Surface Wave Testing of Pavements
Abstract: A novel approach for surface wave testing of pavements is presented. It is a non-destructive testing (NDT) technique that can be used to obtain the thickness and stiffness properties of the different layers in a pavement. With this method structural properties of the pavement can be mapped as a function of time and space, providing a valuable tool in pavement design and management. The technical development is based on a theoretical study of wave propagation in pavement structures and on the reported difficulties experienced with existing methods. A computer based data acquisition system and program for evaluation of layer properties have been developed. From the theoretical study on wave propagation in pavement structures, it is concluded that the nature of wave propagation has been oversimplified in previous studies. Results show that the measurable wave field at the surface of a pavement structure is dominated by leaky quasi-Lamb waves in the first and second layer. The fundamental anti-symmetric mode of vibration is the dominating mode generated in the stiff top layer. This mode drives the complete system and continuity across the boundaries generates higher order modes in the embedded second layer. The interaction of leaky Lamb waves in the first two layers results in large variations in the excitability and the attenuation, so that only the waves corresponding to certain portions of the dispersion curves are measurable at the pavement surface. These portions of dispersion curves (mode branches) are critical for a refined NDT technique for pavements. To resolve the different mode branches it is necessary to record the complete wave field on the pavement surface. In this study the multichannel data acquisition method is replaced by multichannel simulation with one receiver (MSOR). This method uses only one accelerometer-receiver and a light hammer-source, to generate a synthetic receiver array. The recorded data is automatically and objectively transformed to a phase velocity spectrum through the multichannel analysis of surface waves (MASW) processing scheme. The top layer thickness, and stiffness properties are obtained automatically in the field by a Lamb wave analysis of the measured phase velocity spectrum. The inversion of deeper embedded layers is based on the full phase velocity spectrum. The main benefit from the developed inversion procedure is that the raw field data can be automatically processed and inverted without any subjective user input to identify discrete dispersion curves. The viscoelastic properties of the asphalt layer are included to produce the asphalt stiffness as a function of frequency, a mastercurve. In this study the presented NDT technique is applied to pavements and concrete structures. However, generic findings here may also be useful in other fields. Possible applications are; ultrasonic testing of coated materials and sandwich structures, surface wave soil site characterization, and medical applications.
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