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Ultrasound sensors should be embedded into concrete for monitoring concrete properties. These new longitudinal wave sensors with a center frequency of 60 kHz were examined regarding their suitability for ultrasonic measurements in concrete structures in terms of emission characteristics, sensitivity and frequency ränge.
For the measurement of the radiation patterns, the sensors were embedded vertically and horizontally in concrete cylinders. The directivity pattern was measured using a laser vibrometer.
The sensitivity of the sensor was determined in water using different sensors of the same type. It shows changes in the signal amplitude as well as variations in the frequency ränge for different transmitter-receiver combinations.
The attenuation of the concrete affects the achievable resolution of the measurements and thus, the maximum possible spacing of the sensors within a concrete element. Experimental tests helped optimizing the distances with respect to the required resolution and the effort of embedding the sensors. The signal attenuation in the concrete was measured in the frequency ränge of 60 kHz in response to various degrees of reinforcement and grain size. For this purpose, the sensors were cast at different distances in the specimens studied. The recorded Signals were evaluated for their amplitude and frequency spectrum.
The impact-echo method has been successfully applied to identify defects inside concrete. In addition, to detect ungrouted tendon ducts in a large concrete slab, a scanning impact-echo technique is developed. However, since resonant frequencies in the spectrum responsible for the travel paths via defects are only taken into account, the method could lead to erroneous results due to complicated spectra obtained in the tests. Consequently, Stack Imaging of spectral amplitudes Based on Impact-Echo (SIBIE) procedure has been developed to improve the data interpretation. Conventionally, SIBIE is applied to a single measurement data and a point information of defects is obtained at the area, where the impact test is performed. In this study, SIBIE is applied to scanning impact-echo data. Locations of ungrouted tendon ducts embedded in a large concrete specimen are investigated. In order to visualize the whole cross-section tested, the SIBIE analysis is modified, introducing an elliptical integration mode. It is demonstrated that ungrouted tendon ducts are successfully located by the modified SIBIE analysis, whereas results of the conventional B-scan analysis are not so good as the modified SIBIE analysis.