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- 2014 (4) (entfernen)
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- Aggregates (1)
- CFRP (1)
- Concrete bridges (1)
- Construction materials (1)
- Dual-polarization (1)
- Durability (1)
- Ground penetrating radar (GPR) (1)
- Inspection (1)
- Non-destructive testing (1)
- Short time-Fourier transform (STFT) (1)
Trans-European corridors for transport of people and goods require strengthening of bridges to carry increased axle loads in terms of absolute axle loads and of their frequency. Most of the bridges along highways are reinforced or prestressed concrete bridges.
Since the late 1990s, CFRP laminates have been applied to box girder and T-beam bridges to increase the overall capacity. Although regular inspections are carried out, no special method was available to test the bond condition of CFRP laminates after being in Service for more than ten years. The German Center of Competence in Civil Engineering is aware of this need and funded a research project to investigate the bond condition by means of advanced non-destructive testing (NDT).
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.
This paper studies the spatial and temporal spectral absorption of reflector signals of a 1.5 GHz ground penetrating radar (GPR) during a drying process of a brickwall from initial wet to later dry state. The non-stationary GPR signals were processed with short time-Fourier transform (STFT) and wavelet transform (WT) in a novel spatial-timefrequency (STF) domain. Spatial distribution of peak frequency at the direct wave (DW) across the antenna and a backwall reflection was studied to characterize the mechanism of spectral absorption of GPR wave. Results from WT were shown to be more preferred to those from STFT because the WT offers multiple resolutions to cope with both low and high frequency components in GPR wavelets but STFT does not. In addition to the traditional GPR signal interpretation in time-domain and our previous works on time–frequency domain, the analysis method operated in the STF domain provides another possibility of material characterization by GPR in large and field scale.
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.