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- 2015 (7) (entfernen)
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- Ground penetrating radar (3)
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Eingeladener Vortrag
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Das Radarverfahren ist ein schnelles bildgebendes Verfahren für die Untersuchung der inneren Struktur von Stahlbeton-Bauwerken und wird seit längerem erfolgreich für die zerstörungsfreie Untersuchung von Brückenbauwerken eingesetzt. Für die Rekonstruktion der Lage der Bewehrung reicht die Annahme einer homogenen Materialeigenschaft des Betons meistens aus.
Nähere Untersuchungen der empfangenen Streuungen der Radarwellen zeigen, dass neben der starken Streuung der Radarwellen an den metallischen Einbauteilen auch schwache Streuungen existieren. Diese schwachen Streuungen im Beton wurden nun erstmals in dieser Arbeit untersucht und können der Heterogenität des Betons zugeordnet werden. Die schwachen Streuungen entstehen durch den Permittivitätskontrast und die Geometrie der Gesteinskörnung im umgebenden Zementstein des Betons.
Im Rahmen dieser Arbeit wurden die schwachen Streuungen im Beton für unterschiedliche Gesteinskörnungen charakterisiert und der Einfluss des Wellenlängenbereichs der Radarwellen auf die Ergebnisse untersucht. Weiter zeigt die Arbeit, dass die Streuungen an der Gesteinskörnung einen wesentlichen Einfluss auf die maximale Eindringtiefe von Radarwellen in Beton haben.
Detection of air voids in concrete by radar in transmission mode by radar in transmission mode
(2015)
Amplitudes Variation of GPR Rebar Reflection Due to the Influence of Concrete Aggregate Scattering
(2015)
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.
The concreting of prefabricated concrete structures can lead to insufficient bonding or even to remaining cavities. Honeycombs (aggregate clusters without cement) represent potential weakening of the structure and need to be detected non-destructively. In our study we tested the capability of ground penetrating radar (GPR)techniques for this purpose. We applied GPR in reflection mode and zero-offset profiling (ZOP) Transmission mode on a precast concrete twin wall with built-in honeycombs. GPR measurements were performed as twochannel measurement with ground coupled antennas with centre frequencies of 1.5 GHz and 2.6 GHz mounted to an automated scanner system.
Our findings show that ZOP transmission measurements are a more efficient method to detect voids in reinforced concrete structures compared to reflection mode measurements. This holds for both the effort needed for the measurement and the evaluation as well as the validity of the data. Honeycombs (basically representing voids) are usually characterized by strongly reduced amplitudes and earlier arrivals of the transmitted wave.
Amplitudes variation of GPR rebar reflection due to the influence of concrete aggregate scattering
(2015)
Dense GPR measurements of rebar reflection amplitudes Show relative variations, which can be in the order of more than 10% - 20%. Former investigations demonstrated that these variations are caused by the heterogeneity of concrete, i.e. due to the inclusion of aggregates in concrete. These amplitude variations make it difficult to analyse single reflection amplitudes in order to determine the rebar diameter or to estimate the concrete deterioration state. In a systematic study we have quantified the statistical variation of the rebar reflection amplitude for concrete covers of 6 cm, 9 cm, 12 cm, 15 cm and 18 cm, for two different grading curves and for the rebar diameters 12 mm and 28 mm. Also the influence of the wavelength has been investigated by using antennas with different centre frequencies in relation to the aggregate size. The results are discussed with regard to a quantitative amplitude evaluation of GPR measurements and also the potential of using these variations for a characterization of concrete material properties.