Analytische Chemie
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- Ground penetrating radar (4) (entfernen)
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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.
Research and Development in Non-Destructive Testing in Civil Engineering at BAM has de-veloped over more than two decades. Highlights of present research are ultrasound phase array probes, ground penetrating radar applications for the assessment of existing structures and Laser Induced Breakdown Spec-troscopy (LIBS) to determine which chemical elements on the surface of a specimen are. RFID sensor devel-opments for wireless moisture monitoring and data fusion for the combined analysis of sensor data are fast developing areas. In addition, a selection of application cases is briefly described.
We investigated the possibility of surrounding waves
along with perimeter of large diameter tendon ducts during
ground penetrating radar (GPR) testing of concrete structures.
The surrounding waves influence the primary reflection from
tendon duct and alter the phase as well as amplitude of signal
depending on the diameter of the tendon duct. This phenomenon
was studied in detail by conducting several simulations using a
commercial software Reflexw®. Finite Difference Time Domain
method was adopted to analyze reflection patterns from objects
of various diameters located at different depths in concrete
media.
The proposed methodology will be useful in qualitative
assessment of size of metallic objects greater than a diameter of
6 cm. In general, the identification of large diameter tendon ducts
in a civil structure is carried out by prior knowledge of the civil
plan and its diagonal appearance to smaller diameter object
mesh. The proposed methodology will enhance the confidence in
identification of the larger diameter tendon ducts even if the civil
plan is not known accurately.