United States Department of Transportation
i
Corrosion performance of prestressing strands in contact with dissimilar grouts.
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2013-01-01
[PDF-2.25 MB]
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Edition:Final report.
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Abstract:To improve the corrosion protection provided to prestressing strands, anti-bleed grouts are used to fill voids in post-tensioning
ducts that result from bleeding and shrinkage of older Portland Cement grouts. Environmental differences caused by exposing the
strands to dissimilar grouts, however, have the potential to cause rapid corrosion. Portland Cement grout, gypsum grout, and four
commercially available prepackaged grouts were analyzed to determine the chemical composition of the resulting pore solutions
and tested to determine if using a second grout will provide improved corrosion protection for prestressing strands or result in
accelerated corrosion. The potential consequences of leaving the voids unfilled were also evaluated. Pore solutions were analyzed
for pH and sodium, potassium, fluoride, chloride, nitrite, sulfate, carbonate, nitrate, and phosphate ion content. The analyses were
used to develop simulated pore solutions. Selected grouts and simulated pore solutions were paired to evaluate their potential to
cause corrosion of, respectively, grout-wrapped and bare stress-relieved seven-wire prestressing strands using the rapid macrocell
test. Strands were also evaluated in simulated pore solutions containing chlorides and in deionized water. Because exposure of
strands to water or chlorides has the potential to cause rapid corrosion, filling voids in post-tensioning ducts with an anti-bleed
grout is recommended. Gypsum grout, with its low pH and high sulfate content, will cause accelerated corrosion of strands when
used in conjunction with Portland Cement grout or any of the commercially prepackaged grouts tested. When paired with Portland
Cement grout, the prepackaged anti-bleed grouts evaluated in this study resulted in corrosion losses significantly below those
observed for strands exposed to salt or water. The highest corrosion measured for a prepackaged grout occurred for the grout with
the highest pore solution sulfate content.
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