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Identification of viaduct beam parameters using the Ground Penetrating Radar (GPR) technique
NDT&E International 49 (2012) 18–26
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Identification of viaduct beam parameters using the Ground Penetrating Radar (GPR) technique Damian Beben n, Arkadiusz Mordak, Wojciech Anigacz Faculty of Civil Engineering, Opole University of Technology, Katowicka Street 48, 45-061 Opole, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2011 Received in revised form 1 March 2012 Accepted 2 March 2012 Available online 14 March 2012
The paper presents the application of the Ground Penetrating Radar (GPR) technique to determine the parameters of viaduct beams. It is a non-destructive method that is widely used for testing various engineering structures. The principle of electromagnetic wave dispersion was used for measurement with application of the GPR method. The wave propagation velocity during the GPR test was chosen on the basis of a preliminary test and literature review. The subject of this study is a three-span road viaduct located over a railway line. The main objective of experimental tests was to determine the geometric parameters of viaduct beams and establish the steel reinforcements appearing in them. Conclusions drawn from the tests can be helpful in measurements using the GPR technique, especially for tests of reinforced concrete bridges. & 2012 Elsevier Ltd. All rights reserved.
Keywords: GPR technique Field test Measurement Viaduct beam Non-destructive method Electromagnetic wave Reinforcing bar
1. Introduction Old reinforced concrete (RC) bridge structures require fast and effective repair or strengthening. In many cases, there is a lack of basic information on the cross-section of the bridge. The choice of modernization method requires knowledge of the load carrying capacity of the existing structures, which may be obtained using destructive and/or non-destructive methods of testing. Taking into consideration the advantages and disadvantages of nondestructive test methods for bridges presented by McCann and Forde [1], the Ground Penetrating Radar (GPR) technique was applied in this research. GPR is a high-resolution electromagnetic (EM) technique, which originated in the 1970s. It is designed for non-destructive investigations of the shallow subsurface of soil, structural elements, roads, and bridges. The non-destructive assessment of the condition of engineering structures using GPR has lately become quite an attractive and effective method of testing [2,3]. The principle of EM wave scattering is used in the GPR method. The fundamental principles and theory of GPR operation have evolved over many years, resulting from the development of electrical engineering, and geophysical and seismic testing.
n
Corresponding author. Tel./fax: þ 48 77 454 33 18. E-mail addresses: [email protected] (D. Beben), [email protected] (A. Mordak), [email protected] (W. Anigacz). 0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2012.03.001
In favourable conditions, the GPR technique can provide precise information on the nature of underground facilities, the depth of their location, the occurrence of structural anomalies such as voids in the testing object, etc. The GPR method can be applied to locate any object (element) in another material non-invasively. It is commonly used for environmental engineering, geological, civil engineering, and archaeological investigations. It is also widely used to locate underground facilities, such as pipes and cables, and it can be applied in shallow geotechnical and geological surveys (e.g., determination of substructure layers, foundations, geological strata, caves, and caverns). The GPR technique can also be used for very precise scanning of walls and foundations to seek rebars, pipes, cables, and local internal discontinuities [4–6]. The application of the GPR technique in non-destructive assessment of the condition of masonry arch bridges is described by Diamanti et al. [7] and Solla et al. [8,9]. As for concrete bridges, Hugenschmidt [3] and Hugenschmidt and Mastrangelo [10] present some aspects of GPR measurements. They used the GPR method to obtain information on the rebar depth, asphalt pavement thickness, and concrete damage beneath the pavement. Varela-Ortiz et al. [11] present a GPR evaluation of reinforced concrete bridges on military installations, where GPR testing yielded data on the internal reinforcement as well as the condition of the concrete. Chang et al. [12] propose radius measurement of rebars in concrete. The subject of this study is a three-span RC road viaduct, which is planned to modernization, located over railway lines.
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In relation to the adopted reinforcement method and the lack of design documentation for the existing viaduct, it was necessary to obtain basic information on the beams occurring in the object. The main purpose of the experimental tests using the GPR technique was to determine the geometric parameters of beams and reinforcing bars within them. The distances between particular reinforcing bars and the depth of their position were also to be determined. Two selected RC beams were thoroughly tested (the first one located in the central span and the second in the left span). The velocity of the wave propagation during the GPR test was estimated based on a preliminary test and literature review.
2. Description of tested viaduct The road viaduct consists of three spans made of simply supported RC beams. Each span consists of 25 main beams with unknown geometrical and strength parameters. All spans have the same length of 11.50 m. The total object length is 39.00 m and its width is 13.00 m. The object intersection angle in relation to the railway line axis under the viaduct is about 701. The main beams are directly situated on the piers (without bearings) and are probably anchored to the heads of piers. The viaduct repair project foresaw the change of the static scheme by adding a new layer of continuous RC slab on the whole object. This change was planned to permit the movement of heavy trucks weighing 150 kN during normal service of the viaduct. However, the owner of the viaduct decided to increase its carrying capacity to 300 kN. For this reason, detailed testing of existing main beams was necessary. During the repair works, the asphalt layers and levelling concrete were removed to uncover the upper surfaces of the existing main beams. This was done to minimize the influence of other elements which could disturb the GPR measurement. The tested surface was dry, and the atmospheric conditions did not influence the accuracy of the results.
3. Theoretical fundamentals of the GPR technique The Aladdin GPR system was used in the field tests [13]. The GPR technique involves the propagation of short EM pulses (tr1 ns¼10 9 s) in the medium being tested. The pulses reflect variations in the EM properties of the medium, including the magnetic permeability, electric conductivity, and dielectric permittivity. These changes are related to the presence of heterogeneities inside the medium (including voids, cracks, and steel rebars in concrete), changes in the materials (such as shallow geological layers), or changes in the physical properties of the medium (e.g., in the water content). The correct interpretation of the GPR results (radargrams)
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requires comparison of the emitted waves to the recorded waves. Differences between them depend on the EM properties of the medium. The velocity of the wave is a characteristic that is highly dependent on these EM properties. It also depends on the amplitude of the reflected wave and its frequency [14]. Penetration depth, vertical resolution, and horizontal resolution usually limit GPR applications. Penetration depth is defined as the depth achieved when the amplitude has been attenuated by a factor e–1. It indicates the capability of the radar signal to penetrate into the studied medium. It depends mainly on the attenuation factor and on EM wave changes. The horizontal resolution indicates the capability of the GPR system to detect two different elements in the direction of the antenna movement. From a practical point of view, it is seen as two different anomalies in the GPR record. It depends on the antenna frequency, the penetration depth, and the EM properties of the tested medium. In turn, the vertical resolution is so-called resolution in time, defined as the capability of the antenna to detect two horizontal discontinuities as separate anomalies. This parameter also depends on the wave velocity and its length [14,15]. As the GPR (antenna) approaches an object (element), which is a discontinuity medium, the distance d–n, d–1, etc. decreases (Fig. 1). Thus, the time taken for the wave reflected from the object (element) to return to the receiver also decreases. From the combination of distance (time) data for various points on the object, xn, x–1, etc., a hyperbola is created, which is a reflection of the object on the radargram (Fig. 1). From the theoretical point of view, the GPR signal, f(k), passing through an object (element) consisting of several layers can be analysed as the sum of scaled and timedelayed replicas of the incident signal, x(k), as shown by Eq. (1): f ðkÞ ¼
X
Ai xðkt i Þ þ nðkÞ,
ð1Þ
i
where Ai is an attenuation factor, ti the time delay, and n(k) the added noise. The factors Ai and ti depend mainly on the dielectric properties of the material and the layer thicknesses [16]. As the EM wave propagates in concrete, it is modified due to the EM properties of the material. This modification concerns the electrical conductivity s, the dielectric permittivity e, and the magnetic permeability m. It should be noted that concrete is a material with nonmagnetic properties, so it has a magnetic permeability similar to that which can be seen in a vacuum (m ¼4p 10–7 H/m) [17,18]. It is also important that the principal factor influencing the concrete dielectric permittivity is the amount of moisture contained within the concrete pores, which often varies with increasing depth of the tested element. The relative dielectric permittivity of concrete usually varies from a value of about 6 for naturally dry concrete to 12 for saturated concrete [18].
Fig. 1. General principles of GPR operation: (a) measurement, (b) graphic interpretation of measurement, (c) final radargram.
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D. Beben et al. / NDT&E International 49 (2012) 18–26
Perez-Gracia et al. [14] introduced the following relation (2) based on Maxwell’s equations: 00 s þjoe ¼ s0 þ oe00 þjo e0 so , ð2Þ where j is the imaginary unit, s is the complex electric conductivity (s0 and s00 are the real part and the imaginary part, respectively), and e is the complex dielectric permittivity (e0 and e00 are the real part and the imaginary part, respectively). In order to describe the EM properties of concrete, the following formulae can be used: 0
00
sðoÞ ¼ s ðoÞ þ js ðoÞ,
ð3Þ
eðoÞ ¼ e0 ðoÞ þje00 ðoÞ,
ð4Þ
o ¼ 2pf ,
ð5Þ
j2 ¼ 1,
ð6Þ
where o is the angular frequency and f is the wave frequency. From these equations, it is clear that the properties of the EM waves are related to the dispersion [17]. The relative dielectric constant e0G and loss factor e00G influence the wave velocity v passing through the material. Therefore, one can introduce Eq. (7), expressing the velocity of an EM wave as a function of e0G , e00G , and c (velocity of EM waves in free space) [17]: pffiffiffi c 2 : ð7Þ nðoÞ ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e0G ðoÞ2 þ e00 ðoÞ2 þ e0G ðoÞ In media with low losses, like concrete, Eq. (7) can be presented in the form: c
ffi, nðoÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffi e0G ðoÞ
calculated from Eq. (10): vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u uðf ðkÞrðkÞÞ2 NRMSE ¼ t , ðf ðkÞmf Þ2 where mf is the mean value of the GPR signal f(k) [16]. 4. Description of equipment
Each GPR apparatus consists of a few essential components. The main component of the whole measuring system is a control unit. The basic tasks of this unit are the following: control of the antenna, signal receiving and converting (from analog to digital), and cooperation with a computer. The second component of the GPR configuration is the antenna, which is especially important from the user’s point of view. GPR antennas consist of suitably prepared steel plates called dipoles. Antennas differ from one another primarily in the operation frequency. The differences in frequencies also result in variations in antenna dimensions. From a practical point of view, the antenna’s operation frequency demonstrates physical parameters of the EM wave. The measurement accuracy and the depth of penetration directly depend on the operation frequency of the antenna. The frequency range of GPR antennas is from more than 10 to 2000 MHz. Low frequency antennas are usually large and allow deep penetration but with low accuracy (resolution). In turn, the highest-frequency antennas (about 2000 MHz) are considerably smaller and allow high-accuracy testing, but at shallow depth. Middle-frequency antennas (100–1000 MHz) are the most universal and are usually applied, among other things, to the detection of underground infrastructure.
ð8Þ
and Eq. (8) explicitly shows that the EM wave velocity in concrete depends only on dielectric constant e0G [17]. Reflection of the emitted energy (the EM wave) is due to intense changes of the dielectric parameters. The reflection coefficient is strictly related to the dielectric constant of the given material (element). This coefficient determines the part of the incident wave that is reflected. The sum of the transmission and reflection coefficients is equal to one, so, the following relationship can be introduced: pffiffiffiffiffiffi pffiffiffiffiffiffi er1 er2 R ¼ pffiffiffiffiffiffi pffiffiffiffiffiffi , ð9Þ er1 þ er2 where er1 and er2 are the relative permittivity of the upper and lower layers of the element, respectively [14]. The normalized root-mean-square error between the original GPR signal, f(k), and the de-noised GPR signal, r(k), can be
ð10Þ
Fig. 3. Transverse profiles (T) of the viaduct beam.
Fig. 2. Inspection traces on the tested viaduct beams.
D. Beben et al. / NDT&E International 49 (2012) 18–26
The GPR set applied to the test consists of the following elements [19]:
‘‘FastWave’’ control unit with frequency of the EM pulse at 400 kHz;
shielded antenna with operation frequency of 2000 MHz,
which is a bipolar antenna, i.e., it has two pairs of transmitter– receiver sets with frequency of 2000 MHz placed perpendicularly to each other; antenna’s handle with the distance measurement wheel; computer with data-collection software; accumulator and indispensable linking cables.
5. The methodology of GPR tests and results analysis 5.1. Test description Generally, there are no standards for GPR testing of RC structures. Some aspects of the American standard ASTM D 4748 [20] that
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concern the problem of determining the thickness of road pavement can be applied. However, in most cases each structure should be analysed individually. General information on the application of the GPR method can be found in the standards [21,22]. Prior to testing, the velocity of EM wave propagation in the analysed medium should be determined. This may be done experimentally. The time of EM wave propagation through an element of known thickness (measured with accuracy of 75 mm) should be estimated. This study should be carried out on an unreinforced structural element. Based on the EM wave transit time, the velocity of wave propagation in the tested medium can be calculated. It should be noted that the propagation velocity of an EM wave in concrete depends on the porosity, moisture content, and composition of the concrete. The common mid-point (CMP) method is another way of measuring the EM wave velocity. It can be used in cases where two independent antennas (transmitting and receiving) are used in measurements. This method consists of repeated measurements of pulse transit time at different locations of antennas around a given point. The wide-angle reflection and
Fig. 4. The processed radargrams: ( a) L1, (b) L6, and (c) L11.
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ranking (WARR) method is a similar technique for determining the EM wave velocity. In the case of the viaduct, because of the difficulties in accessing the non-reinforced beam element, the traditional pre-tests of calibration of the EM wave propagation velocity were not conducted. However, a few preliminary GPR tests using various wave velocities (v(o)¼ 0.08 m/ns, 0.10 m/ns, 0.12 m/ns, and 0.15 m/ns) were carried out. Generally, concrete is not a homogeneous material and should be considered as a composite material composed of aggregate, cement paste (often with some mineral additives, e.g., fly ash), and some air and water (mainly inside the pores). As it turned out, the best results were achieved for the wave velocity of 0.10 m/ns. Therefore, the velocity of wave propagation in concrete was adopted from the preliminary test and from a literature review [1,23]. Additionally, it should be noted that at the test preparation stage, the beam cross-section and the physical and chemical parameters of the concrete were unknown. Two main beams placed in the longitudinal axis of the viaduct were chosen for testing, one from the external span of the viaduct and the other from the central span. Each of these beams was a separate quarter of the coordinate system, where the longer side was the T-axis and the shorter was the L-axis. Parallel to the T-axis, longitudinal scanning (L) was executed, while parallel to the L-axis, transverse scanning (T) was carried out (Fig. 2). After uncovering the upper part of the beams, it turned out that their width was about 0.45 m and length about 11.50 m. It was decided to scan a band of width 0.50 m and length 12.00 m, because it was very hard to accurately define the beginning and the end of individual beams and there was a lack of information about the shape of the beam cross-sections. Individual scans were executed in increments of 0.05 m to obtain the best accuracy of the radargrams. Eleven longitudinal profiles (L1–L11) with length of 12.00 m were generated for the first measurement. These profiles fully covered beam no. 1.
Fig. 5. The cross-section of the viaduct beam based on the radargrams (longitudinal profiles).
Additionally, two sections of length 1.00 m were chosen, of which 42 transverse scans (T) were made in increments of 0.05 m. The selected sections were located between 0.00 and 1.00 m (T12–T32) and between 6.00 and 7.00 m (T33–T53) along the beam length. Fig. 3 shows the course of the transverse scanning (T). Due to the partial exposure of beam no. 2, the measurements were shortened to a length of about 6.00 m. In the next step, 11 longitudinal profiles of length 6.00 m were executed, once again in increments of 0.05 m. As for the first beam, two sections of length 1.00 m were chosen (between 0.00 and 1.00 m and between 4.00 and 5.00 m along the beam length). Both sections were scanned in the transverse direction (T12–T53) in increments of 0.05 m (Fig. 2). Generally, the longitudinal scans (L) were used to find the position of the reinforcement bars (stirrups) placed in the transverse direction of the beam. In turn, the transverse scans (T) were used to locate the main longitudinal reinforcement bars and possible anomalies in the beams. 5.2. Results of the GPR tests and their analysis The GPR results were processed using the GRED 3D Utilities software and using the proper filters and signal gains. All GPR results were processed by the following filters [13]: 1. Move start time – filter moving all scanning results to the 0 level 2. Background removal – filter removing the unnecessary background (disturbances) in the form of horizontal lines 3. Vertical band pass filter (TD) – sliding filter for selected frequencies (from 1600 to 2400 MHz) 4. Linear gain – improving quality of the results displaying 5. Smoothed gain – improving the radargram sharpness 5.2.1. Beam no. 1 The selected results of the radargrams for the longitudinal scanning (L) are presented in Fig. 4(a)–(c). All longitudinal radargrams (L) show that the first level of reinforcing bars (stirrups) is on a similar depth, i.e., 0.055 m from the upper edge of the beam. Possible differences could be the result of unevenness of the tested surface (the levelling concrete layer); in these places, the stirrups are situated a little bit deeper. In order to define finally the position of the stirrups, it was necessary to analyse the beginning fragments of each radargram in detail. In these places, the tested beam was fully uncovered and the stirrups were clearly visible on the radargram. Additionally, the layer at a depth of 0.12 m, which is clearly visible in Fig. 4(a)–(c), turned out to be the upper edge of an internal hole in the beam. In the same figure, the layer at a depth of about 0.40 m is also visible and is probably the bottom edge of the internal hole. Moreover, all the radargrams show the layer at a depth of 0.56 m, which is the bottom edge of the main beam. A few anomalies were
Fig. 6. Top view of the beam with marked longitudinal profiles and stirrups.
D. Beben et al. / NDT&E International 49 (2012) 18–26
detected, e.g., reflections from an undefined object (element) are visible at a depth of 0.30 m (marked with circles in Fig. 4(b)). This type of anomaly is probably connected with the internal hole existing in the beam and could be due to reflections of the EM waves from the edges or bends in the internal hole. Moreover, four bigger longitudinal anomalies were observed (marked by ellipsis in Fig. 4(c)) at a depth of 0.07 m, which are probably due to the main upper longitudinal reinforcing bars in the beams. On the basis of the results obtained, the initial cross-section of the main beam of the viaduct was established (Fig. 5). The distances between the reinforcing bars (stirrups) were estimated based on the part of the radargram where they were most visible, i.e., on the length of 2 m of each scanning. In these places, the main beam was fully uncovered (Fig. 4(a)). In the remaining part of the beam, only single stirrups were visible and it was difficult to find the repeated their locations on the radargrams. The average distance between the stirrups was 0.17 m; the largest distance was 0.20 m (in the middle part of the beam) and the
Fig. 7. The processed transverse radargram (T22) of beam no. 1.
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smallest was 0.13 m (at both ends of the beam). Fig. 6 shows the top view of the stirrup arrangement inside the beam. As mentioned earlier, transverse scans (T) were also executed. These scans confirmed that the first level of reinforcing bars and the location of the internal hole were at the same depth. Moreover, transverse scanning allowed us to estimate the range of the internal hole inside the beam. Considering the small length of scanning and numerous reflections, the visualization colour was changed in order to obtain improved sharpness. An example of a transverse radargram is shown in Fig. 7. In the transverse scans (T), two reinforcing bars were detected in the upper part of the beam at a depth of 0.06 m. These rebars existed in the corners of the main beam. Moreover, the edges of the hole inside the beam were visible on all radargrams, the upper edge at a depth of 0.12 m and the lower edge at a depth of 0.40 m. The beginning of the hole is visible about 0.18 m from the beginning of the scan, and it ends at 0.37 m of the wide scan. In the majority of radargrams, reflection on the beginning of the scan is visible at a depth of 0.055 m. These are probably the stirrups situated at a similar depth to the main reinforcing bars. The bottom level of the main reinforcing bars could not be determined accurately. However, at a depth of 0.50 m, some reflections of the EM waves were observed, probably due to existing reinforcement bars. The arrangement of upper reinforcement bars and the edges of the internal hole in beam no. 1 are presented in Fig. 8. 5.2.2. Beam no. 2 The results obtained for beam no. 2 confirm the results obtained for beam no. 1. The distance between the stirrups was estimated based on radargram L4 (Fig. 9), where the fragment of the rebar group is most visible. The average distance between stirrups was about 0.18 m. Larger spacing was detected in the middle part and smaller distances at the ends of the beam. Besides the stirrups at a depth of 0.055 m, an anomaly was also detected at a depth of 0.07 m (falling down). These are probably the upper longitudinal reinforcing bars. Based on the presented radargrams, it was difficult to determine the depth of layers constituting the edges of the internal hole in the beam. For better identification of these edges, the radargrams were processed omitting the background removal filter, which removes the horizontal continuous lines. As a result, quite a good image was obtained, where the internal hole was accurately located and the second (bottom) level of reinforcement bars was also observed (Fig. 10). In the case of the second beam, the rebar level and edges of the internal hole were more visible on transverse radargrams (T). Two reinforcing bars were also detected for this beam at a depth of about 0.06 m. The first was 0.06 m and the second 0.40 m from the beginning of the scanning area. Moreover, the internal hole at a depth of 0.12 m is quite well visible. The internal hole reaches
Fig. 8. Top view of the main longitudinal rebars and the edges of the hole, received from the transverse profile (T) for beam no. 1.
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a depth of 0.40 m, where the image structure changes visibly. The second level of rebars is also visible (unfortunately, the number of rebars cannot be distinguished, probably due to the large number of rebars merging in the image). The bottom edge of the beam is also clearly visible at a depth of 0.56 m (Fig. 11). Finally, the location of upper reinforcing bars and the edges of the internal hole inside beam no. 2 are presented in Fig. 12. 5.3. Discussion of the GPR results
Fig. 9. The L4 radargram of beam no. 2 with well visible stirrups.
Based on the research, we determined that the main upper reinforcing bars were situated at a depth of about 0.06 m from the top edges of beams. The possible differences in their positions could result from inequalities of the test surface on which the measurement antenna was moved. Spacing of the main upper longitudinal reinforcing bars was determined quite accurately. After detailed analyses of both considered beams, only two longitudinal reinforcing bars were
Fig. 10. The L3 radargram of beam no.2 processed without the background remover filter.
Fig. 11. Processed transverse radargrams: (a) T20 and (b) T45.
D. Beben et al. / NDT&E International 49 (2012) 18–26
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Fig. 12. Top view of the longitudinal rebars and the edges of the hole, received from the transverse profiles (T) for beam no. 2.
Fig. 13. Comparison of cross-sections of the viaduct beams received from: (a) the GPR tests and (b) the catalogue of typical beams of Gromnik type.
detected. The analysis indicates that only two longitudinal rebars should be taken into account in calculations of the current carrying capacity of the viaduct. The rebars were positioned in both corners – about 0.06 m from the beginning (and end) of tested beams. The distance between rebars ranged from 0.36 to 0.38 m. Additionally, it was discovered that the upper part of transverse reinforcing bars (stirrups) was positioned at a depth of about 0.055 m. Spacing of stirrups was approximately constant. However, they were more densely positioned at the ends of the beams (0.13 m) and sparser in the middle part (0.20 m). The main bottom reinforcing bars were situated at a level of about 0.50 m from the top edges of the beams. The numbers and spacing of rebars could not be accurately determined due to the complex cross-section of the beams. At the end of the analysis process, the location of the internal hole was identified inside each of the beams. The top edge of the hole was at a depth of about 0.12 m and the lower edge at about 0.40 m. In the transverse direction of the beam, the aperture starts at about 0.18 m and ends at 0.37 m, counting from the beginning of the beam. After analysis of all GPR results for both beams, it was concluded that Gromnik-type beams were used to construct the viaduct. Fig. 13 shows a comparison between the catalogue beam of Gromnik-type and results of the GRP research. Some differences were detected, for example in the number of upper reinforcement bars. In order to confirm the number of rebars, it was decided to uncover the concrete layer in one of the damaged beams. It turned out that only two longitudinal rebars existed in the top part of the beam. This confirms the necessity of applying the GPR method to the detection of reinforcement bars, especially in the case of old RC bridges, where differences between the project assumptions and reality can occur. These differences should be taken into consideration in repair projects, especially in the calculations of the necessary rebars. Due to a complicated
beam cross-section (the internal hole and some edges), the number of the bottom reinforcing bars could not be determined accurately. The geometry of the tested beams influenced the number of reflections of the EM waves from different layers occurring at the depth of beams. Taking into account the difficulty in accessing the concrete beams and the lack of basic information about the cross-section of the viaduct beam, as well as considering the assumption of the velocity of the wave propagation, the received results are quite correct. 6. Conclusions As a result of the tests conducted on viaduct beams using the GPR technique, the following conclusions can be drawn: 1. The performed research allowed us to identify the locations of: (i) the main upper and bottom reinforcing bars, (ii) the stirrups and their spacing (also the differences in their spacing in midspan and support zones), and (iii) an internal hole in the main beams. The test results showed quite good compatibility with the Gromnik-type beam. Disparities in the results were due to the relative complexity of the beam cross-section (many layers and edges causing reflection of the EM waves). This complexity had a negative influence on the ability to scan efficiently through the whole depth of the beam. 2. The GPR results were affected by the complexity of the viaduct beam cross-section, which consisted of various media (concrete, steel, hole filled by air). Generally, concrete is not a homogeneous material and should be considered as a composite material. Therefore, the velocities of the wave propagation through the concrete elements vary according to their physical and chemical properties. In this case study, the application of the wave velocity on the level of 0.10 m/ns allowed us to obtain quite acceptable results.
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3. Taking into account the advantages and disadvantages of nondestructive methods, presented by McCann and Forde [1], it can be concluded that the GPR technique is the most suitable method for bridge testing in order to detect the cross-section of the beams. GPR measurements using a bipolar antenna with a high frequency of 2000 MHz proved to be a quite effective method of determining the parameters of the RC viaduct beams. The GPR technique also allowed detection of discontinuities in the concrete structure (e.g., the hole in the beam). It should be noted that the analysis of radargrams requires skill to interpret the data. Research on accurate ways of determining the diameter of rebars occurring in concrete should be continued.
Acknowledgements The authors of this paper would like to thank OUTech and INZBUD from Opole for co-financing the tests. Special thanks are addressed to SejsCom s.c. from Cracow for their support during experimental testing.
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[7] Diamanti N, Giannopoulos A, Forde MC. Numerical modelling and experimental verification of GPR to investigate ring separation in brick masonry arch bridges. NDT&E Int 2008;41(5):354–63. [8] Solla M, Lorenzo H, Riveiro B, Rial FI. Non-destructive methodologies in the assessment of the masonry arch bridge of Traba, Spain. Eng Fail Anal 2011;18(3):828–35. [9] Solla M, Lorenzo H, Novo A, Rial FI. GPR evaluation of the Roman masonry arch bridge of Lugo (Spain). NDT&E Int 2011;44(1):8–12. [10] Hugenschmidt J, Mastrangelo RGPR. Inspection of concrete bridges. Cem Concr Compos 2006;28(4):384–92. [11] Varela-Ortiz, W, Cintro´n, CYL, Vela´zquez, GI, Stanton, TR. Load testing and GPR assessment for concrete bridges on military installations. Constr Build Mater, doi:10.1016/j.conbuildmat.2010.09.044, in press. [12] Chang CW, Lin CH, Lien HS. Measurement radius of reinforcing steel bar in concrete using digital image GPR. Constr Build Mater 2009;23(2):1057–63. [13] GRED Software. Georadar data elaboration. User manual. Ingegneria Dei Sistemi S.p.A., Pisa, October 2009. [14] Perez-Gracia V, Garcıa Garcıa F, Rodriguez Abad I. GPR evaluation of the damage found in the reinforced concrete base of a block of flats: a case study. NDT&E Int 2008;41(5):341–53. [15] Daniels, JJ. Ground Penetrating Radar Fundamentals. Prepared as an appendix to a Report to the U.S.EPA, Region V. Nov. 25, 2000 (Ohio State University, Columbus, 2000). [16] Baili J, Lahouar S, Hergli M, Al-Qadi IL, Besbes K. GPR signal de-noising by discrete wavelet transform. NDT&E Int 2009;42(8):696–703. [17] Klysz G, Balayssac JP, Laurens S. Spectral analysis of radar surface waves for non-destructive evaluation of cover concrete. NDT&E Int 2004;37(3):221–7. [18] Davis, J, Huang, Y, Millard, SG, Bungey, JH. Determination of dielectric properties of in-situ concrete at radar frequencies. In: International Symposium on Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin, September 16–19, 2003. [19] K2 Fast Wave Software. Data acquisition. User’s guide. Ingegneria Dei Sistemi S.p.A., Pisa, November 2008. [20] ASTM D. 4748. American Testing and Material Standards. Standard test method for determining the thickness of bound pavement layers using short-pulse radar. West Conshohocken, PA. 2006. [21] ASTM D. 6432-99. American Testing and Material Standards. Standard guide for using the surface ground penetrating radar method for subsurface investigation. West Conshohocken, PA. 2005. [22] Instruction leaflet about the radar method for nondestructive-testing in ¨ ¨ civil engineering (Merkblatt uber das Radarverfahren zur Zerstorungsfreien Prufung im Bauwesen). German Society for Nondestructive-Testing, November 2001 [in German]. [23] Drobiec L, Jasinski R, Piekarczyk A. Diagnosis of Reinforced Concrete Structures. Warsaw: Scientific Publisher PWN; 2010.
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NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Identification of viaduct beam parameters using the Ground Penetrating Radar (GPR) technique Damian Beben n, Arkadiusz Mordak, Wojciech Anigacz Faculty of Civil Engineering, Opole University of Technology, Katowicka Street 48, 45-061 Opole, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2011 Received in revised form 1 March 2012 Accepted 2 March 2012 Available online 14 March 2012
The paper presents the application of the Ground Penetrating Radar (GPR) technique to determine the parameters of viaduct beams. It is a non-destructive method that is widely used for testing various engineering structures. The principle of electromagnetic wave dispersion was used for measurement with application of the GPR method. The wave propagation velocity during the GPR test was chosen on the basis of a preliminary test and literature review. The subject of this study is a three-span road viaduct located over a railway line. The main objective of experimental tests was to determine the geometric parameters of viaduct beams and establish the steel reinforcements appearing in them. Conclusions drawn from the tests can be helpful in measurements using the GPR technique, especially for tests of reinforced concrete bridges. & 2012 Elsevier Ltd. All rights reserved.
Keywords: GPR technique Field test Measurement Viaduct beam Non-destructive method Electromagnetic wave Reinforcing bar
1. Introduction Old reinforced concrete (RC) bridge structures require fast and effective repair or strengthening. In many cases, there is a lack of basic information on the cross-section of the bridge. The choice of modernization method requires knowledge of the load carrying capacity of the existing structures, which may be obtained using destructive and/or non-destructive methods of testing. Taking into consideration the advantages and disadvantages of nondestructive test methods for bridges presented by McCann and Forde [1], the Ground Penetrating Radar (GPR) technique was applied in this research. GPR is a high-resolution electromagnetic (EM) technique, which originated in the 1970s. It is designed for non-destructive investigations of the shallow subsurface of soil, structural elements, roads, and bridges. The non-destructive assessment of the condition of engineering structures using GPR has lately become quite an attractive and effective method of testing [2,3]. The principle of EM wave scattering is used in the GPR method. The fundamental principles and theory of GPR operation have evolved over many years, resulting from the development of electrical engineering, and geophysical and seismic testing.
n
Corresponding author. Tel./fax: þ 48 77 454 33 18. E-mail addresses: [email protected] (D. Beben), [email protected] (A. Mordak), [email protected] (W. Anigacz). 0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2012.03.001
In favourable conditions, the GPR technique can provide precise information on the nature of underground facilities, the depth of their location, the occurrence of structural anomalies such as voids in the testing object, etc. The GPR method can be applied to locate any object (element) in another material non-invasively. It is commonly used for environmental engineering, geological, civil engineering, and archaeological investigations. It is also widely used to locate underground facilities, such as pipes and cables, and it can be applied in shallow geotechnical and geological surveys (e.g., determination of substructure layers, foundations, geological strata, caves, and caverns). The GPR technique can also be used for very precise scanning of walls and foundations to seek rebars, pipes, cables, and local internal discontinuities [4–6]. The application of the GPR technique in non-destructive assessment of the condition of masonry arch bridges is described by Diamanti et al. [7] and Solla et al. [8,9]. As for concrete bridges, Hugenschmidt [3] and Hugenschmidt and Mastrangelo [10] present some aspects of GPR measurements. They used the GPR method to obtain information on the rebar depth, asphalt pavement thickness, and concrete damage beneath the pavement. Varela-Ortiz et al. [11] present a GPR evaluation of reinforced concrete bridges on military installations, where GPR testing yielded data on the internal reinforcement as well as the condition of the concrete. Chang et al. [12] propose radius measurement of rebars in concrete. The subject of this study is a three-span RC road viaduct, which is planned to modernization, located over railway lines.
D. Beben et al. / NDT&E International 49 (2012) 18–26
In relation to the adopted reinforcement method and the lack of design documentation for the existing viaduct, it was necessary to obtain basic information on the beams occurring in the object. The main purpose of the experimental tests using the GPR technique was to determine the geometric parameters of beams and reinforcing bars within them. The distances between particular reinforcing bars and the depth of their position were also to be determined. Two selected RC beams were thoroughly tested (the first one located in the central span and the second in the left span). The velocity of the wave propagation during the GPR test was estimated based on a preliminary test and literature review.
2. Description of tested viaduct The road viaduct consists of three spans made of simply supported RC beams. Each span consists of 25 main beams with unknown geometrical and strength parameters. All spans have the same length of 11.50 m. The total object length is 39.00 m and its width is 13.00 m. The object intersection angle in relation to the railway line axis under the viaduct is about 701. The main beams are directly situated on the piers (without bearings) and are probably anchored to the heads of piers. The viaduct repair project foresaw the change of the static scheme by adding a new layer of continuous RC slab on the whole object. This change was planned to permit the movement of heavy trucks weighing 150 kN during normal service of the viaduct. However, the owner of the viaduct decided to increase its carrying capacity to 300 kN. For this reason, detailed testing of existing main beams was necessary. During the repair works, the asphalt layers and levelling concrete were removed to uncover the upper surfaces of the existing main beams. This was done to minimize the influence of other elements which could disturb the GPR measurement. The tested surface was dry, and the atmospheric conditions did not influence the accuracy of the results.
3. Theoretical fundamentals of the GPR technique The Aladdin GPR system was used in the field tests [13]. The GPR technique involves the propagation of short EM pulses (tr1 ns¼10 9 s) in the medium being tested. The pulses reflect variations in the EM properties of the medium, including the magnetic permeability, electric conductivity, and dielectric permittivity. These changes are related to the presence of heterogeneities inside the medium (including voids, cracks, and steel rebars in concrete), changes in the materials (such as shallow geological layers), or changes in the physical properties of the medium (e.g., in the water content). The correct interpretation of the GPR results (radargrams)
19
requires comparison of the emitted waves to the recorded waves. Differences between them depend on the EM properties of the medium. The velocity of the wave is a characteristic that is highly dependent on these EM properties. It also depends on the amplitude of the reflected wave and its frequency [14]. Penetration depth, vertical resolution, and horizontal resolution usually limit GPR applications. Penetration depth is defined as the depth achieved when the amplitude has been attenuated by a factor e–1. It indicates the capability of the radar signal to penetrate into the studied medium. It depends mainly on the attenuation factor and on EM wave changes. The horizontal resolution indicates the capability of the GPR system to detect two different elements in the direction of the antenna movement. From a practical point of view, it is seen as two different anomalies in the GPR record. It depends on the antenna frequency, the penetration depth, and the EM properties of the tested medium. In turn, the vertical resolution is so-called resolution in time, defined as the capability of the antenna to detect two horizontal discontinuities as separate anomalies. This parameter also depends on the wave velocity and its length [14,15]. As the GPR (antenna) approaches an object (element), which is a discontinuity medium, the distance d–n, d–1, etc. decreases (Fig. 1). Thus, the time taken for the wave reflected from the object (element) to return to the receiver also decreases. From the combination of distance (time) data for various points on the object, xn, x–1, etc., a hyperbola is created, which is a reflection of the object on the radargram (Fig. 1). From the theoretical point of view, the GPR signal, f(k), passing through an object (element) consisting of several layers can be analysed as the sum of scaled and timedelayed replicas of the incident signal, x(k), as shown by Eq. (1): f ðkÞ ¼
X
Ai xðkt i Þ þ nðkÞ,
ð1Þ
i
where Ai is an attenuation factor, ti the time delay, and n(k) the added noise. The factors Ai and ti depend mainly on the dielectric properties of the material and the layer thicknesses [16]. As the EM wave propagates in concrete, it is modified due to the EM properties of the material. This modification concerns the electrical conductivity s, the dielectric permittivity e, and the magnetic permeability m. It should be noted that concrete is a material with nonmagnetic properties, so it has a magnetic permeability similar to that which can be seen in a vacuum (m ¼4p 10–7 H/m) [17,18]. It is also important that the principal factor influencing the concrete dielectric permittivity is the amount of moisture contained within the concrete pores, which often varies with increasing depth of the tested element. The relative dielectric permittivity of concrete usually varies from a value of about 6 for naturally dry concrete to 12 for saturated concrete [18].
Fig. 1. General principles of GPR operation: (a) measurement, (b) graphic interpretation of measurement, (c) final radargram.
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D. Beben et al. / NDT&E International 49 (2012) 18–26
Perez-Gracia et al. [14] introduced the following relation (2) based on Maxwell’s equations: 00 s þjoe ¼ s0 þ oe00 þjo e0 so , ð2Þ where j is the imaginary unit, s is the complex electric conductivity (s0 and s00 are the real part and the imaginary part, respectively), and e is the complex dielectric permittivity (e0 and e00 are the real part and the imaginary part, respectively). In order to describe the EM properties of concrete, the following formulae can be used: 0
00
sðoÞ ¼ s ðoÞ þ js ðoÞ,
ð3Þ
eðoÞ ¼ e0 ðoÞ þje00 ðoÞ,
ð4Þ
o ¼ 2pf ,
ð5Þ
j2 ¼ 1,
ð6Þ
where o is the angular frequency and f is the wave frequency. From these equations, it is clear that the properties of the EM waves are related to the dispersion [17]. The relative dielectric constant e0G and loss factor e00G influence the wave velocity v passing through the material. Therefore, one can introduce Eq. (7), expressing the velocity of an EM wave as a function of e0G , e00G , and c (velocity of EM waves in free space) [17]: pffiffiffi c 2 : ð7Þ nðoÞ ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e0G ðoÞ2 þ e00 ðoÞ2 þ e0G ðoÞ In media with low losses, like concrete, Eq. (7) can be presented in the form: c
ffi, nðoÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffi e0G ðoÞ
calculated from Eq. (10): vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u uðf ðkÞrðkÞÞ2 NRMSE ¼ t , ðf ðkÞmf Þ2 where mf is the mean value of the GPR signal f(k) [16]. 4. Description of equipment
Each GPR apparatus consists of a few essential components. The main component of the whole measuring system is a control unit. The basic tasks of this unit are the following: control of the antenna, signal receiving and converting (from analog to digital), and cooperation with a computer. The second component of the GPR configuration is the antenna, which is especially important from the user’s point of view. GPR antennas consist of suitably prepared steel plates called dipoles. Antennas differ from one another primarily in the operation frequency. The differences in frequencies also result in variations in antenna dimensions. From a practical point of view, the antenna’s operation frequency demonstrates physical parameters of the EM wave. The measurement accuracy and the depth of penetration directly depend on the operation frequency of the antenna. The frequency range of GPR antennas is from more than 10 to 2000 MHz. Low frequency antennas are usually large and allow deep penetration but with low accuracy (resolution). In turn, the highest-frequency antennas (about 2000 MHz) are considerably smaller and allow high-accuracy testing, but at shallow depth. Middle-frequency antennas (100–1000 MHz) are the most universal and are usually applied, among other things, to the detection of underground infrastructure.
ð8Þ
and Eq. (8) explicitly shows that the EM wave velocity in concrete depends only on dielectric constant e0G [17]. Reflection of the emitted energy (the EM wave) is due to intense changes of the dielectric parameters. The reflection coefficient is strictly related to the dielectric constant of the given material (element). This coefficient determines the part of the incident wave that is reflected. The sum of the transmission and reflection coefficients is equal to one, so, the following relationship can be introduced: pffiffiffiffiffiffi pffiffiffiffiffiffi er1 er2 R ¼ pffiffiffiffiffiffi pffiffiffiffiffiffi , ð9Þ er1 þ er2 where er1 and er2 are the relative permittivity of the upper and lower layers of the element, respectively [14]. The normalized root-mean-square error between the original GPR signal, f(k), and the de-noised GPR signal, r(k), can be
ð10Þ
Fig. 3. Transverse profiles (T) of the viaduct beam.
Fig. 2. Inspection traces on the tested viaduct beams.
D. Beben et al. / NDT&E International 49 (2012) 18–26
The GPR set applied to the test consists of the following elements [19]:
‘‘FastWave’’ control unit with frequency of the EM pulse at 400 kHz;
shielded antenna with operation frequency of 2000 MHz,
which is a bipolar antenna, i.e., it has two pairs of transmitter– receiver sets with frequency of 2000 MHz placed perpendicularly to each other; antenna’s handle with the distance measurement wheel; computer with data-collection software; accumulator and indispensable linking cables.
5. The methodology of GPR tests and results analysis 5.1. Test description Generally, there are no standards for GPR testing of RC structures. Some aspects of the American standard ASTM D 4748 [20] that
21
concern the problem of determining the thickness of road pavement can be applied. However, in most cases each structure should be analysed individually. General information on the application of the GPR method can be found in the standards [21,22]. Prior to testing, the velocity of EM wave propagation in the analysed medium should be determined. This may be done experimentally. The time of EM wave propagation through an element of known thickness (measured with accuracy of 75 mm) should be estimated. This study should be carried out on an unreinforced structural element. Based on the EM wave transit time, the velocity of wave propagation in the tested medium can be calculated. It should be noted that the propagation velocity of an EM wave in concrete depends on the porosity, moisture content, and composition of the concrete. The common mid-point (CMP) method is another way of measuring the EM wave velocity. It can be used in cases where two independent antennas (transmitting and receiving) are used in measurements. This method consists of repeated measurements of pulse transit time at different locations of antennas around a given point. The wide-angle reflection and
Fig. 4. The processed radargrams: ( a) L1, (b) L6, and (c) L11.
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D. Beben et al. / NDT&E International 49 (2012) 18–26
ranking (WARR) method is a similar technique for determining the EM wave velocity. In the case of the viaduct, because of the difficulties in accessing the non-reinforced beam element, the traditional pre-tests of calibration of the EM wave propagation velocity were not conducted. However, a few preliminary GPR tests using various wave velocities (v(o)¼ 0.08 m/ns, 0.10 m/ns, 0.12 m/ns, and 0.15 m/ns) were carried out. Generally, concrete is not a homogeneous material and should be considered as a composite material composed of aggregate, cement paste (often with some mineral additives, e.g., fly ash), and some air and water (mainly inside the pores). As it turned out, the best results were achieved for the wave velocity of 0.10 m/ns. Therefore, the velocity of wave propagation in concrete was adopted from the preliminary test and from a literature review [1,23]. Additionally, it should be noted that at the test preparation stage, the beam cross-section and the physical and chemical parameters of the concrete were unknown. Two main beams placed in the longitudinal axis of the viaduct were chosen for testing, one from the external span of the viaduct and the other from the central span. Each of these beams was a separate quarter of the coordinate system, where the longer side was the T-axis and the shorter was the L-axis. Parallel to the T-axis, longitudinal scanning (L) was executed, while parallel to the L-axis, transverse scanning (T) was carried out (Fig. 2). After uncovering the upper part of the beams, it turned out that their width was about 0.45 m and length about 11.50 m. It was decided to scan a band of width 0.50 m and length 12.00 m, because it was very hard to accurately define the beginning and the end of individual beams and there was a lack of information about the shape of the beam cross-sections. Individual scans were executed in increments of 0.05 m to obtain the best accuracy of the radargrams. Eleven longitudinal profiles (L1–L11) with length of 12.00 m were generated for the first measurement. These profiles fully covered beam no. 1.
Fig. 5. The cross-section of the viaduct beam based on the radargrams (longitudinal profiles).
Additionally, two sections of length 1.00 m were chosen, of which 42 transverse scans (T) were made in increments of 0.05 m. The selected sections were located between 0.00 and 1.00 m (T12–T32) and between 6.00 and 7.00 m (T33–T53) along the beam length. Fig. 3 shows the course of the transverse scanning (T). Due to the partial exposure of beam no. 2, the measurements were shortened to a length of about 6.00 m. In the next step, 11 longitudinal profiles of length 6.00 m were executed, once again in increments of 0.05 m. As for the first beam, two sections of length 1.00 m were chosen (between 0.00 and 1.00 m and between 4.00 and 5.00 m along the beam length). Both sections were scanned in the transverse direction (T12–T53) in increments of 0.05 m (Fig. 2). Generally, the longitudinal scans (L) were used to find the position of the reinforcement bars (stirrups) placed in the transverse direction of the beam. In turn, the transverse scans (T) were used to locate the main longitudinal reinforcement bars and possible anomalies in the beams. 5.2. Results of the GPR tests and their analysis The GPR results were processed using the GRED 3D Utilities software and using the proper filters and signal gains. All GPR results were processed by the following filters [13]: 1. Move start time – filter moving all scanning results to the 0 level 2. Background removal – filter removing the unnecessary background (disturbances) in the form of horizontal lines 3. Vertical band pass filter (TD) – sliding filter for selected frequencies (from 1600 to 2400 MHz) 4. Linear gain – improving quality of the results displaying 5. Smoothed gain – improving the radargram sharpness 5.2.1. Beam no. 1 The selected results of the radargrams for the longitudinal scanning (L) are presented in Fig. 4(a)–(c). All longitudinal radargrams (L) show that the first level of reinforcing bars (stirrups) is on a similar depth, i.e., 0.055 m from the upper edge of the beam. Possible differences could be the result of unevenness of the tested surface (the levelling concrete layer); in these places, the stirrups are situated a little bit deeper. In order to define finally the position of the stirrups, it was necessary to analyse the beginning fragments of each radargram in detail. In these places, the tested beam was fully uncovered and the stirrups were clearly visible on the radargram. Additionally, the layer at a depth of 0.12 m, which is clearly visible in Fig. 4(a)–(c), turned out to be the upper edge of an internal hole in the beam. In the same figure, the layer at a depth of about 0.40 m is also visible and is probably the bottom edge of the internal hole. Moreover, all the radargrams show the layer at a depth of 0.56 m, which is the bottom edge of the main beam. A few anomalies were
Fig. 6. Top view of the beam with marked longitudinal profiles and stirrups.
D. Beben et al. / NDT&E International 49 (2012) 18–26
detected, e.g., reflections from an undefined object (element) are visible at a depth of 0.30 m (marked with circles in Fig. 4(b)). This type of anomaly is probably connected with the internal hole existing in the beam and could be due to reflections of the EM waves from the edges or bends in the internal hole. Moreover, four bigger longitudinal anomalies were observed (marked by ellipsis in Fig. 4(c)) at a depth of 0.07 m, which are probably due to the main upper longitudinal reinforcing bars in the beams. On the basis of the results obtained, the initial cross-section of the main beam of the viaduct was established (Fig. 5). The distances between the reinforcing bars (stirrups) were estimated based on the part of the radargram where they were most visible, i.e., on the length of 2 m of each scanning. In these places, the main beam was fully uncovered (Fig. 4(a)). In the remaining part of the beam, only single stirrups were visible and it was difficult to find the repeated their locations on the radargrams. The average distance between the stirrups was 0.17 m; the largest distance was 0.20 m (in the middle part of the beam) and the
Fig. 7. The processed transverse radargram (T22) of beam no. 1.
23
smallest was 0.13 m (at both ends of the beam). Fig. 6 shows the top view of the stirrup arrangement inside the beam. As mentioned earlier, transverse scans (T) were also executed. These scans confirmed that the first level of reinforcing bars and the location of the internal hole were at the same depth. Moreover, transverse scanning allowed us to estimate the range of the internal hole inside the beam. Considering the small length of scanning and numerous reflections, the visualization colour was changed in order to obtain improved sharpness. An example of a transverse radargram is shown in Fig. 7. In the transverse scans (T), two reinforcing bars were detected in the upper part of the beam at a depth of 0.06 m. These rebars existed in the corners of the main beam. Moreover, the edges of the hole inside the beam were visible on all radargrams, the upper edge at a depth of 0.12 m and the lower edge at a depth of 0.40 m. The beginning of the hole is visible about 0.18 m from the beginning of the scan, and it ends at 0.37 m of the wide scan. In the majority of radargrams, reflection on the beginning of the scan is visible at a depth of 0.055 m. These are probably the stirrups situated at a similar depth to the main reinforcing bars. The bottom level of the main reinforcing bars could not be determined accurately. However, at a depth of 0.50 m, some reflections of the EM waves were observed, probably due to existing reinforcement bars. The arrangement of upper reinforcement bars and the edges of the internal hole in beam no. 1 are presented in Fig. 8. 5.2.2. Beam no. 2 The results obtained for beam no. 2 confirm the results obtained for beam no. 1. The distance between the stirrups was estimated based on radargram L4 (Fig. 9), where the fragment of the rebar group is most visible. The average distance between stirrups was about 0.18 m. Larger spacing was detected in the middle part and smaller distances at the ends of the beam. Besides the stirrups at a depth of 0.055 m, an anomaly was also detected at a depth of 0.07 m (falling down). These are probably the upper longitudinal reinforcing bars. Based on the presented radargrams, it was difficult to determine the depth of layers constituting the edges of the internal hole in the beam. For better identification of these edges, the radargrams were processed omitting the background removal filter, which removes the horizontal continuous lines. As a result, quite a good image was obtained, where the internal hole was accurately located and the second (bottom) level of reinforcement bars was also observed (Fig. 10). In the case of the second beam, the rebar level and edges of the internal hole were more visible on transverse radargrams (T). Two reinforcing bars were also detected for this beam at a depth of about 0.06 m. The first was 0.06 m and the second 0.40 m from the beginning of the scanning area. Moreover, the internal hole at a depth of 0.12 m is quite well visible. The internal hole reaches
Fig. 8. Top view of the main longitudinal rebars and the edges of the hole, received from the transverse profile (T) for beam no. 1.
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D. Beben et al. / NDT&E International 49 (2012) 18–26
a depth of 0.40 m, where the image structure changes visibly. The second level of rebars is also visible (unfortunately, the number of rebars cannot be distinguished, probably due to the large number of rebars merging in the image). The bottom edge of the beam is also clearly visible at a depth of 0.56 m (Fig. 11). Finally, the location of upper reinforcing bars and the edges of the internal hole inside beam no. 2 are presented in Fig. 12. 5.3. Discussion of the GPR results
Fig. 9. The L4 radargram of beam no. 2 with well visible stirrups.
Based on the research, we determined that the main upper reinforcing bars were situated at a depth of about 0.06 m from the top edges of beams. The possible differences in their positions could result from inequalities of the test surface on which the measurement antenna was moved. Spacing of the main upper longitudinal reinforcing bars was determined quite accurately. After detailed analyses of both considered beams, only two longitudinal reinforcing bars were
Fig. 10. The L3 radargram of beam no.2 processed without the background remover filter.
Fig. 11. Processed transverse radargrams: (a) T20 and (b) T45.
D. Beben et al. / NDT&E International 49 (2012) 18–26
25
Fig. 12. Top view of the longitudinal rebars and the edges of the hole, received from the transverse profiles (T) for beam no. 2.
Fig. 13. Comparison of cross-sections of the viaduct beams received from: (a) the GPR tests and (b) the catalogue of typical beams of Gromnik type.
detected. The analysis indicates that only two longitudinal rebars should be taken into account in calculations of the current carrying capacity of the viaduct. The rebars were positioned in both corners – about 0.06 m from the beginning (and end) of tested beams. The distance between rebars ranged from 0.36 to 0.38 m. Additionally, it was discovered that the upper part of transverse reinforcing bars (stirrups) was positioned at a depth of about 0.055 m. Spacing of stirrups was approximately constant. However, they were more densely positioned at the ends of the beams (0.13 m) and sparser in the middle part (0.20 m). The main bottom reinforcing bars were situated at a level of about 0.50 m from the top edges of the beams. The numbers and spacing of rebars could not be accurately determined due to the complex cross-section of the beams. At the end of the analysis process, the location of the internal hole was identified inside each of the beams. The top edge of the hole was at a depth of about 0.12 m and the lower edge at about 0.40 m. In the transverse direction of the beam, the aperture starts at about 0.18 m and ends at 0.37 m, counting from the beginning of the beam. After analysis of all GPR results for both beams, it was concluded that Gromnik-type beams were used to construct the viaduct. Fig. 13 shows a comparison between the catalogue beam of Gromnik-type and results of the GRP research. Some differences were detected, for example in the number of upper reinforcement bars. In order to confirm the number of rebars, it was decided to uncover the concrete layer in one of the damaged beams. It turned out that only two longitudinal rebars existed in the top part of the beam. This confirms the necessity of applying the GPR method to the detection of reinforcement bars, especially in the case of old RC bridges, where differences between the project assumptions and reality can occur. These differences should be taken into consideration in repair projects, especially in the calculations of the necessary rebars. Due to a complicated
beam cross-section (the internal hole and some edges), the number of the bottom reinforcing bars could not be determined accurately. The geometry of the tested beams influenced the number of reflections of the EM waves from different layers occurring at the depth of beams. Taking into account the difficulty in accessing the concrete beams and the lack of basic information about the cross-section of the viaduct beam, as well as considering the assumption of the velocity of the wave propagation, the received results are quite correct. 6. Conclusions As a result of the tests conducted on viaduct beams using the GPR technique, the following conclusions can be drawn: 1. The performed research allowed us to identify the locations of: (i) the main upper and bottom reinforcing bars, (ii) the stirrups and their spacing (also the differences in their spacing in midspan and support zones), and (iii) an internal hole in the main beams. The test results showed quite good compatibility with the Gromnik-type beam. Disparities in the results were due to the relative complexity of the beam cross-section (many layers and edges causing reflection of the EM waves). This complexity had a negative influence on the ability to scan efficiently through the whole depth of the beam. 2. The GPR results were affected by the complexity of the viaduct beam cross-section, which consisted of various media (concrete, steel, hole filled by air). Generally, concrete is not a homogeneous material and should be considered as a composite material. Therefore, the velocities of the wave propagation through the concrete elements vary according to their physical and chemical properties. In this case study, the application of the wave velocity on the level of 0.10 m/ns allowed us to obtain quite acceptable results.
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3. Taking into account the advantages and disadvantages of nondestructive methods, presented by McCann and Forde [1], it can be concluded that the GPR technique is the most suitable method for bridge testing in order to detect the cross-section of the beams. GPR measurements using a bipolar antenna with a high frequency of 2000 MHz proved to be a quite effective method of determining the parameters of the RC viaduct beams. The GPR technique also allowed detection of discontinuities in the concrete structure (e.g., the hole in the beam). It should be noted that the analysis of radargrams requires skill to interpret the data. Research on accurate ways of determining the diameter of rebars occurring in concrete should be continued.
Acknowledgements The authors of this paper would like to thank OUTech and INZBUD from Opole for co-financing the tests. Special thanks are addressed to SejsCom s.c. from Cracow for their support during experimental testing.
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