## 1 Introduction  
### Non-Destructive Testing for Concrete  
It has been generally recognized that preventive and proactive maintenance ideas are necessary for such social infrastructure as bridges and tunnels to develop rational life-cycle scenarios. For reinforced concrete (RC) members, essential issues involve the establishment of a maintenance system with appropriate measurements before evolving the serious damage and failure. For non-destructive testing (NDT), to qualify the materials' property or structural integrity of concrete, several NDT methods have been applied.  
Since sufficient and effective maintenance of infrastructure is desired under the restriction of budget, appropriate inspections by means of NDT must be applied adequately. As for the damage assessment and estimation of repair and retrofit recovery in concrete structures, in addition to developing effective NDTs, innovative methods corresponding to the damage characteristics must be established in parallel. Also, it is strongly demanded to propose appropriate management systems for ageing concrete infrastructures, adapting the aforementioned accurate and ready-to-use inspection techniques for the damage in concrete.  
In concrete structures of bridges such as decks, the most problematic type of damage is fatigue damage due to numerous repetitions of excessive traffic overloads [1; 2]. The final form of damage due to cyclic loads is well known as horizontally induced large cracks in steel-concrete composite slabs [3]. As it is difficult to detect interior damage, which potentially affects structural performance, by visual inspection [4], this becomes a very serious problem for highway structures composed of reinforced concrete (RC) [5]. For the RC bridge slabs in service, inspection work is generally allowed to implement the bottom side, not including the top side, to avoid controlling the traffic. Therefore, a one-side access NDT method is strongly demanded to quantify or assess the damage in RC members. When the special permission to access both sides is given for in-depth inspections or experimental purposes, tomographic approaches with elastic waves have been applied to visualize the internal damages. Specifically, AE (acoustic emission) sensors or accelerometer arrays are installed on the bottom side and artificial excitations are made on the top side so the internal damage can be visualized with such elastic wave parameters as P-wave velocity and energy attenuation [6; 7]. One-side measurements with surface wave techniques have thus been employed to investigate the interior damage of concrete [8]. However, specific depths could not so far be identified with this technique. Meanwhile, the impact-echo method is the most well-used non-destructive testing for concrete structures to characterize the surface area of interest [9; 10]. The SIBIE (Stack Imaging of spectral amplitudes Based on Impact-Echo) procedure is an advanced visualization technique using the data of impact echo testing [11; 12]. SIBIE demonstrates a two-dimensional (2D) internal image of concrete simply with one receiver while leaving such issues to study such as 3D visualization and more accurate damage identification.