IEEE SENSORS JOURNAL, VOL. 17, NO. 19, OCTOBER 1, 2017 6153

Monitoring of Compression and BendingProcess of Reactive Powder Concrete Using

MWCNTs/PDMS Composite SensorsYongquan Zhang, Yong Ge, Yuanbo Du, and Lili Yang

Abstract— In the structural healthy monitoring of bridges,pavements of concrete or asphalt, flexible conductive sensorsexhibit so many advantages, such as low cost, high durabil-ity, and large deformation monitoring. Herein, we report alow cost flexible composite sensor of multi-walled carbon nan-otubes/Polydimethylsilosane (MWCNTs/PDMS) nanocompositesvia solution casting method. Results show that MWCNTs/PDMSreveals excellent sensitivity and good stability when the fractionof MWCNTs is larger than the threshold (∼2.5 wt%). Thecompressive strain of reactive powder concrete was lineal withthe resistance change of sensor (�R/R0) in the range of elasticdeformation, while the deflection of reactive powder concretecould be predicted by the fitting exponential equation fromthe electric resistance change of MWCNTs/PDMS. These resultshighlight MWCNTs/PDMS nanocomposite as strain sensor inthe structural health monitoring of reactive powder concretestructures.

Index Terms— Reactive powder concrete, MWCNTs/PDMSnanocomposite, compressive strain, deflection.

I. INTRODUCTION

DUE to the great loss in economic and safetyhazard from the increasing accidents, the process of

nondestructive evaluation (NDE) or structural health monitor-ing (SHM) [1], [2] of civil infrastructure system has attractedmuch interest, such as bridges [3], pavement, or steel struc-tures [4]. The key materials in the structural health monitoringsystem are strain sensors which are attached to or embeddedin the structure, such as fiber optical sensors [5]–[7], piezo-electric sensors [8]–[11], or piezoresistive materials [12]–[14].However, large deformation often happens, for example thelarge deformation of asphalt-based concrete under tempera-ture change, the large deformation of cement-based concreteexpansion joint, et al. Traditional optical or piezoelectricsensors would rupture when large deformation occurs. Com-pared with other kinds of strain sensors, flexible conductivecomposites-based sensors are of increasing importance withwider applications due to their flexibility, low cost, high dura-bility and great compatibility with structural materials [15].

Manuscript received June 1, 2017; revised August 4, 2017; acceptedAugust 11, 2017. Date of publication August 15, 2017; date of currentversion September 8, 2017. This work was supported by the NationalNatural Science Fund of China under Grant 51278157. The associate editorcoordinating the review of this paper and approving it for publication wasProf. Octavian Postolache. (Corresponding authors: Lili Yang; Yong Ge.)

Y. Zhang, Y. Ge, and Y. Du are with the School of Transportation Scienceand Engineering, Harbin Institute of Technology, Harbin 150090, China(e-mail: zyqzygzs@126.com; hitbm@163.com; 915439482@qq.com).

L. Yang is with the College of Materials Science and Engineering, DonghuaUniversity, Shanghai 201620, China (e-mail: liliyang@dhu.edu.cn).

Digital Object Identifier 10.1109/JSEN.2017.2740325

Among these materials, composites with carbon nanotubes asfiller have been intensely studied due to their excellent andmechanical properties [16]–[19].

Reactive powder concrete (RPC) is a cement-based com-posite with ultra-low water to cementitious materials ratio,and composed of ultra-fine reactive powder, high-quality fineaggregate, high volume fraction of steel fiber or hybrid fibers.Due to the great flexural strength, deformability and toughness,reactive powder concrete is widely used in bridges, pipes,airport slabs, blast resisting panels, corrosion proof structure,and so on. The peak compressive strain, ultimate compressivestrain, and bending cracking strain of reactive powder concretewere obviously superior to ordinary portland concrete. Con-ductive elastomer composite has arising sufficient regards dueto their wide working temperature range, large deformationand stable electrical resistance-time characteristic [20]–[22].

In this research, sensitivity of MWCNTs/PDMS nanocom-posites were characterized, showing significant change inelectrical resistance when subject to large tensile strains(E =50%), and repeatable resistance change in response tocyclic loadings of mechanical strain. Furthermore, their real-time detecting of compressive strain, monitoring of deflection,and first-crack strength of reactive powder concrete werefurther studied. The experimental results showed that therelationship between resistance change of MWCNTs/PDMSnanocomposite and compressive strain of RPC was linear inthe range of elastic deformation, and the deflection of reactivepowder concrete can be calculated from resistance change ofMWCNTs/PDMS nanocomposite.

II. MATERIALS AND EXPERIMENTS

A. Materials and Proportions

Polydimethylsilosane (PDMS) was purchased fromDow Corning (Sylgard 184), and Multi-walled carbonnanotubes (MWCNTs) from Timesnano Inc (outer diameter:10-20 nm, length: 10-30 μm). The MWCNTs/PDMS nan-ocomposite was prepared by solution casting method. Firstly,PDMS base polymer was added into tetrahydrofuran (THF)solutions and magnetically stirred, and MWCNTs were addedinto another THF solutions and dispersed with ultrasonic.Secondly two solutions were combined in an open containerand went through further dispersing with ultrasonic, andthen magnetically stirred while THF slowly evaporated ata hot plate. After removing THF, PDMS-curing agent wasadded into the mixture to obtain a conformal elastomernanocomposite. In this research, nanocomposites containing

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6154 IEEE SENSORS JOURNAL, VOL. 17, NO. 19, OCTOBER 1, 2017

TABLE I

MIX PROPORTIONS OF REACTIVE POWDER CONCRETE

Fig. 1. Illustration of experimental system of (a) compressive stress, strain monitoring of reactive powder concrete and (b) stress, flexural strain monitoringof reactive powder concrete.

about 0, 0.5wt%, 1.0wt%, 2.0wt%, 3.0wt%, 4.0wt%, 5.0wt%,6.0wt%, 8.0wt%,and 10.0wt% of multi-walled carbonnanotubes were prepared. The obtained nanocomposite wascut into 55 mm×5 mm×1 mm specimen for testing, and onegroup consists of five samples.

The reactive powder concrete was utilized in order toimplement this study. P·O 42.5 Ordinary Portland cementand silica fume were used as the cementitious material inthe experiment. The aggregate was quartz sand with threedifferent grain sizes of 0.16∼0.315 mm, 0.315∼0.63 mm, and0.63∼1.25 mm. A poly carboxylate based super plasticizerwas used to enhance the workability. The steel fibers with13 mm length and 0.20mm diameter were used in this study,and the designed mix proportions of reactive powder concretefor experiment are listed in Table 1.

A laboratory type mixer having a vertical axis and3 L capacity was used for concrete production. First,the cementious materials and quartz sand were poured intothe mixer and mixed for 1 min. Secondly, the steel fiberswere added to the former mixture in the dry phase, and amixture process was performed for another 1 min until a nearlyhomogeneous mixture was obtained. Finally, water and theplasticizer were added to the mixture and the wet mixture washomogeneously mixed for 2 min. During the study, cubic spec-imens with dimensions of 70.7 mm×70.7 mm×70.7 mm andcuboid specimens with dimensions of 40mm×40mm×160mmwere prepared. Following a 3 day steam curing at 70° and4 day standard curing process, the specimens were readyfor testing.

B. ExperimentsIn order to achieve repeatable results, all the experiments

were conducted in a controlled laboratory environment. Con-ductivity of MWCNTSs/PDMS nanocomposite was calculated

from sheet resistance measurements using a four point probe.Copper sheets were stick to the ends of specimen usingcolloidal silver paste as electrodes to form a two-probe mea-surement setup. Two-probe electrode connections were madeat both ends using copper sheet. As the MWCNTs/PDMSnanocomposite sample was stretched with the help of aCSS-88000 testing machine, its resistance was recorded in realtime using a UT805A digital multimeter with a measurementrange of 1×10−5� to 20M�. Temperature was controlled byrefrigerator and heat oven, relative humidity was control byclimatic test chambers, the resistance of samples was alsomeasured using a UT805A digital multimeter.

Three specimens of 70.7 mm × 70.7 mm × 70.7 mmwere used in detecting compressive strain of reactive powderconcrete under uniaxial compression test. The compressivestrength was measured by ordinary pressure testing machinewith the capacity of 1000kN, according to (Method of test-ing cements-determination of strength-ISO) GB/T 17671-2011of China, and the loading rate was 0.2MPa/s. Traditionalstrain gauge (Testing Instrument Factory Huangyan Zhejiang,BX120-50AA) and MWCNTs/PDMS nanocomposite samplewere stick to the same side of reactive powder concretespecimen by resin. The value of pressure, strain and resistanceof MWCNTs/PDMS nanocomposite sample were simulta-neously recorded until to the fracture of reactive powderconcrete specimen. During the loading, the compressive strainwas measured by use of traditional strain gauges parallel tothe loading direction, and the MWCNTs/PDMS nanocom-posite sample was also parallel to the loading direction.The strain signals were collected using DH3821 Net staticstrain testing system, and the resistances of MWCNTs/PDMSnanocomposite sample were collected using UT805A digitalmultimeter. The specific illustration of test system was shownin Fig. 1a.

ZHANG et al.: MONITORING OF COMPRESSION AND BENDING PROCESS OF RPC 6155

Fig. 2. Relationship between electrical conductivity of MWCNTs/PDMSnanocomposite and mass fraction of MWCNTs.

Three specimens of 40 mm × 40 mm ×160 mm sizewere used in monitoring the deformation and crack of reac-tive powder concrete under three point bending test, . Theflexural strength was measured by universal electronic testingmachine MTS-810, according to (Method of testing cements-determination of strength-ISO) GB/T 17671-2011 of China.The distance between supports was 100mm, and the loadingrate was 0.02 mm/min. The specimen of MWCNTs/PDMSnanocomposite was stick to the geometry center of the bottomof reactive powder concrete samples by resin. In this experi-mental test, the values of load, displacement, and electricalresistance were recorded simultaneously by MTS-110 andUT805A digital mutimeter respectively until to the fracture ofMWCNTs/PDMS nanocomposite samples, MWCNTs/PDMSnanocomposite was perpendicular to the loading direction.The specific illustration of test system was shown in Fig. 1b.The crack width of reactive powder concrete was measuredby ZBL-F800 crack test instrument when MWCNTs/PDMSnanocomposite was fractured.

III. RESULTS AND DISCUSSION

A. Percolation Threshold of MWCNTs/PDMSNanocomposites

Fig. 2 showed the electrical conductivity at room tem-perature of MWCNTs/PDMS composites with the increaseof mass fraction. There was a steep increase of electricalconductivity when the mass fraction of MWCNTs increasedfrom 1.0wt% to 4.0wt%. According to the slope fitting andpercolation theory, the critical point of 2.5wt% was thepercolation threshold, which means the formation of chan-nels for electron transport in the whole nanocomposite. Theconductive polymer nanocomposites usually were used nearthis critical point because the sensitivity is the highest at thepercolation threshold [23], MWCNTs/PDMS nanocompositeswith different mass fractions of 3.0wt%, 4.0wt%, 5.0wt%and 6.0wt% above the threshold were chosen, and propertiesand application of MWCNTs/PDMS nanocomposites werediscussed in the follow sections.

B. Sensitivity of Nanocomposites Under Axial TensileStrain and Cyclic Loading

The sensitivity of smart sensing materials using in structuralhealth monitoring lies on the fast and regular electric response

under external load environments. Fig. 3a shows the resistancechange of MWCNTs/PDMS nanocomposites with differentmass fractions of MWCNTs under uniaxial tensile strain from0 to 50%. When MWCNTs/PDMS nanocomposite sampleswere elongated from 0 to 50% of tensile strain, the resistancechange of nanocomposite with 3.0wt% mutiwalled carbonnanotubes was the largest, increased by 302.8%. The resistancechange of nanocomposite with 6.0wt% MWCNTs was thesmallest, increased by 70.4%. This can be explained as fol-lows, the tunneling effect governs the conductivity mechanismwhen the fraction is low and percolation theory usually domi-nates when the fraction is high. Although the resistance changeof MWCNTs/PDMS nanocomposite was not perfectly linearwith tensile strain, the sensing factors of nanocomposites canbe calculated according to the equation of SF = (�R/R0)/ε,were 6.0, 1.98, 1.95, and 1.41 respectively, where SF issensing factor, �R, R0, and ε is the resistance change, initialresistance and tensile strain of MWCNTs/PDMS, respectively.With the increased mass fraction of MWCNTs, the sensitivitycoefficient of MWCNTs/PDMS nanocomposites decreased butthe stability of electric-strain response of nanocomposites wasenhanced due to the weaker structural relaxation.

To further characterize the strain-dependent resistance ofMWCNTs/PDMS nanocomposites, multiple cycles of stretch-ing and relaxing were carried out on the automated motionstage at a constant strain rate (0.04 mm/s). When a uniaxialtensile strain is applied to the sensing material, resistanceincreased because of loss of contact points and wideningof intertubular distances between neighboring CNTs. Simi-larly, when sensing material was relaxed, electron conductionpaths were restored, resistance dropped along with decreasingstrain [24]. Fig. 3b shows 5 cycles indicating that upon load-ings of tensile strain to 50%, the MWCNTs/PDMS nanocom-posite with 3.0wt% of MWCNTs shows largest resistancechange, i.e. 88.0%. The resistance changes of nancompositeswith 4.0wt%, 5.0wt%, and 6.0wt% of MWCNTs were 55.9%,60.2%, and 63.7% respectively. The ability of PDMS-basedcomposites to endure repeated large deformation without anymechanical failure presents a potential advantage over otherstrain-sensing materials.

C. Detection of Compressive Strain of RPCUnder Compressing

In this section, representative samples of reactive powderconcrete with 1.5vol% of steel fiber were selected to study onreal-time detection of compressive strain of reactive powderconcrete using MWCNTs/PDMS nanocomposites under axialcompressive loading, because this volume concentration ismost usually used in engineering applications. Fig. 4 showedthat the resistance change of MWCNTs/PDMS nanocompositewas linearly changed with the compressive strain of reactivepowder concrete in the range of elastic deformation. Due tothe damaged surface, the traditional strain gauge does notaccurately measure the compress strain of reactive powderconcrete out the range of linear elastic deformation. Other-wise, the sensing factors of nanocomposites with differentmass fractions of MWCNTs were calculated according tothe equation used in Section 3.2, were 2.54, 1.84, 1.68,

6156 IEEE SENSORS JOURNAL, VOL. 17, NO. 19, OCTOBER 1, 2017

Fig. 3. Resistance changes of MWCNTs/PDMS nanocomposites with different mass fractions of MWCNTs (a) as a function of tensile strain and (b) undercyclic loadings of tensile strain.

TABLE II

PARAMETERS AND R-SQUARES OF FITTING EQUATIONS

and 0.64 respectively, showed good sensitivity in detectingcompressive strain of reactive powder concrete.

In order to further research the sensitivity of MWCNTs/PDMS nanocomposite in detecting compressive strain ofreactive powder concrete under uniaxial compressive loading,the linear function (1) was fitted by the relationship betweenresistance change of MWCNTs/PDMS nanocomposite andcompressive strain of reactive powder concrete, the pink lineshowed in Fig. 4 was the fitting fuction.

�R/R0 = A + Bμε (1)

where A, and B are coefficients for the linear lines, �R/R0isthe resistance change of MWCNTs/PDMS nanocompos-ite (in %), μεis compressive strain of reactive powder concrete.Fig. 4 and Table 2 demonstrated that compressive of reactivepowder concrete under constant compressive loading ratecould be predicted from the linear equation easily.

D. Monitoring Deflection of RPC

Representative samples of reactive powder concrete with1.5vol% fraction of steel fiber were also used in this section.The deflection, load of reactive powder concrete and theelectric resistance of MWCNTs/PDMS nanocomposite weresimultaneously collected until to the maximal stress of reactivepowder concrete. Fig. 5 showed the stress and resistancechange of MWCNTs/PDMS nanocomposite with the increaseof deflection. It is observed that the resistance change ofMWCNTs/PDMS nanocomposite remained unchanged in theOA stage due to no crack occurred. The electrical resistanceof MWCNTs/PDMS nanocomposite slightly increased at thecritical point A, an initial single crack was found by thehigh-power microscope in the bottom surface of reactive

TABLE III

RESULTS OF ULTRASONIC TEST

powder concrete sample, because the occurrence of micro-cracks below a width of 50-100μm that is visible to thehuman eye was not detected [25], [26]. Ultrasonic test verifiedthe occurred of micro-crack from another point of view,the results were shown in Table 3, where t0 is the initialacoustic transit time in reactive powder concrete, and t f sg isthe initial acoustic transit time in reactive powder concretewhen micro-crack occurred. In the AB stage, the micro-crackappeared and expanded with the increase of deflection and theelectrical resistance of MWCNTs/PDMS nanocomposite wasalso increased.

In order to further research the sensitivity of MWCNTs/PDMS nanocomposite in monitoring the deflection of reactivepowder concrete under bending load, the relationship betweenresistance change of MWCNTs/PDMS nanocomposite anddeflection of reactive powder concrete was fitted by theexponential function (2), the pink curve showed in Fig. 5 wasthe fitting fuction.

�R/R0 = A1 + B1 exp((δ − C1)/D1) (2)

Where A1, B1, C1 and D1 are coefficients for the expo-nential curves, �R/R0is the resistance change of MWCNTs/PDMS nanocomposite (in %), δ is the deflection of reactivepowder concrete. Fig. 5 and Table 4 demonstrated that thedeflection of reactive powder concrete under the three pointbending test could be predicted from the exponential equationeasily. The results here indicated that the MWCNTs/PDMSnanocomposites as smart sensing materials could monitor thedeflection of reactive powder concrete when subjected to thebending load.

The deflection, load of reactive powder concrete andthe electric resistance of MWCNTs/PDMS nanocompos-ite were simultaneously collected until to the fracture of

ZHANG et al.: MONITORING OF COMPRESSION AND BENDING PROCESS OF RPC 6157

Fig. 4. Resistance change of MWCNTs/PDMS nanocomposite versus compressive strain of reactive powder concrete under axial compressive loading.The pink line is the linear fitting curve.

Fig. 5. Resistance change of MWCNTs/PDMS nanocomposite versus deflection of reactive powder concrete under three point bending test. The pink lineis the fitting curve.

nanocomposite sample. Fig. 6 showed the stress and resistancechange of nanocomposite with the increase of deflection. It isobserved that the resistance of MWCNTs/PDMS nanocompos-ite increased quickly in the BC stage after the obviously visiblecrack occurred. A macroscopic crack was found at the point Bin the bottom surface of reactive powder concrete sample.At the point C, MWCNTs/PDMS nanocomposite specimenwas fractured because of the propagation of crack of reactive

powder concrete sample, the range of crack width was from1.22 mm to 2.70mm.

In order to further research the sensitivity of MWCNTs/PDMS nanocomposite in monitoring the deflection of reac-tive powder concrete from maximal stress to the fractureof nanocomposite under bending load, the exponential func-tion (3) was also fitted by the relationship between resistancechange of MWCNTs/PDMS nanocomposite and deflection of

6158 IEEE SENSORS JOURNAL, VOL. 17, NO. 19, OCTOBER 1, 2017

Fig. 6. Resistance change of MWCNTs/PDMS nanocomposite versus deflection of reactive powder concrete under three point bending test. The pink lineis the fitting curve.

TABLE IV

PARAMETERS AND R-SQUARES OF FITTING EUQATIONS

TABLE V

PARAMETERS AND R-SQUARES OF FTTING EUATIONS

TABLE VI

COMPARISON OF SENSORS WITH DIFFERENT TYPES

FOR DEFORMATION OF CONCRETE

reactive powder concrete, the pink curve showed in Fig. 6 wasthe fitting fuction.

�R/R0 = A2 + B2 exp((δ − C2)/D2) (3)

where A2, B2, C2and D2 are coefficients for the exponentialcurves. Fig. 6 and Table 5 demonstrated that the deflection of

reactive powder concrete during binding could be predictedfrom the exponential equation easily. The results here alsoindicated that the MWCNTs/PDMS nanocomposites as smartsensing materials could monitor the deflection of reactivepowder concrete after the maximal bending load.

From previous studies, the commonly used sensors areoptical fiber sensors (OFS) [27], [28], cement/carbon nan-otubes composite (CCNTC) [29], [30], cement/carbon fibercomposite (CCFC) [31], cement/nickel composite (CNC) [32]and so on. The comparisons of these sensors were summarizedin Table VI. Optical fiber sensor has been widely used instructural health monitoring because its outstanding proper-ties, but its cost is very high. Piezoresistive cement-basedcomposite hold a restrict problem is the dispersion of fillerin cement matrix, and its cost is also high. MWCNTs/PDMScomposite in this paper is low cost, shows linear responseand exponential response to compressive strain and deflectionof reactive powder concrete. Furthermore, MWCNTs/PDMScomposite could be as a smart film stick to the surfaceof concrete structure to achieve the long term monitoring.In addition, large/huge deformation happens to local concreteduring the compression/deflection even the deformation ofwhole concrete is small. Therefore, MWCNTs/PDMS showsenormous advantages due to its great deformation capacity,low cost and high durability.

IV. CONCLUSION

MWCNTs/PDMS nanocomposites were fabricated as flex-ible sensors through a simple solution casting method. Thesensitivity and their applications in monitoring compressionand bending of reactive powder concrete were experimentallystudied. The results show that MWCNTs/PDMS nanocompos-ite displayed significant resistance change under large defor-mation, and cyclic loadings of tensile strain yielded repeatable

ZHANG et al.: MONITORING OF COMPRESSION AND BENDING PROCESS OF RPC 6159

resistive responses. The resistance change of MWCNTs/PDMSnanocomposite showed good linear relationship with compres-sive strain of reactive powder concrete in the range of elasticdeformation. According to the fitting exponential function atthe AB stage and BC stage, the deflection of reactive powderconcrete could be predicted by the numerical equation fromthe electric resistance change of MWCNTs/PDMS nanocom-posite. Therefore, all of these experimental results proved thehighly potential application of MWCNTs/PDMS nanocom-posites as flexible sensor in structural health monitoring ofreinforced concrete structures.

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Yongquan Zhang received the B.S. degree from Qiqihar University in2007, and the M.S. degree from the Department of Materials Science andEngineering, Harbin Institute of Technology, in 2011, where he is currentlypursuing the Ph.D. degree with the School of Transportation Science andEngineering. His current research interests include thin films, flexible sensorsand their applications in concrete.

Yong Ge received the B.S. and M.S. degrees from the Harbin ConstructionEngineering Institute in 1982 and 1986, respectively, and the Ph.D. degreefrom the Department of Materials Science and Engineering, Harbin Instituteof Technology, Harbin, China, in 2009. He is currently a Professor withthe School of Transportation Science and Engineering, Harbin Institute ofTechnology. He has expertise in high performance concrete, constructionfunctional materials, and green building materials. He holds five projectsfunded from the National Science Foundation and the Ministry of Transportof China.

Yuanbo Du received the B.S. and M.S. degrees from the Department of CivilEngineering, Shijiazhuang Tiedao University, in 2011 and 2014, respectively.He is currently pursuing the Ph.D. degree with the School of TransportationScience and Engineering, Harbin Institute of Technology. His current researchinterests include the thermal properties of concrete.

Lili Yang received the B.S., M.S., and Ph.D. degrees in materials sciencefrom the Harbin Institute of Technology in 2003, 2005, and 2009, respectively.Then, from 2010 to 2016, she was an Associate Professor with the School ofTransportation Science and Engineering, Harbin Institute of Technology. Sheis currently an Associate Professor with Donghua University, Shanghai. Hermain research interest includes the preparation of carbon-based nanocompos-ites in sensors, energy storage.

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