## 1 Introduction  
Many concrete structures are built for multiple purposes. These structures include railway track slabs, Portland cement concrete (PCC) pavements, bridge decks and piers, building columns, pipes, and so on. However, old concrete structures may possess severe damage, such as cracks, exfoliation, and voids, and it becomes essential to investigate the damage of old structures before attempting to repair or reconstruct them. Railway concrete slab tracks are a particular concrete structure that can be exposed to harsh operating and environment conditions during service after construction. They experience repetitive and heavy wheel loading and severe temperature variation all day and night and even seasonally. Railway authorities have recently taken interest in new technologies to detect gaps generated at the interface of railway track slabs and track bed soil foundations. It was pointed out that because of subgrade settlements, which is the main cause of making gaps, it is necessary to further optimize the slab-soil interaction [1]. Among the many causes of generating the gaps, soil settlement under railway track structures is noted as the main reason for inducing instabilities in a railway track [2]. All defects and damages including gaps in railway track slabs may decrease the design life of a railway track and prohibit the stable operation of a railway. Thus, it is important to detect gaps between railway track slabs and tracked soil foundations accurately. Presently available nondestructive test (NDT) methods however, are not effective tools to find gaps generated at the interface of railway track slabs and track bed soil foundations. Most available NDT methods used for detecting defects in concrete structures are based on sensors that should be installed on and be fully in contact with the surface of the structure. In order to obtain reasonable and reliable test data from this kind of sensor-based NDT method, the structure surface must be carefully prepared. A rough concrete slab surface may impose difficult problems on the NDT tester. It takes time for the NDT tester to prepare the concrete surface to be clean, planar, and smooth by grinding and polishing to get reliable test data. One alternative technique is to adapt a noncontact NDT method that excludes physical direct surface contact between the sensor and the structure to be tested. The noncontact NDT method can reduce the time for treating data collection and data processing. This kind of NDT test method includes RADAR electro-magnetic pulsed waves, X-ray, and infrared thermography [3]. However, it is not easy for the non-contacting NDT methods to display the location and size of defects or gaps in or underneath concrete track slabs. It is difficult to distinguish a gap from the slab when it is surrounded by soils. An NDT method based on elastic stress waves can detect defects in concrete structures using sensors placed directly on the surface of the structure to obtain propagated elastic stress waves. Such an NDT method used for concrete structures has been adapted in the field to measure slab thickness [4; 5; 6] or identify the location (depth) and size of the defects in a structure and also to monitor strength of a concrete structure [7; 8]. Various types and concepts of NDT methods based on non-contacting acoustic waves have been proposed and developed [9; 10; 11]. An NDT method based on leaky Rayleigh acoustic waves (NDT-RAWs) is a noncontact method since it uses a microphone to catch leaky Rayleigh acoustic waves in the air above the structure. In short, the NDT-RAW method has the merit of using air waves without direct contact with and influence of the surface of a structure. A comprehensive study was performed recently for investigating and summarizing state-of-the art developments in the field of nondestructive testing of materials in civil engineering that were mainly related with the detection of flaws and defects in concrete parts and structures and specifically acoustic methods mostly used in the field [10].