- Email: [email protected]
Active IR-applications in civil engineering
Infrared Physics & Technology 43 (2002) 233–238 www.elsevier.com/locate/infrared
Active IR-applications in civil engineering H. Wiggenhauser
*
Federal Institute for Materials Research and Testing (BAM), Division IV.4, Unter den Eichen 87, D-12205 Berlin, Germany
Abstract Applications of IR-thermography in civil engineering are not limited to the identification of heat losses in building envelopes. As it is well known from other areas of non-destructive testing, active IR-thermographic methods such as cooling down or lock-in thermography improves the results in many investigations. In civil engineering these techniques have not been used widely. Mostly thermography is used in a quasi-static manner. The interpretation of moisture measurements with thermography on surfaces can be very difficult due to several overlapping effects: emissivity changes due to composition, heat transfer through wet sections of the specimen, cooling through air flow or reflected spurious radiation sources. These effects can be reduced by selectively measuring the reflection in two wavelength windows, one on an absorption band of water and another in a reference band and then combining the results in a moisture index image. Cooling down thermography can be used to identify subsurface structural deficiencies. For building materials like concrete these measurements are performed on a much longer time scale than in flash lamp experiments. A quantitative analysis of the full cooling down process over several minutes can reliably identify defects at different depths. Experiments at BAM have shown, that active thermography is capabale of identifying structural deficiencies or moist areas in building materials much more reliable than quasi-static thermography. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Active thermography; Non-destructive testing; Quantitative analysis; Civil engineering; Moisture measurement
1. Introduction IR-thermography has been widely used in civil engineering for the identification of heat losses in building envelopes. In numerous cases this technique was applied successfully for all kinds of buildings and many sources of heat loss have been successfully detected.
*
Tel.: +49-30-8104-1440; fax: +49-30-8104-1447. E-mail address: [email protected] (H. Wiggenhauser).
In some cases, such investigations have also revealed other information, such as sections of excess moisture, delaminations of plaster or the presence of wooden framework under plaster [1]. Consequently, IR-thermography was also used as a non-destructive technique to test buildings and structures. There are many reports on the successful use of thermography to detect delaminations of asphalt or concrete on bridges and highways. Moisture detection was another area of research and application [2]. In non-destructive testing of modern materials such as carbon fibres IR-thermography has become a very powerful tool for testing the integrity
1350-4495/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 4 9 5 ( 0 2 ) 0 0 1 4 5 - 7
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H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
of composites. In contrast to the conventional use where natural temperature gradients are utilized, the NDT-applications take an active approach. A heat pulse is applied and the surface temperature is monitored and analysed. Typically, the temperature distribution at the surface at the time of maximum contrast is used for the detection of any defects [3]. Lock-in thermography also is an active technique, where a periodic heat wave is applied to the test object. The temperature distribution is observed over many cycles of the excitation and analysed in the frequency domain. The frequency and the phase of the observed temperature modulation on the surface is used to extract depth information about the specimen [4]. Building materials such as concrete, bricks or mortar do have a very slow response to temperature changes due to their low heat conductivity. Experiments with active thermography therefore take place on a long timescale. Cooling down processes in concrete e.g. may need up to an hour or even more. Also, the controlled heating of objects needs high energy and cannot be done with flash lamps. Other methods must be utilized to generate enough temperature difference between the surface and the interior. There have been several approaches of active IR-thermography researched at BAM in recent years. Moisture detection on surfaces was investigated with the analysis of the reflection of modulated infrared light on surfaces [5]. Also, the use of cooling down thermography to detect defects in building parts was investigated.
Reflection measurements with one light source suffers from one drawback: it is impossible to distinguish between changes of the emissivity of the material and the influence of moisture. In addition, scattered light from the environment contributes to the signal. To solve this problem, a method was used which allows to separate the moisture effect from other factors using modulated light sources. The radiation fluxes, which contribute to the signal, are summarized in Fig. 1. Transmitted, emitted and reflected radiation are unavoidable noise sources. The two additional light sources kref and kwater are indicated as modulated light sources. All fluxes are measured by the camera, which cannot distinguish between the components. In order to separate between stray lights and reflected light, the additional light sources are modulated with an appropriate, distinct frequency. A frequency analysis (lock-in principle) on the detector side allows the separation between each of the modulated and any other light sources. Emissivity changes due to moisture are distinguished from other causes by using light of two wavelengths, one which matches an absorption band of water (1980 nm) and a second one for reference outside any influence from water (1000 nm). The images taken from the object at these two wavelengths are then extracted by using the frequency analysis. For each pixel, the formula ðSref Swater Þ= ðSref þ Swater Þ þ 0:5 is used to calculated the pixel
2. Moisture detection on surfaces Moisture detection in building materials is typically done by taking cores and measuring the weight loss after heating. Non-destructive methods are also used, e.g. neutron diffraction, radar, microwaves or electrical methods. IR-thermography has also been applied successfully in many cases [2]. Sections with excess moisture can be identified using passive thermography by the change in heat conductivity. On surfaces, the change in remission between dry and moist material is used [6].
Fig. 1. Radiation fluxes contributing to measurements with a camera. kref and kwater are two light sources which are modulated. Sref and Swater are the corresponding signals which are extracted from the camera signal using a frequency analysis (FFT).
H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
235
Fig. 2. Brick specimens with different moisture content (from left to right: 100%, 36%, 57%, 79%, 14%). Left: photograph, right: thermogram [7].
Fig. 3. Images of specimens in the reference wavelength (1000 nm) (left top) and in the absorption band of water (1980 nm) (left bottom) and synthetic image of specimens from a combination of both images [7] (see text).
values for the moisture index image. This moisture index image is visualized as a grey scale image where increasing surface moisture content is visualized as increasing grey value (Figs. 2 and 3). First measurements on historical objects verified, that this method is suitable to measure the moisture on surfaces of masonry.
3. Cooling down thermography Cooling down thermography is an established testing method in NDT of non-metallic materials
[8]. Especially parts made from carbon fibres in aviation are tested using flash lamp excitation. Materials used in civil engineering have very different properties and typically the thickness of objects is much larger. Consequently, the time scale for cooling down tests for concrete objects is very long. An example is shown in Fig. 4, where a tiled wall is heated for 3 min to 50 °C. After switching off the radiator, the cooling down process is monitored using a conventional IR-camera. The thermograms in the sequence have little contrast, the main feature shows the edges of the tiles.
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H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
Fig. 4. Visualisation of cooling down analysis: top row––thermograms of a tiled wall (subset of series of 4500 thermograms) as recorded. Bottom row: visualisation of one fit parameter from numerical analysis of cooling down process in the indicated time span.
All thermograms in the sequence, consisting of 4500 images are digitised and a rectangular area chosen for data analysis. For each pixel, the temperature over time data is extracted. All those curves are fitted with exponential functions. The results of the fitting parameters for each pixel can then be visualized in images, which can directly be compared to the thermograms. It is obvious, that this analysis reveals very clearly the effect of water, which was injected between the tiles to generate a hidden moisture spot. The best contrast is achieved by analysing the first minute only after the heater has been switched off. This defect is totally invisible at the surface. This experiment shows, that cooling down experiments can be analysed using numerical procedures which summarize the whole cooling down process in a few numbers. In contrast to an analysis which utilises only the thermograms at the time of maximal contrast, the full process is analysed. A research project, funded by the German Research Foundation (DFG), was started at BAM to develop a methodology for the investigation of typical building parts using cooling down thermography. In addition to the analysis using numerical fits to the temperature transients in each point of the thermograms, it is also possible to perform an
analysis in the frequency domain. The frequency components in the temperature transients at each point are extracted using FFTs. This method is also known as pulse phase thermography [9]. The results for each frequency component are then treated as in lock-in thermography [4]. In Fig. 5, the amplitude and phase images of a test specimen are shown for two different frequencies. The test specimen is a concrete block with 1:3 m 1:3 m 0:5 m in size. There are eight artificial defects embedded with concrete covers between 5 and 10 cm.
4. Induction thermography Another interesting technique used in civil engineering is the induction thermography [10]. With this technique it is possible to locate reinforcement by heating the metal with electric current, induced by a very strong magnetic field. The inductor is a powerful electric coil which induces the strong electric current in the rebars. As the rebars are heated themselves, they act as the source of heat and after a while the temperature at the surface increases over the rebars. Due to the heavy equipment and the high electric power needed for the inductor, this active thermographic technique is not used very widely for civil engineering applications.
H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
237
Fig. 5. Amplitude (top) and phase (bottom) images of frequency analysis of cooling down process. The phase image on the left (0.13 mHz) corresponds to a depth of 13 cm, the right one (0.28 mHz) to 9 cm (specimen: concrete block with artificial voids, 1:3 m 1:3 m 0:5 m).
5. Conclusions For civil engineering problems, thermography can be used as active techniques to address different testing problems such as moisture on surfaces or hidden structural features. In research projects it was shown, that these techniques might have the potential to become a valuable tool for nondestructive testing of structures.
Acknowledgements The presented work was supported by the Deutsche Forschungsgemeinschaft within the research project Investigation of structure and moisture content in the surface near region of building structures with impulse thermography (WI 1785/1-
1). Special thanks to Dr. F. Weritz for providing the images in Fig. 5.
References [1] B. Petterson, B. Axen, Thermography; Testing of the thermal insulation and airtightness of buildings, Swedish Council for Building Research, Spangbergs Tryckerier AB, Stockholm, 1980. [2] B. B€ ottcher, H. Richter, Ein Beitrag zum Nachweis von Feuchtigkeit in Mauerwerk mit Hilfe einer Infrarotkamera, Materialpr€ ufung 24 (1) (1982) 5–9. [3] G. Zenzinger, Thermografie––Neue M€ oglichkeiten in der ZfP, DACH-Zeitung 63/10 (1998) 58–61. [4] D. Wu, G. Busse, Lockin-Thermographie in der zerst€ orungsfreien Werkstoffpr€ ufung, DACH-Zeitung (DGZfP) 58 (1998) 67–72. [5] K. B€ uscher, W. Wild, H. Wiggenhauser, Feuchtemessung im Bauwesen mit der amplitudenempfindlichen
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H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
Modulationsthermografie, Technisches Messen 66 (4) (1999) 39–145. [6] E. Geyer, D. Arndt, L. Vogel, Bestimmung des Remissionsgrades von trockenen und befeuchteten Baustoffen im Wellenl€ angenbereicbis 1,3 bis 13 lm f€ ur die IR-reflektometrische Ermittlung von Oberfl€achenfeuchte, in: H. Wiggenhauser, H.S. M€ uller, N. Girlich (Eds.), Tagungsbericht Feuchtetag 21.09.1993, DGZfP, Berlin, 1993, pp. 223– 231. [7] K. B€ uscher, Einsatz der Modulationsthermografie zum Nachweis von Feuchtigkeit in Baustoffen, Dissertation Uni Rostock, 1999.
[8] X.P.V. Maldague, Nondestructive evaluation of materials by infrared thermography, Springer-Verlag, London, 1993. [9] X. Maldaque, J.-P. Couturier, S. Marinetti, A. Salerno, D. Wu, Advances in pulse phase thermography, in: G. Busse, D. Balageas, G.M. Carlomagno (Eds.), Quantitative Infrared Thermography, QIRT 96, Edizione ETS, Pisa, 1997, pp. 377–382. [10] B. Hillemeier, Induktionsthermographie zur Ortung von Bewehrungsst€ahlen, in: G. Schickert, D. Schnitger (Eds.), Zerst€ orungsfreie Pr€ ufung im Bauwesen, Tagungsbericht ZfPBau-Symposium 2. und 3. Oktober 1985, Berlin, BAM, DGZfP, Berlin, 1986, pp. 178–192.
Active IR-applications in civil engineering H. Wiggenhauser
*
Federal Institute for Materials Research and Testing (BAM), Division IV.4, Unter den Eichen 87, D-12205 Berlin, Germany
Abstract Applications of IR-thermography in civil engineering are not limited to the identification of heat losses in building envelopes. As it is well known from other areas of non-destructive testing, active IR-thermographic methods such as cooling down or lock-in thermography improves the results in many investigations. In civil engineering these techniques have not been used widely. Mostly thermography is used in a quasi-static manner. The interpretation of moisture measurements with thermography on surfaces can be very difficult due to several overlapping effects: emissivity changes due to composition, heat transfer through wet sections of the specimen, cooling through air flow or reflected spurious radiation sources. These effects can be reduced by selectively measuring the reflection in two wavelength windows, one on an absorption band of water and another in a reference band and then combining the results in a moisture index image. Cooling down thermography can be used to identify subsurface structural deficiencies. For building materials like concrete these measurements are performed on a much longer time scale than in flash lamp experiments. A quantitative analysis of the full cooling down process over several minutes can reliably identify defects at different depths. Experiments at BAM have shown, that active thermography is capabale of identifying structural deficiencies or moist areas in building materials much more reliable than quasi-static thermography. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Active thermography; Non-destructive testing; Quantitative analysis; Civil engineering; Moisture measurement
1. Introduction IR-thermography has been widely used in civil engineering for the identification of heat losses in building envelopes. In numerous cases this technique was applied successfully for all kinds of buildings and many sources of heat loss have been successfully detected.
*
Tel.: +49-30-8104-1440; fax: +49-30-8104-1447. E-mail address: [email protected] (H. Wiggenhauser).
In some cases, such investigations have also revealed other information, such as sections of excess moisture, delaminations of plaster or the presence of wooden framework under plaster [1]. Consequently, IR-thermography was also used as a non-destructive technique to test buildings and structures. There are many reports on the successful use of thermography to detect delaminations of asphalt or concrete on bridges and highways. Moisture detection was another area of research and application [2]. In non-destructive testing of modern materials such as carbon fibres IR-thermography has become a very powerful tool for testing the integrity
1350-4495/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 4 9 5 ( 0 2 ) 0 0 1 4 5 - 7
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H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
of composites. In contrast to the conventional use where natural temperature gradients are utilized, the NDT-applications take an active approach. A heat pulse is applied and the surface temperature is monitored and analysed. Typically, the temperature distribution at the surface at the time of maximum contrast is used for the detection of any defects [3]. Lock-in thermography also is an active technique, where a periodic heat wave is applied to the test object. The temperature distribution is observed over many cycles of the excitation and analysed in the frequency domain. The frequency and the phase of the observed temperature modulation on the surface is used to extract depth information about the specimen [4]. Building materials such as concrete, bricks or mortar do have a very slow response to temperature changes due to their low heat conductivity. Experiments with active thermography therefore take place on a long timescale. Cooling down processes in concrete e.g. may need up to an hour or even more. Also, the controlled heating of objects needs high energy and cannot be done with flash lamps. Other methods must be utilized to generate enough temperature difference between the surface and the interior. There have been several approaches of active IR-thermography researched at BAM in recent years. Moisture detection on surfaces was investigated with the analysis of the reflection of modulated infrared light on surfaces [5]. Also, the use of cooling down thermography to detect defects in building parts was investigated.
Reflection measurements with one light source suffers from one drawback: it is impossible to distinguish between changes of the emissivity of the material and the influence of moisture. In addition, scattered light from the environment contributes to the signal. To solve this problem, a method was used which allows to separate the moisture effect from other factors using modulated light sources. The radiation fluxes, which contribute to the signal, are summarized in Fig. 1. Transmitted, emitted and reflected radiation are unavoidable noise sources. The two additional light sources kref and kwater are indicated as modulated light sources. All fluxes are measured by the camera, which cannot distinguish between the components. In order to separate between stray lights and reflected light, the additional light sources are modulated with an appropriate, distinct frequency. A frequency analysis (lock-in principle) on the detector side allows the separation between each of the modulated and any other light sources. Emissivity changes due to moisture are distinguished from other causes by using light of two wavelengths, one which matches an absorption band of water (1980 nm) and a second one for reference outside any influence from water (1000 nm). The images taken from the object at these two wavelengths are then extracted by using the frequency analysis. For each pixel, the formula ðSref Swater Þ= ðSref þ Swater Þ þ 0:5 is used to calculated the pixel
2. Moisture detection on surfaces Moisture detection in building materials is typically done by taking cores and measuring the weight loss after heating. Non-destructive methods are also used, e.g. neutron diffraction, radar, microwaves or electrical methods. IR-thermography has also been applied successfully in many cases [2]. Sections with excess moisture can be identified using passive thermography by the change in heat conductivity. On surfaces, the change in remission between dry and moist material is used [6].
Fig. 1. Radiation fluxes contributing to measurements with a camera. kref and kwater are two light sources which are modulated. Sref and Swater are the corresponding signals which are extracted from the camera signal using a frequency analysis (FFT).
H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
235
Fig. 2. Brick specimens with different moisture content (from left to right: 100%, 36%, 57%, 79%, 14%). Left: photograph, right: thermogram [7].
Fig. 3. Images of specimens in the reference wavelength (1000 nm) (left top) and in the absorption band of water (1980 nm) (left bottom) and synthetic image of specimens from a combination of both images [7] (see text).
values for the moisture index image. This moisture index image is visualized as a grey scale image where increasing surface moisture content is visualized as increasing grey value (Figs. 2 and 3). First measurements on historical objects verified, that this method is suitable to measure the moisture on surfaces of masonry.
3. Cooling down thermography Cooling down thermography is an established testing method in NDT of non-metallic materials
[8]. Especially parts made from carbon fibres in aviation are tested using flash lamp excitation. Materials used in civil engineering have very different properties and typically the thickness of objects is much larger. Consequently, the time scale for cooling down tests for concrete objects is very long. An example is shown in Fig. 4, where a tiled wall is heated for 3 min to 50 °C. After switching off the radiator, the cooling down process is monitored using a conventional IR-camera. The thermograms in the sequence have little contrast, the main feature shows the edges of the tiles.
236
H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
Fig. 4. Visualisation of cooling down analysis: top row––thermograms of a tiled wall (subset of series of 4500 thermograms) as recorded. Bottom row: visualisation of one fit parameter from numerical analysis of cooling down process in the indicated time span.
All thermograms in the sequence, consisting of 4500 images are digitised and a rectangular area chosen for data analysis. For each pixel, the temperature over time data is extracted. All those curves are fitted with exponential functions. The results of the fitting parameters for each pixel can then be visualized in images, which can directly be compared to the thermograms. It is obvious, that this analysis reveals very clearly the effect of water, which was injected between the tiles to generate a hidden moisture spot. The best contrast is achieved by analysing the first minute only after the heater has been switched off. This defect is totally invisible at the surface. This experiment shows, that cooling down experiments can be analysed using numerical procedures which summarize the whole cooling down process in a few numbers. In contrast to an analysis which utilises only the thermograms at the time of maximal contrast, the full process is analysed. A research project, funded by the German Research Foundation (DFG), was started at BAM to develop a methodology for the investigation of typical building parts using cooling down thermography. In addition to the analysis using numerical fits to the temperature transients in each point of the thermograms, it is also possible to perform an
analysis in the frequency domain. The frequency components in the temperature transients at each point are extracted using FFTs. This method is also known as pulse phase thermography [9]. The results for each frequency component are then treated as in lock-in thermography [4]. In Fig. 5, the amplitude and phase images of a test specimen are shown for two different frequencies. The test specimen is a concrete block with 1:3 m 1:3 m 0:5 m in size. There are eight artificial defects embedded with concrete covers between 5 and 10 cm.
4. Induction thermography Another interesting technique used in civil engineering is the induction thermography [10]. With this technique it is possible to locate reinforcement by heating the metal with electric current, induced by a very strong magnetic field. The inductor is a powerful electric coil which induces the strong electric current in the rebars. As the rebars are heated themselves, they act as the source of heat and after a while the temperature at the surface increases over the rebars. Due to the heavy equipment and the high electric power needed for the inductor, this active thermographic technique is not used very widely for civil engineering applications.
H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
237
Fig. 5. Amplitude (top) and phase (bottom) images of frequency analysis of cooling down process. The phase image on the left (0.13 mHz) corresponds to a depth of 13 cm, the right one (0.28 mHz) to 9 cm (specimen: concrete block with artificial voids, 1:3 m 1:3 m 0:5 m).
5. Conclusions For civil engineering problems, thermography can be used as active techniques to address different testing problems such as moisture on surfaces or hidden structural features. In research projects it was shown, that these techniques might have the potential to become a valuable tool for nondestructive testing of structures.
Acknowledgements The presented work was supported by the Deutsche Forschungsgemeinschaft within the research project Investigation of structure and moisture content in the surface near region of building structures with impulse thermography (WI 1785/1-
1). Special thanks to Dr. F. Weritz for providing the images in Fig. 5.
References [1] B. Petterson, B. Axen, Thermography; Testing of the thermal insulation and airtightness of buildings, Swedish Council for Building Research, Spangbergs Tryckerier AB, Stockholm, 1980. [2] B. B€ ottcher, H. Richter, Ein Beitrag zum Nachweis von Feuchtigkeit in Mauerwerk mit Hilfe einer Infrarotkamera, Materialpr€ ufung 24 (1) (1982) 5–9. [3] G. Zenzinger, Thermografie––Neue M€ oglichkeiten in der ZfP, DACH-Zeitung 63/10 (1998) 58–61. [4] D. Wu, G. Busse, Lockin-Thermographie in der zerst€ orungsfreien Werkstoffpr€ ufung, DACH-Zeitung (DGZfP) 58 (1998) 67–72. [5] K. B€ uscher, W. Wild, H. Wiggenhauser, Feuchtemessung im Bauwesen mit der amplitudenempfindlichen
238
H. Wiggenhauser / Infrared Physics & Technology 43 (2002) 233–238
Modulationsthermografie, Technisches Messen 66 (4) (1999) 39–145. [6] E. Geyer, D. Arndt, L. Vogel, Bestimmung des Remissionsgrades von trockenen und befeuchteten Baustoffen im Wellenl€ angenbereicbis 1,3 bis 13 lm f€ ur die IR-reflektometrische Ermittlung von Oberfl€achenfeuchte, in: H. Wiggenhauser, H.S. M€ uller, N. Girlich (Eds.), Tagungsbericht Feuchtetag 21.09.1993, DGZfP, Berlin, 1993, pp. 223– 231. [7] K. B€ uscher, Einsatz der Modulationsthermografie zum Nachweis von Feuchtigkeit in Baustoffen, Dissertation Uni Rostock, 1999.
[8] X.P.V. Maldague, Nondestructive evaluation of materials by infrared thermography, Springer-Verlag, London, 1993. [9] X. Maldaque, J.-P. Couturier, S. Marinetti, A. Salerno, D. Wu, Advances in pulse phase thermography, in: G. Busse, D. Balageas, G.M. Carlomagno (Eds.), Quantitative Infrared Thermography, QIRT 96, Edizione ETS, Pisa, 1997, pp. 377–382. [10] B. Hillemeier, Induktionsthermographie zur Ortung von Bewehrungsst€ahlen, in: G. Schickert, D. Schnitger (Eds.), Zerst€ orungsfreie Pr€ ufung im Bauwesen, Tagungsbericht ZfPBau-Symposium 2. und 3. Oktober 1985, Berlin, BAM, DGZfP, Berlin, 1986, pp. 178–192.