United States Department of Transportation
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Alternative Title:FHWA Special Study: Post-Tensioning Tendon Grout Chloride;Thresholds;
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Abstract:"Elevated levels of chloride were recently discovered in a commercially available pre-bagged grout product made for
post-tensioned (PT) tendons. Chloride concentrations were reported to be as high as 5.27 percent by weight of cement.
These numbers exceed the current limits set by all of the domestic and international regulatory committees. For
example, the chloride limits for prestressed concrete imposed by American Concrete Institute committees are either
0.06 percent water-soluble chloride by weight of cement or 0.08 percent acid-soluble chloride by weight of cement.
Chloride threshold is defined as a minimum chloride concentration needed to induce corrosion. However, there is
limited information reported in the literature pertaining to actual chloride threshold of seven-wire PT strands. For this
reason, the Federal Highway Administration sponsored a 6-month accelerated corrosion testing program to determine
chloride threshold(s) of PT strands exposed to chloride-contaminated grout.
Based on the test results obtained through the 6-month accelerated corrosion testing program and subsequent autopsy
of the specimens, two chloride threshold values were determined for the PT strands fully encased in normal grout. The
first threshold is 0.4 percent chloride concentration by weight of cement, which is the lowest amount needed to initiate
corrosion of strands. At this threshold, rust spots can be formed, and a small number of pits start to form beneath some
of the rust spots. The second critical chloride threshold is found in association with corrosion propagation. At this
threshold, corrosion starts to intensify in terms of the number of pits and pit depth. Test data and autopsy results
obtained in this study indicated that 0.8 percent chloride by weight of cement is the critical threshold. Once chloride
concentration exceeds the critical threshold, significant corrosion damage can occur rapidly, and structural integrity of
the PT bridges may be compromised in the near future. The actual deterioration rate will be influenced by many
factors. These threshold values are applicable to normal grout condition only. They may not be adequate in other
conditions such as carbonated grout, segregated grout, duct cracks, grout voids filled with water with or without
chloride ions, or free sulfate ions in contact with the strands. In these circumstances, corrosion should start below the
proposed threshold values."
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