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The Usage of Smart Materials for Skin-diagnostics of Building Structures While Their Monitoring
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 172 (2017) 119 – 126
Modern Building Materials, Structures and Techniques, MBMST 2016
The usage of smart materials for skin-diagnostics of building structures while their monitoring Bolshakov V.I.a, Vaganov V.E.b, Bier Th.A.c, Bausk Ie.A.a*, Matiushenko I.M.a, Ozhyshchenko O.A.a, Popov M.Yu.b, Sopilniak A.M.a a
State Higher Education Establishment “Prydniprovs`ka State Academy of Civil Engineering and Architecture”, 24A, Chernyshevs’kogo str., Dnipropetrovs’k 49005, Ukraine b Vladimir State University, Gorky st 87, Vladimir, 600000,Russian Federation c TU Bergakademie Freiberg, Institut für Keramik, Glas- u. Baustofftechnik, Leipziger Straße 28, Freiberg 09599, Germany
Abstract This study investigates the usage of smart materials for skin-diagnostics with the view of monitoring of building structures parameters characterizing their mode of deformation. The method is based on changes in electrical resistance of the smart coat owing to external influences. Conductivity achieving was performed by introducing of carbon nanotubes (CNTs) into the initial material. Composites on the basis of sodium silicate, epoxy resin, acrylic and cement have been worked out. Resistivity-number of CNTs dependence was obtained for sodium silicate and epoxy resin. The evaluation of the uniformity of the CNTs distribution in the bulk of samples by simple mechanical mixing was executed. The influence of the measurement of the electrical resistance by direct and alternating current was checked. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016. Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: skin-diagnostic, carbon nanotubes, monitoring, smart materials, electrical resistance.
* Corresponding author. Tel.: +3-805-624-702-63; fax: +3-805-624-702-63. E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016
doi:10.1016/j.proeng.2017.02.033
120
V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126
1. Introduction Monitoring of building structures is one of the most time-consuming and long-term operations in the operation during maintenance of buildings and constructions. According to [1] under the monitoring of building structures the constant monitoring of their condition is implied, which can be achieved only with the usage of automated systems of technical diagnostics. I.e. the main task of the system of technical diagnostics and, consequently, monitoring of building structures is to monitor the parameters that allow controlling their technical condition. The main parameters of building structures, which can be permanently monitored, are [1]: geometric, force, dynamic, temperature, longevity parameters. Monitoring of force, dynamic and longevity parameters is a time-consuming process in terms of its organization, i.e. the system of installation. These observations are used in solitary cases after justifying the necessity, when it is impossible to obtain the desired results by measuring geometric parameters. Monitoring of the temperature change is obligatory practically in all cases of the organization of diagnostics system to assess the effect of this parameter on the others and their subsequent correlations. Monitoring of geometric parameters of building structures is the most common kind of observation in the organization of diagnostic systems and nomenclature of monitoring means over the given parameters is rather diverse [1,2]: fiber optic sensors, piezo resistive sensors (strain gauges), hydrostatic leveling system, geodetic systems and others. The above mentioned monitoring means have both advantages and disadvantages. The main disadvantages of these systems are: x x x x x x x
the difficulty of installation and adjustment; the impossibility to install in the places with limited access; the instability to mechanical damage; the impact of external influences on the readings; the difficulty in localizing of structure deformation; the lack of sensitivity; the instability of the readings and others.
Therefore, the development of effective monitoring systems with a minimum of drawbacks is rather urgent task. One solution to this problem is the development of smart materials that have the ability to react to the changes of geometric size, the distribution of local stresses, the formation and development of cracks, the moisture content and even the presence of chlorides. The successful implementation of this material is possible with the help of the usage of carbon nanostructures (graphite, graphene, nanotubes and nanofibers) according to two schemes: uniform distribution in the bulk of the structure material or in the composition of the coverage material applied to the surface (part of the surface) of the diagnosed structure (skin diagnostics). Despite the considerable amount of researches in this area it may be noted with confidence that the issue is still urgent. The evidence of this can be the inconsistent results of studies by various authors, in which the mechanical and conductive properties of concretes reinforced by carbon nanotubes can vary significantly. In [3] the grafting of functional groups was performed by boiling in a solution of sulfuric and nitric acids. The formation on the surface of oxygenated groups has led to the increase of the strengthening properties of the concrete while compression and bending to 19 and 25%, respectively. However, in [4] there was a decrease in durability while implementation of composite cement into the composition. According to the authors, the observed reduction of durability (7 times) is due to the absorption of water by carbon nanotubes and as a consequence of incomplete hydration of clinker. The double improvement of the durability of concrete has been found in [5]. According to the authors in the course of the acid treatment, concurrently with the process of grafting of functional groups onto the surface of the carbon nanotubes, there is the formation of carboxylated carbon fragments whose presence helps to reduce the reinforcing effect from the usage of carbon nanotubes. The removal of the mixing of mentioned fragments from the water has increased the strength of the cement composite from 36 MPa to 71 MPa, at the concentration of carbon nanotubes of 0.03% by the weight of the composite.
V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126
In the further numerous studies [6-9] it has been shown that the average increase in the durability of the composites was 30-50%. At the same time in the works of various authors the carbon nanotubes concentration, which provides the maximum durability increase, ranged from 0.005% to 1% by weight of the binder. In recent years, a large number of studies were aimed at studying not only the mechanical properties of concrete, but also their electrical properties. The common concrete in certain temperature and humidity conditions has the ability to conduct electricity, but this property is unstable. While seasonal variations in temperature and humidity the electrical resistance of conventional concrete changes to 6-8 orders of magnitude. Imparting of conductivity to the concretes, particularly, as it has already been noted, is due to the possibility of development of self-diagnostics materials and structures on their basis. In recent years, this kind of research is actively conducted abroad, in particular in the United States, Germany and other countries. Moreover, the real objects of transport and energy infrastructure have already been constructed of this concrete. As an active development of this area the last conference, held in Chicago, "Nanotechnology in Construction 2015" [15-19] can be given as an example, which presented the reports of the leading specialists devoted to the study of concretes conductive properties. The other principle of obtaining conductive composites – the introduction of conductive additives – is the basis of this study. It should be noted that in this regard the considerable progress has been made on polymeric materials, which have already found practical usage in the form of polymer products having conductive properties, smart materials and others [10,11]. Similar studies [12-14] are carried out for the metal and ceramic composites to increase strengthening and heat and electrical conducting properties. 2. Materials and methods As previously noted development of smart materials using carbon nanostructure is possible according two essentially different ways: adding of carbon nanotubes into the construction material by insuring of uniform distribution in the amount of structure or elaboration of coating material using carbon nanotubes with its surface application on the structure The development of the material according to the first scheme is valid only for new structures and does not cover the need for monitoring of existing structures. Furthermore, with the propagation of plastic deformations or cracking in the structure material, changing of humidity and other information obtained through this method will not be sufficient for an adequate assessment of the structure condition. The optimal solution of such problems is the application as surface coatings, which is the usage of skin-diagnostics. To study the possibility of the usage of coatings that could assess and monitor the stress-strain state of the structure, the primary (for testing the hypothesis) experiment was conducted according to the following scheme (Figure 1) – two bands of different width (10 mm and 14 mm) of conductive composition (adhesive mixture: carbon black: water – 1: 1: 1) were applied to the surface of the stretched area of a flexible element. a
b
Fig. 1. (a) Photo and (b) testing scheme of the beam with a surface coating of an electrically conductive composition.
After drying of the electrically conductive composition the phased beam-strength with simultaneous measuring of the conductive coatings resistance and deflection monitoring of crack formation, both in the beam and in the coating,
121
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V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126
was carried out. In order to evaluate the elastic properties of the material the partial removal of the loading (10%) was conducted, after which the resistance change was recorded. According to results of hereof experiment possible problems and disadvantages of coatings were identified for the purpose of the follow-up study adjustment. In view of primary experiment results sodium silicate, epoxy resin, acrylic sealant, cement-sand mixture with the addition of PVA were used as the basic materials for obtaining the samples according to the scheme of skindiagnostics. Multilayer carbon nanotubes with the characteristics shown below (Table 1) were used as conductive additives. Table 1. The characteristics of the applied multilayer carbon nanotubes. Indices
Norm according to specifications
Exterior view
Lightweight black powder. Small amount of impurity loose lumps, easily crumbling into powder under pressure, is admitted. Point inclusions incorporating a small amount of white or yellow particles (unreacted catalyst residues) are admitted.
Apparent density, g/dm3
20-40
Weight ash content, %
8-22
Specific surface of untreated nanotubes, m2/g
200-400
Specific surface of nanotubes after acid refined from mineral impurities, m2/g
200-400
Outer diameter, nm
10-40
Electrical resistivity of compressed powder of unrefined nanotubes, Оhm/сm
0.05-0.15
Electrical resistivity of the compressed powder refined from mineral impurities nanotubes, Оhm/сm
0.05-0.10
Temperature loss of 5% weight after cleaning from mineral impurities
560-620
Actual value
28
200
Figure 2 shows microphotographs of used multilayer carbon nanotubes. a
b
Fig. 2. Microphotographs of used carbon nanotubes.
The concentration of introduced nanotubes was varied from 0.03 % to 3 % by binding material weight. Nanotubes adding into the composites was realized by mechanical blending according to research results [20]. After the samples hardens nanotubes uniform distribution in the amount of material was verified using scanning electron microscope (SEM), also electric resistance was measured. The resistance measurement was conducted by alternate and direct current for impact assessment of measurement methods. Experiment series two weeks apart was conducted for impact assessment of hardening time on electric resistance.
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3. Discussion of research results The primary experiment result is the confirmation of deformation continuity of the coating made of informative (conductive) material and stretching strain of a flexible element, i.e. workability of conductive coatings according to the scheme of skin-diagnostics. The change in resistance of conductive coatings while the propagation of deformation of the beam stretched area is shown in Figure 3. Changes in resistance of conductive coatings while flexible element cracking and coating craking were fixed during the testing.
appearance off cracks in the beam
aappearance of cracks in bands
Fig. 3. The change of the conductive structure resistance and the beam deflection under the loading.
The problems of the used coating have been identified on the results of the initial experiment: x the propagation of plastic deformations while tension in the early stages of loading (the resistance changes are insignificant while partial removal of the load and decrease of the deflection); x the presence of mixing water in the original composition, which would distort the measurement readings because of water evaporation and hydration of the base material components; That is the main problem in the usage of the skin-diagnostics is a smart material itself that would satisfy the conditions of exploitation of the structure: alternating temperatures, increased or high temperatures, changing of the ambient humidity, cyclic loadings, alternate opening / closing of cracks (for example, for spans of bridges, concrete walling structures of the NPP safety systems, crane beams, etc.). Undoubtedly, so universal material in terms of operating conditions is difficult to implement, respectively, for these technologies several options for smart materials should be offered. According the primary experiment results 14 sample series were made (three samples for each series) using different binding materials with addition of carbon nanotubes with different concentration. Figure 4 shows the obtained samples.
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V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126 Fig. 4. Experimental samples.
Figure 5 shows SEM pictures of the samples of sodium silicate (Fig. 5a), acrylic (Fig. 5b) and cement (Fig. 5c). It should be noted that in the samples of sodium silicate relatively even allocation of nanotubes on the surface of destruction can be seen (Fig. 5a). Acrylic composition is characterized by high coverage (Fig. 5b), which does not allow to see the allocation of nanotubes. Uniform allocation of nanofibers coated by C-H-S gel is viewed in the sample on the basis of the cement (Fig. 5c). a
b
c
Fig. 5. SEM pictures of CNT composites: a) sodium silicate, b) acrylic, c) cement.
Measurement results of resistivity of the samples without giving a specific value of the direct and alternating current are summarized in Table 2. Regarding the measurement repeatability, it may be note that the most part of results keep within acceptable testing error. Maximum measured error for each series is less than 6 %. Concerning the testing results the samples of sodium silicate and epoxy resin are the most interesting in terms of clarity of the results. The dependence of their resistance and of nanotube concentration is illustrated on Fig. 6. The resistance values set to arithmetical average of results for each sample series. It should be noted that drastic fall of sample resistance was marked with changing of nanotube concentration from 0 to 1% during the testing by alternate and direct current. Despite different maximum values of electric resistance of samples on sodium silicate and epoxy resin this parameter decrease on two value order while increasing of nanotube concentration up to 1%. With further increasing concentration of nanotubes the difference of measurements decreases. For samples with cement and acrylic the procedure of measuring at different frequencies of current remained practically constant. Table 2. The results of resistivity measurements. №
Resistance (DC), kΩ
Resistance (AC), kΩ
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% canbonic nanotubes by the weights of the binding
1
2
3
1
2
3
1
0.00
232000
236000
230000
84000
95000
78000
2
0.03
130000
128000
132000
35000
36000
34000
0.93
2040
1870
1930
950
1000
1000
4
2.00
140
128
127
20
20
21
5
2.89
0.726
0.728
0.722
1
0.96
1.02
6
0.00
5300
5160
5200
5000
5050
4900
1.00
536
527
526
500
500
500
2.00
293
304
302
200
200
200
0
>1200000
>1200000
>1200000
>1200000
>1200000
>1200000
0.1
>1200000
>1200000
>1200000
1200000
1200000
1200000
1
1000000
1000000
1000000
1000000
1000000
1000000
Name of the binding
3
7
Sodium silicate
Epoxy resin
8 9
Acrylic
10 11
Cement
Fig. 6. Dependence of resistivity from the concentration of nanotubes.
4. Conclusion The usage of smart materials allows to qualitatively record deformations of structures with high accuracy. When processing the data there will be the opportunity to detect deflections of structures and calculate the value of local stresses, to determine the time of cracking and the width of the crack opening. The conducted study confirms the possibility to use multilayer carbon nanotubes to create smart material composites for skin-diagnostics. The method is based on changes of the electrical conductivity of the composites by applying an external loading or the impact of external conditions on the material. A variant of deformations
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observations by coating the surface of the structure with a thin layer of electrically conductive material of sodium silicate, acrylic, cement-sand mixture as well as epoxy resin with CNT with apparent density 28 g/dm3 and specific surface of untreated nanotubes 200m2/g was examined. For all formulations a series of tests to measure the electrical resistance of samples with different concentrations of carbon nanostructures has been conducted. The percentage of CNTs relative to binder weight was varied from 0 to 3%. For the studied compositions threshold concentrations of carbon nanotubes, leading to an abrupt increase of electrical conductivity measurements by constant and alternating current were determined. In the case of materials based on sodium silicate and epoxy resin the addition of CNT from 0 to 1% by weight of binder results in significant changes in the resistivity of the material, while a further increasing of carbon nanotubes, their conductivity change in a less degree. As for the samples based on cement and acrylic, the order of measurements at different current frequencies remained almost constant. References [1] Konrad Bergmeister, Monitoring and safety evaluation of existing concrete structures, Lausanne, Fédération internationale du béton, 2003. 297 p. [2] Antonia Moropoulou, Kyriakos C. Labropoulos, Ekaterini T. Delegou, Marina Karoglou, Asterious Bakolas, Non-destructive techniques as a tool for the protection of built cultural heritage, Construction and Building Materials 48 (2013) 1222–1239. [3] G. Y. Li, Mechanical behavior and microstructure of cement composites incorporating surface-treated multi-walled carbon nanotubes, Carbon 43 (2005) 1239–1245. [4] S. Musso, Influence of carbon nanotubes structure on the mechanical behavior of cement composites, / S. Musso, J. M. Tulliani, G. Ferro, A. Tagliaferro // Composites Science and Technology 69 (2009) 1985–1990. [5] L. I. Nasibulina, Effect of Carbon Nanotube Aqueous Dispersion Quality on Mechanical Properties of Cement Composite / L. I. Nasibulina, I. V. Anoshkin, A. G. Nasibulin, A. Cwirzen, V. Penttala, E. I. Kauppinen // Journal of Nanomaterials 2012 (2012) 1–6. [6] Rashid K. Abu Al-Rub, Ahmad I. Ashour, Bryan M. Tyson, On the aspect ratio effect of multi-walled carbon nanotube reinforcements on the mechanical properties of cementitious nanocomposites, Construction and Building Materials 35 (2012) 647–655. [7] Maria S. Konsta-Gdoutos, Zoi S. Metaxa, Surendra P. Shah, Highly dispersed carbon nanotube reinforced cement based materials, Cement and Concrete Research 40 (2010) 1052–1059. [8] Е. В. Королев, Нанотехнологии в строительном материаловедении. Анализ состояния и достижений, Пути развития / Королев Е. В // Строительные материалы 11 (2014) 47–79. (Korolev E. Nanotechnology in construction materials. Analysis of the status and achievements. Ways of development / Korolev E. // Building Materials) [9] S.Petrunin, Cement Composites Reinforced with Functionalized Carbon Nanotubes / S. Petrunin, V. Vaganov, K. Sobolev // Materials Research Society Symposium Proceedings 1611 (2014) 133 – 138. [10] Kai Yu, Zhichun Zhang, Yanju Liu, and Jinsong Leng, Carbon nanotube chains in a shape memory polymer/carbon black composite: To significantly reduce the electrical resistivity. Applied physics letters 98, 074102 (2011). [11] Osayuki Osazuwa, Marianna Kontopoulou, Peng Xiang, Zhibin Ye, Aristides Docoslis, Electrically conducting polyolefin composites containing electric field-aligned multiwall carbon nanotube structures: The effects of process parameters and filler loading Osayuki Osazuwa a, Marianna Kontopoulou, Carbon 72 ( 2014 ) 89 –99. [12] Kaleem Ahmad, Wei Pan, and Sui-Lin Shi, Electrical conductivity and dielectric properties of multiwalled carbonnanotube and alumina composites, Applied physics letters 89, 133122 (2006). [13] Guo-Dong Zhan* and Amiya K. Mukherjee, Carbon Nanotube Reinforced Alumina-Based Ceramics with Novel Mechanical, Electrical, and Thermal Properties, Int. J. Appl. Ceram. Technol., 1 [2] (2004) 161–71. [14] Kaleem Ahmad*, Wei Pan, Dramatic effect of multiwalled carbon nanotubes on the electrical properties of alumina based ceramic nanocomposites, Composites Science and Technology 69 (2009) 1016–1021. [15] Isaac Wong, Kenneth J. Loh, Rongzong Wu, and Navneet Garg, Effects of Ultra-low Concentrations of Carbon Nanotubes on the Electromechanical Properties of Cement Paste. [16] Bérengère Lebental, Boutheina Ghaddab, Fulvio Michelis, Nanosensors for Embedded Monitoring of Construction Materials: The “2D Conformable” Route. [17] Chrysoula A. Aza, Panagiotis A. Danoglidis, and Maria S. Konsta-Gdoutos, Self Sensing Capability of Multifunctional Cementitious Nanocomposites. [18] Hongjian Du and Sze Dai Pang, Mechanical Response and Strain Sensing of Cement Composites Added with Graphene Nanoplatelet Under Tension. [19] Ana Guerrero , Jose Luis García Calvo , Pedro Carballosa , Gloria Perez , Virginia R. Allegro , Edurne Erkizia , and Juan Jose Gaitero, An Innovative Self-Healing System in Ultra- high Strength Concrete Under Freeze-Thaw Cycles. [20] Zoi S. Metaxa, Jung-Woo T. Seo, Maria S. Konsta-Gdoutos, Mark C.Hersam, Surendra P.Shah, Highly concentrated carbon nanotube admixture for nano-fiber reinforced cementitious materials. Cement & Concrete Composites 34 (2012) 612–617.
ScienceDirect Procedia Engineering 172 (2017) 119 – 126
Modern Building Materials, Structures and Techniques, MBMST 2016
The usage of smart materials for skin-diagnostics of building structures while their monitoring Bolshakov V.I.a, Vaganov V.E.b, Bier Th.A.c, Bausk Ie.A.a*, Matiushenko I.M.a, Ozhyshchenko O.A.a, Popov M.Yu.b, Sopilniak A.M.a a
State Higher Education Establishment “Prydniprovs`ka State Academy of Civil Engineering and Architecture”, 24A, Chernyshevs’kogo str., Dnipropetrovs’k 49005, Ukraine b Vladimir State University, Gorky st 87, Vladimir, 600000,Russian Federation c TU Bergakademie Freiberg, Institut für Keramik, Glas- u. Baustofftechnik, Leipziger Straße 28, Freiberg 09599, Germany
Abstract This study investigates the usage of smart materials for skin-diagnostics with the view of monitoring of building structures parameters characterizing their mode of deformation. The method is based on changes in electrical resistance of the smart coat owing to external influences. Conductivity achieving was performed by introducing of carbon nanotubes (CNTs) into the initial material. Composites on the basis of sodium silicate, epoxy resin, acrylic and cement have been worked out. Resistivity-number of CNTs dependence was obtained for sodium silicate and epoxy resin. The evaluation of the uniformity of the CNTs distribution in the bulk of samples by simple mechanical mixing was executed. The influence of the measurement of the electrical resistance by direct and alternating current was checked. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016. Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: skin-diagnostic, carbon nanotubes, monitoring, smart materials, electrical resistance.
* Corresponding author. Tel.: +3-805-624-702-63; fax: +3-805-624-702-63. E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016
doi:10.1016/j.proeng.2017.02.033
120
V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126
1. Introduction Monitoring of building structures is one of the most time-consuming and long-term operations in the operation during maintenance of buildings and constructions. According to [1] under the monitoring of building structures the constant monitoring of their condition is implied, which can be achieved only with the usage of automated systems of technical diagnostics. I.e. the main task of the system of technical diagnostics and, consequently, monitoring of building structures is to monitor the parameters that allow controlling their technical condition. The main parameters of building structures, which can be permanently monitored, are [1]: geometric, force, dynamic, temperature, longevity parameters. Monitoring of force, dynamic and longevity parameters is a time-consuming process in terms of its organization, i.e. the system of installation. These observations are used in solitary cases after justifying the necessity, when it is impossible to obtain the desired results by measuring geometric parameters. Monitoring of the temperature change is obligatory practically in all cases of the organization of diagnostics system to assess the effect of this parameter on the others and their subsequent correlations. Monitoring of geometric parameters of building structures is the most common kind of observation in the organization of diagnostic systems and nomenclature of monitoring means over the given parameters is rather diverse [1,2]: fiber optic sensors, piezo resistive sensors (strain gauges), hydrostatic leveling system, geodetic systems and others. The above mentioned monitoring means have both advantages and disadvantages. The main disadvantages of these systems are: x x x x x x x
the difficulty of installation and adjustment; the impossibility to install in the places with limited access; the instability to mechanical damage; the impact of external influences on the readings; the difficulty in localizing of structure deformation; the lack of sensitivity; the instability of the readings and others.
Therefore, the development of effective monitoring systems with a minimum of drawbacks is rather urgent task. One solution to this problem is the development of smart materials that have the ability to react to the changes of geometric size, the distribution of local stresses, the formation and development of cracks, the moisture content and even the presence of chlorides. The successful implementation of this material is possible with the help of the usage of carbon nanostructures (graphite, graphene, nanotubes and nanofibers) according to two schemes: uniform distribution in the bulk of the structure material or in the composition of the coverage material applied to the surface (part of the surface) of the diagnosed structure (skin diagnostics). Despite the considerable amount of researches in this area it may be noted with confidence that the issue is still urgent. The evidence of this can be the inconsistent results of studies by various authors, in which the mechanical and conductive properties of concretes reinforced by carbon nanotubes can vary significantly. In [3] the grafting of functional groups was performed by boiling in a solution of sulfuric and nitric acids. The formation on the surface of oxygenated groups has led to the increase of the strengthening properties of the concrete while compression and bending to 19 and 25%, respectively. However, in [4] there was a decrease in durability while implementation of composite cement into the composition. According to the authors, the observed reduction of durability (7 times) is due to the absorption of water by carbon nanotubes and as a consequence of incomplete hydration of clinker. The double improvement of the durability of concrete has been found in [5]. According to the authors in the course of the acid treatment, concurrently with the process of grafting of functional groups onto the surface of the carbon nanotubes, there is the formation of carboxylated carbon fragments whose presence helps to reduce the reinforcing effect from the usage of carbon nanotubes. The removal of the mixing of mentioned fragments from the water has increased the strength of the cement composite from 36 MPa to 71 MPa, at the concentration of carbon nanotubes of 0.03% by the weight of the composite.
V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126
In the further numerous studies [6-9] it has been shown that the average increase in the durability of the composites was 30-50%. At the same time in the works of various authors the carbon nanotubes concentration, which provides the maximum durability increase, ranged from 0.005% to 1% by weight of the binder. In recent years, a large number of studies were aimed at studying not only the mechanical properties of concrete, but also their electrical properties. The common concrete in certain temperature and humidity conditions has the ability to conduct electricity, but this property is unstable. While seasonal variations in temperature and humidity the electrical resistance of conventional concrete changes to 6-8 orders of magnitude. Imparting of conductivity to the concretes, particularly, as it has already been noted, is due to the possibility of development of self-diagnostics materials and structures on their basis. In recent years, this kind of research is actively conducted abroad, in particular in the United States, Germany and other countries. Moreover, the real objects of transport and energy infrastructure have already been constructed of this concrete. As an active development of this area the last conference, held in Chicago, "Nanotechnology in Construction 2015" [15-19] can be given as an example, which presented the reports of the leading specialists devoted to the study of concretes conductive properties. The other principle of obtaining conductive composites – the introduction of conductive additives – is the basis of this study. It should be noted that in this regard the considerable progress has been made on polymeric materials, which have already found practical usage in the form of polymer products having conductive properties, smart materials and others [10,11]. Similar studies [12-14] are carried out for the metal and ceramic composites to increase strengthening and heat and electrical conducting properties. 2. Materials and methods As previously noted development of smart materials using carbon nanostructure is possible according two essentially different ways: adding of carbon nanotubes into the construction material by insuring of uniform distribution in the amount of structure or elaboration of coating material using carbon nanotubes with its surface application on the structure The development of the material according to the first scheme is valid only for new structures and does not cover the need for monitoring of existing structures. Furthermore, with the propagation of plastic deformations or cracking in the structure material, changing of humidity and other information obtained through this method will not be sufficient for an adequate assessment of the structure condition. The optimal solution of such problems is the application as surface coatings, which is the usage of skin-diagnostics. To study the possibility of the usage of coatings that could assess and monitor the stress-strain state of the structure, the primary (for testing the hypothesis) experiment was conducted according to the following scheme (Figure 1) – two bands of different width (10 mm and 14 mm) of conductive composition (adhesive mixture: carbon black: water – 1: 1: 1) were applied to the surface of the stretched area of a flexible element. a
b
Fig. 1. (a) Photo and (b) testing scheme of the beam with a surface coating of an electrically conductive composition.
After drying of the electrically conductive composition the phased beam-strength with simultaneous measuring of the conductive coatings resistance and deflection monitoring of crack formation, both in the beam and in the coating,
121
122
V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126
was carried out. In order to evaluate the elastic properties of the material the partial removal of the loading (10%) was conducted, after which the resistance change was recorded. According to results of hereof experiment possible problems and disadvantages of coatings were identified for the purpose of the follow-up study adjustment. In view of primary experiment results sodium silicate, epoxy resin, acrylic sealant, cement-sand mixture with the addition of PVA were used as the basic materials for obtaining the samples according to the scheme of skindiagnostics. Multilayer carbon nanotubes with the characteristics shown below (Table 1) were used as conductive additives. Table 1. The characteristics of the applied multilayer carbon nanotubes. Indices
Norm according to specifications
Exterior view
Lightweight black powder. Small amount of impurity loose lumps, easily crumbling into powder under pressure, is admitted. Point inclusions incorporating a small amount of white or yellow particles (unreacted catalyst residues) are admitted.
Apparent density, g/dm3
20-40
Weight ash content, %
8-22
Specific surface of untreated nanotubes, m2/g
200-400
Specific surface of nanotubes after acid refined from mineral impurities, m2/g
200-400
Outer diameter, nm
10-40
Electrical resistivity of compressed powder of unrefined nanotubes, Оhm/сm
0.05-0.15
Electrical resistivity of the compressed powder refined from mineral impurities nanotubes, Оhm/сm
0.05-0.10
Temperature loss of 5% weight after cleaning from mineral impurities
560-620
Actual value
28
200
Figure 2 shows microphotographs of used multilayer carbon nanotubes. a
b
Fig. 2. Microphotographs of used carbon nanotubes.
The concentration of introduced nanotubes was varied from 0.03 % to 3 % by binding material weight. Nanotubes adding into the composites was realized by mechanical blending according to research results [20]. After the samples hardens nanotubes uniform distribution in the amount of material was verified using scanning electron microscope (SEM), also electric resistance was measured. The resistance measurement was conducted by alternate and direct current for impact assessment of measurement methods. Experiment series two weeks apart was conducted for impact assessment of hardening time on electric resistance.
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3. Discussion of research results The primary experiment result is the confirmation of deformation continuity of the coating made of informative (conductive) material and stretching strain of a flexible element, i.e. workability of conductive coatings according to the scheme of skin-diagnostics. The change in resistance of conductive coatings while the propagation of deformation of the beam stretched area is shown in Figure 3. Changes in resistance of conductive coatings while flexible element cracking and coating craking were fixed during the testing.
appearance off cracks in the beam
aappearance of cracks in bands
Fig. 3. The change of the conductive structure resistance and the beam deflection under the loading.
The problems of the used coating have been identified on the results of the initial experiment: x the propagation of plastic deformations while tension in the early stages of loading (the resistance changes are insignificant while partial removal of the load and decrease of the deflection); x the presence of mixing water in the original composition, which would distort the measurement readings because of water evaporation and hydration of the base material components; That is the main problem in the usage of the skin-diagnostics is a smart material itself that would satisfy the conditions of exploitation of the structure: alternating temperatures, increased or high temperatures, changing of the ambient humidity, cyclic loadings, alternate opening / closing of cracks (for example, for spans of bridges, concrete walling structures of the NPP safety systems, crane beams, etc.). Undoubtedly, so universal material in terms of operating conditions is difficult to implement, respectively, for these technologies several options for smart materials should be offered. According the primary experiment results 14 sample series were made (three samples for each series) using different binding materials with addition of carbon nanotubes with different concentration. Figure 4 shows the obtained samples.
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V.I. Bolshakov et al. / Procedia Engineering 172 (2017) 119 – 126 Fig. 4. Experimental samples.
Figure 5 shows SEM pictures of the samples of sodium silicate (Fig. 5a), acrylic (Fig. 5b) and cement (Fig. 5c). It should be noted that in the samples of sodium silicate relatively even allocation of nanotubes on the surface of destruction can be seen (Fig. 5a). Acrylic composition is characterized by high coverage (Fig. 5b), which does not allow to see the allocation of nanotubes. Uniform allocation of nanofibers coated by C-H-S gel is viewed in the sample on the basis of the cement (Fig. 5c). a
b
c
Fig. 5. SEM pictures of CNT composites: a) sodium silicate, b) acrylic, c) cement.
Measurement results of resistivity of the samples without giving a specific value of the direct and alternating current are summarized in Table 2. Regarding the measurement repeatability, it may be note that the most part of results keep within acceptable testing error. Maximum measured error for each series is less than 6 %. Concerning the testing results the samples of sodium silicate and epoxy resin are the most interesting in terms of clarity of the results. The dependence of their resistance and of nanotube concentration is illustrated on Fig. 6. The resistance values set to arithmetical average of results for each sample series. It should be noted that drastic fall of sample resistance was marked with changing of nanotube concentration from 0 to 1% during the testing by alternate and direct current. Despite different maximum values of electric resistance of samples on sodium silicate and epoxy resin this parameter decrease on two value order while increasing of nanotube concentration up to 1%. With further increasing concentration of nanotubes the difference of measurements decreases. For samples with cement and acrylic the procedure of measuring at different frequencies of current remained practically constant. Table 2. The results of resistivity measurements. №
Resistance (DC), kΩ
Resistance (AC), kΩ
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% canbonic nanotubes by the weights of the binding
1
2
3
1
2
3
1
0.00
232000
236000
230000
84000
95000
78000
2
0.03
130000
128000
132000
35000
36000
34000
0.93
2040
1870
1930
950
1000
1000
4
2.00
140
128
127
20
20
21
5
2.89
0.726
0.728
0.722
1
0.96
1.02
6
0.00
5300
5160
5200
5000
5050
4900
1.00
536
527
526
500
500
500
2.00
293
304
302
200
200
200
0
>1200000
>1200000
>1200000
>1200000
>1200000
>1200000
0.1
>1200000
>1200000
>1200000
1200000
1200000
1200000
1
1000000
1000000
1000000
1000000
1000000
1000000
Name of the binding
3
7
Sodium silicate
Epoxy resin
8 9
Acrylic
10 11
Cement
Fig. 6. Dependence of resistivity from the concentration of nanotubes.
4. Conclusion The usage of smart materials allows to qualitatively record deformations of structures with high accuracy. When processing the data there will be the opportunity to detect deflections of structures and calculate the value of local stresses, to determine the time of cracking and the width of the crack opening. The conducted study confirms the possibility to use multilayer carbon nanotubes to create smart material composites for skin-diagnostics. The method is based on changes of the electrical conductivity of the composites by applying an external loading or the impact of external conditions on the material. A variant of deformations
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observations by coating the surface of the structure with a thin layer of electrically conductive material of sodium silicate, acrylic, cement-sand mixture as well as epoxy resin with CNT with apparent density 28 g/dm3 and specific surface of untreated nanotubes 200m2/g was examined. For all formulations a series of tests to measure the electrical resistance of samples with different concentrations of carbon nanostructures has been conducted. The percentage of CNTs relative to binder weight was varied from 0 to 3%. For the studied compositions threshold concentrations of carbon nanotubes, leading to an abrupt increase of electrical conductivity measurements by constant and alternating current were determined. In the case of materials based on sodium silicate and epoxy resin the addition of CNT from 0 to 1% by weight of binder results in significant changes in the resistivity of the material, while a further increasing of carbon nanotubes, their conductivity change in a less degree. 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