Ji Sang Yahng

Grout voids in external tendons of post-tensioned bridges are a critical issue, as they may result in the corrosion of the steel strands and significantly reduce tendon strength. Therefore, preventing tendon failure necessitates thorough inspection for these voids during both construction and operation. Terahertz electromagnetic wave testing is an effective method for detecting voids between the protective duct and the grout in external tendons, as terahertz waves can penetrate through the protective duct. This study introduces a D-band electronic frequency-modulated continuous-wave terahertz A-scanner for enhanced real-time inspection. The proposed method offers key advantages such as miniaturization, cost-effectiveness, and robustness, while providing effective detection of voids beneath the duct in external tendons. It is indicated that voids with a thickness of approximately 2.5 mm or greater can be detected using the D-band THz A-scanner.

Grout voids in the tendons of a post-tensioning bridge reduce their strength. Grout voids are also severe flaws causing corrosion of the steel strands in the tendons. Detecting voids during construction and operation of the tendons is essential to prevent tendon failure, which is critical to bridge safety. This study presents a method for inspecting external tendons for voids in the grout below the protective duct pipe using terahertz electromagnetic waves. Due to low attenuation in the high-density polyethylene duct and the large reflectivity difference between the duct/grout and the duct/void interfaces, terahertz waves are suitable for detecting voids in the grout inside tendons. For this study, we developed a mobile frequency-domain terahertz A-scanner that can be used to measure terahertz A-scan data in real time. It is shown that the mobile terahertz A-scanner can be used to assess the area of the grout void in external bridge tendons.

Composite materials are increasingly being utilized in many products, such as aircrafts, wind blades, etc. Accordingly, the need for nondestructive inspection of composite materials is increasing and technologies that allow nondestructive inspection are being studied. Existing ultrasound methods are limited in their ability to detect defects due to high attenuation in composite materials, and radiographic examination methods could pose a danger to human health. Terahertz (THz) wave technology is an emerging approach that is useful for imaging of concealed objects or internal structures due to high transmittance in non-conductive materials, straightness, and safety to human health. Using high-speed THz tomography systems that we developed, we have obtained THz tomographic images of glass-fiber-reinforced polymer (GFRP) laminates with artificial internal defects such as delamination and inclusion. The defects have various thicknesses and sizes, and lie at different depths. We present THz tomographic images of GFRP samples to demonstrate the extent to which the defects can be detected with the THz tomography systems.