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Electrical impedance of carbon nanofiber aggregates
Electrical impedance of carbon nanofiber aggregates
14
Y.L. Mo a , Avinash Gautam a , Yuhua Chen b , Jinghong Chen b , Bhagirath Joshi a a Civil and Environmental Engineering Department, University of Houston, Houston, TX, United States; bElectrical and Computer Engineering Department, University of Houston, Houston, TX, United States
Chapter outline 1. 2. 3. 4. 5.
Introduction 333 Fiber reinforced concrete 334 Carbon nanofiber (CNF) reinforcement 334 Development of carbon nanofiber aggregate (CNFA) Optimization for CNFA 338 5.1 5.2
336
AC circuit for CNFA and AC frequency optimization 340 CNFs content optimization 343
6. Conclusion 348 Acknowledgments 348 References 348
1. Introduction A structure should be safe, durable, serviceable and sustainable during its lifetime service. During the service period, it may be subjected to various loading conditions and the worst cases of earthquakes and strong winds. These loads either reduce the performance of the structure or severely damage or completely collapse it. Concrete is one of the most used construction material for structures. In regards to enhancing the particular concrete behavior, various methods have been adopted and advanced from the very beginning of its use. In one hand when increasing the strength of concrete has been one of the key fields of investigation, enhancement of the sensing ability of concrete is another vast field of concrete research. The sensing ability of concrete that senses its environment and respond to changes in strain, temperature, moisture, pH, and electric or magnetic fields is an area of interest for a big fraction of scientists and researchers in the concrete study. These sensing abilities of makes the concrete smart enough to be used to monitor its health regarding stress-strain, temperature, pH, moisture, electrical/magnetic responses, and the damage in concrete and surrounding. This property is utilized in Smart Nanoconcretes and Cement-Based Materials. https://doi.org/10.1016/B978-0-12-817854-6.00014-3 Copyright © 2020 Elsevier Inc. All rights reserved.
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Structural Health Monitoring (SHM). The technology of structural health monitoring helps in providing the capability of non-destructive flaw detection allowing concrete to be repaired before it is too late (Chen & Chung, 1993a, 1993b). This evaluation of safety and the durability of a structure is essential during its lifetime. SHM system is composed of smart sensing tool, data acquisition, and transmission system and database for effective data management and health diagnosis (including data processing, data mining, damage detection, model updating, safety evaluation, and reliability analysis) (Ou & Li, 2010).
2.
Fiber reinforced concrete
The use of fibers in concrete as an admixture is a rapidly developing field for the study of sensitive concrete. Hydrated cement as a major constituent of concrete on itself is a brittle material that is stronger in compression than tension and to compensate for this weakness, a reinforcement consisting typically of rebars or fibers are added to concrete. It was the use of straws in mud mortar that initiated the reinforcement of brittle construction materials.In the 1960s, the fiber research and industrialization started with the use of steel fibers to reinforced concrete structures.This field of studyprogressed in the 1970s, 80s, and 90s, with the addition of polymeric fibers, glass fibers, and carbon fibers respectively (Li, 2002). Fibers recover brittle constituentsof structural industry such as concrete by enhancing tensile strength, ductility, and toughness (Shah & Naaman, 1976). In concrete, they do this by arresting cracks which initiate with the onset of cracks in nano-level of concrete. These nano-cracks grow together to form localized microcracks, which in turn grow together to form macro-cracks. These macro-cracks widen to form cracks visible with the naked eye. Fibers check these cracks by forming bridges across them. The increase in tensile stresses stretches these cracks.The same bridging action and fiber pull out a phenomenon that improves the tensile strength, ductility, and toughness in concrete can be used to turn the concrete itself into a strain sensor. Short-fiber composites, such as carbon nanofiber (CNF) concretes, were found to be a new class of strain sensor. These worked on the basis that the short electrically conducting fiber pull-out that accompanies slight and reversible crack opening. Fig. 14.1 shows the bridging action of fibers across micro and macro-cracks in concrete. Fig. 14.2 presents the crack bridging in cement/multi-walled carbon nanotubes (MWCNT) composite.
3.
Carbon nanofiber (CNF) reinforcement
The CNF reinforcement to concrete has many advantageous mechanical and electrical properties. In addition to increased strength and ductility, which is found in all fibere reinforced concretes, concrete composites containing micro and nanofibers, made of various materials including carbon, have been found to have unique electrical
Electrical impedance of carbon nanofiber aggregates
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Fig. 14.1 Bridging action of fibers across micro (A) and macro-cracks (B). Reproduced from Memon, I. A., Jhatial, A. A., Sohu, S., Lakhiar, M. T., Zahid Hussain Khaskheli, Z. H., 2018. Influence of fibre length on the behaviour of polypropylene fibre reinforced cement concrete. Civil Engineering Journal 4 (9), 2124e2131. https://doi.org/10. 28991/cej-03091144. Licensed under a Creative Commons Attribution 4.0 International License. (CC BY 4.0).
Fig. 14.2 SEM image of (A) MWCNTs bridging hydrates, (B) evidence showing MWCNTs bridging a nano-crack. Reproduced with permission from Naqi, A., Abbas, N., Zahra, N., Hussain, A., Shabbir, S.Q., 2018. Effect of multi-walled carbon nanotubes (MWCNTs) on the strength development of cementitious materials. Journal of Materials Research and Technology 8 (1), 1203e1211. https://doi.org/10.1016/j.jmrt.2018.09.006. Copyright 2018, Elsevier.
properties (Chen & Chung, 1993a, 1993b; Li et al., 2004, 2006, 2007a, 2007b). Because of the tunnel conductivity effect, CNF concrete (CNFC) exhibits properties necessary for strain monitoring and electromagnetic interference (EMI) shielding (Gao et al., 2009). In addition to strain sensing, carbon fiber composites have been used to monitor temperature (Chung, 2000) and create self-healing composites (Chang et al., 2009; Chung, 2004). Chen et al. (2004) studied the effects of hydration and relative humidity on carbon fiber reinforced cement-based composites with inconclusive results. Han et al. (2010) examined the change in Electric Resistance Variation (ERV) of cement-based materials containing carbon fibers and carbon black during
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the hydration process. Smart materials sense their environment and respond to changes in strain, temperature, moisture, pH, and electric or magnetic fields. CNT/CNF composites qualify as smart materials since they can be used to measure strain and temperature (Li et al., 2004, 2007a, 2007b; Gao et al., 2009; Chung, 1995, 2000; Howser et al., 2011; Yang & Chung, 1992) as shown in Fig. 14.3. Fiber research in concrete construction is a rapidly developing field, and the electrical properties of CNF concrete aggregate (CNFA) is studiedin this chapter.
4.
Development of carbon nanofiber aggregate (CNFA)
The University of Houston (UH) has successfully demonstrated strain sensing abilities of that self-consolidating carbon nanofiber concrete (SCCNFC) (Gao et al., 2009; Howser et al., 2011). This led to the development of carbon nanofiber aggregate (CNFA). The development of a CNFA made it is possible to utilize the strain sensing abilities of SCCNFC with a reduced cost since only the CNFAs placed in the structure would contain carbon nanofibers (CNFs) instead of whole structure. SCCNFC’s expenses are almost 20 times as much as regular concrete. This chapter describes the optimization of both the CNFA configuration and the mortar design with the electrical response of CNFAs in different stress levels. Howser et al. (2011) after testing more than 100 specimens demonstrated that the optimum CNF content for CNFAs was 0.7% of the weight of cement. They used four probe resistance measurement of CNFA in a DC circuit as shown in Fig. 14.4. The optimized CNFA as a 2.54 cm 2.54 cm 2.54 cm (1 in. 1 in. 1 in.) cubes with four wire meshes. Meshes were made of 6.35 mm. 6.35 mm (0.25 in. 0.25 in.) 23 gauge welded galvanized steel hardware cloth cut into 19.05mm 19.05 mm (0.75in. 0.75in) squares with the leg extending on each. The gauge 24 wire was soldered to the meshes. The formwork had the drilled holes in which two legs from each mesh were inserted to hold the meshes vertically and at a fixed distance during and after casting as shown in Fig. 14.5. We at the University of Houston developed the CNFA with (Howser et al., 2011) dissertation as a background for our research. The CNFA is primarily the CNF mortar mix. The CNFA ingredients consist of Type III Portland cement along washed, dried and screened fine aggregate mix with admixtures. The first and primary admixture was CNFs, which allows the mortar to be self-sensing. The second admixture used was super plasticizers aka High Range Water Reducer (HRWR). Gao et al. (2009) showed that HRWRs are capable of creating self-consolidating concrete (SCC) also aiding for dispersion of fibers. The third admixture used as a constituent was silica fume. Chen et al. (Chen & Chung, 1993b; Chen et al., 1997) have shown that silica fume aids to the dispersion of fibers in cement-based materials. The final admixture is Kim Krystol which is a hydrophilic crystalline admixture used to create permanently waterproof concrete. This was an additional ingredient than (Howser et al., 2011) approach for CNFA development.
Electrical impedance of carbon nanofiber aggregates
(A)
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0
0
–0.2 –50
–0.6 –0.8
–100
–1 –150
–1.2 –1.4
Strain (10–6)
Stress (MPa)
–0.4
–200
–1.6 –1.8 –2 0
Electrical resistivity (104Ω.cm)
(B)
50
100 150 Time (s)
–250 250
200
6 5 4 3
Cooling
2 Heating
1 0 0
5
10
15
20 25 30 35 Temperature (°C)
40
45
50
Fig. 14.3 (A) Variation of the stress and strain with time during dynamic compressive loading for carbon-fiber latex cement paste, (B) Electrical Behavior during the Heating and Cooling of a Carbon Microfiber Silica Fume Cement Paste. Reproduced with permission from Chung, D.D.L., 2000. Cement reinforced with short carbon fibers: a multifunctional material. Composites Part B: Engineering 31, 511e526. Copyright 2000, Elsevier.
The mixing procedure of CNFA is a hybrid of dry and wet mixing of concrete. The CNF, HRWR, and water are mixed to get a homogeneous slurry. This slurry is kept in a separate container. The cement, sand, silica and Kim Krystol is then added to obtain a homogeneous mixture. Then, one-half of the slurry is poured into the dry mix and mixed for 30 s in a mixer. The remaining half of the slurry is poured into the mix in two equal proportions with mixing. Excessive mixing is avoided to make sure the CNFs do not start clumping. Then the homogenous mixture of CNFA is poured into the formwork with wire meshes. The developed CNFA with four wire meshes and its schematic is as shown in Fig. 14.6.
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Fig. 14.4 Four probe method for resistance measurement.
Fig. 14.5 (A) Four Meshes with Soldered Wire; (B) Mesh Spacing (units: in.) (C) Meshes inserted the formwork (D) Meshes Inserted into Complete Formwork.
5.
Optimization for CNFA
The electrical properties of CNFAs depend on many factors: type and quantity of CNFs, quality of CNF dispersion, the water content in the mix, and circuit type/setup to measure response. Howser et al. (2011) through the series of over 100 CNFAs tests under compression demonstrated that the CNF concentration to the weight of cement of 0.70% exhibited the most significant change in ERV as shown in Fig. 14.7, which matched the results found by (Gao et al., 2009). Keithley SourceMeter SMU Instruments were used for all the DC measurements.
Electrical impedance of carbon nanofiber aggregates
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Fig. 14.6 Probe CNFA and its Schematic.
Fig. 14.7 Relationship between CNF percentage and ERV (Howser et al., 2011).
The four probes CNFA’s DC resistance was observed with no loading. It was expected that the change in resistance would not have significant change with no stimuli, unexpectedly, that was not the case. The result showed that the electrical DC resistance of CNFAs changed on its own with time without any stimuli as shown in Fig. 14.8. Probably, the previous researches who used DC circuit to monitor ERV in mortar 10.5
Resistance (K-Ohm)
10.4 10.3
At 0 Hour
10.2
After 2 Hours
10.1
After 20 Hours
10.0 9.9 9.8 9.7 9.6 9.5 0
5
10
15
20
25
Time (Minutes)
Fig. 14.8 Variation of DC Resistance of 4-Probe CNFA with time in No-Loading Stage.
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Smart Nanoconcretes and Cement-Based Materials 6
Resistance (M-Ohm)
5 4 3 2 1 0 0
5
10
15
20
25
30
Time (Minutes)
Fig. 14.9 Variation of DC Resistance of 2-Probe CNFA with time in No-Loading Stage.
did not study the steady-state response of CNFAs. The change in resistance was observed to be faster initially which slowed down with increasing time. This change in DC resistance was observed to be around 6% without any stress with time independently to the dosage of CNFs, and this change was not stable even after half an hour of no-load observation. The research team at UH, considered for alternatives as the study of ERV in DC circuit would not lead to correct stress-strain monitoring of CNFA embedded structures without steady-state response.It should be noted that without the stable steady-state response,i.e., a unique ERV for unique strain for a CNFA, the study of ERV with varying stress is not correctly estimated. With the concept of changing DC study to AC study, the four mesh configuration was also changed to two mesh configuration. The four probe method is basically to eliminate the contact resistance and lead resistance in a circuit to measure very low resistances. CNFA usually has a very high magnitude of resistance in the range of several Kilo to Mega Ohms. Thus the contact and lead resistance has very less effect on the total magnitude of CNFA resistance. Thus two probes of resistance measurement can be implemented in our CNFAs. Fig. 14.9 shows the DC resistance of CNFA with two probe measurement technique. The resistance is increasing with time in 2-probe CNFA under no loading. The change in resistance was observed to be around 150% without any stress within 30 min of observation. This led to the study CNFAs in AC circuit to find a stable steady state for each unique load/strain.
5.1
AC circuit for CNFA and AC frequency optimization
The AC circuit was introduced for CNFA to look for the steady-state response with no loading and at different unique stress/strain level. Two probe CNFAs were selected for impedance measurement with varying frequencies on unloaded conditions. Keysight E4980AL Precision LCR Meter with unit AC voltage as input was used for the measurements.
Electrical impedance of carbon nanofiber aggregates
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Fig. 14.10 shows the impedance (Z) of 2-Probe CNFAs with different frequencies in no loading stage. CNFAs had CNF content of 0.7% to the weight of cement. The magnitude of the impedance is still changing, but the magnitude of variation is a lot lower than that of DC. The change in impedance with 30 min was 1% between maximum and minimum impedances which are way less than the DC resistance change.The real and imaginary components of impedance, i.e. resistance and reactance as seen from observed data also did not vary more than 1% for 30 min for a particular frequency. The reason for minimum variation of impedance with time can be further justified by studying the characteristics of CNFA for resistance (R) and reactance (X) independently. Figs. 14.11 and 14.12 show the resistance (R) and reactance (X) of CNFAs with different frequencies respectively. The magnitude of resistance and reactance plotted in Figs. 14.11, and 14.12 is average of 30-min observation
500Hz
33.0 32.9 32.8 0
5
10
15
20
25
30
Z (K-Ohm)
Z (K-Ohm)
20Hz 33.1
26.8 26.7 26.6 26.5 0
5
Time (Minutes)
25.7 25.6 25.5 5
10
15
20
25
30
20
25
30
Z (K-Ohm)
Z (K-Ohm)
18.7
0
5
14.5 20
Time (Minutes)
25
30
Z (K-Ohm)
Z (K-Ohm)
30
25
30
25
30
16.7 0
5
10
15
20
300KHz
14.6
15
20
16.8
200KHz
10
15
Time (Minutes)
14.7
5
10
16.9
Time (Minutes)
0
25
22.1
100KHz
18.8
15
30
22.2
50KHz
10
25
Time (Minutes)
18.9
5
20
22.3
Time (Minutes)
0
15
10KHz Z (K-Ohm)
Z (K-Ohm)
1KHz 25.8
0
10
Time (Minutes)
13.3 13.2 13.1 0
5
10
15
20
Time (Minutes)
Fig. 14.10 Variation of AC Impedance (Z) of 2-Probe CNFA with time in No-Loading Stage in different frequencies.
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Resistance (K-Ohm)
35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 0
50
100
150
200
250
300
Frequency (KHz)
Fig. 14.11 Electrical Resistance (R) of 2-Probe CNFA in AC Circuit as a function of Frequency. 9.0
Reactance (K-Ohm)
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0
50
100
150
200
250
300
Frequency (KHz)
Fig. 14.12 Electrical Reactance (X) of 2-Probe CNFA in AC Circuit as a function of Frequency.
from a set of data of Fig. 14.10. The reactance value was always observed to be negative, dictating that CNFAs possessed the internal capacitance. It is seen from Fig. 14.11 that the electrical resistance (R)of CNFAdecreases with an increase in AC frequency and Fig. 14.12 shows that the magnitude of the reactance of CNFA increases with an increase in AC frequency. Here, the decrease in resistance is less than the increase in reactance of CNFA. As impedance is the square root of summation of squares (SRSS) of R and X, the decrease of resistance and increase of reactance with time limits the significant change in impedance.These comparatively stable results allow the use of AC circuit for 2-probe CNFAs study. Fig. 14.13 shows the variation of impedance 2-probe CNFA with frequency. Due to instrumental limitation, AC frequencies higher than 300 KHz were not able to be observed, but above a frequency of 100 KHz, the impedance response is already getting stable even with the increase in frequency. At lower frequencies, the change in impedance is significant for even small change in AC frequency, but at frequencies higher than 100 KHz, the magnitude of impedance starts getting independent of applied frequency. Thus, for further studies, this chapter has considered the frequency higher than 100 KHz.
Electrical impedance of carbon nanofiber aggregates
343
Impedance (K-Ohm)
35 30 25 20 15 10 5 0 0
50
100
150
200
250
300
Frequency (KHz)
Fig. 14.13 Electrical Impedance (Z) of 2-Probe CNFA in AC Circuit as a function of Frequency.
The concept of steady-state response was also checked for the various load/stress stages.This is an important step as this observation assures that the CNFAs can be used to monitor long term loads. The specimens were subjected to different loading stages and were held at each load step for some time interval where the observation for impedance was made as in Fig. 14.14. The CNFAs have stable impedance within a few minutes of connection to the circuit. The linear trend of the impedance explains the reliability of CNFAs of being used to monitor static loads/stresses. The distinct lines for impedance at each load stage ensures the unique response to a unique loading stage. The overlapping lines will not be accurate to determine the stress level. Fig. 14.14 shows that the frequency of 300 KHz has overlapping between stress levels. Thus this frequency for monitoring is not recommended. The impedance response between 100 KHz and 200 KHz frequency is more reliable, and thus 100 KHz and 200 KHz of AC frequency can be selected as the optimum frequency to evaluate the response of 2-probe CNFAs with CNF content of 0.7% of the weight of cement in AC circuit.
5.2
CNFs content optimization
The impedance of CNFA is much higher than that of the wires; two probe impedance measurement is quite accurate to determine the electrical impedance variation of the CNFAs under loads/stresses. The calculation of variation is quite simple, and the electrical impedance variation (EZV) can be determined as, EZV ¼
Zi Z0 Z0
where. EZV: Electrical Impedance Variation Z0 : Initial Impedance Zi : Impedance at Step i
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(A)
100KHz
Impedance (K-Ohm)
116 115
No Load
114
3.86 MPa
113
4.83 MPa 112
5.79 MPa
111
7.72 MPa
110
9.65 MPa 11.03 MPa
109 0
5
10
15
20
Time (In Minutes)
(B)
200KHz
62.5
Impedance (K-Ohm)
62
No Load
61.5
3.86 MPa 61
4.83MPa
60.5
5.79MPa
60
7.72 MPa
59.5
9.65 MPa 11.03 MPa
59 0
5
10
15
20
Time (In Minutes)
Impedance (K-Ohm)
(C)
300KHz
44.2 44 43.8 43.6 43.4 43.2 43 42.8 42.6 42.4 42.2 42
No Load 3.86 MPa 4.83 MPa 5.79 MPa 7.72 MPa 9.65 MPa 11.03 MPa 0
5
10
15
20
Time (In Minutes)
Fig. 14.14 Static Stress Impedance Analysis for Different Frequencies: (A) 100 KHz; (B) 200 KHz; (C) 300 KHz.
Several researchers have been studying to find the optimized content of CNF in concrete and mortar. Chen and Chung (Chen & Chung, 1993a) studied the carbon microfiber’s electrical and mechanical properties. They observed that in 0.5% by weight of cement, the electrical resistivity decreased by 83%. This study later (Chen and Chung, 1996) elaborated with the study of microfibers with varying concentration.
Electrical impedance of carbon nanofiber aggregates
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Fig. 14.15 CNFA with Varying CNF content (0.1%, 0.2%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% & 1.0% of CNF by the weight of cement from left to right).
They studied properties of mortar and concrete with 0e4% and 0.5e3% by weight of cement respectively which showed that these could act as reversible damage sensor by measuring ERV. They observed that increasing fiber content increased the ERV in concrete but the while increasing fiber content did not appreciably increase ERV in the mortar. Gao et al. (2009) showed that 0.7% of cement of CNF in concrete was optimal and higher than that caused fiber clumping in concrete. Several 2-probe CNFAs with varying CNFs contents were tested under compression, and electrical impedance variation was determined to find the optimal CNFs percentage in the AC circuit. Each CNFAs were cast as explained above. After casting, each CNFAs were cured for 14 days and then air dried for the next 24 h. To remove all excess water including pore water in CNFAs, they were oven dried at 100 C (212 F). Fig. 14.15 shows the series of oven dried CNFAs with varying CNF content (0.1%, 0.2%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% & 1.0% of CNF by the weight of cement). It is seen that higher the CNF concentration, darker is the CNFA. Fig. 14.16 shows the test setup for the compression test. Each CNFA was loaded in Instron 5960 Series Universal Testing Systems up to 50 kN (11,250 lbf) force capacity. The CNFA’s top and bottom surface were smoothened with grit to ensure even contact surface. Two strain gauge were pasted to the surface of CNFAs to measure average
Fig. 14.16 Test setup for CNFA’s CNF optimization.
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Fig. 14.17 Peeling of strain gauge from CNFA surface under compression.
strain from the Model P3 Strain Indicator and Recorder during the test. The CNFAs were tested in compression at a constant displacement rate of 0.0508 mm/min (0.002 in/mm) until the load dropped to 40% of peak load. The electrical impedance was measured using a two-probe technique with varying frequencies from Keysight E4980AL Precision LCR Meter. Fig. 17.17 shows the peeling of strain gauge from the surface of CNFA under loading. The compression test showed the promising trend between the concentration of CNF and maximum EZV recorded. The maximum ERV at each percentage of CNF to the weight of cement is the average response of CNFAs at peak load. The standard deviation was then calculated. Figs. 14.18 and 14.19 show the averaged ERV with one standard deviation on each side of average.
0.125
MAXIMUM EZV
0.115
Average EZV
0.105 Average EZV+STD 0.095 Average EZV-STD
0.085 0.075 0.065 0.055 0.045 0.035 0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
% CNF BY WEIGHT OF CEMENT
Fig. 14.18 CNF optimization of 2-probe CNFA for 100 KHz AC frequency.
Electrical impedance of carbon nanofiber aggregates
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0.11
MAXIMUM EZV
0.1
Average EZV
0.09 Average EZV+ STD 0.08 Average EZV-STD 0.07 0.06 0.05 0.04 0.03 0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
% CNF BY WEIGHT OF CEMENT
Fig. 14.19 CNF optimization of 2-probe CNFA for 200 KHz AC frequency. Table 14.1 Optimized 2-probe CNFA mix design. Materials
Percentage of total mortar weight
Fine Aggregate
52.3%
Cement
28.4%
Water
12.1%
Silica Fume
4.2%
HRWR
2.0%
CNFs
0.3%
Kim Krystol (waterproofing)
0.6%
Figs. 14.18 and 14.19 demonstrates that, for 100 KHz and 200 KHz of AC frequency respectively, the CNFA concentration with respect to the weight of cement of 0.8% exhibited the most distinct change in EZV, which is slightly higher than the results found by Howser et al. (2011) for four-probe CNFAs in DC Circuit.This may be due to factors like changing 4-probe measurement technique to 2-probe, use of AC circuit and the addition of new admixture Kim Krystol with the target of waterproofing. The response of CNFAs with 0.1% and 0.2% of CNF to the weight of cement is excluded from these figures as they had a minimum response than those represented here. The variance in the above figure might be due to factors like temperature and remaining water in CNFA micropores even after drying. By the above tests, the optimum CNF content for 2-probe CNFAs in AC was found to be 0.8% to the weight of cement,i.e., 0.3% of total mortar weight. Table 14.1. Below shows the final optimized 2-probe CNFA mix design.
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Conclusion
The study of carbon nanofiber aggregate (CNFA) in DC circuit tests showed the unstable resistance variation resulting in unreliable data acquisition for stress/strain monitoring. This led to the development of two probe CNFA in AC circuit which was developed which is capable of sensing load/stresses with stable impedance measurements. The developed CNFA was 2.54 cm 2.54 cm x2.54 cm (1.00 in. 1.00 in. 1.00 in) cube with two steel wire meshes.The CNFAs were tested in different load stages to check the variation of impedance with time. The 30-min observation showed that the change in impedance was within 1% ensuring the reliability of stress-impedance data obtained from tests. The optimized AC frequency for response measurement was found to be between 100 KHz and 200 KHz while the optimum carbon nanofiber (CNF) content was found to be 0.8% of the weight of cement and based on this study, the optimized CNFA mix design was developed.
Acknowledgments The research described in this book chapter is financially supported by the Hurricane Resilience ResearchInstitute program. The opinions expressed in this study are those of the authors and do not necessarily reflect the views of the sponsor.
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Gao, D., Sturm, M., Mo, Y.L., 2009. Electrical resistance of carbon-nanofiber concrete. Smart Materials and Structures 18 (9). Han, B., Zhang, L., Ou, J., 2010. Influence of water content on conductivity and piezo resistivity of cement-based material with both carbon fiber and carbon black. Journal of Wuhan University of Technology-Materials Science Edition 25 (1), 147e151. Howser, R.N., Dhonde, H.B., Mo, Y.L., 2011. Self-sensing of carbon nanofiber concrete columns subjected to reversed cyclic loading. Smart Materials and Structures 20 (8), 085031. Li, H., Xiao, H., Yuan, J., Ou, J., 2004. Microstructure of cement mortar with nano-particles. Composites Part B: Engineering 35 (2), 185e189. Li, H., Zhang, M., Ou, J., 2006. Abrasion resistance of concrete containing nano particles for pavement. Wear 260 (11e12), 1262e1266. Li, H., Zhang, M., Ou, J., 2007a. Flexural fatigue performance of concrete containing nanoparticles for pavement. International Journal of Fatigue 29 (7), 1292e1301. Li, G.Y., Wang, P.M., Zhao, X., 2007b. Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites. Cement and Concrete Composites 29 (5), 377e382. Li, V.C., 2002. Large volume, high-performance applications of fibers in civil engineering. Journal of Applied Polymer Science 83, 660e686. Memon, I.A., Jhatial, A.A., Sohu, S., Lakhiar, M.T., Zahid Hussain Khaskheli, Z.H., 2018. Influence of fibre length on the behaviour of polypropylene fibre reinforced cement concrete. Civil Engineering Journal 4 (9), 2124e2131. https://doi.org/10.28991/cej-03091144. Naqi, A., Abbas, N., Zahra, N., Hussain, A., Shabbir, S.Q., JanuaryeMarch 2019. Effect of multi-walled carbon nanotubes (MWCNTs) on the strength development of cementitious materials. Journal of Materials Research and Technology 8 (1), 1203e1211. https://doi.org/ 10.1016/j.jmrt.2018.09.006. Ou, J., Li, H., 2010. Structural health monitoring in mainland China: review and future trends. Structural Health Monitoring 9 (3), 219e231. Shah, S.P., Naaman, A.E., 1976. Mechanical properties of glass and steel fiber reinforced mortar. ACI Journal Proceedings 73 (1), 50e53. Yang, X., Chung, D.D.L., 1992. Latex-modified cement mortar reinforced by short carbon fibers. Composites 23 (6), 453e460.
14
Y.L. Mo a , Avinash Gautam a , Yuhua Chen b , Jinghong Chen b , Bhagirath Joshi a a Civil and Environmental Engineering Department, University of Houston, Houston, TX, United States; bElectrical and Computer Engineering Department, University of Houston, Houston, TX, United States
Chapter outline 1. 2. 3. 4. 5.
Introduction 333 Fiber reinforced concrete 334 Carbon nanofiber (CNF) reinforcement 334 Development of carbon nanofiber aggregate (CNFA) Optimization for CNFA 338 5.1 5.2
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AC circuit for CNFA and AC frequency optimization 340 CNFs content optimization 343
6. Conclusion 348 Acknowledgments 348 References 348
1. Introduction A structure should be safe, durable, serviceable and sustainable during its lifetime service. During the service period, it may be subjected to various loading conditions and the worst cases of earthquakes and strong winds. These loads either reduce the performance of the structure or severely damage or completely collapse it. Concrete is one of the most used construction material for structures. In regards to enhancing the particular concrete behavior, various methods have been adopted and advanced from the very beginning of its use. In one hand when increasing the strength of concrete has been one of the key fields of investigation, enhancement of the sensing ability of concrete is another vast field of concrete research. The sensing ability of concrete that senses its environment and respond to changes in strain, temperature, moisture, pH, and electric or magnetic fields is an area of interest for a big fraction of scientists and researchers in the concrete study. These sensing abilities of makes the concrete smart enough to be used to monitor its health regarding stress-strain, temperature, pH, moisture, electrical/magnetic responses, and the damage in concrete and surrounding. This property is utilized in Smart Nanoconcretes and Cement-Based Materials. https://doi.org/10.1016/B978-0-12-817854-6.00014-3 Copyright © 2020 Elsevier Inc. All rights reserved.
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Structural Health Monitoring (SHM). The technology of structural health monitoring helps in providing the capability of non-destructive flaw detection allowing concrete to be repaired before it is too late (Chen & Chung, 1993a, 1993b). This evaluation of safety and the durability of a structure is essential during its lifetime. SHM system is composed of smart sensing tool, data acquisition, and transmission system and database for effective data management and health diagnosis (including data processing, data mining, damage detection, model updating, safety evaluation, and reliability analysis) (Ou & Li, 2010).
2.
Fiber reinforced concrete
The use of fibers in concrete as an admixture is a rapidly developing field for the study of sensitive concrete. Hydrated cement as a major constituent of concrete on itself is a brittle material that is stronger in compression than tension and to compensate for this weakness, a reinforcement consisting typically of rebars or fibers are added to concrete. It was the use of straws in mud mortar that initiated the reinforcement of brittle construction materials.In the 1960s, the fiber research and industrialization started with the use of steel fibers to reinforced concrete structures.This field of studyprogressed in the 1970s, 80s, and 90s, with the addition of polymeric fibers, glass fibers, and carbon fibers respectively (Li, 2002). Fibers recover brittle constituentsof structural industry such as concrete by enhancing tensile strength, ductility, and toughness (Shah & Naaman, 1976). In concrete, they do this by arresting cracks which initiate with the onset of cracks in nano-level of concrete. These nano-cracks grow together to form localized microcracks, which in turn grow together to form macro-cracks. These macro-cracks widen to form cracks visible with the naked eye. Fibers check these cracks by forming bridges across them. The increase in tensile stresses stretches these cracks.The same bridging action and fiber pull out a phenomenon that improves the tensile strength, ductility, and toughness in concrete can be used to turn the concrete itself into a strain sensor. Short-fiber composites, such as carbon nanofiber (CNF) concretes, were found to be a new class of strain sensor. These worked on the basis that the short electrically conducting fiber pull-out that accompanies slight and reversible crack opening. Fig. 14.1 shows the bridging action of fibers across micro and macro-cracks in concrete. Fig. 14.2 presents the crack bridging in cement/multi-walled carbon nanotubes (MWCNT) composite.
3.
Carbon nanofiber (CNF) reinforcement
The CNF reinforcement to concrete has many advantageous mechanical and electrical properties. In addition to increased strength and ductility, which is found in all fibere reinforced concretes, concrete composites containing micro and nanofibers, made of various materials including carbon, have been found to have unique electrical
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Fig. 14.1 Bridging action of fibers across micro (A) and macro-cracks (B). Reproduced from Memon, I. A., Jhatial, A. A., Sohu, S., Lakhiar, M. T., Zahid Hussain Khaskheli, Z. H., 2018. Influence of fibre length on the behaviour of polypropylene fibre reinforced cement concrete. Civil Engineering Journal 4 (9), 2124e2131. https://doi.org/10. 28991/cej-03091144. Licensed under a Creative Commons Attribution 4.0 International License. (CC BY 4.0).
Fig. 14.2 SEM image of (A) MWCNTs bridging hydrates, (B) evidence showing MWCNTs bridging a nano-crack. Reproduced with permission from Naqi, A., Abbas, N., Zahra, N., Hussain, A., Shabbir, S.Q., 2018. Effect of multi-walled carbon nanotubes (MWCNTs) on the strength development of cementitious materials. Journal of Materials Research and Technology 8 (1), 1203e1211. https://doi.org/10.1016/j.jmrt.2018.09.006. Copyright 2018, Elsevier.
properties (Chen & Chung, 1993a, 1993b; Li et al., 2004, 2006, 2007a, 2007b). Because of the tunnel conductivity effect, CNF concrete (CNFC) exhibits properties necessary for strain monitoring and electromagnetic interference (EMI) shielding (Gao et al., 2009). In addition to strain sensing, carbon fiber composites have been used to monitor temperature (Chung, 2000) and create self-healing composites (Chang et al., 2009; Chung, 2004). Chen et al. (2004) studied the effects of hydration and relative humidity on carbon fiber reinforced cement-based composites with inconclusive results. Han et al. (2010) examined the change in Electric Resistance Variation (ERV) of cement-based materials containing carbon fibers and carbon black during
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the hydration process. Smart materials sense their environment and respond to changes in strain, temperature, moisture, pH, and electric or magnetic fields. CNT/CNF composites qualify as smart materials since they can be used to measure strain and temperature (Li et al., 2004, 2007a, 2007b; Gao et al., 2009; Chung, 1995, 2000; Howser et al., 2011; Yang & Chung, 1992) as shown in Fig. 14.3. Fiber research in concrete construction is a rapidly developing field, and the electrical properties of CNF concrete aggregate (CNFA) is studiedin this chapter.
4.
Development of carbon nanofiber aggregate (CNFA)
The University of Houston (UH) has successfully demonstrated strain sensing abilities of that self-consolidating carbon nanofiber concrete (SCCNFC) (Gao et al., 2009; Howser et al., 2011). This led to the development of carbon nanofiber aggregate (CNFA). The development of a CNFA made it is possible to utilize the strain sensing abilities of SCCNFC with a reduced cost since only the CNFAs placed in the structure would contain carbon nanofibers (CNFs) instead of whole structure. SCCNFC’s expenses are almost 20 times as much as regular concrete. This chapter describes the optimization of both the CNFA configuration and the mortar design with the electrical response of CNFAs in different stress levels. Howser et al. (2011) after testing more than 100 specimens demonstrated that the optimum CNF content for CNFAs was 0.7% of the weight of cement. They used four probe resistance measurement of CNFA in a DC circuit as shown in Fig. 14.4. The optimized CNFA as a 2.54 cm 2.54 cm 2.54 cm (1 in. 1 in. 1 in.) cubes with four wire meshes. Meshes were made of 6.35 mm. 6.35 mm (0.25 in. 0.25 in.) 23 gauge welded galvanized steel hardware cloth cut into 19.05mm 19.05 mm (0.75in. 0.75in) squares with the leg extending on each. The gauge 24 wire was soldered to the meshes. The formwork had the drilled holes in which two legs from each mesh were inserted to hold the meshes vertically and at a fixed distance during and after casting as shown in Fig. 14.5. We at the University of Houston developed the CNFA with (Howser et al., 2011) dissertation as a background for our research. The CNFA is primarily the CNF mortar mix. The CNFA ingredients consist of Type III Portland cement along washed, dried and screened fine aggregate mix with admixtures. The first and primary admixture was CNFs, which allows the mortar to be self-sensing. The second admixture used was super plasticizers aka High Range Water Reducer (HRWR). Gao et al. (2009) showed that HRWRs are capable of creating self-consolidating concrete (SCC) also aiding for dispersion of fibers. The third admixture used as a constituent was silica fume. Chen et al. (Chen & Chung, 1993b; Chen et al., 1997) have shown that silica fume aids to the dispersion of fibers in cement-based materials. The final admixture is Kim Krystol which is a hydrophilic crystalline admixture used to create permanently waterproof concrete. This was an additional ingredient than (Howser et al., 2011) approach for CNFA development.
Electrical impedance of carbon nanofiber aggregates
(A)
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0
0
–0.2 –50
–0.6 –0.8
–100
–1 –150
–1.2 –1.4
Strain (10–6)
Stress (MPa)
–0.4
–200
–1.6 –1.8 –2 0
Electrical resistivity (104Ω.cm)
(B)
50
100 150 Time (s)
–250 250
200
6 5 4 3
Cooling
2 Heating
1 0 0
5
10
15
20 25 30 35 Temperature (°C)
40
45
50
Fig. 14.3 (A) Variation of the stress and strain with time during dynamic compressive loading for carbon-fiber latex cement paste, (B) Electrical Behavior during the Heating and Cooling of a Carbon Microfiber Silica Fume Cement Paste. Reproduced with permission from Chung, D.D.L., 2000. Cement reinforced with short carbon fibers: a multifunctional material. Composites Part B: Engineering 31, 511e526. Copyright 2000, Elsevier.
The mixing procedure of CNFA is a hybrid of dry and wet mixing of concrete. The CNF, HRWR, and water are mixed to get a homogeneous slurry. This slurry is kept in a separate container. The cement, sand, silica and Kim Krystol is then added to obtain a homogeneous mixture. Then, one-half of the slurry is poured into the dry mix and mixed for 30 s in a mixer. The remaining half of the slurry is poured into the mix in two equal proportions with mixing. Excessive mixing is avoided to make sure the CNFs do not start clumping. Then the homogenous mixture of CNFA is poured into the formwork with wire meshes. The developed CNFA with four wire meshes and its schematic is as shown in Fig. 14.6.
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Fig. 14.4 Four probe method for resistance measurement.
Fig. 14.5 (A) Four Meshes with Soldered Wire; (B) Mesh Spacing (units: in.) (C) Meshes inserted the formwork (D) Meshes Inserted into Complete Formwork.
5.
Optimization for CNFA
The electrical properties of CNFAs depend on many factors: type and quantity of CNFs, quality of CNF dispersion, the water content in the mix, and circuit type/setup to measure response. Howser et al. (2011) through the series of over 100 CNFAs tests under compression demonstrated that the CNF concentration to the weight of cement of 0.70% exhibited the most significant change in ERV as shown in Fig. 14.7, which matched the results found by (Gao et al., 2009). Keithley SourceMeter SMU Instruments were used for all the DC measurements.
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Fig. 14.6 Probe CNFA and its Schematic.
Fig. 14.7 Relationship between CNF percentage and ERV (Howser et al., 2011).
The four probes CNFA’s DC resistance was observed with no loading. It was expected that the change in resistance would not have significant change with no stimuli, unexpectedly, that was not the case. The result showed that the electrical DC resistance of CNFAs changed on its own with time without any stimuli as shown in Fig. 14.8. Probably, the previous researches who used DC circuit to monitor ERV in mortar 10.5
Resistance (K-Ohm)
10.4 10.3
At 0 Hour
10.2
After 2 Hours
10.1
After 20 Hours
10.0 9.9 9.8 9.7 9.6 9.5 0
5
10
15
20
25
Time (Minutes)
Fig. 14.8 Variation of DC Resistance of 4-Probe CNFA with time in No-Loading Stage.
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Resistance (M-Ohm)
5 4 3 2 1 0 0
5
10
15
20
25
30
Time (Minutes)
Fig. 14.9 Variation of DC Resistance of 2-Probe CNFA with time in No-Loading Stage.
did not study the steady-state response of CNFAs. The change in resistance was observed to be faster initially which slowed down with increasing time. This change in DC resistance was observed to be around 6% without any stress with time independently to the dosage of CNFs, and this change was not stable even after half an hour of no-load observation. The research team at UH, considered for alternatives as the study of ERV in DC circuit would not lead to correct stress-strain monitoring of CNFA embedded structures without steady-state response.It should be noted that without the stable steady-state response,i.e., a unique ERV for unique strain for a CNFA, the study of ERV with varying stress is not correctly estimated. With the concept of changing DC study to AC study, the four mesh configuration was also changed to two mesh configuration. The four probe method is basically to eliminate the contact resistance and lead resistance in a circuit to measure very low resistances. CNFA usually has a very high magnitude of resistance in the range of several Kilo to Mega Ohms. Thus the contact and lead resistance has very less effect on the total magnitude of CNFA resistance. Thus two probes of resistance measurement can be implemented in our CNFAs. Fig. 14.9 shows the DC resistance of CNFA with two probe measurement technique. The resistance is increasing with time in 2-probe CNFA under no loading. The change in resistance was observed to be around 150% without any stress within 30 min of observation. This led to the study CNFAs in AC circuit to find a stable steady state for each unique load/strain.
5.1
AC circuit for CNFA and AC frequency optimization
The AC circuit was introduced for CNFA to look for the steady-state response with no loading and at different unique stress/strain level. Two probe CNFAs were selected for impedance measurement with varying frequencies on unloaded conditions. Keysight E4980AL Precision LCR Meter with unit AC voltage as input was used for the measurements.
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Fig. 14.10 shows the impedance (Z) of 2-Probe CNFAs with different frequencies in no loading stage. CNFAs had CNF content of 0.7% to the weight of cement. The magnitude of the impedance is still changing, but the magnitude of variation is a lot lower than that of DC. The change in impedance with 30 min was 1% between maximum and minimum impedances which are way less than the DC resistance change.The real and imaginary components of impedance, i.e. resistance and reactance as seen from observed data also did not vary more than 1% for 30 min for a particular frequency. The reason for minimum variation of impedance with time can be further justified by studying the characteristics of CNFA for resistance (R) and reactance (X) independently. Figs. 14.11 and 14.12 show the resistance (R) and reactance (X) of CNFAs with different frequencies respectively. The magnitude of resistance and reactance plotted in Figs. 14.11, and 14.12 is average of 30-min observation
500Hz
33.0 32.9 32.8 0
5
10
15
20
25
30
Z (K-Ohm)
Z (K-Ohm)
20Hz 33.1
26.8 26.7 26.6 26.5 0
5
Time (Minutes)
25.7 25.6 25.5 5
10
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Z (K-Ohm)
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25
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200KHz
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14.7
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10
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30
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10
25
Time (Minutes)
18.9
5
20
22.3
Time (Minutes)
0
15
10KHz Z (K-Ohm)
Z (K-Ohm)
1KHz 25.8
0
10
Time (Minutes)
13.3 13.2 13.1 0
5
10
15
20
Time (Minutes)
Fig. 14.10 Variation of AC Impedance (Z) of 2-Probe CNFA with time in No-Loading Stage in different frequencies.
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Resistance (K-Ohm)
35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 0
50
100
150
200
250
300
Frequency (KHz)
Fig. 14.11 Electrical Resistance (R) of 2-Probe CNFA in AC Circuit as a function of Frequency. 9.0
Reactance (K-Ohm)
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0
50
100
150
200
250
300
Frequency (KHz)
Fig. 14.12 Electrical Reactance (X) of 2-Probe CNFA in AC Circuit as a function of Frequency.
from a set of data of Fig. 14.10. The reactance value was always observed to be negative, dictating that CNFAs possessed the internal capacitance. It is seen from Fig. 14.11 that the electrical resistance (R)of CNFAdecreases with an increase in AC frequency and Fig. 14.12 shows that the magnitude of the reactance of CNFA increases with an increase in AC frequency. Here, the decrease in resistance is less than the increase in reactance of CNFA. As impedance is the square root of summation of squares (SRSS) of R and X, the decrease of resistance and increase of reactance with time limits the significant change in impedance.These comparatively stable results allow the use of AC circuit for 2-probe CNFAs study. Fig. 14.13 shows the variation of impedance 2-probe CNFA with frequency. Due to instrumental limitation, AC frequencies higher than 300 KHz were not able to be observed, but above a frequency of 100 KHz, the impedance response is already getting stable even with the increase in frequency. At lower frequencies, the change in impedance is significant for even small change in AC frequency, but at frequencies higher than 100 KHz, the magnitude of impedance starts getting independent of applied frequency. Thus, for further studies, this chapter has considered the frequency higher than 100 KHz.
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Impedance (K-Ohm)
35 30 25 20 15 10 5 0 0
50
100
150
200
250
300
Frequency (KHz)
Fig. 14.13 Electrical Impedance (Z) of 2-Probe CNFA in AC Circuit as a function of Frequency.
The concept of steady-state response was also checked for the various load/stress stages.This is an important step as this observation assures that the CNFAs can be used to monitor long term loads. The specimens were subjected to different loading stages and were held at each load step for some time interval where the observation for impedance was made as in Fig. 14.14. The CNFAs have stable impedance within a few minutes of connection to the circuit. The linear trend of the impedance explains the reliability of CNFAs of being used to monitor static loads/stresses. The distinct lines for impedance at each load stage ensures the unique response to a unique loading stage. The overlapping lines will not be accurate to determine the stress level. Fig. 14.14 shows that the frequency of 300 KHz has overlapping between stress levels. Thus this frequency for monitoring is not recommended. The impedance response between 100 KHz and 200 KHz frequency is more reliable, and thus 100 KHz and 200 KHz of AC frequency can be selected as the optimum frequency to evaluate the response of 2-probe CNFAs with CNF content of 0.7% of the weight of cement in AC circuit.
5.2
CNFs content optimization
The impedance of CNFA is much higher than that of the wires; two probe impedance measurement is quite accurate to determine the electrical impedance variation of the CNFAs under loads/stresses. The calculation of variation is quite simple, and the electrical impedance variation (EZV) can be determined as, EZV ¼
Zi Z0 Z0
where. EZV: Electrical Impedance Variation Z0 : Initial Impedance Zi : Impedance at Step i
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(A)
100KHz
Impedance (K-Ohm)
116 115
No Load
114
3.86 MPa
113
4.83 MPa 112
5.79 MPa
111
7.72 MPa
110
9.65 MPa 11.03 MPa
109 0
5
10
15
20
Time (In Minutes)
(B)
200KHz
62.5
Impedance (K-Ohm)
62
No Load
61.5
3.86 MPa 61
4.83MPa
60.5
5.79MPa
60
7.72 MPa
59.5
9.65 MPa 11.03 MPa
59 0
5
10
15
20
Time (In Minutes)
Impedance (K-Ohm)
(C)
300KHz
44.2 44 43.8 43.6 43.4 43.2 43 42.8 42.6 42.4 42.2 42
No Load 3.86 MPa 4.83 MPa 5.79 MPa 7.72 MPa 9.65 MPa 11.03 MPa 0
5
10
15
20
Time (In Minutes)
Fig. 14.14 Static Stress Impedance Analysis for Different Frequencies: (A) 100 KHz; (B) 200 KHz; (C) 300 KHz.
Several researchers have been studying to find the optimized content of CNF in concrete and mortar. Chen and Chung (Chen & Chung, 1993a) studied the carbon microfiber’s electrical and mechanical properties. They observed that in 0.5% by weight of cement, the electrical resistivity decreased by 83%. This study later (Chen and Chung, 1996) elaborated with the study of microfibers with varying concentration.
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Fig. 14.15 CNFA with Varying CNF content (0.1%, 0.2%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% & 1.0% of CNF by the weight of cement from left to right).
They studied properties of mortar and concrete with 0e4% and 0.5e3% by weight of cement respectively which showed that these could act as reversible damage sensor by measuring ERV. They observed that increasing fiber content increased the ERV in concrete but the while increasing fiber content did not appreciably increase ERV in the mortar. Gao et al. (2009) showed that 0.7% of cement of CNF in concrete was optimal and higher than that caused fiber clumping in concrete. Several 2-probe CNFAs with varying CNFs contents were tested under compression, and electrical impedance variation was determined to find the optimal CNFs percentage in the AC circuit. Each CNFAs were cast as explained above. After casting, each CNFAs were cured for 14 days and then air dried for the next 24 h. To remove all excess water including pore water in CNFAs, they were oven dried at 100 C (212 F). Fig. 14.15 shows the series of oven dried CNFAs with varying CNF content (0.1%, 0.2%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% & 1.0% of CNF by the weight of cement). It is seen that higher the CNF concentration, darker is the CNFA. Fig. 14.16 shows the test setup for the compression test. Each CNFA was loaded in Instron 5960 Series Universal Testing Systems up to 50 kN (11,250 lbf) force capacity. The CNFA’s top and bottom surface were smoothened with grit to ensure even contact surface. Two strain gauge were pasted to the surface of CNFAs to measure average
Fig. 14.16 Test setup for CNFA’s CNF optimization.
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Fig. 14.17 Peeling of strain gauge from CNFA surface under compression.
strain from the Model P3 Strain Indicator and Recorder during the test. The CNFAs were tested in compression at a constant displacement rate of 0.0508 mm/min (0.002 in/mm) until the load dropped to 40% of peak load. The electrical impedance was measured using a two-probe technique with varying frequencies from Keysight E4980AL Precision LCR Meter. Fig. 17.17 shows the peeling of strain gauge from the surface of CNFA under loading. The compression test showed the promising trend between the concentration of CNF and maximum EZV recorded. The maximum ERV at each percentage of CNF to the weight of cement is the average response of CNFAs at peak load. The standard deviation was then calculated. Figs. 14.18 and 14.19 show the averaged ERV with one standard deviation on each side of average.
0.125
MAXIMUM EZV
0.115
Average EZV
0.105 Average EZV+STD 0.095 Average EZV-STD
0.085 0.075 0.065 0.055 0.045 0.035 0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
% CNF BY WEIGHT OF CEMENT
Fig. 14.18 CNF optimization of 2-probe CNFA for 100 KHz AC frequency.
Electrical impedance of carbon nanofiber aggregates
347
0.11
MAXIMUM EZV
0.1
Average EZV
0.09 Average EZV+ STD 0.08 Average EZV-STD 0.07 0.06 0.05 0.04 0.03 0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
% CNF BY WEIGHT OF CEMENT
Fig. 14.19 CNF optimization of 2-probe CNFA for 200 KHz AC frequency. Table 14.1 Optimized 2-probe CNFA mix design. Materials
Percentage of total mortar weight
Fine Aggregate
52.3%
Cement
28.4%
Water
12.1%
Silica Fume
4.2%
HRWR
2.0%
CNFs
0.3%
Kim Krystol (waterproofing)
0.6%
Figs. 14.18 and 14.19 demonstrates that, for 100 KHz and 200 KHz of AC frequency respectively, the CNFA concentration with respect to the weight of cement of 0.8% exhibited the most distinct change in EZV, which is slightly higher than the results found by Howser et al. (2011) for four-probe CNFAs in DC Circuit.This may be due to factors like changing 4-probe measurement technique to 2-probe, use of AC circuit and the addition of new admixture Kim Krystol with the target of waterproofing. The response of CNFAs with 0.1% and 0.2% of CNF to the weight of cement is excluded from these figures as they had a minimum response than those represented here. The variance in the above figure might be due to factors like temperature and remaining water in CNFA micropores even after drying. By the above tests, the optimum CNF content for 2-probe CNFAs in AC was found to be 0.8% to the weight of cement,i.e., 0.3% of total mortar weight. Table 14.1. Below shows the final optimized 2-probe CNFA mix design.
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Conclusion
The study of carbon nanofiber aggregate (CNFA) in DC circuit tests showed the unstable resistance variation resulting in unreliable data acquisition for stress/strain monitoring. This led to the development of two probe CNFA in AC circuit which was developed which is capable of sensing load/stresses with stable impedance measurements. The developed CNFA was 2.54 cm 2.54 cm x2.54 cm (1.00 in. 1.00 in. 1.00 in) cube with two steel wire meshes.The CNFAs were tested in different load stages to check the variation of impedance with time. The 30-min observation showed that the change in impedance was within 1% ensuring the reliability of stress-impedance data obtained from tests. The optimized AC frequency for response measurement was found to be between 100 KHz and 200 KHz while the optimum carbon nanofiber (CNF) content was found to be 0.8% of the weight of cement and based on this study, the optimized CNFA mix design was developed.
Acknowledgments The research described in this book chapter is financially supported by the Hurricane Resilience ResearchInstitute program. The opinions expressed in this study are those of the authors and do not necessarily reflect the views of the sponsor.
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