Surface Wave Testing of Pavements

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

Studies Into the Use of Non-linear Ultrasound to Detect Corroded Steel Plates Embedded in Concrete

In some nuclear power plants, the containment liners are embedded in concrete. The purpose of the containment liner is to minimize the risk of radioactive leakage by granting a leak-tight containment building. In the presence of flaws in the surrounding concrete, such as voids, or foreign objects embedded in the concrete, there is a risk that the liner plate might corrode. This could degrade the leak tightness of the containment building, leading to an increased risk of radioactive leakage. A project is being carried out at Lund University whose purpose is to assess the capability of non-linear ultrasound to detect corroded steel plates embedded in concrete. Second harmonic analysis has been used to determine if general corrosion induced an increase in acoustical non-linearity. This was done by inspecting a steel plate with varying grades of corrosion immersed in water. Results from this study indicated that severe corrosion increases the relative parameter of quadratic non-linearity ( Beta’). Beta' is given by the ratio between the second harmonic amplitude and the fundamental amplitudesquared. Building on this result, initial findings from modulation experiments on concrete cylinders with embedded steel plates with varying grades of corrosion are presented

Resonant frequency testing of cylindrical asphalt samples

There is a need to develop simple and quick non-destructive tests to measure the complex dynamic modulus of asphalt over a wide frequency and temperature range, i.e. mastercurve. In this study results from free-free resonant frequency measurements on cylindrical disk shaped asphalt samples are presented. The resulting mastercurve, obtained by resonant frequency testing at different temperatures, compares well with reference values within the high modulus range. The proposed method is simple and repeatable

The influence of Poisson's ratio on the excitability of the first higher symmetric (S1) zero-group velocity Lamb mode.

The impact echo (IE) method is widely used in non-destructive testing of concrete plate like structures but has proved difficult to apply on asphalt pavements. Observed low IE resonance amplitudes on asphalt have been attributed to larger material damping but the present study shows that it may also be an effect from a larger Poisson's ratio. The IE frequency corresponds to the stationary resonance of the S1 Lamb mode at zero-group velocity (ZGV). It has been previously shown that the S1 ZGV Lamb mode only exists below a Poisson's ratio of 0.45. [Prada et al., J. Acoust. Soc. Am. 124, 203-212 (2008).] The present study investigates the magnitude of the excitability and damping ratio of the S1 ZGV mode as a function of Poisson's ratio. The frequency response from a point source is predicted using a finite element model. Results show that the excitability decreases with increasing Poisson's ratio. The excitability of the S1 ZGV mode is about twice as high at a Poisson's ratio of 0.18 compared to 0.30, which are representative values for concrete and asphalt, respectively. The modal damping ratio increases with Poisson's ratio, causing an overall lower IE resonance amplitude at higher values of Poisson's ratio

Seismic joint analysis for non-destructive testing of asphalt and concrete slabs

A seismic approach is used to estimate the thickness and elastic stiffness constants of asphalt or concrete slabs. The overall concept of the approach utilizes the robustness of the multichannel seismic method. A multichannel-equivalent data set is compiled from multiple time series recorded from multiple hammer impacts at progressively different offsets from a fixed receiver. This multichannel simulation with one receiver (MSOR) replaces the true multichannel recording in a cost-effective and convenient manner. A recorded data set is first processed to evaluate the shear wave velocity through a wave field transformation, normally used in the multichannel analysis of surface waves (MASW) method, followed by a Lambwave inversion. Then, the same data set is used to evaluate compression wave velocity from a combined processing of the first-arrival picking and a linear regression. Finally, the amplitude spectra of the time series are used to evaluate the thickness by following the concepts utilized in the Impact Echo (IE) method. Due to the powerful signal extraction capabilities ensured by the multichannel processing schemes used, the entire procedure for all three evaluations can be fully automated and results can be obtained directly in the field. A field data set is used to demonstrate the proposed approach

Guided wave propagation in three-layer pavement structures

A study on guided waves in a. layered half-space with large velocity contrasts and a decreasing velocity with depth is presented. Multiple mode dispersion curves are calculated in the complex wave number domain, taking into consideration the attenuation caused by leakage into the underlying half-space. The excitability of the modes by a vertical point force on the surface is also calculated. Results show that the measurable wave field at the surface of a pavement structure is dominated by leaky quasi-Lamb waves in the top and second layers. The fundamental antisymmetric 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 remote from the source at the pavement surface. It is concluded that these portions of dispersion curves can be individually resolved in practice, by using multichannel processing techniques. This holds the potential for a refined nondestructive testing technique for pavements

Near Field Effects and Estimation of Poisson's Ratio in Impact-Echo Thickness Testing

Based on the polarization and shape of the first symmetric Lamb mode at the zero-group velocity point (S1-ZGV), a novel approach for Impact-Echo thickness testing is proposed. In this new approach all three plate parameters (thickness, velocity and Poisson's ratio) are estimated from the same wave mode (S1-ZGV). A traditional approach based on a measurement of the longitudinal-and Rayleigh wave velocity along the surface is also studied, and is found to be influenced by near field effects leading to an underestimated thickness. In addition, an alternative approach which estimates Poisson's ratio from the frequency ratio of the S1-ZGV and A2-ZGV point is discussed. The three studied approaches are demonstrated in a realistic field case

Short- and long-term measurements of resonance frequencies on prestressed concrete beams

Currently, no method exists for determining the prestress forces in prestressed concrete structures. According to the theory of acoustoelasticity, the modulus of elasticity of concrete is stress dependent, which means that the resonance frequency of a concrete structure also is stress dependent. In this study, both short-term and long-term measurements of the resonance frequencies of four prestressed concrete beams were performed. The results show that the acoustoelastic effect in concrete is measureable and that the resonance frequencies increase with the applied compressive stress and that the prestress losses can be monitored through measurements of the resonance frequencies

An adaptive frequency domain finite element model for surface wave testing of pavements

The proposed adaptive frequency domain finite element (FE) model makes it possible to simulate wave propagation in a pavement structure over a wide frequency range using a normal size computer. The geometry and mesh size are optimized and regenerated for each frequency using a simple script in a commercial FE software package. Results from the FE model agree well with normal mode dispersion curves for a 3-layer reference pavement model. The proposed approach is fast and computationally efficient and intended to be used as a forward model for iterative surface wave inversion