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The phenomenon of slow dynamics has been observed in a variety of materials which are considered as relatively homogeneous that exhibit nonlinearity due to the presence of defects or cracks within ...

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.

Surface wave velocity is measured and evaluated along a straight survey line in order to compare two different data acquisition methods. Results from a rolling microphone array are compared to data acquired using a conventional accelerometer. Results from the two different data acquisition methods are shown to be similar. However, it is demonstrated that the results are very sensitive to misalignments between the microphone array and the measured surface. Practices to overcome problems with misalignments are discussed and demonstrated. QC 20160109

Impact hammer and non-contact speaker excitation were applied to an asphalt concrete, a PVC-U and a concrete specimen to measure the fundamental longitudinal resonance frequency at different strain levels. The impact and the noncontact excitation methods resulted in similar resonance frequencies for the undamaged asphalt concrete and for the PVC-U specimen. However, the two excitation approaches gave different results for the concrete specimen, which was shown to have a nonlinear response to increasing strain levels. A reduction and a following recovery of the resonance frequency of the asphalt concrete were shown after the specimen was exposed to a small amount of damage. However, no fast nonlinear dynamics were observed for the asphalt concrete through the speaker measurements.

The dynamic Young’s modulus of asphalt concrete is directly related to pavement quality and is used in thickness design of pavements. There is a need for a nondestructive laboratory method to evaluate the complex modulus, which can be linked to nondestructive field measurements. This study applies seismic measurements to an asphalt concrete beam where resonant acoustic spectroscopy and optimization of frequency response functions are used to estimate the complex moduli. A good estimation of the master curve is obtained.

Impact Echo is commonly used to determine thickness of concrete plate like structures. The method is based on the generation and detection of the plate thickness resonance frequency, where the group velocity of the first higher symmetric Lamb mode goes to zero (S1-ZGV). When using air-coupled microphones as receivers it is hard to determine the correct resonance frequency due to low signal to noise ratio. In this study multichannel signal processing is used to identify the S1-ZGV frequency, based on backward wave propagation instead of the conventional amplitude spectrum approach.The original PDF file of this article, as supplied to AIP Publishing, contained some minor font problems within Figures 1, 4, 7, 8, and 9. An updated PDF file using the correct font within those figures was issued on June 3, 2013. There are no other changes to the scientific content.

Pre‐stressed concrete is used to increase the load capacity in many civil engineering structures. There is a need for non‐destructive methods to estimate the potential loss of pre‐stress in these structures. In this study we investigate the acoustoelastic effect on the natural resonance frequencies of a pre‐stressed concrete beam. Measured resonant frequencies increase with increasing compressive stress. Results agree with predicted frequency shifts obtained from a non‐linear finite element model based on Murnaghan’s third order elastic theory.

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.

Non‐destructive prediction of pavement layer thickness and material properties is described in this paper. Three non‐destructive evaluation (NDE) techniques are applied and evaluated through field tests carried out on various full‐scale pavements. The pavement sections vary in material type, thickness, inter‐layer bond condition and underlying base material type. The following NDE methods are considered: conventional and modified impact‐echo, multi‐channel analysis of surface waves (MASW), a hybrid seismic/guided wave method, and ground penetrating radar (GPR). The evaluation of MASW data is based on a Lamb wave approach in frequency domain utilizing both propagating and non‐propagating waves. In the hybrid seismic/guided wave method the velocity of guided waves is evaluated in time‐domain and the thickness is estimated from the combined measured velocity and resonant frequency of the Sl Lamb wave. The technical bases, equipment, data interpretation and field testing procedure are described for each method. The NDE results are presented and compared to ground‐truth core samples. The performance and accuracy of the NDE methods are presented for each pavement type.

We present experiments with a rolling multichannel microphone array where measurements can be taken continuously while moving. Leaky air‐coupled surface waves are measured with ordinary non‐directional audio microphones. Results show that microphones can be successfully used to produce a realistic phase velocity spectrum even while moving along the surface. The paper presents some theoretical background along with initial experimental results using the moving microphone array.

The basis of a new non‐destructive method for concrete pavements is described. The method uses a hybrid approach that combines frequency domain (impact‐echo) and time domain (seismic) data. The equipment and data interpretation for the hybrid approach are described. The dynamic response data are interpreted in terms of guided plate wave (Lamb wave) theory, which are shown to follow the S1 Lamb mode behavior. The developed field testing procedure is then described. Results from verification tests, carried out on full‐scale concrete slabs, show that the new method provides accurate thickness estimates. On average the error between predicted thickness and actual thickness determined by cores is less than 6mm, although some individual estimates exceed this error value.

A method is presented for evaluating the thickness and stiffness of multilayered pavement structures from guided waves measured at the surface. Data is collected with a light hammer as the source and an accelerometer as receiver, generating a synthetic receiver array. The top layer properties are evaluated with a Lamb wave analysis. Multiple layers are evaluated by matching a theoretical phase velocity spectrum to the measured spectrum. So far the method has been applied to the testing of pavements, but it may also be applicable in other fields such as ultrasonic testing of coated materials.