APPLICATION OF INNOVATIVE TESTING METHODS FOR THE QUASI NON-DESTRUCTIVE ASSESSMENT OF THE MATERIAL CONDITION IN HYDRAULIC ENGINEERING STRUCTURES

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1 APPLICATION OF INNOVATIVE TESTING METHODS FOR THE QUASI NON-DESTRUCTIVE ASSESSMENT OF THE MATERIAL CONDITION IN HYDRAULIC ENGINEERING STRUCTURES Frank Weise (1), Stephan Pirskawetz (1) and Birgit Meng (1) (1) Federal Institute for Materials Research and Testing (BAM), Berlin Abstract The current assessment of the material condition in hydraulic engineering structures is mainly based on visual survey and knocking on the surface of the structure in apparently damaged areas with subsequent extraction of drilling cores. The drilling cores are evaluated visually and also tested in the laboratory concerning their material properties with conventional methods (e.g. mechanical testing and chemical analysis). The results of these selective investigations are related to the entire structure. In this paper new possibilities are highlighted using innovative testing methods for the non-destructive assessment of the material condition in hydraulic engineering structures. For example the combined application of radar measurements and ultrasonic measurements may provide much more information about the structural state than the current assessment methods. Furthermore new perspectives for laboratory damage analysis are presented which are issued by the application of 3D X-ray tomography and micro X-ray fluorescence analysis. Keywords Quasi non-destructive assessment, Hydraulic engineering structures, radar, ultrasonic and X- ray 1. DEFINITION OF PROBLEM AND OBJECTIVE The intended rehabilitation of the double-chamber lock in Üfingen was the reason for the investigations. It was built from 1938 until 1940 as a gravity dam. Each chamber of the lock has a dimension of m by 12.0 m by 13.5 m (Figure 1a). The outer aspect of the damage on the chamber walls is characterized by spalling and by cracks extending in parallel with the platform and the block joint, with partially very distinctly developed scaling (Figure 1b). 483

2 The drawing up of an acceptable rehabilitation concept required a comprehensive condition and damage analysis. In the course of this, by order of the Federal Institute for Hydraulic Engineering (Bundesanstalt für Wasserbau BAW) in close cooperation of the Patitz Engineering Office (Ingenieurbüro Patitz IGP), of the Company for Geophysical Investigations (Gesellschaft für geophysikalische Untersuchungen GGU) Karlsruhe and of the Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und -prüfung BAM), there had to be tested the potential of various innovative nondestructive testing techniques in the structural and laboratory investigations. So there had to be verified if near-surface structural damage may be located by the non-destructive indirect investigation methods of radar and ultrasonics in large areas and economically. Furthermore there had to be verified the potential of 3D X-ray tomography (3D CT) and micro X-ray fluorescence analysis (MRFA) in damage analysis in the laboratory. 2. INVESTIGATION CONCEPT Figure 2 gives a general view of the performed structural and laboratory investigations. block joint platform spalling a. general view b. detailed view 3. STRUCTURAL INVESTIGATIONS cracks Figure 1: Western lock Üfingen In the following there will be presented the structural investigations with radar and ultrasonics, based on [1], with the severely damaged chamber wall block 5 as an example. The verification of the quality of the non-destructive tests was carried out on drilling cores. 3.1 Radar First there was used the radar method in reflection configuration for the fast large-surface scanning of the chamber lock wall. The principle of measurement is depicted in Figure 3. It is based on the method of a pair of antennas (transmitter and receiver) along a sweep. Short electromagnetic pulses are emitted by the transmitting antenna that will be reflected at boundary layers of materials with different dielectric properties. The reflected signals are registered by the receiving antenna. The representation of the radar results normally is performed as a radargram (B scan). Therein the intensity of the reflected signals over the travel path and the depth of the building component is represented by colour-coding. From the 484

3 general inspection with radar(igp, GGU) detailed investigations with ultrasonics (BAM) definition of the extraction points BUILDING extraction of drilling cores investigations on drilling cores LABORATORY validation of NDTmeasurements (IGP, BAM) investigation of cause of damage (BAM) visual inspection ultrasonics X-ray 3D-CT light microscopy micro -RFA REM -EDX macrostructure microstructure and microchemistry Figure 2: Investigation concept radargrams that are taken along all the sweeps that extend in parallel there may be generated a data cube. From this there may be obtained time slices or C scans, respectively These scans that extend in parallel to the surface are well suited for the large-surface depth-resolved representation of the inner structure of the examination item. Figure 4 gives an impression of the practical on-site activities. The scanning of the lock chamber wall was carried out from a lifting platform that was mounted on a ship. The radar investigations were performed with 1.5 GHz antennas to achieve the highest possible local resolution. The pair of antennas was travelled exclusively along vertical sweeps at an interval of 5 cm each. On this basis there was performed a differentiated analysis of the radar measurements with a sweep interval of 5, 10, 20 and 50 cm. (a) traverse path (b) traverse path (c) traverse path S E S E cylindric reflector depth (run time) depth (run time) time slice (C-scan) radargram (B-scans) Figure 3: Principle of measurement of radar (a) Measurment set-up, (b). Radargram (B scan) with hyperbolic diffraction, (c) Generation of a C scan from B scans 485

4 a. Access technique b. Travelling of the pair of antennas (1.5 GHz) along vertical sweeps Figure 4: Practical execution of the radar examinations on the structure source: Ingenieurbüro Patitz (IGP) The calibration of the radar measurements was carried out on built-in metallic parts of the lock chamber wall with a known depth position. The scanning of an area of 3 m by 4 m with 30 lines takes about 0.5 hours. Figure 5 shows selected radargrams from block 5. Comparing these radargrams over the height of the lock chamber wall one will notice that both the intensity and the depth position of the reflected signals are significantly higher in the upper region then in the lower B-scan 1 B-scan 11 B-scan 16 B-scan x 10 cm 30 x 10 cm 300 cm run time [ns] run time [ns] run time [ns] run time [ns] a. Examined region with measurement grid b. B scans Figure 5: Selected radar results at the severely damaged block 5 (B scans)(data source: GGU Gesellschaft für geophysikalische Untersuchungen mbh) 486

5 Thus in the upper region there are found reflexions up to a maximum depth of about 40 cm and in the lower region up to a depth of about 20 cm. This suggests a more severe and deeper structural damage in the upper region. This may show itself as structural loosening, crack formation and shell separation in the concrete. The generation of depth averaged C scans for defined depth regions proved practicable for the large-surface representation of the depth-dependent structural damages in the lock chamber wall (Figure 6). In the C scans the bright and red regions represent reflexes of high signal intensity. These are indicators for inner structural damages in the concrete. It is evident that in the zone of alternating water level are large structural damages in the concrete up to a depth of about 30 cm. In contrast, the structural damages in the concrete in the underlying region reach only up to a maximum depth of 20 cm, with a decreasing tendency from top to bottom. For the verification of the result of the radar examinations drilling cores were drawn selectively at chosen regions (Figure 6). The results of these examinations can be found in part 3.3. BK 5-1 depth: 0 5 cm 5 10 cm cm cm BK 5-1 BK 5-1 BK 5-1 BK cm BK 5-4 BK 5-4 BK 5-4 BK 5-4 BK cm low intensity of the reflected signals high Figure 6: Depth averaged C scans from block 5 (data source: GGU Gesellschaft für geophysikalische Untersuchungen mbh) 3.2 Ultrasonics Starting out from the results of the radar examinations, selected regions of the lock chamber wall were minutely examined with ultrasonics. The principle of measurement of this method is based on the reflection of ultrasonic waves at interfaces of materials. From the run time of the reflected pulses with known ultrasonic velocity there may be determined the depth position of the interface of materials. Novel point-contact probes were used for the lock examinations. They produce transverse waves with a centre frequency of 55 khz. In contrast with conventional probes a coupling means between the component surface and the probe is not needed. Because of this and because of the resilient bedding of the probes a coupling to rough surfaces is also possible. In practical use the probe array is passed along a measurement grid. A data record recorded at one measuring point is referred to as an A scan. In Figure 7a there is plotted the signal amplitude over the signal run time and additionally represented with colour coding. The A scans combined along one sweep are referred to as a B scan (Figure 7b). The ultrasonic velocity needed for the translation of the run time into the component depth was determined on drilling cores from the lock chamber wall. 487

6 high intensity A-scan B-scan intensity of the reflected signals signal run time low a. A scan b. B scan Figure 7: Representation of the measurement data The scanning of an area of 3 m by 4 m with a measuring grid of 10 cm by 10 cm takes about 1.5 hours. Figure 8 shows selected B scans of the ultrasonic examinations in the red marked measurement grid with a mesh width of 10 cm. Because of the large thickness of the lock chamber wall no slap-back was expected in the ultrasonic scans. In all B scans there can be seen that the number of reflections with high intensities tends to decrease from top to bottom. This suggests that the extent of structural damage of the concrete is significantly higher in the upper region (water change zone) than below. This inner structural condition corresponds with the outward appearance in this wall region. However, with a complete shell separation a further structural reconnaissance is impossible because of the lacking acoustical contact with the concrete behind. The results of the ultrasonic examinations, analogously with the radar examinations, were validated on drilling cores. BK B-scan 9 B-scan 10 B-scan B-scan cm BK BK cm a. Examined region with measurement grid intensity of the reflected signals b. B scans with identification of the extraction regions of the drilling cores Figure 8: Selected ultrasonic results at the severely damaged block 5 low high 488

7 3.3 Extraction of drilling cores and validation of the results The validation of the results of the non-destructive structural investigations will be exemplified by means of two drilling cores. Drilling core BK 5-1 (Figure 9a) shows the structural damages detected both with radar and ultrasonics in the region of the water change zone. Only a moderate grain bonding can be seen. The drilling core is characterized by a very strong crack formation perpendicular to the axis of the drilling core over the entire length of the drilling core. Due to these structural damages the foremost region of the drilling core up to a depth of 12 cm could not be recovered. The drilling core BK 5-4 shown in Figure 9b, from the lower region of the lock chamber wall, in contrast shows a significantly better grain bonding and no significant structural damage. The separation crack is likely to be due to the drilling process. This result also confirms the results of the radar and ultrasonic examinations. a. Drilling core BK 5-1 b. Drilling core BK 5-4 Figure 9: Selected drilling cores from block 5 of the chamber wall 4. LABORATORY INVESTIGATIONS Besides the validation of the non-destructive structural investigations, the laboratory investigations on the drilling cores served the determination of the cause of the damage. Also in the course of this, besides traditional methods, various innovative testing techniques were used. Thus X-ray computed tomography was used for the verification of the loosening of the structure as well as for selectively choosing the regions for the subsequent microscopic and microchemical investigation. During the latter the micro X-ray fluorescence analysis (MRFA) as a novel technique in the field of building materials was used for the establishment of largesized element distribution images. The investigations were performed on an already severely damaged drilling core from block 11 of the lock chamber wall (Figure 10). 180 cm Figure 10: Local classification and outward appearance of the drilling core for laboratory investigations 489

8 4.1 X-ray 3D computed tomography (3D CT) The principle of measurement of 3D CT is based on the penetration of the test item with X- radiation from different angles over its entire perimeter. As a result one obtains a variety of individual images with the distribution of the radiographic density. By means of special reconstruction algorithms from these subsequently there is calculated the image of the geometric distribution of the absorption coefficients in the test item (Figure 11). detector X-Ray source specimen 3D-absorption picture Figure 11: Principle of measurement of 3D CT on an X-ray basis A 3D micro-focus installation with a 320 kv X-ray tube as well as an α-si flat detector with a size of 1024 x 1024 pixels was used for the performed investigations. A local resolution of about 100 µm results from this for the 100 mm outer diameter drilling cores examined herein. Figure 12 exemplifies virtual vertical cross sections through the severely damaged drilling core from the upper region of the lock chamber wall of block 11, measured by the 3D CT. In the grey-scale images the regions with high radiographic density as well as stone graining appear bright, cracks and pores in contrast dark because of their lower radiographic density. The grey-scale values of the cement stone matrix are between both extreme values. In all images there can be seen a distinct crack formation. The crack pattern indicates a combined structural damage due to freeze and alkali-silica reaction. The latter can be recognized especially in the first cutting plane. Thus here in the emphasized region there can be seen a severe damage to a larger stone grain and the cracks starting out from there. 1 2 ASR-damage 1 2 Ø 100 mm section 1-1 section 2-2 Figure 12: Selected results of 3D CT of a severely damaged drilling core 490

9 4.2 Microscopy and microchemistry Based on the described tomographic examinations, regions with severe crack formation and abnormal absorptions of X-radiation in the concrete structure were examined more extensively. The objective was to complement the structural parameters obtained by 3D CT such as the beginning and the course of cracks by information on the material composition of the damaged regions, by means of microscopy and microchemistry. Large-sized polished and thin-ground sections (100 mm x 75 mm) of the severely damaged region were made for the investigations. These were examined with a polarizing microscope and a scanning electron microscope with an energy-dispersive detector (REM-EDX) as well as with the micro X-ray fluorescence analysis (MRFA) that up to date has been scarcely used in the field of building materials [2]. The severely damaged grain depicted in Figure 13 (upper left hand side) is heavily decomposed siliceous schist with cryptocrystalline quartz and chalcedony. Under the polarizing microscope there can be seen the typical gel at the ends of cracks and also in the interior of the grain. The element distribution images (Figure 13) obtained by means of MRFA show beyond all doubt that the denser boundary zone of the grain consists of alkali-silica gel. The result of MRFA is also confirmed by the results of more extensive light-optical microscope examinations. So there were found various reactive aggregates. Notably there are extraordinary concentrations of sulphur in the cracks and pores. Selective examinations with the polarizing microscope and the scanning electron microscope showed that this sulphur-containing phase is ettringite. The formation of ettringite in the course of an alkali-silica reaction has already been described repeatedly (e.g. [3] to [5]). However, up to date it has been impossible to clarify definitely if a there is an actual damage to concrete by the formation of ettringite. Durchlicht Light microscopy 5 mm MRFA silicon MRFA potassium MRFA sulfur Figure 13: Result of the microchemical investigations with MRFA 491

10 5. SUMMARY The investigations that were performed exemplarily on a lock structure show that the use of innovative testing techniques at the structure and in the laboratory opens up totally new possibilities in the condition and damage analysis on massive concrete structures. Thus there was shown in the structural investigations that a non-destructive qualitative determination of different degrees of damage in the interior of the lock wall is possible with radar. The secure penetration depth was about 40 cm. The sweep interval of about 10 cm can be recommended for the localization of conspicuous regions, a sweep interval of 20 cm is sufficient for a rough overview. The detailed ultrasonic examinations with a relatively new testing technique also enabled a qualitative analysis of the degree of damage in the interior of the lock wall. Thus the B scans determined comparatively with radar and ultrasonics in the same structural regions match very well. Both investigation methods have the fact in common that their calibration requires the extraction of drilling cores. The use of the innovative testing techniques also proved to be very effective in the determination of the damage causes. Thus not only the cracks but also the decomposition of aggregates, induced by the alkali-silica reaction, could be geometrically visualized on the drilling core by 3D CT. This simplifies the selection of regions for the preparation of polished thin-sections for microscopic and microchemical investigations. The large-surface element distribution images determined with MRFA proved to be very useful for the identification of the damage by alkali-silica reaction. ACKNOWLEDGEMENTS We would like to thank the Federal Institute for Hydraulic Engineering for the assignment of this interesting task, the responsible Patitz Engineering Office for the inclusion of the BAM in this project as well as the Company for Geophysical Investigations, Karlsruhe, for the radar examinations. Furthermore we would like to thank our colleagues for the tomographic measurements as well as for the microscopic and microchemical investigations. REFERENCES [1] Weise, F., Patitz, G. und T. Reschke: Zerstörungsarme baustoffliche Zustandsbewertung von AKR-geschädigten Wasserbauwerken mit innovativen Prüftechniken. In: Tagungsband der Fachtagung des DAfStb Zerstörungsfreie Prüfverfahren und Bauwerksdiagnose im Betonbau, 10./11. März 2005, Berlin. [2] Müller, U.: The micro-xrf - A new technique for the analysis of building materials. In: Construction Technology in Europe 26: (2004) 3-4. [3] Menéndez, E.: Cracking and sulphate attack in field concrete in Spain. In: Scrivener K, Skalny J (eds) Internal Sulfate Attack and Delayed Ettringite Formation, vol. RILEM, Villars, Swizerland, (2002) [4] Owsiak, Z.: Alkali-aggregate reaction in concrete containing high-alkali cement and granite aggregate. Cement and Concrete Research 34 (1): (2004) 7-11 [5] St. John, DA, Poole, AB, Sims, I.: Concrete petrography: a handbook of investigative techniques, vol. John Wiley & Sons, New York, (1998). 492