United States Department of Transportation
i
Corrosivity of Water-Soluble Sulfate Ions in Simulated Pore Water Solutions and Different Types of Grout Samples
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2021-05-01
[PDF-3.33 MB]
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Abstract:While sulfate-induced corrosion problems have been observed in segregated grout, the existing literature lacks useful information about the role of sulfate ions in the corrosion process. To produce empirical data on the corrosivity and concentration of water-soluble sulfate ions under different grout conditions, researchers performed a laboratory study. In low-pH aqueous solutions, simulating carbonated bleed water and grout pore water, water-soluble sulfate ions were as detrimental as chloride to post-tensioned (PT) strands if the strands were not protected by high-pH grout. In high-pH aqueous solutions, sulfate was less corrosive than chloride. When chloride and sulfate of equal concentrations coexisted in different pH aqueous solutions, chloride was the dominant species in determining corrosivity of the solutions. Like chloride ions, water-soluble sulfate ions showed temperature-dependent corrosivity in that corrosion was more severe at the elevated than ambient temperature. Mean corrosion rates at the near-freezing temperature were less than 10 percent of those measured at the ambient temperature, regardless of sulfate concentration. The content of water-soluble sulfate in raw grout powders ranged from 0.35 to 1.0 percent by weight of sample, but most of the sulfate became bound to the hardened grout and the water-soluble sulfate reduced to 0.1 percent. Aged/expired raw grout was more prone to segregation and readily released water-soluble sulfate ions into the segregated grout. Petrographic examination and chemical analyses revealed that pore water and soft/wet segregated grout retained 100 times the water-soluble sulfate and more than five times the total sulfur content than dried and hardened grout did. This is because water-soluble sulfate ions can migrate via water movement toward areas with more moisture. If the segregated grout maintains a low-pH environment by carbonation, corrosion can be initiated on a strand in direct contact with a carbonated layer of segregated grout with a concentration of water-soluble sulfate ions as low as 0.05 percent by weight of sample. Just a thin film of carbonation, in intimate contact with the steel, is sufficient to induce the intensive corrosion.
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