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  • Characterization of Coarse Backfill Materials for Prevention

    of Corrosion of MSE Metallic Wall Reinforcement

    Research Report 0-6359-1

    TXDOT Project Number 0-6359

    Performed in cooperation with the

    Texas Department of Transportation and

    the Federal Highway Administration

    April 2013

    Center for Transportation Infrastructure Systems The University of Texas at El Paso

    El Paso, TX 79968 (915) 747-6925

    http://ctis.utep.edu

  • TECHNICAL REPORT STANDARD TITLE PAGE 1. Report No. FHWA/TX 11/0-6359-1

    2. Government Accession No.

    3. Recipient's Catalog No.

    4. Title and Subtitle Characterization of Coarse Backfill Materials for Prevention of Corrosion of MSE Metallic Wall Reinforcement

    5. Report Date October 2012 Revised April 2013

    6. Performing Organization Code

    7. Author(s) David Borrok, Arturo Bronson and Soheil Nazarian

    8. Performing Organization Report No. 0-6359-1

    9. Performing Organization Name and Address Center for Transportation Infrastructure Systems The University of Texas at El Paso, El Paso, Texas 79968-0516

    10. Work Unit No. (TRAIS)

    11. Contract or Grant No. Project No. 0-6359

    12. Sponsoring Agency Name and Address Texas Department of Transportation Research and Technology Implementation Office 125 E. 11th St., Austin, TX 78701-2483

    13. Type of Report and Period Covered Technical Report (Final) September 2008 – August 2012

    14. Sponsoring Agency Code

    15. Supplementary Notes Research Performed in Cooperation with the Texas Department of Transportation and Federal Highway Administration Research Study Title: Characterization of Backfill Materials for Prevention of Corrosion of MSE Metallic Wall Reinforcement 16. Abstract The service life of mechanically stabilized earth (MSE) walls depends on the rate of corrosion of the metallic reinforcements used in their construction. Assessment of corrosion potential requires the accurate evaluation of pH, resistivity, and ionic (e.g., sulfate and chloride) concentrations of aqueous solutions in contact with the surrounding aggregate. There is a tendency to utilize larger-size aggregates that contain only a small amount of fine material (passing No. 40 sieve) in the backfill. Evaluation of the electrochemical parameters of coarse aggregates is challenging because traditional methods utilize only fine-grained material. The effectiveness of traditional soil characterization techniques for use with coarse aggregates was evaluated by performing leaching experiments with coarse limestone and dolomite aggregates from six materials in Texas. Chemical differences were isolated from size-related kinetic leaching effects by comparing results from the same-sized material collected in the field versus material derived from the crushing of larger (≥ 3/8”) aggregates in the laboratory. Testing demonstrated that the fines collected from the field were enriched in chemicals that when exposed to water decreased pH and resistivity and increased chloride or sulfate concentrations relative to the bulk rock. This phenomenon can bias traditional soil testing results and therefore the assessment of corrosion potential. In this report a more representative geochemical testing protocol is recommended. The rate of corrosion was primarily controlled by the chloride content developed from the geochemistry of the backfill and the rate was predicted from the measured conductivity, as well as a two-step corrosion model was determined for the MSE.

    17. Key Words MSE Wall Reinforcement Corrosion Prevention Backfill Material Characterization

    18. Distribution Statement No restrictions. This document is available to the public through the National Technical Information Service, 5285 Port Royal Road, Springfield, Virginia 22161, www.ntis.gov

    19. Security Classif. (of this report) Unclassified

    20. Security Classif. (of this page) Unclassified

    21. No. of Pages: 137

    22. Price

    Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

  • DISCLAIMERS The contents of this report reflect the view of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Texas Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, a specification or a regulation. The material contained in this report is experimental in nature and is published for informational purposes only. Any discrepancies with official views or policies of the Texas Department of Transportation or the Federal Highway Administration should be discussed with the appropriate Austin Division prior to implementation of the procedures or results. NOT INTENDED FOR CONSTRUCTION, BIDDING, OR PERMIT PURPOSES David Borrok, PhD Arturo Bronson, PhD Sergio Rocha, BSEE Soheil Nazarian, Ph.D., PE (66495)

  • Characterization of Coarse Backfill Materials for Prevention of Corrosion of MSE Metallic Wall

    Reinforcement

    by

    David Borrok, PhD Arturo Bronson, PhD Sergio Rocha, BSEE

    and Soheil Nazarian, PhD, PE

    Conducted for Texas Department of Transportation

    Project Number 0-6359

    Characterization of Backfill Materials for Prevention of Corrosion MSE of Metallic Wall Reinforcement

    Research Report 0-6359-1

    April 2013

    Center for Transportation Infrastructure Systems The University of Texas at El Paso

    El Paso, TX 79968-0516

  • i

    Acknowledgements The authors would like to express their sincere appreciation to Mr. Marcus Galvan for his outstanding support and guidance on this project. The authors also thank the Project Management Committee of this project, consisting of Mike Arellano, Claudia Kern and Jimmy Si for their ever-present support. We are grateful to a number of TxDOT district personnel for their support and cooperation with the interviews, material collection and field testing including:

     Austin District: Rodney Matthews  Dallas District: Duke Hall  Waco District: Jacob Bell (TxDOT) and James B. Toon (W.W. Webber)

    We are also thankful to a number of graduate and undergraduate research assistances and CTIS staff that worked very hard to generate the information presented. These students include:

     Carlos Castillo, MSME  Jose Garibay, MSCE  Jesus Hinojos, MSME  Adalberto Ordanez, BSCE  Anita Thapalia, MSGS

    Special gratitude goes to Dr. Ken Fishman for his support and helpful comments.

  • ii

  • iii

    Abstract The service life of mechanically stabilized earth (MSE) walls depends on the rate of corrosion of the metallic reinforcements used in their construction. Assessment of corrosion potential requires the accurate evaluation of pH, resistivity, and ionic (e.g., sulfate and chloride) concentrations of aqueous solutions in contact with the surrounding aggregate. There is a tendency to utilize larger-size aggregates that contain only a small amount of fine material (passing No. 40 sieve) in the backfill. Evaluation of the electrochemical parameters of coarse aggregates is challenging because traditional methods utilize only fine-grained material. The effectiveness of traditional soil characterization techniques for use with coarse aggregates was evaluated by performing leaching experiments with coarse limestone and dolomite aggregates from six materials in Texas. Chemical differences were isolated from size-related kinetic leaching effects by comparing results from the same-sized material collected in the field versus material derived from the crushing of larger (≥ 3/8”) aggregates in the laboratory. Testing demonstrated that the fines collected from the field were enriched in chemicals that when exposed to water decreased pH and resistivity and increased chloride or sulfate concentrations relative to the bulk rock. This phenomenon can bias traditional soil testing results and therefore the assessment of corrosion potential. In this report a more representative geochemical testing protocol is recommended. The rate of corrosion was primarily controlled by the chloride content developed from the geochemistry of the backfill and the rate was predicted from the measured conductivity, as well as a two-step corrosion model was determined for the MSE.

  • iv

  • v

    Implementation Statement The products of this study are proposed new procedures that can be implemented by TxDOT. Recommendations have been made in terms of improving the process of estimating the corrosion rates of MSE reinforcement strands. This study seems to be one of the first studies focused on coarse backfill materials in the US. However, the recommendations are based on a rigorous study of a limited number of coarse backfill. The implementable products can consist of:

     Implementing the recommendations at a number of actual construction sites in Texas to fine-tune the specification

     Developing a database of electrochemical properties of the more common coarse backfill materials in Texas using the test protocols proposed

     Providing training-oriented presentations at annual construction confe

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