###### Abstract  
Obtaining the required homogeneity, including uniform thickness and density, is very crucial for controlling the quality of flexible asphalt layers. Although non-destructive testing (NDT) methods are time-saving and less labor-intensive, they only provide indirect measurement data under testing area conditions and strongly depend on the explanations by prediction models. In this study, in terms of the three-dimensional air-launched Ground Penetrating Radar (GPR) technique, the dielectric constant of asphalt concrete base with dry conditions in pavements was detected and calculated by different methods (the Coring Method, Reflection Amplitudes Method and Common Mid-Point Method). According to the calculated dielectric constant, the thickness and density of asphalt concrete base were further calculated and assessed. Comparing with the Coring Method, the Common Mid-Point Method was recommended to calculate dielectric constants in order to obtain reliable thickness of asphalt pavement base. Among the Birefringence, Boettcher, Linearity indicator, and Rayleigh models, the Rayleigh model was suggested to predict the density, and the predicted density exhibited a good correlation coefficient with the measured one. Furthermore, by choosing these proper calculation methods, an assessment was successfully conducted to evaluate homogeneity of a constructed field pavement in practice. ground penetrating radar; asphalt concrete base; homogeneity assessment; dielectric constant; model 29 September 2021 15 November 2021 17 November 20  
waves signals into pavement layers and detects the reflection waves from the surface and interface of asphalt pavement. The frequency of GPR signal is typically in the range of 10 to 5000 MHz [10; 11]. The EM waves of GPR are formed by coupled electric and magnetic fields propagating into asphalt pavement materials. Changes in dielectric constants of the materials scatter can be reflected by the EM waves. The GPR receiver detects these scattered and reflected signals [1; 12; 13; 14]. These reflected waves are processed through digital signal processing to calculate physical parameters and generate two or three dimensional (2D/3D) images of asphalt pavements. In the 1980s, the Federal Highway Administration (FHWA) developed one of the first vehicle mounted GPR system for pavement detection [15]. During the 1990s, as the development of processing software with high-resolution, time-efficient and reliability, GPR technology for road detection was developed to have faster systems operating at higher frequencies [16; 17; 18; 19]. Previous studies thus showed that GPR can be used to assess the thickness and density of asphalt concrete layers [20; 21; 22; 23; 24; 25]. The information from the GPR should be explained and then used to calculate and predict dielectric constant, thickness, and density by using prediction models. As an important parameter detected from GPR, previous studies found that the dielectric constants were influenced by pavement conditions (i.e., the overlapping structure and the presence of surface moisture). Moreover, the dielectric constants calculated by using different methods (i.e., the Coring Method, Reflection Amplitudes Method and Common Mid-Point Method) influence the accuracy of the predicted thickness [26; 27]. Furthermore, when using different density prediction models to estimate density, such as the Birefringence model, Boettcher model, Linearity indicator model and Rayleigh model [28; 29; 30], it might vary considerably. In literature, less information is given to present the estimation variation of such different prediction models and to choose an optimum existing model for the explanation of the EM waves from GPR. The objective of this study was to choose usable and accurate methods and models to evaluate the thickness and density homogeneity of an asphalt concrete base in terms of GPR for quality control and quality assurance.