Detecting crack and damage location in self-sensing fiber reinforced cementitious composites

Construction and Building Materials 240 (2020) 117973

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Detecting crack and damage location in self-sensing fiber reinforced cementitious composites Huy Viet Le a,b, Dong Joo Kim a,⇑ a b

Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143–747, South Korea Department of Civil Engineering, Hanoi University of Mining and Geology, Hanoi, Viet Nam

h i g h l i g h t s
The location of the crack can be determined by using Multi-channel DC measurement.
The electrical resistivity in the cracked part generated a noticeable decrease.
The reduction in the electrical resistivity was sensitive to the location of the crack.

a r t i c l e

i n f o

Article history: Received 12 August 2019 Received in revised form 16 December 2019 Accepted 27 December 2019

Keywords: Detect crack Crack location Self-sensing Fiber reinforced cementitious composite Electrical resistivity

a b s t r a c t This paper investigated the feasibility of using direct current (DC) measurements for detecting the cracking and identifying the location of cracks in self-sensing fiber-reinforced cementitious composites (SSFRCCs) based on the electrical response of SS-FRCCs. The electro-mechanical response of SS-FRCCs with three different types of specimens (with a cold joint, a double-edged notch, and two double-edged notches) under direct tension is investigated. The test results indicated that the location of the crack (one-crack or multi cracks or main crack opening) could be notably determined by measuring the electrical resistivity of SS-FRCCs using two or multi-channel DC measurements. Under direct tension, the SSFRCCs produced a clear reduction in electrical resistivity of cracked zone, but not in that of the un-cracked zone. Moreover, the amount of reduction in the electrical resistivity of cracked zone decreased as the location of the crack was close to the inner electrode. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction There has been significant interest in structural health monitoring systems since 1990s owing to the increasing frequency of catastrophic structural collapses, e.g., the collapse of the Silver Bridge in the United States (1967), Seongsu Bridge in South Korea (1994), and Qijiang Rainbow Bridge in China (1998). The collapses of civil infrastructure and buildings result in significant damages to the human society, in the form of both human deaths and property damage. Abbreviations: CJ, cold joint; DC, direct current; DE, double-edged; ECC, engineered cementitious composite; HPFRCC, high performance fiber-reinforced cementitious composite; LCJ, long specimen with a cold joint; LVDT, linear variable differential transformer; PVA, polyvinyl alcohol; SFRC, steel fiber reinforced cementitious composite; SS-FRCC, self-sensing fiber-reinforced cementitious composite; UHPC, ultra-high-performance concrete; UHPFRC, ultra-high-performance fiber-reinforced concrete; UTM, universal testing machine. ⇑ Corresponding author. E-mail address: [email protected] (D.J. Kim). https://doi.org/10.1016/j.conbuildmat.2019.117973 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

In order to prevent the catastrophic collapse of structures or structural systems, the safety of infrastructure and buildings are carefully investigated by performing periodic inspection, repair, and maintenance [1]. In particular, to monitor the status of structural systems and to detect any abnormal responses, structural health monitoring systems have been applied to important civil infrastructures and buildings. Current monitoring systems generally utilize attached or embedded sensors to detect damages and cracks in structural systems [2–9]. However, the attached or embedded sensors have relatively low durability in comparison to the life of the infrastructure involved, and require thus frequent exchange, which eventually leads to higher costs. To overcome the limitations associated with monitoring systems based on attached or embedded sensors, various smart construction materials with self-sensing capabilities have been intensively investigated during the last decade [10–33]. Smart construction materials with self-sensing capabilities have demonstrated the ability to sense stress/strain, damage, and crack

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formation by sensing the changes in electrical resistivity under mechanical loads [10,11]. Carbon fiber reinforced composites [12–19], cement based composites including functional nanofillers [20–27], and self-sensing fiber-reinforced cementitious composites (SS-FRCCs) [28–37] have all been shown to exhibit such self-sensing capacity. The application of smart construction materials is expected to overcome the limitations of current monitoring system since no additional sensors with limited durability are required. Among the various smart construction materials developed to date, SS-FRCCs are of particular interest as they are able to sense their damage even after the first cracking, whereas other materials typically lose their sensing capability after cracking. SSFRCCs have displayed a very high tensile strength and strainhardening behavior, and these characteristics are accompanied by the generation of multiple micro-cracks [28–33,38,39]. The self-sensing capabilities of SS-FRCCs have recently been reported by several researchers, including Lin et al. [28], Ranade et al. [29], Nguyen et al. [30], Song et al. [31], Kim et al. [32], and Le et al. [33]. Generally, the electrical resistivity of SS-FRCCs starts to change as the tensile strain of SS-FRCCs increases, and this change is accompanied by the formation of multiple microcracks. Among the SS-FRCCs investigated to date, engineered cementitious composites (ECCs) with electrically non-conductive polyvinyl alcohol (PVA) fibers have been shown to generate an increase in electrical resistivity corresponding to the generation of multiple micro-cracks during the tensile strain hardening response [28,29]. By contrast, high performance fiber-reinforced cementitious composites (HPFRCCs) and ultra-high-performance fiber-reinforced concretes (UHPFRCs) containing electrically conductive steel fibers produce a reduction in electrical resistivity during the strain hardening response and result in the formation of multiple microcracks [30–33]. The mechanism underlying the reduction in electrical resistivity of steel fiber-reinforced cementitious composites (SFRCs) via matrix cracking was explained in Nguyen et al. [30] and Song et al. [31]. Nguyen et al. [30] and Song et al. [31] investigated the effects of fiber type and fiber volume contents on the self-sensing capabilities of HPFRCCs containing steel fibers. Kim et al. [32] evaluated recently the self-sensing capability of UHPFRCs with highly dense ultra-high performance concrete (UHPC) matrix. The source of change in electrical resistivity mediated by matrix cracking was investigated by Le et al. [33]. Although several studies have recently reported the self-sensing capability of SS-FRCCs, reports that focus on determining the location of cracks and damages by measuring electrical resistivity remain very limited. Hou et al. [35] reported that electrical impedance tomography can be used to detect and localize the damage in a polymeric fiber reinforced cementitious composite. However, in the study by Hou et al. [35], the application of electrical impedance tomography required the use of finite element analysis in order to determine the damage location. Downey et al. [26,27] recently proposed an innovative biphasic DC measurement approach, with significantly reduced polarization time, to damage localization in cementitious composites containing multi-walled carbon nanotubes. In addition, effect of different crack location in a gauge length on the electrical resistivity response of SS-FRCCs under tension has not been investigated. The main aim of the present study is to investigate the feasibility of using multichannel DC measurements as a tool for detecting the location of cracking in SS-FRCCs and the effects of crack location on the electrical resistivity response of SS-FRCCs. The specific objectives are 1) to investigate the effect of crack location within the gauge length on the changes in electrical resistivity of SSFRCCs using single channel DC measurement, and 2) to detect the location of pre-designated or random cracks using multichannel DC measurements.

2. Typical electro-mechanical response of SS-FRCCs containing steel fibers The electro-mechanical response of SS-FRCCs depends significantly on the matrix cracking and the number of multiple microcracks formed [30–33]. Fig. 1a shows the typical electromechanical response of SS-FRCCs, which is accompanied by the formation of multiple micro-cracks [30–32]; while Fig. 1b shows the response observed for an SS-FRCC with one cold joint (1CJ) and a single pre-engineered crack under direct tension [33]. It is clear from Fig. 1a that as the tensile strain increases from 0 to ecc at the first cracking point, the tensile stress increases linearly from 0 to rcc, while the electrical DC resistivity decreases slightly from qo to qcc. After the first cracking point, the tensile stress increases further to rpc (post cracking tensile strength) and the electrical resistivity decreases markedly to the minimum electrical resistivity value (qmin). The reduction in electrical resistivity during the strain hardening response of SS-FRCCs is clearly correlated with the number of cracks [30–32]. By contrast, it is apparent from Fig. 1b that the electromechanical response of SS-FRCCs with a single pre-designated crack can be divided into three distinct stages: elastic stage, fiber debonding stage, and fiber pullout stage [33]. In the elastic stage, i.e., the period prior to the cracking point, the bonding between the fibers and the matrix can be characterized as linear elastic, and the electrical resistivity remains virtually unchanged. In the fiber debonding stage (i.e., after the cracking point), the electrical resistivity decreases significantly to the minimum value of electrical resistivity, and the embedded fibers are fully debonded. Finally, in the fiber pullout stage, the electrical resistivity starts to increase after the complete fiber debonding, and the fiber pullout load is governed only by friction at the interface between the fibers and matrix. The reduction in electrical resistivity of SS-FRCCs incorporating a crack depends significantly on the fiber–matrix debonding process after matrix cracking [33,37]. The presence of conductive steel fibers bridging the crack generated the reduction in the electrical resistivity of specimens. According to the theoretical model proposed by Song [31] for the electrical resistance of SS-FRCCs after matrix cracking, the total electrical resistance of SS-FRCCs can be divided to two parts including the un-cracked region (composite) and cracked region (steel fibers bridging the crack). At the cracked region, the electrical current would only transfer through the debonded steel fibers. Thus, after matrix cracking, the electrical resistance of the cracked part (conductive steel fibers) was significantly lower than that of un-cracked part (composite); consequently, the total resistance of specimens within the gauge length would decrease. However, nonconductive fibers bridging a crack would result in an opposite trend. When the electrical resistivity of specimens is measured using a DC multimeter with four-probes during tension, a direct current passes through the two outer electrodes and voltage is measured between the two inner electrodes. The electrical resistivity (q) of the specimens is calculated from the measured electrical resistance (R) using Eq. (1):

q¼R

A L

ð1Þ

where A is the cross-sectional area and L is the gauge length between the two inner electrodes. The maximum reduction in electrical resistivity (Dqmax, calculated as qoqmin) and the maximum percentage reduction in electrical resistivity (calculated as Dqmax/ qo.100,%) are determined to evaluate the electrical resistivity response of the composites under tension.

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3

Fig. 1. The typical electromechanical behavior of SS-FRCCs.

cracks position, and 2DE specimens for detecting random-multicracks and main crack opening position.

3. Materials and methods Fig. 2 shows the series of experiments designed to investigate the effects of crack or damage location on the electrical resistivities of SS-FRCCs under direct tension. Specifically, one or two DC multimeters were applied simultaneously to the specimens to identify the location of cracks by measuring the electrical resistivity of SSFRCCs under direct tension. Fig. 3 shows the four types of specimens employed: (i) long specimen with one cold joint (LCJ), (ii) specimen with one cold joint (1CJ), (iii) specimen with one double-edged notch (1DE), and (iv) specimen with two doubleedged notches (2DE). One-cold joint specimens (LCJ and 1CJ) were designed to create a crack at the cold joint position under tension whereas specimens with double-edged notch (1DE and 2DE) were designed to generate multi-cracks at double-edged notch position under tension. The LCJ specimens (Fig. 3a) were prepared to investigate the effects of crack location on the electrical resistivity by changing the cold joint position: the distance from pre-designed crack position (cold joint position) to the inner electrode of P5, P15, P25, and P35 specimens was 5, 15, 25, and 35 mm, respectively. The 1CJ, 1DE, and 2DE specimens (Fig. 3b, c, and d, respectively) were prepared to investigate the feasibility of determining the location of crack using multiple DC multimeters simultaneously or multichannel DC measurements: 1CJ specimens for detecting one crack position, 1DE specimens for detecting multi-

3.1. Materials and specimen preparation Table 1 shows the composition of the UHPC matrix (reported as weight ratios), and Table 2 summarizes the properties of the smooth steel fibers. Cement Type I (according to ASTM standard) and polycarboxylate based superplasticizer (30% solid content and 70% water) were used. The silica sand had an average diameter of 0.2 mm. Two types of steel fibers including long smooth steel fibers (30 mm in length and 0.3 mm in diameter) and short smooth steel fibers (6 mm in length and 0.2 mm in diameter) were used in this study. Long smooth steel fibers were used as functional fillers in the UHPC matrix. They were embedded at the pre-designated crack position of cold joint specimens (LCJ and 1CJ in Fig. 3a and b) and were randomly distributed in the matrices of 1DE and 2DE specimens (Fig. 3c and d). Short smooth steel fibers (2 vol%) were used only in the cold joint specimens (LCJ and 1CJ in Fig. 3a and b) to reinforce their matrices for preventing cracking of the matrix, with the exception of the pre-designated crack position [33]. Thus, under tension, the crack in the cold joint specimens (LCJ and 1CJ) would be generated at the cold joint position since the tensile resistance of cold joint position (containing 49 long smooth fibers, which was equivalent to an approximately 1 vol%)

Fig. 2. Experimental program.

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Fig. 3. Designed specimens.

Table 1 Composition of matrix by weight ratio and compressive strength. Notation

Cement (Type)

Silica fume

Silica powder

Silica sand

Water

Super plasticizer

Strength (MPa)

UHPC

1 (Ι)

0.25

0.3

1.1

0.2

0.067

180

Table 2 Properties of high strength steel fibers. Fiber type

Diameter (mm)

Length (mm)

Tensile strength (MPa)

Elastic modulus (GPa)

Long smooth (LS) fiber Short smooth (SS) fiber

0.3 0.2

30 6

2428 2104

200 200

was significantly weaker than that of the other parts (containing 2 vol% short smooth fibers). The cross section of each specimen in the gauge length measurements was 25 mm  25 mm. Fig. 3a and b show the geometries of one-cold joint specimens (LCJ specimen with 525 mm in length and 1CJ specimen with 190 mm in length). Specimens with one cold joint (LCJ and 1CJ) were prepared using the procedure reported by Le et al. [33]. The preparation of the specimens involved two different mixings: one half of the specimens (left/right part in Fig. 3a or b) was first cast and subsequently cured in a hot water tank at 90 °C for 3 days; and then the second half of specimens was prepared. Two parts of each specimen were connected through embedded fibers (49 long smooth fibers) and bond of mortars of two parts. The section between two parts is notated as ‘cold joint position’. The cold joint position of different series (P5, P15, P25, and P35 specimens) was adjusted by changing the position of the embedded fibers in the molds. The foam sheets and PVC plate were used to align and fix position of fibers in the mold during mixing and were removed during demolding. The detail of embedded fibers using the foam sheets and PVC plate can be found in Ref. [33]. The matrix of each specimen contained 2.0 vol% short smooth fibers (6 mm in length). Fig. 3c and d show the geometries of 1DE and 2DE specimens, respectively. Specimens 1DE and 2DE were prepared using an UHPC matrix containing 1% of long smooth steel fibers by volume. The preparation of both 1DE and 2DE specimens required only a single mixing step. Single or double notches (8 mm in depth) were made in these specimens as shown in Fig. 3c and d.

A Hobart type laboratory mixer with 20-L capacity was used in preparing the UHPC mixtures. In each case, silica fume and silica sand were first dry-mixed for 5 min, and subsequently cement and silica powder were added to the mixture, which was stirred for further 5 min. Water and super-plasticizer were added next and the mixture was stirred for 5 to 10 min. Since the mortar showed suitable workability and viscosity, the fibers were dispersed carefully by hand into the mortar mixture and mixed for 1 min. Short smooth steel fibers were utilized in both LCJ and 1CJ specimens, whereas long smooth steel fibers were used in 1DE and 2DE specimens. The appropriate mortar mixture containing the fibers was poured into the molds. All specimens were covered with plastic sheets and stored at room temperature (20 ± 2 °C) for 48 h prior to demolding. After demolding, the specimens were cured in water at 90 °C for 3 days. 3.2. Test setup Fig. 4 shows the set-up employed for measuring the electrical resistance of LCJ specimens using a DC multimeter in order to investigate the effects of different crack locations on the measured electrical resistivity. The uniaxial tensile tests were carried out using a universal testing machine (UTM) with 2943 kN (300 tf) capacity, while the electrical resistivity during the tests was measured using a Fluke 8846A (FL) multimeter. The speed of displacement was maintained at 1.0 mm/min during the tests. During the tests, the load was obtained from a load cell with a capacity of 98.1

H.V. Le, D.J. Kim / Construction and Building Materials 240 (2020) 117973

5

Fig. 4. A multimeter measurement test set-up investigated effect of crack position.

kN (10 tf) attached to the top of the specimens, while the elongation was obtained by averaging the signals from two linear variable differential transformers (LVDTs). Fig. 5 shows the set-up utilized for measuring the electrical resistance of 1CJ, 1DE, and 2DE specimens using two different DC multimeters simultaneously. The tensile tests were performed using an electro-mechanical UTM with a capacity of 4905 N (500 kgf), and the speed of displacement was maintained at 1.0 mm/ min during the tests. The elongation was measured using an LVDT attached to the upper grip of the set-up, while the load was measured using the load cell with 4905 N (500 kgf) capacity. Two DC multimeters (Fluke 8846A and Keysight 3458A) were used to measure the electrical resistivity of both the cracked and crack-free parts of specimens. The Fluke 8846A multimeter was used to measure the electrical resistivity of part F (lower part in Fig. 5) containing the cold joint in 1CJ specimens, and double-edged notch in specimens 1DE and 2DE. By contrast, a Keysight 3458A (KS) multimeter was used to measure the electrical resistivity of part K (upper part in Fig. 5) without the cold joint in 1CJ, without doubleedged notch in 1DE, and with other notch in 2DE specimens. Prior to tests, the electrical resistivity of the specimens was stabilized at least for 30 min to minimize the polarization effect [30]. Each multimeter employed four probes to measure the electrical resistivity of specimens by minimizing the contact resistance between electrodes and self-sensing concrete [40]. Electrodes were made on the surfaces of the specimens using copper tape and silver paint. The positions of electrodes on the surfaces of the specimens were shown in Figs. 4 and 5.

4. Results and discussion 4.1. The effect of crack location on self-sensing capability of SS-FRCCs The effect of crack location on changes in electrical resistivity was investigated using SS-FRCCs containing a LCJ. To this end, Fig. 6 shows the effect of crack location on the changes in electrical resistivity observed during testing, while Fig. 7a displays the maximum percentage reduction in electrical resistivity of LCJ specimens observed as a function of crack position. The distance from the designated crack position (CJ) to an inner electrode was varied systematically from 5 to 35 mm (P5 to P35) while the gauge length between two inner electrodes was maintained at 70 mm. The P35 specimens had a crack position that was at the very center of the gauge. As summarized in Table 3, the initial electrical resistivity of all specimens was quite similar, irrespective of the crack positions: 794.8, 774.6, 772.5, and 812.8 kX-cm for specimens P35, P25, P15, and P5, respectively. The slight difference in the initial electrical resistivity of these specimens could be due to the different crack location and/or heterogeneous property of matrices. The maximum percentage reduction in electrical resistivity determined during testing was found to be different depending on the location of the crack within gauge length, although all specimens generally exhibited a reduction in electrical resistivity during testing, as shown in Fig. 6. The maximum percentage reduction in electrical resistivity was the highest when the crack was located at the center of the gauge length, and it decreased progressively as the location of the crack approached the inner

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Fig. 5. The two multimeters measurement test set-up.

electrode. As shown in Table 3, as the distance between the crack location and the inner electrode decreased from 35 mm (P35 specimen) to 5 mm (P5 specimen), the average maximum percentage reduction in electrical resistivity decreased from 6.5% to 2.0% while the average maximum reduction in electrical resistivity decreased from 50.4 kX-cm to 16.2 kX-cm, respectively. The different reduction in the electrical resistivity could be attributed to the correlation between the crack location and the debonded fiber length within the gauge length. Le et al. [33] reported that the reduction in the electrical resistivity of SS-FRCCs was occurred owing to the change in electrical resistivity from the high electrical resistivity of composite to the low electrical resistivity of debonded steel fibers at the cracked zone. This reduction was significantly influenced by the debonded fiber length in the fiber–matrix interface at the cracked zone: as the debonded fiber length within the gauge length increased, the reduction in electrical resistivity obviously increased. In this study, the debonded fiber length in fiber–matrix interface of P5 specimen within the gauge length (between two inner electrodes) was clearly shorter than that of P35 specimen. As can be seen in Fig. 7b, a part of debonded fiber length of the P5 specimen was within two inner electrodes while other part was outside whereas the debonded fiber length of the P35 specimen was fully within two inner electrodes. The shortened debonded fiber length within the gauge length generated the lower reduction in electrical resistivity of P5 specimen. These test results confirm that the self-sensing capability of SS-FRCCs is sensitive to the location of the crack. Specifically, as the crack location was close to the inner electrode, the amount of reduction in electrical resistivity decreased. Hence, based on this investigation, the source of change in the electrical resistivity of SS-FRCCs under tension could be more clearly understood. Despite this information, however, designating the crack location within the gauge based on the obtained the amount of reduction in electrical resistivity using DC multimeter measurements still remains challenging.

4.2. The electrical resistivity of SS-FRCCs obtained from two different DC multimeters Fig. 8 compares the electrical polarization of SS-FRCCs (1CJ specimens) measured using a single multimeter and two multimeters simultaneously, while Table 4 reports the values of initial electrical resistivity (qo) of 1CJ, 1DE, and 2DE specimens. It is apparent from Fig. 8 that using two multimeters simultaneously had negligible effect on the results. The curves (empty circles and squares) were obtained using both FL and KS multimeters simultaneously while the curves (solid circles and squares) were measured using single multimeter. The electrical resistivity curves obtained from two multimeters were found to be very similar to the curves measured from single multimeter. In all measurements, the polarization time of the specimens was about 30 to 40 min. The electrical resistivity measured using a single multimeter or two multimeters simultaneously was slightly different initially, and the differences became less apparent after the polarization time was reached. For instance, after 30 min of polarization, the values of initial electrical resistivity (qo) measured using a single multimeter and two multimeters simultaneously were 1093.0 and 1067.9 kX-cm for part F, and 2283.8 and 2266.9 kX-cm for part K, respectively. Table 4 shows the effects of using different multimeters on the measured values of electrical resistivity of 1CJ, 1DE, and 2DE specimens. The initial electrical resistivity of part K of 1CJ and 2DE specimens measured using a KS multimeter was clearly higher than that of part F measured by the FL multimeter. The higher initial electrical resistivity of part K of 1CJ and 2DE specimens could be attributed to the different current sources (10 lA vs. 5 lA) of the FL and KS multimeters, respectively. In addition, the higher initial electrical resistivity of part K of 1CJ specimens could be caused by the lower conductivity of the matrix. The part F incorporates 2% of short smooth fibers by volume and 49 long smooth fibers embedded at

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a) P5-sp1

b) P15-sp2

c) P25-sp3

d) P35-sp3

Fig. 6. Effect of different crack positions on the electrical resistivity response of specimens with a cold joint.

Fiber debonding part-measured

25

30

35

Crack position 70

Inner electrode

P35 specimen Fiber debonding part-measured

25

20

5

Crack position

Inner electrode

70 P5 specimen

a) Percentage reduction in electrical resistivity

b) Fiber debonding part in the gauge length

Fig. 7. Effect of crack position on self-sensing ability of SS-FRCCs.

crack position, while part K contains 2% of short smooth fibers by volume only, and no long smooth fibers. Thus, the conductivity in part F was clearly higher than that in part K. In other words, the elec-

trical resistivity of part K measured using the KS multimeter was higher than that of part F determined using the FL multimeter. However, the electrical resistivity of part F in 1DE specimens was higher

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Table 3 Effect of crack position on the electrical resistivity of SS-FRCCs. Notation LCJ-P5 Sp1 Sp2 Sp3 Average LCJ-P15 Sp1 Sp2 Sp3 Average LCJ-P25 Sp1 Sp2 Sp3 Average LCJ-P35 Sp1 Sp2 Sp3 Sp4 Average

qo (kO-cm)

qmin (kO-cm)

Dqmax (kO-cm)

Dqmax/qo.100 (%)

Pmax (N)

855.4 800.2 819.4 825.0

841.9 795.1 789.5 808.8

13.5 5.1 29.9 16.2

1.6 0.6 3.7 2.0

1998.3 1986.2 1690.1 1891.5

802.2 717.3 747.0 755.5

791.8 690.4 720.1 734.1

10.4 26.9 26.8 21.4

1.3 3.7 3.6 2.9

1826.4 1822.3 1842.3 1830.3

798.9 785.3 814.2 799.5

776.7 755.5 778.7 770.3

22.1 29.8 35.5 29.1

2.8 3.8 4.4 3.7

1827.5 1745.8 1732.5 1768.6

878.2 740.7 787.3 773.0 794.8

860.3 660.2 731.5 725.6 744.4

17.8 80.5 55.8 47.4 50.4

2.0 10.9 7.1 6.1 6.5

1719.7 2013.2 2196.2 2337.5 2066.6

4.3. Electro-mechanical response of 1CJ specimens

2800

Single multimeter - part K

2400 2000

Simultaneous two multimeters - part K 1600

Single multimeter - part F

1200 800

Simultaneous two multimeters - part F 400 0 0

5

10

15

20

25

30

35

40

Time (min) Fig. 8. Effects of using two multimeters on the electrical resistivity of specimens with a cold joint (1CJ).

than that of part K, as shown in Table 4, although the current application in part F (measured using FL multimeter) was higher than that of part K (using KS multimeter). In the 1DE specimens, both parts F and K contained the same matrix composition (the matrix contained 1% of conductive long smooth fibers by volume). The presence of a double-edged notch in part F reduced not only the cross section but also the number of conductive fibers. Thus, it led to a decrease in the conductivity of the matrix, or, in other words, to an increase in the electrical resistivity of part F.

Fig. 9 shows the electrical resistivity measured using two DC multimeters simultaneously as a function of the slip and the tensile load of one cold joint specimen (1CJ-sp2). Fig. 9a shows the electrical resistivity of part F containing a cold joint, and Fig. 9b displays the resistivity of the joint-free part K. During the measurements, crack formation was observed at the cold joint in part F but, as expected, no crack was observed in part K. As can be seen in Fig. 9, the responses of 1CJ specimens with a crack could be divided into three stages: elastic, fiber debonding, and fiber pullout stages, as described previously in Fig. 1b. As shown in Table 5, the average value of tensile load at the first cracking point (Pcc) was 1091.5 N, while the maximum tensile load was 2328.7 N. The average values of elongation and tensile load at the full debonding point were 2.5 mm and 2068.8 N, respectively. Fig. 9a and 9b show clearly the differences in electrical resistivity responses between two parts of specimens—i.e., part F with a crack and part K without a crack. As can be seen from Fig. 9a, the resistivity of part F exhibited the three stages that are characteristic for SS-FRCCs with a crack. Specifically, the electrical resistivity was almost constant in the first stage, decreased markedly in the second stage, and gradually increased after the full debonding of fibers in the third stage. By contrast, the resistivity of part K changed very little during the testing, as can be observed in Fig. 9b. As summarized in Tables 4 and 6, the average values of qo, qcc, qmin, and q5 (the electrical resistivity of a point 5 mm in the slip) were 1161.5, 1161.8, 1129.5, and 1221.3 kX-cm for part F, and 2456.5, 2458.4, 2451.4, and 2463.7 kX-cm for part K, respectively. The maximum percentage reduction in electrical resistivity of part F

Table 4 Two multimeters test result 1. Specimens

The initial electrical resistivity (qo) of SS-FRCCs specimens (kO-cm) One cold joint (1CJ)

Sp1 Sp2 Sp3 Average

A double-edged notches (1DE)

Two double-edged notches (2DE)

Part F

Part K

Part F

Part K

Part F

Part K

1194.8 1072.5 1217.2 1161.5

2483.4 2481.8 2404.4 2456.5

1379.3 1222.3 936.0 1179.2

1028.1 917.1 670.7 872.0

1131.1 1137.7 1188.6 1152.5

1727.9 1721.3 1880.0 1776.4

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a) Part F – cracked region

b) Part K – uncracked region

Fig. 9. Electromechanical response of specimens with a cold joint (1CJ-sp2) measured by using two multimeters.

Table 5 Two multimeters test result 2. Notation

Tensile load (N)

Slip (mm)

Pcc One cold joint specimens (1CJ) Sp1 1514.4 Sp2 1005.7 Sp3 754.5 Average 1091.5 A double-edged notch specimens (1DE) Sp1 1188.4 Sp2 1034.0 Sp3 1206.5 Average 1142.9 Two double-edged notches specimens (2DE) Sp1 1243.5 Sp2 1149.5 Sp3 769.3 Average 1054.1

Number of crack (op)

Pmax

Pdb

Dcc

DM

Ddb

Part F

Part K

2711.7 1973.4 2311.3 2328.7

2515.5 1617.8 2073.1 2068.8

1.5 0.7 0.6 0.9

2.7 1.5 1.4 1.9

2.8 2.4 2.2 2.5

1 (op) 1 (op) 1 (op) 1

0 0 0 0

3173.9 2366.7 3129.6 2890.1

2296.1 2250.6 2314.7 2287.1

1.4 0.5 0.5 0.8

1.7 1.4 1.3 1.5

3.2 1.7 1.5 2.1

3 (op) 2 (op) 3 (op) 2.7

0 0 0 0

2739.5 1802.4 2367.5 2303.1

2538.9 1790.1 2267.7 2198.9

0.3 0.4 0.4 0.4

1.3 1.4 1.3 1.3

1.5 1.4 1.5 1.5

1 1 1 1

1 (op) 1 (op) 1 (op) 1

(Pcc, Dcc), (Pmax, DM), (Pdb, Ddb): load and slip values at a first cracking, maximum tensile load, and minimum electrical resistivity points, correspondingly Part F: using Fluke 8846A multimeter Part K: using the Keysight 3458A multimeter; (op): crack opening at this part.

Table 6 Two multimeters test result 3. Notation

qcc (kO-cm) Part F

qmin (kO-cm) Part K

One cold joint specimens (1CJ) Sp1 1199.9 2487.4 Sp2 1070.7 2483.4 Sp3 1214.8 2404.3 Average 1161.8 2458.4 A double-edged notch specimens (1DE) Sp1 1211.3 1093.6 Sp2 1216.4 937.0 Sp3 902.3 649.2 Average 1110.0 893.3 Two double-edged notches specimens (2DE) Sp1 1132.0 1747.0 Sp2 1140.0 1786.2 Sp3 1187.7 1912.9 Average 1153.2 1815.4

q5 (kO-cm)

Dqmax/qo.100 (%)

Part F

Part K

Part F

Part K

Part F

Part K

1179.4 1017.9 1191.1 1129.5

2480.4 2480.4 2393.4 2451.4

1341.8 1080.4 1241.6 1221.3

2499.4 2495.5 2396.2 2463.7

1.3 5.1 2.1 2.8

0.1 0.1 0.5 0.2

820.2 1057.7 475.2 784.3

1021.2 885.0 622.4 842.8

893.2 1767 865 1175.1

1036.8 916.3 631.9 861.7

40.5 13.5 49.2 34.4

0.7 3.5 7.2 3.8

1111.6 1088.3 1042.9 1080.9

1593.4 1632.4 1745.9 1657.2

1147.9 1108.2 1093.0 1116.4

1927.6 2145.9 1955.1 2009.5

1.7 4.3 7.3 4.4

7.8 5.2 8.2 7.1

Dqmax = qo  qmin. qcc, qmin, and q5: the resistivity at a first cracking, minimum electrical resistivity, and 5 mm in slip points, correspondingly.

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was 2.8%; the maximum percentage reduction of part K was significantly lower, at 0.2%. The different electrical resistivity responses of different parts (F and K) of 1CJ specimens can be attributed to the different mechanical responses of these parts during the tests. Prior to matrix cracking (elastic region), the elongation was essentially identical in all parts of the specimens (F and K). In the elastic region, the electrical resistivity of both the F and K parts of the specimens changed only slightly. However, after the matrix cracking, the elongation and tensile stress of the specimen were clearly dependent on the crack location. As the tensile load increased, the fibers in part F (i.e., the cracked part) were debonded and pulled out from matrix, whereas part K (i.e., the crack-free part) was still in the elastic stage. Thus, the electrical resistivity response of part F changed in accordance with the debonding and pullout stages, whereas the electrical resistivity of part K remained in the elastic stage—i.e., the first stage in the typical electrical resistivity response of a SS-FRCC with a crack (Fig. 1b). Hence, these results demonstrate that the application of two DC multimeters (or multichannel DC measurement) allows the detection of crack location based on the differences in electrical resistivity responses at different parts during testing. 4.4. Electro-mechanical response of 1DE specimens Fig. 10 shows the electro-mechanical responses of different parts (F and K) of a double-edged notch specimen (1DE-sp2) determined using two DC multimeters simultaneously. Fig. 10a shows the electro-mechanical response of part F containing a doubleedged notch, whereas Fig. 10b shows the response of part K without the double-edged notch. As can be seen from Fig. 10, the tensile response of 1DE specimens also included three main stages, accompanied by the formation of multiple cracks. The first cracking occurred at the notch position in part F. Subsequent cracks were generated continuously around the notch position. By contrast, no cracks could be found in part K. The average tensile load value of the first matrix cracking point (Pcc) was 1715 N, while the average maximum tensile load value was 2644.2 N. The electrical resistivity of part F with cracks was noticeably different from that of the part K, which did not contain any cracks. It is apparent from Fig. 10a that the electrical resistivity response observed for part F exhibited clear changes over the three distinct stages; these changes are in accordance with the typical electrical resistivity response of specimens containing multiple cracks (see Fig. 2a). In particular, the electrical resistivity changed only slightly in the first stage, and decreased markedly in the second stage. Finally, the

a) Part F – cracked region

electrical resistivity increased gradually in the third stage. However, the electrical resistivity of part K remained essentially unchanged during the tests. As shown in Tables 4 and 6, the average values of qo, qcc, qmin, and q5 were determined to be 1179.2, 1110, 784.3, and 1175.1 kX-cm for part F, and 872.0, 893.3, 842.8, and 861.7 kX-cm for part K, respectively. The maximum percentage reduction in electrical resistivity measured for part F was 34.4% whereas that determined for part K was dramatically lower, at 3.8%. The slight reduction in electrical resistivity (even though there was no visible crack on the surface of part K in 1DE-specimen) might be originated from minor interfacial cracks between fibers and matrix inside the composites. Le et al. [33] reported that the reduction in the electrical resistivity of SSFRCCs containing steel fibers was attributed to the interfacial debonding at the fiber–matrix interface and it increased with longer debonded length. On the other hands, the change in the electrical resistivity of part K in the 1DE-specimen was clearly different with that in the 1CJ-specimen. The maximum percentage reduction in the electrical resistivity of part K in the 1CJspecimen was very small (0.2%) because no minor interfacial cracks could be occurred inside the matrix owing to the addition of 2.0 vol% short smooth fibers. In addition, the significant reduction in electrical resistivity of part F in 1DE-specimens could be due to the generation of both multi cracks and minor interfacial cracks inside the matrix within the gauge length. The differences in electrical resistivity responses measured for parts F and K in the 1DE specimens are associated with their different mechanical responses during applied tension. Two or three micro cracks were produced near the location of the notch in part F as a result of the reduction in the cross sectional area. The numbers and positions of the cracks formed in part F are summarized in Table 5. Prior to the formation of the first crack, all parts of the specimen were in the elastic region, and the resistivities of part F and part K changed only slightly. During the formation of multiple cracks in part F, part K (without any cracks) remained in the elastic region. Thus, the electrical resistivity in part F decreased notably, whereas that in part K changed little. After the maximum tensile load was reached, tensile stress decreased gradually in both parts, and the main crack in part F started to open, while part K remained in the elastic region. The electrical resistivity of part F during this stage increased gradually as the fiber slip increased. By contrast, the electrical resistivity of part K changed very little. Thus, the results demonstrate that the use measurements employing two DC multimeters or multichannel DC measurement facilitates easy detection of the location of multiple cracks in SS-FRCCs during tension.

b) Park K – uncracked region

Fig. 10. Electromechanical response of specimens with a double-edged notch (1DE-sp2) measured by using two multimeters.

H.V. Le, D.J. Kim / Construction and Building Materials 240 (2020) 117973

a) Part F with a crack

11

b) Part K with a crack opening

Fig. 11. Electromechanical response of specimens with two double-edged notch (2DE-sp3) measured by using two multimeters.

4.5. Electro-mechanical response of 2DE specimens Specimens with two double-edged notches (2DE) containing 1% of long smooth fibers by volume were tested using two DC multimeters or via multichannel DC measurements. In these specimens, both the F and K parts contained a double-edged notch, as shown in Fig. 3d. Thus, the cracks were generated in part F or part K, or in fact in both parts during the testing. Fig. 11 shows the tensile response of specimen 2DE (2DE-sp3) with multiple cracks. As shown in Table 5, the average tensile load value at the first matrix cracking point was 1054.1 N, while the maximum tensile load was 2198.9 N. During the testing, one crack was generated in part F at the double-edged notch position, while the second crack was created and subsequently opened in part K. The number of cracks and the details of their opening are summarized in Table 5. Fig. 11 compares the electro-mechanical responses measured for parts F and K of the 2DE specimen (2DE-sp3) using two DC multimeters simultaneously. It is apparent from Fig. 11a and 11b that both the F and K parts exhibited a significant reduction in electrical resistivity during the tests as a result of cracking. The electrical resistivities of the two parts changed only slightly during the first stage, and decreased notably in the second stage. However, the electrical resistivities of these parts were markedly different in the third stage: the electrical resistivity of part K with a crack opening increased significantly whereas that of part F remained virtually unchanged. For instance, as summarized in Tables 4 and 6, the average values of qo, qcc, qmin, and q5 of the 2DE specimen were 1152.5, 1153.2, 1080.9, and 1116.4 kX-cm for part F, and 1776.4, 1815.4, 1657.2, and 2009.5 kX-cm for part K, respectively. The maximum percentage reduction in electrical resistivity measured for the part with the open crack was clearly higher than that of part F, as summarized in Table 6. Specifically, the average maximum percentage reduction in electrical resistivity measured for the 2DE specimen was 7.1% for part K containing the main crack opening, and 4.4% for part F containing the second crack (but no opening). The differences in electrical resistivity responses obtained for parts K and F with and without a crack opening, respectively, can be attributed again to the differences in their tensile responses during testing. The significant increase in electrical resistivity observed for part K in the third stage was caused by the widening of the main crack. By contrast, the width of crack formed in part F did not increase, and thus the electrical resistivity remained unchanged. The electrical resistivity at each crack decreased significantly as the fiber debonding was initiated and progressed to full

fiber debonding, whereas it increased gradually during the fiber pullout tests [33]. Thus, the lower maximum percentage reduction in electrical resistivity can be explained by the fact that fibers bridging the crack in part F were not fully debonded, whereas the fibers bridging the crack in part K were fully debonded and pulled out during the tests. Overall, the results suggest that the crack location can be established based on the electrical resistivity responses determined during tests employing two DC multimeters or multichannel DC measurements. Further, the main crack opening position also can be detected based on the electrical resistivity responses of SS-FRCCs in the different gauge lengths.

5. Conclusions This experimental study investigated the feasibility of using multichannel DC measurements as a tool for detecting the location of the cracks of SS-FRCCs and the effects of crack location on the measured the composite electrical resistivity of SS-FRCCs under tension. The results showed that the reduction in the electrical resistivity of SS-FRCCs under tension as a result of cracking was sensitive to the location of the crack within to the gauge length. The amount of reduction in the electrical resistivity of cracked zone decreased as the location of the crack was close to the inner electrode. Based on these results, the source of change in the electrical resistivity of SS-FRCCs under tension could be more clearly understood. The location of the crack (one-crack or multi cracks or main crack opening) could be determined by measuring the electrical resistivity of SS-FRCCs using two or multi-channel DC measurements. The electrical resistivity in the cracked part generated a noticeable decrease in the response, whereas the resistivity of the crack-free zone changed very little during the tests. Overall, the present results demonstrate that the location of cracks in tensile region can be determined successfully using multichannel DC measurements based on the electrical resistivity responses measured from different parts of SS-FRCCs. However, the cracking behavior of concrete in practical structure is very complex, i.e., cracks not only occur in the tensile region but also the compressive or shear regions owing to external effects or shrinkage. Thus, to apply multichannel DC measurements for detecting the crack in practical structures, further researches need to be carefully investigated. Huy Viet LE conducted experiments and prepared first manuscript. Dong Joo KIM designed the experimental program and revised the manuscript.

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Declarations of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the Basic Research Program through the National Research Foundation of Korea (NRF) funded by the MSIT(2019R1A4A1021702). The opinions expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsors. References [1] C.C. Comisu, N. Taranu, G. Boaca, M.C. Scutaru, Structural health monitoring system of bridges, Procedia Eng. 199 (2017) 2054–2059, https://doi.org/ 10.1016/j.proeng.2017.09.472. [2] M. Imai, M. Feng, Sensing optical fiber installation study for crack identification using a stimulated Brillouin-based strain sensor, Struct. Heal. Monit. 11 (2012) 501–509, https://doi.org/10.1177/1475921712442440. [3] Z. Chen, F. Ansari, Fiber optic acoustic emission distributed crack sensor for large structures, J. Struct. Control. 7 (2000) 119–129, https://doi.org/10.1002/ stc.4300070108. [4] N.K. Mutlib, S. Baharom, M.Z. Nuawi, A. El-Shafie, Ultrasonic surface wave monitoring for steel fibre-reinforced concrete using gel-coupled piezoceramic sensors: a Case Study, Arab. J. Sci. Eng. 41 (2016) 1273–1281, https://doi.org/ 10.1007/s13369-015-1925-1. [5] S.C. Huang, W.W. Lin, M.T. Tsai, M.H. Chen, Fiber optic in-line distributed sensor for detection and localization of the pipeline leaks, Sens. Actuat., A Phys. 135 (2007) 570–579, https://doi.org/10.1016/j.sna.2006.10.010. [6] K.S.C. Kuang, T.W.K. Goh, Crack sensing and healing in concrete beams based on wire-crack sensors and healing tube, Constr. Build. Mater. 132 (2017) 395– 411, https://doi.org/10.1016/j.conbuildmat.2016.12.007. [7] D. Ai, H. Zhu, H. Luo, Sensitivity of embedded active PZT sensor for concrete structural impact damage detection, Constr. Build. Mater. 111 (2016) 348–357, https://doi.org/10.1016/j.conbuildmat.2016.02.094. [8] H. Abdel-Jaber, B. Glisic, Analysis of the status of pre-release cracks in prestressed concrete structures using long-gauge sensors, Smart Mater. Struct. 24 (2015) 1–12, https://doi.org/10.1088/0964-1726/24/2/025038. [9] A. Billon, J.M. Henault, M. Quiertant, F. Taillade, A. Khadour, R.P. Martin, K. Benzarti, Qualification of a distributed optical fiber sensor bonded to the surface of a concrete structure: A methodology to obtain quantitative strain measurements, Smart Mater. Struct. 24 (2015) 1–13, https://doi.org/10.1088/ 0964-1726/24/11/115001. [10] B. Han, X. Yu, J. Ou, Self-sensing concrete in smart structures, Butterworth Heinemann, Elsevier, Kidlington (2014), https://doi.org/10.1016/C2013-014456-X. [11] B. Han, S. Ding, X. Yu, Intrinsic self-sensing concrete and structures: a review, Measurement. 59 (2015) 110–128, https://doi.org/10.1016/j.measurement. 2014.09.048. [12] P. Chen, D.D.L. Chung, Concrete as a new strain-stress sensor, Compos. Part B 27 (1996) 11–23. [13] Z.Q. Shi, D.D.L. Chung, Carbon fiber-reinforced concrete for traffic monitoring and weighing in motion, Cem. Concr. Res. 29 (1999) 435–439, https://doi.org/ 10.1016/S0008-8846(98)00204-X. [14] S. Wen, D.D.L. Chung, Uniaxial tension in carbon fiber reinforced cement, sensed by electrical resistivity measurement in longitudinal and transverse directions, Cem. Concr. Res. 30 (2000) 1289–1294, https://doi.org/10.1016/ S0008-8846(00)00304-5. [15] X. Fu, W. Lu, D.D.L. Chung, Improving the strain-sensing ability of carbon fiberreinforced cement by ozone treatment of the fibers, Cem. Concr. Res. 28 (1998) 183–187, https://doi.org/10.1016/S0008-8846(97)00265-2. [16] D.M. Bontea, D.D.L. Chung, G.C. Lee, Damage in carbon fiber-reinforced concrete, monitored by electrical resistance measurement, Cem. Concr. Res. 30 (2000) 651–659, https://doi.org/10.1016/S0008-8846(00)00204-0. [17] D.D.L. Chung, Piezoresistive cement-based materials for strain sensing, J. Intell. Mater. Syst. Struct. 13 (2002) 599–609, https://doi.org/10.1106/104538902031861. [18] A. Al-Dahawi, O. Öztürk, F. Emami, G. Yildirim, M. Sßahmaran, Effect of mixing methods on the electrical properties of cementitious composites incorporating different carbon-based materials, Constr. Build. Mater. 104 (2016) 160–168, https://doi.org/10.1016/j.conbuildmat.2015.12.072.

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