This paper deals with an innovative technique, which improves the pulse echo testing of concrete by using
specially designed ultrasonic probes
pulse compression technique
ultrasonic wave detection by laser doppler interferometer
random speckle modulation
space time signal processing methods
The pulse echo technique is carried out by sending frequency modulated chirp signals and performing a cross correlation between the received and the transmitted signal. In combination with the application of recently available ultrasonic concrete probes as transmitter, this leads to an improvement of the signal to noise ratio. A laser doppler interferometer, equipped with a random speckle modulator, is used as detector of the ultrasound. Finally, the data sets can be processed with various methods, involving the time signals of several space points. Examples are the space averaging and the synthetic aperture focussing technique (SAFT).
The advantage of the suggested technique is demonstrated by practical measurements. The improvement compared to standard laser interferometric measurements will increase the feasibility of laser interferometric detection for non-destructive testing in civil engineering.