## 1 Introduction  
The structural behavior of in-service concrete pavements can be considerably affected by the combined influence of traffic loading, climate, and environmental conditions [1]. The dominant distress in the concrete pavement could be considered slab cracking, faulting, spalling, poor ride, delamination, etc. In order to minimize and even eliminate the effects of these distresses, several concrete pavement rehabilitation techniques may be used to prolong and enhance the concrete pavement life, which can be divided into (1) Concrete Pavement Restoration (CPR) techniques; (2) asphalt overlay over existing or fractured concrete pavement; (3) cracking and seating techniques before constructing the asphalt overlay; and (4) bonded/unbonded concrete overlays [2, 3]. Meanwhile, the CPR technique also involves full-depth/partial-depth patching, joint and crack resealing, slab stabilization, diamond grinding, load transfer restoration, and cross-stitching longitudinal cracks [4]. Among these rehabilitation techniques, the asphalt overlay over existing concrete pavement is the most common concrete pavement rehabilitation strategy preferred by many agencies, which enhances the structural capacity to extend the service life, lessens the deterioration of pavement, and also minimizes traffic disruptions, especially in urban areas [5]. However, there still exists distress, such as transverse cracking, which could be frequently observed in the asphalt overlay. These distresses are directly affected by three key factors when designing asphalt surfaces: surface preparation, overlay thickness, and material [6]. Therefore, it is significant to carefully design and determine the most targeted pavement rehabilitation process. Such a decision depends on the condition of the concrete pavement and its evaluation. From an assessment of the concrete pavement's condition, it is possible to identify the critical area that has a considerable effect on the structure's performance [7]. Once the evaluation of concrete pavement condition is obtained, which mainly involves strength assessment and internal condition analysis, the rehabilitation process of asphalt overlay could be reasonably determined. During pavement condition evaluation, both direct and indirect measurement methods are frequently employed to characterize the performance of concrete pavement. Traditionally, the direct measurement method involves extracting core samples from the field, followed by laboratory strength and modulus tests to obtain the necessary characterizations. While this method is straightforward and easy to implement, it has notable drawbacks, including compromising the integrity of the pavement. In contrast, non-destructive testing (NDT) technologies offer the ability to characterize material performances without causing damage to the pavement, thereby preserving the integrity of the pavement system. Due to their increased efficiency, cost-effectiveness, and environmental sustainability, NDT methods are gaining popularity. Currently, the Falling Weight Deflectometer (FWD), as a commonly used NDT technique, is regarded as a standard method for measuring the elastic modulus of the pavement, base, and subbase layers [8]. However, the back-calculation process in the FWD contains some unsolved uncertainties and highly depends on the initial values, such as the layer thickness [9]. Also, it is difficult to obtain the modulus value, which varies with the depth of the pavement. These shortcomings would reduce the accuracy of the measured modulus results. During the 1980s, [PERSON] and his research team developed a new NDT technique to determine the properties of pavements, the Spectral-Analysis-of-Surface-Waves (SASW) method [10; 11; 12]. The SASW method came from observing how the Rayleigh waves were used in seismology for the Earth's crust. The researchers found a similar concept could be used for more localized investigations of material stiffness. Over time, several improvements have been made to the SASW method since its inception.