Frequency Wave Number–Domain Analysis of Air-Coupled Impact-Echo Tests in Concrete Slab1

Abstract

An air-coupled (also known as noncontact or contactless) sensing technique in the impact-echo method has been developed and its practical application to the field has been significantly improved. However, unlike the flexural mode frequency, the thickness mode frequency is difficult to measure using the air-coupled sensing technique mainly due to direct acoustic noise from the impact source. The direct acoustic noise is transmitted through the air, while the thickness mode is a portion of leaky Lamb waves from a platelike concrete slab, known as the zero-group velocity (ZGV) or standing wave motion. The noise components and leaky Lamb waves can be separated in the frequency-wave number (f-k) domain because of their different characteristics in phase velocity. In this paper, the f-k domain analysis is validated through both numerical simulations and experiments on two different thicknesses including full depth of concrete slab and shallow delamination. For shallow delamination, a dry-coupled ultrasonic transducer is applied to generate high-frequency ultrasonic waves corresponding to the ZGV frequency of the thickness while a microelectromechanical system (MEMS) is used as an air-coupled sensor. The results demonstrate that the proposed f-k domain analysis scheme and the applied hybrid hardware are capable of clearly detecting the thickness mode frequency and minimize the direct acoustic noise, while the conventional impact-echo method has limits.