3D Microwave Camera for Concrete Delamination and Steel Corrosion Detection

Corrosion of embedded steel reinforcement in concrete leads to concrete cracking and delamination, followed by increased salt and moisture permeation and further damage. Invisibility of the embedded rebar in combination with physical inaccessibility in elevated bridges presents a challenge in the assessment of reinforced concrete (RC) bridge elements. Wideband (3D) microwave synthetic aperture radar (SAR) imaging techniques that can be integrated into an unmanned aerial vehicle (UAV) offer a practical solution to overcome this challenge. Microwave SAR imaging acquires wideband data over a 2D spatial grid by raster (or electronic) scanning a reflectometer - real-time images. The recently-developed microwave camera can produce 3D SAR images at 30 image frames per second (see https://youtu.be/mK_zU-GHxRA). This 45-N microwave camera operates at a frequency range of 20-30 GHz and has an aperture size of 130 x 165 mm. Similar but lighter systems can be designed with aperture size and frequency range optimized for imaging of concrete in bridges and pavements. When mounted on a UAV, microwave cameras can cover a wide area of infrastructure as SAR imaging has been for terrain mapping and remote sensing. However, the position tracking accuracy requirement must be more stringent for SAR imaging in nondestructive evaluation (NDE) applications. The higher accuracy can be achieved in multiple ways (e.g. a more precise positioning device (laser) and a small optical camera). A recent study of concrete specimens with relatively high moisture and high chloride levels indicated great potential of 3D imaging to detect corrosion of the embedded steel bars in concrete. The 2D slice of a 3D image showed two corroded steel bars, embedded at 25 mm deep. Microwave signals can propagate through concrete and be reflected by steel reinforcing bars, delamination and voids. They are attenuated by moisture, ionic solution, and corrosion by-products. The principle of microwave SAR imaging in NDE applications has been well-documented . A wideband antenna is used to scan a bridge element surface following a 2D grid of certain step (sub-Nyquist sampling rate for reduced measurement time). Using a uniquely-designed and patented reflectometer, the collected reflected data (reference to aperture of the antenna) is then fed to a custom 3D SAR imaging algorithm. The resulting image resolution depends upon the overall scanned area dimension, the wavelength inside materials, and the standoff distance. Higher frequencies (or shorter wavelength) render images with higher 3D resolution depending on the operation frequency, bandwidth and the permittivity of the material. This project aims to develop and optimize a 3D microwave camera for bridge inspection on a UAV platform, quantify its performance for steel corrosion evaluation and concrete delamination detection in RC bridge elements, and build a microwave camera prototype that can be installed on a UAV for field applications. The focus of the first year will be to: (1) Prepare concrete specimens with embedded reinforcing bars, (2) Evaluate corrosion rate or mass loss of rebar in 3.5wt.% NaCl solution with EIS tests over time, (3) Periodically take the specimens out of the solution and scan them (for processing 3D images) at various relative humidity levels (measured) to quantify the effect of moisture on delamination, and (4) Optimize critical design parameters of 3D microwave cameras for high fidelity and spatial resolution.


Subject/Index Terms

Filing Info

  • Accession Number: 01646008
  • Record Type: Research project
  • Source Agency: Inspecting and Preserving Infrastructure through Robotic Exploration University Transportation Center
  • Contract Numbers: 69A3551747126
  • Files: UTC, RiP
  • Created Date: Sep 14 2017 11:35AM