# Application of Tooth Gear Impact-Echo System for Repeated and Rapid Data Acquisition
## 1 Introduction  
Delamination-like defects are a major concrete bridge-deck deterioration concern. The internal delamination causes degradation of the structural elements in bridges as well as problems of serviceability by exposing the surface. Therefore, early detection of the internal defects is required to repair bridge decks. Impact-echo (IE) tests are a well-used non-destructive testing (NDT) method to evaluate delamination-like defects in concrete. Compared to other NDT methods, the IE test is a relatively simple testing configuration, which includes a hammer-type source and a vibration sensor. The hammer-type source literally makes an impact onto the surface of the concrete and the corresponding echo is measured. The IE test identifies the presence of delamination with depth information, and so it has been applied to bridge-deck inspection [1].  
Since the IE test has proven its potential for internal damage detection [2; 3], various studies have been conducted to understand the damage mechanism as well as to improve measuring systems. [PERSON] and [PERSON] discovered the physical relationship between the measured frequency and the depth of a plate-like structure, suggesting the effect of Poisson's ratio [4]. [PERSON] and [PERSON] proposed adding air-coupled sensing to the IE test, demonstrating the leakage of the vibration mode into the air [5]. [PERSON] et al. investigated the application of the air-coupled IE test in the case of shallow delamination and the visualization of its vibration modes [6]. [PERSON] et al. also explained the boundary condition effect on shallow delamination [7]. Further efforts have been made on the application ofair-coupled sensing through the combination of different automated systems [8; 9], their research reaching into efficient impact sources with air-coupled sensing. [PERSON] et al. showed chain dragging as an impact source while [PERSON] et al. used a ball-chain system to filter out dragging noise [10; 11].  
Studies on impact sources are crucial for rapid examination of the field. [PERSON] et al. presented an interesting ice ball as an impactor [10]. The disposal of ice into concrete generated mechanical vibrations indicating that shallow delamination had been detected. [PERSON] and [PERSON] also demonstrated the potential of a rolling impactor for the air-coupled IE test [12]. As established from previous studies, impact sources for the IE test could be found from surrounding products. In this study, a commercially available tooth gear was used as an impactor, generating continuous vibrational forces. A hand-controlled IE system was developed, including microelectromechanical systems (MEMS) as receivers. The aim of this paper is to demonstrate that tooth gears can be applied to air-coupled IE tests and to propose a rapid scanning test configuration including acoustic noise reduction by IE system development, measurement of thickness mode frequency and data visualization. To generate IE vibration modes, the contact duration onto the surface of concrete was experimentally evaluated and excited modes were measured from intact and delamination cases in concrete slabs. Consequently, system designs will improve and extend the understanding of field applications of the IE method.