Ultrasonic and electromagnetic evaluation of corrosion-induced damage in reinforced concrete structures

• Main Author: Cheng, Weixia
• Other Authors: Fan Zheng, David
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/170116
• Institution: Nanyang Technological University

Corrosion in steel bars has been a major cause of the degradation of reinforced concrete (RC) structures. At the early stage of corrosion, rust grows from the reinforcing bar and builds up tensile stress within the concrete, leading to cracking of the concrete cover and reduced cross-sectional areas of the bars. To extend the service life of RC structures, non-destructive testing methods are required to assess early corrosion damage, such as internal cracks or reduced cross-sectional areas of reinforcing bars, so that timely repair can be carried out. Among various non-destructive evaluation tools that are commonly applied to RC structures, ultrasonic methods are sensitive to voids and cracks, while electromagnetic methods can be used to assess reinforcing bars. However, there still exist significant challenges to quantitatively evaluate corrosion-induced damage in concrete at an early stage, due to its heterogeneous properties. To overcome this limitation, this thesis aims to explore both ultrasonic and electromagnetic methods for detecting and characterising corrosion-induced cracks, as well as the cross-sectional loss of reinforcing bars. For the inspection and characterisation of corrosion-induced cracks in RC structures, two ultrasonic methods based on diffuse coda waves and Rayleigh waves are investigated. In the diffuse coda wave-based method, the scattering cross-section of the crack is reconstructed with the Locadiff imaging technique. Based on the assumption that both crack tips have the same scattering cross-section, the size of the crack can be estimated when the location of the reinforcing bar is known. Excellent accuracy has been demonstrated by performing numerical simulations on cracks with different lengths, orientations, and shapes. The validation experiment on corroded concrete samples with different crack sizes shows very good agreement with X-ray CT imaging results. It should be noted that the diffuse coda wave-based method is limited to assessing a single crack and requires the pristine condition of the specimen as a baseline. Hence, this thesis also explores a reference-free method to detect multiple internal cracks in concrete based on ultrasonic Rayleigh waves. In this method, the energy spectrum of the Rayleigh wave is analysed using continuous wavelet transform and correlated to the size of the internal crack. This method is not sensitive to the surface roughness of concrete structures and does not need accurate identification of the Rayleigh wave components. The performance of the method has been examined using corroded RC specimens with different cover depths and aggregate sizes. As a comparison, electrochemical tests have also been carried out to map the corrosion potential and corrosion current density, showing very good agreement with the corrosion damage map generated from the proposed Rayleigh wave-based method. In order to determine the cross-sectional loss of reinforcing bars caused by corrosion, an electromagnetic method based on ground-penetrating radar (GPR) array is explored. The diameter of the reinforcing bar is reconstructed using a multiple-input-multiple-output (MIMO) GPR array. This method employs a linear array of ultra-wideband (UWB) antennas to acquire the full-matrix MIMO data of the reinforcing bar embedded in concrete. The diffraction stacking algorithm is applied to the obtained MIMO data to reconstruct the reinforcing bar. The 3 decibels (dB) drop technique is used to measure the chord length that passes through the peak intensity point of reconstruction images of reinforcing bars. Following that, the diameter of reinforcing bars can be determined based on their cover depth and chord length. Numerical studies and experiments have been carried out on reinforcing bars with different diameters and cover depths, showing excellent sizing accuracy. Subsequently, this thesis extends the application of the MIMO GPR array method to monitor the corrosion of reinforcing bars. The reconstructed image of reinforcing bars from MIMO GPR array would be affected by the corrosion, which introduces rust and cracks to RC structures. Therefore, by monitoring the area enclosed by the contour line, it is possible to indicate the development of corrosion from a healthy state. Experimental corrosion monitoring has been carried out to show the feasibility of the proposed GPR array monitoring method. In addition, ultrasonic measurements have been conducted during the corrosion monitoring test. The analysis of coda wave interferometry and spectrum of transmitted Rayleigh wave serves to verify the results obtained from the GPR array imaging method, exhibiting good agreement.