Electrochemical/Microstructural Studies on the Corrosion of Prestressed Steel Strand in Concrete with Naturally Ingressing Chlorides

The occurrence of environmentally assisted cracking (EAC) is as dependent on the aggressiveness of the environment as the susceptibility of the material and the presence of static tensile stresses. However, the influence of the environment has not been adequately considered in past investigations on EAC mechanisms of prestressing strands. This study utilizes various characterization techniques to evaluate the surface/bulk deterioration of corroded pretensioned concrete (PTC) specimens after natural chloride exposure (by diffusion through cover concrete). Corroded strands in two PTC prism specimens (3,000 mm x 150 mm x 200 mm) were characterized using electrochemical impedance spectroscopy (EIS) and other microanalytical techniques. The EIS and scanning electron microscope (SEM) images obtained after 1 y and 2 y of exposure revealed a negligible residual protectiveness of the passivated surface (although the concretes possessed high resistivity). Raman spectra, SEM, and x-ray computed tomography images of extracted corroded strands also provided unique insights into the pattern of corrosion propagation in PTC systems subjected to realistic chloride exposure. Microcracks in the bulk metal beneath flatbottomed corrosion pits revealed the possible EAC at low chloride levels expected in service (<0.6% by weight of binder). The findings serve as a basis to define chloride-induced passive-to-active transition as the end of risk-free service life of PTC structures, and for considering it as the limit state for both service life design and corrosion assessment to avoid the onset of EAC.