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Investigation on electrically conductive aggregates produced by incorporating carbon fiber and carbon black
Construction and Building Materials 144 (2017) 106–114
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation on electrically conductive aggregates produced by incorporating carbon fiber and carbon black Binmeng Chen a, Bo Li b,⇑, Yan Gao a, Tung-Chai Ling c, Zeyu Lu a, Zongjin Li a a
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China Department of Civil Engineering, The University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo, China c College of Civil Engineering, Hunan University, Changsha, China b
h i g h l i g h t s Electrically conductive aggregates (ECAs) are fabricated by pelletization technique. Carbon fiber and/or carbon black are well dispersed by semi-dry mixing method. The threshold percolation of carbon fiber and carbon black are identified for ECAs. ECAs exhibit excellent resistivity, acceptable strength and water absorption.
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 10 March 2017 Accepted 18 March 2017
Keywords: Conductive aggregate Carbon fiber Carbon black Pelletization Electrical resistivity
a b s t r a c t This paper reports on an investigation of newly developed electrically conductive aggregates (ECAs) through the semi-dry mixing method and the pelletization technique. Carbon fiber and carbon black were incorporated into the aggregates as conductive fillers, while ordinary Portland cement and fly ash were used as matrix materials. The effects of carbon fiber and/or carbon black dosages on the electrical resistivity, water absorption and crushing strength of ECAs were studied. For ECAs with carbon fiber only, the threshold percolation of carbon fiber was identified to be 1.0% by volume. The ECAs with 1.0% carbon fiber exhibited 3.4 Om electrical resistivity, 13.08% water absorption and 1.57 MPa crushing strength. Moreover, the effect of carbon black content was investigated when the content of carbon fiber was kept at 0.5 vol.%. The threshold percolation of carbon black for ECAs with 0.5 vol.% carbon fiber was 2.0% by weight. These ECAs possessed 7.34 Om resistivity, 24.41% water absorption and 0.95 MPa crushing strength. Scanning electron microscope was employed to study the conductive network formed by two conductive components, which helped to illustrate the conductive mechanism of carbon fiber and carbon black inside the ECAs. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Electrically conductive concrete is a cement-based composite that contains electronically conductive components to achieve a stable and relatively low electrical resistivity [1]. It is a relatively new type of functional material which has drawn much attention due to its good performance in electrical conductivity and mechanical properties [2,3]. With the function of electrical conductivity, electrically conductive concrete has been proposed for the applications of deicing, antistatic flooring, electromagnetic shielding, cathodic protection of steel reinforcement in concrete structure and health monitoring of buildings [3–6]. Since the electrical ⇑ Corresponding author. E-mail address: [email protected] (B. Li). http://dx.doi.org/10.1016/j.conbuildmat.2017.03.168 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
conductivity of normal concrete is very low due to the very limited conductivity of C-S-H and the highly tortuous pore structure [7,8], electrically conductive concrete is normally developed through adding a certain portion of conductive fillers, including steel slag, stainless steel fiber, graphite, carbon fiber, carbon black, etc. Several studies have already focused on the optimization of conductive fillers to reduce the electrical resistivity of concrete [9–18]. Conduction of electricity in the hardened concrete mainly depends on the movement of electrons, which requires a good contact between conductive components. Monteiro et al. [9] prepared a cement-based composite for structural monitoring through adding carbon black particles. Test results showed that the resistivity could be significantly decreased as the increase of the carbon black dosage, which enhanced the accuracy of resistivity monitoring. However, the authors also reported that the carbon black was
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seldom exclusively used in the conductive concrete. Dong et al. [10] incorporated short-cut super-fine stainless wire (SSSW) in reactive powder concrete to improve the electrical conductivity. The optimal content of SSSW in the reactive powder concrete was identified to be 0.5 vol.% for achieving an excellent conductivity of 0.44 Om. Al-Dahawi et al. [11] compared the influence of carbon fiber length on the electrical properties of cementitious composites and revealed that the cementitious composites with the longer carbon fibers exhibited better electrical conductivity than composites with the short ones. Xie et al. [12] also reported that the longer fibers in the non-conductive matrix were more effective to conduct electricity than shorter ones. Thus, the dosage of fibers could be minimized to achieve a certain conductivity value. Chung [1] compared the effectiveness of various electrically conductive components, including steel fibers, steel dust, carbon fibers, carbon nanofiber, coke powder and graphite, on the electrical conductivity of cement-based materials. It was found that steel fiber with 8 lm diameter was the most effective conductive filler for lowering the electrical resistivity. In respect to carbon-based materials, carbon fiber with 15 lm diameter was more effective than carbon nanofiber, coke powder or graphite powder in improving the electrical conductivity. For the electrically conductive concrete with a single conductive component, conductive fiber was more effective for conducting electricity than conductive filler. However, the combined use of conductive fibers and fillers had been recognized to produce a more efficient conductive system in the electrically conductive concrete. Wen and Chung [13] reported that a 50% replacement of carbon fiber by carbon black in the cement matrix composites could maintain the electrical conductivity of the system with 100% carbon fiber, but it reduced the strain sensing effectiveness. Tuan and Yehia [14] developed electrically conductive concrete for bridge deck deicing through adopting steel fiber, steel shaving and carbon-based materials. Carbon and graphite materials were recommended to replace steel shavings due to their drawbacks in consistency of size and composition. The resulting conductive concrete has been demonstrated to work well in deicing. Garcia et al. [15] examined the conductivity of asphalt mortar produced with graphite and steel wool. The conductive fibers were more effective for conducting electricity than the conductive fillers. Comparatively, a combination network consisting of fibers to reach the optimum conductivity and a small volume of filler to stabilize the resistivity were suggested. Wu et al. [16] investigated the effects of filler type, filler content and mixed fillers on the electrical resistivity of the asphalt concrete. It was found that the asphalt conductive concrete produced with pure carbon fiber exhibited the best conductive performance. However, the authors recommended the combined use of small amounts of expensive carbon fibers and large amounts of cheap carbon black or graphite as a cost-effective conductive system for asphalt concrete. Ding et al. [17] proposed the use of nano-carbon black and carbon fiber in developing the electrically conductive concrete as conductive components. The degree of strain and damage of the beam cast by the developed electrically conductive concrete was successfully correlated with electrical properties. Wu et al. [18] developed a threephase composite conductive concrete containing steel fiber, carbon fiber and graphite for pavement deicing. Dispersion of carbon fiber and concrete voids were identified as the main factors affecting the conductivity of concrete. An optimal mix formulation containing 1.0 vol.% steel fiber, 0.4 vol.% carbon fiber and 4.0 wt.% graphite achieved an electrical resistivity of 3.22 Om. Generally, the electrically conductive concrete with combined conductive components exhibited superior performance in terms of efficient and stable electrical conductivity. Conduction of electricity requires the continuous contact of conductive components within the concrete. Conventional aggregates
occupying 50–70% volume of concrete may inhibit the conductive of the media, as they are nonconductive. The electrical resistivity of common coarse aggregates ranged from 300 Om to 1500 Om [3]. Thus, the combination of conductive coarse aggregates with the conventional conductive paste may produce a more continuous, and thus more conductive concrete. In addition, replacing small amount of conventional aggregates by lightweight aggregates would not significantly affect the mechanical properties of concrete [19]. Thus, replacement of normal aggregates with conductive aggregates might be promising to fabricate conductive concrete. Currently, few studies have considered coarse aggregates to be electrically conductive. He et al. [20] manufactured the conductive aggregates by sintering clay with the incorporation of graphite powder. It demonstrated that the resistivity of conductive aggregate mortar was much lower than that of conventional mortar. Unfortunately, the conductive aggregates prepared in the study had a tablet shape which was significantly different from natural coarse aggregates. Moreover, the calcination process to produce conductive aggregates in their study consumed quite a bit of energy. Thus, it is of great interest to produce electrically conductive coarse aggregates with proper manufacturing methods and satisfactory properties. Application of waste materials in producing conductive aggregates would also positively impact environment and cost. In this paper, a pelletization technique was employed to produce conductive coarse aggregates, using a process of agglomeration of fine particles into pellets. Prior to this, a semi-dry mixing method was used to disperse the carbon fibers in the matrix. Ordinary Portland cement and fly ash were used as the matrix materials for pelletization. Carbon fiber and carbon black were incorporated into the matrix materials as the conductive components. Physical, mechanical and electrical properties were estimated and compared for the conductive aggregates. Conductive aggregates prepared by the semi-dry mixing and the pelletization exhibited excellent electrical conductivity with satisfactory mechanical properties. 2. Experimental study 2.1. Raw materials The main materials used to prepare aggregates in this study were ordinary Portland cement (OPC), fly ash (FA). Carbon fiber and/or carbon black were incorporated into the mix as conductive components. The OPC has the specific gravity and surface area of 3.15 and 3310 cm2/g, respectively. The FA complying with ASTM class F ash was adopted in the study. The specific gravity and surface area of the FA were 2.31 and 3960 cm2/g, respectively. The chemical compositions of the OPC and the FA are presented in Table 1. Carbon fiber (CF) with diameter of 12–15 lm and length of 1.0 mm was used in this study. The density of CF was about 1.76 g/cm3 and the volume resistivity of CF was around Table 1 Chemical compositions of OPC and PFA. Chemical compositions
OPC (wt.%)
PFA (wt.%)
SiO2 Al2O3 CaO SO4 Fe2O3 MgO K2O TiO2 Na2O
19.85 3.68 65.14 5.40 2.90 1.78 0.91 0.27 –
55.81 23.17 7.31 1.57 6.68 2.27 1.78 1.29 –
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0.015 Ocm. The tensile strength of the CF was 3800 MPa while the Young’s Modulus was between 220 GPa and 240 GPa. The carbon black (CB) had partial size of 20–50 lm. The density of CB was about 1.3–1.6 g/cm3 and the volume resistivity was 1.5–2.0 Ocm. SEM photos of the CF and CB are shown in Fig. 1.
weight. The constant content of CF in this series was determined based on the test results from series one, which was optimized to achieve a proper electrical conductivity. Mix formulations for both series are presented in Tables 2 and 3. 2.3. Test methods
2.2. Preparation of aggregates Aggregates were prepared by the process consisting semi-dry mixing and pelletization as shown in Fig. 2. Dry powders of OPC and FA with conductive materials were first mixed at the semidry condition to improve the dispersion of CF and CB. At this mixing stage, one third of total water required in the mix formulation was added. After that, raw materials with carbon fibers and/or carbon black were mixed under high shear rate for five minutes to ensure the uniform distribution of conductive components. Semidry mixing method is effective to disperse carbon fibers in the mixed raw materials with the following two actions. Wetting process reduced surface energy of the mixed powders based on the surface energy theory [21], which was beneficial to separate the powder particles. Carbon fibers were attached to the wetted powders due to their attraction forces. During the mixing process, carbon fibers were gradually attached to the cement particles. However, the amount of carbon fibers to be dispersed highly depended on the weight of wetted powders. Agglomeration of carbon fibers was about to happen when the carbon fiber content was relatively high. Semi-dry mixed raw materials were subsequently fed into the pelletizer to produce aggregates. Rest two thirds of water was sprayed into the pelletizer to bind fine particles, which formed the round-shaped aggregates. Pelletization is a process involving in agglomeration of fine particles into the pellets [22,23]. Small grains are firstly formed with small dosage of water and grow up gradually when more powder and water are added [24]. During this process, green pellets are formed through agitation granulation at initial stage and compaction at advanced stage [25]. The manufactured aggregates as shown in Fig. 3 were curing at room temperature for 28 days. Two series of mix formulations were studied according to conductive components used. CF was only adopted in series one while CF and CB were used in series two. The weight ratio of OPC to FA was fixed at 0.25 for all mix formulations. Water to solid material ratio was also fixed at 0.3. Carbon fiber was added at eight dosages at 0, 0.25%, 0.5%, 0.75%, 1.0%, 1.25%, 1.5% and 2.0% by volume in series one. In series two, part of carbon fiber was replaced by carbon black. The content of CF in the aggregate was kept constant while CB was investigated at the dosage of 0, 0.5%, 1.0%, 1.5% and 2.0% by
2.3.1. Electrical resistivity The two-probe method of measuring electrical resistivity was adopted in this study. Schematic view of measurement is illustrated in Fig. 4. Rectangular samples of aggregates were prepared for easy measurement. Resistivity of aggregates could directly affect the electrical resistivity of concrete as the main conductive medium. Thus, the conductive aggregate was first polished from a round shape to a rectangular shape sample. To reduce the influence of the contact resistivity, conductive silver paint was applied on both ends of cross section of the sample. The electrical resistivity of conductive aggregates was measured at 28th day after preparation. The voltage drop V across the sample and current I through the sample were recorded. The electrical resistivity then could be calculated by the Eq. (1).
q¼
VA IL
ð1Þ
where q is the electrical resistivity. V and I are the electrical voltage and current, respectively. A and L are the area of cross section and the length of the sample, respectively. For each mix formulation, six samples were tested and the averaged electrical resistivity was obtained. 2.3.2. Water absorption The water absorption of conductive aggregates was measured according to BS EN 1097-6 [26]. After 28-day water curing, aggregates were first removed from the water and dried in the oven at a temperature of 105 °C until it reached constant mass. The weight of the aggregate at dry condition was recorded as Mdry. Afterwards, the same batch of aggregates was immersed in the water for 24 h before it was removed from the water and dried on the surface by gentle rolling in the cloth. The weight of saturated surface-dried aggregates was recorded as Msat. Then, the 24-h water absorption of the aggregates could be calculated by the following equation.
WA24 h ¼
M sat M dry M dry
ð2Þ
where Mdry is the mass of the oven dried aggregates, and Msat is the mass of 24-h saturated surface-dried aggregates.
Fig. 1. SEM photos of CF and CB.
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Fig. 2. Manufacturing process of conductive aggregates.
3. Results and discussion 3.1. Electrical resistivity of CF-based aggregate
Fig. 3. Photo of manufactured electrically conductive aggregates.
Table 2 Mix formulations for aggregates with carbon fiber only. Mix
OPC
FA
w/b
CF Vol.%
CF0 CF25 CF50 CF75 CF100 CF125 CF150 CF200
1 1 1 1 1 1 1 1
4 4 4 4 4 4 4 4
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
0% 0.25% 0.5% 0.75% 1.0% 1.25% 1.5% 2.0%
2.3.3. Crushing strength The crushing strength of each single aggregate was estimated by uniaxial compression between two parallel rigid plates [27– 29]. Loading rate was set at 0.5 mm/min. The crushing strength of each aggregate S is calculated by the following equation.
S¼
2:8 Pc
ð3Þ
pX 2
where Pc is the load at fracture and X is the sphere diameter of individual aggregate. Averaged strength of ten samples was adopted.
Fig. 5 shows electrical resistivity of the conductive aggregates against content of incorporated carbon fiber. It was obvious that the aggregates had a high electrical resistivity (around 105 Om) when the CF content was less than 0.5 vol.%. It indicated that carbon fibers at those dosages could not form continuous conductive paths inside the aggregate. There was a sharp decrease in the resistivity when content of CF was increased from 0.5 vol.% to 0.75 vol.%. Quantitatively, electrical resistivity decreased from 7.5 104 Om for the aggregates with 0.5 vol.% CF to 23 Om for that with 0.75 vol.%, which was several magnitude degree drops in the electrical resistivity. Further increase in the content of CF marginally reduced the resistivity of aggregates. For instance, the electrical resistivity was reduced from 23 Om to 3.4 Om when content of CF increased from 0.75 vol.% to 1.0 vol.%. Therefore, the percolation threshold of CF in the conductive aggregates was found to be between 0.5% and 1.0% by volume. Similar finding was reported by Al-Dahawi et al. [30]. However, incorporation of CF at a dosage higher than 1.0 vol.% slightly increased the electrical resistivity of aggregates. For instance, the resistivity of aggregates with CF at 1.25 vol.%, 1.5 vol.% and 2.0 vol.% were 7.5 Om, 10.4 Om and 16.0 Om, respectively. This was mainly attributed to the agglomeration of CF at high content inside the aggregates, which reduced the conductive paths and then slightly increased the resistivity of aggregates. This identified that 1.0 vol.% of CF incorporation might be the optimum value for the current aggregate mix design.
3.2. Electrical resistivity of CF&CB-based aggregate Fig. 6 shows the electrical resistivity of aggregates prepared with both CF and CB against the content of CB. The content of CF was fixed at 0.5% by volume, which was slight lower than the percolation threshold of CF. It was expected that the combined use the CF at dosage of 0.5 vol.% and the CB could achieve a good electrical conductivity with the effective conductive paths formed by
Table 3 Mix formulations for aggregates with carbon fiber and carbon black. Mix
OPC
FA
w/b
CF Vol.%
CB wt.%
CF50CB0 CF50CB5 CF50CB10 CF50CB15 CF50CB20
1 1 1 1 1
4 4 4 4 4
0.3 0.3 0.3 0.3 0.3
0.5% 0.5% 0.5% 0.5% 0.5%
0% 0.5% 1% 1.5% 2%
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Fig. 4. Two probes method to measure electrical resistance of cement-based specimen.
3.3. Water absorption
Resistivity ( ·m)
1,00,000 10,000 1,000 100 10 1 0.00% 0.25% 0.50% 0.75% 1.00% 1.25% 1.50% 1.75% 2.00%
Carbon fiber content by volume Fig. 5. Electrical resistivity of aggregate with CF only.
1,00,000
Resistivity ( ·m)
10,000 1,000 100 10 1 0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
Carbon black content by mass Fig. 6. Electrical resistivity of aggregate with CF and CB.
conductive fillers. It was noted that higher content of CF (e.g. 1.0 vol.%) had achieved the suitable electrical conductivity for the applications of deicing. In consideration of cost of ECAs and dispersion of CF, half of CF in the aggregates with proper electrical conductivity was substituted by CB. As seen in Fig. 6, the electrical resistivity of aggregates with CF and CB remained at high level of 3.8–4.8 104 Om when the content of CB was less than 1.0 wt.%. It revealed that the conductive network consisting of CF and CB was not established inside the aggregate. It meant that the CF could not be continuously connected by the CB. When the content of CB increased from 1.0 wt.% to 2.0 wt.%, a sharp decrease in the resistivity was observed. It demonstrated that the percolation threshold of CB for electrical conductivity was located within the above range while the CF was fixed at 0.5 vol.%. More conductive paths were formed inside the aggregates as the incorporation of more CB. However, further increase of the content of CB only slightly improved the resistivity of aggregates. For instance, the electrical resistivity of aggregates dropped from 7.3 Om to 6.4 Om when increasing the content of CB from 2.0 wt.% to 2.5 wt.%.
Fig. 7 shows the water absorption of CF-modified aggregates against the content of carbon fiber. It was readily seen that there were two levels of water absorption for the aggregates with various CF. When the content of CF was less than 1.0 vol.%, the water absorption of aggregates was under low level and was slightly varied from 11.13% to 13.08%. Once the content of CF reached 1.25 vol. %, the water absorption of aggregates dramatically increased to 26.11%, which was almost double of that with 1.0 vol.% CF. It was a turning for both electrical resistivity and water absorption. Further increase in the content of CF slightly increased the water absorption of the aggregates. For instance, the water absorptions of aggregates with 1.5 vol.% and 2.0 vol.% were 27.32% and 28.31%, respectively. This was mainly attributed to the agglomeration of CF, which induced high porosity of the aggregates. This phenomenon could be also verified by the SEM microscopies in the later section. In summary, it was suggested to limit the content of CF below 1.25 vol.% to achieve a better water absorption for the electrically conductive aggregates. Fig. 8 illustrates the water absorption of aggregate containing 0.5 vol.% CF and different content of CB. For the aggregates with the same content of CF at 0.5 vol.%, incorporating CB increased the water absorption of the aggregates, even if the amount of incorporated CB was small. For instance, water absorption was increased from 10.54% to 15.98% when 0.5 wt.% CB was added into the aggregates. Generally, water absorption of the aggregates with both CF and CB gradually increased as the content of CB increased. It demonstrated that CB had adverse effect on the water absorption of aggregates. This was mainly attributed to the water absorption of CB itself inside the aggregates. Specifically, water absorptions of the aggregates with at least 1.5 wt.% CB were at least two times of that with 0.5 vol.% CF only. In addition, it could be found that 30% 25%
Water absorption
10,00,000
20% 15% 10% 5% 0%
0.00% 0.25% 0.50% 0.75% 1.00% 1.25% 1.50% 2.00%
Carbon fiber content by volume Fig. 7. Water absorption of aggregate with CF only.
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30%
2 1.8
Crushing strength (N/mm2)
Water absorption
25% 20% 15% 10% 5%
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2
0%
0.00%
0.50%
1.00%
1.50%
2.00%
0
2.50%
Carbon black content by mass
0.25% 0.50% 0.75% 1.00% 1.25% 1.50% 2.00%
Carbon fiber content by volume
Fig. 8. Water absorption of aggregate with CF and CB.
Fig. 9. Crushing strength of aggregate with CF only.
1.8 1.6
Crushing strength (N/mm2)
the effect of CB on the water absorption of aggregate was much more significant than that of CF. Thus, the content of CB should be well controlled to produce the aggregates with proper water absorption. In general, water absorption of the produced ECAs ranged from 10% to 30%, which depended on the dosage of CF and CB. This was comparable to that of lightweight aggregates in the literature [27,31]. Tuan et al. [32] also suggested that 24-h water absorption of lightweight aggregates should be below 20%. When referring to standard GB/T 17431.1 [33], 1-h water absorption of lightweight aggregates was specified in the range of 10% and 30%. This was related to the type and density of lightweight aggregates. Thus, the produced ECAs exhibited satisfied water absorption despite incorporation of either CF or CF&CB. To overcome the shortage in water absorption, it was suggested to have surface treatment for ECAs or control the amount of ECAs in concrete.
0%
1.4 1.2 1 0.8 0.6 0.4 0.2 0
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
Carbon black mass content
3.4. Crushing strength
Fig. 10. Crushing strength of aggregate with CF and CB.
Fig. 9 shows the crushing strength of aggregates incorporated of CF only. It was readily seen that the crushing strength of aggregates decreased as the increase of carbon fiber content. When incorporating 0.25 vol.% CF into the aggregates, the crushing strength was decreased by 10.5%. This was the first obvious decrease in the crushing strength. Subsequently, crushing strength of aggregates dropped gradually with the increase of CF content from 0.25 vol.% to 1.0 vol.%. There was second significant drop in the crushing strength when the content of CF increased to 1.25%. For instance, crushing strength of aggregates decreased from 1.56 N/mm2 to 1.27 N/mm2, which was about 20% decrease in crushing strength. Further increase in the content of CF continuously reduced the crushing strength. This was probably attributed to the agglomeration of CF at a high dosage that could lead a higher porosity in aggregate. The crushing strength decreased to 0.97 N/ mm2 when the content of CF reached 2.0 vol.%, which was the third obvious drop in crushing strength. Generally, incorporation of CF has detrimental effect on the crushing strength of aggregates. This was mainly attributed to the more porous matrix due to the large aspect ratio of CF [34]. Similarly, it was recommended that the content of CF should be limited to 1.0% in volume by considering the crushing strength of aggregates. Fig. 10 shows the crushing strength of aggregates with constant 0.5 vol.% CF and various dosages of CB. Similar to the aggregates with CF only, the crushing strength of aggregates decreased as the CB content increased. The conductive aggregates without CB exhibited the highest crushing strength at 1.64 N/mm2. The decrease in the crushing strength was almost linear with the increase of CB content in the aggregates. It indicated that CB also
had detrimental influence on the crushing strength of aggregates. When CB was added at high dosage (e.g. 2.0 wt.% or higher), the degradation of crushing strength was more significant. For instance, crushing strength of aggregates with 2.0 wt.% and 2.5 wt.% CB were 0.95 N/mm2 and 0.73 N/mm2, respectively. In respective of crushing strength, it was suggested that the content of CB inside the aggregates should be no more than 2.0 wt.%. Crushing strength of ECAs incorporated either with CF only or CF&CB ranged from 0.7 MPa to 2.0 MPa, which depended on the dosage of conductive fillers. There was a wide variation in crushing strength of aggregates made with different materials and processes. However, it was generally accepted that the crushing strength of artificial lightweight aggregates should be above 0.5 MPa when comparing to commercial lightweight aggregates [27]. The produced ECAs satisfied the above general requirement. Gennaro et al. [35] had demonstrated that lightweight aggregates with a compressive strength around 1.0 MPa could be used to produce structural concrete. However, the crushing strength of ECAs could be further enhanced by optimizing the composition of aggregates. For the application in concrete, the amount of ECAs should be properly controlled with the aim of ensuring concrete strength. 3.5. SEM microscopy Microstructures of ECAs with CF and/or CB were examined by SEM. Fig. 11 shows the SEM microscopies of aggregates with different contents of CF. Generally, CF was much easier to be
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(a) 0% CF
(b) 0.25% CF
(c) 0.5% CF
(d) 0.75% CF
(e) 1.0% CF
(f) >1.0% CF
Fig. 11. SEM microscopies of aggregate with CF only.
(a) 0% CB
(b) < 1.0% CB
(c) 1.5% CB
(d) 2.5% CB
Fig. 12. SEM microscopies of aggregate with CF and CB.
detected in the aggregates when dosage of CF was high. For the aggregates with less than 0.75 vol.% CF (Fig. 11(a)–Fig. 11(c)), carbon fibers were found to be disconnected. That confirmed the high electrical resistivity of these aggregates. On the other hand, it also
verified that these disconnected carbon fibers did not significantly reduce the crushing strength as well as increase the water absorption. The connective paths found in Fig. 11(d) confirmed the dramatic change in the electrical resistivity of aggregates with
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0.75 vol.% CF. With more CF in the aggregates (e.g. 1.5 vol.% and 2.0 vol.%), agglomeration of CFs was clearly found as shown in Fig. 11(e) and Fig. 11(f). That would be main reason for the reduced crushing strength and increased water absorption of aggregates. It also demonstrated that the fibers could be dispersed well by the proposed semi-dry mixing method when a proper fiber dosage was used. Fig. 12 shows the SEM microscopes of the aggregates with CF and CB. As compared to the aggregate in Fig. 12(a), CB particles could be clearly found in the aggregates in Fig. 12(b)–Fig. 10(d). When the content of CB was less than 1.0 wt.%, the conductive paths could not be formed even if the CB particles were uniformly distributed. It was evidenced by the sparsely distribution of CB. As the increase of CB content inside the aggregate, continuous conductive paths could be formed. Meanwhile, CF inside the aggregates was connected by new CB paths, resulting in a conductive network. However, CB would not participate in the hydration process of OPC and the pozzolanic reaction of FA. Thus, adding more CB into the aggregates dramatically reduced the crushing strength. Moreover, CB particles surrounding the OPC and FA particles would negatively affect their reactions. 4. Conclusions In this paper, a type of conductive aggregates was developed by pelletization technique. FA and OPC were used for pelletization as matrix materials. Carbon fiber and carbon black were incorporated into the aggregates as conductive components during the manufacturing process. Properties of electrically conductive aggregates with different contents of carbon fiber and/or carbon black, including electrical resistivity, water absorption, and crushing strength, were investigated. The following conclusions can be drawn from the study. (1) Electrical conductive aggregates could be successfully produced by pelletization method with carbon fiber and/or carbon black incorporated as conductive components. The electrical resistivity of aggregates could be reduced to 3.4 Om and 7.3 Om for CF-modified ECAs and CF&CBmodified ECAs respectively, which was several magnitude lower than conventional aggregates. (2) It was demonstrated that semi-dry mixing method was effective to disperse carbon fiber and/or carbon black inside the aggregates. (3) The threshold percolation of carbon fiber for electrical resistivity was identified at 1.0% by volume. For the aggregates with constant carbon fiber of 0.5 vol.% and carbon black, the threshold percolation of carbon black for electrical resistivity was found to be 2.0% by weight. (4) Incorporation of carbon fiber and/or carbon black was detrimental to increase the water absorption and to decrease the crushing strength of ECAs. However, these degradations could be minimized by incorporating proper dosages of CF and/or CB. (5) In general, the ECAs produced with 1.0 vol.% CF only or with 0.5 vol.% CF and 2.0 wt.% CB were recommended. However, the ECAs produced with 1.0 vol.% CF only exhibited slightly lower resistivity, lower water absorption and higher crushing strength than those with CF&CB.
Acknowledgements The financial supports from the Ministry of Science and Technology of China under the Grand of 2015CB655100 and the technical supports from Materials Lab of Department of Civil Engineering
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at The Hong Kong University of Science and Technology is greatly acknowledged. The second author would also acknowledge the supports from the Faculty of Science and Engineering at The University of Nottingham Ningbo China via New Researchers Grant.
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Li, Estimate the relative electrical conductivity of C-S-H gel from experimental results, Constr. Build. Mater. 71 (2014) 392–396. [8] H. Ma, Z. Li, Realistic pore structure of Portland cement paste: experimental study and numerical simulation, Comput. Concrete 11 (4) (2013) 317–336. [9] A.O. Monteiro, P.B. Cachim, P.M.F.J. Costa, Electrical properties of cementbased composites containing carbon black particles, Mater. Today: Proc. 2 (1) (2015) 193–199. [10] S. Dong, B. Han, J. Ou, Z. Li, L. Han, X. Yu, Electrically conductive behaviors and mechanisms of short-cut super-fine stainless wire reinforced reactive powder concrete, Cem. Concr. Compos. 72 (2016) 48–65. [11] A. Al-Dahawi, O. Ozturk, F. Emami, G. Yildlrim, M. Sahmaran, Effect of mixing methods on the electrical properties of cementitious composites incorporating different carbon-based materials, Constr. Build. Mater. 104 (2016) 160–168. [12] P. Xie, P. Gu, J.J. Beaudoin, Electrical percolation phenomena in cement composites containing conductive fibres, J. Mater. Sci. 31 (15) (1996) 4093– 4097. [13] S.H. Wen, D.D.L. Chung, Partial replacement of carbon fiber by carbon black in multifunctional cement-matrix composites, Carbon 45 (3) (2007) 505–513. [14] C.Y. Tuan, S. Yehia, Evaluation of electrically conductive concrete containing carbon products for deicing, ACI Mater. J. 101 (4) (2004) 287–293. [15] A. Garcia, E. Schlangen, M. van de Ven, Q.T. Liu, Electrical conductivity of asphalt mortar containing conductive fibers and fillers, Constr. Build. Mater. 23 (10) (2009) 3175–3181. [16] S.P. Wu, L.T. Mo, Z.H. Shui, Z. Chen, Investigation of the conductivity of asphalt concrete containing conductive fillers, Carbon 43 (7) (2005) 1358–1363. [17] Y.N. Ding, Z.P. Chen, Z.B. Han, Y.L. Zhang, F. Pacheco-Torgal, Nano-carbon black and carbon fiber as conductive materials for the diagnosing of the damage of concrete beam, Constr. Build. Mater. 43 (2013) 233–241. [18] J.M. Wu, J.G. Liu, F. Yang, Three-phase composite conductive concrete for pavement deicing, Constr. Build. Mater. 75 (2015) 129–135. [19] M. Gesoglu, E. Guneyisi, T. Ozturan, H.O. Oz, D.S. Asaad, Permeation characteristics of self compacting concrete made with partially substitution of natural aggregates with rounded lightweight aggregates, Constr. Build. Mater. 59 (2014) 1–9. [20] Y.J. He, L.N. Lu, S. Jin, S.G. Hu, Conductive aggregate prepared using graphite and clay and its use in conductive mortar, Constr. Build. Mater. 53 (2014) 131– 137. [21] W.Z. Tan, Structure Engineering Materials, Tsinghua University Press & Springer-Verlag Press, Chapter 1: pp. 16–17. [22] A. Sivakumar, P. Gomathi, Pelletized fly ash lightweight aggregate concrete: a promising material, J. Civil Eng. Constr. Technol. 3 (2) (2012) 42–48. [23] K.I. Hankrishnan, K. Ramamurthy, Influence of pelletization process on the properties of fly ash aggregates, Waste Manage. 26 (8) (2006) 846–852. [24] J.M.J.M. Bijen, Manufacturing processes of artificial lightweight aggregates from fly ash, Int. J. Cem. Compos. Lightweight Concrete 8 (3) (1986) 191–199. [25] W. Pietsch, An interdisciplinary approach to size enlargement by agglomeration, Powder Technol. 130 (1–3) (2003) 8–13. [26] BS EN 1097-6:2013 Tests for Mechanical and Physical Properties of Aggregates Part 6: Determination of Particle Density and Water Absorption. [27] B. Gonzalez-Corrochano, J. Alonso-Azcarate, M. Rodas, Effect of prefiring and firing dwell times on the properties of artificial lightweight aggregates, Constr. Build. Mater. 53 (2014) 91–101. [28] S.H. Zhang, L.B. Liu, K.F. Tan, L.H. Zhang, K.J. Tang, Influence of burning temperature and cooling methods on strength of high carbon ferrochrome slag lightweight aggregate, Constr. Build. Mater. 93 (2015) 1180–1187. [29] C.H. Lu, J.C. Chen, K.H. Chuang, M.Y. Wey, The different properties of lightweight aggregates with the fly ashes of fluidized-bed and mechanical incinerators, Constr. Build. Mater. 101 (2015) 380–388. [30] A. Al-Dahawi, M.H. Sarwary, O. Öztürk, G. Yıldırım, A. Akın, M. S ß ahmaran, M. Lachemi, Electrical percolation threshold of cementitious composites
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possessing self-sensing functionality incorporating different carbon-based materials, Smart Mater. Struct. 25 (10) (2016) 1–15. [31] V. Strokova, I. Zhernovsky, Y. Ogurtsova, A. Maksakov, M. Kozhukhova, K. Sobolev, Artificial aggregates based on granulated reactive silica powders, Adv. Powder Technol. 25 (3) (2014) 1076–1081. [32] B.L.A. Tuan, C.L. Hwang, K.L. Lin, Y.Y. Chen, M.P. Young, Development of lightweight aggregate from sewage sludge and waste glass powder for concrete, Constr. Build. Mater. 47 (2013) 334–339.
[33] GB/T 17431.1. Lightweight Aggregates and its Test Methods – Part 1: Lightweight Aggregates. 2010, p. 6. [34] S.B. Park, Experimental-study on the engineering properties of carbon-fiber reinforced cement composites, Cem. Concr. Res. 21 (4) (1991) 589–600. [35] R. Gennaro, A. Langella, M. D’Amore, M. Dondi, A. Colella, P. Cappelletti, A. Gennaro, Use of zeolite-rich rocks and waste materials for the production of structural lightweight concretes, Appl. Clay Sci. 41 (1–2) (2008) 61–72.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation on electrically conductive aggregates produced by incorporating carbon fiber and carbon black Binmeng Chen a, Bo Li b,⇑, Yan Gao a, Tung-Chai Ling c, Zeyu Lu a, Zongjin Li a a
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China Department of Civil Engineering, The University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo, China c College of Civil Engineering, Hunan University, Changsha, China b
h i g h l i g h t s Electrically conductive aggregates (ECAs) are fabricated by pelletization technique. Carbon fiber and/or carbon black are well dispersed by semi-dry mixing method. The threshold percolation of carbon fiber and carbon black are identified for ECAs. ECAs exhibit excellent resistivity, acceptable strength and water absorption.
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 10 March 2017 Accepted 18 March 2017
Keywords: Conductive aggregate Carbon fiber Carbon black Pelletization Electrical resistivity
a b s t r a c t This paper reports on an investigation of newly developed electrically conductive aggregates (ECAs) through the semi-dry mixing method and the pelletization technique. Carbon fiber and carbon black were incorporated into the aggregates as conductive fillers, while ordinary Portland cement and fly ash were used as matrix materials. The effects of carbon fiber and/or carbon black dosages on the electrical resistivity, water absorption and crushing strength of ECAs were studied. For ECAs with carbon fiber only, the threshold percolation of carbon fiber was identified to be 1.0% by volume. The ECAs with 1.0% carbon fiber exhibited 3.4 Om electrical resistivity, 13.08% water absorption and 1.57 MPa crushing strength. Moreover, the effect of carbon black content was investigated when the content of carbon fiber was kept at 0.5 vol.%. The threshold percolation of carbon black for ECAs with 0.5 vol.% carbon fiber was 2.0% by weight. These ECAs possessed 7.34 Om resistivity, 24.41% water absorption and 0.95 MPa crushing strength. Scanning electron microscope was employed to study the conductive network formed by two conductive components, which helped to illustrate the conductive mechanism of carbon fiber and carbon black inside the ECAs. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Electrically conductive concrete is a cement-based composite that contains electronically conductive components to achieve a stable and relatively low electrical resistivity [1]. It is a relatively new type of functional material which has drawn much attention due to its good performance in electrical conductivity and mechanical properties [2,3]. With the function of electrical conductivity, electrically conductive concrete has been proposed for the applications of deicing, antistatic flooring, electromagnetic shielding, cathodic protection of steel reinforcement in concrete structure and health monitoring of buildings [3–6]. Since the electrical ⇑ Corresponding author. E-mail address: [email protected] (B. Li). http://dx.doi.org/10.1016/j.conbuildmat.2017.03.168 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
conductivity of normal concrete is very low due to the very limited conductivity of C-S-H and the highly tortuous pore structure [7,8], electrically conductive concrete is normally developed through adding a certain portion of conductive fillers, including steel slag, stainless steel fiber, graphite, carbon fiber, carbon black, etc. Several studies have already focused on the optimization of conductive fillers to reduce the electrical resistivity of concrete [9–18]. Conduction of electricity in the hardened concrete mainly depends on the movement of electrons, which requires a good contact between conductive components. Monteiro et al. [9] prepared a cement-based composite for structural monitoring through adding carbon black particles. Test results showed that the resistivity could be significantly decreased as the increase of the carbon black dosage, which enhanced the accuracy of resistivity monitoring. However, the authors also reported that the carbon black was
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seldom exclusively used in the conductive concrete. Dong et al. [10] incorporated short-cut super-fine stainless wire (SSSW) in reactive powder concrete to improve the electrical conductivity. The optimal content of SSSW in the reactive powder concrete was identified to be 0.5 vol.% for achieving an excellent conductivity of 0.44 Om. Al-Dahawi et al. [11] compared the influence of carbon fiber length on the electrical properties of cementitious composites and revealed that the cementitious composites with the longer carbon fibers exhibited better electrical conductivity than composites with the short ones. Xie et al. [12] also reported that the longer fibers in the non-conductive matrix were more effective to conduct electricity than shorter ones. Thus, the dosage of fibers could be minimized to achieve a certain conductivity value. Chung [1] compared the effectiveness of various electrically conductive components, including steel fibers, steel dust, carbon fibers, carbon nanofiber, coke powder and graphite, on the electrical conductivity of cement-based materials. It was found that steel fiber with 8 lm diameter was the most effective conductive filler for lowering the electrical resistivity. In respect to carbon-based materials, carbon fiber with 15 lm diameter was more effective than carbon nanofiber, coke powder or graphite powder in improving the electrical conductivity. For the electrically conductive concrete with a single conductive component, conductive fiber was more effective for conducting electricity than conductive filler. However, the combined use of conductive fibers and fillers had been recognized to produce a more efficient conductive system in the electrically conductive concrete. Wen and Chung [13] reported that a 50% replacement of carbon fiber by carbon black in the cement matrix composites could maintain the electrical conductivity of the system with 100% carbon fiber, but it reduced the strain sensing effectiveness. Tuan and Yehia [14] developed electrically conductive concrete for bridge deck deicing through adopting steel fiber, steel shaving and carbon-based materials. Carbon and graphite materials were recommended to replace steel shavings due to their drawbacks in consistency of size and composition. The resulting conductive concrete has been demonstrated to work well in deicing. Garcia et al. [15] examined the conductivity of asphalt mortar produced with graphite and steel wool. The conductive fibers were more effective for conducting electricity than the conductive fillers. Comparatively, a combination network consisting of fibers to reach the optimum conductivity and a small volume of filler to stabilize the resistivity were suggested. Wu et al. [16] investigated the effects of filler type, filler content and mixed fillers on the electrical resistivity of the asphalt concrete. It was found that the asphalt conductive concrete produced with pure carbon fiber exhibited the best conductive performance. However, the authors recommended the combined use of small amounts of expensive carbon fibers and large amounts of cheap carbon black or graphite as a cost-effective conductive system for asphalt concrete. Ding et al. [17] proposed the use of nano-carbon black and carbon fiber in developing the electrically conductive concrete as conductive components. The degree of strain and damage of the beam cast by the developed electrically conductive concrete was successfully correlated with electrical properties. Wu et al. [18] developed a threephase composite conductive concrete containing steel fiber, carbon fiber and graphite for pavement deicing. Dispersion of carbon fiber and concrete voids were identified as the main factors affecting the conductivity of concrete. An optimal mix formulation containing 1.0 vol.% steel fiber, 0.4 vol.% carbon fiber and 4.0 wt.% graphite achieved an electrical resistivity of 3.22 Om. Generally, the electrically conductive concrete with combined conductive components exhibited superior performance in terms of efficient and stable electrical conductivity. Conduction of electricity requires the continuous contact of conductive components within the concrete. Conventional aggregates
occupying 50–70% volume of concrete may inhibit the conductive of the media, as they are nonconductive. The electrical resistivity of common coarse aggregates ranged from 300 Om to 1500 Om [3]. Thus, the combination of conductive coarse aggregates with the conventional conductive paste may produce a more continuous, and thus more conductive concrete. In addition, replacing small amount of conventional aggregates by lightweight aggregates would not significantly affect the mechanical properties of concrete [19]. Thus, replacement of normal aggregates with conductive aggregates might be promising to fabricate conductive concrete. Currently, few studies have considered coarse aggregates to be electrically conductive. He et al. [20] manufactured the conductive aggregates by sintering clay with the incorporation of graphite powder. It demonstrated that the resistivity of conductive aggregate mortar was much lower than that of conventional mortar. Unfortunately, the conductive aggregates prepared in the study had a tablet shape which was significantly different from natural coarse aggregates. Moreover, the calcination process to produce conductive aggregates in their study consumed quite a bit of energy. Thus, it is of great interest to produce electrically conductive coarse aggregates with proper manufacturing methods and satisfactory properties. Application of waste materials in producing conductive aggregates would also positively impact environment and cost. In this paper, a pelletization technique was employed to produce conductive coarse aggregates, using a process of agglomeration of fine particles into pellets. Prior to this, a semi-dry mixing method was used to disperse the carbon fibers in the matrix. Ordinary Portland cement and fly ash were used as the matrix materials for pelletization. Carbon fiber and carbon black were incorporated into the matrix materials as the conductive components. Physical, mechanical and electrical properties were estimated and compared for the conductive aggregates. Conductive aggregates prepared by the semi-dry mixing and the pelletization exhibited excellent electrical conductivity with satisfactory mechanical properties. 2. Experimental study 2.1. Raw materials The main materials used to prepare aggregates in this study were ordinary Portland cement (OPC), fly ash (FA). Carbon fiber and/or carbon black were incorporated into the mix as conductive components. The OPC has the specific gravity and surface area of 3.15 and 3310 cm2/g, respectively. The FA complying with ASTM class F ash was adopted in the study. The specific gravity and surface area of the FA were 2.31 and 3960 cm2/g, respectively. The chemical compositions of the OPC and the FA are presented in Table 1. Carbon fiber (CF) with diameter of 12–15 lm and length of 1.0 mm was used in this study. The density of CF was about 1.76 g/cm3 and the volume resistivity of CF was around Table 1 Chemical compositions of OPC and PFA. Chemical compositions
OPC (wt.%)
PFA (wt.%)
SiO2 Al2O3 CaO SO4 Fe2O3 MgO K2O TiO2 Na2O
19.85 3.68 65.14 5.40 2.90 1.78 0.91 0.27 –
55.81 23.17 7.31 1.57 6.68 2.27 1.78 1.29 –
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0.015 Ocm. The tensile strength of the CF was 3800 MPa while the Young’s Modulus was between 220 GPa and 240 GPa. The carbon black (CB) had partial size of 20–50 lm. The density of CB was about 1.3–1.6 g/cm3 and the volume resistivity was 1.5–2.0 Ocm. SEM photos of the CF and CB are shown in Fig. 1.
weight. The constant content of CF in this series was determined based on the test results from series one, which was optimized to achieve a proper electrical conductivity. Mix formulations for both series are presented in Tables 2 and 3. 2.3. Test methods
2.2. Preparation of aggregates Aggregates were prepared by the process consisting semi-dry mixing and pelletization as shown in Fig. 2. Dry powders of OPC and FA with conductive materials were first mixed at the semidry condition to improve the dispersion of CF and CB. At this mixing stage, one third of total water required in the mix formulation was added. After that, raw materials with carbon fibers and/or carbon black were mixed under high shear rate for five minutes to ensure the uniform distribution of conductive components. Semidry mixing method is effective to disperse carbon fibers in the mixed raw materials with the following two actions. Wetting process reduced surface energy of the mixed powders based on the surface energy theory [21], which was beneficial to separate the powder particles. Carbon fibers were attached to the wetted powders due to their attraction forces. During the mixing process, carbon fibers were gradually attached to the cement particles. However, the amount of carbon fibers to be dispersed highly depended on the weight of wetted powders. Agglomeration of carbon fibers was about to happen when the carbon fiber content was relatively high. Semi-dry mixed raw materials were subsequently fed into the pelletizer to produce aggregates. Rest two thirds of water was sprayed into the pelletizer to bind fine particles, which formed the round-shaped aggregates. Pelletization is a process involving in agglomeration of fine particles into the pellets [22,23]. Small grains are firstly formed with small dosage of water and grow up gradually when more powder and water are added [24]. During this process, green pellets are formed through agitation granulation at initial stage and compaction at advanced stage [25]. The manufactured aggregates as shown in Fig. 3 were curing at room temperature for 28 days. Two series of mix formulations were studied according to conductive components used. CF was only adopted in series one while CF and CB were used in series two. The weight ratio of OPC to FA was fixed at 0.25 for all mix formulations. Water to solid material ratio was also fixed at 0.3. Carbon fiber was added at eight dosages at 0, 0.25%, 0.5%, 0.75%, 1.0%, 1.25%, 1.5% and 2.0% by volume in series one. In series two, part of carbon fiber was replaced by carbon black. The content of CF in the aggregate was kept constant while CB was investigated at the dosage of 0, 0.5%, 1.0%, 1.5% and 2.0% by
2.3.1. Electrical resistivity The two-probe method of measuring electrical resistivity was adopted in this study. Schematic view of measurement is illustrated in Fig. 4. Rectangular samples of aggregates were prepared for easy measurement. Resistivity of aggregates could directly affect the electrical resistivity of concrete as the main conductive medium. Thus, the conductive aggregate was first polished from a round shape to a rectangular shape sample. To reduce the influence of the contact resistivity, conductive silver paint was applied on both ends of cross section of the sample. The electrical resistivity of conductive aggregates was measured at 28th day after preparation. The voltage drop V across the sample and current I through the sample were recorded. The electrical resistivity then could be calculated by the Eq. (1).
q¼
VA IL
ð1Þ
where q is the electrical resistivity. V and I are the electrical voltage and current, respectively. A and L are the area of cross section and the length of the sample, respectively. For each mix formulation, six samples were tested and the averaged electrical resistivity was obtained. 2.3.2. Water absorption The water absorption of conductive aggregates was measured according to BS EN 1097-6 [26]. After 28-day water curing, aggregates were first removed from the water and dried in the oven at a temperature of 105 °C until it reached constant mass. The weight of the aggregate at dry condition was recorded as Mdry. Afterwards, the same batch of aggregates was immersed in the water for 24 h before it was removed from the water and dried on the surface by gentle rolling in the cloth. The weight of saturated surface-dried aggregates was recorded as Msat. Then, the 24-h water absorption of the aggregates could be calculated by the following equation.
WA24 h ¼
M sat M dry M dry
ð2Þ
where Mdry is the mass of the oven dried aggregates, and Msat is the mass of 24-h saturated surface-dried aggregates.
Fig. 1. SEM photos of CF and CB.
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Fig. 2. Manufacturing process of conductive aggregates.
3. Results and discussion 3.1. Electrical resistivity of CF-based aggregate
Fig. 3. Photo of manufactured electrically conductive aggregates.
Table 2 Mix formulations for aggregates with carbon fiber only. Mix
OPC
FA
w/b
CF Vol.%
CF0 CF25 CF50 CF75 CF100 CF125 CF150 CF200
1 1 1 1 1 1 1 1
4 4 4 4 4 4 4 4
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
0% 0.25% 0.5% 0.75% 1.0% 1.25% 1.5% 2.0%
2.3.3. Crushing strength The crushing strength of each single aggregate was estimated by uniaxial compression between two parallel rigid plates [27– 29]. Loading rate was set at 0.5 mm/min. The crushing strength of each aggregate S is calculated by the following equation.
S¼
2:8 Pc
ð3Þ
pX 2
where Pc is the load at fracture and X is the sphere diameter of individual aggregate. Averaged strength of ten samples was adopted.
Fig. 5 shows electrical resistivity of the conductive aggregates against content of incorporated carbon fiber. It was obvious that the aggregates had a high electrical resistivity (around 105 Om) when the CF content was less than 0.5 vol.%. It indicated that carbon fibers at those dosages could not form continuous conductive paths inside the aggregate. There was a sharp decrease in the resistivity when content of CF was increased from 0.5 vol.% to 0.75 vol.%. Quantitatively, electrical resistivity decreased from 7.5 104 Om for the aggregates with 0.5 vol.% CF to 23 Om for that with 0.75 vol.%, which was several magnitude degree drops in the electrical resistivity. Further increase in the content of CF marginally reduced the resistivity of aggregates. For instance, the electrical resistivity was reduced from 23 Om to 3.4 Om when content of CF increased from 0.75 vol.% to 1.0 vol.%. Therefore, the percolation threshold of CF in the conductive aggregates was found to be between 0.5% and 1.0% by volume. Similar finding was reported by Al-Dahawi et al. [30]. However, incorporation of CF at a dosage higher than 1.0 vol.% slightly increased the electrical resistivity of aggregates. For instance, the resistivity of aggregates with CF at 1.25 vol.%, 1.5 vol.% and 2.0 vol.% were 7.5 Om, 10.4 Om and 16.0 Om, respectively. This was mainly attributed to the agglomeration of CF at high content inside the aggregates, which reduced the conductive paths and then slightly increased the resistivity of aggregates. This identified that 1.0 vol.% of CF incorporation might be the optimum value for the current aggregate mix design.
3.2. Electrical resistivity of CF&CB-based aggregate Fig. 6 shows the electrical resistivity of aggregates prepared with both CF and CB against the content of CB. The content of CF was fixed at 0.5% by volume, which was slight lower than the percolation threshold of CF. It was expected that the combined use the CF at dosage of 0.5 vol.% and the CB could achieve a good electrical conductivity with the effective conductive paths formed by
Table 3 Mix formulations for aggregates with carbon fiber and carbon black. Mix
OPC
FA
w/b
CF Vol.%
CB wt.%
CF50CB0 CF50CB5 CF50CB10 CF50CB15 CF50CB20
1 1 1 1 1
4 4 4 4 4
0.3 0.3 0.3 0.3 0.3
0.5% 0.5% 0.5% 0.5% 0.5%
0% 0.5% 1% 1.5% 2%
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Fig. 4. Two probes method to measure electrical resistance of cement-based specimen.
3.3. Water absorption
Resistivity ( ·m)
1,00,000 10,000 1,000 100 10 1 0.00% 0.25% 0.50% 0.75% 1.00% 1.25% 1.50% 1.75% 2.00%
Carbon fiber content by volume Fig. 5. Electrical resistivity of aggregate with CF only.
1,00,000
Resistivity ( ·m)
10,000 1,000 100 10 1 0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
Carbon black content by mass Fig. 6. Electrical resistivity of aggregate with CF and CB.
conductive fillers. It was noted that higher content of CF (e.g. 1.0 vol.%) had achieved the suitable electrical conductivity for the applications of deicing. In consideration of cost of ECAs and dispersion of CF, half of CF in the aggregates with proper electrical conductivity was substituted by CB. As seen in Fig. 6, the electrical resistivity of aggregates with CF and CB remained at high level of 3.8–4.8 104 Om when the content of CB was less than 1.0 wt.%. It revealed that the conductive network consisting of CF and CB was not established inside the aggregate. It meant that the CF could not be continuously connected by the CB. When the content of CB increased from 1.0 wt.% to 2.0 wt.%, a sharp decrease in the resistivity was observed. It demonstrated that the percolation threshold of CB for electrical conductivity was located within the above range while the CF was fixed at 0.5 vol.%. More conductive paths were formed inside the aggregates as the incorporation of more CB. However, further increase of the content of CB only slightly improved the resistivity of aggregates. For instance, the electrical resistivity of aggregates dropped from 7.3 Om to 6.4 Om when increasing the content of CB from 2.0 wt.% to 2.5 wt.%.
Fig. 7 shows the water absorption of CF-modified aggregates against the content of carbon fiber. It was readily seen that there were two levels of water absorption for the aggregates with various CF. When the content of CF was less than 1.0 vol.%, the water absorption of aggregates was under low level and was slightly varied from 11.13% to 13.08%. Once the content of CF reached 1.25 vol. %, the water absorption of aggregates dramatically increased to 26.11%, which was almost double of that with 1.0 vol.% CF. It was a turning for both electrical resistivity and water absorption. Further increase in the content of CF slightly increased the water absorption of the aggregates. For instance, the water absorptions of aggregates with 1.5 vol.% and 2.0 vol.% were 27.32% and 28.31%, respectively. This was mainly attributed to the agglomeration of CF, which induced high porosity of the aggregates. This phenomenon could be also verified by the SEM microscopies in the later section. In summary, it was suggested to limit the content of CF below 1.25 vol.% to achieve a better water absorption for the electrically conductive aggregates. Fig. 8 illustrates the water absorption of aggregate containing 0.5 vol.% CF and different content of CB. For the aggregates with the same content of CF at 0.5 vol.%, incorporating CB increased the water absorption of the aggregates, even if the amount of incorporated CB was small. For instance, water absorption was increased from 10.54% to 15.98% when 0.5 wt.% CB was added into the aggregates. Generally, water absorption of the aggregates with both CF and CB gradually increased as the content of CB increased. It demonstrated that CB had adverse effect on the water absorption of aggregates. This was mainly attributed to the water absorption of CB itself inside the aggregates. Specifically, water absorptions of the aggregates with at least 1.5 wt.% CB were at least two times of that with 0.5 vol.% CF only. In addition, it could be found that 30% 25%
Water absorption
10,00,000
20% 15% 10% 5% 0%
0.00% 0.25% 0.50% 0.75% 1.00% 1.25% 1.50% 2.00%
Carbon fiber content by volume Fig. 7. Water absorption of aggregate with CF only.
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30%
2 1.8
Crushing strength (N/mm2)
Water absorption
25% 20% 15% 10% 5%
1.6 1.4 1.2 1 0.8 0.6 0.4 0.2
0%
0.00%
0.50%
1.00%
1.50%
2.00%
0
2.50%
Carbon black content by mass
0.25% 0.50% 0.75% 1.00% 1.25% 1.50% 2.00%
Carbon fiber content by volume
Fig. 8. Water absorption of aggregate with CF and CB.
Fig. 9. Crushing strength of aggregate with CF only.
1.8 1.6
Crushing strength (N/mm2)
the effect of CB on the water absorption of aggregate was much more significant than that of CF. Thus, the content of CB should be well controlled to produce the aggregates with proper water absorption. In general, water absorption of the produced ECAs ranged from 10% to 30%, which depended on the dosage of CF and CB. This was comparable to that of lightweight aggregates in the literature [27,31]. Tuan et al. [32] also suggested that 24-h water absorption of lightweight aggregates should be below 20%. When referring to standard GB/T 17431.1 [33], 1-h water absorption of lightweight aggregates was specified in the range of 10% and 30%. This was related to the type and density of lightweight aggregates. Thus, the produced ECAs exhibited satisfied water absorption despite incorporation of either CF or CF&CB. To overcome the shortage in water absorption, it was suggested to have surface treatment for ECAs or control the amount of ECAs in concrete.
0%
1.4 1.2 1 0.8 0.6 0.4 0.2 0
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
Carbon black mass content
3.4. Crushing strength
Fig. 10. Crushing strength of aggregate with CF and CB.
Fig. 9 shows the crushing strength of aggregates incorporated of CF only. It was readily seen that the crushing strength of aggregates decreased as the increase of carbon fiber content. When incorporating 0.25 vol.% CF into the aggregates, the crushing strength was decreased by 10.5%. This was the first obvious decrease in the crushing strength. Subsequently, crushing strength of aggregates dropped gradually with the increase of CF content from 0.25 vol.% to 1.0 vol.%. There was second significant drop in the crushing strength when the content of CF increased to 1.25%. For instance, crushing strength of aggregates decreased from 1.56 N/mm2 to 1.27 N/mm2, which was about 20% decrease in crushing strength. Further increase in the content of CF continuously reduced the crushing strength. This was probably attributed to the agglomeration of CF at a high dosage that could lead a higher porosity in aggregate. The crushing strength decreased to 0.97 N/ mm2 when the content of CF reached 2.0 vol.%, which was the third obvious drop in crushing strength. Generally, incorporation of CF has detrimental effect on the crushing strength of aggregates. This was mainly attributed to the more porous matrix due to the large aspect ratio of CF [34]. Similarly, it was recommended that the content of CF should be limited to 1.0% in volume by considering the crushing strength of aggregates. Fig. 10 shows the crushing strength of aggregates with constant 0.5 vol.% CF and various dosages of CB. Similar to the aggregates with CF only, the crushing strength of aggregates decreased as the CB content increased. The conductive aggregates without CB exhibited the highest crushing strength at 1.64 N/mm2. The decrease in the crushing strength was almost linear with the increase of CB content in the aggregates. It indicated that CB also
had detrimental influence on the crushing strength of aggregates. When CB was added at high dosage (e.g. 2.0 wt.% or higher), the degradation of crushing strength was more significant. For instance, crushing strength of aggregates with 2.0 wt.% and 2.5 wt.% CB were 0.95 N/mm2 and 0.73 N/mm2, respectively. In respective of crushing strength, it was suggested that the content of CB inside the aggregates should be no more than 2.0 wt.%. Crushing strength of ECAs incorporated either with CF only or CF&CB ranged from 0.7 MPa to 2.0 MPa, which depended on the dosage of conductive fillers. There was a wide variation in crushing strength of aggregates made with different materials and processes. However, it was generally accepted that the crushing strength of artificial lightweight aggregates should be above 0.5 MPa when comparing to commercial lightweight aggregates [27]. The produced ECAs satisfied the above general requirement. Gennaro et al. [35] had demonstrated that lightweight aggregates with a compressive strength around 1.0 MPa could be used to produce structural concrete. However, the crushing strength of ECAs could be further enhanced by optimizing the composition of aggregates. For the application in concrete, the amount of ECAs should be properly controlled with the aim of ensuring concrete strength. 3.5. SEM microscopy Microstructures of ECAs with CF and/or CB were examined by SEM. Fig. 11 shows the SEM microscopies of aggregates with different contents of CF. Generally, CF was much easier to be
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(a) 0% CF
(b) 0.25% CF
(c) 0.5% CF
(d) 0.75% CF
(e) 1.0% CF
(f) >1.0% CF
Fig. 11. SEM microscopies of aggregate with CF only.
(a) 0% CB
(b) < 1.0% CB
(c) 1.5% CB
(d) 2.5% CB
Fig. 12. SEM microscopies of aggregate with CF and CB.
detected in the aggregates when dosage of CF was high. For the aggregates with less than 0.75 vol.% CF (Fig. 11(a)–Fig. 11(c)), carbon fibers were found to be disconnected. That confirmed the high electrical resistivity of these aggregates. On the other hand, it also
verified that these disconnected carbon fibers did not significantly reduce the crushing strength as well as increase the water absorption. The connective paths found in Fig. 11(d) confirmed the dramatic change in the electrical resistivity of aggregates with
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0.75 vol.% CF. With more CF in the aggregates (e.g. 1.5 vol.% and 2.0 vol.%), agglomeration of CFs was clearly found as shown in Fig. 11(e) and Fig. 11(f). That would be main reason for the reduced crushing strength and increased water absorption of aggregates. It also demonstrated that the fibers could be dispersed well by the proposed semi-dry mixing method when a proper fiber dosage was used. Fig. 12 shows the SEM microscopes of the aggregates with CF and CB. As compared to the aggregate in Fig. 12(a), CB particles could be clearly found in the aggregates in Fig. 12(b)–Fig. 10(d). When the content of CB was less than 1.0 wt.%, the conductive paths could not be formed even if the CB particles were uniformly distributed. It was evidenced by the sparsely distribution of CB. As the increase of CB content inside the aggregate, continuous conductive paths could be formed. Meanwhile, CF inside the aggregates was connected by new CB paths, resulting in a conductive network. However, CB would not participate in the hydration process of OPC and the pozzolanic reaction of FA. Thus, adding more CB into the aggregates dramatically reduced the crushing strength. Moreover, CB particles surrounding the OPC and FA particles would negatively affect their reactions. 4. Conclusions In this paper, a type of conductive aggregates was developed by pelletization technique. FA and OPC were used for pelletization as matrix materials. Carbon fiber and carbon black were incorporated into the aggregates as conductive components during the manufacturing process. Properties of electrically conductive aggregates with different contents of carbon fiber and/or carbon black, including electrical resistivity, water absorption, and crushing strength, were investigated. The following conclusions can be drawn from the study. (1) Electrical conductive aggregates could be successfully produced by pelletization method with carbon fiber and/or carbon black incorporated as conductive components. The electrical resistivity of aggregates could be reduced to 3.4 Om and 7.3 Om for CF-modified ECAs and CF&CBmodified ECAs respectively, which was several magnitude lower than conventional aggregates. (2) It was demonstrated that semi-dry mixing method was effective to disperse carbon fiber and/or carbon black inside the aggregates. (3) The threshold percolation of carbon fiber for electrical resistivity was identified at 1.0% by volume. For the aggregates with constant carbon fiber of 0.5 vol.% and carbon black, the threshold percolation of carbon black for electrical resistivity was found to be 2.0% by weight. (4) Incorporation of carbon fiber and/or carbon black was detrimental to increase the water absorption and to decrease the crushing strength of ECAs. However, these degradations could be minimized by incorporating proper dosages of CF and/or CB. (5) In general, the ECAs produced with 1.0 vol.% CF only or with 0.5 vol.% CF and 2.0 wt.% CB were recommended. However, the ECAs produced with 1.0 vol.% CF only exhibited slightly lower resistivity, lower water absorption and higher crushing strength than those with CF&CB.
Acknowledgements The financial supports from the Ministry of Science and Technology of China under the Grand of 2015CB655100 and the technical supports from Materials Lab of Department of Civil Engineering
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at The Hong Kong University of Science and Technology is greatly acknowledged. The second author would also acknowledge the supports from the Faculty of Science and Engineering at The University of Nottingham Ningbo China via New Researchers Grant.
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