sensors Article
Effects of Ambient Temperature and Relative Humidity 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 2
*
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.) 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 wind, 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 Figure 1. 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
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techniques such as impact-echo, coin tapping, ultrasonic surfaceor waves or ground penetrating (NDT) such(NDT) as impact-echo, coin tapping, ultrasonic surface waves ground penetrating radar. radar.of One of the most significant advantages of technique the IRT technique its short time detection time One the most significant advantages of the IRT is its shortisdetection compared compared to other techniques, and it is to qualitatively evaluate thedepth area of and depth of a to other techniques, and it is possible to possible qualitatively evaluate the area and a subsurface subsurface delamination [4–7]. quantifying However, quantifying the delamination depth appears to be a delamination [4–7]. However, the delamination depth appears to be a considerable considerable only a few have attempted to evaluate the depth of challenge, andchallenge, only a fewand researchers haveresearchers attempted to evaluate the depth of delamination using delamination using concrete active IRT to inspect concrete structures. Inenvironmental addition, while changes in active IRT to inspect structures. In addition, while changes in conditions have conditions have a significant effect on experimental results, thehumidity, ambient clouds, temperature, aenvironmental significant effect on experimental results, the ambient temperature, relative wind relativeand humidity, clouds, wind speed, and solar during radiation should be [8]. considered during field studies testing speed, solar radiation should be considered field testing Currently, previous [8]. Currently, previous just focused on the passive IRT wheninfluence considering have just focused on thestudies passivehave IRT technique when considering thetechnique environmental on the environmental influence on the concrete mentioned in detail in Section 2, while concrete detection, as mentioned in detaildetection, in Section as 2, while the impact of the environment on the impactIRT of the environment the active IRT technique is still an open question. active technique is still on an open question.
Figure 1. 1. Crack, Crack, delamination, delamination, and and spalling spalling caused caused by by expansion Figure expansion of of corroded corroded steel. steel.
The aim of of this this study studyisisto tosurvey surveyconcrete concretestructural structuralcomponents components that are not exposed directly that are not exposed directly to to the sunlight and are less affected by the wind, such as inside a bridge box-girder, areas between the sunlight and are less affected by the wind, such as inside a bridge box-girder, areas between the T theI-girder T or I-girder and diaphragms, andstructural other structural components in the of interior of buildings and or and diaphragms, and other components in the interior buildings and factories, factories, and so on. The impulse thermography technique, one of the active IRT techniques, is and so on. The impulse thermography technique, one of the active IRT techniques, is applied with applied withhalogen six 500-watt halogen lamps workingheat as external sources to transfer into six 500-watt lamps working as external sources heat to transfer heat flux intoheat the flux concrete the concrete specimen. impulse thermography is throughout considered throughout cases specimen. The impulseThe thermography technique istechnique considered twelve casestwelve of different of different ambientand temperature and relative humidity conditions concrete Our slabpurpose in the ambient temperature relative humidity conditions on a concrete slab in on the alaboratory. laboratory. this study is to describe several important characteristics heat transfer in this studyOur is topurpose describeinseveral important characteristics of heat transfer that affectofthe inspection that affect the inspection results of concrete with different environmental conditions, and propose results of concrete with different environmental conditions, and propose a preliminary selection of aa preliminary selection of a suitable weather conditions inspection when using the active suitable weather conditions for site inspection when usingfor thesite active thermography technique. thermography technique. 2. Related Work 2. Related Work An early application of IRT in the civil engineering field related to health monitoring of concrete structures wasapplication performedofby Clemena et al. in 1978 [9]. Their research found that more severely An early IRT in the civil engineering field related to health monitoring of concrete delaminated regions induceby a stronger and delaminated wasmore warmer than structures was performed Clemenathermal et al. incontrast 1978 [9]. Their research concrete found that severely non-delaminated concrete directly exposed to sunlight. Other studies were by delaminated regions inducewhen a stronger thermal contrast and delaminated concrete wasconducted warmer than Cielo et al. in 1987concrete [10] and when by Maldague 1993 [11], focusing on delamination depth. In theseby studies, non-delaminated directlyin exposed to sunlight. Other studies were conducted Cielo they changes in the surfacein temperature of delaminated layers anddepth. the observation time to et al.observed in 1987 [10] and by Maldague 1993 [11], focusing on delamination In these studies, estimate the depth of delamination. 2013, Vaghefi tested fourlayers concrete with delaminations they observed changes in the surfaceIntemperature of[5] delaminated andslabs the observation time to placed at the various depths to confirm the technique proposed by Maldague [11].slabs In 2008, et al. [4] estimate depth of delamination. In 2013, Vaghefi [5] tested four concrete withPollock delaminations conducted active depths thermaltoimaging onproposed six concrete specimens[11]. withInembedded artificial placed at various confirm inspections the technique by Maldague 2008, Pollock et al. air voids and post-tensioning ducts (PT-ducts) to on simulate post-tensioned girder bridges. It was [4] conducted active thermal imaging inspections six concrete specimensbox with embedded artificial shown that thepost-tensioning PT-ducts and simulated voids in the 20 cm thick specimens were in air voids and ducts (PT-ducts) to simulate post-tensioned box more girderapparent bridges.than It was the 30 cm specimens. Also, PT-ducts were and more visible inmore the thermal images shown thatthick the PT-ducts and simulated voids inmuch the 20clearer cm thick specimens were apparent than of specimens. However, was found that much two factors need be considered: in the the thinner 30 cm thick specimens. Also,it PT-ducts were clearer andtomore visible in accessibility the thermal (whereby source needs to be placed inside thefound box girder) andfactors cost (whereby is images ofthe theheat thinner specimens. However, it was that two need tothe be technique considered: time-consuming when testing relatively deep airplaced voids).inside the box girder) and cost (whereby accessibility (whereby the heatfor source needs to be For assessing the influence ofwhen environmental the air passive IRT results, in 2009 Washer the technique is time-consuming testing forconditions relativelyon deep voids). et al.For [12]assessing developed how to use theconditions technologyonand it could be applied by theguidelines influence on of environmental thewhere passive IRT results, in 2009 characterizing the developed environmental conditions (i.e.,tosolar diurnaland temperature variations, wind Washer et al. [12] guidelines on how use loading, the technology where it could be applied 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
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speed,a and humidity) necessary forwith effective thermography the field. In Washer’s project, project, largerelative concrete test block was made embedded targets toin model the subsurface defects a large concrete test block was made with embedded targets to model the subsurface defects at depths ranging from 25 to 127 mm (1 to 5 inches) in concrete. Data were collected for six months at depths from to 127 mmthe (1 toenvironmental 5 inches) in concrete. Datathat werewould collected for six months of and and wereranging analyzed to25determine conditions make detection were analyzed to determine the environmental conditions that would make detection of subsurface subsurface delaminations more possible during practical bridge inspections. This research trend delaminations morewith possible practical bridge research trend continued continued until 2013, Rilyaduring Rumbayan’s work [13].inspections. RumbayanThis developed a numerical modeluntil to 2013, thermal with Rilya Rumbayan’s work [13]. Rumbayanbased developed a numerical model to Washer’s predict thermal predict contrasts in subsurface delamination on a given set of data from [12] contrasts in subsurface delamination based on passive a given IRT set of data from Washer’s [12] research. These research. These studies have just focused on the technique when considering the impact have justonfocused on the passive IRT technique considering theextensive impact of environment of studies environment the concrete detection. Therefore,when in this study, an experimentalon the concrete detection. Therefore, in this study, an extensive experimental investigation was carried investigation was carried out on the size and depth of sub-surface delaminations with variousout on the size and depth of sub-surface delaminations with various environmental conditions focusing environmental conditions focusing on the active IRT technique as mentioned above. on This the active above.the fundamentals of infrared thermography are paperIRT is technique organizedasasmentioned follows. First, This paper is organized as follows. the fundamentals of infraredforthermography introduced in Section 3 with discussion on First, the appropriate input parameters setting up theare introduced in Section 3 with discussion on the appropriate input parameters for setting up infrared (IR) camera. Section 4 contains the preparation of testing specimen, testing method, andthe infrared (IR) camera. Section 4 contains thediscussed preparation of testing specimen, testing method, and instrumentation. The experimental results are in Section 5, where the analysis focuses on instrumentation. The experimental results are discussed in Section 5, where the analysis focuses the impact of meteorological factors on the testing results on both area and depth detection of on the impactFinally, of meteorological factors the testing results on both area and depth detection of delaminations. the conclusion is in on Section 6. delaminations. Finally, the conclusion is in Section 6.
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 Figure 2.
Figure 2. Principle infrared (IR) thermal camera receiving radiation. Figure 2. Principle of of anan infrared (IR) thermal camera receiving radiation.
The total radiation ) received camera comprises the emission the object,ε·τε·W τW The total radiation (W(W tot)tot received byby thethe IRIR camera comprises the emission ofof the object, obj,obj , emission the surroundings andreflected reflectedbybythe theobject, object,(1(1−−ε)τW ε)·τ·W , and emission thethe emission ofof the surroundings and refl,refl and thethe emission of of thethe atmosphere, − τ)atm ·W, atm , [14–16] as follows: atmosphere, (1 (1 − τ)·W [14–16] as follows: Wtot=εε·ττ · W 1 ε τ W · W 1 +τ (1W−atmτ) · W Wtot Wobj obj + (1 − ε) · τrefl re f l , atm , 4 4 4 + 4 1 +τ (1 4 Tobj (11−ε ε)τ· τσ · T = ε· ετ ·τσ σ· T σrefl· Tre )4 · σ · Tatm f l σ− Tτatm obj
(1)
(1)
The objectcan canbebe calculated from Equation (2), where is the StefanThetemperature temperatureof of an object calculated from Equation (2), where σ is theσStefan-Boltzmann 4). Other parameters must be set up in the camera, including Boltzmann constant × 10−82W/m constant (=5.67 × (=5.67 10−8 W/m ·K4 ).2·K Other parameters must be set up in the camera, 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).
Tobj
4
4 4 Wtot 1 ε τ σ Trefl 1 τ σ Tatm
ε τ σ
,
(2)
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emissivity of the object material (ε), the transmittance of the atmosphere (τ), the reflected temperature (Trefl ), and the atmospheric/ambient temperature (Tatm ). s Sensors 2017, 17, 1718
Tobj =
4
4 − (1 − τ) · σ · T 4 Wtot − (1 − ε) · τ · σ · Tre atm fl
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(2) ε·τ·σ According to Equation (2), three parameters are needed to automatically calculate the According to Equation (2), parameters are needed to automatically atmospheric atmospheric transmission (τ) three using the IR camera: survey distance, calculate relative the humidity, and transmission (τ) using the IR camera: survey distance, relative humidity, and atmospheric temperature. atmospheric temperature. The formula used by the FLIR camera is as follows [17,18]: The formula used by the FLIR camera is as follows [17,18]: d , K atm exp d 1 1 1 K atm exp d 2 2 , (3) h √ h √ i √ i √ τ (d, ω ) = Katm · exp − d α1 + β 1 ω + (1 − Katm ) · exp − d α2 + β 2 ω , (3)
,
2 3 % , Tatm % exp h1 h2 Tatm h3 Tatm h4 Tatm ,
(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 water 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 (K = 1.9), attenuation for atmosphere without water vapor (α , α2 ), attenuation for water vapor (β1, β2atm ), and h1 = 1.5587, h2 = 6.939 × 10−2, h3 = −2.7816 × 10−4, and h4 =1 6.8455 × 10−7. vapor (β1 , β2 ), and h1 = 1.5587, h2 = 6.939 × 10−2 , h3 = −2.7816 × 10−4 , and h4 = 6.8455 × 10−7 . The characteristics were specified using Equations (3) and (4) by Minkina 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. Figure 3 with specific parameters of α1 = 0.0066, α2 = 0.0126, β1 = −0.0023, and β2 = −0.0067.
(a)
(b)
Figure 3. Characteristics Figure 3. Characteristics of of atmospheric atmospheric transmission transmission coefficient coefficient ττ == f(d) f (d) for for longwave longwave band band LW LW ThermaCAM PM 595 camera: (a) Function of the relative humidity ω%; (b) function of the ThermaCAM PM 595 camera: (a) Function of the relative humidity ω%; (b) function of the atmospheric atmospheric temperature Tatm [17]. temperature T [17]. atm
As shown in Figure 3, the atmospheric transmission (τ) is close to 1.0 when the survey distance As shown in Figure 3, 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 (Trefl ) in Equation 2 are necessary for measuring the target object under a survey at a near distance. Furthermore, 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
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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, 2017, specimen 17, 1718 5 of 18 aSensors concrete 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 diffusivity (α) (α) of of the the concrete concrete specimen specimenisisestimated estimatedto tobe be1.636 1.636cm cm22/min /min as Twelve diffusivity as described described below. below. Twelve delaminations were simulated with different dimensions and depths embedded inside the concrete delaminations were simulated with different dimensions and depths embedded inside the concrete specimen. The study are are square square shapes shapes of of polystyrene, polystyrene, denoted denoted specimen. The delaminations delaminations considered considered in in this this study 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 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 at 1 cm deep (D9 to D12), 2 cm deep (D5 to D8), and 3 cm deep (D1 to D4) from the specimen's 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 surface as shown in Figure 4. The delaminations are simulatedare using polystyrene, the thermal the specimen's surface as shown in Figure 4. The delaminations simulated using polystyrene, conductivity of which is 0.027 W/m·°C, which is similar to that of air (0.024 W/m·°C [21]) and is [21]) thus ◦ the thermal conductivity of which is 0.027 W/m· C, which is similar to that of air (0.024 W/m ·◦ C expected to behave in a similar manner to that of an air void when receiving heat flux [22]. 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. Figure 4. Concrete specimen with embedded artificial delaminations.
4.2. 4.2. Testing Testing Method Method Twelve Twelve cases cases are are considered considered in in aa series series of of experiments experiments that that are are carried carried out out in in the the laboratory laboratory on on different days under different environmental conditions. Six halogen lamps (providing a maximum different days under different environmental conditions. Six halogen lamps (providing a maximum energy asas illustrated in in Figure 5 were placed at 1.2 from the specimen and were energyoutput outputofof3000 3000watts) watts) illustrated Figure 5 were placed atm 1.2 m from the specimen and applied as an external heat source providing heat fluxheat to theflux concrete For this technique, were applied as an external heat by source by providing to thespecimen. concrete specimen. For this the specimen heated for min and IR thermal camera, FLIR SC660 [23],FLIR was technique, thewas specimen was15heated fora long-wavelength 15 min and a long-wavelength IR thermal camera, used to record the variation in temperatures of the specimen every 10 s during both the heating and SC660 [23], was used to record the variation in temperatures of the specimen every 10 s during both cooling periods. major technical of the cameradata are listed Table 1.are The camera was placed the heating and The cooling periods. Thedata major technical of theincamera listed in Table 1. The at 2.9 m from the specimen and was set at a constant emissivity value of 0.95 for all cases of 0.95 testing. camera was placed at 2.9 m from the specimen and was set at a constant emissivity value of for The ambient temperature was measured using an ambient temperature humidity meter (CEM all cases of testing. The ambient temperature was measured using anand ambient temperature and ◦ C to 21.9 ◦ C for the twelve cases, and the corresponding relative humidity DT-615), humidityranging meter from (CEM15.8 DT-615), ranging from 15.8 °C to 21.9 °C for the twelve cases, and the was varied from 71% tohumidity 35%. Half ofvaried the experimental rainy dayscases whilewere the corresponding relative was from 71% tocases 35%.were Half tested of the on experimental other tested sunny days. The detailed valueson of the meteorological conditions for the testedhalf on was rainy dayson while the other half was tested sunny days. The detailed values of IRT the inspection are given in Table 2. The atmospheric transmission can be automatically calculated by the meteorological conditions for the IRT inspection are given in Table 2. The atmospheric transmission camera based on the calculated information camera temperature, and relative can be automatically byof the cameradistance, based onambient the information of camera distance,humidity, ambient as mentioned previously. temperature, and relative humidity, as mentioned previously.
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 can be2017, automatically calculated by the camera based on the information of camera distance, ambient Sensors 17, 1718 6 of 18 temperature, and relative humidity, as mentioned previously.
Figure 5. 5. Photograph Photograph of of the the thermography thermography system. system. Figure Table 1. Technical data of FLIR camera SC660 [23]. Items IR resolution Thermal sensitivity/NETD Field of view (FOV) Spatial resolution (IFOV) Focal Plane Array (FPA) Wavelength Temperature range Accuracy
Parameters 640 × 480 pixels
