Low frequency coded waveform for the inspection of concrete structures

Low frequency piezocomposite transducers have been used in combination with pulse compression methods to penetrate 35 cm into concrete-type materials. A chirp signal with a frequency range 100 - 200 kHz and of 10 ms duration was used to excite a pair of 54 mm diameter piezocomposite transducers in patch-catch mode. The centre frequency of the transducers was 170 kHz. Using this arrangement, good signals could be obtained, but there was a problem with variability due to the high degree of scattering - even small changes in transducer location changed the measurement. Thus, an additional signal processing method was introduced, whereby the total energy reaching the receiver was calculated as a function of time from the cross-correlation outputs.