| With the rapid development of urban road construction and maintenance management, the precise exploration of some important road quality evaluation indicators has become the development direction of measurement technology including thickness, moisture content, porosity and degree of compaction for the road structural layer. The ground penetrating radar is a physical geography exploration method to explore medium distribution of underground shallow layer and ultra-shallow layer using ultrahigh frequency pulse electromagnetic wave. The application of ground penetrating radar is gradually becoming more and more diversified and it has also been widely applied to the evaluation of road conditions due to its features of no damage, efficiency, high anti-interference ability, convenient operation, low cost, less limitation of detecting condition and high detection precision etc. However, the development of urban road non-destructive detecting technology based on ground penetrating radar is restricted seriously by the lack of relevant research on dielectric property of urban road structural layer material and calculation model of multi-phase composite medium dielectric constant that is reasonably applied to the material of road structural layer. The research achievements are still restricted only to the research on the thickness of single road structural layer. However, the research on the thickness of multiple road structural layer is very insufficient. At present, the road quality evaluation indicators, such as moisture content, porosity and degree of compaction etc., are still under the exploration stage.Based on the above problems exist in the application of the ground penetrating radar on city road, this paper carries out a model experiment, it analyzes the electromagnetic wave's propagation velocity in city road structural layer materials and also studies the influence from the dielectric constant and ratio of the components of the road structural layer materials on electromagnetic wave speed. Research on the application of normal materials'dielectric permittivity model on road structural layer materials like subgrade soil, cement concrete, and bituminous concrete. Besides, by ray tracing method, it implements a forward simulation on disease like low compactness, void, cavity and fissure; by genetic algorithm, it carries out an inversion study on dielectric property of the road structural layer materials; by field based comprehensive detection, it compares the measuring image and forward simulating image of the disease; and by the inversion program, it analyzes the thickness of the asphalt pavement. The main research results and conclusions it achieves are as follows:(1) Citv road structure layer material dielectric properties are studied. In view of the common cement concrete, asphalt concrete, and subgrade soil etc. road structure layer material, analyze the relationship between the dielectric properties and ages, moisture content, mixture ratio, grading, air voids, asphalt content and degree of compaction etc. quality evaluation index. Experimental results show that the key factors influencing the detection of electromagnetic wave velocity in concrete is the distribution of pore water, bound water and free water also have significant different influences on the detect of concrete electromagnetic wave speed. Ground penetrating radar (GPR) detects that the electromagnetic wave speed in concrete is rapidly increased with the decrease of moisture content. The dielectric constant of asphalt concrete decreases with the increase of void fraction, and presents linear correlation. Influence on the dielectric constant of asphalt concrete most is aggregate (gravel), it is mainly due to that aggregate volume and quality occupy the largest in asphalt mixture, dielectric properties of the aggregate determine the main dielectric properties of the asphalt concrete. Soil particle size level matching has certain influence to the soil dielectric constant, but less affected. The dielectric constant of soil is increased with the increase of soil moisture content and soil compaction degree. It reveals the relationship between dielectric property of road structural layer material and project quality evaluation indicators including moisture content, porosity and degree of compaction etc.(2) Composite dielectric permittivity calculation model of city road structure layer material is studied. In view of the common cement concrete, asphalt concrete, and subgrade soil structure etc. road layer material, the dielectric constant calculation rationality and adaptability of existing CRIM model, Brown and Looyenga model has been carried out.CRIM model, Brown model and Looyenga model is improved respectively. Calculation and test results show that deviation between dielectric constant calculated by using this improved value calculating model and the measured dielectric constant value is decreased significantly compared with the existing calculation model.(3) Proceed the forward modeling for the common urban roads loosen, void, holes etc. concealed diseases and road cracks, and then get the GPR profile record. Through research the propagation rules of radar wave medium in these diseases, analyze the characteristics of the radar reflected wave signal including diseases. Ground penetrating radar (GPR) forward modeling results show that the electromagnetic wave will produce strong reflection and certain diffraction in the area of less dense disease, waveform become disorder in disease area, poor regularity, and event correlation is very poor.900MHZ radar antenna can identify air void almost higher than wavelength, namely the void is greater than 5 cm. Due to the big water dielectric constant, low speed, its wavelength is about of the air, around, so from the point of wavelength resolution, can identify 3cm water filling void.900MHZ antenna can identify pavement cracks of 2cm, 1cm width, but need appropriate gain anomaly characteristics then could appear on the radar profile. When the pavement crack width of 5cm,4cm,3cm, radar profile can clearly reflect the anomalous characteristics of cracks.(4) Conduct forward modeling aims at the factors affecting the ground penetrating radar (GPR) detect resolution, the results show that the irregular medium, inhomogeneous medium with different dielectric parameters, uneven medium and different antenna frequency etc, has a great influence on detection of ground penetrating radar (GPR). Irregular medium with the increase of diseases buried depth, and expansion of the medium area, diffraction curve become widen. The greater electrical property difference between medium filling medium electrical and the surrounding medium, the more obvious medium characteristics, the stronger boundary diffraction signal. Inhomogeneous medium, the correlation of the event is very poor, medium area waveform is disorder, strong interference phenomenon exists between the diffraction wave. The higher antenna frequency, the shorter wavelength, the higher resolution of the ground penetrating radar (GPR).(5) By means of MATLAB to write the inversion program with genetic algorithm of road structure layer material dielectric constant and thickness, which have important implications for the linearization of nonlinear relations, and also greatly reduce the variability and uncertainty of the algorithm, thus promoting the practical application of ground penetrating radar (GPR); Through the inversion program used for road structure layer material dielectric constant and the thickness tests, the results show that the dielectric constant and the thickness are largely equal with the measured model values after inversion model calculation, relatively presents a more accurate road structure layer material dielectric properties, confirmed the inversion algorithm for road structure layer material of ground penetrating radar (GPR) dielectric constant and the thickness inversion calculation is reasonable and effective.(6) Through the practical application of comprehensive testing in certain northeast and southeast urban road, verified the feasibility and credibility of the forward modeling and inversion calculation method in urban road disease detection in this paper, successfully completed the comprehensive urban road detection work. |
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