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GPR evaluation of the Roman masonry arch bridge of Lugo (Spain)
NDT&E International 44 (2011) 8–12
Contents lists available at ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
GPR evaluation of the Roman masonry arch bridge of Lugo (Spain) M. Solla n, H. Lorenzo, F.I. Rial, A. Novo Department of Natural Resources and Environmental Engineering, University of Vigo, A Xunqueira, 36005 Pontevedra, Spain
a r t i c l e in f o
a b s t r a c t
Article history: Received 30 April 2010 Received in revised form 4 August 2010 Accepted 15 August 2010 Available online 27 October 2010
Many masonry arch bridges are historical constructions still in use within the transportation infrastructure. As their modern functionalities are different from those for which they were originally designed, and because of their age, some have suffered structural transformations over time. Consequently, there has been a recent and continuous increase in the use of non-destructive testing to evaluate historical masonry arch bridges using methods that will not change the historical characteristics of these structures. In Spain, a significant number of masonry arch bridges were constructed between the Roman period and the early 19th century. The Roman bridge of Lugo (NW Spain) was surveyed with GPR to analyze its inner structure from three points of view: historical, archaeological and structural. GPR measurements were performed using 250 and 500 MHz shielded antennas. Interpretation of the data included simulation of FDTD models based on the external geometric measures of the structure. Results revealed the ancient profile of the bridge, as well as many subsoil zones with different fills. GPR provided the unknown inner construction details, which may represent noteworthy information for archaeologists and civil engineers. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Ground penetrating radar NDT methods FDTD simulations Historical bridges Archaeology
1. Introduction Several organizations dedicated to the conservation, protection, enhancement and appreciation of cultural heritage, such as ICOMOS and UNESCO, have asserted the importance of nondestructive testing (NDT) to evaluate historical monuments, reflecting societal concern for their maintenance and conservation [1]. Consequently, there has been a continuous increase in the use of NDT methods to detect defects and anomalies on historical buildings in the last decade [2,3]. Many of the masonry arch bridges in Spain are listed as historical monuments, and are considered to be important structures from several points of view: architectural, historic, economic, symbolic and aesthetic [4]. They can characterize the landscape of a region and represent an integral part of its traditional architectonic heritage [5]. In Galicia (NW Spain), the distribution of historical bridges is very extensive and a Roman or medieval bridge can be found in any village, valley or stream. At the moment, there are about 250 historical bridges catalogued in the Galician territory [6]. Many of these bridges are still in use within the transportation infrastructure, and as a result some have potentially lost their original utility or have been modified for new functions [7]. These changes require a constant diagnosis because they are normally subjected to potentially destructive
n
Corresponding author. Tel.: + 34 986 801908; fax: + 34 986 801907. E-mail address: [email protected] (M. Solla).
0963-8695/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2010.08.004
conditions, such as increase in traffic loads, intense vibrations and their age, which have resulted in some structural decay [8]. Without conservation, a number of the remaining bridges are at a risk of total loss, not to mention those that are in the process of ruin and destruction due to their changing functions. This situation has posed an important task for engineers and scientists to evaluate the state of conservation of historical bridges using methods that will not change the historical character of the structures [9]. Ground penetrating radar (GPR) is one of the most frequently recommended NDT methods in bridge inspection because it is a relatively quick technique that gives an overall qualitative internal image and provides high penetration depth [10,11]. However, to date there have been few published studies on the evaluation of masonry arch bridges, probably because nondestructive testing on ancient civil engineering structures with GPR is a relatively new subject since the 1990s [12] in addition to the geometrical and interpretational difficulties of these complex structures. Mainly, these studies have shown good potential for providing valuable information about bridge foundations, ring stone thickness, moisture content and fill conditions [13–18]. The objective of this work was to analyze the viability and effectiveness of GPR in evaluating historical masonry arch bridges. ˜ o River, The Roman bridge of the Lugo council (Fig. 1), over the Min was surveyed in order to obtain structural and geometrical characteristics, mainly in terms of detecting hidden features and construction details of different building phases over history. Additionally, some finite-difference time-domain (FDTD)
M. Solla et al. / NDT&E International 44 (2011) 8–12
9
Fig. 1. Location of the Lugo Bridge in Galicia (Europe).
Fig. 2. General view of the Lugo Bridge from the downstream side.
Fig. 4. GPR survey composed of two parallel profiles through the bridge, back and forth. Data acquisition setup composed of 250 MHz shielded antenna.
Table 1 Data processing applied to the GPR data acquired with the 250 and 500 MHz shielded antennas. GPR data processing (ReflexW v5.0 software)
Fig. 3. (A) Profile evolution of the Lugo Bridge throughout the time and (B) the external building materials composition at the upstream side (adaptated from [19]).
simulations were performed to obtain information on the fill material within the structure. This bridge was selected because of its importance in providing contemporary strategic road access to the city. Therefore, a structural assessment is required because it supports traffic loads much heavier than in ancient times. The Lugo Bridge has eight gothic or barrel arches spanning from 5.6 to 10.4 m (Fig. 2). Although this bridge presents a horizontal profile at present, some authors inform us that the Lugo Bridge had a medieval double slope profile until the end of the XIX century as shown in Fig. 3A. Several restoration and reconstruction tasks have been performed over time [19]. Consequently, different building materials such as granitic and slate are observed in the stonework surface (Fig. 3B).
2. GPR data acquisition and processing The GPR survey was carried out with 250 and 500 MHz shielded antennas using a RAMAC/GPR system from MALA˚
250 MHz
500 MHz
1. Time-zero correction 2. Dewow filtering 3. Gain function (linear 1.66 and exponential 0.47) 4. Subtracting average (150 average traces) 5. Band-pass Butterworth (low cutoff: 100 MHz & upper cutoff: 450 MHz)
1. Time-zero correction 2. Dewow filtering 3. Gain function (linear 1.58 and exponential 0.45) 4. Subtracting average (400 average traces) 5. Band-pass Butterworth (low cutoff: 200 MHz & upper cutoff: 900 MHz)
Geoscience. These antennas were chosen as the most suitable ones because of their optimum compromise between penetration and resolution. Two longitudinal parallel profiles in opposite directions along the bridge, each 122 m in length, were collected using both antennas (Fig. 4). Data acquisition was based on constant distance intervals following the common offset mode, and the survey parameters selected with 250 MHz were 2 cm inline spacing, time windows of 210 ns, a sampling frequency of 2577 MHz and 540 samples/trace, whereas the acquisition parameters for the 500 MHz antenna were 2 cm in-line spacing, time windows of 100 ns, a sampling frequency of 6802 MHz and 677 samples/trace. Before interpretation, GPR processing filters were employed in order to reduce clutter or any unwanted noise in the raw-data, to enhance extraction of information from the signals received and to produce an image of the subsurface including all the features and/or targets of interest, which makes GPR data interpretation easier. Each radargram was filtered by ReflexW v5.0 software by applying the processing sequences described in Table 1.
10
M. Solla et al. / NDT&E International 44 (2011) 8–12
Fig. 5. Processed radargram acquired with the 250 MHz antenna (A) and 500 MHz antenna (B).
3. Results and discussion Fig. 5 shows two radargrams acquired with the 250 and 500 MHz antennas. Both radargrams were filtered by applying the processing sequences described in Table 1. At first glance, it is possible to appreciate the different GPR signal responses over the arches along the structure (Fig. 5A). Clear signal attenuation was observed over some arches. The 250 MHz antenna provided more structural details, probably because of its better penetration. The reflections from the arch–air interfaces for the first, fifth and eighth arches were nearly completely attenuated (Fig. 6), although they were placed at the same level as the others (Fig. 2). Additionally, a slightly sloping constant reflector was identified at both margins of the bridge. The geometry, condition and size of this reflector allowed us to conclude that the Lugo Bridge had a double slope profile in former times as documented in the specialized literature [19]. This ancient profile was filled at both margins of the river to create the actual horizontal pathway (Fig. 6). The corner reflections caused by the perpendicular interfaces between the top of the vaults and the water level were also identified [20]. Finally, Fig. 7A illustrates in detail the reflections produced by the transverse reinforcing beams used for supporting the cantilever metal parapet of this bridge (Fig. 7B). These metallic structures lean in pilasters from the buttresses. Once this repair task was performed in 1989, the pathway became wider [19]. The different signal response that we observed could be an indication of different stonework and fill materials within the bridge structure. These differences were most likely caused by several reconstructions and restorations over the arches throughout its lifetime. According to historical references, some arches have a granitic vault (1, 2, 4, 7 and 8 from the right margin in the downstream side), whereas the others have a slate vault [19]. The signal attenuation produced can be explained by this fact because vaults built of slate materials, such as arches 5 and 6, presented more attenuation than the others built with granite (arches 4 and 7). The fill material used for leveling the pathway, most probably composed of granitic and slate fills, could also have produced the signal attenuation observed for the first, second and eighth arches (Fig. 6). Inner granitic and slate fills were assumed due to the external bridge stonework, historical references and material availability in this area. Additionally, the asymmetric
Fig. 6. Radargram acquired with the 250 MHz antenna. Interpretation of the main reflectors detected.
shape observed in the reflection generated from the arch–air interface of the barrel arches, as in the case of the fourth arch, could be a consequence of this lack of homogeneity (Fig. 6). This non-homogeneity in fills resulted in several GPR signal velocities over the arch. Using the structural dimensions provided by geometric measurements of the bridge with a tape measure, it was possible to estimate signal velocities in different zones of the structure, ranging from 8.5 to 14.0 cm/ns. Due to the asymmetric reflection patterns on either side of the keystone, velocity for these cases was estimated by adapting a diffraction hyperbola to each half of the reflection, resulting in two different signal velocities as shown in Fig. 8. Thus, a lower signal velocity for the right half (10.83 cm/ns) was determined in comparison to the left branch (13.37 cm/ns). The typical average velocity of a GPR pulse is reported in the literature as 12.0–15.0 cm/ns in dry granite and 9.0–10.0 cm/ns in dry slate [21]. Therefore, the differences in the velocity values estimated could be explained by the presence of different fills of either slate or granite (at the right and left branch, respectively), over the arch at both sides of the keystone. In complex structures such as this, comparison of real data with synthetic models allows for extraction of subtle interpretational information to produce an exhaustive data interpretation [22,23]. Several FDTD models were built to confirm or reject the hypothesis considered to explain the radar wave attenuation and the asymmetric shape in the reflection of the arch observed in the GPR real data. A two-span bridge was simulated considering granitic and slate fills in different locations over arches (Fig. 9). These synthetic models were built using the external geometry of the bridge provided by measurements. The purpose was to design a realistic model. Table 2 shows the electromagnetic properties
M. Solla et al. / NDT&E International 44 (2011) 8–12
11
Fig. 7. Identification of reinforcing beams: (A) association to the radar data obtained with the 250 MHz antenna (in detail) and (B) location on the bridge structure.
Fig. 8. Example of velocity determination by adapting a diffraction hyperbola to each half part of the reflection.
Fig. 9. Some FDTD simulations to analyze the influence of granitic and slate fills over the arches on the radar wave response.
employed for media characterization. These parameters were calculated using the velocity values estimated, and the relative dielectric permeabilities estimated for granitic and slate materials are comparable to the values proposed by other authors [24]. The synthetic models simulated showed a slight signal attenuation in the reflection from the arch–air interface for the
slate fill compared to the granitic fill (Fig. 9). Additionally (Fig. 9C) considering both fills over the second arch illustrates the asymmetric reflection pattern on either side of the keystone observed in the real data (Fig. 8). Thus, this irregularity in shape is most likely produced by different radar wave velocities over the arch on each side. Most importantly for our study, the synthetic
12
M. Solla et al. / NDT&E International 44 (2011) 8–12
Table 2 Electromagnetic properties assumed for media characterization. Material
Conductivity (s m
Air Granite (masonry) Granitic fill Slate fill Water
0.0 0.0001 0.0001 0.01 0.5
1
)
Relative permeability 1 6.0 5.0 8.0 80.0
results together with the velocity values estimated seem to confirm the existence of both granitic and slate fills in the bridge structure. Therefore, the differing signal behavior observed in the real data facilitates mapping of those areas in the bridge regarding fill material composition. These changes in fills would most probably be associated with reconstruction or restoration tasks performed over time [19].
4. Conclusions GPR was confirmed to be an effective NDT method to reveal the details of inner bridge construction and modification over time. The geometry of an ancient profile of a bridge from medieval times was determined. The former double slope profile had been filled to obtain the actual horizontal pathway using a fill material, which was most probably different from the original one. Additionally, GPR results revealed the presence of diverse areas in the entire bridge structure, depending on their different fills, related to several restoration and reconstruction tasks that have been conducted over its lifetime. This information is useful for civil engineers engaged in developing future conservation and strengthening measures because it can be used to take decisions about stability. Differences in stonework and fill materials can affect the structural reinforcement of the bridge in terms of durability and strength of the materials involved. Future replacement of the fill would therefore be needed to provide better structural stability. The 500 MHz antenna that might have produced more precise information of the shallower fill material, however, was not capable of transmitting energy to the depth necessary within the corresponding signal attenuation observed for slate fill. The inner granitic and slate fills were established with the signal attenuation observed and the velocity values estimated. The FDTD provided important additional information that could be directly compared to the GPR real data, which were crucial in the interpretation. Although GPR can usually distinguish between different subsoil zones depending on their composition, specific identification of materials involved is more difficult. In order to confirm the fill materials it would be interesting to perform an endoscopic survey. On observing the GPR results collected, it is possible to select several locations of interest to perform an endoscopic survey to obtain a general view of the materials within the bridge structure, which will allow for an exhaustive data interpretation. Thus, the combination of GPR and endoscopy is very useful to reduce invasive interventions if the endoscopy is perfectly set.
Acknowledgments The authors gratefully acknowledge the financial support of the Spanish Ministry of Science and Innovation (Grant no. BIA2009-08012). The authors wish to acknowledge the useful suggestions provided by Antonios Giannopoulos. References [1] ICOMOS, Recommendations for the analysis, conservation and structural restoration of architectural heritage, 2001. [2] Garcı´a F, Ramı´rez M, Rodrı´guez I, Martı´nez R, Tort I, Benlloch J, Montalva´ JL. GPR technique as a tool for cultural heritage restorations: San Miguel de los Reyes Hieronymite Monastery, 16th century (Valencia, Spain). Journal of Cultural Heritage 2007;8(1):87–92. [3] Orlando L, Slob E. Using multicomponent GPR to monitor cracks in a historical building. Journal of Applied Geophysics 2009;67(4):327–34. [4] Ferna´ndez JA. Obras Pu´blicas y Monumentos. Revista de Obras pu´blicas y Monumentos 1995;142(3347):7–13. ¨ . Turkish historical arch bridges and ¨ A, _Iskender O [5] Ural A, Oruc- S- , Do˘gangun their deteriorations and failures. Engineering Failure Analysis 2008;15: 43–53. [6] Alvarado S, Dura´n M, Na´rdiz C. Puentes histo´ricos de Galicia. Colegio Oficial de Ingenieros de Caminos, Canales y Puertos. Xunta de Galicia, 1989. [7] Forde MC. Bridge research in Europe. Construction and Building Materials 1998;12(2–3):85–91. [8] Bhandari NM, Kumar P. Structural health monitoring and assessment of masonry arch bridges, in: Proceedings of the advances in bridge engineering conferrence, 2006, p. 115–32. [9] Melbourne C, McKibbins LD, Sawar N, Sicilia Gaillard C. Masonry arch bridges: condition appraisal and remedial treatment. London: Documentation, CIRIA; 2006. [10] McCann DM, Forde MC. Review of NDT methods in the assessment of concrete and masonry structures. NDT&E International 2001;34:71–84. [11] Orba´n Z, Gutermann M. Assessment of masonry arch railway bridges using nondestructive in-situ testing methods. Engineering Structures 2009;31(10): 2287–98. [12] Flint RC, Jackson PD, McCann DM. Geophysical imaging inside masonry structures. NDT&E International 1999;32:469–79. [13] Colla C, Das PC, McCann D, Forde M. Sonic electromagnetic and impulse radar investigation of stone masonry bridges. NDT&E International 1997;30(4): 249–54. [14] Pe´rez-Gracia V, Radar de Subsuelo. Evaluacio´n para aplicaciones en arqueologı´a y en patrimonio histo´rico-artı´stico. PhD Thesis, Universidad ˜ a, 2001. Polite´cnica de Catalun [15] Fernandes F. Evaluation of two novel NDT techniques: microdrilling of clay bricks and ground penetrating radar in masonry. PhD thesis, Universidade do Minho, 2006. [16] Arias P, Armesto J, Di-Capua D, Gonza´lez-Drigo R, Lorenzo H, Pe´rez-Gracia V. Digital photogrammetry, GPR and computational analysis of structural damages in a medieval bridge. Engineering Failure Analysis 2007;14: 1444–57. [17] Lubowiecka I, Armesto J, Arias P, Lorenzo H. Historic bridge modelling using laser scanning, ground penetrating radar and finite element methods in the context of structural dynamics. Engineering Structures 2009;31(11): 2667–76. [18] Solla M, Lorenzo H, Rial FI, Novo A, Riveiro B. Masonry arch bridges evaluation by means of GPR. IEEE Xplore, in press, doi:10.1109/icgpr.2010. 5550194. [19] Dura´n M. La construccio´n de Puentes Romanos en Hispania, Xunta de Galicia, 2005. [20] Martinaud M, Frappa M, Chapoulie R. GPR signal for the understanding of the shape and filling of man-made underground masonry, in: Proceedings of the 10th international conference on ground penetrating radar, 2004. [21] David J, Annan P. Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophysical Prospecting 1989;37:531–51. [22] Giannopoulos A. Modelling ground penetrating radar by GprMax. Construction and Building Materials 2005;19:755–62. [23] Jol HM. Ground penetrating radar: theory and applications. Elsevier Science; 2009 524pp. [24] Daniels DJ. Ground penetrating radar. London: The institution of Electrical Engineers; 2004.
Contents lists available at ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
GPR evaluation of the Roman masonry arch bridge of Lugo (Spain) M. Solla n, H. Lorenzo, F.I. Rial, A. Novo Department of Natural Resources and Environmental Engineering, University of Vigo, A Xunqueira, 36005 Pontevedra, Spain
a r t i c l e in f o
a b s t r a c t
Article history: Received 30 April 2010 Received in revised form 4 August 2010 Accepted 15 August 2010 Available online 27 October 2010
Many masonry arch bridges are historical constructions still in use within the transportation infrastructure. As their modern functionalities are different from those for which they were originally designed, and because of their age, some have suffered structural transformations over time. Consequently, there has been a recent and continuous increase in the use of non-destructive testing to evaluate historical masonry arch bridges using methods that will not change the historical characteristics of these structures. In Spain, a significant number of masonry arch bridges were constructed between the Roman period and the early 19th century. The Roman bridge of Lugo (NW Spain) was surveyed with GPR to analyze its inner structure from three points of view: historical, archaeological and structural. GPR measurements were performed using 250 and 500 MHz shielded antennas. Interpretation of the data included simulation of FDTD models based on the external geometric measures of the structure. Results revealed the ancient profile of the bridge, as well as many subsoil zones with different fills. GPR provided the unknown inner construction details, which may represent noteworthy information for archaeologists and civil engineers. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Ground penetrating radar NDT methods FDTD simulations Historical bridges Archaeology
1. Introduction Several organizations dedicated to the conservation, protection, enhancement and appreciation of cultural heritage, such as ICOMOS and UNESCO, have asserted the importance of nondestructive testing (NDT) to evaluate historical monuments, reflecting societal concern for their maintenance and conservation [1]. Consequently, there has been a continuous increase in the use of NDT methods to detect defects and anomalies on historical buildings in the last decade [2,3]. Many of the masonry arch bridges in Spain are listed as historical monuments, and are considered to be important structures from several points of view: architectural, historic, economic, symbolic and aesthetic [4]. They can characterize the landscape of a region and represent an integral part of its traditional architectonic heritage [5]. In Galicia (NW Spain), the distribution of historical bridges is very extensive and a Roman or medieval bridge can be found in any village, valley or stream. At the moment, there are about 250 historical bridges catalogued in the Galician territory [6]. Many of these bridges are still in use within the transportation infrastructure, and as a result some have potentially lost their original utility or have been modified for new functions [7]. These changes require a constant diagnosis because they are normally subjected to potentially destructive
n
Corresponding author. Tel.: + 34 986 801908; fax: + 34 986 801907. E-mail address: [email protected] (M. Solla).
0963-8695/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2010.08.004
conditions, such as increase in traffic loads, intense vibrations and their age, which have resulted in some structural decay [8]. Without conservation, a number of the remaining bridges are at a risk of total loss, not to mention those that are in the process of ruin and destruction due to their changing functions. This situation has posed an important task for engineers and scientists to evaluate the state of conservation of historical bridges using methods that will not change the historical character of the structures [9]. Ground penetrating radar (GPR) is one of the most frequently recommended NDT methods in bridge inspection because it is a relatively quick technique that gives an overall qualitative internal image and provides high penetration depth [10,11]. However, to date there have been few published studies on the evaluation of masonry arch bridges, probably because nondestructive testing on ancient civil engineering structures with GPR is a relatively new subject since the 1990s [12] in addition to the geometrical and interpretational difficulties of these complex structures. Mainly, these studies have shown good potential for providing valuable information about bridge foundations, ring stone thickness, moisture content and fill conditions [13–18]. The objective of this work was to analyze the viability and effectiveness of GPR in evaluating historical masonry arch bridges. ˜ o River, The Roman bridge of the Lugo council (Fig. 1), over the Min was surveyed in order to obtain structural and geometrical characteristics, mainly in terms of detecting hidden features and construction details of different building phases over history. Additionally, some finite-difference time-domain (FDTD)
M. Solla et al. / NDT&E International 44 (2011) 8–12
9
Fig. 1. Location of the Lugo Bridge in Galicia (Europe).
Fig. 2. General view of the Lugo Bridge from the downstream side.
Fig. 4. GPR survey composed of two parallel profiles through the bridge, back and forth. Data acquisition setup composed of 250 MHz shielded antenna.
Table 1 Data processing applied to the GPR data acquired with the 250 and 500 MHz shielded antennas. GPR data processing (ReflexW v5.0 software)
Fig. 3. (A) Profile evolution of the Lugo Bridge throughout the time and (B) the external building materials composition at the upstream side (adaptated from [19]).
simulations were performed to obtain information on the fill material within the structure. This bridge was selected because of its importance in providing contemporary strategic road access to the city. Therefore, a structural assessment is required because it supports traffic loads much heavier than in ancient times. The Lugo Bridge has eight gothic or barrel arches spanning from 5.6 to 10.4 m (Fig. 2). Although this bridge presents a horizontal profile at present, some authors inform us that the Lugo Bridge had a medieval double slope profile until the end of the XIX century as shown in Fig. 3A. Several restoration and reconstruction tasks have been performed over time [19]. Consequently, different building materials such as granitic and slate are observed in the stonework surface (Fig. 3B).
2. GPR data acquisition and processing The GPR survey was carried out with 250 and 500 MHz shielded antennas using a RAMAC/GPR system from MALA˚
250 MHz
500 MHz
1. Time-zero correction 2. Dewow filtering 3. Gain function (linear 1.66 and exponential 0.47) 4. Subtracting average (150 average traces) 5. Band-pass Butterworth (low cutoff: 100 MHz & upper cutoff: 450 MHz)
1. Time-zero correction 2. Dewow filtering 3. Gain function (linear 1.58 and exponential 0.45) 4. Subtracting average (400 average traces) 5. Band-pass Butterworth (low cutoff: 200 MHz & upper cutoff: 900 MHz)
Geoscience. These antennas were chosen as the most suitable ones because of their optimum compromise between penetration and resolution. Two longitudinal parallel profiles in opposite directions along the bridge, each 122 m in length, were collected using both antennas (Fig. 4). Data acquisition was based on constant distance intervals following the common offset mode, and the survey parameters selected with 250 MHz were 2 cm inline spacing, time windows of 210 ns, a sampling frequency of 2577 MHz and 540 samples/trace, whereas the acquisition parameters for the 500 MHz antenna were 2 cm in-line spacing, time windows of 100 ns, a sampling frequency of 6802 MHz and 677 samples/trace. Before interpretation, GPR processing filters were employed in order to reduce clutter or any unwanted noise in the raw-data, to enhance extraction of information from the signals received and to produce an image of the subsurface including all the features and/or targets of interest, which makes GPR data interpretation easier. Each radargram was filtered by ReflexW v5.0 software by applying the processing sequences described in Table 1.
10
M. Solla et al. / NDT&E International 44 (2011) 8–12
Fig. 5. Processed radargram acquired with the 250 MHz antenna (A) and 500 MHz antenna (B).
3. Results and discussion Fig. 5 shows two radargrams acquired with the 250 and 500 MHz antennas. Both radargrams were filtered by applying the processing sequences described in Table 1. At first glance, it is possible to appreciate the different GPR signal responses over the arches along the structure (Fig. 5A). Clear signal attenuation was observed over some arches. The 250 MHz antenna provided more structural details, probably because of its better penetration. The reflections from the arch–air interfaces for the first, fifth and eighth arches were nearly completely attenuated (Fig. 6), although they were placed at the same level as the others (Fig. 2). Additionally, a slightly sloping constant reflector was identified at both margins of the bridge. The geometry, condition and size of this reflector allowed us to conclude that the Lugo Bridge had a double slope profile in former times as documented in the specialized literature [19]. This ancient profile was filled at both margins of the river to create the actual horizontal pathway (Fig. 6). The corner reflections caused by the perpendicular interfaces between the top of the vaults and the water level were also identified [20]. Finally, Fig. 7A illustrates in detail the reflections produced by the transverse reinforcing beams used for supporting the cantilever metal parapet of this bridge (Fig. 7B). These metallic structures lean in pilasters from the buttresses. Once this repair task was performed in 1989, the pathway became wider [19]. The different signal response that we observed could be an indication of different stonework and fill materials within the bridge structure. These differences were most likely caused by several reconstructions and restorations over the arches throughout its lifetime. According to historical references, some arches have a granitic vault (1, 2, 4, 7 and 8 from the right margin in the downstream side), whereas the others have a slate vault [19]. The signal attenuation produced can be explained by this fact because vaults built of slate materials, such as arches 5 and 6, presented more attenuation than the others built with granite (arches 4 and 7). The fill material used for leveling the pathway, most probably composed of granitic and slate fills, could also have produced the signal attenuation observed for the first, second and eighth arches (Fig. 6). Inner granitic and slate fills were assumed due to the external bridge stonework, historical references and material availability in this area. Additionally, the asymmetric
Fig. 6. Radargram acquired with the 250 MHz antenna. Interpretation of the main reflectors detected.
shape observed in the reflection generated from the arch–air interface of the barrel arches, as in the case of the fourth arch, could be a consequence of this lack of homogeneity (Fig. 6). This non-homogeneity in fills resulted in several GPR signal velocities over the arch. Using the structural dimensions provided by geometric measurements of the bridge with a tape measure, it was possible to estimate signal velocities in different zones of the structure, ranging from 8.5 to 14.0 cm/ns. Due to the asymmetric reflection patterns on either side of the keystone, velocity for these cases was estimated by adapting a diffraction hyperbola to each half of the reflection, resulting in two different signal velocities as shown in Fig. 8. Thus, a lower signal velocity for the right half (10.83 cm/ns) was determined in comparison to the left branch (13.37 cm/ns). The typical average velocity of a GPR pulse is reported in the literature as 12.0–15.0 cm/ns in dry granite and 9.0–10.0 cm/ns in dry slate [21]. Therefore, the differences in the velocity values estimated could be explained by the presence of different fills of either slate or granite (at the right and left branch, respectively), over the arch at both sides of the keystone. In complex structures such as this, comparison of real data with synthetic models allows for extraction of subtle interpretational information to produce an exhaustive data interpretation [22,23]. Several FDTD models were built to confirm or reject the hypothesis considered to explain the radar wave attenuation and the asymmetric shape in the reflection of the arch observed in the GPR real data. A two-span bridge was simulated considering granitic and slate fills in different locations over arches (Fig. 9). These synthetic models were built using the external geometry of the bridge provided by measurements. The purpose was to design a realistic model. Table 2 shows the electromagnetic properties
M. Solla et al. / NDT&E International 44 (2011) 8–12
11
Fig. 7. Identification of reinforcing beams: (A) association to the radar data obtained with the 250 MHz antenna (in detail) and (B) location on the bridge structure.
Fig. 8. Example of velocity determination by adapting a diffraction hyperbola to each half part of the reflection.
Fig. 9. Some FDTD simulations to analyze the influence of granitic and slate fills over the arches on the radar wave response.
employed for media characterization. These parameters were calculated using the velocity values estimated, and the relative dielectric permeabilities estimated for granitic and slate materials are comparable to the values proposed by other authors [24]. The synthetic models simulated showed a slight signal attenuation in the reflection from the arch–air interface for the
slate fill compared to the granitic fill (Fig. 9). Additionally (Fig. 9C) considering both fills over the second arch illustrates the asymmetric reflection pattern on either side of the keystone observed in the real data (Fig. 8). Thus, this irregularity in shape is most likely produced by different radar wave velocities over the arch on each side. Most importantly for our study, the synthetic
12
M. Solla et al. / NDT&E International 44 (2011) 8–12
Table 2 Electromagnetic properties assumed for media characterization. Material
Conductivity (s m
Air Granite (masonry) Granitic fill Slate fill Water
0.0 0.0001 0.0001 0.01 0.5
1
)
Relative permeability 1 6.0 5.0 8.0 80.0
results together with the velocity values estimated seem to confirm the existence of both granitic and slate fills in the bridge structure. Therefore, the differing signal behavior observed in the real data facilitates mapping of those areas in the bridge regarding fill material composition. These changes in fills would most probably be associated with reconstruction or restoration tasks performed over time [19].
4. Conclusions GPR was confirmed to be an effective NDT method to reveal the details of inner bridge construction and modification over time. The geometry of an ancient profile of a bridge from medieval times was determined. The former double slope profile had been filled to obtain the actual horizontal pathway using a fill material, which was most probably different from the original one. Additionally, GPR results revealed the presence of diverse areas in the entire bridge structure, depending on their different fills, related to several restoration and reconstruction tasks that have been conducted over its lifetime. This information is useful for civil engineers engaged in developing future conservation and strengthening measures because it can be used to take decisions about stability. Differences in stonework and fill materials can affect the structural reinforcement of the bridge in terms of durability and strength of the materials involved. Future replacement of the fill would therefore be needed to provide better structural stability. The 500 MHz antenna that might have produced more precise information of the shallower fill material, however, was not capable of transmitting energy to the depth necessary within the corresponding signal attenuation observed for slate fill. The inner granitic and slate fills were established with the signal attenuation observed and the velocity values estimated. The FDTD provided important additional information that could be directly compared to the GPR real data, which were crucial in the interpretation. Although GPR can usually distinguish between different subsoil zones depending on their composition, specific identification of materials involved is more difficult. In order to confirm the fill materials it would be interesting to perform an endoscopic survey. On observing the GPR results collected, it is possible to select several locations of interest to perform an endoscopic survey to obtain a general view of the materials within the bridge structure, which will allow for an exhaustive data interpretation. Thus, the combination of GPR and endoscopy is very useful to reduce invasive interventions if the endoscopy is perfectly set.
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