Study On Microwave Tomography Of Steel Bars In Concrete

Steel bars are an important part of concrete structure.It ensures the safety and durability of concrete structures.Although the traditional damage detection method is direct,it can damage the structure and affect the safety of the structure.There are many methods of nondestructive testing for steel bars in concrete.Microwave imaging technology has its unique advantages in radar detection.It adopts microwave to irradiate the measured objects.According to the properties that different substances has different dielectric constant within the object being measured,the distribution of complex dielectric constant inside the object is reconstructed by detecting the scattering data of scattering field.It is of great significance for nondestructive testing of concrete structures.In this thesis,the nondestructive testing development of reinforced concrete and the research results of microwave imaging technology at home and abroad are summarized.First,the theoretical basis of microwave imaging was studied which includes microwave imaging methods,synthetic aperture radar technology,and ultra-wideband antenna research.The whole microwave imaging system was designed.HFSS simulation software was used to simulate the performance of horn antenna.The design requires that the working bandwidth of the double-ridged horn antenna is 0.8~6GHz,the average gain is greater than 10 dBi,and the average standing wave ratio is less than 3.A microwave imaging system based on vector network analyzer and double ridged horn antenna was utilized to test the reinforced concrete model.A transmission system with easy operation,high precision,and intelligent control was designed combined with PLC.It drives the antenna to move and realizes the accurate position,and the error of each move is less than 0.5 mm.The internal structure of concrete was imaged by utilizing ?-k algorithm in synthetic aperture microwave imaging method.The echo signal of the object surface contains a strong direct wave,which has a great influence on the target resolution.So the elimination method is used to remove the direct wave interference.Because of the fixed position of the steel bars in the concrete test block and the single experimental condition,the experimental parameters is determined by comparison of multiple groups of experiments and the accuracy of the imaging results can be verified by altering the position of the steel bars.A carton box model of450mm×500mm×400mm was designed to simulate concrete.Combining the theory and the comparison analysis of the imaging results,the appropriate imaging parameters of the system and antenna placement direction were selected.The steel bars with the diameter of the 14 mm at different positions and different depths are imaged exactly.The difference between the image position of steel bars and the real position is less than 2cm.A real reinforced concrete of 400mm×300mm×200mm was tested.It verifies the feasibility of using ultra-wideband continuous wave microwave signal in detecting steel bars in concretes.