Embedded Ultrasound Sensors for Monitoring Concrete Structures

Ultrasound sensors can be embedded into concrete for monitoring concrete properties. 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 range. For the measurement of the radiation patterns, the sensors were embedded in concrete cylinders parallel to the symmetry axis and orthogonal to it. 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 range 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 range of 55 kHz and 70 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. A case study was carried out at a concrete bridge in Izmir/Turkey to demonstrate the possibilities of an embedded ultrasound sensor network. This contribution briefly describes the installation of the network and the measurements. This installation was carried out in the frame of a research project.

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