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Eingeladener Vortrag
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Die drei Beispiele zerstörungsfreier Prüfungen an Spannbetonbrücken zeigen unterschiedliche Problemstellungen und deren Lösung mit unterschiedlichen Ansätzen.
Allen drei Beispielen gemeinsam ist die Verwendung kommerzieller Geräte. Diese werden von verschiedenen Anbietern bereitgestellt. Eine weitere Gemeinsamkeit ist, dass die Grundgeräte durch zusätzliche Maßnahmen ergänzt. Erst dann ist der gewünschte Untersuchungserfolg sicher gestellt.
Im ersten Beispiel wurde die vom Auftraggeber geforderte Präzision dadurch erreicht, dass systematische Voruntersuchungen stattfanden. Die Übertragung der Lokalisierung direkt auf die Betonoberfläche stellte sicher, dass keinen Informationsverluste an die nachfolgenden Aufgaben auftraten.
Das zweite Beispiel zeigt den Nutzen von Automatisierung und Kombination von Verfahren. Damit können dem Auftraggeber die notwendigen Angaben über Einbauteile mit großer Detaitreue und leicht verständlicher Visualisierung aufbereitet werden. Der abschließende Bericht zeigt, wie vorhandene Auswertungen sinnvoll durch noch in der Forschung befindliche Ansätze bereichert werden.
Insgesamt ist die zerstörungsfreie Prüfung in der Lage, an Brücken preisrelevante Fragestellungen zu lösen. Voraussetzung hierfür ist eine sorgfältig geplante Vorgehensweise und ein sicheres Verständnis der Grundlage der eingesetzten Geräte und Auswertungsschritte.
Collecting different geophysical data sets at the same object and site offers the opportunity to reduce uncertainties and ambiguities in data analysis and interpretation. To be effective, the different available data sets should be linked during the model-generation process, e.g. by cooperative inversion. In this study, we apply a recently developed zonal cooperative inversion approach based on fuzzy c-means cluster analysis to a non-destructive testing experiment. After briefly reviewing the fundamentals of the inversion strategy, we present a synthetic study investigating the potential of the method to detect air-filled voids in masonry by using ultrasonic and georadar traveltime data. Then, we present and discuss laboratory experiments including the results of cooperatively inverted ultrasonic and georadar traveltimes collected at a masonry test specimen. The geometry of the specimen is known and is thus an ideal test object for a first-time real application of the novel zonal cooperative inversion procedure. Compared to the results of separate inversions of ultrasonic and georadar traveltimes, the zonal cooperative inversion allows for an improved delineation of the size and position of the cavities. The P-wave and georadar velocities determined for the model regions corresponding to the cavities are also improved.
Non-destructive tests (NDT) are an essential tool used in special inspections to gather detailed information about the condition of a bridge. The inspection of bridge decks is a critical task, and, currently, can be successfully carried out using a wide range of NDT techniques. Nevertheless, some of these techniques are excessively expensive and time consuming. One of these techniques, the ground penetrating radar (GPR), has been used for some decades in the non-destructive inspection and diagnosis of concrete bridges. GPR is useful to find general information about the true position of reinforcement and tendon ducts, and check the quality of the construction and materials. A significant number of reinforced and prestressed concrete bridges are deteriorating at a rapid rate and need to be repaired and strengthened. During these rehabilitation processes, designers are often faced with a lack of original design plans and unawareness of the real position of reinforcement and tendon ducts. In this paper, three case studies of the use of GPR techniques for the inspection of concrete bridges are presented and analysed. The main aim of this research is to show the strong need and usefulness of these techniques, which can provide non-visible information about structural geometry and integrity required for strengthening and rehabilitation purposes.
Moisture ingress is one of major damaging factors for masonry buildings. As the complex resistivity (CR) is sensitive to textural properties as well as to the pore fluid chemistry of wet porous media, its non-destructive application can provide helpful information for conservators. In a comprehensive laboratory study it has been shown that CR might even be able to distinguish between salt content and saturation degree in only one measurement. The combined use of electrical and electromagnetic measurement techniques in two field-scale flooding experiments has shown some unexpected differences. Possible reasons are discussed and it is shown that bringing together the information of both methods leads to a clearer picture.
Time slices of very dense GPR measurements carried out at concrete with a real point distance of less than 10 mm in x and y direction show weak reflection patterns besides the strong reflections of reinforcements. A repetition of these measurements with the same local geometrical precision at the same specimen shows the same reflection pattem. We suppose that the heterogeneity of concrete given by aggregates (e.g. gravel, broken granite) causes local weak Scattering of the GPR waves and leads to reflection patterns in the GPR data. These reflection patterns can be explained by a Superposition of multiple scatterings of single aggregates. So far these reflection patterns have been not recognized in Standard time slices due to the large spacing (> 5 cm) between single profiles and because of the interpolation between the profiles.
We investigate the characteristics of the volume scattering effects, caused by aggregates, at concrete blocks with two different grading curves. In a more general investigation we study the volume scattering of aggregates by using wooden boxes filled with only typical aggregates and an included defined reflector (metal plate). The thickness of the aggregate layer above the metal plate was varied between 20 cm and 40 cm. With a very dense measuring grid at the surface we are able to calculate the effective volume scattering and to analyze the distribution of the reflection amplitude of the included metal plate for different aggregate set-ups. Results of this study confirm that the weak reflection patterns in concrete can be explained by aggregate scattering and have a direct correlation to the penetration depth of GPR in concrete.
Air voids in concrete like honeycombs are one of the major concerns regarding quality assurance for the construction of infrastructure buildings like bridges or tunnels. This paper shows that voids in reinforced concrete walls or slabs can be detected by two standard ground coupled GPR antennas in transmission mode using a two-side zero-offset profiling configuration like in borehole radar applications. For the detection of voids only the amplitudes of the direct wave in transmission mode are evaluated. Even when the depth of the void can not be detected in this configuration, the major advantage of the zero-offset profiling in transmission mode is a lower interference with the surrounding reinforcement compared to a regular one-side reflection profiling. The capability of the two-side zero-offset profiling is demonstrated on a test specimen with a set of polystyrene balls of two different sizes representing voids like honeycombs in concrete. GPR measurements are realized by an automated scanning system in order to allow for a synchronous movement of the antennas. In transmission mode voids can be detected at greater depths compared to reflection mode, since the travel path of the direct wave is half as long. Another characteristic of the transmission mode is that the direct wave through the air voids is faster and arrives earlier than the direct wave in the surrounding concrete. Hence it can be separated from the strong reflection of the reinforcement. Finally the measurements in transmission mode are less sensitive to the antenna frequency and polarization than in reflection mode.