- Email: [email protected]
Ultrasonic imaging of concrete members using an array system
NDT&E International 34 (2001) 403±408
www.elsevier.com/locate/ndteint
Ultrasonic imaging of concrete members using an array system M. Krause a,*, F. Mielentz a, B. Milman a, W. MuÈller b, V. Schmitz b, H. Wiggenhauser a a
b
Federal Institute for Materials Research and Testing (BAM), D 12200 Berlin, Germany Fraunhofer Institute for Non-destructive Testing (IZFP), D 66123 SaarbruÈcken, Germany
Abstract The use of an ultrasonic array system is described, which can be used combined with 3D reconstruction calculations. In this way ultrasonic re¯ection and backscatter from the inside of concrete members can be imaged and interpreted. The application of the system is demonstrated for two examples: measuring the concrete cover of utility pipes in a tunnel and the examination of transversal ducts in a bridge plate. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ultrasonic echo; Array technique; Synthetic aperture focusing technique; Tunnels; Reinforced concrete; Prestressing ducts
1. Introduction Ultrasonic echo methods are nowadays often used for analyzing the internal structure of concrete members and buildings [1±7]. The enormous progress, which has been achieved in the last decade to overcome the dif®culties characteristic of testing concrete is mainly due to the development of new types of broad-band transducers operating in the frequency range of 50±250 kHz and to the application of the principle of synthetic aperture. The latter means, that ultrasonic measurements are carried out at many points of the surface and are evaluated by means of reconstruction calculations, which show the location and distribution of acoustic re¯ectors and scatterers in the volume of the concrete member tested. Brie¯y summarized, the state of the art is as follows: Thickness measurement and localization of construction features having a diameter of about 100 mm and larger are feasible aims by means of single A-scan interpretation, when the site conditions are not too dif®cult. This means, that the maximum aggregate size is 16 mm or less and the non-prestressed reinforcement is not too dense (e.g. mesh size of 150 mm). Additionally the concrete porosity has not to be too excessive [8]. In many cases these conditions are not ful®lled. Concrete with a maximum aggregate size of 32 mm is widely used, the mesh size of the rebars is often 100 mm or less and the rebar diameter is 24 or 32 mm. Under these conditions many
* Corresponding author. E-mail address: [email protected] (M. Krause).
A-scans must be collected at different points on the surface and reconstruction calculations have to be performed. This is also important, when ducts in prestressed concrete have to be analyzed. For this aim a scanning laser interferometer can successfully be used to localize injection faults in tendon ducts. In this contribution, the principle and the application of an ultrasonic array are described. It consists of an ensemble of broad-band transducers which are successively used as transmitters and receivers and permit fast data acquisition. Thus large data sets can be collected and reconstruction calculation by means of synthetic aperture focusing technique (SAFT) is applicable. 2. Equipment The ultrasonic array used for data acquisition consists of an array of 10 broad-band transducers (diameter 50 mm, frequency range of about 50±250 kHz). In most cases the transducers are positioned on the concrete surface using a template, as shown in Fig. 1. They are used in a transmit± receive con®guration using an electronic multiplexer. The principle of the con®guration is shown in Fig. 2. The function generator allows to produce any pulse form with adjustable repetition frequency. Depending on the testing problem, it used a transmitting pulse having a time duration from about 20 to 40 ms. The variation of the pulse form and duration allows to ®t the center frequency to the desired transducer vibration. A typical transmitting pulse is shown in Fig. 2 (left) leading to a broad-band pulse with a center frequency of 85 kHz. A band pass ®lter is usually applied and working with a lower cut-off frequency
0963-8695/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0963-869 5(01)00007-X
404
M. Krause et al. / NDT&E International 34 (2001) 403±408
Fig. 1. Array of 10 broad-band transducers.
of 20 kHz and an upper cut-off frequency of 300 kHz. This is placed before the digital oscilloscope (Fig. 2) in order to suppress low frequency vibration, high frequency noise and aliasing effects. The measurement and data acquisition is governed by a portable PC. In this way, 45 pairs of transmitting±receiving positions Ð that means 90 different A-scans Ð are registered in a short time and can be used for the reconstruction calculations. The equipment can be installed in a van and cables up to 25 m length can be used. The use of broad-band transducers allows to optimize the frequency band for the exciting pulse which is programmed in the function generator. In order to have a good depth resolution, usually frequencies of 150±200 kHz are used (corresponding to the wavelength (l ) of 27±20 mm in concrete with cl 4000 m/s). If the attenuation of the waves is too high, lower frequencies can be chosen (80 kHz, l 50 mm). A typical excitation pulse and the result of the phasecorrected superposition (see below) of 100 A-scans is
shown in Fig. 3 (left). It is the result of the thickness measurement of a concrete plate made with maximum aggregate size of 32 mm. The (normalized) transmitting pulse and the received pulse (showing a phase shift due to the re¯ection at the interface concrete/air) can be observed in this ®gure. When an imaging of the internal structure of the concrete member is planned, a data set may consist of many different positions of the template. Depending on the desired resolution, the template is moved in steps between 10 and 100 mm over the surface. In this way data sets of several thousands of A-scans can be recorded in a relatively short time. The coupling of the transducers to the concrete surface is a dif®cult point for the ultrasonic measurements. A good coupling of every transducer of the array has to be carefully realized for each location of the template. For horizontal surfaces liquid coupling agents as glycerin or water may have an advantage. For the ultrasonic non-destructive applications described in this paper, the array technique was used for data acquisition. During earlier examinations of ducts positioned rather deep below the surface, adjacent areas of the surface were scanned by a laser interferometer working as an ultrasonic detector [9]. In this way a continuous scanning of the surface was obtained with only a few locations of transmitting transducers. For the applications discussed in this paper ducts were expected to have a concrete cover of only 80±120 mm. In this case it seemed to be more convenient to move the whole array over the surface, rather than changing often the location of the ultrasonic sending transducer and readjusting the laser interferometer. 3. Reconstruction calculations When the desired resolution for an experimental measurements is not too demanding, for example for thickness measurement or localization of large construction
Fig. 2. Principle sketch of the array equipment.
M. Krause et al. / NDT&E International 34 (2001) 403±408
405
Fig. 3. Measurement of a concrete specimen (0.50 m thick) with a maximum aggregate size of 32 mm. Left: time-of-¯ight corrected superposition of 100 Ascans. t0: expected arrival time of the echo signal. Right: pulse energy vs. calculated thickness for the integration interval which is indicated at the left side.
elements, a time-of-¯ight corrected superposition of Ascans at one or few positions of the template is performed. It results in the distribution of the re¯ected energy vs. depth for the volume below the template (usually a surface of 250 mm £ 350 mm). This is realized by integrating the square of the ultrasonic intensity during the pulse length. An example is shown in Fig. 3 (right) for the thickness measurement of a concrete plate. When a better resolution of the reconstruction calculation is necessary, for example for the examination of ducts in prestressed concrete members, a three-dimensional (3D) version of the SAFT method is applied [10]. This evaluation method was already successfully used for analyzing injection and compaction faults during a round robin test [9,11]. Brie¯y described, it is an integration of the time-of-¯ight surface in the x-y-z-t data ®eld for each voxel of the specimen. It results in a 3D representation of the backscatter intensity from the inside of the specimen. To interpret such data ®elds, projection planes are plotted, which are the well-known B-scans (ultrasonic intensity along the scan-axis vs. depth; perpendicular to the surface) and C-scans (ultrasonic intensity parallel to the
surface in an adjustable depth). The computer time required for a 3D-SAFT reconstruction depends on the number of Ascans in the data set and the reconstruction depth. For the investigation of transversal ducts in a bridge plate as described later, typically 3500 A-scans were collected. A typical time for such a reconstruction calculation on a 500 MHz PC is in the order of 20 min. With this method, axial and depth resolution in the order of one wavelength can be achieved. 4. Experimental and results In this section, two examples of the application of the array system and the reconstruction calculation are described. 4.1. Measurement of the concrete cover of utility pipes The task was to localize and measure the depth of ducts for utility installations in the wall of a tunnel. The ducts (diameter 200 mm) that should have been installed at
Fig. 4. Result of measuring the concrete cover of a duct for utility installations. Left: re¯ected ultrasonic energy vs. depth indicating a clear maximum at 360 mm. Right: corresponding time-of-¯ight corrected superpositions of 90 A-scans for the depth of 360 mm, t0: time-of-¯ight for the re¯ected pulse for that depth.
406
M. Krause et al. / NDT&E International 34 (2001) 403±408
Fig. 5. Reference measurement near the element joint over the bore hole with a known depth.
20 cm depth were instead installed towards the outer side of the wall (at 40 cm depth in a wall thickness of 0.8 m). It became visible at an element joint. At another part of the tunnel the ducts had to be localized in order to know, if the fault continued. At the element joint new holes were placed having the correct depth for the tubes. At this point a reference measurement was performed.
The measurement was strongly in¯uenced by dense nonprestressed reinforcement (diameter 28 mm, mesh size about 100 mm). For this reason the reinforcement was ®rst localized using a scanning magnetic device, and the transducers of the array were manually placed between the rebars on the surface. Two different locations of the array (180 A-Scans) were used to obtain a data ®eld for the time-of¯ight corrected superposition. Fig. 4 shows the re¯ected ultrasonic energy vs. depth indicating a clear maximum at 360 mm. In the range between 100 and 200 mm disturbing re¯ections caused by the reinforcement are visible. The corresponding time-of-¯ight corrected superposition for the depth of 350 mm is shown at the right side of Fig. 4. The multiple re¯ections caused by the reinforcement have been suppressed by digital ®ltering prior to the evaluation. Fig. 5 shows the reference experiment over a core with a known duct cover of 180 mm. The depth measurement (150 mm) is very clear, the error of 30 mm is caused by the fact, that calculations were performed with an estimated velocity and by the overlap with re¯ections and stray signals from the rebars. The knowledge of the exact ultrasonic velocity was not necessary for the described question, because the measurements had only to estimate, if the ducts were
Fig. 6. Test specimen for the application of 3D-SAFT reconstruction.
M. Krause et al. / NDT&E International 34 (2001) 403±408
407
Fig. 9. Imaging of an ungrouted duct in a concrete plate: B-scan from 3D SAFT reconstruction.
Fig. 7. B-scan from array measurement and SAFT reconstruction of the test specimen shown in Fig. 6.
placed about 200 mm (correct position) or 400 mm (wrong position) below the surface. Using the ultrasonic array it could be clearly demonstrated that the ducts were wrongly placed, deeply below the surface. 4.2. Examination of transversal ducts in a bridge plate Prior to a planned site application on a bridge deck it was to be veri®ed, if transversal ducts can be investigated with the system described combined with 3D-SAFT reconstruction calculations. As mentioned above the ducts to be analyzed had a concrete cover of only 80±120 mm. In order to test the principal function of the array system and the 3D-SAFT reconstruction, a test specimen with a step in thickness and a steel tube (diameter 40 mm) was constructed (Fig. 6) The array was scanned in steps of 30 mm over the surface using glycerin as coupling agent. The B- and C-scans resulting from the 3D-SAFT reconstruction are presented in Figs. 7 and 8. In the B-scan the step in the thickness is clearly seen, whereas the tube is not
represented in the reconstruction. As expected, at the border of the aperture the intensity is small, because the outer regions of the specimen are not homogeneously irradiated by the ultrasonic waves. The signals below the surface re¯ection are partly due to disturbing pulses produced by the transducers and multiple re¯ections. The C-scan shows that the reconstruction of the back wall is relatively homogeneous indicating that the system can be used for imaging plain surfaces. The variations of about 6 dB are due to the inevitable changes in the coupling conditions. It can be seen that for imaging ducts the aperture in the direction of the ducts has to be larger. In order to test the system for imaging ducts, a second test specimen was constructed. It contains three ducts having the depth expected at the bridge deck. Fig. 9 represents the B-scan of an empty duct (diameter 40 mm, concrete cover: 100 mm). The back wall and the duct are clearly represented. The slight bowing of the duct of about 20 mm towards the center of the specimen corresponds with the real situation in the specimen. The additional signals about 80 mm below the duct and the back wall, respectively, are due to the wave mode conversion from longitudinal to transversal waves. This can be proved by a reconstruction calculation taking into account the corresponding velocities. The change in the intensity of ultrasonic re¯ection from both the duct and the back wall is evident: Although the measured surface was very smooth, the reason is probably the changing of the coupling conditions. The existence of the clear back wall echo can help in this interpretation. Assuming that the concrete quality does not strongly depend on the location, a normalization to the back wall intensity was performed. The result (Fig. 10) shows a relatively constant re¯ection intensity of the empty duct except in the region around x 0.8 m. Here an anomaly of the concrete quality is assumed which will be examined later. 5. Summary and conclusions
Fig. 8. C-scam from array measurement and SAFT reconstruction of the test specimen shown in Fig. 6.
The use of an ultrasonic array system is described, which
408
M. Krause et al. / NDT&E International 34 (2001) 403±408
References
Fig. 10. Imaging of an ungrouted duct in a concrete plate: B-scan from 3D SAFT reconstruction. Same data as in Fig. 9, the intensity is normalized to the back wall re¯ection.
consists of 10 broad-band transducers working each one successively as transmitter and receiver. The practical application of the system on site shows that it is possible to measure the concrete cover of large constructions elements even behind dense reinforcing bars. To do this, the data are evaluated by means of time-of-¯ight corrected superposition. It is demonstrated, that the array system together with 3D-SAFT reconstruction calculation can be used for the examination of transversal prestressing ducts having a concrete cover of about 100 mm. The change of the coupling conditions can be compensated by a normalization, so that the analysis of grouting and compaction faults in and around the duct can be performed by considering the re¯ection intensity. The system has already been used on site successfully.
Acknowledgements The authors acknowledge the assistance of BAM VII.1 in specimen design and manufacturing. Ing D. Schaurich has assisted with graphical work.
[1] Kroggel O, Jansohn R, Ratmann M. Novel ultrasound system to detect voides ducts in posttensioned bridges. In: Forde MC, editor. Proceedings of the 6th International Conference on Structural Faults and Repair, UK, vol. 1. London: Engineering Technics Press, 1995. p. 203±8. [2] Schickert M. The use of ultrasonic A-scan and B-scan and SAFT techniques for testing concrete elements. Second International Conference on NDT of Concrete in the Infrastucture, Nashville, Tennessee, June 12±14, 1996. p. 135±42. [3] Krause M, BaÈrmann R, Frielinghaus R, Kretzschmar F, Kroggel O, Langenberg K, Maierhofer C, MuÈller W, Neisecke J, Schickert M, Schmitz V, Wiggenhauser H, Wollbold F. Comparison of pulse±echo methods for testing concrete. NDT & E Int 1997;30(4):195±204. [4] Wollbold F, Neisecke J. Ultrasonic testing of concrete with fast imaging pulse±echo-technique. Proceedings ECNDT Conference, May 26±29, 1998, Copenhagen, 1998. [5] Kapphahn G, Kaschmierzeck K. WeserbruÈcke DrakenburgÐVoruntersuchungen an den 41 Meter langen, vorgespannten HauptlaÈngstraÈgern. Wissenschaftliche BeitraÈge der MFPA 1998, vol. 3, Leipzig, 1998. [6] Maierhofer C, Krause M, Wiggenhauser H. Non-destructive investigation of sluices using radar and ultrasonic impulse echo. NDT & E Int 1998;31(6):421±7. [7] Krieger J, Krause M, Wiggenhauser H. Tests and assessments of NDT methods for concrete bridges. In: Medlock RD, Laffrey DC, editors. Structural materials technology III. Proceedings of SPIE, Bellingham, 3400, 1998. p. 258±69. [8] Krieger J, Krause M, Wiggenhauser H. Erprobung und Bewertung zerstoÈrungsfreier PruÈfmethoden fuÈr BetonbruÈcken. Bremerhaven: Wirtschafts NW, 1998. [9] Krause M, MuÈller W, Wiggenhauser H. Ultrasonic inspection of tendon ducts in concrete slabs using 3D-SAFT. Acoust Imaging 1997;23:433±9. [10] Schmitz V, KroÈning M, Langenberg K. Quantitative NDT by 3D image reconstruction. Acoust Imaging 1996;22:735±44. [11] Krause M, Wiggenhauser H, MuÈller W, Kostka J, Langenberg K. Ultrasonic bridge inspection using 3D-SAFT. In: Fahlstedt K, editor. Proceedings of the CIB World Building Congress, Symposium A, GaÈvle, Schweden, 7±12. June 1998, KTH Built Environment, GaÈvle, 1998. p. A 87.
www.elsevier.com/locate/ndteint
Ultrasonic imaging of concrete members using an array system M. Krause a,*, F. Mielentz a, B. Milman a, W. MuÈller b, V. Schmitz b, H. Wiggenhauser a a
b
Federal Institute for Materials Research and Testing (BAM), D 12200 Berlin, Germany Fraunhofer Institute for Non-destructive Testing (IZFP), D 66123 SaarbruÈcken, Germany
Abstract The use of an ultrasonic array system is described, which can be used combined with 3D reconstruction calculations. In this way ultrasonic re¯ection and backscatter from the inside of concrete members can be imaged and interpreted. The application of the system is demonstrated for two examples: measuring the concrete cover of utility pipes in a tunnel and the examination of transversal ducts in a bridge plate. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ultrasonic echo; Array technique; Synthetic aperture focusing technique; Tunnels; Reinforced concrete; Prestressing ducts
1. Introduction Ultrasonic echo methods are nowadays often used for analyzing the internal structure of concrete members and buildings [1±7]. The enormous progress, which has been achieved in the last decade to overcome the dif®culties characteristic of testing concrete is mainly due to the development of new types of broad-band transducers operating in the frequency range of 50±250 kHz and to the application of the principle of synthetic aperture. The latter means, that ultrasonic measurements are carried out at many points of the surface and are evaluated by means of reconstruction calculations, which show the location and distribution of acoustic re¯ectors and scatterers in the volume of the concrete member tested. Brie¯y summarized, the state of the art is as follows: Thickness measurement and localization of construction features having a diameter of about 100 mm and larger are feasible aims by means of single A-scan interpretation, when the site conditions are not too dif®cult. This means, that the maximum aggregate size is 16 mm or less and the non-prestressed reinforcement is not too dense (e.g. mesh size of 150 mm). Additionally the concrete porosity has not to be too excessive [8]. In many cases these conditions are not ful®lled. Concrete with a maximum aggregate size of 32 mm is widely used, the mesh size of the rebars is often 100 mm or less and the rebar diameter is 24 or 32 mm. Under these conditions many
* Corresponding author. E-mail address: [email protected] (M. Krause).
A-scans must be collected at different points on the surface and reconstruction calculations have to be performed. This is also important, when ducts in prestressed concrete have to be analyzed. For this aim a scanning laser interferometer can successfully be used to localize injection faults in tendon ducts. In this contribution, the principle and the application of an ultrasonic array are described. It consists of an ensemble of broad-band transducers which are successively used as transmitters and receivers and permit fast data acquisition. Thus large data sets can be collected and reconstruction calculation by means of synthetic aperture focusing technique (SAFT) is applicable. 2. Equipment The ultrasonic array used for data acquisition consists of an array of 10 broad-band transducers (diameter 50 mm, frequency range of about 50±250 kHz). In most cases the transducers are positioned on the concrete surface using a template, as shown in Fig. 1. They are used in a transmit± receive con®guration using an electronic multiplexer. The principle of the con®guration is shown in Fig. 2. The function generator allows to produce any pulse form with adjustable repetition frequency. Depending on the testing problem, it used a transmitting pulse having a time duration from about 20 to 40 ms. The variation of the pulse form and duration allows to ®t the center frequency to the desired transducer vibration. A typical transmitting pulse is shown in Fig. 2 (left) leading to a broad-band pulse with a center frequency of 85 kHz. A band pass ®lter is usually applied and working with a lower cut-off frequency
0963-8695/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0963-869 5(01)00007-X
404
M. Krause et al. / NDT&E International 34 (2001) 403±408
Fig. 1. Array of 10 broad-band transducers.
of 20 kHz and an upper cut-off frequency of 300 kHz. This is placed before the digital oscilloscope (Fig. 2) in order to suppress low frequency vibration, high frequency noise and aliasing effects. The measurement and data acquisition is governed by a portable PC. In this way, 45 pairs of transmitting±receiving positions Ð that means 90 different A-scans Ð are registered in a short time and can be used for the reconstruction calculations. The equipment can be installed in a van and cables up to 25 m length can be used. The use of broad-band transducers allows to optimize the frequency band for the exciting pulse which is programmed in the function generator. In order to have a good depth resolution, usually frequencies of 150±200 kHz are used (corresponding to the wavelength (l ) of 27±20 mm in concrete with cl 4000 m/s). If the attenuation of the waves is too high, lower frequencies can be chosen (80 kHz, l 50 mm). A typical excitation pulse and the result of the phasecorrected superposition (see below) of 100 A-scans is
shown in Fig. 3 (left). It is the result of the thickness measurement of a concrete plate made with maximum aggregate size of 32 mm. The (normalized) transmitting pulse and the received pulse (showing a phase shift due to the re¯ection at the interface concrete/air) can be observed in this ®gure. When an imaging of the internal structure of the concrete member is planned, a data set may consist of many different positions of the template. Depending on the desired resolution, the template is moved in steps between 10 and 100 mm over the surface. In this way data sets of several thousands of A-scans can be recorded in a relatively short time. The coupling of the transducers to the concrete surface is a dif®cult point for the ultrasonic measurements. A good coupling of every transducer of the array has to be carefully realized for each location of the template. For horizontal surfaces liquid coupling agents as glycerin or water may have an advantage. For the ultrasonic non-destructive applications described in this paper, the array technique was used for data acquisition. During earlier examinations of ducts positioned rather deep below the surface, adjacent areas of the surface were scanned by a laser interferometer working as an ultrasonic detector [9]. In this way a continuous scanning of the surface was obtained with only a few locations of transmitting transducers. For the applications discussed in this paper ducts were expected to have a concrete cover of only 80±120 mm. In this case it seemed to be more convenient to move the whole array over the surface, rather than changing often the location of the ultrasonic sending transducer and readjusting the laser interferometer. 3. Reconstruction calculations When the desired resolution for an experimental measurements is not too demanding, for example for thickness measurement or localization of large construction
Fig. 2. Principle sketch of the array equipment.
M. Krause et al. / NDT&E International 34 (2001) 403±408
405
Fig. 3. Measurement of a concrete specimen (0.50 m thick) with a maximum aggregate size of 32 mm. Left: time-of-¯ight corrected superposition of 100 Ascans. t0: expected arrival time of the echo signal. Right: pulse energy vs. calculated thickness for the integration interval which is indicated at the left side.
elements, a time-of-¯ight corrected superposition of Ascans at one or few positions of the template is performed. It results in the distribution of the re¯ected energy vs. depth for the volume below the template (usually a surface of 250 mm £ 350 mm). This is realized by integrating the square of the ultrasonic intensity during the pulse length. An example is shown in Fig. 3 (right) for the thickness measurement of a concrete plate. When a better resolution of the reconstruction calculation is necessary, for example for the examination of ducts in prestressed concrete members, a three-dimensional (3D) version of the SAFT method is applied [10]. This evaluation method was already successfully used for analyzing injection and compaction faults during a round robin test [9,11]. Brie¯y described, it is an integration of the time-of-¯ight surface in the x-y-z-t data ®eld for each voxel of the specimen. It results in a 3D representation of the backscatter intensity from the inside of the specimen. To interpret such data ®elds, projection planes are plotted, which are the well-known B-scans (ultrasonic intensity along the scan-axis vs. depth; perpendicular to the surface) and C-scans (ultrasonic intensity parallel to the
surface in an adjustable depth). The computer time required for a 3D-SAFT reconstruction depends on the number of Ascans in the data set and the reconstruction depth. For the investigation of transversal ducts in a bridge plate as described later, typically 3500 A-scans were collected. A typical time for such a reconstruction calculation on a 500 MHz PC is in the order of 20 min. With this method, axial and depth resolution in the order of one wavelength can be achieved. 4. Experimental and results In this section, two examples of the application of the array system and the reconstruction calculation are described. 4.1. Measurement of the concrete cover of utility pipes The task was to localize and measure the depth of ducts for utility installations in the wall of a tunnel. The ducts (diameter 200 mm) that should have been installed at
Fig. 4. Result of measuring the concrete cover of a duct for utility installations. Left: re¯ected ultrasonic energy vs. depth indicating a clear maximum at 360 mm. Right: corresponding time-of-¯ight corrected superpositions of 90 A-scans for the depth of 360 mm, t0: time-of-¯ight for the re¯ected pulse for that depth.
406
M. Krause et al. / NDT&E International 34 (2001) 403±408
Fig. 5. Reference measurement near the element joint over the bore hole with a known depth.
20 cm depth were instead installed towards the outer side of the wall (at 40 cm depth in a wall thickness of 0.8 m). It became visible at an element joint. At another part of the tunnel the ducts had to be localized in order to know, if the fault continued. At the element joint new holes were placed having the correct depth for the tubes. At this point a reference measurement was performed.
The measurement was strongly in¯uenced by dense nonprestressed reinforcement (diameter 28 mm, mesh size about 100 mm). For this reason the reinforcement was ®rst localized using a scanning magnetic device, and the transducers of the array were manually placed between the rebars on the surface. Two different locations of the array (180 A-Scans) were used to obtain a data ®eld for the time-of¯ight corrected superposition. Fig. 4 shows the re¯ected ultrasonic energy vs. depth indicating a clear maximum at 360 mm. In the range between 100 and 200 mm disturbing re¯ections caused by the reinforcement are visible. The corresponding time-of-¯ight corrected superposition for the depth of 350 mm is shown at the right side of Fig. 4. The multiple re¯ections caused by the reinforcement have been suppressed by digital ®ltering prior to the evaluation. Fig. 5 shows the reference experiment over a core with a known duct cover of 180 mm. The depth measurement (150 mm) is very clear, the error of 30 mm is caused by the fact, that calculations were performed with an estimated velocity and by the overlap with re¯ections and stray signals from the rebars. The knowledge of the exact ultrasonic velocity was not necessary for the described question, because the measurements had only to estimate, if the ducts were
Fig. 6. Test specimen for the application of 3D-SAFT reconstruction.
M. Krause et al. / NDT&E International 34 (2001) 403±408
407
Fig. 9. Imaging of an ungrouted duct in a concrete plate: B-scan from 3D SAFT reconstruction.
Fig. 7. B-scan from array measurement and SAFT reconstruction of the test specimen shown in Fig. 6.
placed about 200 mm (correct position) or 400 mm (wrong position) below the surface. Using the ultrasonic array it could be clearly demonstrated that the ducts were wrongly placed, deeply below the surface. 4.2. Examination of transversal ducts in a bridge plate Prior to a planned site application on a bridge deck it was to be veri®ed, if transversal ducts can be investigated with the system described combined with 3D-SAFT reconstruction calculations. As mentioned above the ducts to be analyzed had a concrete cover of only 80±120 mm. In order to test the principal function of the array system and the 3D-SAFT reconstruction, a test specimen with a step in thickness and a steel tube (diameter 40 mm) was constructed (Fig. 6) The array was scanned in steps of 30 mm over the surface using glycerin as coupling agent. The B- and C-scans resulting from the 3D-SAFT reconstruction are presented in Figs. 7 and 8. In the B-scan the step in the thickness is clearly seen, whereas the tube is not
represented in the reconstruction. As expected, at the border of the aperture the intensity is small, because the outer regions of the specimen are not homogeneously irradiated by the ultrasonic waves. The signals below the surface re¯ection are partly due to disturbing pulses produced by the transducers and multiple re¯ections. The C-scan shows that the reconstruction of the back wall is relatively homogeneous indicating that the system can be used for imaging plain surfaces. The variations of about 6 dB are due to the inevitable changes in the coupling conditions. It can be seen that for imaging ducts the aperture in the direction of the ducts has to be larger. In order to test the system for imaging ducts, a second test specimen was constructed. It contains three ducts having the depth expected at the bridge deck. Fig. 9 represents the B-scan of an empty duct (diameter 40 mm, concrete cover: 100 mm). The back wall and the duct are clearly represented. The slight bowing of the duct of about 20 mm towards the center of the specimen corresponds with the real situation in the specimen. The additional signals about 80 mm below the duct and the back wall, respectively, are due to the wave mode conversion from longitudinal to transversal waves. This can be proved by a reconstruction calculation taking into account the corresponding velocities. The change in the intensity of ultrasonic re¯ection from both the duct and the back wall is evident: Although the measured surface was very smooth, the reason is probably the changing of the coupling conditions. The existence of the clear back wall echo can help in this interpretation. Assuming that the concrete quality does not strongly depend on the location, a normalization to the back wall intensity was performed. The result (Fig. 10) shows a relatively constant re¯ection intensity of the empty duct except in the region around x 0.8 m. Here an anomaly of the concrete quality is assumed which will be examined later. 5. Summary and conclusions
Fig. 8. C-scam from array measurement and SAFT reconstruction of the test specimen shown in Fig. 6.
The use of an ultrasonic array system is described, which
408
M. Krause et al. / NDT&E International 34 (2001) 403±408
References
Fig. 10. Imaging of an ungrouted duct in a concrete plate: B-scan from 3D SAFT reconstruction. Same data as in Fig. 9, the intensity is normalized to the back wall re¯ection.
consists of 10 broad-band transducers working each one successively as transmitter and receiver. The practical application of the system on site shows that it is possible to measure the concrete cover of large constructions elements even behind dense reinforcing bars. To do this, the data are evaluated by means of time-of-¯ight corrected superposition. It is demonstrated, that the array system together with 3D-SAFT reconstruction calculation can be used for the examination of transversal prestressing ducts having a concrete cover of about 100 mm. The change of the coupling conditions can be compensated by a normalization, so that the analysis of grouting and compaction faults in and around the duct can be performed by considering the re¯ection intensity. The system has already been used on site successfully.
Acknowledgements The authors acknowledge the assistance of BAM VII.1 in specimen design and manufacturing. Ing D. Schaurich has assisted with graphical work.
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