Experimental Study on Electrode Method for Electrical Resistivity Survey to Detect Cavities under Road Pavements

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1 sustainability Article Experimental Study on Electrode Method for Electrical Resistivity Survey to Detect Cavities under Road Pavements Chang-Seon Park 1,2 ID, Jin-Hoon Jeong 3 ID, Hae-Won Park 3 and Kyoungchul Kim 4, * ID 1 Department of Civil Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Korea; cseon90@kistec.or.kr 2 Currently Bridge & Tunnel Safety Section, Safety Division, Korea Infrastructure Safety & Technology Corporation, Sadeul-ro, Gyeongsangnam-do, Jinju 52852, Korea 3 Department of Civil Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Korea; jhj@inha.ac.kr (J.-H.J.); spro374@naver.com (H.-W.P.) 4 R&D Center, KOLON Global Corporation, 32 Songdogwahak-ro, Yeonsu-gu, Incheon 21984, Korea * Correspondence: kckim@kolon.com; Tel.: Received: 16 October 2017; Accepted: 10 December 2017; Published: 13 December 2017 Abstract: There are two types of electrode methods for resistivity survey (ERS): pole electrode method (PEM) and flat electrode method (FEM). During past few decades, most studies were conducted by using PEM for various purposes while only a few were conducted by using FEM. Laboratory and field experiments were performed in this study to investigate advantage of FEM in detecting cavities under pavements. In laboratory experiment, results of PEM and FEM were compared graphically and statistically. A significant difference between results of PEM and FEM was observed for concrete at an age of seven days, while re was no significant difference in results for soil materials. Electrical resistivity could not be obtained from asphalt because it is an insulator. In a field experiment, four different cases were simulated: field ground with/without cavity and concrete pavement with/without cavity. The results of PEM and FEM for se cases were compared using 2D resistivity contour images. It was observed that distribution of resistivity obtained using FEM was wider than that using PEM. Moreover, locations of cavities artificially made in ground and under pavement were accurately detected using both PEM and FEM. Keywords: resistivity; pole electrode method; flat electrode method; cavity; pavement 1. Introduction Electrical resistivity survey (ERS) is a time- and cost-effective non-destructive method of producing shape and location image of an object. It can be used in various applications such as in detecting underground water and cavities, faults, and cracks [1 7]. Generally, ERS is performed by installing an electrode made of a conductive material. As shown in Figure 1a, pole electrode method (PEM) performed by inserting a pole conductor into an object has been commonly used for ERS [3,8,9]. Since pavement collapse frequently occurs due to cavities generated under pavements, various nondestructive survey methods have been widely used in preobservation of status under pavements [2,10 12]. However, it is difficult to apply ERS to pavements using PEM as pole conductor may seriously damage pavement during insertion. To overcome disadvantage of PEM, flat electrode method (FEM), which involves contacting a flat conductor to an object as shown in Figure 1b, is recommended when applying ERS to pavements [3,5,8,9]. Moreover, application of FEM can prevent secondary damage occurred by pavement surface damage and preserve sustainability of infrastructure lost due to pavement collapse disasters. Sustainability 2017, 9, 2320; doi: /su

2 Sustainability 2017, 9, of 20 Sustainability application 2017, 9, of 2320 FEM can prevent secondary damage occurred by pavement surface damage 2 of 21 and preserve sustainability of infrastructure lost due to pavement collapse disasters. Figure 1. Electrode installation methods for resistivity survey: PEM; FEM. Figure 1. Electrode installation methods for resistivity survey: PEM; FEM. During past few decades, most of studies used PEM for detecting cavities under During pavements under past few various decades, conditions, most of while only studies a few used used PEM FEM for [4 6,13 16]. detectingpark cavities et al. under [4] examined pavements under various applicability conditions, of three-dimensional while only aers fewto used detect FEM cavities [4 6,13 16]. under a ground Park etsubsidence al. [4] examined area by applicability comparing of three-dimensional results of borehole ERS tests toperformed detect at cavities field experiment under asites. ground As subsidence cavities were area by comparing mostly filled results with water of borehole and clays tests in performed test sites in at accordance field experiment with results sites. of As borehole cavities test, were a low resistivity was observed at locations. Thus, it was shown that ERS was effective mostly filled with water and clays in test sites in accordance with results of borehole test, in detecting cavities underground subsidence areas. a low resistivity was observed at locations. Thus, it was shown that ERS was effective in Farooq et al. [5] examined resistivity of Portland cement mortar specimens with detecting various cavities water-cement underground (w/c) ratios subsidence by using areas. PEM for evaluating cement mortar grouted locations. A Farooq significant et correlation al. [5] examined between resistivity resistivity and w/c ofwas Portland observed. cement Although mortar specimens with resistivity various water-cement was proportional (w/c) to ratios curing by time using of PEM mortar, forit evaluating was inversely cement proportional mortar to w/c. grouted locations. Thus, Ait significant was shown correlation that ERS is between applicable to cement mortar resistivity grouted andlocations. w/c was Gambetta observed. et al. Although [6] observed resistivity geophysical wasresponse proportional of a shallow to cave curing by using time PEM. of The mortar, results it showed was inversely clear proportional geophysical to responses w/c. Thus, of large it cave waspassages shownby that high ERS is applicable resistivity. Moreover, to cement it was mortar confirmed grouted locations. that locations Gambetta of voids et al. were [6] observed precisely detected geophysical using ERS even response under complex of a shallow environments. cave by using PEM. Recently, Cai et al. [14] developed an empirical equation based on correlation between The results showed clear geophysical responses of large cave passages by high resistivity. resistivity and reactive magnesia content to predict resistivity of carbonated soils. Moreover, it was confirmed that locations of voids were precisely detected using ERS even under The resistivity computed using that equation in accordance with t and / was complex compared environments. with experimentally obtained resistivity. Reasonable degrees of correlation Recently, were found Cai between al. [14] developed model and an empirical experimental equation results. based on correlation between resistivitygenerally, and reactive as concrete magnesiand content asphalt to predict pavements have resistivity high of carbonated resistance, soils. contact The resistance between resistivity electrode computed and pavement using that surface equation is most in accordance important factor within tan and ERS w[5,9]. 0 /c was compared In particular, with FEM, experimentally which has a wider obtained contact area, is more resistivity. influenced Reasonable by contact resistance degrees of than correlation PEM were [3,8]. foundin between this study, laboratory model and field experimental experiments were results. performed to investigate advantage of Generally, FEM in detecting as concrete cavities and asphalt under pavements. pavements The have correlation high between resistance, properties contact resistance and constituents of pavement layers were obtained from laboratory experiments. The applicability of between electrode and pavement surface is most important factor in an ERS [5,9]. In particular, FEM for detecting cavities was investigated by applying method to field site where FEM, which has a wider contact area, is more influenced by contact resistance than PEM [3,8]. concrete pavement and cavity were simulated artificially. In this study, laboratory and field experiments were performed to investigate advantage of FEM in detecting 2. Theoretical cavities under Background pavements. of ERS The correlation between properties and constituents of pavement layers were obtained from laboratory experiments. The applicability of FEM for detecting cavities 2.1. was Outline investigated of ERS by applying method to field site where concrete pavement and cavity were The simulated amount of artificially. flowing current is determined when a constant voltage is applied to pavement. If materials under pavement are homogeneous, resistivity would be 2. Theoretical considered Background as true resistivity of ERS ρ (Ω m) expressed as Equation (1) [7,17]: 2.1. Outline of ERS The amount of flowing current is determined when a constant voltage is applied to pavement. If materials under pavement are homogeneous, resistivity would be considered as true resistivity ρ (Ω m) expressed as Equation (1) [7,17]: ρ = VA IL. (1)

3 Sustainability 2017, 9, of 21 The typical resistivities of various types of geomaterials are listed in Table 1. The resistivities exhibit an extensive range from 1 Ω m for clay to 740,000,000 Ω m for sandstone [7]. The resistivity of concrete and asphalt depends on mix proportions because of influence of constituent materials (aggregates proportion, content of water and clay, content of ions in water, and pore void, etc.). Therefore, analyzing correlation between resistivity and constituents of pavement materials is important in using ERS [1,3,4,6,9,13,14,16,17]. Table 1. Typical resistivity of geomaterials. Material Electrical Resistivity (Ω m) Granite 300 1,300,000 Sandstone 1 740,000,000 Clay Topsoil Gravel Alluvium and sand Dry sandy soil Sandy clay/clayey sand Sand and gravel Electrode Array and Installation Method ERS usually requires four electrodes: a positive potential electrode (P+), negative potential electrode (P ), positive current electrode (C+), and negative current electrode (C ). The difference between two potential electrodes helps in determining true resistivity. The purpose of ERS is to determine true resistivity of materials under pavement. However, as measured resistance depends on array of electrodes, and actual materials under pavement are not completely homogeneous, measured resistivity is considered as apparent resistivity (ρ a, Ω m) [1,5,7,17]. Apparent resistivity can be defined as equivalent resistance obtained from a heterogeneous medium, which corresponds to resistance of a homogeneous medium as given in Equation (2): ρ a = K V I = KR, (2) where K is a geometric factor. Usually, apparent resistivity changes depending on electrode array because of change in geometric factor of heterogeneous materials under pavement. However, if materials under pavement are homogeneous, measured apparent resistivity can be considered as true resistivity [3,7,17]. In this study, dipole dipole array method (DDAM) was used since DDAM surveys in both vertical and horizontal directions simultaneously for two-dimensional analysis [3,7]. As shown in Figure 2, survey is performed by increasing initial spacing a between current electrodes (C+, C ) and potential electrodes (P+, P ) to 2a, 3a,..., na [7]. By increasing electrode spacing a and n, effective survey depth increases and measured resistivity could be related to a specific position in accordance with electrode spacing [3,8]. The geometric factor (K) is also affected by electrode spacing as shown in Equation (3): K = n(n + 1)(n + 2)πaR. (3) FEM, which has a wider contact area, is more influenced by contact resistance than PEM [3,8]. In addition, PEM is inefficient since inserting electrodes into pavement surface is technically difficult and time-consuming compared with that of FEM. Moreover, damaged pavement surface caused by electrode insertion could seriously deteriorate in time [3]. However, FEM requires

4 Sustainability 2017, 9, of 21 Sustainability Sustainability 2017, 2017, 9, 9, of of consideration of contact resistance since disturbance of current can increase as contact area increases; using a wider contact area in FEM can contact contact area area increases; increases; using using a wider wider contact contact area area in in FEM FEM can can disturb disturb current current more more than than in in PEM PEM [3,8,9]. [3,8,9]. Therefore, it is Therefore, it it is is necessary necessary to to compare compare results results obtained obtained from from both both PEM PEM and and FEM FEM to to determine determine applicability of offem to topavements. In Inthis this study, study, experiments experiments were were performed performed by by PEM PEM using using applicability of FEM to pavements. In this study, experiments were performed by PEM using mm-long steel steel nails nails and and by by FEM FEM using using hex a hex wrench wrench and and 5-mm-wide a 5-mm-wide copper copper plate plate to make to make mm-long steel nails and by FEM using a hex wrench and a 5-mm-wide copper plate to make comparison. comparison. comparison. Figure 2. Dipole dipole array method Laboratory Experiments Laboratory experiments were performed using both FEM and PEM to to correlate properties of of pavement materials to to constituent materials analyzed. The results of of laboratory experiments will be be used used as as a reference reference for for analyzing analyzing field field experiments experiments results. results. Specimens were were prepared in in a different different size size to to obtain obtain reliable reliable resistivity resistivity data data by by considering resistance characteristics of of each each material material as as shown shown in in Figure Figure Figure 3. Cont.

5 Sustainability 2017, 9, of 21 Sustainability 2017, 9, of 20 Sustainability 2017, 9, of 20 (c) Figure 3. Specimens for laboratory experiments: soil material; concrete; (c) asphalt. Figure 3. Specimens for laboratory experiments: soil material; concrete; (c) asphalt Soil Specimens 3.1. Soil Specimens (c) Sieve analysis Figure was 3. Specimens carried for out laboratory identify experiments: characteristics soil material; of concrete; soil materials (c) asphalt. as shown in Sieve analysis was carried out to identify characteristics of soil materials as shown in Table [18]. The type of field ground soil (nature ground soil), subgrade soil, and subbase soil was Table 2 classified 3.1. [18]. Soil as Specimens The type of field ground soil (nature ground soil), subgrade soil, and subbase soil poorly graded sand (SP), well-graded sand (SW), and poorly graded gravel (GP), was classified as poorly graded sand (SP), well-graded sand (SW), and poorly graded gravel (GP), respectively, Sieve in analysis accordance was carried with out standard. to identify The characteristics compaction of curve soil for materials each type as shown of soil in was respectively, in accordance with standard. The compaction curve for each type of soil was generated generated Table as 2 shown [18]. The in type Figure of field 4 [19]. ground Moreover, soil (nature maximum ground soil), dry subgrade unit weight soil, and ( subbase ) and soil optimum was as shown in Figure 4 [19]. Moreover, maximum dry unit weight (γ water classified content ( as poorly ) are summarized graded sand in (SP), Table well-graded 3. sand (SW), and dmax poorly ) and optimum graded gravel water (GP), content (ω opt ) are respectively, summarized in accordance in Table 3. with standard. The compaction curve for each type of soil was generated as shown in Figure 4 [19]. Table Moreover, 2. Results of sieve maximum analysis. dry unit weight ( ) and optimum water content ( ) are summarized Table in 2. Results Table 3. of sieve analysis. Percentage Passing Percentage Passing Uniformity Coefficient of Type of Soil Classification (No. Percentage 4 Sieve, Passing %) Percentage (No. 200 Sieve, %) Coefficient Cu Curvature Cg (1<, 3>) Type of Soil Table 2. Passing Results of sieve Uniformity analysis. Coefficient of Classification Field ground soil (No Sieve, %) (No Sieve, %) Coefficient 7.43 (>6) Cu Curvature Cg (1<, 3>) SP Percentage Passing Percentage Passing Uniformity Coefficient of Subgrade Field ground Type soil of soil Soil (>6) (>6) Classification SP SW (No. 4 Sieve, %) (No. 200 Sieve, %) Coefficient Cu Curvature Cg (1<, 3>) Subbase Subgrade soil soil (>6) (>4) SW GP Field ground soil (>6) SP Subbase Subgrade soil soil (>4) (>6) GP SW Subbase soil Table Properties of soils. (>4) 0.35 GP Type of Soil Max. Dry Unit Weight Table 3. Properties (, /cm of soils. Table 3. Properties of soils. 3 ) Optimum Water Content (, %) Field ground soil Type Type of of Soil Soil Max. Max. Dry Dry Unit Unit Weight ( (γ dmax, g/cm 3, /cm) 3 ) Optimum Water Water Content Content (ω opt (, %), %) Subgrade soil Field Field ground ground soil soil Subbase Subgrade soil soil soil Subbase Subbase soil soil Figure Figure 4. Compaction 4. Compaction curve curve of of soil soil materials. In many studies, it was indicated that properties of soil correlate with its In In many studies, it was indicated that properties of soil correlate with its constituents many studies, and itphysical was indicated property that (minerals content, properties water content, of soilclay correlate content, with pore itsvoid, constituents etc.) and constituents physical and physical property (minerals content, water content, clay content, pore void, etc.) [1,3,4,6,13,14,17]. property (minerals In particular, content, water water content content, has clay most content, significant poreinfluence void, etc.) on [1,3,4,6,13,14,17]. In [1,3,4,6,13,14,17]. particular, resistivity water of In content particular, soil since has water most conductivity content significant has is related influence most to significant onmobility of influence ions resistivity dissolved on of in soil resistivity water of that fills soil since pores of soil conductivity [3,17]. Therefore, is related four cylindrical to mobility specimens of (150 ions mm dissolved in diameter in water and that 125 fills mm in pores height) of with soil different [3,17]. water Therefore, content four were cylindrical prepared in specimens order to investigate (150 mm in diameter effect and 125 mm in height) with different water content were prepared in order to investigate effect

6 Sustainability 2017, 9, of 21 since conductivity is related to mobility of ions dissolved in water that fills pores of soil [3,17]. Therefore, four cylindrical specimens (150 mm in diameter and 125 mm in height) with different water content were prepared in order to investigate effect of water content on resistivity of field ground soil, subgrade soil, and subbase soil [19]. The water content of soils used in this study is given in Table 4 [20]. Table 4. Water content of soils (%). Type of Soil Case No Field ground soil Subgrade soil Subbase soil Concrete Specimens The composition of concrete is an important factor influencing resistivity. In many studies, effect of w/c on resistivity of concrete was investigated since current flows in concrete by ions dissolved in pore water [5,8,9]. The w/c is an element of mix design because it affects strength and workability of concrete. However, only few research on effect of fine aggregate modulus (S/a) on resistivity of concrete has been conducted although it is used in mix design to control segregation, slump, and air void [5,8,9,21]. Fine aggregate modulus is absolute volume ratio of coarse aggregate and fine aggregate in mixture proportion of concrete. Therefore, effect of S/a on resistivity of concrete was investigated using PEM and FEM in this study. Cylindrical specimens (150 mm in diameter and 300 mm in height) for three mixture proportions with various S/a [18,22] as indicated in Table 5 were prepared. Table 5. Mix proportions of concrete. Case No. Gmax (mm) w/c (%) S/a (%) Unit Quantity (kg/cm 3 ) Water (W) Cement (C) Sand (S) Gravel (G) (>6) SP (>6) 1.45 SW (>4) 0.35 GP 3.3. Asphalt Specimens In general, conductivity is proportional to ionic concentration, ionic mobility, viscosity of liquid, and quality of conducting paths determined by textural properties of a material. However, petroleum products (oil, kerosene, gasoline, etc.) are considered as insulators as re is no mobility of ions. Electrical resistivity tends to substantially increase in most cases for petroleum-contaminated soils by exceeding 5,000,000 Ω m [5,23]. Therefore, asphalt concrete, which consists of aggregate and asphalt binder, could be considered as an insulator with properties of electric insulation, adhesion, and waterproofing [16,23]. In this study, applicability of ERS to asphalt was investigated by performing PEM and FEM in laboratory. Cylindrical specimens (150 mm in diameter and 110 mm in height) with different porosities and asphalt content were prepared as indicated in Table 6 [18]. Table 6. Mix proportions of asphalt. Case No. 1 2 Porosity 4% 10% Asphalt content 5.8% 5.0%

7 Sustainability 2017, 9, of Instrumentation of Electrode Electrode spacing could affect resistivity of specimens with small sizes (150 mm in diameter). A very small electrode spacing could result in interference between electrodes, while an extremely wide electrode spacing could result in disturbance of current field owing to small space between electrode and edge of specimen [5,8]. Therefore, effect of electrode spacing on resistivity was investigated by varying electrode spacing. In this study, six types of electrode spacing were used: (1) a = 20 mm, n = 1 (20-1); (2) a = 20 mm, n = 2 (20-2); (3) a = 20 mm, n = 3 (20-3); (4) a = 30 mm, n = 1 (30-1); (5) a = 30 mm, n = 2 (30-2); (6) a = 40 mm, n = 1 (40-1). Each experiment was repeated five times for each material to improve reliability of results. Terrameter SAS 1000 (ABEM, Sundbyberg, Sweden) was used to measure resistivity for both PEM and FEM Results of Laboratory Experiments Each resistivity with PEM and FEM was compared graphically and statistically. The correlation was analyzed by paired t-test with significance level of 5% (α = 0.05; α means significance level in statistics). The average values of five repeated experiments were used in analysis Field Ground Soil The resistivity obtained with both PEM and FEM decreases as water content increases. However, resistivity converges to lowest value when water content exceeds optimum water content ω opt (9.80%) in both methods as shown in Figure 5. When water content is 4.53%, resistivity through FEM showed significant scattering from 116 Ω m to 245 Ω m, compared to resistivity through PEM from 145 Ω m to 205 Ω m. Overall, average resistivities of two methods are similar, while re are slight differences in maximum and minimum resistivities at low water content. The order of resistivity by electrode spacing using FEM is 20-2, 20-3, 30-1, 20-1, 40-1, and 30-2, while that using PEM is 20-2, 30-1, 20-3, 30-2, 40-1, and 20-1 showing a similar order for two methods. However, resistivities using FEM are more scattered than those using PEM at low water content. The resistivity could be disturbed when pore void in soil is not sufficiently filled with water and particle surface is dried. In addition, loss of adhesion between soil particles owing to low water content could also cause disturbance of current [3,14,17]. If current in FEM, which disperses current into a wide contact area, flows on surface of se conditions, it will be more disturbed by resistance than PEM, which concentrate current on one place. Thus, it is suggested that disturbance of current at low water content of field ground soil should be considered in conducting FEM. In addition to graphical comparison, statistical analysis was also performed to investigate applicability of FEM by analyzing correlation of PEM results with FEM. The paired t-test was used for analysis by assuming a linear correlation between results of PEM and FEM [24]. The level of statistical significance was set to α = 0.05 (5% error level). Figure 6 and Table 7 summarize statistical analysis results. The relationship of resistivity of two methods is almost linear and correlation coefficient (R) is 0.951, indicating a high correlation. In addition, significance probability (p-value) of is also higher than significance level (0.05), indicating a high correlation between results of PEM and FEM.

8 increases. However, resistivity converges to lowest value when water content exceeds optimum water content (9.80%) in both methods as shown in Figure 5. When water content is 4.53%, resistivity through FEM showed significant scattering from 116 Ω m to 245 Ω m, compared to resistivity through PEM from 145 Ω m to 205 Ω m. Overall, Sustainability average 2017, 9, 2320 resistivities of two methods are similar, while re are slight differences 8in of 21 maximum and minimum resistivities at low water content. Sustainability 2017, 9, of 20 The order of resistivity by electrode spacing using FEM is 20-2, 20-3, 30-1, 20-1, 40-1, and 30-2, while that using PEM is 20-2, 30-1, 20-3, 30-2, 40-1, and 20-1 showing a similar order for two methods. However, resistivities using FEM are more scattered than those using PEM at low water content. The resistivity could be disturbed when pore void in soil is not sufficiently filled with water and particle surface is dried. In addition, loss of adhesion between soil particles owing to low water content could also cause disturbance of current [3,14,17]. If current in FEM, which disperses current into a wide contact area, flows on surface of se conditions, it will be more disturbed by resistance than PEM, which concentrate current on one place. Thus, it is suggested that disturbance of current at low water content of field ground soil should be considered in conducting FEM. In addition to graphical comparison, statistical analysis was also performed to investigate applicability of FEM by analyzing correlation of PEM results with FEM. The paired t-test was used for analysis by assuming a linear correlation between results of PEM and FEM [24]. The level of statistical significance was set to = 0.05 (5% error level). Figure 6 and Table 7 summarize statistical analysis results. The relationship of resistivity of two methods is almost linear and correlation coefficient (R) is 0.951, indicating a high correlation. In addition, significance probability Figure 5. (p-value) Electrical resistivity of is of also field higher ground than soil using significance PEM and level FEM. (0.05), indicating a high correlation Figure between 5. Electrical results resistivity of PEM of and field FEM. ground soil using PEM and FEM. Figure Correlation between resistivityof offield field groundsoil soil using using FEM FEMand and PEM. PEM. Table 7. Results of paired t-test between PEM and FEM. Table 7. Results of paired t-test between PEM and FEM. Mean Standard Standard Error Correlation Significance Type of Material t-score ( ) Deviation Standard ( ) Standard (SE) Coefficient Correlation(R) Probability Significance (p-value) Type of Material Mean (µ) t-score Field ground soil Deviation (σ) Error (SE) Coefficient (R) Probability (p-value) Subgrade Field ground soil soil Subbase soil Subgrade soil days Concrete Subbase 28 days soil days Concrete Subgrade 28 days Soil Similar to experiment for field ground soil, four values of water content (4.30%, 6.86%, 9.28%, and 15.00%) were used to investigate its effect on resistivity of subgrade soil. The resistivity obtained by electrode spacing decreases as water content increases in both PEM and FEM. When water content exceeds (9.80%), resistivity converges to lowest value as shown in Figure 7. Although resistivity of field ground soil

9 Sustainability Sustainability 2017, 2017, 9, , of 9 20 of 21 resistivity obtained using FEM ranges from 253 Ω m to 561 Ω m at 4.30% and from 135 Ω m to Subgrade Soil Ω m at 6.86%. Overall, minimum resistivities obtained from both methods are similar regardless Similar tof experiment water content, forwhile field ground maximum soil, value fourobtained values of using water FEM content is larger (4.30%, than 6.86%, that 9.28%, using and PEM %) were used to investigate its effect on resistivity of subgrade soil. The The order resistivity of obtained resistivity by electrode by electrode spacing spacing decreases for PEM as is 20-1, water 20-3, content 20-2, increases 40-1, 30-1, inand both PEM30-2 and at 4.30% FEM. When and 20-2, 20-1, water 30-1, content 40-1, 30-2, exceeds and ω20-3 opt (9.80%), at 6.86% of water content. resistivity The order converges of to lowest resistivity value using as shown FEM in is Figure 20-1, 20-2, 7. Although 30-1, 20-3, 30-2, and 40-1 resistivity at 4.30% of and 20-2, field 20-3, ground 20-1, soil 30-1, obtained 40-1, and 30-2 at 6.86% showing a similar order to PEM. As with field ground soil, using FEM scatters only at water content of 4.53%, which of subgrade soil obtained by electrode resistivity using FEM is more scattered than that using PEM because of its larger contact area, causing spacing significantly scatters at two amounts of water content of 4.30% and 6.86% in both PEM and greater disturbance at low water content. FEM as shown in Figure 7. The scattered resistivity obtained using PEM ranges from 253 Ω m Statistical analysis was also performed to investigate correlation between results of PEM to 426 Ω m at 4.30% and from 157 Ω m to 301 Ω m at 6.86%, while scattered resistivity and FEM. The results are summarized in Figure 8 and Table 7. A linear relationship of obtained using FEM ranges from 253 Ω m to 561 Ω m at 4.30% and from 135 Ω m to 314 Ω m at 6.86%. resistivities of PEM and FEM is observed and its R value is 0.862, indicating a high correlation. In Overall, addition, minimum p-value of is resistivities higher than obtained α of 0.05, from indicating both methods high correlation are similar between regardless two of water methods. content, while maximum value obtained using FEM is larger than that using PEM. Figure 7. Electrical resistivity of subgrade soil using PEM and FEM. Figure 7. Electrical resistivity of subgrade soil using PEM and FEM. The order of resistivity by electrode spacing for PEM is 20-1, 20-3, 20-2, 40-1, 30-1, and 30-2 at 4.30% and 20-2, 20-1, 30-1, 40-1, 30-2, and 20-3 at 6.86% of water content. The order of resistivity using FEM is 20-1, 20-2, 30-1, 20-3, 30-2, and 40-1 at 4.30% and 20-2, 20-3, 20-1, 30-1, 40-1, and 30-2 at 6.86% showing a similar order to PEM. As with field ground soil, resistivity using FEM is more scattered than that using PEM because of its larger contact area, causing greater disturbance at low water content. Statistical analysis was also performed to investigate correlation between results of PEM and FEM. The results are summarized in Figure 8 and Table 7. A linear relationship of

10 Sustainability 2017, 9, of 21 resistivities of PEM and FEM is observed and its R value is 0.862, indicating a high correlation. In addition, p-value of is higher than α of 0.05, indicating high correlation between two Sustainability methods. 2017, 9, of 20 Figure 8. Correlation between resistivity of subgrade soil using FEM and PEM. Figure 8. Correlation between resistivity of subgrade soil using FEM and PEM Subbase Soil Subbase Soil Similar to field ground soil and subgrade soil, effect of water content on Similar to field ground soil and subgrade soil, effect of water content on resistivity of subbase soil was investigated by using four values of water content (3.22%, 5.71%, 7.62%, resistivity of subbase soil was investigated by using four values of water content (3.22%, 5.71%, 7.62%, and 10.10%). The resistivity obtained by electrode spacing decreases as water content and 10.10%). The resistivity obtained by electrode spacing decreases as water content increases in both PEM and FEM. When water content exceeds (6.20%), increases in both PEM and FEM. When water content exceeds ω opt (6.20%), resistivity resistivity converges to lowest value as shown in Figure 9. Based on similar trends observed for converges to lowest value as shown in Figure 9. Based on similar trends observed for field field ground soil, subgrade soil, and subbase soil, it is shown that resistivity is not ground soil, subgrade soil, and subbase soil, it is shown that resistivity is not affected affected by water content larger than, since adhesion of soil particles is stabilized by by water content larger than ω opt, since adhesion of soil particles is stabilized by water water content [1,3,17]. content [1,3,17]. The resistivity of subbase soil obtained by electrode spacing significantly scatters in both PEM and FEM when water content is lower than 3.22%. The scattered resistivity ranges from 90 Ω m to 228 Ω m for PEM and 84 Ω m to 220 Ω m for FEM as shown in Figure 9. At low water content, since subbase soil containing large particles of gravel has a smaller adhesion than or soil, adhesion between pole electrode and soil was weakened and current in PEM was disturbed like FEM. Overall, resistivities obtained using both methods are similar. The resistivity is high in order of 20-2, 20-1, 30-1, 20-3, 40-1, and 30-2 in both PEM and FEM showing a small effect of electrode spacing. The effect of electrode spacing on resistivity of subbase soil is similar to that of field ground soil and subgrade soil. In order to investigate correlation between PEM and FEM, statistical analysis was also performed. The results are shown in Figure 10 and Table 7. The relationship of resistivity of two methods is almost linear and value of R is 0.961, indicating a high correlation. In addition, p-value of is higher than α of 0.05, which indicates that re is a high correlation between PEM and FEM.

11 and 10.10%). The resistivity obtained by electrode spacing decreases as water content increases in both PEM and FEM. When water content exceeds (6.20%), resistivity converges to lowest value as shown in Figure 9. Based on similar trends observed for field ground soil, subgrade soil, and subbase soil, it is shown that resistivity is not Sustainability affected by 2017, water 9, 2320 content larger than, since adhesion of soil particles is stabilized by 11 of 21 water content [1,3,17]. Sustainability 2017, 9, of 20 The resistivity of subbase soil obtained by electrode spacing significantly scatters in both PEM and FEM when water content is lower than 3.22%. The scattered resistivity ranges from 90 Ω m to 228 Ω m for PEM and 84 Ω m to 220 Ω m for FEM as shown in Figure 9. At low water content, since subbase soil containing large particles of gravel has a smaller adhesion than or soil, adhesion between pole electrode and soil was weakened and current in PEM was disturbed like FEM. Overall, resistivities obtained using both methods are similar. The resistivity is high in order of 20-2, 20-1, 30-1, 20-3, 40-1, and 30-2 in both PEM and FEM showing a small effect of electrode spacing. The effect of electrode spacing on resistivity of subbase soil is similar to that of field ground soil and subgrade soil. In order to investigate correlation between PEM and FEM, statistical analysis was also performed. The results are shown in Figure 10 and Table 7. The relationship of resistivity of two methods is almost linear and value of R is 0.961, indicating a high correlation. In addition, p-value of is higher than α of 0.05, which indicates that re is a high correlation Figure 9. Electrical resistivity of subbase soil using PEM and FEM. between PEM and Figure FEM. 9. Electrical resistivity of subbase soil using PEM and FEM. Figure Figure Correlation Correlation between between resistivity resistivity of of subbase subbase soil soil using using FEM FEM and and PEM. PEM Concrete Concrete Pore Pore water water is is most most important important factor factor affecting affecting resistivity resistivity of of concrete concrete since since current current is is transported transported by by ions ions dissolved dissolved in in pore pore water. water. However, However, pore pore water water in in concrete concrete decreases decreases and and concrete concrete gets gets dry dry by by generating generating hydration hydration products products and and evaporation evaporation with with increase increase in in curing curing time time [5,9]. [5,9]. To To accurately accurately analyze analyze resistivity resistivity of of concrete, concrete, effect effect of of curing time should be considered. Therefore, resistivity of concrete was observed in accordance with curing time (seven and 28 days). The effect of S/a (38.50%, 39.50%, and 40.50%) on resistivity of concrete was investigated. The resistivity increases in accordance with increase of age of concrete and S/a in both PEM and FEM as shown in Figures 11 and 12. There is a small difference in resistivity between PEM and FEM. The overall magnitude and variation of resistivity in

12 Sustainability 2017, 9, of 21 curing time should be considered. Therefore, resistivity of concrete was observed in accordance with curing time (seven and 28 days). The effect of S/a (38.50%, 39.50%, and 40.50%) on resistivity of concrete was investigated. The resistivity increases in accordance with increase of age of concrete and S/a in both PEM and FEM as shown in Figures 11 and 12. There is a small difference in resistivity between PEM and FEM. The overall magnitude and variation of resistivity in accordance with S/a at age of seven days are smaller than those at age of 28 days in both PEM and FEM. At age of seven days, resistivities obtained using FEM are more scattered than those obtained using PEM in accordance with S/a being influenced by electrode spacing. The scattered resistivities obtained using FEM range from 16 Ω m to 32 Ω m at 38.50%, from 16 Ω m to 36 Ω m at 39.50%, and from 18 Ω m to 46 Ω m at 40.50% of S/a. On or hand, at age of 28 days, scattering of resistivity obtained using PEM and FEM is similar in accordance with S/a. This scattering of resistivity may be caused by change in chemical activity owing to age, which can affect current in concrete [5,9]. Sustainability 2017, 9, of 20 Figure 11. Electrical resistivity of concrete at age of seven days using PEM and FEM. Figure 11. Electrical resistivity of concrete at age of seven days using PEM and FEM.

13 Sustainability 2017, 9, of 21 Figure 11. Electrical resistivity of concrete at age of seven days using PEM and FEM. Sustainability 2017, 9, of 20 Figure 12. Electrical resistivity of concrete at age of 28 days using PEM and FEM. Figure 12. Electrical resistivity of concrete at age of 28 days using PEM and FEM. The maximum, minimum, and average resistivities obtained using PEM and FEM at age of The seven maximum, and 28 days minimum, were compared and average without considering resistivities obtained electrode using spacing. PEM The and FEM at age of seven and 28 days were compared without considering electrode spacing. The resistivities of concrete range from 11 Ω m to 46 Ω m at age of seven days and from 26 Ω m to resistivities of concrete range from 11 Ω m to 46 Ω m at age of seven days and from 26 Ω m to Ω m at age of 28 days within range of S/a from 38.50% to 40.50%. Although maximum Ω m at age of 28 days within range of S/a from 38.50% to 40.50%. Although maximum resistivity of FEM is higher than that of PEM at age of seven days, re is only a small resistivity of FEM is higher than that of PEM at age of seven days, re is only a small difference of resistivity between two methods in or conditions. Statistical analysis difference of resistivity between two methods in or conditions. Statistical analysis was performed to investigate correlation between results of PEM and FEM. The results are was performed to investigate correlation between results of PEM and FEM. The results are summarized in Figure 13 and Table 7. At age of seven days, R value of is lower than that summarized in Figure 13 and Table 7. At age of seven days, R value of is lower than that in soil materials and p-value of is lower than α of 0.05, which indicate low correlation of in soil materials and p-value of is lower than of 0.05, which indicate low correlation of resistivity of PEM with that of FEM. At age of 28 days, although R value of is resistivity of PEM with that of FEM. At age of 28 days, although R value of still lower than that in soil materials, p-value of becomes higher than α of 0.05, showing a is still lower than that in soil materials, p-value of becomes higher than of 0.05, showing more reliable linear relationship. a more reliable linear relationship.

14 was performed to investigate correlation between results of PEM and FEM. The results are summarized in Figure 13 and Table 7. At age of seven days, R value of is lower than that in soil materials and p-value of is lower than of 0.05, which indicate low correlation of resistivity of PEM with that of FEM. At age of 28 days, although R value of Sustainability is still lower 2017, than 9, 2320 that in soil materials, p-value of becomes higher than of 0.05, showing 14 of 21 a more reliable linear relationship. Sustainability 2017, 9, of 20 Figure 13. Correlation between resistivity of concrete using FEM and PEM: age of seven Figure 13. Correlation between resistivity of concrete using FEM and PEM: age of seven days; age of 28 days. days; age of 28 days Asphalt Asphalt Since asphalt is an insulator, measurement of resistivity was carried out after a Since asphalt is an insulator, measurement of resistivity was carried out after a period period of time following salt water spraying to improve conductivity of asphalt mixture. of time following salt water spraying to improve conductivity of asphalt mixture. Although Although salt water was sprayed sufficiently on surface of asphalt mixture specimen, a salt water was sprayed sufficiently on surface of asphalt mixture specimen, a negative resistivity, negative resistivity, which should be considered as an error, was measured as shown in Table 8 which should be considered as an error, was measured as shown in Table 8 [16,23,25,26]. Therefore, it is [16,23,25,26]. Therefore, it is difficult to apply ERS directly to an asphalt surface for investigating difficult to apply ERS directly to an asphalt surface for investigating status under pavement. status under pavement. Table 8. Electrical resistivity of asphalt mixtures. Table 8. Electrical resistivity of asphalt mixtures. 4. Field Experiments 4.1. Experimental Method Case Case Time Time (min)(min) Field experiments were performed to investigate applicability of FEM for ERS instead of PEM to detect cavities under pavements. As shown in Figure 14, four different cases are simulated in

15 Sustainability 2017, 9, of 20 Twelve rectangular concrete specimens (55 mm in width and 15 mm in depth and height) were prepared in accordance with mix design of Case 1 given in Table 5 to simulate a concrete Sustainability 2017, 9, of 21 pavement (1650 mm in width and 300 mm in depth and height) between electrode numbers 12 and 20. A cubic cavity (300 mm in width, depth, and height) was simulated as an empty space under simulated 4. Field concrete Experiments pavement between electrode numbers 12 and 14. ERS was 4.1. carried Experimental out using Method thirty electrodes with a spacing of 200 mm, and n = 7: total survey length is 5800 mm in width and 1200 mm in height. The results of ERS were interpreted by DIPRO for Field experiments were performed to investigate applicability of FEM for ERS instead of PEM Windows to V.4.01 detect cavities (HSGEO, underseongnam, pavements. As Korea), shownwhich in Figure is 14, a general-purpose four different cases interpretation are simulated in asoftware. This software field site: inverts field ground measured without cavity, fieldresistance ground withinto cavity, a high-resolution concrete pavement 2D without image cavity, based on finite element and concrete method pavement as shown with cavity. in Figures (c) (d) Figure 14. Figure Cases 14. for Cases field forexperiment: field experiment: field field ground ground without cavity; field field ground ground with with cavity; cavity; (c) concrete (c) pavement concrete pavement without without cavity; cavity; (d) concrete (d) concrete pavement with cavity.

16 Sustainability 2017, 9, of 21 To artificially simulate a cavity in field ground, two types of objects were buried: Styrofoam (400 mm in width, 240 mm in depth, and 140 mm in height) and aluminum foil ball (200 mm in diameter). Generally, resistivity of a cavity may be substantially high or low depending on disturbance of current [2,6,13]. Therefore, se objects were used to verify location of cavity since Styrofoam is considered as an insulator with an extremely low conductivity, while aluminum foil is considered as a conductor showing a substantially high conductivity. When cavity was simulated, excavated area was minimized and carried out natural compaction for a week for minimizing effect of voids and compaction. Twelve rectangular concrete specimens (55 mm in width and 15 mm in depth and height) were prepared in accordance with mix design of Case 1 given in Table 5 to simulate a concrete pavement (1650 mm in width and 300 mm in depth and height) between electrode numbers 12 and 20. A cubic cavity (300 mm in width, depth, and height) was simulated as an empty space under simulated concrete pavement between electrode numbers 12 and 14. ERS was carried out using thirty electrodes with a spacing of 200 mm, and n = 7: total survey length is 5800 mm in width and 1200 mm in height. The results of ERS were interpreted by DIPRO for Windows V.4.01 (HSGEO, Seongnam, Korea), which is a general-purpose interpretation software. This software inverts measured resistance into a high-resolution 2D image based on finite element method as shown in Figures Sustainability 2017, 9, of 20 Figure 15. 2D resistivity contour images for field ground without cavity: PEM; FEM. Figure 15. 2D resistivity contour images for field ground without cavity: PEM; FEM.

17 Sustainability 2017, 9, of 21 Figure 15. 2D resistivity contour images for field ground without cavity: PEM; FEM. Figure 16. 2D resistivity contour images for field ground with cavity: PEM; FEM. Figure 16. 2D resistivity contour images for field ground with cavity: PEM; FEM. Sustainability 2017, 9, of 20 Figure 17. 2D resistivity contour images for concrete pavement without cavity: PEM; Figure 17. 2D resistivity contour images for concrete pavement without cavity: PEM; FEM. FEM.

18 Sustainability 2017, 2320 Figure 17.9,2D resistivity contour images for concrete pavement without cavity: PEM; 18 of 21 FEM. Figure 18. 2D resistivity contour images for concrete pavement with cavity: PEM; Figure 18. 2D resistivity contour images for concrete pavement with cavity: PEM; FEM. FEM Results of Field Experiments The results of ERS for field experiments using PEM and FEM were compared by using 2D resistivity contour images generated by software. The analysis of condition under pavement according to changes in resistivity (high or low resistivity) was carried out with reference to laboratory experiments results Field Ground without Cavity PEM and FEM were used as ERS methods to observe status in field ground without any cavity, which will be used as reference condition for experiments. Figure 15 shows 2D resistivity contour images of field ground without a cavity. The general resistivity of field ground ranges from 100 Ω m to 300 Ω m. Referring to Table 1, field ground may consist of clay, gravel, or sand materials. Within dashed box in figure, resistivity is below 100 Ω m, which could mean a high clay and water content. This trend is similar for both PEM and FEM Field Ground with Cavity ERS was carried out to determine wher FEM could be used to detect a cavity existing in a field ground. Figure 16 shows results of ERS obtained using PEM and FEM. The dashed circle in figure represents approximate locations of Styrofoam and aluminum foil ball buried in field ground. In general, resistivity contours obtained using both PEM and FEM are similar to that of field ground without cavity except for high and low resistivity locations. A high resistivity between 1000 Ω m and 2000 Ω m is observed in both methods at location where Styrofoam was buried at 300 mm depth between electrode numbers 9 and 11. It may be associated with disturbance of current caused by Styrofoam with low conductivity. However, a low resistivity is observed in both methods at location

19 Sustainability 2017, 9, of 21 where aluminum foil ball was buried at 300 mm depth between electrode numbers 20 and 22. The shape of distribution of low resistivity below 80 Ω m obtained using PEM is slightly different from that using FEM. The shape of distribution obtained using PEM is horizontally long, while that using FEM is vertically long Concrete Pavement without Cavity The results of ERS performed using PEM and FEM for concrete pavement without cavity are shown in Figure 17. The dashed box represents approximate location of concrete pavement slabs simulated by rectangular specimens. A high resistivity from 300 Ω m to 400 Ω m is observed in both methods at concrete slab situated between electrode numbers 12 and 20. It may be caused by effect of gap between concrete specimens. However, a low resistivity is observed at lower location between electrode numbers 6 and 14 and between 16 and 24 in both methods. The appearance of low resistivity between electrode numbers 6 and 14 is similar to case of field ground without cavity, while that between electrode numbers 16 and 24 is different. It may be caused by ground disturbance during installation of concrete specimens Concrete Pavement with Cavity ERS was performed to detect cavity simulated under concrete pavement using PEM and FEM as shown in Figure 18. The dashed circle represents approximate location of cavity. A high resistivity from 1000 Ω m to 3000 Ω m is observed at location of cavity between electrode numbers 9 and 17 in both methods. However, shape of cavity observed using FEM is slightly larger than that obtained using PEM. It may be caused by effect of contact resistance on current owing to wide contact area of FEM, as PEM uses a point contact by inserting electrode into concrete specimen. The wide contact area caused current to spread widely. 5. Conclusions The resistivity of soil materials obtained using FEM and PEM in laboratory decreased as water content increased. There was little change of resistivity when water content exceeded optimum water content (ω opt ). The resistivity by electrode spacing using FEM was significantly more scattered than that using PEM at low water content. However, scattered resistivity converged to lowest value as water content increased. The resistivities in accordance with electrode spacing for FEM and PEM showed similar trends. The resistivity of concrete for both methods increased as fine aggregate modulus (S/a) and curing time increased. However, resistivity obtained using FEM at age of seven days significantly scattered differently from PEM. The resistivity obtained at age of 28 days was not scattered in both methods. Although salt water was sprayed on surface of asphalt specimen as an electrolyte, a reasonable resistivity could not be obtained. Statistically, a low correlation between PEM and FEM was observed only for concrete at age of seven days, while a high correlation between two methods was observed for or materials (field ground soil, subgrade soil, subbase soil, and concrete at age of 28 days). In field experiments, a high resistivity was observed for cavity simulated by Styrofoam or empty space, while a low resistivity was observed for aluminum foil ball. The distribution of high or low resistivity in 2D contour caused by Styrofoam, empty space, and aluminum foil ball obtained using FEM was wider than that using PEM. However, results using both methods exactly predicted actual location of cavity simulated in ground and under pavement. Based on results, it is shown that using FEM for ERS instead of PEM is acceptable as difference in results between two methods is small. In addition, FEM is more advantageous since it is easier to use and causes less damage to pavement surface compared with PEM.

20 Sustainability 2017, 9, of 21 Acknowledgments: This research was supported by a research grant of INHA University and by Basic Science Research Program of National Research Foundation (NRF) of Korea funded by Korean Ministry of Education (NRF-2014R1A1A ). Author Contributions: Regarding author s participation in this research, Chang-Seon Park planned and performed laboratory and field experimental tests. He also drafted manuscripts. Jin-Hoon Jeong and Hae-Won Park performed field and laboratory tests and also analyzed experiments. Kyoungchul Kim guided this study, provided valuable comments and revised it. Conflicts of Interest: The authors declare no conflict of interest. References 1. Kalinski, R.J.; Kelly, W.E. Electrical-Resistivity Measurements for Evaluating Compacted-Soil Liners. J. Geotech. Eng. 1994, 120, [CrossRef] 2. Louis, I.F.; Louis, F.I.; Bastou, M. Accurate Subsurface Characterization for Highway Applications Using Resistivity Inversion Methods. J. Electr. Electron. Eng. 2002, Samouëlian, A.; Cousin, I.; Tabbagh, A.; Bruand, A.; Richard, G. Electrical Resistivity Survey in Soil Science: A Review. Soil Tillage Res. 2005, 83, [CrossRef] 4. Park, S.G.; Kim, C.R.; Son, J.S.; Kim, J.H.; Yi, M.J.; Cho, S.J. Detection of Lime Silicate Cavities by 3-D Resistivity Survey. Econ. Environ. Geol. 2006, 39, Farooq, M.; Park, S.G.; Song, Y.S.; Kim, J.H. Mortar Characterization using Electrical Resistivity Method. Geophys. Geophys. Explor. 2009, 12, Gambetta, M.; Armadillo, E.; Carmisciano, C.; Stefanelli, P.; Cocchi, L.; Caratori Tontini, F. Determining Geophysical Properties of a Nearsurface Cave through Integrated Microgravity Vertical Gradient and Electrical Resistivity Tomography Measurements. J. Cave Karst Stud. 2010, 73, [CrossRef] 7. Reynolds, J.M. An Introduction to Applied and Environmental Geophysics; John Wiley & Sons: New York, NY, USA, 2011; ISBN Millard, S.G. Reinforced Concrete Resistivity Measurement Techniques. Proc. Inst. Civ. Eng. 1991, 91, [CrossRef] 9. Polder, R.B. Test Methods for on Site Measurement of Resistivity of Concrete A RILEM TC-154 Technical Recommendation. Constr. Build. Mater. 2001, 15, [CrossRef] 10. Park, J.-Y.; Sohn, D.-S.; Lee, J.-H.; Jeong, J.-H. A Study on Joint Position at Concrete Pavement with Box Culverts. Int. J. Highw. Eng. 2012, 14, [CrossRef] 11. Yeom, W.S.; Jeong, H.S.; Yan, Y.; Sohn, D.S.; Lee, J.H.; Jeong, J.H. Optimal Joint Position in Concrete Pavement Slab over Skewed Box Culvert. Int. J. Highw. Eng. 2013, 15, [CrossRef] 12. Sohn, D.S.; Lee, J.H.; Jeong, H.S.; Park, J.Y.; Jeong, J.H. A Method for Evaluation of Hollow Existence in Sublayers of Concrete Pavement Considering Pavement Stiffness. Int. J. Highw. Eng. 2013, 15, [CrossRef] 13. Kim, J.-H.; Hong, W.-P.; Kim, G.-B. Interpretation of Soft Ground Deformation under Embankment using Electrical Resistivity Survey. J. Eng. Geol. 2011, 21, [CrossRef] 14. Cai, G.H.; Du, Y.J.; Liu, S.Y.; Singh, D.N. Physical Properties, Electrical Resistivity, and Strength Characteristics of Carbonated Silty Soil Admixed with Reactive Magnesia. Can. Geotech. J. 2015, 52, [CrossRef] 15. Sentenac, P.; Hogson, T.; Keenan, H.; Kulessa, B. Small Scale Monitoring of a Bioremediation Barrier Using Miniature Electrical Resistivity Tomography. J. Appl. Geophys. 2015, 115, [CrossRef] 16. Pan, P.; Wu, S.; Xiao, F.; Pang, L.; Xiao, Y. Conductive Asphalt Concrete: A Review on Structure Design, Performance, and Practical Applications. J. Intell. Mater. Syst. Struct. 2015, 26, [CrossRef] 17. Park, S.G.; Kim, J.H.; Cho, S.J.; Yi, M.J.; Son, J.S. Electrical Resistivity Characteristic of Soils. Geophys. Geophys. Explor. 2004, MLTMA (Ministry of Land, Transport and Maritime Affairs). Standard Specification of Road Construction; Ministry of Land, Transport and Maritime Affairs: Sejong City, Korea, ASTM D698-12e2. Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft 3 (600 kn-m/m 3 )); ASTM International: West Conshohocken, PA, USA, [CrossRef] 20. ASTM D Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass; ASTM International: West Conshohocken, PA, USA, [CrossRef]

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