Passivation of Prestressed Steel in Concrete With Slag and Exposed to Chlorides

ABSTRACT The usage of slag in prestressed concrete remains contentious because of contradictory results on the early‐stage passivation behaviour of steel in concrete with slag. In this study, six lollipop test specimens with prestressing steel wire (with/without axial stress) in mortar blended with 50% ordinary Portland cement and 50% slag were prepared and then subjected to cyclic wet‐dry chloride exposure and repetitive electrochemical impedance spectroscopy tests. The changes in the resistance of the passive film were monitored for 1 year. These tests revealed that the presence of free sulphides in slag‐based mortar resulted in inadequate early passivation and higher susceptibility to chloride‐induced depassivation within a month. Notably, steel reinforcement in concrete structures may not always get exposed to chlorides at early stages. Long‐term laboratory tests mimicking this field scenario revealed that as time passed, slag systems hydrated further, steel developed good passivation, and even allowed repassivation at later stages (after a few months), resulting in improved corrosion resistance.