High pH Corrosion of Prestressing Steel in Segregated Grout

Protection of prestressing tendons in post-tensioned structures can be ensured by the injection of a grout in the duct; since contact with hardened cement paste provides the conditions for passivity of steel. If ducts are fully injected corrosion protection may be guaranteed for a very long time thanks to the alkalinity of the grout, unless incomplete filling of the duct (and thus the lack of grout in contact with steel) or to penetration of chlorides through defects of sheath or to the heads of prestress occur. Recently, however, cases were reported where severe corrosion conditions took place in correspondence of areas where the grout was segregated. This phenomenon occurred mainly in the highest parts of external prestressing tendons with polyethylene ducts, where a whitish segregated grout was found, characterized by an alkaline pH and high content of sulphate ions. The corrosion attack encountered in these areas appeared much localized and, in some case, led to the failure of prestressing tendons; however, the mechanism of this type of corrosion has not yet been clarified. In order to investigate the corrosion of prestressing strands in segregated grout, corrosion tests were made in the laboratory. The results of tests suggest a complex mechanism which induces initiation of corrosion in the interstices among wires where the availability of oxygen is low. Subsequently propagation of localised attack is favoured by a macrocell that generates between the small depassivated areas and the surrounding passive areas.