GPR AND IR THERMOGRAPHY FOR NEAR-SURFACE DEFECT DETECTION IN BUILDING STRUCTURES

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1 The 12th International Conference of the Slovenian Society for Non-Destructive Testing»Application of Contemporary Non-Destructive Testing in Engineering«September 4-6, 2013, Portorož, Slovenia GPR AND IR THERMOGRAPHY FOR NEAR-SURFACE DEFECT DETECTION IN BUILDING STRUCTURES Patricia Cotič 1, Zvonko Jagličić 2, Vlatko Bosiljkov 3, Ernst Niederleithinger 4 1 Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia, patricia.cotic@imfm.si 2 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, 1000 Ljubljana, Slovenia; Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia, zvonko.jaglicic@imfm.si 3 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, 1000 Ljubljana, vlatko.bosiljkov@fgg.uni-lj.si 4 BAM Federal Institute for Materials Research and Testing, Div. 8.2, Unter den Eichen 87, Berlin, Germany, Ernst.Niederleithinger@bam.de ABSTRACT Ground penetrating radar (GPR) and infrared (IR) thermography techniques have been used in many civil engineering applications for the structural visualization and defect detection. However, validation tests of the methods performance for the defection of defects in the nearsurface region with respect to the defects different material and depth below the surface are lacking. To overcome this, we performed GPR and IR thermography tests where the different material properties, shape and depth of defects were studied on concrete and the evaluation of seismic related damage propagation was assessed on stone masonry walls. The results showed that IR thermography, though being greatly affected by the presence of water in the specimen, outperformed GPR in the detection of defects very close to the surface. However, already at the depth of 3 cm and further up till almost 7.5 cm, the performance of GPR resembles the one of IR thermography for the detection of polystyrene (air) voids. On the plastered masonry walls, IR thermography could detect an air gap resulting from plaster delamination as small as 2 mm. Moreover, structural cracking resulting from the induced lateral load could be detected at an early stage. Key words: Non-destructive testing validation, concrete, masonry, near-surface defects, ground penetrating radar, infrared thermography. 1. Introduction Ground penetrating radar (GPR) and infrared (IR) thermography are well known non-destructive testing (NDT) methods for the structural visualization and defect detection in building structures. GPR is based on the propagation of high frequency electromagnetic impulses produced by an antenna system and their reflection from interfaces between materials with different dielectric properties. With active IR thermography on the other hand, structural visualization is enabled by recording the infrared radiation from the specimen s surface previously being heated. GPR, 225

2 being a contact method, has been found promising in imaging the inner structure at depths of more than 5 cm [1], whereas IR thermography gains potential in resolving defects very close to the surface such as spalling, delamination, structural cracking, voids, as well as the moisture distribution [2, 3]. Although thermography benefits from being a fast technique with the capability of performing remote inspection, its performance decreases rapidly already after a few cm in depth. However, the exact penetration limit is in general dependent on the properties of the tested material as well as of the defects and may vary within the range of 5 to 10 cm [1]. Similar, but contrary behaviour is valid for GPR. In this paper we study the performance of GPR and IR thermography for the detection of nearsurface defects on both concrete and masonry laboratory specimens. Concrete specimens with inbuilt defects featuring various material, shape and size, as well as depth below the surface were taken into account, where the known position of the defects enabled to perform validation tests of both measuring systems. On plastered three-leaf stone masonry walls subjected to in-plane stepwise shear loading, the study of seismic related defects such as gradual plaster delamination and crack propagation was carried on. Multi-leaf masonry that covers a large portion of cultural heritage assets in seismic prone areas [4] is very often subjected to intensive damage due to seismic action. Therefore, to evaluate the structural condition of such masonry (often with attached artistic values such as paintings and frescoes) with minimal interference, the application of NDT is of particular importance. 2. Experimental 2.1 Concrete test specimens Three concrete specimens (S1-S3) with a size of cm 3 and made from the same concrete were studied. The specimens contained inbuilt anomalies varying in the material (polystyrene, air and water), shape, size and depth below the surface (Fig. 1). The cuboids in specimen S1 had a size of cm 3, the size of S2 cuboids was 8 8 cm 2 with varying thickness. The polystyrene plates (simulating delamination) in specimen S3 had a thickness of 1 cm (a) and 2 cm (b) and the diameters of the plastic pipes were 1, 1.5 and 2.5 cm (from left to right in Fig. 1, S3). The concrete cover above all the defects in specimen S3 was between 1.5 and 2 cm, whereas for S1 and S2 the defects depth varied. Defect specifications for all of the specimens are summarized in Table 1. Fig. 1: Sketches of the laboratory concrete test specimens with marked inbuilt defects. Table 1: Material and concrete cover of simulated defects for specimens S1-S3. DEFECT DEFECT MATERIAL CONCRETE COVER [cm] S1a polystyrene 1.5/7.5 S1b polystyrene 3/6 S1c,d polystyrene 4.5 S2a air 6 S2b air 3 S2c water 6 S2d water 3 S3a,b polystyrene S3pipes plastic

3 2.2 Three-leaf stone masonry walls Several plastered three-leaf stone masonry walls measuring cm 3 were investigated (Fig. 2). Due to the unevenness of the masonry surface, the thickness of the two-layer plaster (both coarse and fine lime mortar) varied between cm. External leaves of the masonry were constructed from regular coursed squared ashlar rough tooled lime stone, while the internal core was filled with stone rubble and lime mortar. The specimens varied in the type of connection between the leaves. However, as the paper focuses on the detection of near-surface defects, the detected masonry morphology and geometry using GPR is not presented. For this refer to [5]. The walls were subjected to in-plane cyclic shear test monitored by a 3D digital image correlation (DIC) technique. Though the primary aim of the shear test was to assess the structure s seismic behaviour, the well-defined loading conditions enabled to non-destructively assess the propagation of seismic damage. NDT measurements were performed at the unloaded (reference) state of the specimen, as well as on two loading steps (hereinafter referred to the 1 st and 2 nd loading state). The 1 st loading state corresponded to the induced lateral displacement, where both the first detachment of the plaster and the first visible surface cracks appeared, whereas the 2 nd loading state referred to largely detached or significantly damaged plaster, where a fully collapse of the plaster in the following loading step was assumed. DIC out-of-plane displacements proved a direct correlation to the plaster delamination [5] and were therefore used as a reference for validation of both measuring systems. In the paper we present the results obtained from two walls (referred to walls W1 and W2) with respect to the investigated problem. Fig. 2: The three-leaf stone masonry wall before the plaster application (a); two layers of plaster applied (b); the GPR and IR thermography test set-up under the shear test. 2.3 NDT methods GPR data used for this study was obtained with the equipment from MALÅ Geoscience, using a 1.6 GHz monostatic shielded antenna. A calibrated survey wheel was used and radargrams were collected in both directions with a line spacing of 5 cm and point distance of 0.5 cm. IR thermography data was acquired with an IR camera type FLIR A320, having a thermal sensitivity of 50 mk (at 30 C), spatial resolution of 1.36 mrad and a focal plane array with IR resolution of pixels and spectral range of μm. For a homogeneous heating of the specimens, two IR heaters (1.2 kw each) were moved parallel to the surface at a distance of about 10 cm. The heating time for the masonry walls was approximately 30 min, while the cooling down was recorded for 45 min at approximately 3 m from the specimen s surface. For 227

4 the concrete test specimens, the heating and cooling down times were adjusted in regard to defects depth. All thermal images were recorded at a frame rate of 0.2 Hz. Following Arndt [6], the square pulse thermography (SPT) in the frequency domain technique was used, where the analysis of the recorded temperature evolution versus time of each pixel is performed in the frequency domain, deriving amplitude and phase images. Especially phase images exhibit higher depth penetration, higher resolution and are less sensitive to non-uniform heating than thermal and amplitude images [7] and were therefore used in our study. Since the concrete test specimens contained defects at well-known depths, the quantitative approach of SPT in frequency domain [6] with respect to the material properties of concrete and defects was used, where the depth z of a defect can be expressed by the frequency f ch of the maximum phase or amplitude contrast between defect and sound area through the following equation z = k c α f ch (1) where k c is a correction factor and α is the thermal diffusivity of the investigated material. As a result, phase contrast images could be regarded as depth slices, enabling a direct correlation with the GPR C-scans. However, for masonry walls, the complex structure restricted the application of the quantitative approach; thus phase images were used. 3. Results and discussion 3.1 Results from concrete test specimens Based on ground truth reference from the known position of inbuilt defects, the performance of both NDT methods with respect to the defect material and concrete cover was evaluated quantitatively from GPR C-scans and IR phase contrast images. Sensitivity and specificity values [8] were calculated from the number of true positive (TP), true negative (TN), false positive (FP) and false negative (FN) calls by using sens = TP/(TP + FN) (2) spec = TN/(TN + FP) (3) where positive and negative refer to identified and rejected defect, respectively. Thus, a true positive call means that a pixel was correctly identified as a defect. A combined sensitivity measure c_sens was further calculated by c sens = (a sens + b spec) (a + b) (4) where weights a and b refer to the specimens corresponding defect and non-defect area, respectively. According to Daniels [9], near-field antenna coupling/induction effects affect the radar signal up till the depth of 1.5 λ (λ is the wavelength of the electromagnetic waves), which for the used 1.6 GHz antenna extends up to a depth of almost 10 cm in concrete. Moreover, for the radar depth resolution, 0.25 λ has been proposed for its calculation [10]. However, the results for polystyrene defects in Fig. 3 (full blue marks) show that the performance of GPR is largely decreased at the very near-surface, but increases rapidly already at the depth of 3 cm and varies only to a small extent up till the depth of 7.5 cm. As expected, the sensitivity of IR thermography on the other hand decreases along the depth. However, it has to be noted that the methods good depth penetration results particularly from using phase images. Nevertheless, it is expected that the methods capability would drop remarkably at greater depths. 228

5 From the results in Fig. 3 it can be further observed that the difference between the air and polystyrene properties is more apparent for IR thermography, which is also greatly affected by the presence of water (the sensitivity of IR thermography for the water defect at the depth of 6 cm was largely decreased and is therefore not presented). Although both methods respond to the water defect by a different magnitude, the response is consistent with the expected performance of both methods, where the dielectric constant and the thermal conductivity govern the behaviour of GPR and IR thermography, respectively. Fig. 3: Combined sensitivities (in %) for GPR (full marks) and IR thermography (empty marks) with respect to the defect material and concrete cover. For specimen S3, the GPR depth slice at 1.5 cm and corresponding IR phase contrast image at frequency with marked polystyrene plates are shown in Fig. 4. It can be seen that especially in the region of the plastic pipes, GPR data exhibit poor contrast. This was further justified by sensitivity evaluation of the right (b) polystyrene plate (the left plate seems to have tilted during concreting and was thus not assessed), which yielded 75.4 % and 90.5 % for GPR and IR thermography, respectively. By correlating these results with the ones in Fig. 3 it can be proved that the performance of GPR decreases greatly for the detection of near-surface delamination. Fig. 4: Imaging results from specimen S3 at the depth of 1.5 cm with marked polystyrene plates according to Table 1: GPR depth slice (a) and corresponding IR phase contrast image at frequency Hz. 3.2 Results from three-leaf stone masonry walls As already mentioned, DIC together with visual inspection served as a reference to evaluate the efficiency in detection of gradual plaster delamination on three-leaf masonry walls for both methods. For wall W1, DIC out-of-plane displacements for the loading states taken into account are shown in Fig. 5 together with the NDT results. It can be seen that with the increasing lateral displacement both methods detect larger delaminated area. However, their reliabilities for 229

6 delamination detection differ remarkably. By comparing the IR phase images at the frequency of Hz to DIC results for the specific wall at both loading states, one can see that IR thermography detected an air gap larger than only 2 mm. Contrary, at the 1 st loading state, GPR provided a very unclear delamination pattern and according to Fig. 5i at the 2 nd loading state detected only a delamination larger than approximately 8 mm. Although we present the results from only one wall, it has to be noted that the obtained values were justified also by the results from other investigated walls. Fig. 5: Detection of plaster delamination with DIC, IR thermography and GPR for wall W1: DIC out-of-plane displacements (a,d,g), IR phase images at frequency Hz (b,e,h) and GPR depth slices at 1.3 cm (c,f,i). Images from the first, second and third row refer to the reference state, first and second loading state, respectively. The position of the delamination limit is marked with arrows. As expected, GPR could not account for the estimation of the crack propagation (Fig. 6a), whereas IR thermography was found capable of detecting both surface and subsurface crack patterns. The latter was achieved by using IR phase images at higher frequencies, which enables the detection of small thermal contrasts. On the other hand, lower frequencies carry the 230

7 information from the whole depth profile, resulting in an average thermal behaviour of the specimen. Phase images at the lowest frequency ( Hz) were therefore used to visualize the underlying texture and the delamination patterns, whereas higher frequencies were used for the detection of cracks. An example of a detected vertical surface crack at the 2 nd loading state of specimen W2 is shown in Fig. 6c for IR phase image at frequency Hz. The corresponding phase image at Hz (Fig. 6b) together with the GPR depth slice at the depth of 2 cm (Fig. 6a) visualize the underlying texture and the delamination pattern. Fig. 6: Detection of the underlying texture, plaster delamination and surface cracks with GPR and IR thermography for specimen W2 at the second loading level: GPR depth slice at 2 cm (a), IR phase images at frequencies Hz (b) and Hz (c). 4. Conclusions The challenges of GPR and IR thermography for the detection of near-surface defects were studied on laboratory concrete and masonry specimens. The concrete specimens enabled a quantitative evaluation of the methods performance with respect to the defects material and concrete cover. It was shown that GPR provided a good localization of polystyrene defects already at the depth of 3 cm and responded only slightly to the different defect material with respect to the dielectric constant. On the other hand, the capability of IR thermography worsened to a large extent in the presence of water, whereas it outperformed GPR in the detection of plastic pipes and delamination. In addition, the method exhibited a relatively stable performance up till the depth of 7.5 cm. However, we are aware that the sensitivity results obtained so far cannot be generally applied. Therefore, additional validation tests that will cover a larger depth region and larger variety of defects are currently performed. Results from three-leaf masonry walls showed that IR thermography outperformed GPR in the detection of seismic related defects very close to the surface. Nevertheless, GPR clearly visualized the masonry texture behind the plaster. The methods better performance is also expected in the cases where delamination would appear at greater depths, i.e. for thicker plaster and delamination between leaves. We showed that with a synergetic use of GPR and IR thermography near-surface defects together with the structural geometry could be successfully detected in both concrete and masonry. However, in order to combine the complementary information from both methods on one hand and to ease the data evaluation process and the interpretation of results on the other hand, mathematical data fusion could be applied. For a fusion technique, we have started to use unsupervised clustering methods [11], whereas the development of a complete fusion methodology for NDT data on concrete and masonry is currently under a research and will be published elsewhere. 231

8 5. Acknowledgement The first author acknowledges the financial support of the Slovenian Research Agency through grant and the Slovene Human Resources and Scholarship Fund through grant /2012. The authors wish to thank the Jožef Stefan International Postgraduate School for the use of their radar system, the company Modri Planet d.o.o. for performing DIC, as well as Primož Murn, Damjan Špeglič, Franci Čepon and Assoc. Prof. Violeta Bokan Bosiljkov for extensive measurement support. 6. References [1] Maierhofer Ch., Köpp Ch., Binda L., Zanzi L., Santiago J.R., Knupfer B., Johansson B., Modena C., Porto F., Marchisio M., Gravina F., Falci M., Ruiz J.C., M. Tomazevic M., Bosiljkov V., Hennen Ch.: Project Report EUR EN Onsiteformasonry project On-site investigation techniques for the structural evaluation of historic masonry buildings, European Commission, Office for Official Publications of the European Communities, Brussels, [2] Maierhofer Ch., Arndt R., Röllig M., Rieck C., Walther A., Scheel H., Hillemeier B.: Application of impulse-thermography for non-destructive assessment of concrete structures, Cement Concrete Comp, Vol. 28, No. 4, 2006, [3] Maierhofer Ch., Röllig M.: Active thermography for the characterization of surfaces and interfaces of historic masonry structures, Proceedings of the 7th International Symposium on Non-Destructive Testing in Civil Engineering (NDTCE 09), LCPC, Nantes, 2009, on CD-ROM. [4] Binda L., Saisi A.: Application of NDTs to the diagnosis of historic structures, Proceedings of the 7th International Symposium on Non-Destructive Testing in Civil Engineering (NDTCE 09), LCPC, Nantes, 2009, on CD-ROM. [5] Cotič P., Jagličić Z., Bosiljkov V.: Validation of non-destructive characterization of the structure and seismic damage propagation on multi-leaf stone masonry walls with artistic assets, J Cult Herit (sent for publication). [6] Arndt R.: Square pulse thermography in frequency domain, in: Thermosense XXX, Proceedings of SPIE (Vavilov V.P., Burleigh D.D, eds.), Society of Photo Optical, Orlando, FL, 2008, 69390X-69390X-12. [7] Weritz F., Arndt R., Röllig M., Maierhofer Ch., Wiggenhauser H.: Investigation of concrete structures with pulse phase thermography, Mater Struct, Vol. 38, No. 9, 2005, [8] Altman D.G., Bland J.M.: Statistics Notes: Diagnostic tests 1: sensitivity and specificity, BMJ, Vol. 308, 1994, [9] Daniels J.D., ed.: Ground-penetrating radar - 2nd ed., The Institution of Electrical Engineers, London, [10] Jol H., ed.: Ground Penetrating Radar: Theory and Applications - 1st ed., Elsevier Science, Amsterdam, Oxford, [11] Cotič P., Niederleithinger E., Bosiljkov V., Jagličić Z.: NDT Data Fusion for the Enhancement of Defect Visualization in Concrete, Proceedings of the 10th International Conference on Damage Assessment of Structures (DAMAS 2013), Trinity College, Dublin,