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REPORT
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Publication Number:  FHWA-HRT-14-039    Date:  May 2014
Publication Number: FHWA-HRT-14-039
Date: May 2014

 

An FHWA Special Study: Post-Tensioning Tendon Grout Chloride Thresholds

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FOREWORD

Elevated levels of chloride were recently discovered in a commercially available pre-bagged grout product made for post-tensioned (PT) tendons. These chloride values exceed the current limits set by all of the domestic and international regulatory committees. 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. The findings of this study should be useful to bridge owners who may be responsible for similarly affected PT bridges.

This report presents two chloride threshold values determined for the PT strands that are fully encased in the commercially available grout described above. The first threshold is 0.4 percent chloride concentration by weight of cement, which is the lowest amount to initiate corrosion of strands. At this threshold, the testing program demonstrated that rust spots can form, and a small number of pits start to form beneath some of the rust spots. The second chloride threshold is 0.8 percent chloride by weight of cement in association with corrosion propagation. At this threshold, the testing program demonstrated that corrosion starts to intensify in terms of number of pits and pit depth; therefore, this threshold is considered to be critical. Once chloride concentration exceeds the critical threshold, significant corrosion damage can be anticipated, and the long-term structural integrity of the PT bridges may be compromised. If PT tendons contain 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, corrosion might start below the proposed threshold values.

Jorge E. Pagán-Ortiz
Director, Office of Infrastructure
Research and Development

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

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TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.
FHWA-HRT-14-039
2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle
An FHWA Special Study: Post-Tensioning Tendon Grout Chloride Thresholds
5. Report Date
May 2014
6. Performing Organization Code

 

7. Author(s)
Seung-Kyoung Lee and James Zielske
8. Performing Organization Report No.

 

9. Performing Organization Name and Address
Center for Advanced Infrastructure and Transportation
Rutgers University, The State University of New Jersey
100 Brett Road
Piscataway, NJ 08854-8058
10. Work Unit No. (TRAIS)

 

11. Contract or Grant No.
DTFH61-08-C-00005
12. Sponsoring Agency Name and Address
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered
Final Report
January 2012–September 2013

14. Sponsoring Agency Code
 
15. Supplementary Notes
The Contracting Officer’s Technical Representatives (COTRs) were Y.P. Virmani and Hamid Ghasemi, HRDI-60.
16. 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.

17. Key Words
Chloride threshold, Chloride, Corrosion initiation, Corrosion propagation, Seven-wire strand, Post-tensioning, Grout, Autopsy, Accelerated corrosion testing, Sulfate ions, Void
18. Distribution Statement
No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161.
19. Security Classification (of this report)
Unclassified
20. Security Classification (of this page)
Unclassified
21. No. of Pages
196
22. Price
Form DOT F 1700.7 (8-72)Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors

TABLE OF CONTENTS

LIST OF FIGURES

 

LIST OF TABLES

Table 1. Chemical analysis results
Table 2. Sodium chloride contents for task 2.1 test solutions
Table 3. Chemical compositions for task 2.1 test solutions
Table 4. Sodium chloride weights for tasks 2.2 and 2.3 specimen grout mixes
Table 5. 28-day compressive strength data of grout mixes
Table 6. Test surface area used in this study
Table 7. Corrosion rate criteria
Table 8. Different chloride concentration units reported in this study
Table 9. Summary of corrosion condition observed on single-strand specimens
Table 10. Sulfate ion concentrations in the samples
Table 11. Condition summary of interface segments of multi-strand specimens
Table 12. Condition summary of in-grout strand segments of multi-strand specimens
Table 13. Summary of the lowest chloride concentrations observed in the present study
Table 14. Suggested chloride threshold values for normal grout
Table 15. Adjusted chloride threshold values to be used for field grout powder samples
Table 16. Example of corrosion risk management plan with the suggested chloride
threshold values

 

 

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