Corrosion of Post-Tensioned Tendons with Deficient Grout, Part 3: Segregated Grout with Elevated Sulfate and Vestigial Chloride Content

Severe corrosion of post-tensioned (PT) tendons with pre-packaged thixotropic grout was documented in Florida bridges after less than 8 years of service. In those bridges, corrosion of steel tendon components was associated with deficient grout characterized by high moisture content, high sulfate levels, low chloride levels, and no indication of carbonation. Earlier research described in Parts 1 and 2 of this series of papers had emphasis on the role of sulfate ions in corrosion. However, at the time of observed corrosion of the bridge tendons in deficient grout, there were separate cases of bridges with similar pre-packaged grout materials but containing significant chloride ion contamination that exceeded typical acceptance limits. As a response to the potential risk to the bridge inventory, research was conducted to assess the risk associated with elevated chloride levels. Recommendations were made, but the proposed limits did not directly consider the effect of grout deficiencies such as that observed in the tendon failures in Florida. Therefore, there was interest to verify that the corrosion risk assessment of PT tendons by grout chloride concentration is adequate for adverse grout conditions such as due to segregation. In continuation of this series of papers on corrosion of post-tension strand in physically and chemically deficient grout, the work presented here sought to verify the role of elevated free sulfate ion concentrations in segregated grout containing low-level chlorides. Small and large scale laboratory specimens with geometries allowing for mix water volume displacement produced deficient grout. Physical and chemical deficiencies in segregated grout reduced the extent of chloride binding. Corrosion activity developed in grout with elevated sulfate concentrations. However, there were synergistic effects to reduce corrosion resistance for grouts with the combined presence of sulfate and chloride ions. It was shown that corrosion currents increased for steel in deficient grouts with elevated sulfate levels with the presence of low-level chloride ions (below conventional chloride threshold values). It was therefore evident that the assessment of corrosion susceptibility in deficient grout by chloride values alone is insufficient as sulfate ion presence and grout characteristics are also important.