Use of microwave for investigating the integrity of structural elements has been established as a nondestructive evaluation (NDE) method in civil engineering, especially for detection of invisible damage such as voids and cracks inside concrete and debonding between rebars and concrete caused by corrosions and earthquakes, and objects such as steel rebars or dowels. The authors have found that focused microwaves are much more effective than unfocused ones and thus developed two systems for focusing microwaves: one using dielectric lenses and the other using antenna arrays. The former is referred to as a passive system as the focusing point has to be manually adjusted by moving the lenses, while the latter is an active system as focusing is automatically performed by software without moving the antenna arrays. This paper presents the numerical and experimental results, including measurement of dielectric properties of related materials, the passive microwave imaging technology using dielectric lenses, and the active microwave imaging technology using the antenna arrays with 3-dimensional image reconstruction algorithms. In addition, this paper presents two techniques, multi-frequency technique and incident field extraction technique, for further enhancing the performance of the microwave imaging technology in the field.
Various nondestructive evaluation (NDE) techniques have been studied to locate steel rebars or dowel, and to detect invisible damage such as voids and cracks inside concrete and debonding between rebars and concrete caused by corrosions and earthquakes. In this study, the authors developed 3-dimensional (3D) electromagnetic (EM) imaging technology to detect such damage and to identify exact location of steel rebars or dowel. The authors have developed sub-surface two-dimensional (2D) imaging technique using tomographic antenna array in previous works. In this study, extending the earlier analytical and experimental works on 2D image reconstruction, a 3D microwave imaging system using tomographic antenna array was developed, and multi-frequency technique was applied to improve quality of the reconstructed image and to reduce background noises. This paper presents the analytical expressions of numerical focusing procedures for 3D image reconstruction and numerical simulation to study the resolution of the system and the effectiveness of multi-frequency technique. Also, the design of 4x4 antenna array with switching devices is introduced as a preliminary study for the final design of whole array.
It has been well demonstrated that the structural performance of reinforced concrete (RC) columns can be enhanced by retrofitting using fiber reinforced polymer (FRP) composite jackets. Therefore, an increasingly large number of bridges and building columns have been retrofitted with such jackets. The authors developed a microwave imaging technology for detecting such damage as voids and debonding between the jacket and the column, which may significantly weaken the structural performance of the column. The authors developed surface imaging technology using focused microwave in previous works. In this paper, extending the earlier analytical and experimental works, a microwave sub-surface imaging system using antenna array was developed and verified for its capabilities to assess the damage of concrete structures. The proposed imaging system uses an arrangement consisting of several cylindrical or planar arrayed antennas for transmitting and receiving signals, and a numerical focusing operator is applied to the external signals both in transmitting and in receiving fields. This paper describes a numerical focusing procedure and numerical simulations demonstrated that the sub-surface image can be successfully reconstructed by using the proposed sub-surface imaging technology. For the experimental verification, the prototype of planar antenna array was fabricated and tested on the concrete block, in which the artificial voids were inserted.
As the state of the art in bridge design is advancing toward the performance-based design, it becomes increasingly important to monitor and evaluate the long-term structural performance of bridges, including strains in critical structural members, soil pressures on the abutment back walls and footings, accelerations on the decks and bents, etc. Such information is essential in developing new performance criteria for design. In this research, sensor systems for long-term structural performance monitoring have been installed on two new highway bridges on Orange County, California: the Jamboree Road Overcrossing and the West Street On-Ramp.
Jacketing technology using fiber reinforced polymer (FRP) composites is being applied for seismic retrofit of reinforced concrete (RC) columns designed and constructed under older specifications. In this study, the authors develop an electromagnetic (EM) imaging technology for detecting voids and debonding between the jacket and the column, which may significantly weaken the structural performance of the column otherwise attainable by jacketing. This technology is based on the reflection analysis of a continuous EM wave sent toward and reflected from layered FRP-adhesive-concrete medium: Poor bonding conditions including voids and debonding will generate air gaps which produce additional reflections of the EM wave. In this study, dielectric properties of various materials involved in the FRP-jacketed RC column were first measured. Second, the measured properties were used for a computer simulation of the proposed EM imaging technology. The simulation demonstrated the difficulty in detecting imperfect bonding conditions by using plane waves, as the scattering contribution from the voids and debonding is very small compared to that from the jacketed column. Third, in order to alleviate this difficulty, a special dielectric lens was designed and fabricated to focus the EM wave on the bonding interface. Furthermore, the time gating technique is used in order to reduce the noise resulting from various uncertainties associated with the jacketed columns. Finally, three concrete columns were constructed and wrapped with glass-FRP jackets with various voids and debonding condition artificially introduced in the bonding interface. Using the proposed EM imaging technology with the lens especially designed and installed, these voids and debonding condition were successfully detected.
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