Generation of Rayleigh waves into mortar and concrete samples.

The paper deals with a non-destructive method for characterizing the degraded cover of concrete structures using high-frequency ultrasound. In a preliminary study, the authors emphasized on the interest of using higher frequency Rayleigh waves (within the 0.2-1 MHz frequency band) for on-site inspection of concrete structures with subsurface damage. The present study represents a continuation of the previous work and aims at optimizing the generation and reception of Rayleigh waves into mortar and concrete be means of wedge transducers. This is performed experimentally by checking the influence of the wedge material and coupling agent on the surface wave parameters. The selection of the best combination wedge/coupling is performed by searching separately for the best wedge material and the best coupling material. Three wedge materials and five coupling agents were tested. For each setup the five parameters obtained from the surface wave measurement i.e. the frequency band, the maximal available central frequency, the group velocity error and its standard deviation and finally the error in velocity dispersion characteristic were investigated and classed as a function of the wedge material and the coupling agent. The selection criteria were chosen so as to minimize the absorption of both materials, the randomness of measurements and the systematic error of the group velocity and of dispersion characteristic. Among the three tested wedge materials, Teflon was found to be the best. The investigation on the coupling agent shows that the gel type materials are the best solutions. The "thick" materials displaying higher viscosity were found as the worst. The results show also that the use of a thin plastic film combined with the coupling agent even increases the bandwidth and decreases the uncertainty of measurements.

NDE of two-layered mortar samples using high-frequency Rayleigh waves.

The Spectral Analysis of Surface Waves (SASW) is a popular technique in seismics for imaging the ground subsurface. It uses the dispersive properties of Rayleigh waves in a transversely homogeneous, multilayered medium. The SASW approach is being transposed into the civil engineering domain to characterize subsurface damage in concrete structures. Such a damage consists in a few millimeters thick surface layer with porosity slightly higher than in the sound material. It is induced by contact with moisture or chemicals at the surface of the structure and may facilitate penetration of aggressive agents. In this study, two-layered mortar samples are made to mimic concrete cover damage in real structures. The dispersive behavior of Rayleigh waves arises when the wavelength is comparable to the thickness of the first layer. Given the small thickness of this layer, it requires increasing the frequency up to several hundreds of kHz, which means high attenuation and low signal-to-noise ratio. Rayleigh waves with 0.5 MHz central frequency are generated into the samples by the wedge method. Phase velocity dispersion curves are obtained by broadband phase spectroscopy from signals received at various distances from the source. The signal processing is first validated on simulated signals with known dispersion law. Then, the measured dispersion curves are compared with the theoretical curve for a two-layered medium, following Haskell's approach. The measured curve displays the general behavior expected from theory. However, a three-layered, visco-elastic model would be necessary to get a better fit and to estimate more accurately the parameters of each layer.

Detection of chemical damage in concrete using ultrasound.

This research deals with a non-destructive method for characterizing the degraded cover of concrete structures using high-frequency ultrasound (0.5-1 MHz). Although such a frequency range is unusual in civil engineering, it is well suited to the kind of defect to be detected, as it corresponds to a thin near-to-surface layer with increased porosity and density of microcracks. In order to assess the feasibility of detecting concrete cover degradation, velocity and attenuation measurements were made on both halves of a concrete slab. One half was immersed into an acid solution for 15-45 days, while the other half remained sound. These measurements were made for longitudinal, transverse and surface waves. The results obtained show a 23% decrease of ultrasonic pulse velocity and a 1000% increase of attenuation in the degraded material relative to the sound material. It is thus possible to detect and characterize concrete cover degradation using high-frequency ultrasound. Although attenuation measurements in heterogeneous media are difficult, their sensitivity to degradation is very high.

Assessment of broadband ultrasonic attenuation measurements in inhomogeneous media.

In this paper, we address the problem of evaluating the acoustic attenuation of "difficult" media, i.e. highly attenuating and scattering media. In a broadband, through transmission setup, the signals acquired from such media are characterized by a poor signal-to-noise ratio. Therefore, an accurate estimate of attenuation cannot be obtained from a single measurement, but multiple measurements must be combined. Two methods are considered to yield a single estimate of attenuation from multiple measurements. The first one, the "average attenuation" (AA) method, consists in a simple average of individual attenuation estimates. The second one, the "cross spectrum" (CS) method, is based on a system identification approach. In order to evaluate the estimation errors for these methods, ultrasonic signals transmitted through a material of known attenuation were simulated and mixed with both coherent and incoherent noise. In all tests performed, the "CS" method was found to yield the most accurate estimate. This method, combined time delay compensation, is then applied to real signals measured from a concrete slab. A valid frequency band for the attenuation estimate can be defined based on the coherence function. Results from this research are being applied to characterize the degradation of concrete structures using high-frequency ultrasound.