Assessment of deteriorated concrete cover by high frequency ultrasonic waves: Fid-Bau Portal

Assessment of deteriorated concrete cover by high frequency ultrasonic waves

Fnine, A. / Buyle-Bodin, F. / Goueygou, M. / Piwakowski, B.

The paper presents an ultrasonic method for characterizing the degradation stage of the cover concrete of structures using high-frequency ultrasonic waves (0.5 to 1 MHz). A first sensitivity study was conducted in our laboratory. Mortar and concrete samples were submitted to chemical degradation by immersion into an ammonium nitrate solution for periods of 15, 30 and 45 days. The stage of degradation of material has been characterized by ultrasonic velocity and attenuation, and also by porosity and mechanical measurements. The results of the different measurements are well related. The porosity of the degraded layer seems to be approximately constant. The broadband ultrasonic attenuation is found to be the most sensitive to evaluate degradation stage, especially at high frequency. The present goal of the research is to develop an on site measuring apparatus. A special device is developed to characterise concrete cover by using dispersion and attenuation characteristics of surface waves. This device is at present validated by measurements on laboratory samples, composed of two-layer degraded/sound concrete or mortar. The results enable us to assess the sensitivity of the device to evaluate the degraded layer depth and the rate of degradation. Further measurements will be performed in the conditions i.e. on site bridges and walls. The wedge teflon/coupling gelD combination seems to be the best choice, giving the best results in all the configuration. This combination allows the differenciation between sound and degraded material. The surface wave velocity decrease with degradation. Therefore, this combination has the advantage to get the lowest absorption. At frequency 0.5Mhz and 1Mhz, velocities are rather close for degraded and two-layer samples. The tests on the two-layer samples show that the velocities decrease when the thickness of the first layer increase. At 0.5Mhz and 1Mhz, the velocities are rather close. In situ, the difference between the older (assumed damaged) span and the recent (assumed healthy) is visible. More of the in-situ results were different of our waitings : high frequency seems to be sensitive to damage and degradation. The surface waves seem to be into the coating.