## 7 Conclusions and Suggestions  
This study proposed a temperature-evaluation model for estimating the temperature at various depth of pavement and conducted the frequency-temperature correction for FWD back-calculation using the frequency-temperature superposition principle. The proposed temperature-evaluation  
Figure 17: The back-calculated results of subbase and subgrade layer for cross sections 1–4.  
model was developed through the statistical analysis of temperature measurements in a FWD test site. The in situ cored specimens were obtained from the FWD test site and the samples were subjected to dynamic modulus tests in the laboratory to determine the frequency-temperature shift factors and master curves. The FWD back-calculations were performed with frequency-temperature correction and the back-calculated modulus of the asphalt layer was compared with the master curve. The following are the conclusions and suggestions.  
* The proposed temperature-evaluation model, considering the effect of temperature-transmission delay, can effectively and reliably estimate the temperature at different depths of the pavement structure. The estimated temperature at deep depth remains reliable when compared with the measured temperature (i.e., R\({}^{2}\) = 0.82 at depth of 24.5 cm).
* The frequency-temperature superposition principle was employed to conduct the temperature correction for asphalt material layers. The average percentages of the temperature-corrected back-calculated modulus within \(\pm\)30% range of the master curve are 71.59% and 89.02% for the DGAC and BTB layers, respectively. This result indicates that the frequency-temperature superposition principle can effectively apply to correct the temperature effect of FWD back-calculation for asphalt layer.
* The back-calculated results of the subbase and subgrade layers show that the moduli of subbase and subgrade are not significantly affected by temperature, which is in agreement with the properties of these materials. The back-calculated modulus of the subbase and subgrade are mostly consistent in the same section.
* The proposed temperature-evaluation model in this research is developed based on only one year and local temperature measurements. More temperature measurements should be included in future research to enhance the accuracy and application of model.
* This research combined several layers (i.e., OGAC, SMA and PAC) with a DGAC layer in back-calculation. In future research, the moduli of the OGAC, SMA, and PAC layers should be obtained in a laboratory through dynamic modulus tests and considered as known moduli in back-calculation to improve the back-calculated results of DGAC.  
Conceptualization, C.-W.H.; methodology, C.-W.H.; software, J.L.; validation, J.L.; resources, J.-C.L.; data curation, J.L.; writing--original draft preparation, J.L.; writing--review and editing. J.-C.L. and C.-W.H. visualization, J.L.; supervision, C.-W.H. and J.-C.L.; project administration, C.-W.H.; funding acquisition, J.-C.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Taiwan Freeway Bureau, MOTC, grant number 108C114P009.  
The authors declare no conflict of interest.