| Asphalt concrete pavement of highway is a kind of two-phase medium, which is composed of aggregate, asphalt mortar and air. Aggregate, asphalt mortar and air usually have different sizes, shapes, dielectric properties, and distribute randomly in space. Relative to the wavelength of high-frequency electromagnetic wave, the physical properties of this typical random medium change dramatically. Scattering will happen when high-frequency electromagnetic wave propagates in random medium, and would cause a lot of irrelevant wave arrival, which result in that the being received waveform also has corresponding random characteristics. Due to high dominant frequency of the antenna of Ground Penetrating Radar (GPR), Radar wave has the short wavelength and big scattering, which make the changes of wave field caused by the random distribution of medium’s dielectric properties bigger relatively. So reconstructing the random medium model to approximate actual engineering material, and researching parameters of random medium model and the change law of GPR wave field associated with these parameters are the physical basis for the quantitative evaluation of highway engineering dielectric materials. Besides, when GPR is applied to the nondestructive testing on the quality of the road structural layer, its detecting object is very shallow medium whose depth is less than one meter, even within a few centimeters, which is a typical near-field problem. Therefore, building the model of the bow-tie antenna of GPR and studying its radiation and electromagnetic near-field characteristics in random medium is the theoretical basis of improving the effect and interpretation of ground penetrating radar detection.This work proposes a two-phase discrete random medium that is in line with the physical truth of asphalt concrete and constructs the numerical model of the bow-tie antenna of GPR. Then, the typical corresponding relationships between the wave field characteristics of GPR and the parameters of the two phase discrete random medium, or the electrical parameters of engineering materials, or the typical engineering defects are studied. The main research and their results are as follows:1. A numerical simulation algorithm is designed and implemented using finite-difference time-domain (FDTD) method. This algorithm is based on Maxwell equations and it can be applied to problems of the excitation and the propagation process of GPR electromagnetic waves in random medium. It provides a powerful tool for analyzing the GPR wave field in random medium.2. A finite difference time domain (FDTD) model of commercial bow-ties antenna of ground penetrating radar is set up based on the geometry of the commercial bow-tie antenna, the way of resistive loading, the excitation and the shielding of absorbing mechanism. The effects of the geometry of bow-tie antenna, resistive loading, shielding cavity, wave-absorbing material, and the dielectric characteristics of media on the radiation of the bow-ties antenna are studied through3D numerical simulation, which provides a scientific basis for verifying and improving the radiation performance of GPR bow-ties antenna.3.12core samples of asphalt concrete are collected from highways, and the dielectric constant of these samples" ingredient are measured. By analyzing spatial random distribution characteristics of the dielectric constants of core samples from different structural layers of asphalt concrete pavement. I calculated their autocorrelation functions of random media and estimated their autocorrelation lengths and autocorrelation angle, also classified the type of random media:Then I constructed several models of two-phase discrete random media corresponding to different structural layer media of asphalt concrete pavement by improving the random medium modeling method. The research results provide new theoretical basis for the interpretation and inversion of GPR data.4. Through the3-D numerical simulation of Ground-penetrating radar wave field, we tested the correctness and accuracy of two kinds of model of CRIM and linear approach for estimating the effective dielectric constants of two-phase random media based on volume percentages of components within materials.5. Through of simulating the real asphalt concrete pavement structure and its constituted materials. I constructed the two-phase discrete random medium models with the different void ratio. Then I also studied the effect of asphalt concrete layers of different porosity to GPR wave Held characteristics based on the3-D numerical simulation, and proposed the method to estimate quantitatively void ratio of different structure layer. All of these studies provide theoretical foundation for the quantitative detection of void ratio of asphalt concrete pavement structure using GPR.6. The variation of GPR wave Held caused by the different width vertical crack and different thickness of the interlayer cavity in two-phase discrete random media are studied, which put forward a scientific basis for quantitative estimation of crack width and thickness of the interlaver cavity.The full text is divided into six chapters.The llrst chapter is introduction which briefly describes the purpose and significance of this study. The research status in domestic and foreign and the existing problems are analyzed, and also the research objectives, research contents, research programs, the main research results and my innovations are presented.The second chapter mainly states the basic theory and numerical simulation method of the ground penetrating radar, as well as GPR system. The theory of finite difference in time domain, iterative formula, numerical stability and numerical dispersion conditions are discussed, and the iterative formula of UPML absorbing boundary condition is deduced. Finally, a few of examples show the validitv of the UPML absorbing boundary condition.The third chapter mainly discusses the numerical simulation method of GPR bowtie antenna. The characteristic of GPR bowtie antenna, such as the direction, polarization, ringing effect, shielding design and the bowtie antenna loading on underground media are analyzed. According to the specification of bowtie antenna, for example, geometry, excitation waveform, and the load resistance. I established the Unite difference time domain model of the bowtie antenna, and simulated the effect of the bowtie antenna radiation to its geometry, the change in load resistance, the height of gap from antenna to the ground and the dielectric property of underground medium.The fourth chapter is the research on types and characteristics of the asphalt concrete medium, and its modeling. According to the measurements of asphalt concrete core samples and the distribution of dielectric constant within the transverse and vertical section. I studied the characteristics of spatial random distribution of dielectric constants, such as autocorrelation length, angle and the type of autocorrelation function, in the different structural layer of the asphalt concrete pavement. I suggested the modeling method of two-phase discrete random medium, and established a few of the two-phase discrete random medium models for different structural layer of asphalt concrete pavement.In the fifth chapter, assuming that asphalt concrete is two-phase discrete random medium, the CRIM model compared with linear model to estimate the effective dielectric constant for different porosity of asphalt concrete is more accurate. I also compared characteristics of ground-penetrating radar field between in two-phase discrete random media and in the effective homogeneous medium using three-dimensional numerical simulation. That proved the accuracy of the effective dielectric constant estimation. Quantitative method to estimate the porosity of pavement structural layer is proposed; Study on the relationship between the crack width and radar wave amplitude deduced a conclusion of approximate linear relationship. A similar linear relationship between the thickness of void-layer and the maximum reflection amplitude is got. These results provide scientific basis for quantitative estimation of the crack width and void-layer thickness.Last chapter is the sixth chapter, which comprehensively summarizes my main work and the problems to be solved in the study and the future research directions.The originality of my doctoral work mainly concludes the following three aspects:1. The paper firstly regards asphalt concrete as medium containing randomly distributed dielectric composite in space. I studied the random distribution feature of asphalt concrete media, further confirmed the type of random media and suggested a method of modeling two-phase discrete random media. This model of two-phase discrete random medium tested by numerical simulation can simulate the interior structure of asphalt concrete medium. Thereby, the effective dielectric constant estimated from that and the characteristics of electromagnetic wave propagation in this random media are accordant with the actual measurements of ground-penetrating radar. This study provides a theoretical basis for the quality testing of asphalt concrete highway and paves a new way for studying similar media or materials.2. A quantitative method is proposed to calculate the void ratio of structure layer of asphalt concrete based on the wave field characteristics of the ground-penetrating radar of bowtie antenna. The ground-penetrating radar physical experiments prove that is correct and reliable, which has important significance for quantitative detecting the quality of asphalt concrete highway.3. The influence on the ground-penetrating radar field, caused by the width of the crack or thickness of void-layer in the two-phase discrete random media is studied using3D numerical GPR simulation method with the commercial bowtie antenna as excitation source, which shows approximate line relationship between maximum amplitude of radar wave and with either crack width or void-layer thickness. |
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