation of bleeding under a high concrete column, Cem. Concr. Res. 57 (2014) 61–69. [6] ASTM C 1611-18, Standard Test Method for Slump Flow of Self-Consolidating Concrete, Annual Book of ASTM Standards, 2018.
[7] AASHTO PP58, Static Segregation of Hardened Self-Consolidating Concrete Cylinders, American Association of State Highway and Transportation Officials, 2008. [8] C. Jolicoeur, K.H. Khayat, T.V. Pavate, M. Page, Evaluation of effect of chemical admixture and supplementary materials on stability of concrete-based materials using in-situ conductivity method, ACI Mater. J. 195 (2000) 461–483. [9] T.V. Pavate, K.H. Khayat, C. Jolicoeur, An In-situ conductivity method for monitoring segregation, bleeding and strength development in cement-based materials, ACI Mater. J. 195 (2000) 535–560. [10] K.H. Khayat, T.V. Pavate, J. Assaad, C. Jolicoeur, Analysis of variations in electrical conductivity to assess stability of cement-based materials, ACI Mater. J. 100 (2003) 302–310. [11] L. Shen, Testing static segregation of SCC, in: 2nd North American Conference on The Design and Use of SCC. (2005) 729–737. [12] B. Pierre, On-site concrete segregation estimation using image analysis, J. Adv. Concr. Technol. 6 (1) (2008) 171–180. [13] F.S. Barbosa, A.-L. Beaucour, M.C.R. Farage, S. Ortola, Image processing applied to the analysis of segregation in lightweight aggregate concretes, Constr. Build. Mater. 25 (2011) 3375–3381. [14] L. Xiao, Z. Li, X. Wei, Selection of superplasticizer in concrete mix design by measuring the early electrical resistivities of pastes, Cem. Concr. Compos. 29 (2007) 350–356. [15] M. Heikal, M.S. Morsy, I. Aiad, Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes, Cem. Concr. Res. 35 (2005) 680–687. [16] N. Schwarz, M. DuBois, Na. Neithalath, Electrical conductivity based characterization of plain and coarse glass powder modified cement pastes, Cem. Concr. Compos. 29 (2007) 656–666. [17] D.A. Koleva, O. Copuroglu, K. Van Breugel, G. Ye, J.H.W. de Wit, Electrical resistivity and microstructural properties of concrete materials in conditions of current flow, Cem. Concr. Compos. 30 (2000) 731–744. [18] I.B. Topcu, T. Uygunoglu, I. Hocaoglu, Electrical conductivity of setting cement paste with different mineral admixtures, Constr. Build. Mater. 28 (1) (2012) 414–420. [19] H.J. Yim, H.J. Lee, J.H. Kim, Evaluation of mortar setting time by using electrical resistivity measurements, Constr. Build. Mater. 146 (2017) 679–686. [20] F.K. Boadu, J. Gyamfi, E. Owusu, Determining subsurface fracture characteristics from azimuthal resistivity surveys: a case study at Nsawam, Ghana, Geophysics 70 (5) (2005) B35–B42. [21] M. Benaicha, O. Jalbaud, X. Roguiez, A.H. Alaoui, Y. Burtschell, Prediction of self-compacting concrete homogeneity by ultrasonic velocity, Alexandria Eng. J. 54 (4) (2015) 1181–1191. [22] C. Jolicoeur, M.A. Simard, Chemical admixture-cement interactions: phenomenology and physico-chemical concepts, Cem. Concr. Compos. 20 (1998) 87–101.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Method for evaluating segregation in self-consolidating concrete using electrical resistivity measurements Hong Jae Yim a, Young Hwan Bae b, Jae Hong Kim c,⇑ a
Department of Civil Engineering, Pusan National University, Busan 46241, Republic of Korea Department of Construction and Disaster Prevention Engineering, Kyungpook National University, Sangju 37224, Republic of Korea c Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea b
h i g h l i g h t s A poor mix proportioning of self-consolidating concrete causes undesirable aggregate settling. A column testing setup is proposed to measure the electrical resistivity of fresh concrete. The variation of electrical resistivity in the top and bottom sections is observed with a mix showing aggregate segregation. As a result, the proposed method can sensitively evaluate the relative degree of static segregation in self-consolidating concrete.
a r t i c l e
i n f o
Article history: Received 6 March 2019 Received in revised form 29 August 2019 Accepted 13 October 2019
Keywords: Segregation Self-consolidating concrete Electrical resistivity Square array electrode method
a b s t r a c t Self-consolidating concrete requires a highly fluid binder with sufficient segregation resistance in the fresh mixture. However, careless mix proportioning with various chemical and mineral admixtures can cause instability among the components in the fresh state, including aggregate segregation. The fully or partially segregated mix expectedly exhibits low durability after it hardens. This study attempted to nondestructively evaluate the degree of static segregation in self-consolidating concrete. A columntype test mold was fabricated, which allowed cross-sectional electrical resistivity to be measured in the top and bottom sections using a total of 24 electrodes. The electrical resistivity can sensitively evaluate the relative degree of segregation in concrete mixtures. The experimental results were also compared and discussed using the coarse aggregate segregation index obtained using the conventional method, that requires wet-sieving of self-consolidating concrete in the fresh state. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Self-consolidating concrete (SCC) has high workability by enhancing its fluidity. Its rheological properties, such as viscosity and yield stress, are controlled by sensitively determined the optimum mix proportioning and chemical and mineral admixtures. However, failure to achieve the proper mix-proportion due to high water content, overdosage of superplasticizer, missed viscositymodifying admixture, and other factors can cause segregation of the SCC mixture. Solid aggregate, with its higher specific gravity, tends to settle down relative to the mortar, and water bleeding occurs simultaneously. This phenomenon leads to inhomogeneity in the concrete mixture, which results in potential damage, including degraded and scattered strength, excessive porosity
⇑ Corresponding author. E-mail address: [email protected] (J.H. Kim). https://doi.org/10.1016/j.conbuildmat.2019.117283 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and resultant high drying shrinkage, and unexpected permeability after curing [1]. To achieve a high resistance to segregation, aggregate particles should be distributed by dimension. They need to be well distributed spatially during mixing and placing. Appropriate methods to evaluate the stability of early-age placed concrete have been proposed with respect to aggregate segregation and bleeding [2–5]. Several methods were developed to evaluate dynamic segregation, including relative prediction methods based on the rheological behavior of the fresh state, such as the V-funnel test and L-Box or J-Ring test. Among them, ASTM C 1611 suggests a direct evaluation method of dynamic segregation, which used a visual stability index determined by slump flow [6]. For static segregation of concrete, the column segregation method has been used to measure the content of settled coarse aggregate in a fresh mixture, via wet-sieving of the column segments: ASTM C 1610 [2]. In addition, the penetration test [3], using a hardened visual stability index [7], measurement of electrical
2
H.J. Yim et al. / Construction and Building Materials 232 (2020) 117283
conductivity [8–10], image analysis of the hardened concrete [11,12], and image processing of lightweight aggregate concrete [13] were also developed to evaluate static segregation. On the other hand, electrical resistivity is determined by ion transport through water-filled pores, and many researchers have used this phenomenon to characterize cement hydration under various mix-proportions [14–18]. Recently, the setting and hardening of cement-based materials were also evaluated by measuring electrical resistivity via four-electrode method [19]. The previous experiment indicated that the electrical resistivity of fresh concrete can be used to distinguish the aggregate content in various concrete mixes. To enable more reliable and rapid evaluation of static segregation, this study proposes a nondestructive test method based on the measurement of the electrical resistivity of fresh concrete. A column testing setup was fabricated that can measure the electrical properties of fresh concrete in the top and bottom sections of a column without wet-sieving of coarse aggregates. Rapid measurements of segregation were performed using SCC mixtures with different degrees of segregation, based on the amount of added superplasticizer. The degree of segregation was also evaluated using a conventional test method [2]. The obtained results were then compared and discussed with the results of the settled coarse aggregate. 2. Experimental details 2.1. Sample preparation A total of 6 concrete samples, labeled C0, C1, C2, C3, C4, and C5, were produced. Their mix proportions were identical except for the dosage of superplasticizer, as reported in Table 1. All of the samples had a water-to-cement weight ratio of 0.6. They were 0%, 1%, 2%, 3%, 4% and 5% per cement mass, respectively. The constituent materials used for the concrete samples were as follows: Portland cement type I with a specific gravity of 3.15, river sand for fine aggregate with a maximum size of 5 mm, and crushed gravel for coarse aggregate with a maximum size of 20 mm. The aggregates used were prepared in a surface-dry saturated condition to minimize the effects of water absorption on the test results. The size distribution of the coarse aggregates was controlled to ensure the same proportions at each grade, as reported in Table 2, which expectedly minimized the effect of aggregate size on the segregation test. Polynaphthalene sulfonate was used as a superplasticizer. The surfactant can lead to a better flow of solid particles in the direction of gravity. The samples were mixed using a planetary mixer for 10 min. All samples were poured into a column-type mold, and before testing, tapping by rubber hammer was applied. 2.2. Column segregation measurement A widely used method to evaluate the static segregation of concrete prior to hardening is the column segregation measurement. ASTM C 1610 defines this method that measures the coarse aggregate content in fresh SCC [2]. A freshly-mixed concrete is placed in
Table 2 Particle size distribution of used coarse aggregate. Particle size range (mm)
Probability density function
5.0–9.5 9.5–16 16–20
0.333 0.333 0.333
a cylindrical mold that is divided into three section in the vertical direction, and after tapping and vibrating, the separated coarse aggregates are measured in each section via wet-sieving. Accordingly, the evaluation of static segregation is accomplished by comparing the coarse aggregate in the top and bottom sections as follow:
Ss ¼ 2
CAb CAt 100 CAb þ CAt
ð1Þ
where, Ss is the static segregation ratio in percent, CAt and CAb are the mass of coarse aggregate in the top section and bottom section of the column, respectively. 2.3. Electrical resistivity measurement The mix proportion of concrete determines the initial microstructure of the concrete, including the solid volume fraction, the spacing of solid particles, and water tortuosity through isolated solid particles. The percolated water network determines current flow between inserted electrodes, and measurement of electrical resistivity can evaluate the initial microstructure. The fourelectrode electrical resistivity method can measure the electrical resistivity of fresh cement-based materials without the polarization error caused by the two-electrode method, where electrodes share current and potential [19]. A fabricated column-type mold was prepared to accommodate the multiple-electrode configuration as shown in Fig. 1. This cylindrical mold was made of polyethylene, a non-conducting material, and its dimension complied with the static segregation test equipment (ASTM C 1610). Its inner diameter and height were 150 mm and 540 mm, respectively. A total of 12 brass-electrodes in each section (90 mm away from the top or bottom of the mold) were placed at 30° intervals. Their diameter was 5 mm and they were inserted into the mold about 10 mm. Fig. 2(a) shows the cross-section diagram of the multiple electrodes. Selecting 4 electrodes at the corners of a square developed a circuit. The generating current electrodes were connected through the red dashed line, and the measuring potential electrodes were connected through the blue dashed line in the figure. The square array has been reported to be sensitive to the anisotropy of a geophysical sample [20]. The apparent resistivity (q) of a sample can be obtained from the geometric condition as follows:
q¼
pffiffiffi 2paR pffiffiffi ¼ paR 2 þ 2 ¼ 10:73 aR 2 2
ð2Þ
where a is the spacing between electrodes (here 107.5 mm), and R is electrical resistance obtained by Ohm’s law (R = V/I) from the
Table 1 Mixing ratios of concrete samples. Label
Water (kg/m3)
Cement (kg/m3)
Sand (kg/m3)
Gravel (kg/m3)
AE admixture (kg/m3)
Superplasticizer (kg/m3)
C0 C1 C2 C3 C4 C5
219 219 219 219 219 219
365 365 365 365 365 365
1025 1025 1025 1025 1025 1025
719 719 719 719 719 719
0.011 0.011 0.011 0.011 0.011 0.011
3.65 7.29 10.94 14.59 18.24
3
H.J. Yim et al. / Construction and Building Materials 232 (2020) 117283
duration of the electric potential was limited to 10 ms, eliminating the effect of electrical charge. The current was measured using an alternating-current input module (National Instrument, 9227), and the potential difference was measured at the potential electrodes using a voltage meter (National Instrument, 9222). 3. Results and discussion
Fig. 1. Schematic illustration of fabricated column-type mold.
current and potential electrodes. The cross-sectional resistivity was then averaged with the twelve electrodes sequentially rotating at 30°. The current flow and equipotential lines are described in Fig. 2(b). To measure the stabilized result of electrical resistivity in the mixture, tests were performed 30 min after placing the mix into the cylindrical mold. For the electrical test setup, a sinusoidal potential was generated through the current electrodes using a waveform generator (National Instrument, 9263), which was beneficial to avoid the polarization of water molecules or one-way movement of ion species during the measurement [19]. The peak amplitude voltage was ±1 V and the frequency was 100 Hz. The
To evaluate static segregation, a conventional segregation measurement was performed using six types of prepared mixtures in accordance with ASTM C 1610 [2]. Based on the coarse aggregates (between 5 mm and 20 mm) obtained through wet-sieving in three duplicated mixtures, the averaged coarse aggregate weight in the top and bottom sections are listed with their increased ratio in Table 3. Here following ASTM C 1610, the top and bottom sections represent the upper half and lower half of the mold, respectively. A higher dosage of superplasticizer in the concrete sample decreased the content of coarse aggregate in the top section and
Table 3 Averaged weights of wet-sieved coarse aggregate at top and bottom section of cylinder-type mold. Sample
Top section (kg)
Bottom section (kg)
Increased ratio
C0 C1 C2 C3 C4 C5
1.663 1.547 1.417 1.138 0.917 0.673
1.776 1.798 2.049 2.169 2.288 2.395
1.068 1.162 1.446 1.906 2.495 3.559
Fig. 2. (a) Cross-sectional diagram for electrodes locations and electrical setup, and (b) current flow and equipotential lines for square array.
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increased it in the bottom section [21]. Superplasticizer molecules are adsorbed onto the cement surface by their interaction with the cement. The adsorbed anionic superplasticizer changes the properties of the cement surface and the interaction between the solution phase and other cement particles [22]. This phenomenon induces repulsive forces between adjacent cement particles, and this affects the electrical resistivity of the fresh mixture. The evaluated static segregation ratios using Eq. (1) are compared in Table 4. The segregation ratio increased with the dosage of superplasticizer as expected. For example, the segregation ratio was 112.3% for the C5 sample incorporating a 5% dosage of superplasticizer. The coarse aggregate in the bottom section was 3.55 times larger than that in the top section. The increased ratio of coarse aggregate in the samples is compared in Fig. 3. Using the C0 sample as a reference, the relative content of coarse aggregate in each section is compared in Fig. 4. Here, the top and bottom sections each took 25% of the volume of the column-type mold, and the middle section held 50% of the volume. As can be seen, the ratio
of coarse aggregate changed. The coarse aggregate settled and increased in the middle section as well as the bottom section. The aforementioned result considers the settling of coarse aggregates and the corresponding migration of binder toward the top. However, it hardly reflects the segregation of fine aggregates and bleed water. After placing the mixture on site, static segregation is accompanied by water bleeding, which is related to the consolidation of the mixture. The phenomenon leads to a change in initial electrical resistivity caused by the dissolution of mobile ions from the cement into the pore solution. Previous studies on self-
Table 4 Averaged results of static segregation ratio in accordance with ASTM C1610. Sample
Segregation ratio (%)
C0 C1 C2 C3 C4 C5
6.59 15.01 36.48 62.35 85.55 112.26
Fig. 3. Increased ratio of coarse aggregate in prepared self-consolidating concrete samples.
Fig. 4. Relative content of coarse aggregate due to segregation.
Fig. 5. Measured electrical resistivity and its variation with rotation at each section of sample (a) C0, (b) C2, and (c) C5.
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weight and pressurized bleeding tests have reported that bleeding occurrence converges and stops 30 min after placing [4,5]. To minimize the chemico-physical effects and to get rid of the effect of the hydrating products on the electrodes, this study selected the time of measurement to be 30 min after the mixture’s placing. The electrical resistivity of each section was obtained using the twelve rotating measurements. The values fluctuated slightly due to the cross-sectional inhomogeneity of the concrete mixture. Fig. 5 shows the representative test results (C0, C2, and C5) and their averaged electrical resistivity. The fluctuation due to the inherent inhomogeneity of C0 was within ± 7.65% and ± 5.27% in the top and bottom sections, on average, ± 6.17% and 3.35% in the top and bottom in C2, and ± 2.16% and ± 3.66% in the top and bottom in C5. The range of fluctuation overlapped the difference in the electrical resistivity in the top and bottom section of the C0 sample, and cannot be called segregated. On the other hand, the electrical resistivity in the top and bottom sections of the C5 sample showed a remarkable difference compared to each fluctuation. Obviously it was segregated. Here it should be noted that the electrical resistivity depends on the mix proportion of concrete. Decreasing water content in the mixture increases the tortuosity of the water-filled spaces and consequently electrical resistivity. A larger solid volume in a uniform concrete can break water percolation (cutting electrically conductive wires in the square array), which leads to a higher electrical resistivity. Therefore, the relative electrical resistivity in the top and bottom sections needs to be measured to evaluate the static segregation. The segregation ratio in Eq. (1) is also defined by the relative content of coarse aggregate. Table 5 reports the average cross-sectional electrical resistivity and their increased ratio. Sample C0 (without the superplasticizer) showed similar electrical resistivity as previously discussed. The average was 2.80 X m approximately. The segregation in samples C1 to C5 resulted in an increase in the ratio of electrical resistivity in the bottom section. The measured electrical resistivity increased Table 5 Measured electrical resistivity and increased ratio between top and bottom of cylinder-type mold. Sample
Top section [X m]
Bottom section [X m]
Increased ratio
C0 C1 C2 C3 C4 C5
2.813 2.522 2.485 2.543 2.549 2.423
2.797 2.615 2.699 2.993 3.163 3.232
0.994 1.037 1.088 1.177 1.241 1.334
Fig. 7. (a) Correlation between increased ratios of coarse aggregate and increased electrical resistivity at the bottom section, (b) and correlation between segregation ratio by ASTM C1610 and increased electrical resistivity at bottom section depending on segregation occurrence by superplasticizer.
up to 33.4% with 5% added superplasticizer. Its increased ratio is also comparatively plotted in Fig. 6. The correlation between the conventional test results and the measured electrical resistivity was obvious. Fig. 7(a) shows the correlation between the increased electrical resistivity and the increased coarse aggregate, where their exponential correlation could be found. The segregation ratio defined by ASTM C 1610 was linearly correlated with the increased ratio of electrical resistivity, as shown in Fig. 7(b). Both correlations indicate that the electrical resistivity measurement could possibly be used in place of the conventional wet-sieving technique to evaluate the segregation ratio. The nondestructive test method for measuring electrical resistivity applied here used a column-type mold to verify its accuracy. It can be easily implemented in a real formwork where the concrete mixture is prone to secretion. 4. Conclusion Fig. 6. Increased ratio of electrical resistivity in prepared self-consolidating concrete samples.
This study presents a nondestructive method for rapidly evaluating static segregation in SCC. The proposed column-testing setup
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is based on multiple measurements of electrical resistivity in the top and bottom sections of the column-type mold. The relative electrical resistivity is immediately determined using 12 electrodes in each section. When the amount of added superplasticizer was increased up to 5%, the coarse aggregate settled in the bottom section was 3.5 times greater than in the top section. The measured electrical resistivity increased up to 33.4%. Based on comparison tests between the proposed electrical method and the conventional wet-sieving method, it can be concluded that the proposed method can effectively evaluate the degree of static segregation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1D1A3B07044605). References [1] T. Soshiroda, Segregation characteristics of concrete containing a high-range water-reducing admixture, ACI Mater. J. 68 (1981) 121–138. [2] ASTM C 1610-06, Standard Test Method for Static Segregation of SelfConsolidating Concrete Using Column Technique, Annual Book of ASTM Standards, 2006. [3] ASTM C 1712-14, Standard Test Method for Rapid Assessment of Static Segregation Resistance of Self-Consolidating Concrete Using Penetration Test, Annual Book of ASTM Standards, 2014. [4] H.J. Yim, J.H. Kim, H.G. Kwak, J.K. Kim, Evaluation of internal bleeding in concrete using a self-weight bleeding test, Cem. Concr. Res. 53 (2013) 18–24. [5] H.J. Yim, J.H. Kim, H.G. Kwak, Experimental simulation of bleeding under a high concrete column, Cem. Concr. Res. 57 (2014) 61–69. [6] ASTM C 1611-18, Standard Test Method for Slump Flow of Self-Consolidating Concrete, Annual Book of ASTM Standards, 2018.
[7] AASHTO PP58, Static Segregation of Hardened Self-Consolidating Concrete Cylinders, American Association of State Highway and Transportation Officials, 2008. [8] C. Jolicoeur, K.H. Khayat, T.V. Pavate, M. Page, Evaluation of effect of chemical admixture and supplementary materials on stability of concrete-based materials using in-situ conductivity method, ACI Mater. J. 195 (2000) 461–483. [9] T.V. Pavate, K.H. Khayat, C. Jolicoeur, An In-situ conductivity method for monitoring segregation, bleeding and strength development in cement-based materials, ACI Mater. J. 195 (2000) 535–560. [10] K.H. Khayat, T.V. Pavate, J. Assaad, C. Jolicoeur, Analysis of variations in electrical conductivity to assess stability of cement-based materials, ACI Mater. J. 100 (2003) 302–310. [11] L. Shen, Testing static segregation of SCC, in: 2nd North American Conference on The Design and Use of SCC. (2005) 729–737. [12] B. Pierre, On-site concrete segregation estimation using image analysis, J. Adv. Concr. Technol. 6 (1) (2008) 171–180. [13] F.S. Barbosa, A.-L. Beaucour, M.C.R. Farage, S. Ortola, Image processing applied to the analysis of segregation in lightweight aggregate concretes, Constr. Build. Mater. 25 (2011) 3375–3381. [14] L. Xiao, Z. Li, X. Wei, Selection of superplasticizer in concrete mix design by measuring the early electrical resistivities of pastes, Cem. Concr. Compos. 29 (2007) 350–356. [15] M. Heikal, M.S. Morsy, I. Aiad, Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes, Cem. Concr. Res. 35 (2005) 680–687. [16] N. Schwarz, M. DuBois, Na. Neithalath, Electrical conductivity based characterization of plain and coarse glass powder modified cement pastes, Cem. Concr. Compos. 29 (2007) 656–666. [17] D.A. Koleva, O. Copuroglu, K. Van Breugel, G. Ye, J.H.W. de Wit, Electrical resistivity and microstructural properties of concrete materials in conditions of current flow, Cem. Concr. Compos. 30 (2000) 731–744. [18] I.B. Topcu, T. Uygunoglu, I. Hocaoglu, Electrical conductivity of setting cement paste with different mineral admixtures, Constr. Build. Mater. 28 (1) (2012) 414–420. [19] H.J. Yim, H.J. Lee, J.H. Kim, Evaluation of mortar setting time by using electrical resistivity measurements, Constr. Build. Mater. 146 (2017) 679–686. [20] F.K. Boadu, J. Gyamfi, E. Owusu, Determining subsurface fracture characteristics from azimuthal resistivity surveys: a case study at Nsawam, Ghana, Geophysics 70 (5) (2005) B35–B42. [21] M. Benaicha, O. Jalbaud, X. Roguiez, A.H. Alaoui, Y. Burtschell, Prediction of self-compacting concrete homogeneity by ultrasonic velocity, Alexandria Eng. J. 54 (4) (2015) 1181–1191. [22] C. Jolicoeur, M.A. Simard, Chemical admixture-cement interactions: phenomenology and physico-chemical concepts, Cem. Concr. Compos. 20 (1998) 87–101.