## 1 Introduction
In this technique, Rayleigh waves are generated through ground-surface excitation and subsequently captured by a pair of receivers. Based on the Rayleigh wave signal and its dispersion characteristic, the shear wave velocity profile of each layer in a layered structure can be calculated. The modulus of the material varying with depth can also be determined, as the shear wave velocity is an inherent property of the material [13]. Compared to the other NDT methods, the SASW has several advantages: (1) being less dependent on the initial assumed thickness value; (2) providing more comprehensive information, including gradient modulus varying with depth; and (3) assessing the pavement thicknesses. However, the SASW method requires expert knowledge of wave propagation theory to interpret complex results [14], while the FWD method is generally easier to implement. The SASW method has been successfully used for assessing the bearing capacity of asphalt, subgrade, and base layers over the past decades [15; 16; 17; 18; 19; 20]. In the evaluation of the subgrade and base layer, the SASW method can evaluate the elastic modulus of rigid and granular aggregate bases [21], determine the seismic parameters of soil stiffness [22], and monitor the seasonal changes in modulus of the base and soil foundation during the whole freeze-thaw cycle [23]. For the asphalt pavement, the SASW method was usually combined with APT testing to monitor the asphalt modulus deterioration trend under controlled traffic and environmental conditions [24; 25]. Additionally, as the Rayleigh wave signal can contain the cracking information, it is also possible for the SASW method to study the near-surface damage in the pavement, such as cracks or voids [18; 26; 27; 28; 29]. Meanwhile, for the concrete pavement, the SASW method also has the capability to assess the modulus of the concrete pavement [16; 30]. [PERSON] and [PERSON] [31] conducted SASW tests on multilayered concrete slabs with a finite thin thickness and found the Rayleigh wave velocity would increase with the increasing age of concrete slabs due to the distinct boundary conditionsand reflection at the boundaries. Then, [PERSON] [32] used the SASW method to research the concrete slabs with different boundaries to prove the potential of SASW methods to determine the modulus accurately. Under these circumstances, considering the practicability and advantages of the SASW method, this research aims to employ the SASW method to comprehensively assess the structural health of concrete pavement, and the modulus is used as the important index to perform the evaluation. In this research project, the asphalt overlay rehabilitation technique will be adopted on three distinct in-service concrete pavements. In order to provide the theoretical basis for the rehabilitation procedure, the SASW tests and FWD tests would simultaneously be conducted to evaluate the pavement condition, including the modulus value and internal distress recognition. Meanwhile, the on-site coring was also conducted at the measurement locations. The cylinder compression strength laboratory tests were performed to calculate the equivalent modulus based on the field cores. This research also shows the potential for using the SASW method to provide insights into the maintenance and rehabilitation of concrete pavements.