Self sensing carbon nanotube (CNT) and nanofiber (CNF) cementitious composites for real time damage assessment in smart structures

Cement & Concrete Composites 53 (2014) 162–169

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Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp

Self sensing carbon nanotube (CNT) and nanofiber (CNF) cementitious composites for real time damage assessment in smart structures Maria S. Konsta-Gdoutos ⇑, Chrysoula A. Aza Department of Civil Engineering, Democritus University of Thrace, Xanthi, Greece

a r t i c l e

i n f o

Article history: Received 9 October 2013 Received in revised form 19 May 2014 Accepted 8 July 2014 Available online 16 July 2014 Keywords: Resistivity Piezoresistivity Strain sensing Carbon nanotubes cementitious composites

a b s t r a c t The self sensing properties of cementitious composites reinforced with well dispersed carbon nanotubes and carbon nanofibers were investigated. The electrical resistance of cementitious nanocomposites with w/c = 0.3 reinforced with well dispersed carbon nanotubes (CNTs) and nanofibers (CNFs) at an amount of 0.1 wt% and 0.3 wt% of cement was experimentally determined and compared with resistivity results of nanocomposites fabricated with ‘‘as received’’ nanoscale fibers at the same loading. Results indicate that conductivity measurements, besides being a valuable tool in evaluating the smart properties of the nanocomposites, may provide a good correlation between the resistivity values measured and the degree of dispersion of the material in the matrix. The addition of CNTs and CNFs at different loadings was proven to induce a decrease in electrical resistance, with the nanocomposites containing 0.1 wt% CNTs yielding better electrical properties. Furthermore, conductivity measurements under cyclic compressive loading provided an insight in the piezoresistive properties of selected nanocomposites. Results confirm that nanocomposites, reinforced with 0.1 wt% CNTs and CNFs, exhibited an increased change in resistivity, which is indicative of the amplified sensitivity of the material in strain sensing. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Recent advances in the field of nanotechnology have led to the development of advantageous nanoscale fibers, making possible the development of new multifunctional, high performance, advanced sensing, cement based nanocomposites that could effectively act as sensors to monitor the health of the structures. Depending on their atomic structure, CNTs and CNFs may be metallic or semiconductors. When subject to stress/strain, their electrical properties change, expressing a linear and reversible piezoresistive response [1,2]. These electromechanical characteristics of CNTs and CNFs open new potential applications for cementitious nanocomposites with improved mechanical properties and added multifunctionality in stress monitoring of concrete structures, detecting damage, as well as traffic monitoring in highway structures. Research efforts have been concentrated on developing cementitious composites with sensing capabilities using carbon microfibers [3–10]. Banthia et al. [3] conducted electrical resistivity measurements on cement pastes reinforced with carbon and steel microfibers as well as on several hybrid mixes containing both carbon and steel fibers. The addition of fibers significantly ⇑ Corresponding author. Tel.: +30 25410 79658; fax: +30 25410 79652. E-mail address: [email protected] (M.S. Konsta-Gdoutos). http://dx.doi.org/10.1016/j.cemconcomp.2014.07.003 0958-9465/Ó 2014 Elsevier Ltd. All rights reserved.

improved the conductivity of cement composites to a large extent. Fu and Chung [9] examined the strain sensitivity of carbon fiber/ cement compared to that of normal cement since 1993, while cement composites containing carbon fibers have also been applied for monitoring traffic flow [10]. The results of their studies indicated that under loading, cement-based sensors with carbon fiber reinforcement develop a good correlation between resistivity changes and load changes, increasing in tension and decreasing in compression, sensing crack opening when the resistivity increases and crack closing when resistivity decreases. A few studies have been carried out on the electrical properties, the piezoresistive behavior and sensing ability of cementitious nanocomposites embedded with carbon nanotubes (CNTs). Li et al. [11] conducted experiments using treated with acids (SPCNT) and untreated CNTs (PCNT) as reinforcement and concluded that both types reduce the electrical resistance and enhance the properties of the cementitious composites. In addition, they examined the variation of resistance by imposing cyclic compressive load. Yu and Kwon [12] investigated the electrical properties under compressive load of cement paste reinforced with multiwalled carbon nanotubes (MWCNTs). Results showed that the electrical resistance of the nanocomposite changed synchronously with the compressive stress levels. Recently, Han et al. [13] conducted experiments in the laboratory and in the field. They examined the electrical properties of nanocomposite samples under continuous and instant

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compressive load. Also they investigated the possibility of using the nanocomposite material for traffic control. Their results showed that under repeated compressive loading the electrical resistivity decreases upon loading and increases when unload, while the instant load also presented changes in the electrical resistance. In addition, road test results concluded that this nanocomposite can detect vehicular loads through remarkable changes in electrical resistance. More recently, researchers investigated the effect of the MWNT content and water/cement ratio on the piezoresistive sensitivity of composites [14–16]. The piezoresistive properties are dependent on the conductive network in the composites, which in turn is influenced by the CNT or CNF concentration level and water/cement ratio. Han et al. [14] examined cement nanocomposites with amounts of multi-walled CNT (MWNT) of 0.05, 0.1 and 1 wt%. Experimental results indicated that the piezoresistive sensitivities of the composites first increased and then decreased with the increase of the CNT content concluding that the composite with 0.1 wt% of MWNT presents better sensing property. At the same time, they examined the effect of water/cement ratio on piezoresistivity by fabricating and testing samples with 0.45 and 0.6 water/ cement ratios. The CNT/cement composite with w/c = 0.6 exhibited more sensitive response to compressive stress. Coppola et al. [15] observed that the sensitivity under compressive stress of cement pastes containing different percentages (0.1 and 1.0 wt%) of MWNTs is less evident in the composites with 1.0 wt% concentration of CNTs. Luo et al. [16] used cured multi-walled carbon nanotube (MWCNT) reinforced cement-based composites with 0.1 wt% and 0.5 wt% MWCNT and measured the electrical resistances under cyclic loading and unloading. Results revealed good piezoresistivity and strain sensitivity for both samples though the trendline of fractional change in resistivity (Dq) presented better stability for amounts of 0.5 wt%. The considerable challenge in fully exploiting the properties of carbon nanotubes and carbon nanofibers as reinforcement in a composite is attributed to the lack of the fiber dispersion in the material matrix. The results of experimental studies on CNT/ cement and CNF/cement specimens converge to the point that the addition of carbon nanotubes or nanofibers improves the mechanical [17] and electrical properties of the nanocomposite material by reducing resistivity and providing piezoresistive properties, under the condition that effective dispersion of nanofibers in the cement paste has been achieved [18–21]. Konsta-Gdoutos et al. [18] achieved effective dispersion of CNTs by applying ultrasonic energy along with the use of a surfactant (SFC) and observed that beyond the necessity for ultrasonication there is an optimum ratio of surfactant to CNTs for good dispersion. Considering this dispersion method Metaxa et al. [19] applied it on CNFs embedded in cement composites and investigated the appropriate amount of ultrasonic energy and the surfactant to CNF ratio (SFC/CNF) for effective dispersion. More recently, Han et al. [21] investigated the use of a polycarboxylate superplasticizer for effectively disperse CNTs and CNFs in a cement matrix given that the existing dispersants affect cement hydration and consequently its mechanical properties. Results concluded that proper dispersion is achieved with the use of a superplasticizer, due to its ability to disperse both cement particles and CNTs/CNFs. Through this procedure high performance cementitious nanocomposites with strong piezoresistive characteristics can be successfully fabricated. Another issue that arises is that cementitious composites exhibit the effect of polarization which causes an increase in electrical resistance during measurement while even the current cannot accurately be measured [22]. Solution to this, beyond the use of dry specimens, can be the use of AC measurement method, as applied by Banthia et al. [23].

Finally, recent research focuses its interest in exploring the behavior of carbon nanotube cement based sensors under dynamically varying strain in concrete structures [24]. In this study the electrical resistivities and self sensing properties of cementitious composites reinforced with well dispersed carbon nanotubes and carbon nanbofibers were investigated. The electrical resistance of cementitious nanocomposites with w/c = 0.3, reinforced with well dispersed carbon nanotubes (CNTs) and nanofibers (CNFs) at an amount of 0.1 wt% and 0.3 wt% of cement, was experimentally determined using the 4-pole method, and compared with resistivity results of nanocomposites fabricated with ‘‘as received’’ nanoscale fibers at the same loading. The piezoresistive properties and the sensing ability of the cement-based nanocomposites were also investigated measuring the changes in resistivity under the application of compressive cycling loading. A set of preliminary resistivity experiments were also conducted on nanocomposites reinforced at an amount of 0.048 wt% of cement, for the purpose of determining the optimum applied voltage. 2. Experimental procedure 2.1. Materials The cementitious material used in this study was Type I ordinary Portland cement (OPC). Experiments were conducted with MWCNTs of 20–40 nm diameter and length in the range of 10–100 lm. A type of highly graphitic, Pyrograf-III, carbon nanofibers was used. CNFs exhibit a tensile strength of 7 GPa, tensile modulus of 600 GPa and length range of 30–100 lm. Characteristic properties of MWCNTs and CNFs used can be seen in Table 1. For the preparation of CNT and CNF dispersions, a commercially available surfactant (SFC) Glenium 3030 is used. In a typical procedure of dispersion [18], suspensions are prepared by mixing the CNTs and CNFs in aqueous solution containing the surfactant and the resulting dispersions are sonicated at room temperature. Constant energy is applied to the samples by a 500 W cup-horn high intensity ultrasonic processor with a cylindrical tip and temperature controller. The sonicator is operated at an amplitude of 50% so as to deliver energy of 1900–2100 J/min, at cycles of 20 s in order to prevent overheating of the suspensions. After sonication, cement was added into the CNT and CNF suspensions at a water to cement ratio w/c = 0.3 by weight. The materials were mixed according to ASTM 305 using a mixer capable of operating from 140 ± 5 revolutions/minute (r/min) to 285 ± 10 r/min. After mixing, the paste was cast in 20  20  80 mm oiled molds. For measuring the electrical resistivity of the samples, metallic grids with large opening (3  3 mm) were used as electrodes, which were incorporated into the specimens immediately after casting. Specimens were then covered with membrane until they were demolded after 24 h. Following demolding, the samples were cured in lime-saturated water for 28 days and then put in an oven to dry, first at 60 °C for three days, and then for another three days at 95 °C. The process of specimen drying aims at eliminating the effect of polarization. CNT/cement and CNF/cement nanocomposites were prepared at percentages of 0.1 and 0.3 wt% of cement. At least three specimens were prepared for each mixture (Table 2). Table 1 Properties of multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs). Type

Diameter (nm)

Length (lm)

Tensile strength (GPa)

Tensile modulus (GPa)

Aspect ratio

CNT CNF

20–40 60–150

10–100 30–100

11–200 7

200–1000 600

1600 650

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Table 2 List of specimens and mix proportions. Specimen

w/c

MWCNTs (wt% of cement)

CNFs (wt% of cement)

Dispersion

CP CP + CNTs0.048 wt% CP + CNTs0.1 wt% CP + CNTs0.1 wt% without dispersion CP + CNTs0.3 wt% CP + CNFs0.048 wt% CP + CNFs0.1 wt% CP + CNFs0.1 wt% without dispersion CP + CNFs0.3 wt%

0.3 0.3 0.3 0.3

– 0.048 0.1 0.1

– – – –

– Yes Yes No

0.3 0.3 0.3 0.3

0.3 – – –

– 0.048 0.1 0.1

Yes Yes Yes No

0.3

–

0.3

Yes

2.2. Testing procedures 2.2.1. Method for measuring the electrical resistance The most commonly used methods for measuring the electrical resistance in cement based materials are the two-pole method and the four-pole method with two or four electrodes being embedded in cement paste respectively. In the two-pole method the two electrodes are used for measuring both the current intensity and voltage difference, while in the four-pole method the two inner electrodes are used to measure voltage drop and the two outer for the passing current intensity. In this study, the method of four electrodes (four-pole method) was employed, as it is generally considered a more accurate way to measure small resistances. Han et al. [25] studied the value of electrical resistance in cement composite samples with 3% by volume of carbon fibers, applying both the four-pole and two-pole method in order to compare the two methods. The results of this study showed that the four-pole method yields to a smaller electrical resistance values because of the elimination of the effect of the contact resistance (UR). In addition, unlike in the two-pole method the electrical resistance resulting from the four-pole method meets the connection in series of a DC power circuit. The resistivity of cement-based nanocomposites is calculated by:

q¼

RS L

where R is the resistance of the nanocomposite, S the cross section of the specimen and L the distance between the two inner electrodes. Han et al. [25] observed that the experimental results are consistent when the space between the current and voltage electrodes is bigger than 7.5 mm. In the present study the outer and inner electrodes were put in a distance of 60 mm and 30 mm, respectively (Fig. 1). 2.2.2. Experimental set-up To measure the electrical resistance/conductivity of the cement based nanocomposites two digital multimeters (Mastech MS8218 AC/DC True RMS) with the ability to measure lA and V, and a power supply unit (Mastech HY3005D) capable of supplying direct current (DC) up to 30 V were used. The multimeter connected with the inner electrodes was used to measure the voltage difference while the one connected with the outer electrodes and the power supply unit was used for current intensity measurements. The setup is shown in Fig. 2. Both digital multimeters were connected to a computer via a RS232 port and equipped with the adequate software for direct data acquisition in windows environment. According to the method, direct current is applied to each specimen for approximately 26 min, (electrical resistance in cement based materials is stabilized after 20 min of supplying power),

Fig. 1. Resistivity measurement specimen with embedded electrodes.

Fig. 2. Experimental setup for measuring the electrical resistance.

and values of the voltage difference and current intensity were recorded every 5 s through the multimeters. The piezoresistive properties and the sensing ability of the cement-based nanocomposites were investigated using the aforementioned procedure, while a compressive cyclic load was simultaneously applied to the specimens from a hydraulic testing machine (SCHENK) of maximum operating strain ability 60 kN. A cyclic compressive loading of maximum 2 kN (load should be within the elastic region) was performed in each specimen and every cycle of loading-unloading had duration of approximately 120 s. The values of load, voltage and current intensity were recorded every 1 s, with the experimental procedure lasting 600– 800 s for each specimen. 2.2.3. Testing The resistivity of a material is dependent on several parameters. First, it is a function of the applied voltage. The resistivity also varies as a function of the length of the passing current. The longer the voltage is applied, the higher the resistivity [22]. Factors such as moisture content and temperature also affect a material’s resistivity: in general, the higher the moisture content and the temperature of the specimen, the lower the resistivity. In this work, resistivity experiments with different voltage potentials were conducted, with the purpose of determining the optimum applied

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3. Results and discussion 3.1. Effect of applied voltage

2.5

Cement paste

1.5 1.0

Nanocomposites

0.5 0.0 0

10

1.5 1.0 0.5

CP

CP+CNTs 0.048wt.%

CP+CNFs 0.048wt.%

Fig. 4. Resistivity of plain cement paste and CNT or CNF cementitious nanocomposites, under the application of 20 V.

cement paste and CNT/CNF cementitious nanocomposites, under the application of 20 V are shown in Fig. 4. It can be observed that the use of CNTs and CNFs even at a very low amount (0.048 wt% of cement) in cement based composites was found to substantially decrease the electrical resistance of the material. 3.2. Effect of moisture content-polarization effect The resistitivity/conductivity of the composites greatly depends on the moisture content. To examine the effect of the internal moisture cement paste mixes with w/c = 0.3 and with CNTs and CNFs at a percentage of 0.1 wt% were prepared and dried in the oven at 60 °C, as suggested from previous experimental studies [11,13,29]. It is observed from Fig. 5 that the nanocomposites reinforced with 0.1 wt% CNTs and CNFs exhibit a decrease in their electrical resistance, with the CNT composite demonstrating a larger decrease than the CNF composite. Several reasons could possibly explain this behavior: (a) the amount of individual CNFs corresponding to the same weight is lower than that of the nanotubes, since the CNFs are nearly three times larger in diameter than the nanotubes; (b) CNFs may exhibit lower conductivity compared to CNTs; (c) insufficient dispersion of CNFs in cement paste and the creation of agglomerates in the matrix. This in turn results in a tortuous and discontinuous electrical network that allows electrical current passage with difficulty. Comparing the resistivities of the 0.048 wt% composites with the 0.1 wt% ones (Fig. 6) we observe that the nanocomposites containing CNTs at an amount of 0.1 wt% exhibit lower resistivity than the 0.048 wt% one. It is apparent that the composite’s resistivity depends on the CNT content and clearly the resistivity decreases with increasing CNT amount, resulting in a material with better electrical properties. Comparing the resistivities of the mixes with 0.6

Cement paste (CP) CP+CNTs0.048wt.% CP+CNFs0.048wt.%

2.0

2.0

0.0

20

30

Applying Voltage (V)

Resistivity *10 6 (Ohm.cm)

Resistivity ×106 (Ohm.cm)

Information on the applied voltage during conductivity/resistivity measurements in cementitious nanocomposites is very limited. Based on the information provided by two researchers [27,28] three different voltages 10 V, 20 V and 30 V were applied in w/c = 0.3 OPC and composite cement paste specimens, following the aforementioned procedure for the determination of the electrical resistance. Fig. 3 presents results of the average electrical resistance of three specimens: (i) of plain cement paste; and (ii) cement paste reinforced with CNTs or CNFs at an amount of 0.048 wt% of cement. It is observed that the lowest values of resistivity, for both neat cement paste and nanocomposites, are recorded with the application of 10 V. A 9.3–12% deviation from the values of the other applied voltages was estimated. Values of the average resistivity appear stabilized with the application of 20 and 30 V. According to this, the application of 20 V appears as the optimum amplitude for the measurements of the electrical resistance of cement based materials and was chosen to continue the testing in this study. Initial results of resistivity measurements of plain

2.5

Resistivity ×106 (Ohm.cm)

voltage. Preliminary experiments were conducted on neat cement paste and nanocomposites reinforced at an amount of 0.048 wt% of cement. The specimens were dried at 60 °C for three days. Three different voltages of 10 V, 20 V and 30 V were used, with the duration of each measurement being 26 min, until electrical resistivity reached a plateau [26]. Thereafter, the effect of specimen drying on the resistivity was examined. For this purpose, resistivity measurements took place on specimens reinforced with 0.1 wt% CNTs, dried at 60 °C for three days. A second round of resistivity measurements was conducted after the specimens were put in the oven for three additional days, at 95 °C. Next, the electrical resistivities of the CNT and CNF cementitious composites were compared with the one of the neat cement paste and with the resistivities of cementitious nanocomposites with CNTs and CNFs without dispersion. All specimens were dried at 95 °C and all measurements were conducted at room temperature, while the four-pole method was followed for the measurement of the electrical resistance. In addition, the piezoelectrical behavior of cement nanocomposites was investigated. The change in electrical resistance was evaluated in composite specimens reinforced with 0.1 wt% and 0.3 wt% of cement CNTs and CNFs subjected to a simultaneous cyclic compressive loading. The experiments were conducted for 800 s, with each cycle of loading–unloading lasting 120 s and following the experimental procedure described previously.

0.5 0.4 0.3 0.2 0.1

CP+CNTs0.1wt% CP+CNFs0.1wt%

0.0 0

5

10

15

20

25

Time (min) Fig. 3. Average electrical resistivity of plain cement paste and cement paste reinforced with CNTs or CNFs at an amount of 0.048 wt% of cement under different applied voltages.

Fig. 5. Resistivity of cement nanocomposites reinforced with CNTs and CNFs at a percentage of 0.1 wt% and with w/c = 0.3.

M.S. Konsta-Gdoutos, C.A. Aza / Cement & Concrete Composites 53 (2014) 162–169

Resistivity ×106 (Ohm.cm)

0.6 Specimens dried at 60°C for 3 days 0.5 0.4 0.3 0.2 0.1

Resistivity×106 (Ohm.cm)

166

5.0

CP+CNTs0.1wt.%

4.0

dried at 95oC

3.0 x14

2.0 1.0

dried at 60oC

0.0 0

0.0

5

3.3. Electrical resistance of different loading of CNTs and CNFs In order to further investigate the effect of the different loading of CNTs and CNFs in the electrical resistivity of the nanocomposites, the electrical resistance measurements were performed at 28 days old specimens, containing different amounts of CNTs and CNFs. Cement paste samples, reinforced with dispersed CNTs and CNFs at contents of 0.1 wt% and 0.3 wt% of cement, were prepared and tested after drying at 95 °C for 3 days. The results of the electrical resistance are shown in Fig. 8(a) and (b). It is observed that the nanocomposites reinforced with CNFs exhibit higher resistivity compared to the samples reinforced with the same amount per weight of cement CNTs. As mentioned before, this can be attributed to the different aspect ratio of the two nanomaterials, which

20

25

(a) Resistivity*106 (Ohm.cm)

the same percentage of CNTs and CNFs an increased resistivity can be observed for the CNF composite samples, which can be attributed to an insufficient dispersion. It can also be observed that the difference between the resistivities of the 0.048 wt% CNT and CNF composites is much smaller compared to the ones of the 0.1 wt% composites. This is a very interesting result that could be initially attributed to insufficient dispersion of the CNFs at the amount of 0.1 wt%. A more plausible explanation may include the possibility of a well pronounced polarization effect, mainly due to the amount of water and the dissolved ions in the mixes, which in turn cause the measured resistivity to relatively increase. Konsta-Gdoutos et al. [30] investigated the correlation between the water content in high performance mixes and its conductivity and concluded that the conductivity of the material is greatly affected by the volume of the porosity and the water content. More specifically, increasing the amount of water in the mixes results in the decrease of the electric resistance since the water is a good conductor of electricity and allows the current passing through, thus increasing the conductivity of the material. In this study, to better evaluate the influence of the amount of moisture on the electrical resistance, the appropriate oven temperature for specimen drying was explored, in order to eliminate from the resistivity testing the so-called polarization effect. A protocol was followed for the measurements of the electrical resistance of cementitious nanocomposites with CNTs at an amount of 0.1 wt% as follows: specimens were at first dried at 60 °C for 3 days with a subsequent drying at 95 °C for another 3 days. Results show that the values of electrical resistance after drying at 95 °C are almost 14 times higher than those exhibited by specimens dried at 60 °C (Fig. 7). It was concluded that drying of the specimens at 95 °C results in a significant water removal from the material’s pores and greatly eliminates the polarization effect seen in specimens with higher internal moisture content. Consequently resistivity values dramatically increase.

15

Fig. 7. Electrical resistivity of cement paste reinforced with CNTs at a percentage of 0.1 wt% dried at different temperatures (60 °C and 95 °C).

6 5 4 3 2 1

CP+CNTs0.1wt% CP+CNFs0.1wt%

0 0

5

10

15

20

25

Time (min)

(b) Resistivity*106 (Ohm.cm)

Fig. 6. Average resistivity of cement nanocomposites reinforced with CNTs and CNFs at amounts of 0.048 wt% and 0.1 wt% of cement.

10

Time (min)

CP+CNTs CP+CNFs CP+CNTs CP+CNFs 0.048wt% 0.048wt% 0.1wt% 0.1wt%

8 7 6 5 4 3 2 1

CP+CNTs0.3wt% CP+CNFs0.3wt%

0 0

5

10

15

20

25

Time (min) Fig. 8. Resistivity of cement paste samples with w/c = 0.3 reinforced with dispersed CNTs and CNFs at contents of (a) 0.1 wt% and (b) 0.3 wt% of cement.

results to a lower amount of individual carbon nanofibers reinforcing the matrix. Another plausible explanation could be differences in the electrical conductance between CNTs and CNFs. The average resistivity of cement paste specimens reinforced with CNTs and CNFs at amounts of 0.1 wt% and 0.3 wt% of cement is shown in Fig. 9. Samples with higher loading of reinforcement (0.3 wt% CNTs or CNFs) exhibit relatively higher resistivity than samples containing 0.1 wt% CNTs or CNFs. This can be explained by an insufficient dispersion of the nanomaterials, mainly due to limitations of the ultrasonic processor used. Inadequate dispersion results in the presence of agglomerates in the matrix, which in turn impair the composite’s electrical conductivity. To further evaluate the effect of the nanoscale fibers’ dispersion in the matrix, measurements of the electrical resistance in samples of plain cement paste and cement nanocomposites with CNTs and CNFs without dispersion were performed. Specimens of cement paste with w/c = 0.3 and composites reinforced with non-dispersed, as received, CNTs and CNFs at an amount of 0.1 wt% were casted and dried at 95 °C. Results of the electrical resistivity of CNT/cement and CNF/ cement composites without dispersion (both CNTs and CNFs were

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7

Resistivity*10 6 (Ohm.cm)

Specimens dried at 95°C for 3 days 6 5 4 3 2 1 0 CP+CNTs CP+CNFs CP+CNTs CP+CNFs 0.1wt% 0.1wt% 0.3wt% 0.3wt%

Fig. 9. Average resistivity of cement paste specimens reinforced with CNTs and CNFs at amounts of 0.1% and 0.3 wt% of cement.

Resistivity*10 6 (Ohm.cm)

(a)

12

Specimen B2 Specimen B3 Specimen C1 Specimen C2 Specimen C3

CP+CNTs0.1wt% 10 without dispersion 8 6 4 2 0 0

5

10

15

20

25

Time (min)

Resistivity*10 6 (Ohm.cm)

(b)

12

Specimen Β1 Specimen Β2 Specimen B3 Specimen C1 Specimen C2 Specimen C3

CP+CNFs0.1wt% 10 without dispersion 8 6

3.4. Piezoresistive behavior of CNT and CNF cementitious nanocomposites

4 2 0 0

5

10

15

20

added ‘‘as received’’) tested at room temperature are presented in Fig. 10(a) and (b), respectively. Contrary to the results exhibited by the composites reinforced with well dispersed CNTs at an amount of 0.1 wt% of cement where all specimens examined presented similar and consistent values in resistivity, specimens of cementitious composites with 0.1 wt% ‘‘as received’’ CNTs exhibited inconclusive values of resistivity. Samples of cementitious nanocomposites containing CNFs without dispersion (as received) (Fig. 10(b)) exhibited similar unreliable electric behavior. It is possible that conductivity/resistivity measurements besides being a valuable tool in evaluating the smart properties of the nanocomposites may provide a good correlation between the resistivity values measured and the degree of dispersion of the material in the matrix. More testing is required to further elucidate this finding. The average values of resistivity of the different samples examined are illustrated in Fig. 11. Comparing the results it is obvious that the resistivity of neat cement paste significantly exceeds the average resistivity of all other mixtures containing CNTs and CNFs, which enhance the electrical properties of the cement based material increasing the conductivity. Cement composites with w/c = 0.3, reinforced with dispersed 0.3 wt% CNTs or CNFs develop the highest resistivity compared to nanocomposites with 0.1 wt% CNTs or CNFs even without dispersion. These high resistivity values suggest that the current passes through the material with difficulty, possibly due to insufficient dispersion of CNTs and CNFs and the subsequent creation of agglomerates. Samples with CNFs exhibit higher resistivity than the ones with CNTs at the same content level, probably due to the larger diameter of CNFs compared to CNTs. Cement composites reinforced with well dispersed 0.1 wt% CNTs produced the best results, as they exhibited a significantly lower resistivity with great repeatability.

25

Time (min) Fig. 10. Resistivity of (a) CNT/cement and (b) CNF/cement composites without dispersion at an amount of 0.1 wt% of cement.

To investigate the piezoresistive behavior of cement nanocomposites the change in electrical resistance was measured under the simultaneous application of a compressive load. Specimens of cement paste reinforced with CNTs and CNFs at amounts of 0.1 wt% and 0.3 wt% were subjected to a cyclic compressive loading. The variation in electrical resistance is reversible under cyclic compressive loading within the elastic region. For that reason a preliminary experiment was conducted on nanocomposites to obtain the optimum load application amplitude. Based on this, the amplitude of 0–2 kN was adopted.

Resistivity*106 (Ohm.cm)

12 10 8 6 4 2 0 CP

CP+CNTs 0.1wt% (without dispersion)

CP+CNFs 0.1wt% (without dispersion)

CP+CNTs 0.1wt%

CP+CNFs 0.1wt%

CP+CNTs 0.3wt%

CP+CNFs 0.3wt%

Fig. 11. Average resistivity of plain cement paste, cement composites reinforced with 0.1 wt% and 0.3 wt% well dispersed and ‘‘as received’’ CNTs and CNFs.

168

Stress (MPa) Change in Resistivity (%)

CP+CNTs0.1wt%

5 4

Stress (MPa)

Table 3 Average change in resistivity of cement paste reinforced with CNTs and CNFs at the amounts of 0.1 wt% and 0.3 wt% of cement.

0 -1

3

-2

2

-3

1

-4

0 0

100

200

300

400

Δρ/ρ0 (%)

(a)

M.S. Konsta-Gdoutos, C.A. Aza / Cement & Concrete Composites 53 (2014) 162–169

-5 600

500

Time (sec) Stress (MPa) Change in resistivity (%)

CP+CNFs0.1wt%

5

0 -1

Stress (MPa)

4

-2 3 -3 2 -4 1

Δρ/ρ0 (%)

(b)

-5

0 0

100

200

300

400

-6 600

500

Time (sec) Fig. 12. Piezoresistive behavior of cement composites with w/c = 0.3 reinforced with (a) CNTs and (b) CNFs at an amount of 0.1 wt% of cement.

(a)

Stress (MPa) Change in Resistivity (%)

CP+CNTs0.3wt%

5

0.0

-0.6

3

-0.8 2

-1.0 -1.2

1

-1.4 0 0

100

200

300

400

500

600

700

-1.6 800

Time (sec) Stress (MPa) Change in Resistivity (%)

CP+CNFs0.3wt%

5

0.0

4

-0.5

3

-1.0

2

-1.5

1

-2.0

0 0

100

200

300

400

500

600

700

800

Δρ/ρ0 (%)

Stress (MPa)

(b)

Average change in resistivity (%)

CP + CNTs0.1 wt% CP + CNFs0.1 wt% CP + CNTs0.3 wt% CP + CNFs0.3 wt%

4.5 5.3 1.4 1.9

The results of the piezoresistivity testing of 0.1 wt% CNTs and CNFs composites are shown in Fig. 12(a) and (b), respectively. The graphs reflect the cycles of loading-unloading performed and the corresponding change in resistivity. The vertical axis to the left represents the values of compressive load applied over time. The axis to the right represents the corresponding fractional change of resistivity, (%). The fractional change is obtained by dividing the difference in values of resistivity each time and initial resistivity, Dq, to the value of the initial resistivity, q0. Similarly, the results of the piezoresistive behavior of the 0.3 wt% CNT and CNF nanocomposites are presented in Fig. 13(a) and (b). It is observed that resistivity decreases during loading in the elastic region, where cracks are closing, and increases during unloading of the specimen, where cracks are opening. The average change in resistivity for all composites is summarized in Table 3. The average change in electrical resistivity of cement paste reinforced with CNTs and CNFs at the amount of 0.1 wt% is estimated to 5% while that of cement paste reinforced with CNTs and CNFs at the amount of 0.3 wt% about 1.5%. The calculation of the average value is based on the change of the resistance recorded at each cycle of loading-unloading for each sample. Comparing the average change in resistivity for the 0.1 wt% CNT and CNF samples to those containing CNTs or CNFs at 0.3 wt% it is concluded that the average change in resistivity is higher for the 0.1 wt% composites. An increased change in resistivity (ffi5%) indicates that the response to the application of the cyclic compressive loading is more pronounced; therefore the sample is more sensitive in recognizing the change in the applied stress and the induced mechanical deformation, and generally can considered to be a piezoresistive sensor.

-0.4

Δρ/ρ0 (%)

Stress (MPa)

-0.2 4

Nanocomposites

-2.5 900

Time (sec) Fig. 13. Piezoresistive behavior of cement composites with w/c = 0.3 reinforced with (a) CNTs and (b) CNFs at an amount of 0.3 wt% of cement.

4. Conclusions The resistivity and piezoresistive sensitivity of cement based nanocomposites reinforced with carbon nanotubes (CNTs) and carbon nanofibers (CNFs) was investigated. The application of 20 V appears to be the optimum amplitude for conducting reliable measurements of the electrical resistance of cement based materials, as results under this voltage were consistent. The appropriate oven temperature for specimen drying was explored, in order to eliminate from the resistivity testing the so-called polarization effect. It was shown that drying of the specimens at 95 °C results in a significant water removal from the material’s pores and greatly eliminates the polarization effect seen in specimens with higher internal moisture content. Resistivity measurements performed at 28 days old specimens containing different amounts of CNTs and CNFs showed that the addition of CNTs and CNFs was proven to induce a decrease in electrical resistance, with the nanocomposites containing 0.1 wt% CNTs yielding better electrical properties. Furthermore, a comparison between the resistivity values from composites reinforced with well dispersed carbon nanoscale fibers and those from composites reinforced with carbon nanoscale fibers ‘‘as received’’ indicated that resistivity measurements using the 4-pole method may provide a good correlation between the resistivity values measured and the degree of dispersion of the material in the matrix. Finally, conductivity measurements under cyclic compressive loading pro-

M.S. Konsta-Gdoutos, C.A. Aza / Cement & Concrete Composites 53 (2014) 162–169

vided an insight in the piezoresistive properties of selected nanocomposites. Results confirm that nanocomposites reinforced with 0.1 wt% CNTs and CNFs exhibited an increased change in resistivity, which is indicative of the amplified sensitivity of the material in strain sensing. Acknowledgements The authors would like to acknowledge the financial support of the National Strategic Reference Framework (NSRF)–Research Funding Program ‘‘Thales-Democritus University of Thrace-Center for Multifunctional Nanocomposite Construction Materials’’ (MIS 379496) funded by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program ‘‘Education and Lifelong Learning’’. Applied Sciences Inc is kindly acknowledged for supplying the carbon nanofibers. References [1] Dharap P, Li Z, Nagarajaiah S, Barrera EV. Nanotube film based on single-wall carbon nanotubes for strain sensing. Nanotechnology 2004;15:379–82. [2] Tombler TW, Zhou C, Alexseyev L, Kong J, Dai H, Liu L, et al. Reversible electromechanical characteristics of carbon nanotubes under local-probe manipulation. Nature 2000;405:769–72. [3] Banthia N, Djeridane S, Pigeon M. Electrical resistivity of carbon and steel micro-fiber reinforced cements. Cem Concr Res 1992;22(5):804–14. [4] Chacko R, Banthia N, Mufti AA. Carbon-fiber-reinforced cement-based sensors. Can J Civil Eng 2007;34(3):284–90. [5] Wen S, Chung DDL. Self-sensing characteristics of carbon fiber cement. In: Proceedings of ConMat’05 and Mindess symposium, The University of British Columbia; 2005. [6] Wen S, Chung DDL. Piezoresistivity-based strain sensing in carbon fiberreinforced cement. ACI Mater J 2007;104(2):171–9. [7] Chen PW, Chung DDL. Carbon fiber reinforced concrete as an electrical contact material for smart structures. Covina, CA, USA, Anaheim, CA, USA: SAMPE; 1993. p. 2067–76. [8] Chen PW, Chung DDL. Carbon fiber reinforced concrete for smart structures capable of non-destructive flaw detection. Smart Mater Struct 1993;2(1):22–30. [9] Fu X, Chung DDL. Self-monitoring of fatigue damage in carbon fiber reinforced cement. Cem Concr Res 1996;26(1):15–20. [10] Li Z, Yan X. Application of cement-based Piezoelectric sensors for monitoring traffic flows. J Transp Eng 2006;132(7):565–73. [11] Li GY, Wang PM, Zhao X. Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites. Cem Concr Compos 2007;29:377–82.

169

[12] Yu X, Kwon E. A carbon nanotube/cement composite with piezoresistive properties. Smart Mater Struct 2009;18:055010. [13] Han B, Yu X, Kwon E. A self-sensing carbon nanotube/cement composite for traffic monitoring. Nanotechnology 2009;20:445501. [14] Han B, Yu X, Kwon E, Ou J. Effects of CNT concentration level and water/cement ratio on the piezoresistivity of CNT/cement composites. J Compos Mater 2012;46(1):19–25. [15] Coppola L, Buoso A, Corazza F. Electrical properties of carbon nanotubes cement composites for monitoring stress conditions in concrete structures (Conference Paper). Appl Mech Mater 2011;82:118–23. [16] Luo J, Duan Z, Zhao T, Li Q. Effect of compressive strain on electrical resistivity of carbon nanotube cement-based composites. Key Eng Mater 2011;483:579–83. [17] Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Multi-scale mechanical and fracture characteristics and early-age strain capacity of high performance carbon nanotube/cement nanocomposites. Cem Concr Compos 2010;32(2):110–5. [18] Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Highly dispersed carbon nanotubes reinforced cement based materials. Cem Concr Res 2010;40:1052–9. [19] Metaxa ZS, Konsta-Gdoutos MS, Shah SP. Carbon nanofiber cementitious composites: effect of debulking procedure on dispersion and reinforcing efficiency. Cem Concr Compos 2013;36:25–32. [20] Fu X, Lu W, Chung DDL. Improving the strain-sensing ability of carbon fiberreinforced cement by ozone treatment of the fibers. Cem Concr Res 1997;28(6):183–7. [21] Han B, Zhang K, Yu X, Kwon E, Ou J. Fabrication of piezoresistive CNT/CNF cementitious composites with superplasticizer as dispersant. J Mater Civ Eng 2012;24(6):658–65. [22] Cao J, Chung DDL. Electric polarization and depolarization in cement-based materials, studied by apparent electrical resistance measurement. Cem Concr Res 2004;34:481–5. [23] Azhari F, Banthia N. Cement-based sensors with carbon fibers and carbon nanotubes for piezoresistive sensing. Cem Concr Compos 2012;34:866–73. [24] Materazzi AL, Ubertini F, D’Alessandro A. Carbon nanotube cement-based transducers for dynamic sensing of strain. Cem Concr Compos 2013;37:2–11. [25] Han B, Guan X, Ou J. Electrode design, measuring method and data acquisition system of carbon fiber cement paste piezoresistive sensors. Sens Actuators A 2006;135:360–9. [26] Hou TC. Wireless and electromechanical approaches for strain sensing and crack detection in fiber reinforced cementitious materials. PhD thesis. The University of Michigan; 2008. [27] Gao D, Sturm M, Mo YL. Electrical resistance of carbon-nanofiber concrete. Smart Mater Struct 2009;18:095039. [28] Hansson ILH, Hansson CM. Electrical resistivity measurements of Portland cement based materials. Cem Concr Res 1983;13:675–83. [29] Saafi M. Wireless and embedded carbon nanotube networks for damage detection in concrete structures. Nanotechnology 2009;20:395502. [30] Konsta-Gdoutos MS, Shah SP, Dattatraya DJ. Relationships between engineering characteristics and material properties of high strength-high performance concrete. In: Dhir RV, Newlands MD, Csetenvi LJ, editors. Proceedings of international symposia celebrating concrete: people and practice in sustainable development. Thomas Telford Limited; 2003. p. 37–46.