| Nondestructive testing methods to accurately determine concrete pavement thickness are in increasing demand for verification of performance-based specifications, as the only current technology that provides the required accuracy levels is core extraction. In this work two approaches for pavement thickness NDE are evaluated, one based on frequency-domain analysis and the second based on time-domain analysis. First an alternative interpretation of the Impact-Echo method, based on guided wave theory, is presented. This provides a closer match between theoretical and observed results, and ultimately a more accurate basis for testing. A new, time-domain based method is then proposed to address some practical shortcomings of the Impact-Echo method, such as limited frequency resolution and assumed wave velocity values. A theoretical basis and experimentally verified numerical models are developed for both methods. Although the conceptual basis for each method is different, during the course of the study a common link was identified in the physical phenomena under investigation. In both cases the measured dynamic response of the plate can be described (or interpreted) using the theoretical dispersion behavior of a particular type of guided wave: the first-order symmetrical (S1) Lamb mode. Both experimental techniques involve similar excitation and measurement procedures, namely a spherical steel impact source and resulting out-of plane dynamic response; the S1 mode appears to dominate the dynamic response of a slab under theses circumstances. A series of full-scale experimental tests were undertaken in order to verify and quantify the performance of the two proposed methods. In both cases maximum experimental errors of 3% in terms of average thickness along a 2 m centerline on each of the three slabs were obtained. |
