IEEE SENSORS JOURNAL, VOL. 10, NO. 7, JULY 2010 1243

Concrete Moisture Content Measurement UsingInterdigitated Near-Field Sensors

Md. Nazmul Alam, Student Member, IEEE, Rashed H. Bhuiyan, Student Member, IEEE,Roger A. Dougal, Senior Member, IEEE, and Mohammod Ali, Senior Member, IEEE

Abstract—The efficacy of a meander and a circular interdig-itated sensor in detecting and measuring the moisture presentin wet concrete samples is demonstrated. Analytical, simulation,and measurement results of interelectrode capacitance for sam-ples with different moisture contents show good agreement. Asmoisture content increases from 0% to 6%, the interelectrode ca-pacitance that predicts the moisture content increases by 126.4%for the meander sensor and 187% for the circular sensor. Regres-sion analysis of the measured data demonstrates that for moisturecontent less than 6% the relationship between the measuredcapacitance and the percent moisture content is predominantlylinear.

Index Terms—Concrete, interdigitated sensor, moisture sensor.

I. INTRODUCTION

I N ORDER TO ensure public safety, routine monitoring,and inspection of the health of civil infrastructures such as

bridges, overpasses, and buildings is very important. Accordingto the Bureau of Transportation Statistics (BTS), in 2005 about26.2% bridges were structurally deficient in the U.S. [1]. Themain causes of bridge failure were floods and collisions [2]. Forrehabilitation and maintenance, field data are needed as man-dated by the National Bridge Inspection Program. In most cases,bridge health monitoring is done by using ground penetratingradar (GPR) [3] which is time consuming and costly. Innova-tive sensor technology can provide a cheap and a more reliablesolution. Wireless sensors can be used to detect the presence ofmoisture and crack in concrete or the corrosion in the steel re-inforcement bar, etc. For active wireless sensors, their batteriescan be charged by sending wireless power from outside withoutdamaging the structure [4].

Moisture detection in civil infrastructure has been investi-gated by researchers in the past. Xing et al. [5] developed a2.4-GHz integrated parallel-plate soil moisture sensor system.The sensor detects the phase shift caused by the changes in thedielectric constant of soil which is related to the soil moisturecontent. This sensor needs a number of components, suchas a phase locked loop (PLL), voltage controlled oscillator(VCO), phase shifter, phase detector, microcontroller, and

Manuscript received September 09, 2009; revised December 10, 2009;accepted December 23, 2009. Date of current version May 21, 2010. Thiswork was supported in part by the U.S. Office of Naval Research under GrantN00014-02-1-0623. The associate editor coordinating the review of this paperand approving it for publication was Prof. Okyay Kaynak.

The authors are with the Department of Electrical Engineering, Universityof South Carolina, Columbia, SC 29208 USA (e-mail: alam2@cec.sc.edu;bhuiyan@cec.sc.edu; dougal@cec.sc.edu; alimo@cec.sc.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2010.2040175

power management circuitry on board to be fully functional.In [6], Saxena and Tayal described a parallel-plate capacitor asa moisture sensor. The sensor consists of a Wein bridge oscil-lator, a capacitance bridge, a differential amplifier, a band-passfilter, a rectifier circuit, and an analog meter. Ong et al. [7]proposed a 24 MHz embedded LC resonant sensor for moisturecontent measurement, which requires a large external loop andan impedance analyzer.

To maintain the integrity of concrete structures and to allowminimum disruption, an embedded sensor should occupy assmall space as possible. The sensor should also only have veryfew and small components. To address these issues, we proposea planar interdigitated sensor fabricated on a very thin substrate.Because of its thin planar construction the interdigitated sensorcan be easily embedded in concrete for moisture content mea-surement. Our proposed sensor occupies a very small space.

In recent years, interdigitated sensors have drawn the at-tention of researchers due to their low cost and simplicity.They have found application in humidity sensing [8], chemicalsensing [9], [10], gas detection [11], resin curing [12], etc.In our research group Bhuiyan et al. [13] have designed anddeveloped interdigitated sensors which can detect insulationfaults in unshielded power cables. This was achieved by ap-plying a low frequency signal and then measuring the resultinginterelectrode capacitance of the sensor. It was shown thata half circular sensor was less sensitive to its position withrespect to the sample under test than a meander sensor.

In this paper, the efficacy of two interdigitated sensors inmoisture sensing in concrete samples is studied. First, the me-ander sensor introduced in our earlier work [13] was used. Then,a full circular interdigitated sensor with larger field penetrationdepth was designed, built, and tested. Experimental measure-ments were performed to determine the sensor interelectrode ca-pacitance by varying the moisture content of concrete sampleswith both types of sensors. Measurement results are comparedwith results obtained from analytical equations and simulationsfrom Maxwell 3-D solver.

This paper is organized as follows. In Section II, the basicinterdigitated sensor geometry, operating principles and its ana-lytical model are discussed. In Section III, the details of sensorfabrication and the experimental setup and measurement proce-dure are described. All results are presented and discussed inSection IV followed by the conclusion in Section V.

II. SENSOR GEOMETRY, CIRCUIT, AND ANALYTICAL MODEL

An interdigitated sensor is a coplanar structure consistingof multiple parallel fingers. The sensor can measure materialdielectric constant by applying fringing electric fields into the

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1244 IEEE SENSORS JOURNAL, VOL. 10, NO. 7, JULY 2010

Fig. 1. Typical geometry of an interdigitated sensor.

Fig. 2. (a) Block diagram of sensor circuit and (b) its equivalent circuit.

material. The electrodes of the sensor must be in contact withthe material under test. Fig. 1 shows the geometry of an inter-digitated sensor [13]. To learn more about the sensor structureand its operation, the reader may review [12]. Three types ofelectrodes are present, namely the driving electrode, the sensingelectrode, and the guard electrode. A sinusoidal voltage isapplied to the driving electrode and the output voltage ismeasured from the sensing electrode. Guard electrodes and aconducting backplane are used to shield the sensor from the in-fluence of external fields [14]. Another important term is thepenetration depth of the field which is directly related to thespatial wavelength, . The penetration depth is approximatelyequal to 1/3 of [15].

A popular method to find the dielectric constant of a materialis the short-circuit current method, which is shown in Fig. 2.A low-frequency voltage is applied to the driving electrodewhile the sensing electrode is connected to a precision opamp(AD708). The opamp acts as an inverting amplifier where aknown capacitor is used for feedback.

We used a simplified circuit model in Fig. 2(b) to calculatethe capacitance between the driving and the sensing electrodes.

Fig. 3. Unit cell representation of interdigitated sensor.

The substrate has zero conductivity, so the current is due to thecapacitive effect only. Also, since the opamp is operating in theinverting mode, it is obvious that

(1)

Fig. 3 shows a unit cell of an interdigitated sensor without theconducting backplane. Using the conformal mapping methodScapple [16] derived the total per unit capacitance for an inter-digitated meander sensor as

(2)

where and are the elliptic integrals

of modulus and , respectively [16]. Thefirst part on the right-hand side of the equation resembles thecapacitance of the material under test and the substrate and thesecond part resembles the capacitance of the trapped material inbetween the interelectrode gap. In (2), it is assumed that there isno conducting backplane. If a conducting backplane is presentand the substrate is very thin, then the field lines that penetratethe substrate end at the backplane and not at the sensing elec-trodes. So the substrate capacitance has no contribution to .A more accurate expression for per unit capacitance is found in[13]

(3)

where is the corrected per unit capacitance. is the cor-rection factor. For the meander sensor, the correction factor is

(4)

where is a constant. For the typical meander sensor that wasused in our experiment the value of is 0.02. If the electrodelength is and the total number of unit cells is then the totalcapacitance

(5)

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ALAM et al.: CONCRETE MOISTURE CONTENT MEASUREMENT USING INTERDIGITATED NEAR-FIELD SENSORS 1245

Fig. 4. (a) Experimental setup to measure � for concrete samples and(b) placement of the sensor in the samples.

III. EXPERIMENTAL SETUP

To detect the percentage of moisture in concrete, two interdig-itated sensors were designed and fabricated. One was a meandersensor and the other was a circular sensor. Two sensor configu-rations were chosen to create different field penetration depths.The meander sensor is simpler and has smaller field penetra-tion depth than the circular sensor. For the same field penetra-tion depth, the circular sensor covers a larger surface area thanthe meander sensor. The electrode width, height, gap, and spa-tial wavelength of the meander sensor were 1.125 mm, 17 ,1.125 mm, and 4.5 mm, respectively. Thus, the field penetrationdepth for the meander sensor is about 1.5 mm. For the circularsensor these dimensions were 1.5 mm, 17 , 2 mm, and 7 mm,respectively. Hence, the field penetration depth for the circularsensor is about 7/3 mm. The meander sensor was fabricated ona 10 mil thick Duroid 5880 ( ) substrate and the circularsensor was fabricated on a 31 mil thick Duroid 5880 substrate.Referring to Fig. 1, the meander sensor had seven driving elec-trodes, four sensing electrodes, and two guard electrodes. Thelength of each electrode was 20 mm. For the circular sensor(see Fig. 5), the inner diameter of the innermost sensing elec-trode was 6 mm. The circular sensor had 12 driving electrodes,ten sensing electrodes, and two guard electrodes. Both sensorswere coated with very thin perylene coating to avoid short cir-cuit with the concrete, especially in the wet condition. A 1-kHz,10-V (peak) sinusoidal signal was chosen for . The ampli-tude of the signal was small enough to prevent the opamp fromsaturating. The feedback capacitor was 100 pF.

Fig. 5. Ansoft Maxwell 3-D model of full circular sensor.

Concrete block samples were prepared using concrete mix.The age of the samples was more than one year at the time ofthe measurement. The dimensions of the samples were 15 cm

15 cm 2 cm and 15 cm 15 cm 4 cm.The wet basis for the moisture content in a material is defined

as

(6)

where is the mass of the specimen with water and is themass of the specimen in dry condition. The moisture content ofa material by volume can be calculated as

(7)

where is the density of concrete in dry condition and isthe density of water.

For measurement the sensor was placed between two concretesamples. First, the sensor output voltage was measured forthe samples in dry condition. Since was known, the capac-itance was found using (1). After completion of the mea-surement in dry condition, the concrete samples were weighedin a scale to determine . Then, the samples were placed ina 13.5-l water bucket and were completely submerged in waterfor two days.

Wet samples were taken out from the water bucket. Since ex-cess water present on the concrete surface had the potentials forerroneous results (high and high ) due to the high per-mittivity of water the outer surfaces of the samples were wipedoff using a dry cloth.

A detailed experimental measurement setup is shown inFig. 4. The sensor was placed between the wet concrete sam-ples. At regular periodic intervals, was recorded and theweight of the concrete samples was measured. For eachreading, was found using (1) while the moisture content

was found using (6) and (7). Thus, for each case, a newand its corresponding was found. Since the evaporationrate depended on the environment, the ambient temperature andthe air flow, an external fan was used to make the evaporationprocess faster.

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1246 IEEE SENSORS JOURNAL, VOL. 10, NO. 7, JULY 2010

Fig. 6. Measured capacitance, � versus moisture content,� for (a) themeander sensor and (b) the full circular sensor.

IV. RESULTS

A. Experimental Results

Measured versus data for both the meander and thecircular sensor are shown in Fig. 6. A linear regression analysiswas also performed on the measured data. From Fig. 6(a), indry condition while for ,

. The presence of water in the wet concrete clearly re-sulted in the increase in (126.4% increase). The sensitivityof the meander sensor is 0.519 pF/percent change in moisturecontent. Fig. 6(a) also shows that the coefficient of determina-tion when . Measured versusresults for the circular sensor are shown in Fig. 6(b). Since thefield penetration depth of the circular sensor is larger than thatof the meander sensor the resulting linear regression curve inFig. 6(b) is steeper than the curve in Fig. 6(a). The coefficient ofdetermination . In dry conditionwhile with , (187% increase).

Fig. 7. Comparison between the measured, analytical and simulated data forthe meander sensor.

The sensitivity of the circular sensor is 5.454 pF/ percent changein moisture content.

B. Comparison Between Experimental and Simulation Results

Ansoft Maxwell 3-D was used to simulate the sensor re-sponses for both the meander and the circular sensors. Fig. 5 isan example of the simulation model which was used for the fullcircular sensor. In the simulation, we were not able to accountfor the moisture content directly. In [17], experimental data ofrelative permittivity and conductivity versus frequency for fourdifferent levels of moisture content in concrete (from Fig. 2of [17]) are available. These data are valid for the frequencyrange of 10 MHz to 1 GHz. We extended their curves down to1 kHz using curve fitting in Matlab. Then, we developed linearrelations between the relativity permittivity and conductivitywith percentage of moisture content using curve fitting. Theresulting relative permittivity and conductivity values wereused in our Maxwell simulations. The relative permittivity ( )and conductivity ( ) of the concrete samples that were usedin our simulation and analytical studies (3)–(4) were: 4.4323,0.000224; 6.0333, 0.0009; 7.177, 0.00215; 8.549, 0.00444; and9.693, 0.007 for moisture contents of 0%, 1.816%, 3.1135%,4.67%, and 5.968%, respectively.

In Maxwell 3-D, the solution type was electrostatic. Thedriving electrodes were set to 10 V and the other electrodesincluding the backplane were set to 0 V. The default boundarycondition, the Neumann homogeneous condition was used. Thecapacitances between the driving electrodes and the sensingelectrodes were computed for the four different cases.

Since equations (2) to (5) can only correctly describe the ca-pacitance for the meander sensor, analytical results were ob-tained only for the meander sensor.

Comparison between the experimental, simulation, and ana-lytical data for the meander sensor is shown in Fig. 7. As seen,in general the agreement between all three is quite good. Somedeviation is observed at higher moisture content levels. Fig. 8

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ALAM et al.: CONCRETE MOISTURE CONTENT MEASUREMENT USING INTERDIGITATED NEAR-FIELD SENSORS 1247

Fig. 8. Comparison of measured and simulated capacitance for the full circularsensor.

illustrates the comparison between the experimental and simu-lation results for the circular sensor. Again, agreement is excel-lent for low moisture contents. The measured results are 10%to 20% higher than the simulated results for moisture contentslarger than 4%.

V. CONCLUSION

The prospect of detecting and measuring moisture in concretesamples was investigated using two interdigitated sensors, e.g., ameander sensor and a full circular sensor. The measurement dataclearly indicate that both types of sensors can detect and mea-sure moisture in concrete. There is a distinctly linear relation-ship between the moisture content in the concrete and the mea-sured interelectrode capacitance. Hence, the amount of mois-ture present can be easily predicted from the measured capaci-tance. The disagreement between the simulation and measure-ment data can be due to the following. First, as mentioned, inour Maxwell simulations the exact values of the relative permit-tivity and conductivity of concrete at 10 kHz and as function ofmoisture content was not available. We used curve fitting to ex-trapolate data from the measured permittivity and conductivitydata of concrete that was valid from 10 MHz to 1 GHz. SinceMaxwell does not use moisture content values directly accuratevalues of relative permittivity and conductivity are essential toobtain accurate simulation results. Second, perhaps during thewiping off of the concrete surface there was some standing wateron the concrete surface. Although the outside concrete surfaceswere wiped off with a dry cloth but the uneven surface mighthave allowed for some water to still remain on the surface. Inthe future one may envision that such sensors in miniature formwill be integrated with wireless modules and embedded insideconcrete samples for moisture content measurement in order toprevent premature corrosion of the steel reinforcements.

REFERENCES

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[2] K. Wardhana and F. Hadipriono, “Analysis of recent bridge failuresin the United States,” J. Perform. Construct. Fac., vol. 17, no. 3, pp.144–150, Aug. 2003.

[3] C. Maierhofer, “Nondestructive evaluation of concrete infrastructurewith ground penetrating radar,” J. Mater. Civil Eng., vol. 15, no. 3, pp.287–297, May/Jun. 2003.

[4] K. M. Z. Shams and M. Ali, “Wireless power transmission to a buriedsensor in concrete,” IEEE Sensors J., vol. 7, no. 12, pp. 1573–1577,Dec. 2007.

[5] H. Xing, J. Li, R. Liu, E. Oshinski, and R. Rogers, “2.4 GHz on-boardparallel plate soil moisture sensor system,” in Proc. Sensors IndustryConf., Houston, TX, Feb. 2005, pp. 35–38.

[6] S. C. Saxena and G. M. Tayal, “Capacitive moisture meter,” IEEETrans. Ind. Electron. Contr. Instrum., vol. IECI-28, no. 1, pp. 37–39,Feb. 1981.

[7] J. B. Ong, Z. You, J. Mills-Beale, E. L. Tan, B. D. Pereles, and K. G.Ong, “A wireless, passive embedded sensor for real-time monitoringof water content,” IEEE Sensors J., vol. 8, pp. 2053–2058, Dec. 2008.

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[16] R. Y. Scapple, “A trimmable planar capacitor for hybrid applications,”in Proc. 24th Electron. Component Conf., 1974, pp. 203–207.

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Md. Nazmul Alam (S’09) received the B.Sc. andM.Sc. degrees in electrical engineering from theBangladesh University of Engineering and Tech-nology, Dhaka, Bangladesh, in 2006 and 2008,respectively.

Since 2009, he has been a Graduate Research As-sistant with the Microwave Engineering Laboratory,Department of Electrical Engineering, University ofSouth Carolina, Columbia. His research interests in-clude sensor design and integration, RFID, and de-signing of microwave antennas.

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1248 IEEE SENSORS JOURNAL, VOL. 10, NO. 7, JULY 2010

Rashed H Bhuiyan (S’09) received the B.Sc. andM.Sc. degrees in electrical engineering from theBangladesh University of Engineering and Tech-nology, Dhaka, Bangladesh, in 2003 and 2005,respectively. He is currently working towards thePh.D. degree in the Department of Electrical Engi-neering, University of South Carolina, Columbia.

He is the author of multiple international con-ference and journal papers. He also served as afaculty member with the Bangladesh University ofEngineering and Technology, Dhaka, from 2003 to

2005. His current research interests include miniature electromagnetic sensordesign, antennas, and RF circuits.

Roger A. Dougal (M’82–SM’94) received the Ph.D.degree from Texas Tech University, Lubbock, in1983.

He leads the Power and Energy Group, Elec-trical Engineering Department, University of SouthCarolina (USC), Columbia, which has a principalresearch focus in the general area of power elec-tronics. As a member of the Board of Directorsof the Electric Ship Research and DevelopmentConsortium, he oversees USC’s activities related tonew power generation, processing, and distribution

technologies for ships. He is also the Site Director for a new NSF-spon-sored Industry/University Cooperative Research Center for Grid-connectedAdvanced Power Electronics, which is a joint project between USC and theUniversity of Arkansas. Also, since 1996, under sponsorship of the Office ofNaval Research, he has overseen development of the Virtual Test Bed software,which is a comprehensive simulation and virtual prototyping environmentfor multidisciplinary dynamic systems. This environment has been appliedin studies of electric systems for navy ships, electrochemical power sources,hybrid power sources, power electronics, and controls.

Mohammod Ali (M’93–SM’03) received the B.Sc.degree in electrical and electronic engineering fromthe Bangladesh University of Engineering andTechnology, Dhaka, Bangladesh, in 1987, and theM.A.Sc. and Ph.D. degrees, both in electrical engi-neering, from the University of Victoria, Victoria,BC, Canada, in 1994 and 1997, respectively.

He was with the Bangladesh Institute of Tech-nology, Chittagong, from 1988 to 1992. FromJanuary 1998 to August 2001, he was with EricssonInc., Research Triangle Park, NC. Since August

2001, he has been with the Department of Electrical Engineering, Universityof South Carolina, Columbia, where currently he is an Associate Professor. Hehad also held appointments as a Visiting Research Scientist with the MotorolaCorporate EME Research Laboratory, Plantation, FL, from June to August2004. He established the Microwave Engineering Laboratory at the Universityof South Carolina in 2001. He is the author/coauthor of over 100 publicationsand five granted U.S. patents. His research interests include miniaturizedpackaged (embedded) antennas, meta-materials and their antenna applications,distributed wireless sensors and rectennas, and portable/wearable antennas andtheir interactions with humans (SAR).

Dr. Ali is the recipient of the 2003 National Science Foundation Faculty Ca-reer Award. He is also the recipient of the College of Engineering and Informa-tion Technology Young Investigator Award and the Research Progress Awardfrom the University of South Carolina in 2006 and in 2008, respectively. Hewas the Technical Program Co-Chair of the IEEE Antennas and Propagation So-ciety’s International Symposium in Charleston, SC, in 2009. He has also servedas a member of the Technical Program Committee for the IEEE Antennas andPropagation Society’s International Symposium for a number of years. He hasserved as a reviewer and panelist for grant proposals for a number of federal andlocal funding agencies. He is an Associate Editor for the IEEE ANTENNAS AND

WIRELESS PROPAGATION LETTERS.

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