This study aims to statistically analyze the distribution characteristics of localized corrosion along the length of corroded steel bars with spectral analysis techniques. Steel bars were embedded in a concrete prism and subjected to accelerated corrosion to levels ranging from 5.0 wt.% to 30.0 wt.% mass loss. After the corrosion test, the corroded steel bars were taken out of the concrete and cleaned with a sand blaster, and then scanned with a 3D laser scanner. The scanned point clouds were processed with an image processing software to determine the residual cross-sectional area distribution. Empirical mode decomposition (EMD) was performed by considering the area distribution of the steel bars as a nonlinear and non-stationary time series and the length as time. The intrinsic mode functions (IMFs) extracted from the EMD reveal the characteristics of pitting corrosion of various sizes, and the magnitude of the IMF is related to the cross-sectional area of corrosion pits. To extract the characteristic length of pitting corrosion, the fast Fourier transform (FFT) was performed on each IMF. Results show that the EMD-FFT successfully extracts the spatial distribution characteristics of corroded steel bars including surface irregularity, deformation (ribs and lugs), small corrosion pits and big corrosion pits. Presence of pitting corrosion changes the characteristic length of surface irregularity and rib spacing compared to a non-corroded bar.
Chloride-induced corrosion of steel reinforcement in concrete structures usually experiences two stages: pitting corrosion initiation and propagation.1-2 Initiation of pitting corrosion occurs as the chloride content on the surface of a steel bar exceeds a critical value (or chloride threshold) and breaks down the passive film locally.3-4 Depending on environmental factors at the steel-concrete interface, such as temperature, moisture, and availability of oxygen, pitting corrosion initiation is not stable in the very beginning and some corrosion pits may continue to develop and become stable, while others cease.5 These stable developed corrosion pits continue to grow and become deeper and larger, and may connect to each other over time, which corresponds to the propagation stage. The propagation of corrosion would cause concrete cover cracking, rapid diffusion of oxygen and moisture, and subsequently increased corrosion rate and further enlargement of localized pitting corrosion.6-7