sensors

Article Effects of Ambient and Relative on Subsurface Defect Detection in Concrete Structures by Active Thermal Imaging

Quang Huy Tran 1 ID , Dongyeob Han 1, Choonghyun Kang 1, Achintya Haldar 2 and Jungwon Huh 1,* ID

1 Department of Civil and Environmental Engineering, Chonnam National University, Yeosu, Chonnam 59626, Korea; [email protected] (Q.H.T.); [email protected] (D.H.); [email protected] (C.K.) 2 Department of Civil Engineering and Engineering Mechanics, University of Arizona, Tucson, AZ 85721, USA; [email protected] * Correspondence: [email protected]; Tel.: +82-61-659-7247

Received: 27 June 2017; Accepted: 25 July 2017; Published: 26 July 2017

Abstract: Active thermal imaging is an effective nondestructive technique in the structural health monitoring field, especially for concrete structures not exposed directly to the sun. However, the impact of meteorological factors on the testing results is considerable and should be studied in detail. In this study, the impulse thermography technique with halogen lamps heat sources is used to detect defects in concrete structural components that are not exposed directly to sunlight and not significantly affected by the , such as interior bridge box-girders and buildings. To consider the effect of environment, ambient temperature and relative humidity, these factors are investigated in twelve cases of testing on a concrete slab in the laboratory, to minimize the influence of wind. The results showed that the absolute contrast between the defective and sound areas becomes more apparent with an increase of ambient temperature, and it increases at a faster rate with large and shallow delaminations than small and deep delaminations. In addition, the absolute contrast of delamination near the surface might be greater under a highly humid atmosphere. This study indicated that the results obtained from the active thermography technique will be more apparent if the inspection is conducted on a day with high ambient temperature and humidity.

Keywords: concrete deterioration; active thermal imaging; ambient temperature; relative humidity; infrared thermography; nondestructive testing

1. Introduction Concrete deterioration such as cracking, delamination, and spalling is often the result of a combination of factors such as corrosion of embedded metals, accumulative effect of freeze-thaw successive cycles, chemical attack, alkali-aggregate reactivity, or cracking due to shrinkage [1]. On the other hand, abrasion or overloading on bridge decks and highway pavements due to traffic operations can lead to cracking on the structure’s surface. Of these factors, the corrosion of embedded reinforcing steel in a concrete structure is the most common factor [2]. As a result, delaminations are initially developed at or near the level of the reinforcing bars because rust occupies a greater volume than steel, creating unexpected tensile stresses in the concrete [1,3]. Cracking and delaminations might be invisible upon visual inspection until the deterioration develops into a spalling condition in its later stages, as shown in Figure1. This raises safety concerns, especially for the traffic passing under overpass bridges. To address this problem, thermal imaging/infrared thermography (IRT) has proved to be an effective inspection tool in detecting subsurface deteriorations among other nondestructive techniques

Sensors 2017, 17, 1718; doi:10.3390/s17081718 www.mdpi.com/journal/sensors Sensors 2017, 17, 1718 2 of 18 Sensors 2017, 17, 1718 2 of 18

(NDT)techniques such (NDT) as impact-echo, such as impact-echo, coin tapping, coin tapping ultrasonic, ultrasonic surface wavessurface orwave grounds or ground penetrating penetrating radar. Oneradar. of One the mostof the significant most significant advantages advantages of the IRT of techniquethe IRT technique is its short is detectionits short timedetection compared time tocompared other techniques, to other techniques, and it is possible and it is to possible qualitatively to qualitatively evaluate the evaluate area and the depth area ofand a subsurfacedepth of a delaminationsubsurface delamination [4–7]. However, [4–7]. quantifyingHowever, quanti the delaminationfying the delamination depth appears depth to beappears a considerable to be a challenge,considerable and challenge, only a few and researchers only a few have research attempteders tohave evaluate attempted the depth to evaluate of delamination the depth using of activedelamination IRT to inspect using concreteactive IRT structures. to inspect In addition, concre whilete structures. changes inIn environmental addition, while conditions changes have in aenvironmental significant effect conditions on experimental have a significant results, the effect ambient on experimental temperature, results, relative the humidity, ambient clouds,temperature, wind speed,relative and humidity, solar radiation clouds, wind should sp beeed, considered and solar duringradiation field should testing be [considered8]. Currently, during previous field studiestesting have[8]. Currently, just focused previous on the studies passive have IRT techniquejust focused when on the considering passive IRT the technique environmental when influenceconsidering on the concreteenvironmental detection, influence as mentioned on the concrete in detail detection, in Section 2as, whilementioned the impact in detail of the in environmentSection 2, while on thethe activeimpact IRT of the technique environment is still on an openthe active question. IRT technique is still an open question.

Figure 1. Crack, delamination, and spalling caused by expansion of corroded steel.

The aim ofof thisthis studystudy isis toto surveysurvey concrete concrete structural structural components components that that are are not not exposed exposed directly directly to theto the sunlight sunlight and and are are less less affected affected by theby wind,the wind, such such as inside as inside a bridge a bridge box-girder, box-girder, areas areas between between the T orthe I-girder T or I-girder and diaphragms, and diaphragms, and other and structural other structural components components in the interior in the ofinterior buildings of buildings and factories, and andfactories, so on. and The so impulse on. The thermography impulse thermography technique, onetechnique, of the activeone of IRT the techniques, active IRT is techniques, applied with is sixapplied 500-watt with halogensix 500-watt lamps halogen working lamps as external working heat as external sources heat to transfer sources heat to transfer flux into heat the flux concrete into specimen.the concrete The specimen. impulse The thermography impulse thermography technique is te consideredchnique is throughoutconsidered throughout twelve cases twelve of different cases ambientof different temperature ambient and temperature relative humidity and relative conditions humidity on a concrete conditions slab in on the a laboratory. concrete Ourslab purpose in the inlaboratory. this study Our is to purpose describe in several this study important is to describe characteristics several important of heat transfer characteristics that affect of the heat inspection transfer resultsthat affect of concrete the inspection with different results of environmental concrete with conditions, different environmental and propose a conditions, preliminary and selection propose of a suitablepreliminary weather selection conditions of a suitable for site inspectionweather condit whenions using for the site active inspection thermography when using technique. the active thermography technique. 2. Related Work 2. Related Work An early application of IRT in the civil engineering field related to health monitoring of concrete structuresAn early was application performed of by IRT Clemena in the civil et al.engineering in 1978 [ 9field]. Their related research to health found monitoring that more of concrete severely delaminatedstructures was regions performed induce by a strongerClemena thermal et al. in contrast 1978 [9]. and Their delaminated research concretefound that was more warmer severely than non-delaminateddelaminated regions concrete induce when a stronger directly thermal exposed contrast to sunlight. and delaminated Other studies concrete were was conducted warmer than by Cielonon-delaminated et al. in 1987 concrete [10] and when by Maldague directly inexposed 1993 [11 to], sunlight. focusing onOther delamination studies were depth. conducted In these by studies, Cielo theyet al. observedin 1987 [10] changes and by in theMaldague surface in temperature 1993 [11], focusing of delaminated on delamination layers and depth. the observation In these studies, time to estimatethey observed the depth changes of delamination. in the surface In temperature 2013, Vaghefi of[ 5delaminated] tested four layers concrete and slabs the observation with delaminations time to placedestimate at the various depth depths of delamination. to confirm the In 2013, technique Vaghefi proposed [5] tested by Maldaguefour concrete [11]. slabs In 2008, with Pollock delaminations et al. [4] conductedplaced at various active depths thermal to imaging confirm inspectionsthe technique on proposed six concrete by Maldague specimens [11]. with In embedded 2008, Pollock artificial et al. air[4] conducted voids and post-tensioningactive thermal imaging ducts (PT-ducts) inspections to on simulate six concrete post-tensioned specimens box with girder embedded bridges. artificial It was shownair voids that and the post-tensioning PT-ducts and simulated ducts (PT-ducts) voids in theto si 20mulate cm thick post-tensioned specimens were box moregirder apparent bridges. thanIt was in theshown 30 cm that thick the PT-ducts specimens. and Also, simulated PT-ducts voids were in muchthe 20 clearer cm thick and specimens more visible were in more the thermal apparent images than ofin thethe thinner30 cm thick specimens. specimens. However, Also, it PT-ducts was found were that much two factors clearer need and to more be considered: visible in accessibilitythe thermal (wherebyimages of thethe heat thinner source specimens. needs to beHowever, placed inside it wa thes found box girder) that two and factors cost (whereby need to thebe techniqueconsidered: is time-consumingaccessibility (whereby when testingthe heat for source relatively needs deep to be air placed voids). inside the box girder) and cost (whereby the techniqueFor assessing is time-consuming the influence of when environmental testing for conditions relatively ondeep the air passive voids). IRT results, in 2009 Washer et al.For [12 ]assessing developed the guidelines influence onof environmental how to use the conditions technology on and the where passive it couldIRT results, be applied in 2009 by characterizingWasher et al. [12] the developed environmental guidelines conditions on how (i.e., to solar use loading,the technology diurnal and temperature where it could variations, be applied wind by characterizing the environmental conditions (i.e., solar loading, diurnal temperature variations, wind speed, and relative humidity) necessary for effective thermography in the field. In Washer’s

Sensors 2017, 17, 1718 3 of 18 Sensors 2017, 17, 1718 3 of 18 project,speed, a andlarge relative concrete humidity) test block necessary was made for with effective embedded thermography targets to inmodel the field. the subsurface In Washer’s defects project, at adepths large ranging concrete from test block25 to 127 was mm made (1 to with 5 inches embedded) in concrete. targets Data to model were collected the subsurface for six defectsmonths at anddepths were ranging analyzed from to 25 determine to 127 mm the (1 toenvironmen 5 inches) intal concrete. conditions Data that were would collected make for sixdetection months of and subsurfacewere analyzed delaminations to determine more the possible environmental during conditionspractical bridge that would inspections. make detectionThis research of subsurface trend continueddelaminations until 2013, more with possible Rilya during Rumbayan’s practical work bridge [13]. inspections. Rumbayan Thisdeveloped research a numerical trend continued model untilto predict2013, thermal with Rilya contrasts Rumbayan’s in subsurface work [13 delamination]. Rumbayan based developed on a given a numerical set of data model from to Washer’s predict thermal [12] research.contrasts These in subsurface studies have delamination just focused based on the on passive a given IRT set oftechnique data from when Washer’s considering [12] research. the impact These of studiesenvironment have just on focused the concrete on the passivedetection. IRT techniqueTherefore, when in this considering study, an the extensive impact of environmentexperimental on investigationthe concrete was detection. carried Therefore, out on inthe this size study, and andepth extensive of sub-surface experimental delaminations investigation with was carriedvarious out environmentalon the size and conditions depth of focusing sub-surface on the delaminations active IRT technique with various as mentioned environmental above. conditions focusing onThis the active paper IRT is techniqueorganized as as mentioned follows. First, above. the fundamentals of infrared thermography are introducedThis in paper Section is organized 3 with discussion as follows. on First,the appropriate the fundamentals input parameters of infrared for thermography setting up the are infraredintroduced (IR) camera. in Section Section3 with 4 discussioncontains the on prep thearation appropriate of testing input specimen, parameters testing for settingmethod, up and the instrumentation.infrared (IR) camera. The experimental Section4 contains results are the discussed preparation in ofSection testing 5, specimen,where the analysis testing method, focuses on and theinstrumentation. impact of meteorological The experimental factors on results the test areing discussed results inon Section both area5, where and depth the analysis detection focuses of delaminations.on the impact Finally, of meteorological the conclusion factors is in onSection the testing 6. results on both area and depth detection of delaminations. Finally, the conclusion is in Section6. 3. Fundamentals of Infrared Thermography 3. Fundamentals of Infrared Thermography IRT is known as a technique that measures the infrared radiation emitted by an object and IRT is known as a technique that measures the infrared radiation emitted by an object and converts converts the energy detected into a temperature value [14]. Each pixel then represents a thermal spot the energy detected into a temperature value [14]. Each pixel then represents a thermal spot that is that is shown on a display or image, the so-called thermal infrared image. However, when measuring shown on a display or image, the so-called thermal infrared image. However, when measuring an an object, the IR camera not only receives the radiation emitted from the object, but also the radiation object, the IR camera not only receives the radiation emitted from the object, but also the radiation from other sources such as surrounding objects or the atmosphere, as illustrated in Figure 2. from other sources such as surrounding objects or the atmosphere, as illustrated in Figure2.

FigureFigure 2. Principle 2. Principle of ofan aninfrared infrared (IR) (IR) thermal thermal camera camera receiving receiving radiation. radiation.

TheThe total total radiation radiation (W (Wtot)tot received) received by by the the IR IRcamera camera comprises comprises the the emission emission of of the the object, object, ε·τε ·Wτ Wobj,obj , thethe emission emission of ofthe the surroundings surroundings and and reflected reflected by by the the object, object, (1 (1 −− ε)ετ)·Wτ·Wrefl,refl and, and the the emission emission of ofthe the atmosphere,atmosphere, (1 (1− τ−)·Wτ)atm·W, atm[14–16],[14– as16 ]follows: as follows:

WWετ 1 ετ  W 1 τ  W Wtottot= ε · τ · W obj+(1 − ε) · τ refl· W + (1 − atmτ) · Watm obj re f l , ,(1) (1) = ε · τ · σ · 4444+ ( − ε) · τ · σ · 4 + ( − τ) · σ · 4 ετσTTTTobjobj 11 ετσ  reflTre  f1 l τσ1  atm Tatm

TheThe temperature temperature of of an objectobject can can be be calculated calculated from from Equation Equation (2), where (2), whereσ is the σ Stefan-Boltzmann is the Stefan- Boltzmannconstant (=5.67constant× (=5.6710−8 W/m × 10−82 W/m·K4).2·K Other4). Other parameters parameters must must be setbe set up up in thein the camera, camera, including including the the emissivity of the object material (ε), the transmittance of the atmosphere (τ), the reflected temperature (Trefl), and the atmospheric/ambient temperature (Tatm).

44 WTTtot1 ετσ refl  1 τσ atm T 4 , (2) obj ετσ

Sensors 2017, 17, 1718 4 of 18 emissivity of the object material (ε), the transmittance of the atmosphere (τ), the reflected temperature (Trefl), and the atmospheric/ambient temperature (Tatm). s 4 4 Sensors 2017, 17, 1718 4 Wtot − (1 − ε) · τ · σ · Tre f l − (1 − τ) · σ · Tatm 4 of 18 T = , (2) obj ε · τ · σ According to Equation (2), three parameters are needed to automatically calculate the atmosphericAccording transmission to Equation (2),(τ) threeusing parameters the IR camera: are needed survey to automatically distance, calculaterelative thehumidity, atmospheric and atmospherictransmission temperature. (τ) using the IRThe camera: formula survey used distance,by the FLIR relative camera humidity, is as follows and atmospheric [17,18]: temperature. The formula used by the FLIR camera is as follows [17,18]:  dK, exp  d   1 K exp  d  , atm√11 atm √ 22 (3) h √ i h √ i τ(d, ω) = Katm · exp − d α1 + β1 ω + (1 − Katm) · exp − d α2 + β2 ω , (3) 23 %%1234,expTatm  h  hT atm hT atm hT atm , (4)  2 3  ω(ω%, Tatm) = ω% · exp h1 + h2 · Tatm + h3 · Tatm + h4 · Tatm , (4) Here, ω is the coefficient indicating the content of vapor in the atmosphere and ω% is the Here, ω is the coefficient indicating the content of water vapor in the atmosphere and ω% is relative humidity. Other parameters include distance (d), the scaling factor for the atmosphere the relative humidity. Other parameters include distance (d), the scaling factor for the atmosphere damping (Katm = 1.9), attenuation for atmosphere without water vapor (α1, α2), attenuation for water damping (Katm = 1.9), attenuation for atmosphere without water vapor (α1, α2), attenuation for water vapor (β1, β2), and h1 = 1.5587, h2 = 6.939 × 10−2, h3 = −2.7816 × 10−4, and h4 = 6.8455 × 10−7. vapor (β , β ), and h = 1.5587, h = 6.939 × 10−2, h = −2.7816 × 10−4, and h = 6.8455 × 10−7. The1 characteristics2 1 were specified2 using Equations3 (3) and (4) by Minkina4 et al. [17], and shown The characteristics were specified using Equations (3) and (4) by Minkina et al. [17], and shown in in Figure 3 with specific parameters of α1 = 0.0066, α2 = 0.0126, β1 = −0.0023, and β2 = −0.0067. Figure3 with specific parameters of α1 = 0.0066, α2 = 0.0126, β1 = −0.0023, and β2 = −0.0067.

(a) (b)

Figure 3. CharacteristicsCharacteristics of of atmospheric transmission transmission coefficient coefficient ττ = ff(d)(d) for for longwave longwave band band LW LW ThermaCAM PMPM 595 595 camera: camera: (a) Function(a) Function of the of relative the relative humidity humidityω%; (b) functionω%; (b) of function the atmospheric of the atmospheric temperature Tatm [17]. temperature Tatm [17].

As shown in Figure 3, the atmospheric transmission (τ) is close to 1.0 when the survey distance As shown in Figure3, the atmospheric transmission ( τ) is close to 1.0 when the survey distance is is fairly small (e.g., a common distance for active IRT is from approximately 3 to 5m). Therefore, only fairly small (e.g., a common distance for active IRT is from approximately 3 to 5 m). Therefore, only the emissivity (ε) and the reflected temperature (Trefl) in Equation 2 are necessary for measuring the the emissivity (ε) and the reflected temperature (T ) in Equation 2 are necessary for measuring the target object under a survey at a near distance. Furthermore,refl the emissivity of a dry concrete surface target object under a survey at a near distance. Furthermore, the emissivity of a dry concrete surface has been shown to be a stable value of 0.95 through many studies, and is commonly specified in has been shown to be a stable value of 0.95 through many studies, and is commonly specified in textbooks, camera user manuals [15,16,19], and by the American Concrete Institute (ACI) for infrared textbooks, camera user manuals [15,16,19], and by the American Concrete Institute (ACI) for infrared thermography methods [20]. In addition, an aluminum foil was used to evaluate the emissivity of the thermography methods [20]. In addition, an aluminum foil was used to evaluate the emissivity of concrete specimen, and the result was very close to 0.95. Therefore, in this study, the emissivity is the concrete specimen, and the result was very close to 0.95. Therefore, in this study, the emissivity is fixed at a value of 0.95 throughout all tests. The reflected temperature is the same as the atmospheric fixed at a value of 0.95 throughout all tests. The reflected temperature is the same as the atmospheric temperature for measuring an object with high emissivity in most cases [16]. Hence, for the IR camera set-up, it is considered that the reflected temperature corresponds to the ambient temperature.

4. Experimental Design and Procedure

4.1. Preparation of Testing Specimen To assess the influence of environmental conditions on the subsurface delamination detection, a concrete specimen was fabricated with the design compression strength of 28 MPa. The concrete mixture ratio of water, cement, sand, and aggregate are 0.5, 1.0, 1.2, and 2.6, respectively. The thermal

Sensors 2017, 17, 1718 5 of 18 temperature for measuring an object with high emissivity in most cases [16]. Hence, for the IR camera set-up, it is considered that the reflected temperature corresponds to the ambient temperature.

4. Experimental Design and Procedure

4.1. Preparation of Testing Specimen To assess the influence of environmental conditions on the subsurface delamination detection, aSensors concrete 2017, specimen17, 1718 was fabricated with the design compression strength of 28 MPa. The concrete5 of 18 mixture ratio of water, cement, sand, and aggregate are 0.5, 1.0, 1.2, and 2.6, respectively. The thermal 2 diffusivitydiffusivity ((αα)) ofof thethe concreteconcrete specimenspecimen isis estimatedestimated toto bebe 1.6361.636 cm cm2/min/min as described below. Twelve delaminationsdelaminations were simulatedsimulated with differentdifferent dimensionsdimensions andand depthsdepths embeddedembedded insideinside thethe concreteconcrete specimen.specimen. The delaminations considered in this studystudy are squaresquare shapesshapes ofof polystyrene,polystyrene, denoteddenoted hereafterhereafter as a capital capital letter letter “D”, “D”, with with the the size size of of 10 10 cm cm × 10× cm,10 cm, 7 cm 7 cm× 7 cm,× 7 5 cm, cm 5× cm5 cm,× and5 cm, 3 cm and × 33 cm at× 13 cm cm deep at 1 (D9 cm deepto D12), (D9 2 cm to D12),deep (D5 2 cm to deepD8), and (D5 3 to cm D8), deep and (D1 3 to cm D4) deep from (D1 the to specimen's D4) from thesurface specimen as shown's surface in Figure as shown 4. The in Figuredelamination4. The delaminationss are simulated are using simulated polystyrene, using polystyrene,the thermal theconductivity thermal conductivity of which is of0.027 which W/m·°C, is 0.027 which W/m is·◦ C,sim whichilar to is that similar of air to (0.024 that of W/m·°C air (0.024 [21]) W/m and·◦ C[is thus21]) andexpected is thus to expected behave in to a behave similar in manner a similar to mannerthat of an to air that void of an when air void receiving when heat receiving flux [22]. heat flux [22].

Figure 4. Concrete specimen with embedded artificial delaminations. Figure 4. Concrete specimen with embedded artificial delaminations.

4.2.4.2. Testing MethodMethod TwelveTwelve casescases areare consideredconsidered inin aa seriesseries ofof experimentsexperiments thatthat areare carriedcarried outout inin thethe laboratorylaboratory onon differentdifferent daysdays underunder differentdifferent environmentalenvironmental conditions.conditions. SixSix halogenhalogen lampslamps (providing(providing aa maximummaximum energyenergy outputoutput of of 3000 3000 watts) watts) as as illustrated illustrated in in Figure Figu5re were 5 were placed placed at 1.2 at m 1.2 from m from the specimen the specimen and were and appliedwere applied as an externalas an external heat source heat bysource providing by providing heat flux heat to the flux concrete to the specimen.concrete specimen. For this technique, For this thetechnique, specimen the was specimen heated forwas 15 heated min and for a long-wavelength15 min and a long-wavelength IR thermal camera, IR thermal FLIR SC660 camera, [23], FLIR was usedSC660 to [23], record was the used variation to record in temperaturesthe variation ofin thetemp specimeneratures everyof the 10specimen s during every both 10 the s heatingduring both and coolingthe heating periods. and Thecooling major periods. technical The data major of thetechnical camera data are listedof the incamera Table1 .are The listed camera in Table was placed 1. The atcamera 2.9 m was from placed the specimen at 2.9 m andfrom was the setspecimen at a constant and was emissivity set at a constant value of emissivity 0.95 for all value cases of 0.95 testing. for Theall cases ambient of testing. temperature The ambient was measured temperature using anwas ambient measured temperature using an andambient humidity temperature meter (CEM and ◦ ◦ DT-615),humidity ranging meter from(CEM 15.8 DT-615),C to 21.9 rangingC for from the twelve 15.8 °C cases, to and21.9 the°C correspondingfor the twelve relative cases, humidityand the wascorresponding varied from relative 71% to humidity 35%. Half was of varied the experimental from 71% to cases 35%. were Half testedof the onexperimental rainy days cases while were the othertested half on wasrainy tested days on while sunny the days. other The half detailed was te valuessted on of thesunny meteorological days. The detailed conditions values for the of IRTthe inspectionmeteorological are given conditions in Table for2. the The IRT atmospheric inspection transmission are given in canTable be 2. automatically The atmospheric calculated transmission by the cameracan be automatically based on the calculated information by ofthe camera camera distance, based on ambient the information temperature, of camera and relativedistance, humidity, ambient astemperature, mentioned and previously. relative humidity, as mentioned previously.

Figure 5. Photograph of the thermography system.

Sensors 2017, 17, 1718 5 of 18 diffusivity (α) of the concrete specimen is estimated to be 1.636 cm2/min as described below. Twelve delaminations were simulated with different dimensions and depths embedded inside the concrete specimen. The delaminations considered in this study are square shapes of polystyrene, denoted hereafter as a capital letter “D”, with the size of 10 cm × 10 cm, 7 cm × 7 cm, 5 cm × 5 cm, and 3 cm × 3 cm at 1 cm deep (D9 to D12), 2 cm deep (D5 to D8), and 3 cm deep (D1 to D4) from the specimen's surface as shown in Figure 4. The delaminations are simulated using polystyrene, the thermal conductivity of which is 0.027 W/m·°C, which is similar to that of air (0.024 W/m·°C [21]) and is thus expected to behave in a similar manner to that of an air void when receiving heat flux [22].

Figure 4. Concrete specimen with embedded artificial delaminations.

4.2. Testing Method Twelve cases are considered in a series of experiments that are carried out in the laboratory on different days under different environmental conditions. Six halogen lamps (providing a maximum energy output of 3000 watts) as illustrated in Figure 5 were placed at 1.2 m from the specimen and were applied as an external heat source by providing heat flux to the concrete specimen. For this technique, the specimen was heated for 15 min and a long-wavelength IR thermal camera, FLIR SC660 [23], was used to record the variation in of the specimen every 10 s during both the heating and cooling periods. The major technical data of the camera are listed in Table 1. The camera was placed at 2.9 m from the specimen and was set at a constant emissivity value of 0.95 for all cases of testing. The ambient temperature was measured using an ambient temperature and humidity meter (CEM DT-615), ranging from 15.8 °C to 21.9 °C for the twelve cases, and the corresponding relative humidity was varied from 71% to 35%. Half of the experimental cases were tested on rainy days while the other half was tested on sunny days. The detailed values of the meteorological conditions for the IRT inspection are given in Table 2. The atmospheric transmission Sensorscan be2017 automatically, 17, 1718 calculated by the camera based on the information of camera distance, ambient6 of 18 temperature, and relative humidity, as mentioned previously.

FigureFigure 5.5. Photograph ofof thethe thermographythermography system.system.

Table 1. Technical data of FLIR camera SC660 [23].

Items Parameters IR resolution 640 × 480 pixels Thermal sensitivity/NETD <30 mK @ + 30 ◦C Field of view (FOV) 24◦ × 18◦/0.3 m Spatial resolution (IFOV) 0.65 mrad Focal Plane Array (FPA) Uncooled microbolometer Wavelength 7.5~13 µm Temperature range −40 ◦C~+120 ◦C Accuracy ±1 ◦C or ±1% of reading

Table 2. Various meteorological conditions for the infrared thermography (IRT) Inspection.

Measurement No. Meteorological Conditions Ambient Temperature (◦C) Relative Humidity (%) 1 sunny day 21.9 35 2 sunny day 21.4 44 3 sunny day 21.3 45 4 sunny day 20.7 36 5 cloudy after the rain 19.6 66 6 sunny day 19.0 37 7 light rain 18.7 64 8 cloudy after the rain 18.6 68 9 showers 17.7 70 10 moderate rain 16.4 71 11 sunny day 16.3 58 12 light rain 15.8 70

5. Analysis of Experimental Results In evaluating the defective depth and the percentage of delaminated areas in the concrete specimen, it is critical to consider the parameter of the maximum thermal contrast between the sound and defective areas, herein termed the “absolute contrast”. The thermal image clearly shows that as the absolute contrast increases, the detectability of the sub-surface defects increases. Therefore, this parameter is the main subject for analysis under the influence of atmospheric temperature and relative humidity, with the aim of taking a holistic view when applying this active thermographic method for concrete structures in the field.

5.1. Correlation between Ambient Temperature and Relative Humidity The variation in the relative humidity occurred because the saturated vapor pressure is determined by the air temperature. As the water vapor content stays the same, the temperature increases, the relative humidity decreases substantially, and as the temperature drops, the relative humidity rises substantially [24]. The same behavior was shown for the twelve test cases in this study. When sorting Sensors 2017, 17, 1718 7 of 18 all ambient temperature data in descending order, the general trend of relative humidity increased correspondingly, except for some test cases which showed low values of relative humidity, such as Sensorstests number 2017, 17, 1718 4 and 6 as shown in Figure6. 7 of 18

FigureFigure 6. 6. CorrelationCorrelation between between ambient ambient temp temperatureerature and relative humidity.

The ambient temperature and relative humidity might influence influence the test results in different ways. For For example, example, if if the the surrounding surrounding ambient ambient temper temperatureature is higher is higher than than the thetemperature temperature inside inside the concrete,the concrete, heat heat flux flux will will conduct conduct into into the the concrete concrete to to achieve achieve thermal thermal equilibrium. equilibrium. The The rate rate of of heat transfer from the outside atmosphere to the concrete is controlled by convec convectiontion [12]. [12]. However, However, since this study focuses on structures that are less affe affectedcted by the wind speed, the heat transfer typically depends only on the heat conduction from the ambien ambientt air. In addition, the relative humidity of the atmosphere appears to influence influence the characteristics.characteristics. It is expected that the heat flux flux would be transferred toto thethe concrete concrete more more quickly quickly in in a humida humid atmosphere atmosphere than than in a in dry a atmospheredry atmosphere [25]. [25]. This Thisleads leads to an to increase an increase of the of thermal the thermal gradient gradient between between the outside the outside surfaces surfaces and the and core the of the core concrete. of the concrete.In this study, In this therefore, study, therefore, the absolute the contrast absolute in contrast the defective in the areadefective is separately area is separately investigated investigated under the undereffects the of ambient effects of temperature ambient temperature and relative and humidity relative inhumidity Sections in 5.2 Sections and 5.3 ,5.2 respectively. and 5.3, respectively.

5.2. Effect of Ambient Temperature on the Absolute ContrastContrast From thethe experimental experimental data, data, defective defective areas areas are easily are identifiedeasily identified as hotter thanas hotter other surroundingthan other surroundingsound areas becausesound areas the delamination because the delamination interrupts the in heatterrupts transfer the heat through transfer the concrete. through the The concrete. area can Thebe calculated area can be by calculated counting theby counting number ofthe pixels number on theof pixels thermal on images.the thermal As images. mentioned As previously,mentioned previously,as the thermal as the gradient thermal increases, gradient the increases, defective the boundaries defective boundaries become easier become to identify. easier to In identify. this study, In thisthe temperaturestudy, the , gradient,∆T(t), (also ΔT( termedt), (also thetermed absolute the absolute contrast contrast [6], or [6], thermal or thermal contrast contrast [26]) [26])between between the surface the surface temperature temperature of the of defective the defective area, area,Td(t), T andd(t), thatand ofthat the of background the background sound sound area, Tarea,s(t), T cans(t), be can defined be defined by the by following the following equation equation [6], [6],

∆T(t) = T (t) − T (t), (5) Tt Tdds  t  Ts  t, (5)

The absolute contrast was obtained by taking the averageaverage surface temperature within the boundaries of of an area above the suspectedsuspected defect minus the average temperature on the perimeter around the suspected defect as illustrated in Figure 77[ [5].5]. The sizes of these two squares were changed according to the size of each delamination. This This meth methodod has has been shown to be the most effective as it diminishes the variability in in selectin selectingg only only one one point point [6,27] [6,27] or or a a group of three points in the previous study [22]. [22]. In Washer et al.’s (2009) study on developing passive infrared thermography inspection technology for bridges, it was observed that when the rate of change of the ambient temperature was close to zero (i.e., the ambient temperature is constant), the absolute contrast in the testing concrete slab began to diminish. However, when receiving heat energy from the sun in the morning, the rapid change in the air temperature caused the development of the absolute contrast in the test block [12]. Corresponding to the active IRT technology applied on the concrete specimen in this study, similar outcomes were found to those Washer’s studies using passive thermography. As the ambient temperature increases, the absolute contrast on the defect also increases, and as the ambient

Sensors 2017, 17, 1718 8 of 18 temperature drops, the absolute contrast on the defect also decreases. On the other hand, it is generally recognized that the absolute contrast increases at a faster rate in the large and shallow delaminationsSensors 2017, 17, 1718 than in the smaller and deeper delaminations. 8 of 18

FigureFigure 7. 7. NewNew evaluation evaluation method method to to calculate calculate absolute absolute contrast. contrast.

TheIn Washer results et shown al.’s (2009) in Figure study 8 on illustrate developing the absolute passive infrared contrasts thermography obtained from inspection delaminations technology D4, D8,for bridges,and D12 it having was observed the same that size when of the10 rate× 10 ofcm change and different of the ambient depths temperature under the wastwelve close cases to zero of dissimilar(i.e., the ambient environmental temperature conditions. is constant), The absolute the absolute contrast contrast of the inshallow the testing delamination concrete slab(D12) began shows to adiminish. higher temperature However, when than receiving the other heat delaminations. energy from theIn sunaddition, in the morning,the group the of rapidtwelve change test cases in the displaysair temperature different caused absolute the development contrasts for of theeach absolute delamination contrast depth in the testunder block varying [12]. Corresponding atmospheric conditions.to the active In IRT particular, technology as the applied ambient on temperature the concrete decreases specimen from in this 21.9 study, °C to similar 15.8 °C, outcomes the absolute were contrastfound to of those D12, Washer’s D8, and D4 studies also decreased using passive from thermography. 3.03 to 2.45, from As the1.35 ambient to 1.02, temperatureand from 0.86 increases, to 0.69, respectively,the absolute contrastas depicted on thein Figure defect also8b. This increases, can be and explained as the ambient by considering temperature that, drops,when the the ambient absolute temperaturecontrast on thedecreases, defect also the decreases. thermal gradient On the other between hand, the it issurfaces generally and recognized core of the that concrete the absolute also decreases.contrast increases Consequently, at a faster the rateheat influx the transferre large andd shallowfrom the delaminations heat sources than(halogen in the lamps) smaller to andthe concretedeeper delaminations. core decreased. Thus, the absolute contrast was similarly low under low ambient temperatureThe results with shown the same in Figure size and8 illustrate depth of the defect, absolute and the contrasts same heating obtained time, from type delaminations of heat source, D4, andD8, heat and transfer D12 having energy. the same size of 10 cm × 10 cm and different depths under the twelve cases of dissimilarFigure 9a environmentalillustrates the linear conditions. relationship The absolutebetween contrastabsolute ofcontrast the shallow and ambient delamination temperature, (D12) inshows which a the higher correlation temperature coefficient than values the other (R2) delaminations.give relatively low In values addition, of 0.5352, the group 0.5695, of twelveand 0.5791 test correspondingcases displays to different the delamination absolute contrasts sizes of 10 for × 10 each cm delamination at 1 cm, 2 cm, depthand 3 cm, under respectively. varying atmospheric The slopes ofconditions. the results In of particular, the linear as theequations ambient also temperature indicate the decreases positive from correlation 21.9 ◦C tobetween 15.8 ◦C, the the ambient absolute temperaturecontrast ofD12, and D8,the absolute and D4 alsocontrast decreased that was from developed 3.03 to 2.45, above from the 1.35delaminations to 1.02, and with from different 0.86 to 2 2 depths.0.69, respectively, Higher R values as depicted are obtained in Figure than8b. those This from can be the explained Washer’s by field considering test (with that,R = 0.22). when This the mightambient be temperaturedue to the effects decreases, of wind the speed, thermal differen gradientt solar between loading, the and surfaces cloud andpath core during of the the concrete test. In addition,also decreases. the slope Consequently, tendency theof a heat linear flux line transferred becomes from steeper the heat with sources shallower (halogen delamination. lamps) to theIn particular,concrete core the decreased. slopes of linear Thus, theregression absolute lines contrast are was0.0723, similarly 0.0409, low and under 0.024 low for ambientdelaminations temperature D12, D8,with and the D4 same at 1 sizecm, 2 and cm depthand 3 cm of defect,depths, and respectively. the same heating time, type of heat source, and heat transferThe energy.trend of thermal development on the defective areas due to the increase of ambient temperatureFigure9 isa illustratesalso investigated the linear for relationshipdelaminations between with different absolute sizes, contrast although and ambient with the temperature, same depth asin shown which thein Figure correlation 9b. Less coefficient steep slopes values are (R obtain2) giveed relatively for smaller low sizes values of ofdelamination; 0.5352, 0.5695, for and example, 0.5791 thecorresponding slope of linear to regression the delamination is only sizes0.0325 of for 10 D9 cm (3× × 103 cm), cm atwhereas 1 cm, 2it cm,is 0.047, and 0.0544, 3 cm, respectively. and 0.0723 forThe delaminations slopes of the D10 results (5 × of 5 thecm), linear D11 (7 equations × 7 cm), and also D12 indicate (10 × the10 cm), positive respectively. correlation It can between be stated the thatambient the absolute temperature contrasts and theare absolutefairly low contrast for defe thatcts with was developedsmall size and above deeper the delaminations depth. Hence, with the temperaturedifferent depths. gradient Higher on Rthe2 values defectare due obtained to the change than those of ambient from the temperature Washer’s field is found test (with to be R difficult2 = 0.22). toThis assess. might be due to the effects of wind speed, different solar loading, and cloud path during the test. In addition, the slope tendency of a linear line becomes steeper with shallower delamination. In particular, the slopes of linear regression lines are 0.0723, 0.0409, and 0.024 for delaminations D12, D8, and D4 at 1 cm, 2 cm and 3 cm depths, respectively.

SensorsSensors2017 2017, ,17 17,, 1718 1718 9 of 189 of 18 Sensors 2017, 17, 1718 9 of 18

(a) (a)

(b) (b) Figure 8. (a) Average surface temperature above the defects; and (b) average absolute contrast above Figure 8. (a) Average surface temperature above the defects; and (b) average absolute contrast above Figurethe defects 8. (a )for Average a total of surface 55 min temperature for both the aboveheating the and defects; cooling and time. (b ) average absolute contrast above the defects for a total of 55 min for both the heating and cooling time. the defects for a total of 55 min for both the heating and cooling time.

(a) (a) Figure 9. Cont.

SensorsSensors2017 2017, 17, 17, 1718, 1718 10 of10 18 of 18

(b)

Figure 9. Relationship between absolute contrast and atmospheric temperature: (a) Same size Figure 9. Relationship between absolute contrast and atmospheric temperature: (a) Same size delamination (10 × 10 cm) with different depths of delamination D12 (at 1 cm), D8 (at 2 cm) and D4 delamination (10 cm × 10 cm) with different depths of delamination D12 (at 1 cm), D8 (at 2 cm) (at 3 cm); (b) same depth (1 cm) with different sizes of delamination D12 (10 × 10 cm), D11 (7 × 7 cm), and D4 (at 3 cm); (b) same depth (1 cm) with different sizes of delamination D12 (10 cm × 10 cm), D11 D10 (5 × 5 cm), and D9 (3 × 3 cm). (7 cm × 7 cm), D10 (5 cm × 5 cm), and D9 (3 cm × 3 cm). 5.3. Effect of Relative Humidity on the Absolute Contrast The trend of thermal development on the defective areas due to the increase of ambient temperature The rate of convective heat transfer typically depends on the atmospheric temperature and wind is also investigated for delaminations with different sizes, although with the same depth as shown in speed [12], and is influenced by the relative humidity of the ambient air surrounding the concrete Figure9b. Less steep slopes are obtained for smaller sizes of delamination; for example, the slope of linear specimen [25]. In particular, a noticeable increase is observed in the convective heat transfer × regressioncoefficient is with only an 0.0325 increase for D9of the (3 cm relative3 cm), humidity. whereas In other it is 0.047, words, 0.0544, the effect and 0.0723on the fordetectability delaminations of × × × D10the(5 subsurface cm 5 cm), defect D11 under (7 cm a humid7 cm), atmosphere and D12 might (10 be10 greater cm), respectively. than that under It can a dry be atmosphere. stated that the absoluteIn contraststhis study, are to fairlyinvestigate low for the defects effect of with rela smalltive humidity size and on deeper the active depth. thermographic Hence, the temperature method, gradientdata coinciding on the defect with due ambient to the temperature change of ambient (0.1 °C temperature difference) with is found varying to be relative difficult humidity to assess. are considered. These data are divided into three groups with the average atmospheric temperatures of 5.3. Effect of Relative Humidity on the Absolute Contrast 16.35 °C, 18.65 °C, and 21.35 °C, along with three pairs of relative humidity (71% and 58%), (68% and 64%),The and rate (45% of convective and 44%), heatrespectively. transfer typicallyAll data are depends plotted on in the Figure atmospheric 10, illustrating temperature the change and windof speedabsolute [12], contrast and is influencedunder similar by testing the relative conditions, humidity though of with the ambient varying humidity. air surrounding The difference the concrete of specimenabsolute [25contrast]. In particular, is more clearly a noticeable shown increasein Figure is 10a observed than in in Figure the convective 10b,c, especially heat transfer for shallower coefficient withdelaminations an increase from of the D9 relative to D12. humidity. The absolute In other contra words,st values the effect when on tested the detectability with high humidity of the subsurface show defectlarger under values a humidthan those atmosphere tested in mightthe low be humid greater atmosphere. than that under This might a dry atmosphere.be due to the significant relativeIn this humidity study, todifference investigate shown the in effect Figure of relative10a of approximately humidity on 13%, the active compared thermographic to 4% and 1% method, for other two cases shown in Figure 10b,c, respectively. For deeper delaminations from D2 to D8, the data coinciding with ambient temperature (0.1 ◦C difference) with varying relative humidity are change of absolute contrasts due to the influence of humidity observed from this experiment is not considered. These data are divided into three groups with the average atmospheric temperatures significant. It should be noted that this judgment is based on limited data and that uncertainties of 16.35 ◦C, 18.65 ◦C, and 21.35 ◦C, along with three pairs of relative humidity (71% and 58%), occurred in the environmental conditions during testing. It is recommended that relative humidity (68% and 64%), and (45% and 44%), respectively. All data are plotted in Figure 10, illustrating needs to be considered as a secondary factor in a practical point of view when using active thethermography change of absolute in a field contrast application. under similar testing conditions, though with varying humidity. The difference of absolute contrast is more clearly shown in Figure 10a than in Figure 10b,c, especially for shallower delaminations from D9 to D12. The absolute contrast values when tested with high humidity show larger values than those tested in the low humid atmosphere. This might be due to the significant relative humidity difference shown in Figure 10a of approximately 13%, compared to 4% and 1% for other two cases shown in Figure 10b,c, respectively. For deeper delaminations from D2 to D8, the change of absolute contrasts due to the influence of humidity observed from this experiment is not significant. It should be noted that this judgment is based on limited data and that uncertainties occurred in the environmental conditions during testing. It is recommended that relative

Sensors 2017, 17, 1718 11 of 18 humidity needs to be considered as a secondary factor in a practical point of view when using active thermographySensors 2017, 17 in, 1718 afield application. 11 of 18

(a)

(b)

(c)

Figure 10. Relationship between absolute contrast and relative humidity: (a) for the ambient Figure 10. Relationship between absolute contrast and relative humidity: (a) for the ambient temperature from 16.3 C to 16.4 C; (b) for the ambient temperature from 18.6 C to 18.7 C; (c) for temperature from 16.3 ◦C to 16.4 ◦C; (b) for the ambient temperature from 18.6 ◦C to 18.7 ◦C; (c) for the the ambient temperature from 21.3 C to 21.4 C. ambient temperature from 21.3 ◦C to 21.4 ◦C. In addition, while larger delaminations are normally more expected for higher temperatures thanIn addition,smaller whiletemperatures larger delaminations at a similar aredepth, normally delamination more expected D7 indicates for higher unusual temperatures behavior. than smallerParticularly, temperatures the test at results a similar shown depth, in Figure delamination 10 indicate D7 that indicates the absolute unusual contrast behavior. of delamination Particularly, D7 the (7 cm × 7 cm) has a higher temperature value than that of the larger delamination, D8 (10 cm × 10 cm) at the same depth of 2 cm. This could be due to non-uniform heating on the specimen surface and/or

Sensors 2017, 17, 1718 12 of 18 testSensors results 2017, 17 shown, 1718 in Figure 10 indicate that the absolute contrast of delamination D7 (7 cm × 127 cm)of 18 has a higher temperature value than that of the larger delamination, D8 (10 cm × 10 cm) at the same due to the reflection of the heat from the concrete surface, as reported by Vaghefi [5]. It is also depth of 2 cm. This could be due to non-uniform heating on the specimen surface and/or due to the observed in this research that the existence of tiny air voids in the concrete cover above both defects reflection of the heat from the concrete surface, as reported by Vaghefi [5]. It is also observed in this of D6 and D7 are due to the low compaction when fabricating the specimen. This leads to several research that the existence of tiny air voids in the concrete cover above both defects of D6 and D7 are hotter points above the defect location that might increase the average temperature of the defective due to the low compaction when fabricating the specimen. This leads to several hotter points above area. It is noted that the selection of a suitable defective zone plays an important role in absolute the defect location that might increase the average temperature of the defective area. It is noted that contrast analysis. the selection of a suitable defective zone plays an important role in absolute contrast analysis. It is also important to note that absolute contrast values are significantly dependent on the It is also important to note that absolute contrast values are significantly dependent on the method method by which they are obtained. In the selection of the reference background temperature using by which they are obtained. In the selection of the reference background temperature using the one- the one- or three-point method as mentioned previously, the absolute contrast can easily vary if the or three-point method as mentioned previously, the absolute contrast can easily vary if the reference reference point is moved. To deal with this problem, in this research the reference background point is moved. To deal with this problem, in this research the reference background temperature is temperature is obtained by taking the average of all pixels on the perimeter of the square around the obtained by taking the average of all pixels on the perimeter of the square around the defect; using defect; using this approach, it is expected that mistakes in selecting the background temperature can this approach, it is expected that mistakes in selecting the background temperature can be avoided, be avoided, as shown in Figure 7. as shown in Figure7.

5.4.5.4. DepthDepth DetectionDetection UnderUnder EnvironmentalEnvironmental EffectEffect TheThe depthdepth ofof defectdefect hashas beenbeen quantifiedquantified throughthrough thethe observationobservation timetime (t)(t) shownshown asas thethe verticalvertical dasheddashed arrowsarrows inin FigureFigure 11 11.. ItIt isis defineddefined asas thethe timetime whenwhen thethe absoluteabsolute contrastcontrast remainsremains constantconstant oror increasesincreases toto thethe maximummaximum contrast contrast after after the the heating heating time time [ 5[5,6].,6].

Figure 11. Typical graph for relationship of the observation time and absolute contrast. Figure 11. Typical graph for relationship of the observation time and absolute contrast.

TheThe relationshiprelationship between between the observationthe observation time and time the defectiveand the depthdefective (in the depth first approximation) (in the first canapproximation) be expressed can as [ 6be,12 expressed] as [6,12] z ≈ (t · α)1/2, (6) 1/2 ztα , (6) and the thermal diffusivity can be defined as 

and the thermal diffusivity can be defined as K α = , (7) ρ ·KC α  , (7) ρ  C where specific heat capacity (C) refers to a measure of the ability of materials to absorb heat; thermal conductivitywhere specific (K )heat is the capacity measure (C) of refers the heat to a transfer measure through of the ability materials, of materials and ρ is to the absorb material heat; density thermal of theconductivity target object. (K) is the measure of the heat transfer through materials, and ρ is the material density of theFigure target 12 object. illustrates the estimated defective depth values for delaminations D4, D8, and D12, sortedFigure in the 12 descending illustrates orderthe estimated of ambient defective temperature. depth values The result for delaminations shows that the D4, estimated D8, and depth D12, valuessorted fluctuatein the descending with different order environmental of ambient temper conditions,ature. and The the result fluctuation shows isthat relatively the estimated high for depth deep defects.values fluctuate However, with the overalldifferent trend environmental is maintained, condit despiteions, aand decline the fluctuation of ambient is temperature. relatively high It can for deep defects. However, the overall trend is maintained, despite a decline of ambient temperature. It can be inferred that the depth detection might be less affected by the environmental conditions such as ambient temperature and relative humidity, especially for shallow defects.

Sensors 2017, 17, 1718 13 of 18

Sensors 2017, 17, 1718 13 of 18 Sensors 2017, 17, 1718 13 of 18 be inferred that the depth detection might be less affected by the environmental conditions such as ambientIn this temperature test, the delaminations and relative humidity, D6 and D7 especially show the for maximum shallow defects.contrast immediately after the 15- In this test, the delaminations D6 and D7 show the maximum contrast immediately after the 15- min heatingIn this test,time, the indicating delaminations that the D6 observation and D7 show time theis considerably maximum contrastless than immediately 1 min. Thus, for after these the min heating time, indicating that the observation time is considerably less than 1 min. Thus, for these 15-mindelaminations, heating time,the lack indicating of informat thation the for observation depth detection time isconsiderably caused incorrect less thanestimation, 1 min. Thus,as depicted for these in delaminations, the lack of information for depth detection caused incorrect estimation, as depicted in Figuredelaminations, 13. A number the lack of ofvery information small air forvoids depth that detection exist above caused the delaminations incorrect estimation, could ashave depicted resulted in Figure 13. A number of very small air voids that exist above the delaminations could have resulted inFigure faster 13 thermal. A number dissipation, of very smallleading air to voids a reduction that exist or above even thea loss delaminations of observation could time. have resulted in in faster thermal dissipation, leading to a reduction or even a loss of observation time. faster thermal dissipation, leading to a reduction or even a loss of observation time.

Figure 12. Relationship between depth of defects and ambient temperature. FigureFigure 12. 12.Relationship Relationship between between depth depth of of defects defects and and ambient ambient temperature. temperature.

Figure 13. Absolute contrast versus time for delaminations D8, D7, and D6 at similar depths (case 8). Figure 13. Absolute contrast versus time for delaminations D8, D7, and D6 at similar depths (case 8). Figure 13. Absolute contrast versus time for delaminations D8, D7, and D6 at similar depths (case 8). For small and deep defects, which normally show low absolute contrast (e.g., lower than 0.5 ◦C For small and deep defects, which normally show low absolute contrast (e.g., lower than 0.5 °C accordingFor small to the and American deep defects, Society which for Testing normally and Materials show low (ASTM) absolute requirement contrast (e.g., for fieldlower testing than 0.5 [28 ]),°C according to the American Society for Testing and Materials (ASTM) requirement for field testing itaccording was shown to thatthe theAmerican temperature Society values for Testing with time an containd Materials noises (ASTM) and the requirement thermal curves for arefield not testing clear [28]), it was shown that the temperature values with time contain noises and the thermal curves are enough[28]), it towas estimate shown thethat observation the temperature time [values22]. Therefore, with time while contain the noises depth mightand the have thermal been curves detected are not clear enough to estimate the observation time [22]. Therefore, while the depth might have been incorrectly,not clear enough the defects to estimate on the thermal the observation image are time still visible,[22]. Therefore, except for while the delamination the depth might with have a ratio been of detected incorrectly, the defects on the thermal image are still visible, except for the delamination width-to-depthdetected incorrectly, of equal the to defects or lower on thanthe thermal 1.0 [5,11 imag,22].e The are absolutestill visible, contrast except value for the of delamination delamination with a ratio of width-to-depth of equal to or lower than 1.0 [5,11,22]. The absolute contrast◦ value of D1with (3 a cm ratio× 3of cm width-to-depth at 3 cm deep) of with equal the to width-to-depthor lower than 1.0 ratio [5,11,22]. of 1.0 The is lower absolute than contrast 0.15 C value for all of delamination D1 (3 cm × 3 cm at 3 cm deep) with the width-to-depth ratio of 1.0 is lower than 0.15 °C casesdelamination of testing, D1 which (3 cm is × impossible3 cm at 3 cm to deep) visualize with withthe width-to-depth the naked eye. ratio However, of 1.0 is the lower defect than might 0.15 be °C for all cases of testing, which is impossible to visualize with the naked eye. However, the defect might detectablefor all cases if aof high-quality testing, which IR camerais impossible having to high visualize resolution with andthe naked thermal eye. sensitivity/Noise However, the defect Equivalent might be detectable if a high-quality IR camera having high resolution and thermal sensitivity/Noise Temperaturebe detectable Difference if a high-q (NETD)uality is used,IR camera and the having tests are high conducted resolution under and a less thermal uncertain sensitivity/Noise environment Difference (NETD) is used, and the tests are conducted under a less (e.g.,Equivalent an indoor Temperature test with virtually Difference no wind (NETD) effect). is As used, shown and in Figurethe tests 14, theare averageconducted absolute under contrast a less uncertain environment (e.g., an indoor test with virtually no wind effect). As shown in Figure 14, the uncertain environment (e.g., an indoor test with virtually no wind effect). As shown in Figure 14, the average absolute contrast for the twelve cases of D1 is around 0.12 °C; this is measured by the FLIR average absolute contrast for the twelve cases of D1 is around 0.12 °C; this is measured by the FLIR SC660 camera because most of the values are higher than the thermal sensitivity of the camera (30 SC660 camera because most of the values are higher than the thermal sensitivity of the camera (30

Sensors 2017, 17, 1718 14 of 18

Sensors 2017, 17, 1718 14 of 18 for the twelve cases of D1 is around 0.12 ◦C; this is measured by the FLIR SC660 camera because most mK or 0.03 °C) [29]. Although visually undetectable, the delamination with the width-to-depth ratio of the values are higher than the thermal sensitivity of the camera (30 mK or 0.03 ◦C) [29]. Although of 1.0 might be detected by an image processing method with the correct selection of background visually undetectable, the delamination with the width-to-depth ratio of 1.0 might be detected by an temperature. image processing method with the correct selection of background temperature.

Figure 14. Absolute contrast of defect with width-to-depth ratio of 1.0. Figure 14. Absolute contrast of defect with width-to-depth ratio of 1.0.

ThermalThermal diffusivitydiffusivity mustmust bebe acknowledgedacknowledged asas anan importantimportant parameterparameter influencinginfluencing thethe depthdepth estimates,estimates, suchsuch asas anan observationobservation time.time. Unlike metalmetal materials, the density of concrete relates to airair voidsvoids [[30],30], whichwhich cancan leadlead toto a change in thermal diffusivity. AsAs seenseen inin EquationEquation (7),(7), aa lower material densitydensity cancan causecause a higherhigher diffusivity, thusthus aa shortershorter observationobservation time of thethe defect.defect. In other words, thethe estimatedestimated defectivedefective depthdepth willwill bebe lessless thanthan thethe actualactual depth.depth. ManyMany researchersresearchers [[10,11,31]10,11,31] havehave foundfound thatthat thethe thermalthermal diffusivitydiffusivity of thethe materialmaterial relates to the time of thermal wave propagation. Unfortunately,Unfortunately, thethe resultsresults forfor concreteconcrete differdiffer somewhatsomewhat dependingdepending on the materialmaterial density, typetype ofof aggregate,aggregate, andand thethe methodsmethods usedused toto obtainobtain thisthis parameter. Moreover, thethe diffusiondiffusion processprocess weakensweakens thethe heatheat asas itit movesmoves deeperdeeper intointo thethe materialmaterial [[5].5]. Therefore,Therefore, the diffusivitydiffusivity is difficultdifficult toto estimateestimate forfor concrete.concrete. TheThe referencereference valuesvalues obtainedobtained fromfrom severalseveral previousprevious worksworks areare summarizedsummarized inin TableTable3 .3.

Table 3.3. Values ofof thermalthermal diffusivitydiffusivity ofof concrete.concrete.

Researcher Density, ρ Type of Aggregate ThermalThermal Diffusivity Diffusivitα = K/y ρ C Method Researcher Density, ρ Type of Aggregate Method (kg/m3)α (cm = K/2/min)ρ C Maldague, 1993 [11](kg/ 2400m3) - (cm0.3182/min)- Maldague,Maldague, 1993 2001 [[11]32] 2400 2400 (dry) -- 0.3180.318 - 2500 (moist) - 0.336 - Maldague,Lamond, 20062001 [30 [32]]- 2400 (dry) Quartz - 0.3181.317 CRD-C - 36, 37 2500 (moist) Quartzite- 0.3361.017 CRD-C - 36, 37 Lamond, 2006 [30] - Limestone Quartz 1.3170.917 CRD-CCRD-C 36, 3737 Basalt 0.417 CRD-C 36, 37 Expanded Quartzite shale 1.0170.250 CRD-CCRD-C 36, 3737 Vollmer, 2010 [19]- Limestone- 0.396 0.917 (at 20 ◦C) CRD-C- 36, 37 1 Ahlborn, 2015 [6]- BasaltQuartz 0.4171.425 CRD-CIRT 36, 37 1 Note: Value taken by back analyzing the IRT Expanded on concrete shale slabs for defects with different0.250 sizes, shapes, CRD-C and depths. 36, 37 The value of thermal diffusivity is estimated from the linear regression relationship between observation time and Vollmer,defect depth 2010 (R [19]2 = 0.896). - - 0.396 (at 20 °C) - Ahlborn, 2015 [6] - Quartz 1.425 IRT 1 InNote: this 1 Value research, taken the by concreteback analyzing specimen the IRT was onmade concrete of slabs gneiss for and defects granite with aggregatesdifferent sizes, and shapes, designed withand a mixture depths. ratioThe value of water, of thermal cement, diffusivity sand, andis estimated aggregate from components the linear regression as 0.5, 1.0, relationship 1.2, and 2.6, respectively.between Theobservation sizes of time fine and aggregates defect depth range (R2 from= 0.896). 4.75 to 12.5 mm (approximately 60%), whereas thoseIn of this coarse research, aggregates the concrete range from specimen 12.5 to was 25 mmmade (approximately of gneiss and granite 40%). aggregates and designed with a mixture ratio of water, cement, sand, and aggregate components as 0.5, 1.0, 1.2, and 2.6, respectively. The sizes of fine aggregates range from 4.75 to 12.5 mm (approximately 60%), whereas those of coarse aggregates range from 12.5 to 25 mm (approximately 40%).

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ToTo estimateestimateestimate thethethe thermalthermalthermal diffusivitydiffusivitydiffusivity ofof thethethe concretecoconcretencrete specimen,specimen,specimen, back-analysisback-aback-analysisnalysis from from thethe linearlinear relationshiprelationship betweenbetween between thethe the actualactual actual depthdepth depth ofof of eacheach each delaminationdelamination delamination andand and thethe the observationobservation observation timetime time waswas was carriedcarried carried outout asoutas plottedplotted as plotted inin FigureFigure in Figure 15.15. Missing Missing15. Missing datadata for datafor thethe for overover the-heating-heating over-heating andand depthdepth and depth informinform informationationation areare ignoredignored are ignored inin thethe 2 2 analysis.inanalysis. the analysis. TheThe linearlinear The regressionregression linear regression lineline withwith lineaa highhigh with correlationcorrelation a high correlation coefficientcoefficient valuevalue coefficient (R(R2 == 0.8995)0.8995) value (R indicatesindicates= 0.8995) thethe reliabilityindicatesreliability theofof thethe reliability datadata set.set. of TheThe the diffusivitydiffusivity data set. ofof The thethe diffusivity coconcretencrete specimenspecimen of the concrete cancan bebe es specimenestimatedtimated approximately canapproximately be estimated byby 2 2 invertingapproximatelyinverting thethe slopeslope by inverting ofof thethe equation,equation, the slope αα of== 1/0.61131/0.6113 the equation, == 1.6361.636α cm=cm 1/0.61132/min./min. ThisThis = 1.636 valuevalue cm cancan/min. bebe appliedapplied This value toto estimateestimate can be theappliedthe defectivedefective to estimate depthdepth the forfor defective thethe actualactual depth concreteconcrete for the actual structstruct concreteuresures inin structuresthethe field,field, inhavinghaving the field, thethe having samesame thematerialmaterial same characteristics.materialcharacteristics. characteristics.

Figure 15. Observation time vs square of actual defective depth. Figure 15. Observation time vs square of actual defective depth.

InIn addition,addition, the the loss loss ofof contrastcontrast ( (cc)) isis investigatedinvestigated to to confirm confirmconfirm thethe correctioncorrection of of itsits relationshiprelationship againstagainst thethe cubecube ofof thethe depthdepth asas mentionedmentioned byby MaldagueMaldague [[11][11]11] 1 c  1 , (8) cc≈ 3 ,, (8) (8) zz3 The absolute contrast values versus the invert of the cubic depth (1/z3) of each delamination are The absolute contrast values versusversus thethe invertinvert ofof thethe cubiccubic depthdepth (1/(1/z3)) of of each each delamination delamination are also plotted in Figure 16. Based on the figure, the relationship between the absolute contrast and the also plotted in Figure 1616.. Based on the figure, figure, the relationship between the absolute contrast and the inverse of the cubic depth (1/z3) is relatively accurate with a linear correlation coefficient value (R2) of inverse of the cubic depthdepth (1/(1/zz33)) is is relatively relatively accurate accurate with with a a linear linear correlation correlation coefficient coefficient valuevalue (R(R22) of 0.8004. It should be noted that the data are limited for only three different depths, four different sizes, 0.8004. It should be noted noted that that the the data data are are limited limited for for only only three three different different depths, depths, four four different different sizes, sizes, and twelve cases of dissimilar environmental conditions. and twelve cases of dissimilar environmental conditions.conditions.

Figure 16. Absolute contrast vs. 1/z3. FigureFigure 16.16. AbsoluteAbsolute contrastcontrast vs.vs. 1/1/zz33..

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This type of test should be carried out as many times as possible to obtain a large dataset relating to absolute contrast versus defective depth with a given external heating system (i.e., similar type of heat source, heating time, distribution layout, and distance of heat source). Then, similar to the process for the field test, with known environmental conditions, size of detect (observed from thermal image or camera screen), absolute contrast above defective area, and thermal characteristics of concrete, the depth of defect can be interpolated via the relationship obtained from the known data set without the need for the time-consuming task of analyzing the cooling curve to obtain the observation time for each delamination.

6. Conclusions Active thermography test data obtained from a laboratory experiment were analyzed under different environmental conditions. The research focuses on the effects of different ambient temperatures and relative humidity conditions on the defective depth detection and the absolute contrast between the defective and non-defective areas of delaminations. Results of the experiment show that the absolute contrast between defective and sound areas increases with the increase of the ambient temperature and that the absolute contrast between defective and sound areas decreases with the decrease of the ambient temperature, and it increases at a faster rate with large and shallow delaminations than smaller and deeper delaminations. The existence of a highly humid atmosphere might lead to an increase in the absolute contrast, especially for shallow delaminations of 1 cm deep. For deeper delaminations, the absolute contrast shows similar values for the two cases of the test at 71% and 58%, despite the difference in relative humidity of up to 13%. The results also indicate that, for shallow defects, the depth detection might be less affected by varying ambient temperature and relative humidity. Despite the deeper delaminations fluctuating under different environmental conditions, the overall trend indicates a leveling out with a decline of ambient temperature. The thermal diffusivity of 1.636 cm2/min can be used for concrete structures in the field that have similar material characteristics as those used in this laboratory research. In addition, the loss of contrast versus cube of defective depth (1/z3) can be expressed through a linear relationship for practical purposes, in order to quickly estimate the defective depth based on a known data set, without the need for the time-consuming analysis of the cooling curve to obtain observation time for delaminations.

Acknowledgments: This research was supported by a grant (17RDRP-B076564-04) from the Regional Development Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government. Author Contributions: Jungwon Huh, Dongyeob Han, and Achintya Haldar conceived the idea and designed the framework; Choonghyun Kang and Quang Huy Tran carried out the experiments; Jungwon Huh, Quang Huy Tran and Choonghyun Kang analyzed the data; and Jungwon Huh and Quang Huy Tran wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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