Five pairs of embedded piezoelectric transducers have been installed in the ceiling of the Rogier tunnel in Brussels in June 2019 in order to follow the state of health of concrete repairs.
During 18 months, ultrasonic measurements have been performed continuously with the five pairs of emitters-receivers. Although the measurements have been shown to be sensitive to environmental variations (change of temperature and humidity in the tunnel), we have developed very efficient techniques to filter out these effects  in order to avoid false alarms.
Thanks to these new advances in signal processing, the damage indicators that are computed from the measured ultrasonic waves are made very robust to environmental changes. We have been able to establish statistical threshold for detection of abnormalities in the concrete.
The monitoring system allowed to follow the evolution of the setting and hardening of the repair mortar, and to confirm that the adhesion to the old concrete was excellent. After 18 months, the system is still running and the transducers are still functioning perfectly. Daily reports are sent to our research team at ULB-BATir which confirm that there is no degradation of the repairs on the long-run.