Measurement of ultrasonic attenuation and Rayleigh wave dispersion for testing concrete with subsurface damage

This paper describes a non-destructive method for characterizing the cover of concrete structures using high-frequency ultrasound (0.5 to 1 MHz). Although this frequency range is unusual for such a material, it is well suited to the kind of defect to be detected, i.e. a thin damaged subsurface layer. This research has been carried out in three directions: (i) characterization of concrete samples through measurement of porosity and ultrasonic pulse velocity, (ii) adequate signal processing to access the extremely high attenuation to be measured and (iii) testing of concrete samples using Rayleigh waves. The tested samples have been submitted to chemical degradation. Measured velocities of compression and shear waves are used to derive estimates of elastic moduli. Porosity measurements have also been performed, showing that the observed velocity and stiffness decrease are related to an increase of the damaged layer thickness, not to an increase of porosity in this layer. Furthermore, a frequency domain system identification approach is used to derive an estimate of ultrasonic attenuation from multiple transmitted signals. Finally, high-frequency Rayleigh waves are generated into mortar samples by the wedge method. Several transducer/wedge combinations are tested and the optimal configuration is used to yield dispersion curves. Rayleigh wave dispersion evolves as expected with increasing damage and gives access to depth-dependent characteristics of the degraded layer.