In post-tensioned (PT) concrete, grout is used to protect the steel strand by providing a physical barrier to external moisture and contaminants and allowing for a protective passive film. However, corrosion failures of tendons associated with deficient grout materials occurred in Florida. Recent works demonstrated that elevated sulfate levels in alkaline environments can promote steel corrosion. However, testing in concrete materials sometimes did not reveal the role of sulfates. The purpose of the work here was to identify the influence of early and late sulfate ions presence on the formation of the passive film in alkaline solution. Steel specimens were exposed to saturated calcium hydroxide solution with sulfate levels ranging from 10,000 to 100,000 ppm. Electrochemical tests including open-circuit potential (OCP), linear polarization resistance (LPR), and anodic potentiodynamic polarization scans were made to identify conditions for steel passivation and corrosion initiation. In tests with early addition of sulfate, results indicated higher corrosion activity with greater sulfate concentration. Late sulfate additions after steel prepassivation showed less significant corrosion development below 30,000 ppm and relatively less corrosion in comparison to the early sulfate addition case. It was apparent that the sulfate ion concentrations in deficient grout can impair steel passivity but later sulfate presence due to external contamination would have less aggravating effect on corrosion development.
Effective highway bridge design in terms of strength, service, and economy can be made using prestressed concrete including post-tensioned (PT) construction.1-2 Bonded PT bridge designs incorporate high strength steel strand within tendon ducts and encapsulated with cementitious grout.3-4 The grout provides a protective barrier layer for the steel by preventing direct exposure to the external environment as well as facilitating the natural development of a thin passive oxide layer on the steel surface in presence of the high grout pore water pH. Despite this physical and chemical corrosion protection, some cases of PT strand corrosion that did not conform to conventional corrosion mechanisms associated with chloride-ion contamination, pore water carbonation, or the bleed-water at the grout-to-void interface5-11 occurred in Florida12-15 and elsewhere 16-17. Rather, the steel corrosion was associated with deficient grout materials with elevated sulfate ion concentrations.18-25 The deficient grout was characterized as a moisture-rich, high pH, material with elevated concentrations of sulfate and alkali ions.10,26 It was likely that the high sulfate levels were caused by grout segregation and were present soon after grout placement.27-30
