ABSTRACT

The structural safety, durability and performance of the infrastructure is of primary interest in every country. An efficient system for early and regular structural assessment as well as for quality assurance during and after the construction of new structures and of reconstruction processes is urgently required. At BAM, NDT methods to be applied in civil engineering are developed and their application improved. From the experience of on-site assessments, quality assurance systems and methodologies for regular inspections are elaborated. This work is mainly performed in the frame of externally funded research projects.

1 Introduction

Presently, non-destructive testing (NDT) methods in general are widely used in several industry branches. Aircrafts, nuclear facilities, chemical plants, electronic devices and other safety critical installations are tested regularly and non-destructively requiring fast and reliable testing technologies. Also, as an integral part of quality assurance and quality control implementations, NDT is an indispensable tool. NDT is highly advanced and a variety of methods is available for metallic or composite materials.

In civil engineering NDT methods are still not established for regular inspections. One of the main reasons is that especially methods beyond simple tools like the sounding hammer are not very well known. In recent years, advanced NDT methods like radar, ultrasonics and impact-echo have become available for concrete and masonry structures which can be used for the assessment of existing structures [Ref 1]. But up to now, they are mainly applied by research engineers in few cases mostly for damage assessment.

Like in other areas of applications, NDT methods have a large potential to be part of management systems for infrastructures. This includes quality assurance during and after construction and reconstruction, identification of damages in an early stage in the frame of regular inspections and estimations of need and amount of reconstruction for efficient planning.

At BAM NDT methods are developed and applied for the investigation of structural and material properties of concrete as well as of masonry structures. The strategy is to modify existing technologies from other areas of application (geophysiscs, material testing, medicine, etc) to civil engineering as well as to develop new methods. The primary interest is the on-site applicability of each method. The techniques are optimised and automation is considered where applicable.

On a large variety of structures (e.g. concrete and post-tensioned structures such as bridges, roads, railways, industrial and private housing, historical buildings) diagnostic missions have been carried out with modern, high-performance equipment. Special importance is given to combining the results of different NDT methods [Ref 2, Ref 3, Ref 4].

The development of NDT methods as well as onsite applications are mainly performed in projects externally funded and in close co-operation with partners from industry, public administrations, universities, other research institutions and other BAM departments. In the following, the most important projects are presented concerning

• basic research on the penetration of electromagnetic, sonic and ultrasonic waves through building materials
• development of new NDT methods and system configurations (radar, microwave applications, ultrasonics, impact-echo, active thermography, laser induced breakdown spectroscopy)
• combined and complementary application of NDT methods to infrastructures
• development of fast and on-line data processing and reconstruction software
• development of software for numerical simulation
• automation of NDT methods (on-site scanner)
• development of quality inspection systems
• development of guidelines and recommendations

2 Non-destructive evaluation of concrete structures using acoustic and electromagnetic methods (FOR 384)

For systematic application of NDT methods in civil engineering, further basic research and development is necessary in order to enhance the capacity and the reliability of these methods, also when the site conditions are more complicated. This is the case when the non-prestressed reinforcement is very dense or when multilayer systems have to be investigated (e.g. for non-ballasted railway tracks).

Therefore, BAM participates in a research project sponsored by the German Research Council (Deutsche Forschungsgemeinschaft, DFG) started in 2001, in order to continue basic research and to improve the performance of echo methods for concrete testing [Ref 5, Ref 6]. This research group was founded after a long period of successful co-operation of interested working groups in Germany dealing with ultrasonic echo methods, impact-echo, radar, reconstruction calculation and modelling. In this period, round robin tests [Ref 7, Ref 8] and bridge inspections [Ref 2] funded by the Federal Highway Research Institute (Bundesanstalt für Straßenwesen, BASt) were performed. In the following, some examples of the BAM results are presented.

Ultrasonic impulse-echo
Here, all applied echo methods are based on the time of flight measurement of transmitted and reflected impulses (sonic, ultrasonic and electromagnetic). For ultrasonics, a short electric impulse excites the transducer to radiate an elastic impulse into the specimen. Transducers generating pressure waves (P-waves) are most common, but mode conversion must be considered for each reflection and refraction. In this project, different scanning ultrasonic methods were used and compared:

• scanning across an area with two transducers: a transmitter generating P-waves and a receiver
• application of a transducer array emitting P-waves
• combination of different ultrasonic transducers working as transmitter with a scanning laservibrometer acting as receiver

All methods have been successfully applied for the detection of tendon ducts, reinforcement and inhomogeneities in concrete while the analysis of the grouting condition of tendon ducts is still under work.

Fig 1: B-scan from 3D-SAFT reconstruction of a concrete slab with reinforcement and two artificial flaws. The original data have been recorded with an ultrasonic array.

With these methods, investigations have been performed at a test specimen representing a realistic concrete slab made of reinforced concrete with a maximum gravel size of 32 mm. Behind relatively dense reinforcement (diameter: 25 mm, grid size 12.5 cm), voids simulating honey combing at depth of 10 and 22 cm as well as spherical polystyrene pieces with diameters from 4 to 8 cm could be localised. Figure 1 represents the results obtained with the ultrasonic array after SAFT reconstruction [Ref 9]. Very interesting is the detection of the reinforcement on the far side which enables the determination of concrete cover of reinforcement from the backside. This is an important application for quality assurance of the construction of slabs. For more results on ultrasonics, see the respective papers in this conference [Ref 10, Ref 11].

Impact-Echo
With the impact-echo method a low frequency stress wave is introduced into the structure by hammer impact or steel balls. By that, certain resonance modes of the concrete structure under investigation are excited [Ref 12, Ref 13]. In particular, thickness modes of vibration are primarily used to identify the back-wall or planar flaws. For that, the time domain-signal of displacement or velocity is usually detected a few centimetres beside the impact point. Performing an FFT then leads to significant peaks in the amplitude spectrum which can be associated with the depth of back-wall or flaws if the effective velocity of propagation of longitudinal waves in the structure is known. It is demonstrated that scanning impact-echo testing as first described in [Ref 14] is a significant improvement compared to the usual single point measurements.

An important problem of the interpretation of IE data is concerned with the effects of lateral boundaries of the structure under investigation. These geometrical effects caused by reflections of wave fronts at the outer boundaries of the specimen produce systematic errors in thickness and flaw depth determination. These uncertainties must be taken into account if the measurements are performed at specimens with lateral boundaries lying in the vicinity of the measuring point. For more information on this topic, see the respective paper in this proceedings [Ref 15, Ref 16].

Radar
The radar investigations operating with electromagnetic impulses between 500 MHz and 1.5 GHz were concentrated on the investigation of the influences of reinforcement structure and density and of boundary layers on the reflected impulse characteristics. Measurements in different polarisation configurations of transmitter and receiver have been performed to select the optimum strategy for system configuration for different problems [Ref 17].

Furthermore the requirements of the combinations of radar, ultrasonic and impact-echo were designed. The features resulting from fusing different 3-dimensional datasets have been demonstrated. For more information in this topic, see [Ref 18].

3 Application of impulse-thermography for structural and moisture investigation close to the surface of building elements

Impulse-thermography is an active method for quantitative investigation of the near surface region of various structures and elements. It is well known for material testing in other industry branches like aerospace industry (graphite-epoxy structures) and aluminium industry (aluminium laminates) [Ref 19, Ref 20]. Within the scope of a national funded project sponsored by the German Research Council (Deutsche Forschungsgemeinschaft, DFG) the possible applications of impulse-thermography in civil engineering are analysed. The technique of heating up the surfaces and observing the cooling down process is intended to detect near surface inhomogeneities in building elements, normally defects, and to determine their geometrical and material parameters. Thus, impulse-thermography will be an addition of other NDT methods for volumetric investigation like radar and ultrasonic having high sensitivity and resolution in the surface near region.

For quantitative analysis of experimental data, a computer program for numerical simulation of the heating up and cooling down processes was developed based on Finite Differences. With this program parameter studies have been performed for investigating the influence of environmental conditions, material parameters and geometry on the thermal behaviour. The comparison between experimental and simulated results enables the Inverse Solution [Ref 21].

Figure 2 shows two thermograms of experimental data of a concrete test specimen with tiles recorded 50 s and 300 s after a heating time of 12 min. The grey values of the images were scaled to minimum and maximum temperature in each image. The shallow small delaminations behind the tiles have good contrast after short cooling time (50 s) while the deeper large delaminations behind the mortar appear after a cooling time of 300 s.

Fig 2: Thermograms of a concrete test specimen with tiles in a mortar bed including delaminations behind the tiles (small) and behind the mortar (large) recorded after a heating time of 12 min; left: cooling time 50 s; right: cooling time 300 s.

Fig 3: Thermograms of a concrete test specimen including voids at depths from 1 to 10 cm recorded after a heating time of 10 min; left: cooling time 9 min; right: cooling time 58 min.

Thermograms of a concrete test specimen containing voids at depth between 1 and 10 cm are displayed in figure 3. The shallow voids have good contrast after short cooling time (9 min) while the deeper voids appear after a cooling time of 58 min.

For quantitative analysis, transient curves (surface temperature as a function of time for each pixel) from areas above voids and above homogeneous material were compared and difference curves were calculated. These difference curves usually have a maximum DT_max at a distinct time t_max which depends on the depth of the voids and on the heating time [Ref 22].

Related to the optimisation of the graphical presentation advanced analytical and/or empirical functions will be tested for fitting the transient curves. The analysis of the thermal experimental data will be enhanced by the methods used for pulse phase thermography (PPT) [Ref 23, Ref 24, Ref 25].

4 On-site investigation techniques for the structural evaluation of historic masonry buildings (ONSITEFORMASONRY)

In this project, BAM is developing ultrasonics and impact-echo for the application to inhomogeneous masonry.

Ultrasonics
The interpretation of ultrasonic impulse-echo investigations made on masonry structures is very difficult due to the inhomogeneous material leading to several reflections and refractions. But successful application to concrete structures in the past by using low frequency broad band transducers and transducer arrays [Ref 7, Ref 29] will also be transferable to masonry structures. For adapting ultrasonic impulse-echo and tomography to masonry in ONSITEFORMASONRY, investigations have already been performed by BAM to develop a transducer array taking into account coupling with and without agent. For the selected transducers, the optimised electric impulse will be chosen. Tomographic investigations will be performed focusing on the visualisation of areas having different ultrasonic velocities. For this, time of flight measurements are required and an algorithm for automatic determination of impulse arrival will be developed.

Impact-echo
In the project, measurements with different impactors and sensors have been obtained by BAM at a masonry test specimen and on-site as well. As a real structure, the exterior walls of a historic church (Kirche am Neuendorfer Anger) were investigated (see figure 4 left). This church was built from 1850 to 1852. But since 1899 it was used as storage depot. Few damages were obtained during the 2^nd World War. Since 1999, the church is being reconstructed by the support of a citizens initiative. The measurements were performed with the commercial Olson equipment along several traces at the inner surface of the exterior walls. In figure 4 right, the respective impact-echogram (presentation of the position of the impactor on the surface above the frequency response) is presented showing a very clear reflection at 0.97 kHz which corresponds to the backside at a depth of 1 m. Therefore, in this case the required penetration depth was reached. No other reflections can be seen which might be due to a very homogeneous masonry structure.

Fig 4: Left: Investigations of the outer walls inside the church "Am Neuendorfer Anger" with impact-echo and radar. Right: Impact-echogram as presentation of the position of the impactor on the surface above the frequency response. The reflection from the backside can be observed at a frequency of 0.97 kHz which corresponds to a thickness of about 1 m.

Fig 5: Left: Plan view of the historic masonry specimen at BAM including different masonry structures and inhomogeneities. Right: Position of voids in the test specimen simulated by ceramic vases.

For the validation of the developed methodologies, investigations on test specimen as well as on real historic sites are planned. For these calibration activities, a historic test specimen has been built at BAM in close cooperation with Stiftung Luthergedenkstätten (Wittenberg / Germany), Institut für Diagnostik und Konservierung an Denkmalen in Sachsen und Sachsen-Anhalt e. V. (Halle / Germany) and Institut für angewandte Forschung im Bauwesen (Berlin / Germany). This specimen with the dimensions 7 m x 3 m x 1.5 m has been planned and constructed in consideration of traditional manufacturing techniques, partly using historical materials, stemming from demolitioned buildings (see plan view in figure 5 left). It represents a large variety of problems and characteristics of real historic masonry (mixed masonry, multi-leafed walls, hidden inclusions, cracks, voids, etc.). Each of the characteristics or properties of this Historic Masonry Specimen, which has been erected by a very experienced building company specialised in the restoration of cultural heritage buildings, is known in detail (e. g. see the implementation of voids by ceramic vases in figure 5 right). This specimen is in a way the link between usual masonry specimen and real historic masonry buildings and will enable the validation and calibration of the investigation techniques.

5 Case studies

Location of tendon ducts in a prestressed concrete bridge
For calculating the load carrying capacity of a prestressed concrete bridge the longitudinal tendon ducts had to be investigated related to corrosion. Therefore, the tendon ducts had to be located, small holes had to be drilled and the inner condition of the ducts was then investigated using an endoscope. The tendon ducts were located by radar using the 1.5 GHz and 900 MHz antennas.

Four radar traces were recorded from the underside of the bridge as shown in the cross section in figure 2. Radar traces 1 and 2 as well as 3 and 4 were parallel with a distance of about 1.5 m.

Figure 6 right shows two radargrams of trace no. 4 recorded with the 1.5 GHz (top) and the 900 MHz antennas (bottom). In both radargrams, three hyperbolas are shown which can be related to three longitudinal tendon ducts O1 to O3. From these hyperbolas, the lateral position as well as the depth of the tendon ducts can be calculated. Further small hyperbolas can be related to reinforcing bars close to the surface (3 to 5 cm). At a depth from 12 to 35 cm, the reflection from the surface of the bridge plate can be recognised. This reflection corresponds to the bridge profile as depicted in figure 6 left.

Fig 6: Left: Cross section of the bridge with the four traces. Right: Radargram of trace no. 4, recorded with the 1.5 GHz antenna (top) and the 900 MHz antenna (bottom). O1 to O3 are the reflections from the longitudinal tendon ducts, B is the reflection from the surface of the bridge plate.

Inspection of the girder of a concrete bridge
Practical bridge inspections have been performed in co-operation with the Federal Highway Research Institute (BASt) and other groups [Ref 29, Ref 9, Ref 30]. This section reports on selected results of a research study at a girder of a post tensioned highway bridge [Ref 30]. The experiments were carried out with scanning impact-echo and ultrasonic echo using synthetic aperture and 3D reconstruction calculation (Fraunhofer IZFP). The results of non-destructive testing have been verified by taking cores.

Figure 7 left shows the sketch of a part of the girder with the localisation of three post-tensioned ducts (numbered 1, 2, 3) by means of impact-echo and the position of the cores taken after the evaluation of the results. In figure 7 right the impact-echogram of line B is presented showing the back wall of the girder and direct multireflection signals from the three ducts, from which the location and the concrete cover can be read. Some additional features (which are explained with the condition of the tendon ducts) are clearly visible: The frequency shift of the direct multireflection, the shift of the back wall echo and a kind of side bands near the main reflection peak (for duct 1 and 2).

Fig 7: Left: Localisation of the ducts at the bridge girder with impact-echo, location of cores and position of two lines scanned with impact-echo. Right: Impact-echograms obtained at the bridge girder at x = 13.30 m, corresponding to line A. No. 1, 2, 3 indicate the imaging of the corresponding ducts. The intensity of the echogram has been normalised to the back wall intensity.

The analysis of the cores shows a laterally placed grouting error at duct 1 in core 7 to 9 whereas duct 2 is well grouted with some delaminations appearing in core 5. This and similar results show, that the criteria to indicate badly grouted areas of tendon ducts described in literature [Ref 13] are fulfilled only in some cases and new empiric criteria can be found, which are not yet completely understood [Ref 31]. In order to investigate this thoroughly a scanning impact-echo system seems to be essential.

Using the ultrasonic 3D-method (transducer array which is displaced along the duct axis of duct 2 (figure 8 left)), the latter is imaged as shown in figure 8 right. For this representation, the reflection intensity is normalised to the back wall intensity in order to avoid misinterpretation from the inevitable change of the coupling conditions from point to point. The high reflection intensity at the right end is proved by the delaminations and small voids found in the cores 5 and 6. The very good grouting condition indicated from the low reflection intensity in the middle of the duct was verified by core 4 (including X-ray tomography). At the left side, the high reflection intensity overestimates the existing delaminations.

Fig 8: Left: Ultrasonic array in use at the bridge girder (ultrasonic 3D). Right: C-scan representation of imaging of duct 2 in the bridge girder (ultrasonic 3D) compared to localisation with radar. The areas of high reflection intensity (dark areas) indicate delaminations and small voids (see text).

The results of the impact-echo and ultrasonic measurements have made clear that the non-destructive methods can successfully be used for the condition assessment of concrete members. The measurement results can be converted into condition ratings, according to the German guidelines for bridge inspection. Therefore non-destructive testing plays an important role in the framework of maintenance management.

6 Development of an innovative ground penetrating radar for recognition and identification of subsurface buried objects (SMART RAD)

Within the project Smart-RAD an impulse radar for utility finding is developed by five European project partners. BAM is developing the antenna for the project. The project is financially supported by the EU in the Fifth Framework Program Competitive and Sustainable Growth (GROWTH).

The Smart-RAD project consortium has focused its efforts to the area of utility finding and has identified the main limitations of existing systems as:

• Unreliable detection confidence pending on ground and underground conditions
• Coarse accuracy in the localisation of detected buried objects
• Unavailability of an efficient and cost-effective survey technique
• Difficult on-field radar data interpretation, today only restrained to expert operators

A main point which motivates the Smart-RAD project is that the cost for damages on risky lines are about 50 million EURO per year only in Europe. Impulse radar has shown good results but the market is limited due to the system and service cost and user acceptance. Another point for the market limitation is that radar is used for buried object detection in a depth below 1 m. But the need of utility companies is to detect pipes and tubes of risky lines (gas, power and telecommunication) in depth up to 5 m. Before utility companies are doing excavation work they need a subsurface map showing the presence and accurate location of the lines. So beside the hardware development to improve the radar performance the project consortium will also develop algorithms to interpret the conventional radargrams (B or C scan) of existing systems into the desired utility map.

In a sub-project, BAM is developing a new antenna which is faced to the difficult requirements of utility finding. The two main objectives are to maximise the transmitted power to achieve maximum penetration depth and to minimise the distortion of the transmitting and receiving impulse to increase the accuracy in localisation of detected objects [Ref 32, Ref 33].

7 Re-use of foundation on urban sites (RUFUS)

Todays city centres are full of foundations. Many of them are not older than 30 years and may be reused for new constructions. The EC funded project RUFUS looks for methods to investigate and assess used piles and slabs. Eight partners from five countries are involved [Ref 34].

The focus of the workpackage Materials is on NDT. Several surface and borehole methods are taken into account: surface and borehole radar, ultrasonics, impact-echo, parallel seismics and low strain pile integrity testing. The latter is a NDT method which provides information about the condition of foundations. It is also used as a tool for quality control. A real size test field with sound piles and piles with flaws which will allow calibration of measurement devices and method comparison under real conditions is planned on a BAM testing area at Horstwalde in the south of Berlin [Ref 35].

8 Sustainable improvement in safety of tailings facilities (TAILSAFE)

Tailings are the fine residue of the milling process in the mining industry and appear in slurry form being mixed with water during this process. Large tailings ponds are required to contain them, usually confined by man-made dams. Such tailings facilities pose considerable risk both to the environment and human lives.

A major interdisciplinary research project (TAILSAFE) supported by the European Commission has been initiated with the aim to increase attention towards and reduce the risk posed by tailings facilities [Ref 36, Ref 37]. Methods of parameter evaluation and measurement are being developed within the project and applied for the detection, assessment and improvement of the safety state of tailings dams and ponds.

One of the workpackages has its focus on non-destructive geophysical investigation methods. Geoelectrical (SIP, [Ref 38]), seismic and radar methods will be used to get information on the tailings dam structure and water content.

Figure 9 shows the result of geoelectrical measurements at a tailings pond site in Saxony (Germany). The dark zone in the centre represents the fine part of the tailings. It is covered by coarser material. The underlying crystalline rock (gneiss) is marked by a sharp resistivity increase. The right dam seams to be upstream type as it has low resistivities (fine tailings) beneath 5 to 10 m cover. In the TAILSAFE project the resolution and the interpretation methods will be improved.

Fig 9: Geoelectrical cross-section trough tailing pond. Low resistivities (dark) correspond to fine tailings, high resistivities (bright) to cover (top), dams (left and right) and bedrock (bottom). Data courtesy of Büro für Geophysik Lorenz and BGR.

9 Development of a quality assurance system for fixed railway track beds

In recent years, high-speed railway lines have been constructed in the form of non-ballasted slab-tracks instead of the traditional tracks consisting of concrete sleepers in a bed of ballast. Ballasted tracks are generally considered economic in construction but more expensive in maintenance due to tamping and re-lining up after a certain amount of accumulated axle load tonnage [Ref 39]. With the aim of achieving high-quality track construction work, of developing tools for quality assurance in new construction and for later technical inspection of material condition, a quality strategy has to be developed. For these purposes, BAM has carried out in-situ studies in co-operation with Deutsche Bahn AG (the German Railway national company) using ultrasonic-echo, impact-echo and radar methods on different slab-track constructions [Ref 3, Ref 40].

10 NDT-CE Compendium

The NDT-CE Compendium [Ref 41, Ref 42] is a compilation of the descriptions of 115 methods for non-destructive testing in civil engineering. It is freely accessible on the internet and describes each method within the frame of: short description, classification, characterisation, application, valuation, literature, addresses of distributors and manufacturers and www-links. Additional information is given on handbooks and standards in the area of NDT-CE. The compendium is written in German. An English translation is planned. Any contributions from the readers are welcome.

11 Conclusion and outlook

The described research projects to be performed by BAM clearly demonstrates the wide variety of applications of NDT methodologies and the need for the development of quality assurance systems and of guidelines and recommendations. For exploitation, BAM is working in close co-operation with the industry and is organising periodic national and international conferences (Feuchtetag (Moisture Day), Bauwerksdiagnose (building diagnosis), NDT-CE). For standardisation, BAM is involved in DIN , Committees of the DGZfP (German Society for Non-destructive Testing) and RILEM Committees.

ACKNOWLEDGEMENTS

The presented projects are or have been founded by the European Commission, DFG, Deutsche Bahn AG, Bundeanstalt für Straßenwesen (BASt) and Senator für Stadtentwicklung Berlin. The research has been performed in close team work of all colleagues from BAM IV.4

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33. Kind, T., Krause, I. and Maierhofer, C., 2003, Development of an utility finding impulse radar, this conference
34. http://www.webforum.com/rufus.
35. Niederleithinger, E. and Taffe, A., 2003, Concept for a Reference pile testing site for the development and improvement of NDT-CE, this conference
36. http://www.tailsafe.com.
37. .Niederleithinger, E., McDonald, C., Meggyes, T., Roehl, K. E. and Witt, K. J., 2003, TAILSAFE: Investigation and improvement of tailings facilities, this conference
38. Niederleithinger, E., 2003, Spectral Induced Polarization - a tool for non-destructive testing of soils and building materials, this conference
39. Münchschwander, P., Heinisch, R., Kracke, R., Lehmann, E., Rahn, T. and Stuchly, H., 1997, Feste Fahrbahn. Edition ETR, Darmstadt, Germany.
40. Gardei, A., Mittag, K. and Wiggenhauser, H., 2003, Process development for the quality assurance of concrete-embedded tracks using non-destructive testing methods, this conference
41. www.bam.de/zfpbau-kompendium.htm
42. Wiggenhauser, H. and Borchardt, K., 2003, The NDT-CE compendium on the internet, this conference