Galvanic corrosion of strands in re-grouted, post-tensioned concrete bridges

ABSTRACT

Grouted, post-tensioned (PTd) concrete systems are widely used to construct bridges, typically with an anticipated corrosion-free service life of 100+ years. However, the usage of inadequate grout materials and grouting practices in PTd concrete systems have caused unwanted air voids in ducts, leading to strand-grout-air (SGA) interface, carbonation of exposed grout layer and localized corrosion of strands (say, within about 10 to 20 years). Re-grouting of voids as a tendon repair strategy has led to accelerated galvanic corrosion of the portion of strands at the interface between the carbonated base grout and repair grout with different chemistry, raising concerns and reluctance in re-grouting of voids in tendons. This work focused on understanding and quantifying the galvanic corrosion at the interface of carbonated base grout and repair grout in a re-grouted tendon. The theoretical analysis based on mixed potential theory estimated a galvanic current density of ≈ 2 μA/cm2 and showed that the galvanic coupling can increase the corrosion current density of the prestressing steel in the base grout by about two-fold. The study on prestressed steel in simulated solutions estimated a galvanic current density of ≈ 20 μA/cm2. Then, the study on prestressing steel in grouts and the analytical simulation estimated galvanic current densities around 1.5 to 2 μA/cm2 at 95% external relative humidity and 25 °C. A model relating the galvanic current density in grouted systems as a function of external relative humidity was developed, which showed an exponential increase in the galvanic corrosion with an increase in external relative humidity. Also, a case study showed that if the tendon anchorage region experiences 95% external relative humidity for about 20 years, sufficient strand corrosion could happen, and structural behavior can change from ductile to brittle nature, which could be a serious concern for structures in the coastal zone.