## 5 Conclusions and Future Work  
This research assessed the modulus of three in-service concrete pavements using the SASW method and compared the corresponding modulus measured by FWD with a compressive strength laboratory test. Based on the modulus analysis, the theoretical basis for asphalt overlay schemes on concrete pavements could also be provided. The comprehensive evaluation of concrete pavement condition, including the modulus value and internal distress recognition, has the capability to assist in determining the design of overlay asphalt thickness and material.  
The conclusions are summarized below:  
(1) The SASW method can accurately obtain the elastic modulus of the concrete slab and base layer of the concrete pavements, which can serve as an effective, reliable, and high-precision NDT method for assessing the overall modulus of concrete pavements throughout their entire service life.  
(2) Compared with the concrete slab modulus measured by FWD, the concrete slab modulus measured by the SASW method was much closer to the axial compressive modulus measured by in situ cores and had a high correlation.  
(3) Among the three in-service concrete pavements, pavement Section III performed the best in terms of the concrete slab and base layer. Section II of the pavement needs further damage investigation for both the lowest concrete and base layer. Meanwhile, because the pavement in Section I has a relatively low concrete slab modulus compared to Section III, it was recommended to perform more field NDT testing to determine a more suitable and targeted asphalt overlay scheme.  
(4) It is reasonable that the gradient modulus profile measured by the SASW method could make the inherent distress recognition possible. The modulus distribution has the capability to determine the location of the inherent distress based on the region where the modulus suddenly decreases.  
Although the SASW method has some advantages in evaluating the concrete pavement condition, it still has several drawbacks, such as the high requirements for field testing procedures and manpower engagement, culminating in a complex and time-intensive measurement procedure. In these cases, it is considered necessary to develop a new methodology or improve the measurement process to enhance the efficiency of field testing in future research. Thus, it is anticipated that the comprehensive assessment of pavement conditions might become significantly streamlined and much more accurate.  
Conceptualization, X.W.; methodology, X.W. and M.Q.; software, X.W. and R.H.; formal analysis, M.Q. and R.H.; investigation, R.H.; resources, M.Q.; data curation, X.W. and R.H.; writing--original draft preparation, X.W. and M.Q.; writing--review and editing, X.W.; project administration, M.Q.; funding acquisition, R.H. All authors have read and agreed to the published version of the manuscript.  
This research team would like to acknowledge the financial support from the science and technology project of the Science and Technology Project of Jiangxi Transportation Department (Grant No. 2020H0025). The contents of this paper only reflect the views of the authors and do not reflect the official views or policies of the sponsors.  
Not applicable.  
The authors declare no conflict of interest.