There are some researches about the topic that confirm the relationship between the Strength of unbound pavement layers and the GPR, but I could not find any model to relate them directly.

Maybe you can try Acoustic method. The direct relationship between strength and EM parameters still not clear.

Thanks for your answer Song,  I searched for the related studies, but I could not find any model. I am going to find this relationship in my PhD thesis and want to know about related earlier studies.

Acoustic velocity has been accepted as a effective parameters to estimate the Strength of cement world widely. Is there any relation between acoustic velocity and dielectric constant ? If you can find it, then ...

I would be very surprise if there is a relationship between dielectric constant and pavement strength. Moisture and density and compositional changes in the unbound material have a larger affect on dielectric than any related strength variation in the material itself. Dielectric can be used to map variations in pavement materials and possibly by inference information regarding its condition. Particularly when considered in context with thickness variation and other signal anomalies.  Variations in dielectric can be determine using wave a ratio analysis, provided that sufficient reflection amplitude can be achieved from the material layers in question. Some relavent papers attached.

Ground Penetrating Radar Surveys to Characterize Pavement Layer Thickness Variations at GPS Sites  Kenneth R.Mase 1994

Road evaluation with ground penetrating radar

T Saarenketo, T Scullion - Journal of applied geophysics, 2000