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

Abstract

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 the 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 (3000 × 150 × 200 mm) were characterized using EIS and other microanalytical techniques. The EIS and SEM images obtained after one and two years of exposure revealed a negligible residual protectiveness of the passivated surface (although the concretes possessed high resistivity). Raman spectra, SEM and X-Ray CT images of extracted corroded strands also provided unique insights on the pattern of corrosion propagation in PTC systems subjected to realistic chloride exposure. Microcracks in the bulk metal beneath flat bottomed 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.