Piezoresistivity of smart carbon nanotubes (CNTs) reinforced cementitious composite under integrated cyclic compression and impact

Journal Pre-proofs Piezoresistivity of carbon nanotubes (CNT) reinforced cementitious composites under integrated cyclic compression and impact load Wenkui Dong, Wengui Li, Luming Shen, Zhihui Sun, Daichao Sheng PII: DOI: Reference:

S0263-8223(19)33744-4 https://doi.org/10.1016/j.compstruct.2020.112106 COST 112106

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

3 October 2019 8 January 2020 20 February 2020

Please cite this article as: Dong, W., Li, W., Shen, L., Sun, Z., Sheng, D., Piezoresistivity of carbon nanotubes (CNT) reinforced cementitious composites under integrated cyclic compression and impact load, Composite Structures (2020), doi: https://doi.org/10.1016/j.compstruct.2020.112106

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Piezoresistivity of carbon nanotubes (CNT) reinforced cementitious composites under integrated cyclic compression and impact load Wenkui Donga, Wengui Lia, Luming Shenb, Zhihui Sunc, Daichao Shenga a

School of Civil and Environmental Engineering, University of Technology Sydney, NSW 2007, Australia b

c

School of Civil Engineering, The University of Sydney, NSW 2006, Australia

Department of Civil & Environmental Engineering, University of Louisville, Louisville, KY 40292, USA

Abstract: The cyclic compression and four series of fixed magnitude impact loads with an increment of 50 times were conducted alternatively on the carbon nanotubes (CNTs) reinforced cementitious composites, to evaluate the piezoresistive sensitivity and repeatability of composites after exposed to different drop impact energies. The results show that the impacts procedure suddenly increased in electrical resistivity due to the emerged micro-cracks and pores, and higher impact energy led to faster resistivity increase. On the other hand, when the impact is repeatedly applied, a high impact resistance of the cementitious composites could be observed, which was attributed to the dense microstructures. Furthermore, instead of instable and uneven output of electrical resistivity during cyclical compression, more stable and uniform fractional changes of resistivity were achieved after exposed to impact load. However, severe nonlinearity with swift resistivity reduction of composites under low loads was observed at the beginning and the end of cyclic compression after subjected to many impacts with impact energy of 18.72×10-4 J/cm3. The related outcomes of conductive cementitious composites subjected to cyclic compression and impact will provide a method for stable electrical signal output and promote the applications of cement-based sensors for structural health monitoring under various loading conditions.

Corresponding

author: School of Civil & Environmental Engineering, University of Technology Sydney, NSW 2007, Australia Email: [email protected] (Wengui Li) 1

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Keywords: Carbon nanotube (CNT); Cementitious composite; Cyclic compression; Impact; Sensitivity; Repeatability

1. Introduction Multifunctional cementitious materials have attracted increasing attention in historical architectures protection, road transportation surveillance, structural health monitoring (SHM) and electromagnetic shielding, etc., due to their piezoresistivity, high mechanical strength and durability. In particular, the conductive cement-based composites not only have great potential to be an alternative to the conventional polymer-based or metal-based sensors for forces and deformations monitoring [1-9], but also have great capacity to heat themselves under external voltages for self-deicing or self-healing when applied in pavements [10-12]. Therefore, it can be forecasted that the cement-based composites with acceptable electrical conductivity will be widely used in engineering constructions to achieve the automatic health monitoring and damage detection in the near future. Many researchers have conducted extensive explorations on conductive cement-based composites, and found excellent self-sensing ability especially for cementitious composites reinforced with carbon nanotubes (CNTs) [13-22]. For instance, D'Alessandro et al. [23] proposed a scalable manufacturing procedure for CNTs doped cementitious composites, and obtained the composites with high gauge factor and excellent repeatability of electrical properties. Furthermore, Kim et al. [24] found that the CNTs reinforced cementitious composites with lower water content possessed higher piezoresistivity, as well as better stability and repeatability. Although worse electrical conductivity was found for the composites filled with acid treated CNTs, better compressive sensitivity was observed by Li et al. [25] compared to the composites reinforced with untreated CNTs. In terms of combination of CNTs with other conductive fillers, Azhari et al. [26] investigated 2

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the improvement effect of CNTs on the carbon fibres reinforced cementitious composites, and observed more stable resistance signal output, better piezoresistive reliability and sensitivity. Similarly, by studying the influences of hybrid fillers of CNTs and carbon nanofibers on the electrical conductivity and piezoresistivity of cementitious composites, Konsta-Gdoutos and Aza [27] reported that the appropriate weight content of CNTs to binders was significant to the conductivity and piezoresistive expression due to various dispersion efficiencies of the nanomaterials. However, all of above studies were focused on the static monotonic or cyclic compression. Even though several investigations studied the piezoresistive performance of cementitious composites under tensile load [28-31], very rare studies involved the piezoresistivity of cementitious composites subject to the dynamic loads or impact loads. Dynamic loads are much more prevalent than static loads in real applications, where the load point, magnitudes and frequency of the dynamic loads are variable as a function of loading time [32, 33]. In the case of CNTs filled polymer matrix, Monti et al. [34] observed a sudden and permanent electrical resistivity increase after each impact in a fierce way, and they attributed the increase of resistivity to the CNTs break, generation of micro-cracks and delamination. On the other hand, in order to demonstrate that the monitoring of electrical resistivity of composites was an efficient non-destructive testing method for the damage evaluation, Wang et al. [35] investigated the electrical performance of carbon fibres reinforced polymer composites under drop impact loads, and found that the damage affliction of composites could be successfully assessed by the increased electrical resistivity. Compared to the polymer composites, the investigations on the dynamic loads self-monitoring of cementitious composites were relatively rare. Meehan et al. [36] firstly studied the damage sensing ability of the cementitious composites reinforced with carbon fibres under impact, 3

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and achieved similar results to the polymer composites with stepped growth of resistivity after each impact. Their tests successfully monitored the minor damages caused by the drop impact even without cracks. Furthermore, Materazzi et al. [37] studied the CNTs reinforced cementitious composites under sinusoidal compressive forces and concluded that the electrical resistivity could precisely record the stress alterations. Afterwards, Ubertini et al. [38] extended the application of CNTs reinforced cementitious composites to the soft elastomeric surface sensor, and once again related the electrical resistivity to the input dynamic loads. However, the electrical resistivity showed poor repeatability, and permanently increased resistivity under dynamic loads. In addition, Sasmal et al. [39] investigated the electrical performance of CNTs filled cementitious composites under dynamic loads with different accelerations, and found that the composites were successful to capture the accelerations and fundamental frequencies. These studies more or less involved the sensing efficiency of CNTs reinforced composites when applied to monitor the dynamic loads such as drop impact, but none of them mentioned the piezoresistive sensitivity and repeatability before and after severe impact damages. Drop-weight impacts are commonly encountered dynamic forces for cement-based composites during the service. To understand the piezoresistive performances of conductive cementitious composites subjected to impact loads, the cementitious composites reinforced with different contents of CNTs were investigated on their piezoresistive sensitivity, repeatability and linearity under different times and energy of impact treatment. In particular, the cyclic compression was firstly applied on the cementitious composites without any impact treatment, which was then followed by the alternately performed impact loads and cyclic compression. It is expected that the explorations on the conductive cementitious composites subjected to impacts could contribute to the understanding 4

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of piezoresistivity after impact and promote their practical applications under various loading conditions.

2. Experimental program A set of tests including the cyclic compression and drop impact tests were conducted to evaluate the piezoresistive characteristics of CNTs reinforced cementitious composites. Scanning electron microscope (SEM) was applied to detect the microstructures damages and micro-cracks in the composites after impact loads. The following subsections introduce the raw materials, specimen fabrication, compression and impact regimes and the electrical resistance measurements. 2.1 Raw materials The General purpose cement and silica fume produced by Independent Cement & Lime Pty. Ltd. Australia and Concrete Waterproofing Manufacturing Pty. Ltd. Australia, respectively, were used in this study. The cement and silica fume were dry mixed with a proportion of 9: 1 in mass. The use of silica fume can reduce the porosity of the cementitious composites, as well as promote the dispersion efficiency of nanoparticles [40-42]. Multi-walled CNTs was purchased from Suzhou Tanfeng Graphene Technology Co., Ltd. China, with purity higher than 95%. The physical and electrical properties of CNTs are shown in Table 1. In addition, the polycarboxylate based water reducer was used to ensure the required workability of the composites, as well as promote the dispersion of CNTs in aqueous solutions. There are two mixture groups with one consisting of 0.1% CNTs to the weight of the binders and another including 0.2% CNTs. For all specimens, the water to binder ratio of 0.42 and the water reducer to binder ratio of 0.8% were chosen. Tap water was used throughout the experiments for specimens manufacturing and facility cleaning.

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2.2 Specimen preparation Since CNTs tend to agglomerate together due to the ultrahigh specific area and surface energy, the dispersion of CNTs in aqueous solution is one of the critical steps during the fabrication of cementitious composites. There are three procedures to obtain a well dispersed CNTs solution. First, a determined amount of tap water is prepared in the beaker, and the calculated dosage of superplasticizer was added [43]. The solution was gently stirred to achieve the homogeneity and avoid the generation of air bubbles. Secondly, the weighted CNTs are added into the solution, followed by a mechanical stir to roughly get the suspended solution. Afterwards, the suspended solution is sonicated in an ultrasonic bath to further disperse the CNTs, with the lasting time of 60 min. The ultrasonic frequency chosen was 40 kHz. To prevent the moisture loss during ultrasonic dispersion, the beaker was film sealed in this process. To avert the temperature increases of CNTs solutions during sonication which might affect the nanoparticles dispersion, the tap water in the sonication bath is replaced every 10 min, as shown in Fig. 1(a). After the preparation of well dispersed CNTs solutions, the solutions are poured into a Hobart mixer. Next, the pre weighted dry mix of cement and silica fume was added into the mixer, before the standard mixing methods for cement paste being carried out. Afterwards, moulds in the size of 50 mm 50 mm 50 mm were casted and two copper meshes worked as electrodes were symmetrically embedded into the moulds, with average space of 30 mm. The demolding process was conducted after 1 day curing in the curing chamber with the temperature of 25 °C and 95% relative humidity, and the demolded specimens are further cured in the curing chamber for another 27 days. The preparation of CNTs reinforced cementitious composites is presented in Fig. 1(b).

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2.3 Impact load and cyclic compression 2.3.1 Impact load The impact load was created by the drop-weight test with the free fall of a solid metal ball to the centre of CNTs/cement specimens. The experimental configurations are illustrated in Fig. 2. The solid metal ball was 27 mm in diameter, and weighted 80 grams. Drop heights of 10 cm, 20 cm, and 30 cm controlled by a digital height gauge were used with the corresponding impact energy being 6.24×10-4 J/cm3, 12.48×10-4 J/cm3 and 18.72×10-4 J/cm3, respectively. In particular, there were four regimes for each impact load, with the first regime consisting of 50 times of impact. The later impact schemes were carried out by increments of 50 times of impact on the same specimen, with the second regime of 100 times of impact, the third regime of 150 times of impact and the last regime of 200 times of impact in total. 2.3.2 Integrated compression and impact load The cyclic compression tests on the CNTs reinforced cementitious composites were designed to monitor the piezoresistive sensitivity and repeatability. All the composites were dried beforehand in an oven for 2 days at the temperature of 70 °C. Hence, two stress amplitudes of 4 MPa and 8 MPa under loading rates of 0.33 kN/s and 0.67 kN/s were proposed respectively, from which the piezoresistive characteristics of the cementitious composites under different stress amplitudes and loading rates could be elucidated. For each stress amplitude, there existed three identical compression cycles, as shown in Fig. 3. It shows that the first compression test was carried out without any impact treatment, while the later compression tests were conducted after each impact treatment. The operation of the combined compression and impact loads were applied according to the procedures illustrated in Fig. 4, with the first cyclic compression on the specimen without any 7

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treatment, and the later with intermittently carried out impact loads and the cyclic compression loads. It is necessary to note that the impact induced permanent resistivity growth could only be recorded until the electrical signal output became stabilized due to the resistivity reduction and reversion during each impact. 2.4 Resistance measurement SIGLENT SDM3045X digital multimeter was applied to record the electrical resistivity of CNTs reinforced cementitious composites, with the resistance recording rates being presented as 5 times per second during the cyclic compression and drop-weight impact tests. It is very difficult to record the electrical resistivity alterations in the procedures of impact in details, since the impact process is very swift. However, compared to the relatively long time compression test, the impact process could be considered as a static incident. Only the stable resistance changes before and after the impact treatment are recorded. In addition, the fractional changes of resistivity were calculated based on Eq. (1):

FCR 



0



R R0

(1)

where FCR presents the fractional changes of resistivity;  and R are the changes of electrical resistivity and resistance, respectively;  0 and R0 are the initial electrical resistivity and resistance of CNTs reinforced cementitious composites, respectively. Due to the minor strain/deformation compared to the size of specimens in tests, the fractional changes of resistivity are approximately equal to the fractional changes of resistance.

3. Results and discussions Since the CNTs dispersion efficiency is of significance to both the mechanical and electrical properties of the cementitious composites, description on the CNTs distribution in cement matrix 8

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was firstly discussed. Then the analysis on the compressive strengths of the composites under different numbers and energy of impact were conducted, followed by the main section of piezoresistivity discussion. Finally, the SEM images of the composites subjected to different numbers of impact at energy of 18.72×10-4 J/cm3 are discussed. 3.1 CNTs dispersion in composites Since CNTs possessing ultrahigh surface energy tend to easily attract each other and form agglomeration, the dispersion of nanoparticles before applying into cementitious composites is critical to achieve the composites with all improved physical, mechanical and electrical properties [44-47]. Fig. 5 illustrates the SEM images of CNTs/cementitious composites at the magnification of 2500 and 5000 times. Generally, the distribution of CNTs in the cementitious composites was clearly observed, with the majority of CNTs spread evenly and individually in the cement matrix without agglomeration. It implies that the use of ultra-sonication to disperse CNTs beforehand in the solutions of water and superplasticizer was successful to obtain cementitious composites with well-dispersed CNTs. The network-like distributions of CNTs are critical to the improved electrical conductivity in cementitious composites. Furthermore, bridging effect of CNTs in cementitious composites could be observed to create linkages between hydration products, which could not only improve the mechanical properties of cementitious composites, but also provide the nearby conductive pathways with an effective connection to acquire the piezoresistive property [26]. 3.2 Compressive strength Fig. 6 shows the compressive strength of the 0.1% wt and 0.2% wt CNTs reinforced cementitious composites without impact treatment and after subjecting to different numbers and energy of impacts. For the initial composites without impact, small discrepancy was observed for the composites with 9

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different contents of CNTs, and their ultimate compressive strengths reached 54.8 MPa and 53.0 MPa, respectively. Also, it seems that the impact treatment below 100 times for the cementitious composites could only bring limited negativities on the compressive strength, where their average compressive strengths still reached more than 52 MPa regardless of the impact energy. These are reasonable since it has been reported that visible cracks on the steel fibre reinforced concrete could be observed after more than a thousand times of drop-weight impacts. Even though, slight reduction on the compressive strength was observed with the increase of impact energy from 6.24×10-4 J/cm3 to 12.48×10-4 J/cm3, which was due to the generation of more microstructural damages by the larger impacts. Moreover, different impact resistances did not appear among the composites with various CNTs concentrations until reaching the impact numbers from 100 to 200. It indicates that the compressive strengths of the 0.1% CNTs reinforced cementitious composites under 150 and 200 times of impact with any magnitudes exceeded those of the counterparts with 0.2% CNTs at the same impact conditions. Since CNTs agglomerations have nearly no resistance to forces [24], the relatively lower impact resistance of the cementitious composites reinforced with higher CNTs concentration might be due to more agglomerations generated in the composites, since the identical ultrasonic dispersion duration was applied for these composites during CNTs solutions preparation. 3.3 Piezoresistive characteristics The fractional changes of resistivity of the 0.1% and 0.2% CNT reinforced cementitious composites without impact, and after subjected to impacts of different energy for various numbers of times were investigated by the cyclic compression, through which the electrical resistivity alterations, and the piezoresistive sensitivity and repeatability could be observed and compared. For the sake of easier

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comparison between composites with/without impact, the electrical resistivity increases after impact treatment was displayed in the same images with resistivity decreases during compression. 3.3.1 Composites with 0.1% CNTs Fig. 7 shows the relationship between the compressive stress and fractional changes of the resistivity during alternating cyclic compression and the impact load of 6.24×10-4 J/cm3 for the 0.1% CNTs reinforced cementitious composites. For the composites before impact, the fractional changes of resistivity illustrated unstable alterations by the gradually decreased summits with the compressive cycles of the same stress amplitude. The reason for the relatively higher resistivity changes in the first cycle is possibly due to the brittleness of the cementitious composites. There is irreversible plastic deformation when the composite is compressed in the beginning of compression test. Therefore, the average fractional changes of the resistivity reached approximately 2.4% and 3.8%, respectively for the stress amplitudes of 4 MPa and 8 MPa. After the first 50 times of impact, there was a resistivity growth of approximately 0.24% for the CNTs reinforced cementitious composites (The distance between two blue dotted curves), which was caused by the permanently created micro-cracks or voids. It was found that the fractional changes of the resistivity after the first impact treatment expressed little discrepancy to the counterpart without treatment, which showed similar resistivity changes. In terms of the composites subjected to further 50 times of impact (100 times of impact in total), the resistivity increases rose to 0.46%. It means an incremental increase in damage than the previous composites after 50 times of impact. The results indicate that the weak microstructures which were easier deformed under forces still hold a certain proportion after the first regime of impact, and got further damaged in the second regime of impact. However, in comparison to the composites without impact or under 50 times of impact, more equal and stable outputs of the 11

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resistivity changes in each cycles were observed, which may be attributed to the reduced unstable and weak microstructures in the cementitious composites [48]. Furthermore, the fractional changes of the resistivity of the CNTs reinforced cementitious composites after 150 and 200 times of impact showed similar changes to compressive stress, and illustrated excellent stability and repeatability as well. It is worth noting that the increase rate of the resistivity growth gradually narrowed with the increased times of impact, with the irreversible resistivity reaching 0.31% and 0.23% for the composites under 150 and 200 times of impact, respectively. It was considered that the reduced weaker microstructures in the composites under impact load can be attributed to the decreased resistivity jumps [34]. As compared to all the results without impact or under different times of impact load of 6.24×10-4 J/cm3, it seems that the impact load induced irreversible damages in the microstructures could bring a gentle reduction in the fractional changes of the resistivity, because of the decreased values of nearly 2.2% and 3.6% for the last two cyclic compressions. Fig. 8 presents the resistivity output for the CNT/cementitious composites under the cyclic compression without impact and after 50, 100, 150 and 200 times of impact at the impact energy of 12.48×10-4 J/cm3. Similarly, the composites without impact exhibited slightly higher fractional changes of resistivity in the front cycles for each stress amplitude, with the average values of 1.6% and 3.3%, at 4 MPa and 8 MPa, respectively. After 50 times of impact, the resistivity growth was higher than the counterparts under smaller impact energy (see Fig. 7) and reached 0.29%. In addition, irreversible resistivity emerged after the cyclic compression for the impact treated composites. In comparison to the results of composites subjected to lower impact energy, where very low irreversible resistivity occurred after impact load, the higher irreversibility of composites under cyclic compression indicated that new weak microstructures formed in the process of impact, even 12

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though the weak microstructures were damaged at the same time. Studies have proposed the solution of electrical resistivity detection to monitor the impact damage on conductive cementitious composites [36, 49]. Similarly, by analysing the irreversible resistivity, it can be deduced that the impact energy of 12.48×10-4 J/cm3 could cause micro-cracks or gaps inside the composites after at least 50 times of impact. As for the composites after 100, 150 and 200 times of impact, their electrical resistivity suddenly increased by 0.58%, 0.31% and 0.21%, respectively. The reason for the gradually “improved impact resistance” is similar to the above situation at the impact energy of 6.24×10-4 J/cm3. Although new weak microstructures such as micro-cracks and voids were created after impact, the following cyclic compression could decrease their numbers as revealed by the irreversible resistivity. In other words, not only the impact reduced the weak microstructures, the cyclic compression also caused denser microstructures to improve the resistance to later impact. In terms of piezoresistive sensitivity, the resistivity changes slightly increased with slow down increasing rate as the increase of impact, and arrived at the values of 2.4% and 4.4% under the final compression. The improved sensitivity of CNTs filled cementitious composites by the impact energy of 12.48×10-4 J/cm3 were irreconcilable with the reduced sensitivity for the composites by the impact energy of 6.24×10-4 J/cm3, which demonstrated the electrical anisotropy of cementitious composites [50]. Nevertheless, more stable and equal resistivity in each cycle was achieved to improve the accuracy of the deformation or damage monitoring. The fractional changes of resistivity of CNTs reinforced cementitious composites under alternating compression and impact loads of 18.72×10-4 J/cm3 are illustrated in Fig. 9. Different from the previous curves, composites after being impacted for hundreds of times showed rapid resistivity reduction in the beginning and end of cyclic compression, which can be illustrated by the distance 13

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between these two green dashed curves. For the composites after 50 times of impact, the resistivity growth reached 0.45%, while the rapid resistivity reduction for the followed compression reached 0.56%. It could be assumed that the rapid reduction of the resistivity was partially sourced from the resistivity growth caused by impact treatment. The impact energy of 18.72×10-4 J/cm3 on the CNTs reinforced cementitious composites did cause irreversible microstructural damages and led to the permanent resistivity growth, while the created micro-cracks or voids could be easily recovered under small external forces and result in the sudden resistivity reduction under cyclic compression. After 100 times of impact treatment, much clearer resistivity growth and reduction could be observed, with the variation rate of 1.16% and 1.06%, respectively. Also, for the curves under identical stress amplitude, it was found that the fractional changes of the resistivity at different cycles illustrated smaller discrepancy than the composites without impact or with 50 times of impact, which was consistent to the above conclusions of more stable resistivity output after impact treatment. In terms of the composites after 150 and 200 times of impact, very similar curves of fractional changes of the resistivity were obtained, with the resistivity growths being approximately 1.82% for the former and 1.65% for the latter, respectively. It could be observed that the piezoresistive performances of the initial 0.1% CNT reinforced cementitious composites without impact were slightly different among three impact regimes. This is because the different individuals were applied here to eliminate the effect of loading history. It is easy to understand that the larger irreversible resistivity growth is due to the higher impact energy. In addition, it was predicted that the resistivity growth would gradually decrease with more impacts just like the situation under lower impact energy. However, the largest resistivity growth happened after 150 times of impact at the impact energy of 18.72×10-4 J/cm3 rather than the aforementioned 100 14

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times of impact at the impact energy of 6.24×10-4 J/cm3, indicating that more numbers of impact should be applied for the “improved impact resistance” under the conditions of higher impact energy. As for the rapid resistivity reduction by compression, their values increased up to 2.20% and 2.25%, respectively for the composites after 150 and 200 times of impact, which almost accounted for one half of the total fractional changes of resistivity. In that case, the nonlinear part of the fractional changes of the resistivity to stress will cause false evaluation on the monitored stress and deformations, if the effects of impact forces are underestimated or ignored. 3.3.2 Composites with 0.2 % CNTs The electrical resistivity changes of 0.2% CNTs reinforced cementitious composites under the cyclic compression and impacts at impact energy of 6.24×10-4 J/cm3 are presented in Fig. 10, to exhibit their piezoresistive sensitivity and repeatability after impact treatment. For the composites without impact treatment, their fractional changes of resistivity at stress amplitudes of 4 MPa and 8 MPa reached approximately 3.8% and 6.1%, respectively. These values almost kept constant for the same composites after 50 times of impact, which accompanied by the induced resistivity growth of 0.21%. As for the 100 times of impact treated cementitious composites, largest resistivity growths were detected, reaching the value of 0.61% higher than those of composites after 50 times of impact. The following curves represented the resistivity alterations of composites after 150 and 200 times of impact, while the electrical resistivity growth by impact clearly decreased to 0.46% and 0.29%, respectively. Their fractional changes of resistivity illustrated as good piezoresistive repeatability as the composites without impact treatment. However, slightly higher fractional changes of resistivity could be observed for the composites after 200 times of impact, with the values reaching 4.5% and 7.8%, respectively at the two stress peaks. It was found that less damage was generated in the fourth 15

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impact, while the accumulated micro-cracks or voids from the first to the fourth impact might lead to more contact points between CNTs and their agglomerations. Hence, the higher fractional changes of resistivity or piezoresistive sensitivity might be attributed to the more created contact points. Overall, similar to the results of 0.1% CNTs reinforced cementitious composites under impact energy of 6.24×10-4 J/cm3, the higher resistivity growth under same time and energy of impact demonstrated the lower impact resistance for the 0.2% CNTs filled cementitious composites than that with 0.1% CNTs. Fig. 11 shows the fractional changes of resistivity of the 0.2% CNTs reinforced cementitious composites under the alternating cyclic compression from 4 MPa to 8 MPa and the impact load with energy of 12.48×10-4 J/cm3. In particular, higher and more unstable fractional changes of resistivity could be observed, by approximately 4.7% and 7.0% for the 0.2% CNTs reinforced cementitious composites without impact treatment at the stress amplitudes of 4 MPa and 8 MPa, respectively. The development of resistivity growth of the composites after each 50 times of impact was similar to the counterparts under impact energy of 6.24×10-4 J/cm3. It was observed that the resistivity growth reached 0.25%, 0.72%, 0.61% and 0.32% respectively after the 50, 100, 150 and 200 times of impact. It indicated that the effect of impact with energy of 12.48×10-4 J/cm3 on the electrical resistivity of CNTs cementitious composites was gradually weakened with the increased number of impacts, especially when the impact times were larger than 100. Moreover, excellent resistivity outputs with less fluctuations and discrepancies after 150 times of impact was observed, in which the fractional changes of resistivity at the same stress cycles were almost identical. As for the piezoresistive sensitivity of 0.2% CNTs cementitious composites after impact treatment, it was found that the fractional changes of resistivity increased to 7.3% and 10.1%, respectively at the stress peaks of 4 16

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MPa and 8 MPa, which were increased by almost 1.5 times more than that of the initial composites without impact treatment. Even though the nonlinearity of piezoresistivity still maintained after the impact treatment, it seems that the impact energy of 12.48×10-4 J/cm3 on the 0.2% CNTs reinforced cementitious composites failed to enlarge the nonlinearity but stimulate larger fractional changes of resistivity. Based on the above analysis and discussion, there were two potential reasons for the stimulated fractional changes of the resistivity. The first one might be the mentioned accumulated micro-cracks and voids that offered an opportunity to the connection between CNTs. Since the resistivity changes were obviously swifter under lower compressive stress than under high stress levels, the second reason might be the reversible cracks and voids induced by impact forces. Fig. 12 exhibits the electrical outcomes of the 0.2% CNTs reinforced cementitious composites under cyclic compression and the drop impacts with energy of 18.72×10-4 J/cm3. The initial fractional changes of resistivity reached approximately 4.1% and 7.1% at the stress amplitudes of 4 MPa and 8 MPa, respectively, with acceptable piezoresistive repeatability. However, both resistivity growth by impact and rapid resistivity reduction by compression occurred after the impact treatment, just as the results of the 0.1% CNTs reinforced cementitious composites under the impact energy of 18.72×10-4 J/cm3. Different from the narrowed resistivity growth after 150 times of impact for the counterpart with 0.1% CNTs, it was observed that the electrical resistivity growth by 50 times of impacts gradually increased from the first to the fourth impact. It means that the 0.2% CNTs reinforced cementitious composites was not yet stabilized within the 200 impacts. This is consistent to the above results that the 0.2% CNTs composites were provided with less impact resistance than the composites with 0.1% CNTs. Studies have shown that the dispersion of CNTs was significant to the physical and mechanical properties of cementitious composites [51, 52]. In this study, different 17

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contents of CNTs were dispersed by the ultra-sonication and all lasted for 60 min. The higher content of CNTs with more agglomerations might raise the potential for the uneven microstructures and lower impact resistance for the composites with 0.2% CNTs. To simply describe the resistivity growth by impact and rapid resistivity reduction in the beginning of compression, Fig. 13 illustrates the development of micro-cracks inside the CNT/cementitious composites with the increase of impact numbers, although the fibres could prohibit the cracks growth and propagation [53]. It can be seen that the impact treatment could enlarge the initial cracks and create new micro-cracks, which led to the sudden resistivity growth. On the other hand, during the impact treatment, some already connected CNTs might get detached, especially for the CNTs playing the bridging effect. The reconnection between these CNTs due to the closed cracks in the compression partially contributed to the rapid resistivity reduction. In addition, even for the cracks without CNTs, the easier compressed cracks could also result in the decrease of electrical resistivity. 3.4 Microstructural characterization The microstructural morphology of the 0.1% CNTs reinforced cementitious composites after different times of impact by SEM is shown in Fig. 14, where cracks with different sizes and spreading models was clearly observed. All of these images have the identical magnification of 3000 times. In particular, to better compare the composites with and without impact and magnify their differences, higher impact energy of 18.72×10-4 J/cm3 was chosen to investigate the effects of impacts on the microstructures of cementitious composites. Fig. 14 (a) shows the microstructures of the composites without any load treatment. It was found that relatively denser structures with no cracks were observed in the composites, even though small pores emerged in the composites. These initial pores of cementitious composites include gel pores, capillary pores, air induced pores and the 18

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pores caused by inadequate compaction [54]. In comparison to the morphology of initial cracks and those caused by the later impact and compression, the former are normally in the form of irregular appearances but the latter coincides well with the edges of cracks. Figs. 14 (b) to (e) illustrate the morphology of the composites after 50, 100, 150 and 200 times of impact, respectively. For the composites after 50 times of impact, there were limited cracks in small width within the view. Considering that the compressive strengths of cementitious composites were rarely affected under 50 times of impact, it could be deduced that the majority of micro-cracks are in the form of small and separated individuals, rather than the connected and larger cracks and gaps. However, wider and longer cracks were generated as the impact times increase, thus for the composites after 100 and 150 times of impact, more separated cracks were observed, which gradually spread and almost got connected as shown in Figs. 14 (c) and (d). Moreover, it could be seen that the cracks were much wider than those of the composites with fewer times of impact, which were believed to be responsible for the lower compressive strengths for these composites with more times of impact. In addition to the induced major cracks, secondary cracks along with the major cracks appeared for the composites after 150 times of impact. It was found that the secondary cracks were normally in smaller sizes and some of them were accompanied with wider cracks. Since the cracks played an important role for the impact energy absorption, new cracks were generated from the major cracks and spread with the loose microstructures as the increase of impact energy. In terms of the composites subjected to 200 times of impact, the largest cracks with a width nearly 2 µm and the secondary cracks along with major cracks were clearly observed compared to other composites. These are consistent with the mechanical properties of composites after 200 times of impact which normally possessed the lowest compressive strength due to the largest cracks and voids. As for the 19

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relationship between cracks and electrical piezoresistivity, the largest cracks caused higher permanent resistivity increases, but at the same time, wider cracks had the highest possibility to be compressed and resulted in the reduced electrical resistivity. It is concluded that the easily compressed and recovered cracks were contributed to the swift electrical resistivity reduction at the beginning and end of each cyclic compression.

4. Conclusions The effects of alternately compression and impact load on the mechanical and piezoresistive properties of CNTs reinforced cementitious composites, including the ultimate compressive strength, piezoresistive sensitivity, stability and repeatability, were investigated in this study. The main conclusions can be drawn as follows: (1) Using ultrasonication and superplasticizer can help to disperse the CNTs well in aqueous solutions, which was of importance to manufacture the CNTs reinforced cementitious composites with excellent physical, mechanical and conductive properties. (2) Impact load caused sudden resistivity growth for CNT/cementitious composites, the higher impact energy applied, the larger resistivity increase composites were. The impact resistance was improved with the increase of impact times accompanied by the progressively reduced resistivity growth after the same times of impact. (3) After impact treatment with suitable impact energy, the initially instable and uneven resistivity output of cementitious composites can be improved and replaced by more stable and uniform fractional changes of resistivity under cyclic compression. The results are beneficial to the practical application of CNTs filled cement-based sensors.

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(4) Under the impact energy of 18.72×10-4 J/cm3, cementitious composites after hundreds of times impact exhibited a swift resistivity reduction under low loads during cyclic compression. This is due to the emerged micro-cracks and voids that could be compressed and closed during cyclic compression at low stress magnitude. (5) The 0.1% CNT/cementitious composites exhibited better impact resistance than the composites with 0.2% CNTs which presented slightly worse physical/mechanical properties. Since the CNTs solution preparation and composites manufacturing procedures are exactly identical, the discrepancy might be attributed to the lower dispersion efficiency for the 0.2% CNTs solutions. (6) Wider and longer cracks were produced based on microstructural characterization. Along with the major cracks in CNT/cementitious composites, secondary cracks were produced with more times of impact treatment. It concluded that wider cracks were more likely to be compressed, which led to the sudden resistivity reduction.

Acknowledgements All the authors appreciate the financial supports from the Australian Research Council (ARC) (DE150101751), University of Technology Sydney Research Academic Program at Tech Lab (UTS RAPT), University of Technology Sydney Tech Lab Blue Sky Research Scheme and the Systematic Projects of Guangxi Key Laboratory of Disaster Prevention and Structural Safety (Guangxi University), China (2019ZDX004) and State Key Laboratory of Subtropical Building Science (South China University of Technology), China (2019ZA06).

Conflict of interest The authors declare that they have no conflict of interest.

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[52] Sobolkina A, Mechtcherine V, Khavrus V, Maier D, Mende M, Ritschel M, et al. Dispersion of carbon nanotubes and its influence on the mechanical properties of the cement matrix. Cement and Concrete Composites 2012;34:1104-13. [53] Banthia N, Nandakumar N. Crack growth resistance of hybrid fiber reinforced cement composites. Cement and Concrete Composites 2003;25:3-9. [54] Nambiar EKK, Ramamurthy K. Air ‐ void characterisation of foam concrete. Cement and Concrete Research 2007;37:221-30.

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List of Tables Table 1 Physical and electrical properties of multi-walled carbon nanotubes (CNTs)

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Table 1 Physical and electrical properties of multi-walled carbon nanotubes (CNTs) CNTs

Inner

Outer

Loose

Electrical

density

resistivity

(g/cm3)

(Ω·cm)

2.1

10-2

Length purity

Appearance

diameter

diameter

(nm)

(nm)

3-5

8-15

Specific area (m2/g)

(µm) (wt.) > 95%

Black

3-12

30

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List of Figures Fig. 1. Schematic diagram for specimen preparation: (a) Well-dispersed CNTs solution; (b) Cementitious composites Fig. 2. Set-up for impact test and resistance measurement on CNTs/cement composites Fig. 3. Alternatively applied cyclic compression and impact loads on CNT/cementitious composites Fig. 4. Schematic diagram of cyclic compression and drop-weight impact tests with three different impact energies Fig. 5. Illustration of well dispersed CNTs by ultra-sonication in cementitious composites Fig. 6. Compressive strength of cementitious composite with different contents of CNTs subjected different numbers and energy of impact loads Fig. 7. Fractional changes of resistivity of 0.1% CNT/cementitious composites under cyclic compression after different times of impact at energy of 6.24×10-4 J/cm3 Fig. 8. Fractional changes of resistivity of 0.1% CNT/cementitious composites under cyclic compression after different times of impact at energy of 12.48×10-4 J/cm3 Fig. 9. Fractional changes of resistivity of 0.1% CNT/cementitious composites under cyclic compression after different times of impact at energy of 18.72×10-4 J/cm3 Fig. 10. Fractional changes of resistivity of 0.2% CNT/cementitious composites under cyclic compression after different times of impact at energy of 6.24×10-4 J/cm3 Fig. 11. Fractional changes of resistivity of 0.2% CNT/cementitious composites under cyclic compression after different times of impact at energy of 12.48×10-4 J/cm3 Fig. 12. Fractional changes of the resistivity of 0.2% CNT/cementitious composites under cyclic compression after different times of impact at energy of 18.72×10-4 J/cm3 Fig. 13. Illustration of the micro cracks development in CNT/cementitious composites during impact Fig. 14. Micro-crack development and morphology of cementitious composites under impact

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Fig. 1. Schematic diagram for specimen preparation: (a) Well dispersed CNTs solution; (b) Cementitious composites

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Fig. 2. Set-up for impact test and resistance measurement for CNTs/cement composites

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Compressive stress (MPa)

10

Without impact After the second impact After the first impact After the third impact

8

Impact

Impact

After the fourth impact

Impact

Impact

6

4

2

0 0

100 200 300 400 500 600 700 800 900 1000110012001300140015001600170018001900

Time (s)

Fig. 3. Applied method for monotonic compression and cyclic impact on CNT/cementitious composites

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Fig. 4. Schematic diagram of cyclic compression and drop-weight impact tests with three different impact energies

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Fig. 5. Illustration of well dispersed CNTs by ultra-sonication in cementitious composites

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Compressive strength (MPa)

56 54 52 50 48

0.1% CNTs, impact energy of 6.24 10-4 J/cm3 0.2% CNTs, impact energy of 6.24 10-4 J/cm3

46

0.1% CNTs, impact energy of 12.48 10-4 J/cm3 0.2% CNTs, impact energy of 12.48 10-4 J/cm3

44

0.1% CNTs, impact energy of 18.72 10-4 J/cm3 0.2% CNTs, impact energy of 18.72 10-4 J/cm3

42 0

50

100

150

200

Numbers of impact

Fig. 6. Compressive strength of cementitious composite with different contents of CNTs subjected to different numbers and energy of impact loads

37

Compressive stress (MPa)

12

-20 Stress

Resistivity growth by impact

Resistivity changes

200 times impact in total 100 times impact in total 50 times impact in total 150 times impact in total

10

-16

8 -12 6 -8 4 2 0.46%

0.24%

0 0

200

400

-4

0.23%

0.31%

0 600

800

1000

1200

1400

1600

1800

Fractional changes of resistivity (%)

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Time (s)

Fig. 7. Fractional changes of resistivity of 0.1% CNT/cementitious composites under cyclic compression after different times of impact with energy of 6.24×10-4 J/cm3

38

-20

Compressive stress (MPa)

12 Resistivity growth by impact

Resistivity changes

Stress

100 times impact in total 200 times impact in total 50 times impact in total 150 times impact in total

10

-16

8 -12 6 -8

0.21%

4 0.31%

2

-4

0.58% 0.29%

0 0

200

400

0 600

800

1000

1200

1400

1600

1800

Fractional changes of resistivity (%)

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Time (s)

Fig. 8. Fractional changes of resistivity of 0.1% CNT/cementitious composites under cyclic compression after different times of impact with energy of 12.48×10-4 J/cm3

39

Compressive stress (MPa)

12

Stress

Resistivity changes

-20

Resistivity growth by impact

Rapid resistivity reduction by compression

10

200 times impact in total 100 times impact in total 50 times impact in total 150 times impact in total

8

-16

-12 6 2.25%

4

2.20%

2

0

200

400

-4

1.82%

1.06% 1.16%

0.56% 0.45%

0

-8

1.65%

0 600

800

1000

1200

1400

1600

1800

Fractional changes of resistivity (%)

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Time (s)

Fig. 9. Fractional changes of resistivity of 0.1% CNT/cementitious composites under cyclic compression after different times of impact with energy of 18.72×10-4 J/cm3

40

-40

Compressive stress (MPa)

12 Stress

Resistivity changes

Resistivity growth by impact 200 times impact in total

100 times impact in total

10

150 times impact in total

50 times impact in total

-35 -30

8

-25 -20

6

-15

4

-10 2 0.21%

0 0

200

400

600

800

-5

0.29%

0.46%

0.61%

0 1000

1200

1400

1600

1800

Fractional changes of resistivity (%)

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Time (s)

Fig. 10. Fractional changes of resistivity of 0.2% CNT/cementitious composites under cyclic compression after different times of impact with energy of 6.24×10-4 J/cm3

41

-40

Compressive stress (MPa)

12 Resistivity changes

Stress 10

Resistivity growth by impact 200 times impact in total

100 times impact in total 50 times impact in total

-35 -30

150 times impact in total

8

-25 -20

6

-15

4

-10 2 0.72%

0.25%

0 0

200

400

600

800

-5

0.32% 0.61%

1000

1200

0 1400

1600

1800

Fractional changes of resistivity (%)

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Time (s)

Fig. 11. Fractional changes of resistivity of 0.2% CNT/cementitious composites under cyclic compression after different times of impact with energy of 12.48×10-4 J/cm3

42

Compressive stress (MPa)

12

Stress

Resistivity changes

-40

Resistivity growth by impact

-35

Rapid resistivity reduction by compression 200 times impact in total 100 times impact in total 50 times impact in total 150 times impact in total

10 8

-30 -25

6 4.41%

4 2

200

400

4.54%

-15 -10 -5

1.58%

1.13% 0.95%

0

-20

3.28%

1.82%

0

4.44%

0 600

800

1000

1200

1400

1600

1800

Fractional changes of resistivity (%)

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Time (s)

Fig. 12. Fractional changes of the resistivity of 0.2% CNT/cementitious composites under cyclic compression after different times of impact with energy of 18.72×10-4 J/cm3

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Fig. 13. Schematic diagram of micro-cracks initiation and propagation in CNT/cementitious composites after impact treatment

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(a) Without impact

(b) 50 times of impact

(c) 100 times of impact

(d) 150 times of impact

(e) 200 times of impact

Fig. 14. Microcrack initiation, propagation and morphology of cementitious composites under impact treatment

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Highlights (7) Using ultrasonication and superplasticizer can disperse the CNTs well in aqueous solutions to manufacture CNTs reinforced cementitious composites. (8) Impact load caused sudden resistivity increase for CNT/cementitious composites, the higher impact energy applied, the faster resistivity increase composites were. (9) After impact treatment with suitable impact energy, the initially instable and uneven resistivity of cementitious composites can be more stable and uniform.

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Revised COST_2019_3558

(10)Under the impact energy of 18.72×10-4 J/cm3, cementitious composites after hundreds of impacts exhibited swift resistivity reductions under low loads during the cyclic compression. (11)The 0.1% CNT/cementitious composites exhibited better impact resistance than the one with 0.2% CNTs which presented slightly poorer physical/mechanical properties.

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