Investigation of concrete structures with pulse phase thermography

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1 Available online at Materials and Structures 38 (November 25) Investigation of concrete structures with pulse phase thermography F. Weritz, R. Arndt, M. Röllig, C. Maierhofer and H. Wiggenhauser Federal Institute for Materials Research and Testing (BAM), Berlin, Germany Received: 13 October 24; accepted: 21 February 25 ABSTRACT The applicability of pulse phase thermography (PPT) for the investigation of structures is studied systematically on concrete test specimens and on a plastered sandstone column. In the test specimens, voids and delaminations are implemented in different depths and with different sizes, modelling real voids, honeycombing and debonding. Delaminations of plaster in concrete and masonry and behind tiles on concrete are investigated. PPT is based on the frequency analysis of the cooling down process of actively heated surfaces. Therefore, it is contactless and thus completely non-destructive (if overheating of the surface is prevented), fast and allows the inspection of large surface areas. The interpretation of amplitude and phase images gives semi-quantitative information about the observed defects. The phase images provide a deeper probing up to 1-15 cm in relation to the interpretation of the thermograms and to the amplitude images. In addition, the influence of surface inhomogeneities and non-uniform heating is reduced RILEM. All rights reserved. RÉSUMÉ L applicabilité de la thermographie de phase pulsée à l examen de constructions est systématiquement étudiée sur des échantillons de béton et sur une colonne de grès plâtré. Dans les échantillons testés, des vides et des délaminations sont opérés à différentes hauteurs et différentes tailles, modélisant les vides réels, l instabilité et la décohésion. Les délaminations de plâtre dans le béton et les maçonneries et derrière les sont examinées. La thermographie de phase pulsée est basée sur l analyse de la fréquence du processus de refroidissement des surfaces activement chauffées. L absence de contact la rend donc complètement non-destructive (dès lors que l on empêche de la surface d être surchauffée) et rapide. Cette méthode permet l inspection de grandes surfaces. L interprétation de l amplitude et des images de phase fournit des informations semi-quantitatives sur les défauts observés. Les images de phase sondent plus profondément jusqu à 1-15 cm en relation avec l interprétation des thermogrammes et des images d amplitude. De plus, l influence des inhomogénéités des surfaces et de la chaleur non uniforme est réduite. 1. INTRODUCTION Pulse phase thermography (PPT) is well known for nondestructive testing of materials and structures. It combines features of impulse thermography (IT) and lock-in thermography (LT) [1] and thus enables an easy data acquisition and automatic and fast data analysis. Experimental set-up and data acquisition are identical for IT and PPT: the surface of the structure to be investigated is heated by using a radiation source (also other energy sources are applicable depending on the testing problem). After switching off the heating source, the cooling down behaviour is recorded in real time with an infrared camera. The analysis of temperature differences at the surface in time domain for the detection of defects is a subject of IT. For more information about applications of impulse thermography in civil engineering, see [2]. With PPT, the recorded transient curves (temperature at a point of the surface as a function of cooling down time) are analysed in the frequency domain similar to LT. In a lock-in thermography experiment the object of investigation is heated by a sinusoidal modulated radiation of a given frequency ω. Already during heating an infrared camera records the thermal response. The time delay (phase) and the Editorial note BAM (Germany) is a RILEM Titular Member. Dr. Christiane Maierhofer participates in RILEM TC INR Interpretation of NDT results and assessment of RC structures RILEM. All rights reserved. doi:1.1617/14299

2 844 F. Weritz et al. / Materials and Structures 38 (25) amplitude with respect to the stimulation is analysed. An example of amplitude sensitive modulation-thermography is the detection and quantification of moisture at the surface of building materials [3]. In PPT each pixel (i,j) of a series of thermal images describing the cooling down behaviour after heating is transformed into the frequency domain applying the Fast Fourier Transformation (FFT). The resulting spatial resolved phase and amplitude spectra can also be visualised as images at different frequencies. Basic research on PPT has been accomplished in the group of Maldague, who reports about principles of PPT and gives a comparison between PPT, IT and LT [4]. The method was tested on aluminium specimens and different kinds of polymers. Up to now, to our knowledge PPT has not been used in civil engineering. By the application of PPT on concrete specimens the thermal material parameters and therefore the time scale of the experiment are the main difference to previous works. As concrete has a low thermal conductivity compared with metals and the dimensions are typically much larger than for plastics and laminates, the duration of the heating pulse applied to concrete is in the scale of minutes instead of some milliseconds, the cooling down process is observed for 3 to 12 min, compared to seconds. Also the dimensions of the concrete structures are much larger and defects 1 cm deep under the surface have to be detected, not only covered by a few millimetres. Within the scope of a national project funded by the DFG (German Research Foundation) the application of impulse thermography and pulse phase thermography as nondestructive testing methods in civil engineering is investigated. In a co-operation work between the Technical University of Berlin and BAM the techniques are optimised for the detection of near-surface inhomogeneities and common subsurface defects in typical structural elements. The quantitative determination of their geometrical parameters is the main objective. In this research project practical problems like locating and quantifying voids and honeycombing in concrete locating delaminations of plaster at concrete and masonry locating delaminations and voids behind tiles on concrete embedded in mortar assessment of bonding of carbon fibre reinforced laminates glued on concrete identifying poorly grouted ducts are analysed [5-7]. The influences of different reinforcement densities, of different surface properties and of the moisture content are systematically studied. In this paper, the features of PPT for the application in civil engineering are presented. A concrete test specimen containing voids with different sizes at various depths has been investigated first. Further results are presented from a concrete test specimen, on which cleaving tiles are attached in a mortar embedding. The location of delaminations between mortar and sand stones is demonstrated by an onsite investigation at a column in the Altes Museum in Berlin. 2. THEORETICAL BACKGROUND The heating pulse applied to the surface under investigation causes a non stationary heat flow. The propagation of the heat depends on the material properties like thermal conductivity, heat capacity and density. If there is an inhomogeneity in the near surface region of the object with different thermal properties, the heat flow will slow down or accelerate in these local areas. While observing the temporal changes of the surface temperature distribution with the infrared camera, near surface inhomogeneities will be detected if they give rise to measurable temperature differences on the surface. By applying the Fourier Transformation to all transient curves of each pixel, one obtains amplitude and phase images for all frequencies. Amplitude images show the internal structure of a specimen up to a maximum available depth depending on the frequency (low pass filter behaviour). Phase images show the internal structure within a certain depth range depending on the frequency (band pass filter behaviour) [4, 8]. The main advantages of PPT are given by the following properties of the phase images: Deeper probing as for amplitude images and thus enhanced detectability. For LT it has been demonstrated, that phase images can probe roughly twice the thickness compared to amplitude images [9, 1]. Phase images are less influenced by surface infrared and optical characteristics and less sensitive to nonuniform heating than thermal images and amplitude images [8, 1]. In LT the phase image is related to the propagation time delay of the thermal wave and is therefore independent of surface features. Fast inspection, results are images [4, 8, 1] Enhanced resolution of defect geometry [4, 8, 1] Non-necessity to know a priori position of non-defect areas [4, 8, 1]. This pre-knowledge is required in IT for the computation of contrast images. In our experiment the heating pulse is realised as a square pulse, which can be described as superposition of different frequencies with varying amplitudes. The available energy is concentrated in the low frequencies [4]. The pulse duration determines the frequency spectrum in that way, that for longer pulses lower frequencies contain more energy and therefore more information. This is exceedingly true for the long pulse durations necessary for the investigation of concrete. The maximum frequency is determined by the acquisition rate, the minimum frequency is limited by the recording time. In practice only the first images at low frequencies are of interest, since most of the energy is concentrated here. Higher frequencies exhibit a higher noise level. For a quantitative depth evaluation by using the frequency information different approaches have been reported. Vavilov and Marinetti [1] state that there is a significant dependence between the depth of a defect and the frequency necessary to reveal the defect. They propose phase-defect depth calibration curves resulting from calculations describing the system as 2-dimensional cylindrical problem. Furthermore they conclude that phase images provide information about depths two times deeper than amplitude images and that a material can be depth-probed only within

3 the limits of certain sample thickness. Otherwise the depth can be determined in analogy to LT, where the maximum observation depth for a given frequency ω is estimated by the thermal diffusion length µ [4]: F. Weritz et al. / Materials and Structures 38 (25) µ = 2K ωρ C p with: ω = modulation frequency of the infrared radiation, K = thermal conductivity, ρ = mass density, C p = heat capacity. The thermal diffusion length indicates the length at which the amplitude of the thermal wave has been damped to 1/e of the original value. Approximately up to this depth the thermal wave has enough energy to give a signal (measurable temperature difference) on the surface. The calculated frequencies of the thermal waves can be compared with the thermal diffusion length for the same value of modulation frequency in a LT experiment. Approaches for a quantitative depth determination with PPT have been accomplished with neural networks [11, 12], statistical methods [13] and wavelet transformations [14, 15]. In wavelet transformations the temporal information is not completely lost as in a Fourier Transformation. With the wavelet transformation two parameters become available, a translation factor and a scaling factor. Calibration of the translation factor provides a direct inversion of the defect depth for a given scaling factor. In [16] an approach for quantitative depth evaluation based on phase contrast calculations is proposed. Defects can be detected from Hz to a depth-depending frequency f b, called the blind frequency. The blind frequency is smaller for deeper defects, i.e. shallow defects are visible for a larger frequency range than deeper defects. 3. EXPERIMENTALS 3.1 Experimental setup The experimental set-up consists of a thermal heating unit, an infrared camera and a computer system, which enables digital data recording in real time, and is described in detail elsewhere [2, 5-7]. The experimental set-up is schematically shown in Fig. 1 and a photo is given in Fig. 2. For the laboratory measurements a thermal heating unit consisting of three infrared radiators having an electrical power of 24 W each is used. The heating procedure is usually done dynamically by moving the radiators computer controlled in a distance of about 15 cm to the surface to obtain the best possible homogeneous heating. The heating time varies between 3 to 6 min, resulting in a surface temperature not higher than 5 C, whereas the ambient temperature accords to room temperature. For the onsite measurement a conventional electric fan heater with 2 W is used. The heating time has to be carefully selected to avoid damages of the surface. The cooling down process of the surface is observed with an infrared camera (Inframetrics SC1, PtSi focal plane array detector with 256 x 256 pixel). The camera detects the Fig. 1 - Principle of impulse thermography. Fig. 2 - Experimental set-up for impulse and pulse phase thermography. emitted radiation from the surface of the specimen in a wavelength region from 3 to 5 µm. To obtain a good frequency resolution and for recording as much as possible from the whole cooling down process, the observation time is as long as 12 min. For additional enhancing of the time interval of the transient curves, these can be enhanced by zero padding or extrapolation. The maximum frame rate for image recording is 5 Hz, usually 5 or 1 Hz are used. The data is transferred to a computer in real time with a storage depth of 12 bit of each pixel. After receiving the thermal images the computer is also used to analyse the data with dedicated software programs, either in the time domain (IT) or in the frequency domain (PPT). 3.2 The specimens Concrete specimen with voids A schematic drawing of the concrete test specimen with voids is shown in Fig. 3 a. For simulating compaction faults and voids, cuboids made of Polystyrene foam have been incorporated. These faults vary in size (2 x 2 x 1 cm 3 or 1 x 1 x 1 cm 3 ) and depth (1 cm to 9 cm under the heated surface) inside the specimen. Some of the Polystyrene bodies

4 846 F. Weritz et al. / Materials and Structures 38 (25) (defects number 3 and 4) are not parallel to the surface, but have a tilted orientation due to buoying upwards during concreting Concrete specimen with tiles The concrete test specimen with cleaving tiles has well defined induced delaminations, the positions are shown in Fig. 3 b. The cleaving tiles are 8 to 1 mm thick, the variation resulting from a rippled rear, which should provide a good bond between the cleaving tiles and the mortar. The tiles are embedded in thick bed mortar, which has a thickness of approximately 2 mm. Therefore the system consists of three layers (cleaving tiles, mortar and concrete), with defects either between tiles and mortar (defect 1 to defect 9, defect depth approximately 8-1 mm) or between mortar and concrete (defect 1 to defect 12, defect depth approximately 3 mm) Onsite tests at a column In the frame of the European project ONSITEFORMASONRY sandstone columns in the rotunda of the Altes Museum in Berlin were investigated to locate possible delaminations of the plaster consisting of two different layers. The first layer belongs to lime plaster having a thickness of 2 to 3 cm. This plaster is the carrying basis for the visible thin stucco marble layer of 3 to 6 mm. Thus three different types of delaminations could be anticipated: delaminations of the stucco marble layer, delaminations of the plaster and a combination of both. a) 4. RESULTS 4.1 Concrete specimen with voids In Fig. 4 the transient temperature decay after a 3 min heating pulse is shown for two positions of the concrete test specimen with voids, one above a defect (defect 4 in Fig. 3) and the other above a sound area located close to the defect. The observation time during cooling down was 12 min. For these transient curves the FFT has been calculated. The amplitude spectra are shown in Fig. 5, the phase spectra are shown in Fig. 6. The differences between a 5 45 temperature ( C) reference defect b) Fig. 3 - a) Concrete test specimen with Polystyrene cuboids at different depths (1: 8 cm, 2: 6 cm, 3: 4 cm, 4: 2 cm, 5: 2 cm, 6: 4 cm, 7: 6 cm, 8: 8 cm). b) Concrete test specimen with tiles in mortar; depth of defects 1 to 9: 1 mm; depth of defects 1 to 12: 3 mm time (s) Fig. 4 - Temperature as a function of time above a defect and above the reference area during cooling down.

5 F. Weritz et al. / Materials and Structures 38 (25) amplitude (a. u.) 1 5 reference defect difference difference of amplitudes (a. u.).14 mhz.28 mhz frequency (mhz) Fig. 5 - Amplitude spectra of transient curves in Fig phase (a. u.) 5-5 reference defect difference 1-1 difference of phases (a. u.).42 mhz.56 mhz Fig. 7 - Amplitude images of the concrete test specimen with voids frequency (mhz) Fig. 6 - Phase spectra of transient curves in Fig. 4. defect and a sound area can be seen, as well as that most of the information is contained in the low frequencies (the frequency resolution is.14 mhz). In Figs. 7 and 8 subsequent images of the amplitude and phase sequence are shown, respectively. In each image, the intensity is scaled to the respective minimum and maximum to obtain the best possible contrast. For both the image corresponding to the lowest frequency (.14 mhz) provides the best visibility of all defect areas. For higher frequencies the deeper defects (defects 1 and 2) vanish. This effect occurs at lower frequencies for the amplitude images (.42 mhz compared with.56 mhz in the phase images). This behaviour is in accordance with the observations of Maldague and others, that phase images provide a deeper probing. In amplitude images all defects occur darker than the surroundings, the contrast is decreasing before they are indistinguishable from the surroundings. This frequency is called blind frequency [16]. In phase images the defects are either darker or brighter as the sound areas, which depend on frequency and depth. Additionally the phase images show the defects with better contrast compared with amplitude images at the same frequency, especially the deepest defects. The defects with a tilted orientation in respect to the surface (defects 3 and 4) can be identified in the phase images. As they seem to be half black and half white it can.14 mhz.28 mhz.42 mhz.56 mhz Fig. 8 - Phase images of the concrete test specimen with voids. be deduced that one corner is much deeper than the other. This effect is in accordance with a better geometrical resolution of phase images. Compared with thermal images (Fig. 9) especially phase images provide the advantage that all defects can be clearly seen in the same image, best in the image at the lowest frequency. Defects occur in thermal sequences after a certain time, which depends on the depth. Therefore often not all defects are visible in the same image but the whole thermal sequence has to be evaluated. In Fig. 9 it can be seen that the small defects (defects 5 8), which are covered by cm of concrete are seen best at the beginning of the thermal sequence (after about 15 min), as well as the big voids (defects 3 and 4) with the tilted orientations.

6 848 F. Weritz et al. / Materials and Structures 38 (25) min T min = 19.9 C T max = 4.87 C 39.7 min T min = 19.5 C T max = 32.4 C.26 mhz before water filling.26 mhz after water filling Fig. 1 - Phase images of the concrete test specimen with cleaving tiles and delaminations, before and after water filling in defects 2 and min T min = C T max = C 92.8 min T min = 19.2 C T max = C Fig. 9 - Thermal images of the concrete test specimen with voids. The latter are still visible after a longer observation time. In contrast the small defects show a worse signal and cannot be identified after a certain time (about 8 min). The deep defects (defects 1 and 2) cannot be seen at the beginning of the thermal sequence, but become clearer with time. 4.2 Concrete specimen with tiles To study the influence of moisture two of the smaller defects in the test specimen covered with tiles have been filled with water. Measurements have been accomplished with a 3 min heating pulse before and directly after the water was filled in. In Fig. 1 the phase images are shown. Similar to the results obtained for the concrete test specimen with voids all defects can clearly be seen in the same image, in this case at.26 mhz. All defects (delaminations) except for the water filled ones appear darker than the sound area. The defects filled with water appear brighter, showing the great influence of moisture on the thermal properties. The joints between the tiles appear with very low contrast due to the surface filtering effect of the phase images. 4.3 Onsite tests at a column For the heating of the surface of the columns shown in Fig. 11 top, a conventional electric fan heater was used having a power of 2 W. The heating time was 5 min by moving the heater along the surface (approx. 9 cm x 5 cm [height x width]) and the recording time was 15 min. In contrast to the heating with radiant heaters the Fig. 11 Stucco-marble columns in the rotunda (top) of Altes Museum in Berlin, thermal contrast image (bottom left) and phase image (bottom right) of a selected column section. convective heating is independent from the different emissivities of the surfaces and the surface geometries and causes a steady-going rise of temperature at the surface of the object. It was not possible to heat up the complete investigated area in one cycle due to height of the columns (534 cm). Therefore it was necessary to divide the area in segments and to treat each of the parts individually.

7 F. Weritz et al. / Materials and Structures 38 (25) Fig. 11 shows exemplary results from the investigations carried out at one of the columns. In the middle part of the thermal image recorded 15 s after switching of the heating source (Fig. 11, bottom left) a hot spot is visible. And in the upper part a second inhomogeneity might be accepted. In the phase image at a frequency of.55 mhz (Fig. 11, bottom right) the inhomogeneities appear more clearly, the geometry of the assumed delaminations becomes visible, even on the edge of the picture, where the column bends out of sight. At time, it is not possible to decide whether the delaminations are between sandstone and plaster or between plaster and stucco marble. For further quantitative analysis, numerical simulations are planned. 5. CONCLUSIONS Pulse phase thermography proves to be useful for applications in civil engineering, as shown for laboratory and onsite experiments. The properties of PPT in civil engineering are in accordance with previous works on materials with higher thermal conductivity. Images in the frequency domain facilitate the visibility and detectability of defects. Phase images show only slight influence by surface inhomogeneities and non uniform heating. The geometrical configuration of the defects can be determined from phase as well as from amplitude images. Semi-quantitative estimation of defect depths in relation to the frequency where the defects appear results in the right order of magnitude, but further theoretical work has to be done for enhancing the reliability. The presence of moisture inside a specimen has a clear influence on the thermal properties and thus on the phase image. In the frame of the DFG funded project further systematic investigations will be accomplished. Additional test specimens will be investigated by means of PPT, e.g. carbon fibre reinforced laminates glued on concrete, plaster with different depths, voids with different geometries. The influence of reinforcement, moisture and surface properties will be studied. ACKNOWLEGDEMENTS The presented research has been funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) and by the European Commission (Project Acronym: ONSITEFORMASONRY). REFERENCES [1] Maldague, X., Theory and practice of infrared technology for non-destructive testing, (John Wiley and Sons, Inc., 21). [2] Wedler, G., Brink, A., Röllig, M., Weritz, F. and Maierhofer, C., Active infrared thermography in civil engineering - Quantitative analysis by numerical simulation, Proceedings of the International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin, 23, BB 85-CD, V8 (Deutsche Gesellschaft für Zerstörungsfreie Prüfung e. V., Berlin, Germany, 23). 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