U. S. Department of Transportation Publication No. FHWA-NHI-09-087
 Federal Highway Administration November 2009
 

NHI Courses No. 132042 and 132043 

CORROSION/DEGRADATION OF 

SOIL REINFORCEMENTS FOR 

MECHANICALLY STABILIZED EARTH WALLS 

AND REINFORCED SOIL SLOPES 
NOTICE

The contents of this report reflect the views of the authors, who are responsible for the facts and 

accuracy of the data presented herein. The contents do not necessarily reflect policy of the 

Department of Transportation. This report does not constitute a standard, specification, or 

regulation. The United States Government does not endorse products or manufacturers. Trade 

or manufacturer's names appear herein only because they are considered essential to the object of 

this document. 
 

 

 

Technical Report Documentation Page 

1. REPORT NO. 

FHWA-NHI-09-087 

2. GOVERNMENT 

ACCESSION NO. 

3. RECIPIENT'S CATALOG NO. 

4. TITLE AND SUBTITLE 

Corrosion/Degradation of Soil 

Reinforcements for Mechanically Stabilized 

Earth Walls and Reinforced Soil Slopes 

5. REPORT DATE 

November 2009 

6. PERFORMING ORGANIZATION CODE 

7. AUTHOR(S) 

Victor Elias, P.E., Kenneth L. Fishman, 

Ph.D., P.E., Barry R. Christopher, Ph.D., P.E. 

and Ryan R. Berg, P.E. 

8. PERFORMING ORGANIZATION REPORT NO. 

9. PERFORMING ORGANIZATION NAME AND ADDRESS 

Ryan R. Berg & Associates, Inc. 

2190 Leyland Alcove 

Woodbury, MN 55125 

10. WORK UNIT NO. 

11. CONTRACT OR GRANT NO. 

DTFH61-06-D-00019/T-06-001 

12. SPONSORING AGENCY NAME AND ADDRESS 

National Highway Institute 

Federal Highway Administration 

U.S. Department of Transportation 

Washington, D.C. 

13. TYPE OF REPORT & PERIOD COVERED 

14. SPONSORING AGENCY CODE 

15. SUPPLEMENTARY NOTES: 

This report is an updated version of FHWA SA-96-072 and FHWA-NHI-00-044. 

16. ABSTRACT 

This manual provide’s criteria for evaluating corrosion losses when using coated or uncoated 

steel reinforcements, and for determining aging and installation damage losses when using 

geosynthetic reinforcements. Monitoring methods for in-situ corrosion rates for steel 

reinforcements are evaluated and remote methods using electrochemical methods are 

recommended. Monitoring methods for determinations of in-situ aging of geosynthetics are 

evaluated and protocols for implementation are recommended. 

17. KEY WORDS 

Mechanically Stabilized Earth Walls (MSEW), 

Reinforced Soil Slopes (RSS), soil reinforcement, 

geosynthetics, geotextiles, geogrids, corrosion, 

monitoring, retrievals, polarization resistance, oxidation, 

hydrolysis 

18. DISTRIBUTION STATEMENT 

No restrictions. 

19. SECURITY CLASSIF. 

Unclassified 

20. SECURITY CLASSIF. 

Unclassified 

21. NO. OF PAGES 

142 

22. 
 

 

 

 

 

 

 

SI CONVERSION FACTORS 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

mm 

m 

m 

km 

millimeters 

meters 

meters 

kilometers 

0.039 

3.28 

1.09 

0.621 

inches 

feet 

yards 

miles 

in 

ft 

yd 

mi 

AREA 

mm2 

m

2 

m

2 

ha 

km2 

square millimeters 

square meters 

square meters 

hectares 

square kilometers 

0.0016 

10.764 

1.195 

2.47 

0.386 

square inches 

square feet 

square yards 

acres 

square miles 

in2 

ft2 

yd2 

ac 

mi2 

VOLUME 

ml 

l 

m3 

m3 

millimeters 

liters 

cubic meters 

cubic meters 

0.034 

0.264 

35.71 

1.307 

fluid ounces 

gallons 

cubic feet 

cubic yards 

fl oz 

gal 

ft3 

yd3 

MASS 

g 

kg 

grams 

kilograms 

0.035 

2.202 

ounces 

pounds 

oz 

lb 

TEMPERATURE 

°C Celsius 1.8 C + 32 Fahrenheit °F 

kN/m3 

WEIGHT DENSITY 

kilonewton / cubic m 6.36 poundforce / cubic foot pcf 

FORCE and PRESSURE or STRESS 

N 

kN 

kPa 

kPa 

newtons 

kilonewtons 

kilopascals 

kilopascals 

0.225 

225 

0.145 

20.9 

poundforce 

poundforce 

poundforce / sq in. 

poundforce / sq ft 

lbf 

lbf 

psi 

psf 
 

PREFACE
 

Engineers and specialty material suppliers have been designing reinforced soil structures for 

the past 35 years. During the last decade significant improvements have been made to the 

design methods and in the understanding of factors affecting the durability of the soil 

reinforcements. This work is becoming even more important now that many reinforced soil 

structures are well into their anticipated design life. 

In order to take advantage of recent developments, the FHWA has updated the previous 

version (Elias, 2000) of this manual. The primary purpose of this manual is to serve as the 

FHWA standard reference for highway projects involving reinforced soil structures. This 

manual also supports the national efforts on health monitoring of bridge foundations and 

asset management, providing test techniques and protocols that are being employed to collect 

performance data for earth reinforcements, data interpretation and preliminary information 

available from data that has been collected to date. 

Another purpose of equal importance was to support educational programs conducted by 

FHWA for transportation agencies. 

This Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls 

and Reinforced Soil Slopes manual has evolved from the following FHWA reports and 

previous manuals: 

C Durability/Corrosion of Soil Reinforced Structures; V. Elias, FHWA RD-89-186 

C Testing Protocols for Oxidation and Hydrolysis of Geosynthetics, FHWA RD-97-144. 

C Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls 

and Reinforced Soil slopes: V. Elias, FHWA-NHI-00-044 and FHWA SA-96-072. 

The first author of this manual is the late Mr. Victor Elias, P.E. who dedicated much of his 

professional career to the advancement of reinforced soil technology. As indicated above, he 

was the author of the previous two FHWA manuals on corrosion/degradation of soil 

reinforcements and was the principle investigator for much of the research supporting this 

manual. Mr. Elias was instrumental in the introduction and implementation of reinforced soil 

technology in the U.S., as a Vice President for The Reinforced Earth Company from 1974 to 

1985. His work included major reinforced soil retaining walls. In addition, he expanded the 

applications of soil reinforcement to slabs, dams, storage facilities and bridge abutments. 

FHWA NHI-09-087 

Corrosion/Degradation i November 2009 
Mr. Elias provided significant contributions to the design and construction of safe, costeffective geotechnical works in highway (and private) works. He has been the Principal 

Investigator for several major research and/or implementation projects focused on durability 

of soil reinforcement materials and design specifications for foundations and retaining walls, 

and ground improvement methods. He provided this leadership role in eight design and 

construction reference manuals for FHWA, four corrosion/durability reinforced soil research 

projects, and the recommended revisions of the MSE wall section of the 1990 and 1994 

AASHTO Standard Specifications for Highway Bridges as well as contributions to the 

revisions of the MSE wall sections in the 1997 and 2002 AASHTO Standards. The 

coauthors of this manual, his many colleagues, fellow engineers and friends within the 

geotechnical community will dearly miss Victor’s leadership, insights, staunch opinions, and 

experience. 

The authors also recognize the efforts of Mr. Jerry A. DiMaggio, P.E. who was the FHWA 

Technical Consultant for most of the above referenced publications. Mr. DiMaggio's 

guidance and input to this and the previous works has been invaluable. 

The leadership role of Mr. Rich Barrows, P.E., Mr. Silas Nichols, P.E., and Mr. Dan 

Alzamora, P.E. from the FHWA geotechnical team in the development, facilitation and 

review of this manual is certainly appreciated. 

The authors acknowledge the efforts of the following Technical Working Group members 

who served as a review panel listed in alphabetical order: 

C Tony Allen, P.E. of Washington DOT 

C Christopher Benda, P.E. of Vermont DOT 

C James Brennan, P.E. of Kansas DOT 

C Kathryn Griswell, P.E. of CALTRANS 

C John Guido, P.E. of Ohio DOT 

C Dan Johnston, P.E. of South Dakota DOT 

And the authors acknowledge the contributions of the following industry associations: 

C Association of Metallically Stabilized Earth (AMSE) 

C Geosynthetic Materials Association (GMA) 

C National Concrete Masonry Association (NCMA) 

FHWA NHI-09-087 

Corrosion/Degradation ii November 2009 
TABLE OF CONTENTS
 

CHAPTER 1 INTRODUCTION.............................................................................................. 1-1 

1.1 OBJECTIVES OF MANUAL ............................................................................. 1-1 

1.2 SCOPE ................................................................................................................. 1-2 

1.3 ORGANIZATION ............................................................................................... 1-3 

1.4 PROJECT NCHRP 24-28 "LRFD Metal Loss and Service Life Strength 

Reduction Factors for Metal Reinforced Systems in Geotechnical 

Applications" ....................................................................................................... 1-3 

CHAPTER 2 CORROSION OF METALLIC REINFORCEMENTS ................................ 2-1 

2.1 FUNDAMENTALS OF CORROSION OF METALS IN SOIL ........................ 2-1 

a. Galvanized Coatings ................................................................................ 2-2 

b. Metalization with GALFAN.................................................................... 2-3 

c. Fusion Bonded Epoxy Coatings............................................................... 2-3 

d. Polymeric Barrier Coating with Zinc....................................................... 2-4 

2.2 IDENTIFICATION OF CORROSIVE ENVIRONMENTS ............................... 2-5 

a. Geological................................................................................................ 2-6 

b. Salt Water Intrusion ................................................................................. 2-7 

c. Stray Currents .......................................................................................... 2-8 

d. Other Environmental Factors................................................................... 2-9 

2.3 ELECTROCHEMICAL TEST METHODS...................................................... 2-10 

a. Soil Resistivity....................................................................................... 2-12 

b. Soluble Salts........................................................................................... 2-15 

c. pH .................................................................................................... 2-20 

d. Organic Material .................................................................................... 2-20 

2.4 DESIGN CORROSION RATES AND THEIR APPLICATION ..................... 2-21 

a. Available Data ....................................................................................... 2-21 

b. Design Approach ................................................................................... 2-24 

c. Current Practice ..................................................................................... 2-26 

2.5 MARGINAL FILLS .......................................................................................... 2-29 

2.6 TEMPORARY WALLS .................................................................................... 2-30 

CHAPTER 3 MONITORING METHODS, METALLIC REINFORCEMENTS.............. 3-1 

3.1 CORROSION MONITORING FUNDAMENTALS.......................................... 3-2 

3.2 IMPLEMENTATION OF FIELD CORROSION MONITORING 

PROGRAMS........................................................................................................ 3-5 

a. Plan Development.................................................................................... 3-5 

b. Monitoring Programs............................................................................... 3-6 

c. New Structures......................................................................................... 3-9 

d. Existing Structures (Retrofit)................................................................. 3-12 

e. Materials ................................................................................................ 3-14 

f. Measurement Procedures and Equipment.............................................. 3-19 

FHWA NHI-09-087 

Corrosion/Degradation iii November 2009 
g. Frequency of Measurement.................................................................... 3-24 

3.3 EVALUATION OF CORROSION MONITORING DATA ............................ 3-25 

3.4 APPLICATION OF CORROSION MONITORING TO ASSET 

MANAGEMENT............................................................................................... 3-27 

a. Document Inventory and Prepare Performance Database ..................... 3-27 

b. Data Needs and Data Analysis............................................................... 3-29 

c. Estimate Future Needs for Maintenance, Rehabilitation 

 and Replacement.................................................................................... 3-31 

d. Update Experience with Different Reinforced Fills .............................. 3-31 

3.5 STATE CORROSION MONITORING PROGRAMS ..................................... 3-32 

CHAPTER 4 DURABILITY OF GEOSYNTHETIC REINFORCEMENTS..................... 4-1 

4.1 INTRODUCTION ............................................................................................... 4-1 

a. Overview of Available Products.............................................................. 4-2 

b. Materials Structure and Manufacture....................................................... 4-4 

4.2 FUNDAMENTALS OF POLYMER DEGRADATION .................................... 4-7 

a. Oxidation of Polyolefins (PP and HDPE)................................................ 4-8 

b. Hydrolysis of Polyester (PET)................................................................. 4-9 

c. Stress Cracking ............................................................................ 4-10 

d. UV Degradation..................................................................................... 4-10 

e. Biological Degradation .......................................................................... 4-11 

f. General Chemical Dissolution ............................................................... 4-12 

g. Summary................................................................................................ 4-13 

4.3 IDENTIFICATION OF SOIL ENVIRONMENTS WHICH ACCELERATE 

DEGRADATION .............................................................................................. 4-15 

a. Background............................................................................................ 4-15 

b. Salt-affected Soils .................................................................................. 4-16 

c. Acid-sulphate Soils ................................................................................ 4-16 

d. Calcareous Soils..................................................................................... 4-17 

e. Organic Soils.......................................................................................... 4-17 

f. Soils Containing Transition Metals ....................................................... 4-17 

g. Modified Soils........................................................................................ 4-18 

4.4 IDENTIFICATION OF POLYMER CHARACTERISTICS/ADDITIVES TO 

MITIGATE DEGRADATION AND TESTING METHODS .......................... 4-18 

a. Polyolefins (PP and HDPE)................................................................... 4-19 

b. Polyesters (PET) .................................................................................... 4-21 

4.5 EVALUATION OF INSTALLATION DAMAGE........................................... 4-22 

a. Summary of Available Installation Damage Results............................. 4-23 

b. Summary................................................................................................ 4-25 

4.6 AGING REDUCTION FACTORS.................................................................... 4-27 

a. Field Retrievals ...................................................................................... 4-27 

b. Accelerated Laboratory Testing............................................................. 4-28 

c. Summary................................................................................................ 4-32 

4.7 USE OF DURABILITY DATA FROM "SIMILAR" PRODUCTS ................. 4-35 

FHWA NHI-09-087 

Corrosion/Degradation iv November 2009 
CHAPTER 5 MONITORING METHODS, GEOSYNTHETIC REINFORCEMENTS... 5-1 

5.1 INSTALLATION DAMAGE TESTING ............................................................ 5-1 

5.2 POLYMER DEGRADATION MONITORING.................................................. 5-5 

a. Identification of Site Conditions and Structure Description.................... 5-5 

b. Testing of Control Samples and Retrieved Samples ............................... 5-6 

c. Retrieval Methods.................................................................................... 5-8 

d. Soil Tests................................................................................................ 5-10 

5.3 EVALUATION OF GEOSYNTHETIC DEGRADATION MONITORING 

DATA ................................................................................................................ 5-11 

a. Polyester (PET)...................................................................................... 5-10 

b. Polyolefins (PP and HDPE .................................................................... 5-11 

REFERENCES........................................................................................................................... 6-1 

FHWA NHI-09-087 

Corrosion/Degradation v November 2009 
 

LIST OF FIGURES
 

Figure Page 

Figure 2-1. Metal loss as a function of resistivity (galvanized steel). .................................. 2-16 

Figure 2-2. Metal loss as a function of resistivity (carbon steel).......................................... 2-17 

Figure 2-3. Summary of electrochemical cell test data at 25% saturation ........................... 2-23 

Figure 2-4. Summary of electrochemical cell test data at 50% and 100% saturation. ......... 2-23 

Figure 3-1. Schematic diagram showing locations of coupons and instrumented 

 reinforcement members ....................................................................................... 3-8 

Figure 3-2. Stages of galvanized structure performance. ....................................................... 3-9 

Figure 3-3. Schematic diagram for connection to reinforcing members.............................. 3-13 

Figure 3-4. A portable Copper/Copper Sulfate half cell is hand held on the soil at the base 

of the wall as a reference electrode for multi-meter testing of

 electric potential................................................................................................. 3-16 

Figure 3-5. PR Monitor evaluation of a test location. Note horizontal adjacent access 

 holes for cross testing......................................................................................... 3-15 

Figure 3-6. Schematic diagram for coupons......................................................................... 3-16 

Figure 3-7. Schematic diagram illustrating coupon connection. .......................................... 3-17 

Figure 3-8. Typical installation............................................................................................. 3-20 

Figure 3-9. Automated polarization resistance measurement equipment............................. 3-21 

Figure 3-10. Junction platform and wire spools used to organize connections to inspection 

elements for electrochemical testing.................................................................. 3-36 

Figure 3-11. Minimum reistivity (min) vs. corrosion rate (CR) for corresponding reinforced 

fill and inspection rod locations......................................................................... 3-38 

Figure 3-12. Moisture content (w%) vs. corrosion rate (CR) for corresponding reinforced 

fill and inspection rod locations......................................................................... 3-38 

Figure 3-13. Distribution of corrosion rate from sites in southern California........................ 3-39 

Figure 3-14. Transient response of reinforcements at Dodge and Sweet Home Road Sites.. 3-45 

Figure 3-15. Half-cell potentials from coupons and in-service reinforcements from
 

corrosion monitoring of MSE walls constructed in North Carolina.................. 3-46 

Figure 3-16. Zinc index (ZI) distributions at increasing reinforcement age from corrosion 

monitoring of MSE walls constructed in North Carolina .................................. 3-47 

Figure 4-1. Installation damage vs. backfill size. ................................................................. 4-26 

Figure 4-2. Laboratory aging equipment setup..................................................................... 4-30 

Figure 4-3. Generalized Arrhenius plot used for low-temperature predictions from hightemperature experimental data........................................................................... 4-31 

Figure 5-1. Scheme for sampling test specimens........................................................................ 5-3 

Figure 5-2. Scanning Electron Microscopy (SEM), polyester fibers........................................ 5-13 

Figure 5-3. Scanning Electron Microscopy (SEM), polypropylene fibers. .............................. 5-14 

FHWA NHI-09-087 

Corrosion/Degradation vi November 2009 
 

LIST OF TABLES
 

Table ` Page 

Table 2-1. Aggressive Soil Environments............................................................................. 2-5 

Table 2-2. Recommended Sampling Protocol for Electrochemical Testing for 

 MSE Wall Fill.................................................................................................... 2-12 

Table 2-3. Effect of Resistivity on Corrosion. .................................................................... 2-14 

Table 2-4. Maximum Permissible Levels of Soluble Salts. ................................................ 2-19 

Table 3-1. Summary of Field Corrosion Data, Site 4, Lower Level. .................................. 3-26 

Table 3-2. Summary of State DOT MSEW Corrosion Assessment Programs. .................. 3-34 

Table 3-3. Selected Sites in Southern California. ............................................................... 3-35 

Table 3-4. Summary of Laboratory Data from Caltrans and Comparison with 

 Field Observations. ............................................................................................ 3-36 

Table 3-5. Florida Corrosion Rate Statistics for Sites Subjected to Tidal Fluctuations...... 3-41 

Table 3-6. Florida Corrosion Rate Statistics for 17 Year Old Tallahassee Site.................. 3-41 

Table 3-7. Florida Corrosion Rate Statistics for New Brickell Ave. Site ........................... 3-41 

Table 3-8. Florida Corrosion Rate Statistics for Typical Coastal and Land Sites............... 3-42 

Table 3-9. Electrochemical Properties of Slag/Cinder Ash Reinforced Fill at I990 over 

Dodge and Sweet Home Road Sites in Buffalo, NY ......................................... 3-43 

Table 4-1. Raw Material in Geotextile and Geogrid Production. ......................................... 4-2 

Table 4-2. Geotextile and Geogrid Structure. ....................................................................... 4-2 

Table 4-3. Geosynthetic Applications................................................................................... 4-3 

Table 4-4. Major PP Product Groups. ................................................................................... 4-5 

Table 4-5. HDPE Product Group ......................................................................................... 4-6 

Table 4-6. Major PET Product Groups. ................................................................................ 4-6 

Table 4-7. Commonly Identifiable Degradation Mechanisms. ........................................... 4-14 

Table 4-8. Anticipated Resistance of Polymers to Specific Soil Environments. ................ 4-19 

Table 4-9. Installation Damage Reduction Factors............................................................. 4-25 

Table 4-10. Summary of Product-Specific Studies Needed to Evaluate the Durability of 

Geosynthetic Reinforcement.............................................................................. 4-33 

Table 4-11. Aging Reduction Factors, PET. ......................................................................... 4-33 

FHWA NHI-09-087 

Corrosion/Degradation vii November 2009 
FHWA NHI-09-087 

Corrosion/Degradation viii November 2009 
CHAPTER 1 

INTRODUCTION
 

1.1 OBJECTIVES OF MANUAL 

The use of mechanically stabilized earth (MSE) systems for the construction of retaining wall 

structures and steepened slopes has gained widespread acceptance among owners, as 

evidenced by the many thousands of completed structures. As usage increased in the 1980s 

and 1990s there was, however, a desire by owners and the research community to confirm 

that current methods are valid and that the design models used will ensure that these 

structures will perform as intended for their full design life. Previous editions of this 

reference guide proposed test protocols including electrochemical techniques for condition 

assessment and corrosion monitoring of MSE soil reinforcements. Various researchers, the 

FHWA and state transportation agencies have implemented these test protocols to collect 

information and develop performance databases. These techniques are now considered 

mature technologies that can be implemented for asset management to document 

performance and practices that contribute to effective use of resources and cost savings to 

owners. 

It is important to consider the performance of MSE structures within the context of 

Transportation Asset Management (TAM), as slopes and retaining walls are important 

components of the highway system. Their performance depends on proper selection of 

materials, details of construction and maintenance. These are important considerations and 

their impacts on cost and service life affect decisions inherent to TAM. 

The design of MSE structures requires that the combination of a select soil and reinforcement 

be such that the interaction between the two materials produces a composite structural 

material that combines their best characteristics. The judicious placement of reinforcements 

in the select soil mass serves to restrain the deformation of the soil in the direction parallel to 

the reinforcement. 

The most commonly used soil-reinforcing for retaining walls on transportation projects has 

been galvanized steel, either in strip or grid configuration (~80 to 90 percent of applications 

to date), connected to a precast concrete facing. Polymeric soil reinforcements were 

introduced in the 1970s and early 1980s. They have been used with increasing frequency in 

both MSE walls and reinforced soil slopes (RSS) since their introduction. Today, the 

majority of RSS on transportation projects use geosynthetic soil reinforcements. 

FHWA NHI-09-087 1 – Introduction 

Corrosion/Degradation 1 – 1 November 2009
 
A major design concern for MSE structures has been the durability of reinforcements in the 

soil/water environment in which they are placed. The dual aim of this manual is to provide 

criteria to guide design engineers in evaluating potential corrosion losses when using coated 

or uncoated steel reinforcements, and degradation losses when evaluating the use of 

polymeric reinforcements. The other aim is to guide engineers in implementing field 

evaluation schemes to monitor such corrosion/degradation mechanisms in constructed 

structures. 

The monitoring of corrosion losses in these structures is addressed by implementation of 

non-destructive field evaluation systems using remote electrochemical measuring equipment 

capable of determining in-situ corrosion rates of galvanized and base steel and inferring from 

them the loss of section. The monitoring of degradation losses for polymeric reinforcements 

is addressed by implementation of retrieval protocols and destructive testing of samples to 

measure loss of tensile strength and changes in the polymer structure. This manual was 

originally developed (Elias, 1997) in support of a FHWA Demonstration Project on the 

design, construction and monitoring of MSE walls and slopes. The manual was updated in 

2000 (Elias, 2000). The principal function of this manual is to serve as a reference source for 

the long-term performance of soil reinforcements used in MSE structures. This current 

update incorporates the most recent work in this field, the NCHRP 24-28 study (see Section 

1.4, below). 

The test techniques and procedures described in the manual have been researched and 

developed over the past several decades. These electrochemical test techniques and test 

protocols are mature technologies, and useful tools for asset management. Another objective 

of this manual is to describe how these tools can be used for asset management. The benefits 

of performance monitoring and asset management are demonstrated by several examples 

included in the manual. 

1.2 SCOPE 

The scope of this manual includes: 

• Description of the corrosion/deterioration mechanism that occurs in reinforced soil 

structures constructed with metallic reinforcements, leading to design procedure 

recommendations. 

• Description of techniques and instrumentation designed to measure in-situ corrosion 

rates of steel reinforcements in MSE structures. 

FHWA NHI-09-087 1 – Introduction 

Corrosion/Degradation 1 – 2 November 2009
 
• Review of laboratory test methods for the electrochemical analysis of select 

reinforced fill materials used in MSE structures. Relationships between these test 

variables and predictions of corrosion/degradation are also discussed. 

• Review of criteria to determine survivability of fusion bonded epoxy coatings. 

• Identification of degradation mechanisms consistent with in-ground regimes for 

geosynthetic reinforcements. 

• Monitoring methods and evaluation of degradation mechanisms for geosynthetic 

reinforcements. 

1.3 ORGANIZATION 

Chapter 2 is devoted to the fundamentals of corrosion of metals in soil, identification of 

corrosive environments, and details current design approaches to account for in-ground 

corrosion. 

Chapter 3 details monitoring methods for metallic reinforcements and their application to 

existing and new construction. 

Chapter 4 is devoted to the fundamentals of polymer degradation and identification of in-soil 

regimes that may accelerate degradation. 

Chapter 5 details monitoring methods for geosynthetic reinforcements, and their application 

to existing and new construction. 

Greater detail on topics discussed in Chapters 2 and 3 are detailed fully in FHWA RD 89-186 

Durability/Corrosion of Soil Reinforced Structures (Elias, 1990), a primary source document 

for this manual. 

Greater detail on topics discussed in chapters 4 and 5 are detailed fully in FHWA RD-97-

144, Testing Protocols for Oxidation and Hydrolysis of Geosynthetics (Elias et al., 1997). 

1.4 PROJECT NCHRP 24-28 “LRFD Metal Loss and Service Life Strength 

Reduction Factors for Metal Reinforced Systems in Geotechnical Applications.” 

NCHRP 24-28 aims to: (1) assess and improve the predictive capabilities of existing models 

for corrosion potential, and for estimating metal loss and service life of earth reinforcements, 

and (2) to develop methodology that incorporates the improved predictive models into an 

FHWA NHI-09-087 1 – Introduction 

Corrosion/Degradation 1 – 3 November 2009
 
LRFD approach for the design of MSE. The project scope includes collecting data on the 

performance of metallic reinforcements used in the construction of MSE, developing a 

database for archiving performance data, statistical analysis of performance data, and 

reliability analysis of metal loss estimates used to ensure the specified design life. The test 

techniques and protocols for condition assessment and corrosion monitoring described in this 

reference manual were used in pursuit of performance data for NCHRP 24-28. 

We expect the final report for NCHRP 24-28 to be issued for distribution before September 

2010. Anticipated products from NCHRP Project 24-28 include: 

1. A performance database documenting the attributes and metal loss observed from inservice MSE reinforcements. 

2. Updated metal loss models considering targeted levels of confidence and various site 

conditions. 

3. Recommended resistance factors for use in LRFD designs that account for the estimated 

metal loss over the service life of the structure. 

4. A recommended practice that specifically addresses issues related to metal loss from 

corrosion including required sampling and testing, example design procedure, and 

commentary. 

The field experience, performance data, and insights into factors affecting metal loss gained 

from NCHRP 24-28 contribute to information included in this manual. Retaining walls and 

slopes are important components of the highway system, and the database and test protocols 

resulting from NCHRP 24-28 serve as important tools for asset management. These data 

incorporate effects of climate, soil environment and site conditions, which are significant 

factors in terms of service-life. A major contribution from NCHRP 24-28 is to evaluate 

effects from reinforced fill quality, site conditions, maintenance operations and climatic 

factors on variance, and hence uncertainty, with respect to corrosion and anticipated metal 

loss. 

Changes to the current AASHTO metal loss model are not anticipated; however, confidence 

and model reliability are assessed. The current AASHTO model only applies to galvanized 

reinforcements and reinforced fill meeting stringent electrochemical requirements. Data 

from reinforced fills that don’t necessarily meet all of these requirements (e.g.,  < 3000 -

cm) were also collected during the fieldwork for NCHRP Project 24-28. These data will be 

used to evaluate parameters and other adjustments needed to estimate the impacts of 

marginal quality fills on service life. 

FHWA NHI-09-087 1 – Introduction 

Corrosion/Degradation 1 – 4 November 2009
 
CHAPTER 2 

CORROSION OF METALLIC REINFORCEMENTS
 

The current design approach to account for potential corrosion losses is to add to the required 

structural thickness a sacrificial thickness equal to the projected section loss over the design 

life of the structure. To minimize the sacrificial thickness and reduce uncertainties, a select 

fill with controlled electrochemical properties is specified for the reinforced zone. This 

chapter is intended to provide a background in the fundamentals of corrosion, the 

identification of corrosive environments by electrochemical testing and a review of the basis 

for the currently used design corrosion loss rates. 

2.1 FUNDAMENTALS OF CORROSION OF METALS IN SOIL 

Accelerated or unanticipated corrosion of the reinforcements could cause sudden and 

catastrophic failure of MSE structures, generally along a nearly vertical plane of maximum 

tensile stresses in the reinforcements. This plane is located at a distance varying from 0 to 

0.3H from the facing where H is the height of the structure. Few instances of advanced 

corrosion that have compromised service life of MSE structures have been documented in the 

United States, Europe and South Africa (Blight and Dane, 1989; Elias, 1990; Fishman et al., 

1986; Frondistou-Yannis, 1985; Armour et al., 2004; Gladstone et al, 2006; McGee, 1985; 

Raeburn et al., 2008). 

Corrosion is the deterioration or dissolution of metal or its properties by chemical or 

electrochemical reaction with the environment. When a large surface is affected it can be 

viewed as general corrosion and approximated by an assumed average, uniform rate of 

corrosion per year. If confined to small points so that definite indentations form in the metal 

surface, it is referred to as pitting corrosion and generally reported as maximum pit depth per 

year. 

Corrosion is fundamentally a return of metals to their native state as oxides and salts. Only 

the more noble metals (platinum, gold, etc.) and copper exist in nature in their metallic state 

and are resistant to corrosion. Other metals are refined by applying energy in the form of 

heat. Unless protected from the environment, these metals revert by the corrosion process, 

which is irreversible, from their temporary state to a more natural state. 

Although most chemical elements and their compounds are present in soil, only a limited 

number exert an important influence on corrosion. In areas of high rainfall, the passage of 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 1 November 2009
 
time has resulted in the leaching of soluble salts and other compounds, rendering these soils 

generally acidic. In arid locations, soluble salts are brought to the upper soil layers through 

capillary and evaporative processes, causing the soils to be generally alkaline. (Romanoff, 

1957) 

The authoritative reference work to date on underground corrosion is National Bureau of 

Standards (NBS) Circular 579 (Romanoff, 1957). The corrosion mechanism of ferrous and 

other metals in soils is essentially electrochemical. The corrosion process releases the energy 

the metal gained during its refining in the form of electrical energy. Current flows because 

of a voltage difference between two metal surfaces or two points on the same surface in the 

presence of an electrolyte. Two pieces of metal or two portions of the same metal in an 

electrolyte seldom have the same potential. The amount of potential difference depends on 

the nature of the metal, the condition of the surface, the nature of the electrolyte, and the 

presence of different materials at the interface of the metal and electrolyte. Under these 

conditions, a current will flow from the anodic area through the electrolyte or soil to the 

cathodic area and then through the metal to complete the circuit. The anodic area becomes 

corroded by the loss of metal ions to the electrolyte. 

In general, the most corrosive soils contain large concentrations of soluble salts, especially in 

the form of sulfates, chlorides, and bicarbonates and may be characterized as very acidic (low 

pH) or highly alkaline (high pH). Clayey and silty soils are characterized by fine texture, 

high water-holding capacity, and consequently, by poor aeration and poor drainage. They are 

also prone to be potentially more corrosive than soils of coarse nature such as sand and 

gravel where there is greater circulation of air. Buried metals corrode significantly by the 

process of differential aeration and sometimes by bacterial action. Corrosion by differential 

aeration may result from substantial local differences in type and compaction of the soil or 

variations in the oxygen or moisture content resulting thereof. Such a phenomenon is 

generally associated with fine-grained soils. Microbial induced corrosion is associated with 

the presence of anaerobic sulfate-reducing bacteria that reduce any soluble sulfates present in 

the soil to sulfides. The corrosion process can be slowed or mitigated by the use of coatings. 

a. Galvanized Coatings 

A common method to protect the base metal, carbon steel, from corrosion is to galvanize it 

whereby a layer of zinc on the surface is used to protect the underlying steel. Zinc layers are 

deposited via the hot dip process by dipping the steel member in a bath of molten zinc. 

Coatings of this type initially protect the underlying metal mechanically. When the 

continuity of the coating is destroyed by potential difference on the surface, the underlying 

metal may be protected either galvanically or mechanically by the formation of a protective 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 2 November 2009
 
film of zinc oxides. The protection process is of a sacrificial nature in which zinc acts as the 

sacrificial anode to the bare portions of the steel until it is all consumed. However, there is a 

limit to the distance (throw) along an element between areas of bare steel and zinc coating to 

achieve an effective level of galvanic action. 

b. Metalization with GALFAN 

GALFAN® (a registered trade name of the International Lead Zinc Research Organization 

(ILZRO)) is a zinc-5% aluminum-mischmetal alloy that offers an alternative to 

galvanization. GALFAN is applied similar to the hot dip galvanization process. Some 

observations (CTL, 2001) suggest that GALFAN may perform as well, or better than zinc 

coating, for slightly acidic, normally to moderately corrosive backfill soils. However, 

advantages to using GALFAN are less distinct considering mildly corrosive reinforced fill 

soils, typical of MSE wall construction. Any advantages of using GALFAN versus zinc 

coating has not been demonstrated for burial conditions that represent engineered, select, 

granular soils specified for use as MSE wall fill. Furthermore, the performance of GALFAN 

has not been verified for a wide range of reinforced fill conditions including alkaline soils. 

Jailloux and Anderson (1999) describe application of a protective coating consisting of a zinc 

aluminum alloy (85% Zn-15% Al), called “Dunois”, by thermal spraying. Dunois is proposed 

as an economical solution for applications with reinforced fill soils that are considered 

aggressive relative to the potential for corrosion of plain or galvanized steel. Similar to 

GALFAN, in aggressive environments, Dunois may provide better corrosion protection 

compared to galvanization, however data is not currently available to verify its effectiveness. 

Given the added costs of GALFAN and Dunois compared to zinc, they may only be 

economical solutions where marginal quality materials are used for reinforced fill and the 

benefit of improved performance compared to zinc can be realized. 

c. Fusion Bonded Epoxy Coatings 

As another alternative to galvanized coatings, fusion-bonded epoxy coatings on (nongalvanized) steel reinforcements have been used on a number of projects. Galvanized 

reinforcements may also be epoxy coated. Fusion-bonded epoxy coatings are dielectric. 

They cannot conduct current and therefore deprive the corrosion mechanism of a path for 

galvanic currents to flow, essentially terminating the corrosion process. 

These coatings need to be hard and durable to withstand abrasion under normal construction 

conditions and have strong bonding properties to the base metal to ensure long-term integrity. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 3 November 2009
 
Significant use of fusion-bonded epoxy protection for underground structures has been made 

by the pipeline industry. However, in most cases pipelines also use cathodic protection in 

addition to coatings. 

To be effective, fusion-bonded coatings must be impermeable to gases and moisture and free 

of even microscopically thin gaps at the interface between the metal and the coating. The 

ability to withstand construction induced abrasions must be determined in order to develop 

design recommendations that would ensure longevity. Newer epoxy coatings including 

purple marine epoxy such as 3M Scotchkote 426 are also available. 

These corrosion protection and mitigation systems may be beneficial, and offer improved 

performance, when marginal wall fills are being considered; that do not resist corrosion to 

the extent of those currently specified by AASHTO. . Johnston (2005) describes an MSE 

wall in Deadwood, South Dakota where fusion bonded, epoxy coated reinforcements were 

used and performance was documented. Given the added cost compared to zinc, fusion 

bonded epoxy coatings may be economical solutions where marginal quality materials are 

used for reinforced fill and the benefit of improved performance compared to zinc can be 

realized. This is particularly true if the epoxy coating is applied on top of a galvanized 

surface rendering a double corrosion protection system. 

d. Polymeric Barrier Coating with Zinc 

Galvanized steel reinforcement may be coated with a polymeric barrier as described in 

ASTM A641/A (2004a) and ASTM A975 (2004a). Similar to epoxy coatings, polymeric 

materials are dielectric and therefore deprive the corrosion mechanisms of the needed current 

path. To be effective, coatings must be impermeable to gasses and moisture and free of 

microscopically thin gaps at the interface between the metal and coating. Coatings need to 

be hard and durable to withstand abrasion under normal construction conditions and bond 

strongly to the base metal to ensure long-term integrity. The polymeric barrier is used to 

increase the service-life of the steel and zinc in the MSE reinforcement. The rate of zinc and 

steel loss may be considerably reduced by the presence of the polymeric barrier. The design 

life of the reinforcement depends on the durability of the polymer, time for the zinc coating 

to be consumed, and amount of sacrificial steel. 

HITEC (2002) describes evaluation of a system that includes PVC-coated, galvanized, 

double-twisted wire mesh reinforcement for use within select granular backfill. The PVC 

coating is extruded onto the galvanized wires before they are twisted together forming the 

mesh. The steel wires are galvanized to a minimum of 0.8 oz ft2

, which is equivalent to a 

thickness of approximately 33 m, and then PVC coated to a minimum thickness of 0.5 mm. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 4 November 2009
 
2.2 IDENTIFICATION OF CORROSIVE ENVIRONMENTS 

Escalante (1989) and Fitzgerald (199) discuss the effects of soil characteristics on corrosion. 

General descriptions of soil corrosivity based on pedological descriptions are also possible 

using soil survey data (Miller et al., 1981). Soil environments that are known to be aggressive 

relative to corrosion of galvanized or plain steel reinforcements are summarized in Table 2-1. 

Aggressive soils are identified in terms of electrochemical properties including pH, 

resistivity, and salt content. Details of each of these soil environments and other conditions 

that contribute to corrosive conditions including the presence of stray currents and other 

environmental factors are described in the following subsections. 

Table 2-1. Aggressive Soil Environments. 

Environment Prevalence Characteristics 

Acid-Sulfate Soils Appalachian Regions Pyritic, pH < 4.5, SO4 (1000­

9000 ppm), CL­ (200-600 ppm) 

Sodic Soils Western States pH > 9, high in salts including 

SO4 and ClCalcareous Soils FL, TX, NM and 

Western States 

High in carbonates, alkaline but 

pH <8.5, mildly corrosive 

Organic Soils FL (Everglades), GA, 

NC, MI, WI, MN 

Contain organic material in 

excess of 1% facilitating 

microbial induced corrosion 

Coastal 

Environments 

Eastern, Southern and 

Western Seaboard 

States and Utah 

Atmospheric salts and salt laden 

soils in marine environments 

Road Deicing Salts Northern States Deicing liquid contain salts that 

can infiltrate into soils 

Industrial Fills Slag, cinders, fly ash, 

mine tailings 

Either acidic or alkaline and 

may have high sulfate and 

chloride content 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 5 November 2009
 
a. Geological 

Potentially corrosive environments are usually characterized as being highly acidic, alkaline 

or found in areas containing significant organic matter that promotes anaerobic bacterial 

corrosion. In the United States, acid sulphate soils are often found in areas containing pyritic 

soils, as in many Appalachian regions in the Southeast and Middle Atlantic States. These 

soils are further characterized by a high level of soluble iron (Fe) that can produce highly 

aggressive biogenic iron sulphides. Generally, rock containing pyritic sulfur in excess of 0.5 

percent and little or no alkaline minerals will produce a pH of less than 4.5, which has a 

considerable potential for producing sulfuric acid. 

The predominant anion in acid sulphate soils is sulphates with concentrations ranging from 

1000 to 9000 PPM (parts per million) and the predominant cation is sodium with reported 

concentrations of 1500 to 3000 PPM. Typically, acid sulphate soils contain significant 

soluble levels of iron and chlorides, although levels vary greatly. Chloride levels are 

reported in the range of 200 to 600 PPM. These soils and rocks are identified by the 

presence of noticeable yellow mottles attributable to pyrite oxidation. 

Alkaline soils are described as being either salt affected (sodic) or calcareous. Sodic soils are 

generally found in arid and semiarid regions where precipitation is low and there are high 

evaporation and transpiration rates. In the United States, they primarily occur in seventeen 

western states. 

Sodic soils are characterized by low permeability and thus restricted water flow. The pH of 

these soils is high, usually >9 or 9.5, and the clay and organic fractions are dispersed. The 

major corrosive solute comprising dissolved mineral salts are the cation Na and the anions Cl 

and SO4. 

Calcareous soils are those that contain large quantities of carbonate such as calcite (calcium 

carbonate), dolomite (calcium-magnesium carbonate), sodium carbonates, and sulfates such 

as gypsum. These soils are characterized by alkaline pH, but the pH is less than 8.5. 

Calcareous soils are widespread and occur in Florida, Texas, New Mexico, and many of the 

Western States and are generally mildly corrosive. 

Organic soils are classified as bogs, peats, and mucks. Most organic soils are saturated for 

most of the year unless they are drained. They contain organic soil materials to a great depth. 

The major concentrations are found in the Everglades of Florida, the Okefenokee Swamp in 

Georgia, the Great Dismal Swamp in North Carolina and Virginia, and in the peat bog areas 

of Michigan, Wisconsin, and Minnesota. It is estimated that one-eight of the soils of 

FHWA NHI-09-087 2 – Corrosion 

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Michigan are peats. They are, however, locally widespread throughout the United States. 

Dredged soils, widespread along coastal areas, generally also contain a high percentage of 

organic matter. 

Salt concentrations may be affected during service due to contamination in marine 

environments. The soil along the coast often contains salt. Salt can also directly intrude into 

reinforced fill where structures are constructed directly on the sea front or nearby (i.e., during 

storm surge events). The air in coastal environments often contains a level of salt that can 

deposit on the ground surface and intrude into the soil over time. Salt contamination may 

also occur due to use of deicing salts along roadways in the northeast and other regions 

subject to snow and ice, as discussed in the next section. 

Industrial fills such as slag, fly ash, and mine tailings may be either acidic or alkaline 

depending on their origin. Cinder ash or slag-cinder ash mixtures in particular are likely to 

be acidic and contain significant amounts of sulphates. Slag may or may not be corrosive, as 

the characteristics of slag vary depending on how it is cooled and processed, e.g. air-cooled 

blast furnace slag is alkaline; not acidic. On the other hand, cinder ash is high in salt, may be 

acidic and performance problems of MSE constructed using mixtures of slag and cinder ash 

have been documented by the NYSDOT (Moody, 1993) and Elias (1990)). Modified soils, 

cement, or lime treated can be characterized by a pH as high as 12. Crushed concrete may 

also contain un-hydrated free lime. The corrosive potential of industrial fills should be 

evaluated using the procedures discussed in Chapter 3 before considering the use of these 

materials as reinforced fill. 

b. Salt Water Intrusion 

MSE reinforced fill may be contaminated during service from deicing salts applied to the 

roadways. Sodium chloride is by far the most popular chemical deicer, although calcium 

magnesium acetate (CMA) has been considered as an alternative (TRB, 1991). Chemical 

deicers may be applied alone or mixed with sand. If the salt is able to permeate the reinforced 

fill and comes into contact with steel or galvanized reinforcements, accelerated corrosion can 

occur. The salt intrusion is not uniform and causes the potential along the surface to vary 

with distinctly defined areas of cathodic and anodic activity. This may lead to localized 

attack of the protective layers of oxides adhered to the surface leading to a pitting type of 

corrosion. The advantage to using CMA is that it is less corrosive to steel compared to 

sodium chloride. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 7 November 2009
 
A remedy to salt intrusion is to ensure an impervious barrier exists near the wall top that 

covers the reinforced fill. An impervious barrier (i.e., geomembrane) beneath the pavement 

and above the reinforced fill, and draining to a collection system, is recommended (Berg et 

al., 2009). It is unlikely that the pavement surface itself can provide the needed barrier, even 

if it fully covers the reinforced soil zone, as joints and cracks will inevitably allow infiltration 

during winter deicing salt use. An impervious barrier is particularly important where runoff 

water flows toward the wall and/or where snow piles will exist above the wall. A pavement 

surface could provide protection if the surface is graded such that the runoff flows away from 

the wall, joints and cracks are quickly sealed, the pavement section is well drained, and a 

free-draining (i.e., less than 3 to 5% non plastic fines passing the No. 200 (0.075 mm) sieve, 

e.g. AASHTO No. 57 stone) reinforced fill is used. Even with these precautions, a long-term 

corrosion monitoring program is recommended if a pavement is to provide the barrier to 

deicing salt runoff into the reinforced fill. 

The effect of salt contamination on corrosion rates may also be minimized by the use of free 

draining material (i.e., less than 3 to 5% non plastic fines passing the No. 200 (0.075 mm) 

sieve) for reinforced fill. Timmerman (1990) studied salt infiltration from deicing salts into 

bridge approach embankments and the long-term effects on MSE stability due to potential 

corrosion of the metal system components. Reinforced fill samples were retrieved through 

holes cored in the wall facing of the selected walls in Ohio and tested for chlorides, pH, and 

resistivity. Samples were taken at approximately 3-week intervals between January 1988 and 

April 1989. Of the twenty-seven sampling locations at the three MSE bridge abutment test 

sites, only three locations had measured soil parameters, which could be considered as 

marginally corrosive to the MSE components; and these conditions only existed for limited 

time periods during the winter road-salting season. This result was attributed to the use of 

free-draining reinforced fill specified by the Ohio DOT for MSE wall construction. 

The Florida Department of Transportation observed corrosion rates of walls subjected to tidal 

inundation (Sagues, 2000). Accelerated corrosion rates were not observed, which was 

attributed to the use of free draining backfills and corresponding flushing of chlorides during 

the rainy season. 

c. Stray Currents 

In addition to galvanic corrosion, stray currents may be an additional source of corrosion for 

MSE systems constructed adjacent to electrically powered rail systems or other sources of 

electrical power that may discharge current in the vicinity of these systems, such as existing 

utilities, cathodically protected radio stations, etc. Stray earth currents can be caused by DCpowered transit or other rail systems. These currents are generated by the voltage drop in the 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 8 November 2009
 
running rails, which are used as negative return conductors. This potential difference causes 

differences in track-to-earth potential that varies with time, load (train), location, and other 

factors. Earth-potential gradients are generated by stray current leakage from the rails. The 

magnitude of this current is a function of track-to-earth potential and resistance. The 

magnitude of stray earth current being discharged or accumulated by a source can be 

estimated by measuring earth electrical gradients in the source area. From these 

measurements, the probable effect of stray corrosion can be estimated by a corrosion 

specialist. 

In general, stray currents decrease in magnitude rapidly as they move away from the source 

and are believed not to be a factor 100 to 200 ft (30 to 60 m) away from the source. For 

structures constructed within these distances, AASHTO recommends that a corrosion expert 

evaluate the hazard and possible mitigating features on a project-specific basis. Furthermore, 

it is recommended that a long-term corrosion monitoring program be integrated into the 

design, if steel reinforcements are used. 

For direct current traction power railways, there is some indication that the effect of stray 

currents on the corrosion of metal strip type reinforcements depends on the orientation of the 

strips with respect to the current flow path (Sankey and Anderson, 1999). For many 

installations the strips are orientated perpendicular to the current flow path, and provided the 

resistivity of the backfill is sufficiently high, effects from stray currents may not be a 

significant concern. A corrosion expert should determine whether stray currents are not a 

significant concern for a specific project and a long-term corrosion monitoring program 

should be integrated into the wall design to confirm the recommendations. 

d. Other Environmental Factors 

The level of compaction and grain size distribution of backfills placed around reinforcements 

have an effect on corrosion and corrosion rates. 

Soil Compaction 

Compaction of soil is defined as the reduction of air voids between particles of soil and is 

measured by the mechanical compression of a quantity of material into a given volume. 

When soil compaction occurs evenly, soil resistivity is consistent and corrosivity is generally 

decreased. Soil permeability is reduced with compaction and provided drainage is adequate 

and the soil is non-aggressive (neutral or alkaline), corrosion should be decreased. However, 

the effect of compaction is related to soil cohesiveness. In clay soils, the corrosion rate 

shortly after burial increases with compaction. Well-drained, granular soils with moisture 

contents of less than 5 percent are non-aggressive, but drainage decreases with increasing 

FHWA NHI-09-087 2 – Corrosion 

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compaction, leading to marginal increases of corrosion. These theoretical marginal 

differences have not been quantified to date. 

Moisture Content 

Soil structure, permeability, and porosity determine the moisture content of a soil. Where the 

moisture content of a soil is greater than 25 to 40 percent, the rate of general corrosion is 

increased. Below this value, a pitting type corrosion attack is more likely. 

The corrosion of mild steel increases when soil moisture content exceeds 50 percent of 

saturation. This may be compared to the critical relative humidity (rh) that occurs above 

ground in atmospheric corrosion. Research data strongly suggest that maximum corrosion 

rates occur at saturations of 60 to 85 percent (Darbin et al., 1986). This range of saturation 

for granular materials roughly corresponds to the range of moisture content required in the 

field to achieve needed compaction levels. 

A survey of 14 California sites found saturation levels in MSE fills to be between 30 and 95 

percent, with most samples exceeding 65 percent (Jackura et al., 1987). Therefore the 

placement compaction requirements for MSE structures will be subject to the maximum 

corrosion rates consistent with all other electrochemical criteria. 

2.3 ELECTROCHEMICAL TEST METHODS 

The design of the buried steel elements of MSE structures is predicated on the measurement 

of key index parameters of the reinforced fill, which govern corrosivity, the desired life of 

the structure, and the assessment of such basic environmental factors as location and 

probability of changes in the soil/water environment. Several parameters influence soil 

corrosivity, including soil resistivity (), degree of saturation, pH, dissolved salts, redox 

potential and total acidity. These parameters are interrelated but may be measured 

independently. The direct link between any one soil parameter and a quantitative corrosion 

relationship has not been fully substantiated, but a general consensus has been established 

based on studies of buried metals that resistivity is the most accurate indicator of corrosion 

potential (Romanoff, 1957; King, 1977). Current research projects (2008), NCHRP 21-06 

and 24-28, are focused on developing a better understanding between laboratory 

measurements of index properties and in-situ corrosion rates. 

The frequency and distribution of samples for assessment of electrochemical parameters 

needs to be given careful consideration. The number of samples required increases when 

evaluating more aggressive or marginal backfill materials, and when more confidence is 

FHWA NHI-09-087 2 – Corrosion 

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needed for design (Withiam et al., 2002; Hegazy et al., 2003). Existing data involving 

frequent sample intervals at sites with poor conditions depict a wide scatter in results 

(Whiting, 1986; Fishman, et al, 2006). For moderate to large sized projects, with fill sources 

that are expected to be relatively nonaggressive relative to corrosion (i.e. mildly corrosive 

soils meeting AASHTO criteria), Table 2-2 can be used to determine the number of samples 

that should be taken from each source and evaluated for electrochemical parameters. More 

samples should be retrieved if marginal quality reinforced fills are being contemplated for 

construction (not recommended), or when undertaking performance evaluations at sites with 

poor reinforced fill conditions. In addition to the mean values used for design (i.e., the mean 

of the minimum resistivity (min) values obtained from each test), the distribution and 

variability of the measurements is of significant interest from the standpoint of reliabilitybased design (LRFD). 

Table 2-2 places restrictions on the allowable standard deviations () of the resistivity and 

salt content (see comment 3) measurements. If these standard deviations are exceeded, then 

the sampling should be repeated. If the standard deviation, computed using the total numbers 

of samples, is still outside the limits of Table 2-2, then the backfill source should not be used 

for MSE wall fill. If resistivity less than 3000 -cm is obtained from any test, obtain 

additional samples in the vicinity of this sample location to identify if there are specific areas 

wherein the material is unsuitable. 

Stockpiles should be sampled from the top, middle and bottom portions and an excavator 

with a bucket should be used to remove material from approximately two feet beyond the 

edge of the stockpile. Particular emphasis on sampling needs to be placed at sites where 

different reinforced fill sources and/or types are being considered; and each source should be 

sampled as described in Table 2-2. 

Differences in the electrochemical properties of the soil fill can adversely effect corrosion 

rates, and contribute to more severe and localized occurrences of metal loss. In instances 

where more easily compacted (e.g. open graded) material is placed adjacent to the wall face, 

significant differences in the soil fill conditions may exist with respect to position along the 

reinforcements. For cases where reinforcements are not electrically isolated (e.g., metallic 

facing) variations of backfill types along the height of the wall may also have a significant 

effect on corrosion rates of metallic reinforcements. 

FHWA NHI-09-087 2 – Corrosion 

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Table 2-2. Recommended Sampling Protocol for Electrochemical 

Testing of MSE Wall Fill.
 

Range 

of min 

(-cm) 

General 

Description 

Preconstruction During 

Construction 

No. Comments

Samples 

resistivity 

(-cm) 

Sample 

Interval (yd3) 

> 10, 000 Crushed rock 

and Gravel, 

< 10% passing 

No. 10 sieve 

1 / 31 NA NA 1. pH outside the 

specified limits is not 

allowed for any sample. 

2. backfill sources shall 

be rejected if min 

measured for any 

sample is less than 700 

-cm, Cl­

-

>500 ppm or 

SO4 > 1000 ppm. 

3. For materials with 

min < 5000 -cm,  for 

CL­ and SO4 shall be 

less than 100 ppm and 

200 ppm, respectively. 

5,000 to 

10,000 

Sandy Gravel 

and Sands 

3 / 61 < 2000 4000 / 20001 

< 5,000 Silty sands and 

Clayey sand, 

screenings 

5 / 101 < 1000 2000 / 10001 

1 # resistivity tests / # tests for pH, Cl-

, and SO4 

The influence and measurements techniques for key parameters used in construction control 

can be summarized as follows: 

a. Soil Resistivity 

Soil resistivity is defined as the inverse of conductivity. Resistivity is the convention of 

expressing the resistance of materials in units of ohm-cm. For more practical chemical and 

biological usage, the scientific community uses the algebraic inverse of ohm-cm resistance 

for conductivity expressed in mhos. The current preferred international standard (SI) system 

uses the term electrolytic conductivity expressed in units of siemen per meter (S/m) in which 

100 S/m is equal to 1 mhos/cm. 

The electrolytic behavior of soils is an indirect measurement of the soluble salt content. The 

amount of dissolved inorganic solutes (anions and cations) in water or in the soil solution is 

directly proportional to the solution electrolytic conductivity. The major dissolved anions in 

soil systems are chloride, sulfate, phosphate and bicarbonate, with chloride and sulfate the 

most important anionic constituents in corrosion phenomena. The electrolytic conductivity 

(EC) of the soil solution is the sum of all the individual equivalent ionic conductivities times 

their concentration. 

FHWA NHI-09-087 2 – Corrosion 

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Because soil resistivity governs the effectiveness of the ionic current pathway, it has a strong 

influence on the rate of corrosion, particularly where macro-corrosion cells are developed on 

larger steel members. Corrosion of MSE reinforcements increases as resistivity decreases 

(King, 1977). However, if resistivity is high, localized rather than general corrosion may 

occur. Increased soil porosity and salinity decreases soil resistivity. The importance of and 

interaction between compaction, water content, and resistivity on corrosion processes has 

perhaps been under emphasized in many of the available studies. 

Resistivity should be determined under the most adverse condition (saturated state) in order 

to obtain a comparable resistivity that is independent of seasonal and other variations in soilmoisture content. AASHTO has adopted Method T-288 for measuring resistivity after review 

and analysis of a number of available methodologies. This laboratory test measures 

resistivity of a soil at various moisture contents up to saturation and reports the minimum 

obtained resistivity. Variations of resistivity should be expected between stockpiled soils and 

from subgrades, especially if the soils are friable. AASHTO Method T-288 is used for soil 

fills, and it requires testing on the fraction of the material that passes a No. 10 sieve. This is a 

problem when using coarse reinforced fills that have very little, or no, material finer than the 

No. 10 sieve, because we are interested in the soluble salts from within the fines. 

For coarse fills with a little material passing the No. 10 sieve, a sufficient amount of fines for 

testing might be obtained from sieving a large quantity of the material. For no, or low, 

amount passing the No. 10 sieve sufficient fines may be generated from the construction of a 

test pad using representative construction equipment and techniques. This may be applicable 

if the fill material will be subject to comminution during construction. Note that crushing of 

material to obtain the finer fraction is not appropriate, nor dictated by AASHTO T-288; 

unless some breakage is anticipated during placement and compaction. 

Another approach, similar to that implemented by the North Carolina Department of 

Transportation (Medford, 1999), is to perform specialized resistivity tests on water that has 

been decanted after soaking the aggregate for 24 hours. Resistivity tests are performed on the 

supernate in accordance with ASTM D1125. 

Additional work is needed to define appropriate procedures for obtaining material for and 

testing of coarse fills with little, to no, materials passing the No. 10 sieve. The Texas 

Department of Transportation is sponsoring a study that will address the proper method of 

measuring the electrochemical properties of coarse, reinforced fill materials (Grant # 0­

6359). This research is being conducted at the University of Texas, El Paso with a scheduled 

completion date in 2011. 

FHWA NHI-09-087 2 – Corrosion 

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Adkins and Rutkowsi (1998) compared results from in-situ resistivity measurements to 

laboratory measurements including AASHTO T-288 on samples of reinforced fill extracted 

from MSE structures at twelve different sites. Additionally, results from resistivity and 

chemical tests including chloride and sulfate ion concentrations were compared and 

evaluated. Resistivity is very sensitive to moisture content and good correlations between 

field and laboratory test results were only obtained if special precautions were taken to 

ensure that moisture content of laboratory specimens were not altered compared to in-situ 

conditions. The best comparisons between in-situ measurements and laboratory tests were 

obtained with respect to samples tested onsite (using laboratory techniques), immediately 

upon extraction, rather than from samples that had been transported to the laboratory. The 

minimum resistivity obtained with AASHTO T-288 is always significantly less than insitu 

measurements, however results from this test were found to be very consistent and 

repeatable. 

Results from in-situ testing may be useful for the purpose of comparing observed corrosion 

rates to in-situ conditions. However, using the results from the minimum resistivity test is 

considered prudent for screening reinforced fills sources for MSE construction. Although the 

extremely wet conditions associated with the minimum resistivity test may not represent the 

average moisture content in free draining reinforced fills, the moisture content close to the 

interface between the reinforcement and the reinforced fill is more relevant. In the authors’ 

opinion this could be higher than the average moisture content due to condensation and 

collection of moisture on the surface of the impermeable steel reinforcement. 

The relative level of corrosiveness, commonly accepted by the engineering community as 

indicated by resistivity levels, is shown on Table 2-3. Based on these, resistivity ranges in 

the moderately corrosive to mildly corrosive ranges are generally chosen as lower bound 

values. From the National Bureau of Standards data (Romanoff, 1957) shown on Figures 2-1 

and 2-2, a rough estimate can be made that suggests corrosion rates are roughly increased by 

25 percent in each successive aggressiveness range, all other conditions being essentially 

equal. 

Table 2-3. Effect of Resistivity on Corrosion. (NCHRP, 1978) 

Aggressiveness Resistivity 

(ohm-cm) 

Very corrosive < 700 

Corrosive 700 - 2,000 

Moderately corrosive 2,000 - 5,000 

Mildly corrosive 5,000 - 10,000 

Non-corrosive > 10,000 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 14 November 2009
 
A quantitative limit has been established for MSE reinforced fills when using metallic 

reinforcements requiring a minimum resistivity in the saturated state greater than 3000 ohmcm. This limit has been pragmatically established in recognition that soils meeting this 

criteria are widely distributed and available in the United States. Further, the associated 

corrosion rates are moderate and would not require significant sacrificial steel for the 75-100 

year design life. The New York State Department of Transportation allows resistivity to be 

somewhat lower than 3000 ohm-cm, provided that the chloride and sulfate ion contents are 

below 100 ppm and 200 ppm, respectively (Moody, 1993). The California Department of 

Transportation also allows soils with resistivity less than 3000 ohm-cm, but compensates by 

corresponding increases in sacrificial steel (Jackura et al., 1987) and reduces the design life 

of the structure to 50 years (Caltrans, 2003)). 

b. Soluble Salts 

The amount of dissolved inorganic solutes (anions and cations) in water or soil is directly 

proportional to the solution electrolytic conductivity. Therefore, the electrolytic conductivity 

(inverse of resistivity) of a soil solution is the sum of all the individual equivalent ionic 

conductivities times their concentration. 

Most salts are active participants on the corrosion process, with the exception of carbonate, 

which forms an adherent scale on most metals and reduces corrosion. Chlorides, sulphates 

and sulfides have been identified in the literature as being the chief agents in promoting 

corrosion (Romanoff, 1957). The accurate determination of chloride, sulfate and sulfide 

portions of the total salt content is an important element in determining corrosivity. It should 

be noted that the level of measurable soluble salts in a borrow area or quarry can and often is, 

highly variable and is effected by non chemical variables such as surface area of each particle 

and material soundness during handling. Each of these salts are discussed further in relation 

to available test methods. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 15 November 2009
 
Figure 2-1. Metal loss as a function of resistivity (galvanized steel) (Frondistou-Yannis, 

1985). [1 gr/year/m2 = 0.14 m/year] 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 16 November 2009 
FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 17 November 2009 

Figure 2-2. Metal loss as a function of resistivity (carbon steel). (Elias, 1990) 

 [1 gr/year/m2 = 0.127 m/year] 
Chlorides 

Chloride minerals are very soluble and thus completely removed by an aqueous extract. 

Chloride determination methods can be categorized as electrometric or colorimetric. The 

electrometric methods available include potentiometric titration (i.e. Mohr argentometric), 

coulometric by amperometric automatic titrator, direct reading potential (i.e. selective ion 

electrode), or solution conductance with prior separation by ion exchange. The mercury 

thiocyanate colorimetric method has been devised for application for autoanalyzers. 

AASHTO has adopted an electrometric Method T-291 as the method for measuring chlorides 

concentrations for MSE walls. ASTM D4327 was adopted as a standard test to measure 

anions, including chloride, by ion exchange chromatography. It is the most accurate and 

reproducible of all methods and is well suited for automated laboratories. Most analytical 

labs have this equipment and prefer to run this test rather than AASHTO T-291. The test is 

more automated, less expensive, and provides an indication of the potential for interferences, 

which are not identified by AASHTO Method T-291. The use of ASTM D4327 is 

recommended. Furthermore, it is recommended that agencies clarify which method to use in 

their specifications. 

Sulfates 

The extraction and quantification of soil sulfur imposes a more complex problem than 

chloride. Sulfate represents only one of the fractions in which sulfur can exist in the soil. In 

addition to different sulfur forms, the inorganic sulfate may occur as water soluble (i.e. 

sodium sulfate), sparing soluble (i.e. gypsum) or insoluble (i.e. jarosite) minerals. The 

solubility of sulfate is also restricted in some soils by absorption to clays and oxides or by coprecipitation with carbonates. The water-soluble sulfate will not represent the total sulfate in 

all soils but it is an appropriate choice for quantifying the soil solution activity with regard to 

corrosion potential. 

AASHTO has adopted Method T-290 as the method of measuring water soluble sulfate 

concentrations for MSE walls. This is a chemical titration method. As with chloride 

measurements, ASTM D-4327 methods by ion chromatography are the most accurate and 

reproducible of all methods. The use of ASTM D4327 is recommended. Furthermore, it is 

recommended that agencies clarify which method to use in their specifications. 

Sulfides 

Sulfide containing soils can cause severe deterioration of both steel and concrete. Freshly 

exposed sulfidic materials will have no indication of acid sulfate conditions when analyzed in 

the laboratory. Typical pH values will be from 6 to 8 with a low soluble salt content. Once 

the material is exposed to aeration by disturbance or scalping of the land surface, the sulfides 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 18 November 2009
 
oxidize chemically. Characteristics of active acid sulfate weathering include pH values 

lower than 3 due to free sulfuric acid generation and appearance of salt efflorences. The 

occurrence of sulfidic materials is generally limited to geologic formations derived from 

marine sediments or strata associated with coal and lignitic geologic materials. 

The pyritic sulfur of these materials is quite variable and no simple quantitative method is 

available. If quantitative determinations of pyritic sulfur are desired, ASTM method D-2492 

may be used. 

A qualitative test would involve oxidation of the sulfide with hydrogen peroxide and 

subsequent determination of the converted sulfate. Even though complete conversion is not 

likely by this treatment, sufficient increase in sulfate would be evidence for sulfides. This 

latter procedure is recommended where geological investigations detect the presence of 

pyrite. At present, no standard test method exists for this procedure. 

Maximum Acceptable Levels 

Chlorides, sulfates and other dissolved salts decrease resistivity, promoting the flow of 

corrosion currents and impeding the formation of protective layers. The USDA (1969) and 

Rehm (1980) studied the effect of chlorides and sulfates on resistivity for both theoretical 

considerations and controlled laboratory tests (Elias, 1989). 

From these data, it can be inferred that soils in the moderately corrosive range (5000 to 2000 

ohm-cm resistivity) would be limited to a range of 60 to 180 PPM for chloride ions or 90 to 

280 PPM for sulfates. Where other soluble salts are present, or a combination of chloride 

and sulfates, these concentrations would be reduced. 

Consistent with a minimum resistivity of 3000 ohm-cm, the maximum level of chlorides and 

sulfates in reinforced soil fills with metallic reinforcements has been established in the 

current AASHTO LRFD Bridge Construction Specifications (2004), as shown in Table 2-4. 

Table 2-4. Maximum Permissible Levels of Soluble Salts. (after AASHTO, 2004) 

Soluble Salt Maximum Level Test Method 

Chlorides 100 PPM ASTM D4327 

Sulfates 200 PPM ASTM D4327 

Note: ASTM D4327 test method is recommended, current AASHTO calls 

for use of AASHTO T-290 and T-291 test methods. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 19 November 2009
 
c. pH 

The measurement of pH represents the hydrogen ion concentration in solution. Values of soil 

pH represent the hydrogen activity in the soil solution and are referred to as the intensity 

factor. Soil pH is probably the single most informative measurement that can be made to 

determine soil characteristics (Thomas, 1996), and pH provides more information about a 

soil than merely whether it is acidic or basic. A major factor influencing pH of soils is the 

salt content of the soil solution, and there is often a tendency for the pH of the soil to rise in 

the presence of salts. Soil with a pH of 2 to 3 is usually associated with pyritic minerals, 

which upon oxidation forms sulfuric acid. At the other end of the scale pH is an indicator of 

excess calcium carbonate, and normally soils with pH values of 7.6 to 8.3 are found to be 

calcareous. When pH values stray towards 9, calcium carbonates are no longer controlling 

the system, and soluble salts such as sodium carbonates become the dominant species and 

normal salts of sodium including sodium chloride and sodium sulfate may be present. 

The most widely accepted procedure for measuring the soil pH is by the pH glass electrodecalomel reference electrode pH meter on a 1:1 weight ratio of soil to water, which is 

consistent with AASHTO T-289 test method. The corrosion literature suggests that for bare 

steel and pH between 4 and 10, the corrosion rate is independent of pH and depends only on 

how rapidly oxygen diffuses on the metal surface. Resistivity tends to be higher in acid soils 

than alkaline soils. This effect is associated with moisture content, as highly buffered neutral 

and alkaline soils generally contain a significant clay fraction. This will tend to lead to a 

higher moisture content, the presence of which will reduce the resistivity of the soil. 

Soils that are extremely acidic (pH less than 4.0) or very strongly alkaline (pH greater than 

10) are generally associated with significant corrosion rates. Zinc is vulnerable to corrosion 

in strongly acidic and alkaline soils, whereas steel is passivated in alkaline environments 

(i.e., pH>10) and with a pH less than 13. Therefore, the galvanized coating will have a 

significantly lower life expectancy in either acidic or alkaline regimes. Alkalinity tends to 

favor steel performance, unless it is associated with high concentrations of salts. A 

reasonable allowable pH range may therefore be inferred as being greater than 5 and less 

than 10 when using metallic reinforcements. 

d. Organic Material 

Some soils contain a high proportion of organic material in which general microbial growth 

will reduce this material to organic acids which, when in contact with metals, produce pitting 

corrosion. The inclusion of organics in the reinforced soil fill can initiate the formation of 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 20 November 2009
 
 

anaerobic pockets of soil, which could be contaminated with sulphate-reducing bacteria 

(SRB), thereby initiating microbial attack in the form of severe pitting. 

AASHTO T-267 is normally used to determine organic content on soil fractions smaller than 

the #10 sieve (2 mm). This tests measures organic content by loss on ignition at 450 C for 

six hours. Total organic content to preclude formation of anaerobic pockets should be limited 

to 1 percent by weight of the total soil fraction. 

Materials used for reinforced wall fill that are initially free of organics may become 

contaminated during service. For example, if fertilizers are used to promote vegetation along 

the back and/or side slopes of a retaining wall system, organics may permeate into the fill 

from percolation of rainwater, or from storm water run-off. Planted wall facings may also 

present the possibility of contaminating fill with organics near the wall face. Planted wall 

facings should be avoided when using metallic reinforcements. Accelerated corrosion on 

wire facing systems often used for plantings should also be anticipated and carefully 

evaluated to determine maintenance requirements Use of fertilizers should be avoided, or 

impervious barriers should be employed beneath the topsoil (i.e., on top of the backfill) if 

fertilizers are applied to back and side slopes. 

2.4 DESIGN CORROSION RATES AND THEIR APPLICATION 

a. Available Data 

The most comprehensive data available in the field of underground corrosion are the results 

of extensive field testing on metal pipes and sheet steel buried by the U.S. National Bureau of 

Standards (NBS) in programs originating as early as 1910 (Romanoff, 1957). Additional data 

include the results of many studies conducted in the United States on the performance of 

metal highway culverts and buried piling. These data, generally qualitative rather than 

quantitative, are substantially in good agreement with the extensive burial tests conducted by 

NBS (NCHRP, 1978). A general conclusion of the above studies is that the rate of corrosion 

is greatest in the first few years of burial and then levels off to a steady but significantly 

lower rate. 

Based on these studies, Romanoff at NBS suggested the following exponential equation to 

predict the amount of general corrosion at some time (t) after burial: 

x  K t n (2-1) 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 21 November 2009
 
 

where x is the loss of thickness or pit depth in the metal at time (t) and K and n are constants 

that are soil and site dependent (n is less than unity). For low alloy and carbon steels in a 

number of soil burial conditions, NBS determined a "n" constant varying from 0.5 to 0.6 and 

"K" constants between 150 and 180 μm at the end of the first year. For galvanized steels, "n" 

constants were not evaluated, but "K" constants varying from 5 to 70 μm can be inferred. 

It should be noted that the NBS data was developed from a wide range of burial conditions 

not necessarily reflective of select granular fills required for MSE wall structures. Various 

transportation departments have conducted corrosion studies with reference to metal culvert 

durability, summarized in NCHRP-50 (NCHRP, 1978). This summary indicated that a 

number of analytical methods have been proposed by transportation agencies in California, 

New York, and Utah that appear to be locally satisfactory. However, no method has found 

widespread acceptance. 

Results from carefully controlled French laboratory tests, simulating field burial conditions, 

strongly suggests that for the range of fills utilized in MSE applications, the constant "n" may 

be taken as 0.60 for galvanized steel while the zinc coating is still present and from 0.65 to 1 

for carbon steel once significant corrosion occurs (Darbin et al., 1986). The constant K 

calculated at the end of the first year, for galvanized steel was found to vary between 3 and 

50; with the higher values consistent with soils characterized by lower resistivities and 

highest concentrations of chlorides and sulfates. 

The loss data as a function of time for the French studies are shown on Figures 2-3 and 2-4. 

The data with reference to the constant "K" have been analyzed in an attempt to determine 

any relationship with resistivity and degree of saturation. The scatter is significant, but for 

resistivities greater than 5000 ohm-cm, the range for "K" reduces to 8 to 45, with an average 

value on the order of 25. The data further suggest that once the galvanized zinc coating is 

depleted, the base carbon steel corrodes at the carbon steel rate. 

The Association for Metallically Stabilized Earth (AMSE) has compiled performance data 

from observations of insitu reinforcements collected by several different researchers and state 

departments of transportation (AMSE, 2006; Gladstone et al., 2006). These data document 

performances of reinforcements that have been in service for up to 34 years at various 

locations within the United States and Europe. In general, the observed performance has 

exceeded expectations based on currently available metal loss models whenever good design 

and construction practices as prescribed by current AASHTO specifications were followed. 

The few, well documented, cases of poor performance are always correlated with a very 

aggressive environment; attributed to reinforced fill or to site conditions that do not conform 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 22 November 2009
 
Figure 2-3. Summary of Electrochemical Test Cell Data at 25% Saturation. 

(Darbin et al, 1988) 

Figure 2-4. Summary of Electrochemical Test Cell Data at 50%and 100% Saturation. 

(Darbin et al, 1988) 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 23 November 2009
 
to current AASHTO specifications (Frondistou-Yannis, 1985; McGee, 1985; Whiting; 1986; 

Blight and Dane, 1989; Elias, 1990; Deaver, 1992; Medford, 1999; Wheeler, 2002; Armour 

et al., 2004; Fishman et al., 2006). 

Most of the walls included in the AMSE database are constructed with reinforced fills that 

exceed the minimum requirements described by AASHTO by a wide margin, and the data do 

not reflect a comprehensive geographic distribution of site locations. A currently active 

research project, sponsored by NCHRP (NCHRP Project 24-28), is addressing these database 

limitations. Results from NCHRP Project 24-28 are expected to be available by the end of 

2009 and will describe the statistics and variations of observed corrosion rates and 

correlations with different geographies and corresponding climates. Note that no change in 

recommended design corrosion rates is anticipated. 

Highway agencies in the European Union are undertaking a similar data collection effort. 

Construction of MSE walls in Europe predates that in the United States, and the ages of the 

oldest MSE structures in this data collection effort are approaching 40 to 50 years. These data 

may be compared to observations made 20-years ago on the same structures. These data will 

be useful, with recognition that the practices employed in Europe during the 1960’s and 

1970’s are dissimilar to current U.S. practices; as the reinforcements do not include as much 

zinc and thinner base steel thickness were employed compared to current U.S. design 

standards. 

b. Design Approach 

The generalized corrosion rate relationship developed by Romanoff has been found to be a 

reasonable predictive model to determine the range of corrosion rates for single-phase 

materials for the wide range of soils found in nature. The difficulty in its implementation for 

galvanized steels has been in determining "K" and "n" constants that might reasonably reflect 

the specific environment and integrate the transition in corrosion rates between a galvanized 

state and the subsequent bare steel phase. Such a model has not been adequately studied to 

date, and, therefore, extrapolations are based on limited present data. In addition, since 

corrosion does not occur in a uniform manner, loss of cross-sectional area will be greater 

where significant pitting or greater localized corrosion occurs than a loss computed by 

distributing corrosion losses uniformly over an element. The NBS data, substantiated by 

several studies (Blight and Dane, 1989; Smith et al., 1996), suggest that pitting depths could 

be significantly deeper than depths suggested by uniform loss. Pitting depths, however, are 

somewhat attenuated in uniform backfill environments for galvanized steel as evidenced by 

both NBS data, British studies, and results obtained in France at least in the early stages of 

carbon steel corrosion. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 24 November 2009
 
 

 

 

 

 

Consideration must be given to effects on tensile strength by the pitting mechanism in 

choosing an appropriate "K" and "n" constant when using a uniform rate of corrosion model. 

Alternately, the effect of non-uniform corrosion losses on the tensile strength of reinforcing 

members may be considered statistically, based on test results that relate the relative loss of 

tensile strength to relative average thickness loss. The data developed from buried samples 

of strip type reinforcements that had undergone significant corrosion losses strongly suggest 

that a factor of approximately 2 exists between average thickness loss to average tensile 

strength loss (Elias, 1990). 

Based on theoretical considerations, a factor closer to 3, which is negatively correlated with 

diameter, is more applicable to single reinforcement elements with a circular cross section 

such as used in bar mats and welded wire fabric (Smith et al., 1996). Additional 

corroboration with test data from reinforcement samples collected from the field is needed to 

verify the appropriate factor for geometries other than flat strips. Current design 

specifications presume that the factor of 2, which was initially and specifically determined 

for flat thin galvanized steel strips, also applies to all other types of galvanized steel 

reinforcements, whatever their size and shape. Note that due to redundancy of bar mats, that 

a factor of 2 is also considered reasonable for these reinforcements. 

Using the NBS model, the available data for a wide range of soils suggest that for galvanized 

steel loss determinations using the uniform model concept, the following equation may be 

applicable: 

 X = 25 t 0.65 (Average) (2-2) 

 X = 50 t 0.65(Maximum) (2-3) 

For carbon steels, it appears that the expression should be modified to: 

 X = 40 t 0.80(Average) (2-4) 

 X = 80 t 0.80(Maximum) (2-5) 

Note that these values would be consistent for burial in a wide range of soils, many not 

meeting the restrictive electrochemical requirements for reinforced soil fills. The implication 

of using these relationships would be a predictive life for the galvanization of 7 years on 

average, which would considerably underestimate the results from retrieval tests conducted 

both in the United States and Europe (King, 1977; NCHRP, 1978; Darbin et al., 1986). 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 25 November 2009
 
m

 

Careful study of the NBS corrosion results in soils meeting the stringent requirements for 

MSE wall structures suggests that an alternate simplified model can represent the corrosion 

losses well enough to estimate sacrificial steel requirements for design. This latter model has 

been adopted by AASHTO to predict uniform maximum loss rates for determination of 

sacrificial thickness of galvanized steel reinforcement as follows: 

1) Zinc corrosion rate first 2 years 0.58 mils/yr (15 μm/yr)/side 

2) Zinc corrosion to depletion 0.16 mils/yr (4 μm/yr)/side 

3) Carbon steel rate 0.47 mils/yr (12 μm/yr)/side 

The AASHTO model only applies when reinforcements are galvanized (i.e. not plain 

black steel), and soil fills meet the prescribed AASHTO requirements. The rate of 

carbon steel loss includes a factor of 2 to consider the effect of nonuniform corrosion, but 

this factor is not applied to loss of zinc. This model should not be used if reinforced fill soils 

do not meet AASHTO criteria and the rate of carbon steel loss should not be applied to 

reinforcements that have not been galvanized, i.e. the rate of carbon steel loss considers the 

response of the steel surface subsequent to depletion of zinc. Applicable corrosion rates for 

plain steel reinforcements (i.e. not galvanized) are higher than 0.5 mils/yr (12 m/yr), 

depending on time of exposure and increase in corrosion anticipated from non-uniform, 

pitting type corrosion. 

c. Current Practice 

The recommended design practice as outlined in AASHTO is consistent with the philosophy 

presently in effect worldwide. It considered minimum or maximum mandated long-term 

electrochemical limits for the backfill and the addition of a sacrificial thickness to the 

required structural reinforcement thickness. Potential flow of salts from the retained fill must 

be considered in determining the long-term regime within the reinforced backfill. 

Current practice can be summarized as follows: 

C Recommended Electrochemical Limits (Metallic Reinforcements) 

Property Standard Test Procedures 

Resistivity -cm >3000 AASHTO T-288 

pH >5 <10 AASHTO T-289 

Organic Content 1% Max. AASHTO T-267 

Chlorides < 100 PPM ASTM D4327 

Sulfates < 200 PPM ASTM D4327 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 26 November 2009
 
With minor variations, typically in grain size distribution and occasionally in pH limits, 

AASHTO specifications are used by most state DOTs. Because of significant variability of 

reinforced fill sources, multiple samples must be tested as previously shown in Table 2-2 to 

assess mean conditions. The requisite number of samples depends on the number of 

stockpiles, history of laboratory testing of the material source, and the volume of material 

required for construction. 

C Required Design Life 

For the purpose of determining the sacrificial thickness, the following design life is 

recommended in AASHTO. 

Structure Classification Design Life (yrs) 

Permanent structure 75 

Abutments 100 

C Sacrificial Thickness Requirements 

For MSE structures constructed with select and tested backfills to ensure compliance with the 

electrochemical requirements, the maximum mass presumed to be lost per side due to 

corrosion at the end of the required service life may be computed by assuming a uniform loss 

model that considers the following loss rates: 

- Zinc corrosion rate first 2 years 0.58 mils/yr (15 μm/yr) 

- Zinc corrosion to depletion 0.16 mils/yr (4 μm/yr) 

- Carbon steel rate 0.47 mils/yr (12 μm/yr) 

The resulting sacrificial thickness for a 75-year life based on initial galvanization of 3.4 mils 

(85 μm) is approximately 0.06 in. (1.5 mm) for a steel strip. Since this is a maximum loss 

rate, it is presently assumed that the reduced minimum thickness remains proportional to 

tensile strength and therefore no further reduction is necessary. 

Some systems use ties strips embedded in precast concrete panels to connect the 

reinforcements at the wall face. The tie strips have top and bottom plates that sandwich the 

reinforcements; constituting a double shear connection. Within the tie-strip/strip interface 

there is very little loss of thickness; negligible compared to that of surfaces exposed to the 

soil. Thus, loss of thickness and sacrificial steel requirements are applied the gross cross 

section. 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 27 November 2009
 
 

Bolts, nuts and washers at the connection should be galvanized in accordance with AASHTO 

M 232. In general, the design shear stress expected to be transferred to the bolt cross section 

is well below the allowable shear stress. Therefore, section loss due to corrosion of the bolt is 

often not an important consideration. 

C Use of Resin Bonded Epoxy Coatings 

Use of epoxy coatings for routine corrosion environments provide no greater degree of 

design confidence than galvanization. Where used the following coating standards should be 

followed: 

 Reinforcement Type Coating Standard

 Strip AASHTO M-284 

 Grid ASTM A-884 

The coating thickness should be on the order of 18 mils (450 μm), and the design life should 

be considered as equal to that of a galvanized reinforcement with a coating thickness of 85 

μm (16 years). 

Therefore, for a 75-year design life, a sacrificial thickness of 0.06 in. (1.5 mm) of a structural 

steel strip section is presently recommended. The sacrificial thickness required for bar mat 

wire reinforcement is a function of its initial diameter. 

C Use of Polymeric Barrier Coating 

The effective life of a polymeric barrier coating may be evaluated based on the time it takes 

for the polymer to become brittle and to expose the underlying metal to the reinforced fill. A 

protocol suitable for determining the functional life of polymeric coating is described by 

HITEC (2000). The protocol considers the potential for construction-induced damage, 

chemical compatibility of the polymer, degradation due to oxidation in the soil environment, 

or due to UV radiation if the reinforcement is exposed to the atmosphere (i.e., used as 

facing). Field tests, as described in ASTM D5818 (2004b), may be used to evaluate 

construction-induced damage of polymer coatings. Aging tests, conducted at elevated 

temperatures as described by UL 746B (Underwriters Laboratories, 2000) are used to 

simulate the long-term effects of oxidation. Degradation rates are then computed using 

conventional Arrhenius modeling employing time and retained strength or elongation change 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 28 November 2009
 
C 

from at least three temperatures as the variables. The sensitivity of polymers to UV exposure 

is examined by exposing the material to UV radiation in a controlled test chamber as 

described in ASTM D1499 (2004c) and ASTM D4355 (2004b). 

Use of Welded Wire Steel Facing 

Steel facing is sometimes employed, particularly for walls constructed with welded wire 

facings (WWF) or bar mat reinforcements. Often bar mats or WWF is bent at about a 

90angle at the wall face and a backing mat is applied to control the spacing between wires 

or steel bars at the wall face. Corrosion of the steel facing could cause maintenance issues, 

and corrosion rates will be affected by atmospheric conditions. Typical values of corrosion 

rates applied to steel (i.e., non-galvanized that is used in temporary walls (design life of 3 

years or less)) sheets in contact with soil on one side is 1.0 mil/yr (25 m/yr) per side. And 

these rates should be applied to the case of steel faced walls. Substantially higher rates should 

be used if the wall face will be vegetated, where road salts are used, if atmospheric 

conditions are corrosive such as marine environments or when air quality may be 

compromised buy nearby industrial activity. Corrosion potential can be reduced by using 

open graded stone in the facing. 

Steel facings should be galvanized consistent with the use of galvanized reinforcements. Hot 

dip galvanizing of at least 2 oz/ft2 is expected to protect the steel in atmospheric conditions 

for between 20 and 50 years, depending on atmospheric conditions (AGA, 2004). Forty to 

fifty years are expected in rural and suburban environments, 25 to 30 years in coastal areas, 

and approximately 20 years if located in proximately to industrial areas where the 

atmosphere may be acidic. These zinc lives are considerably higher compared to those 

anticipated for galvanized elements buried in soils. 

Hardware cloth that is sometimes used with welded wire facings to contain fill material may 

be vulnerable to degradation from UV radiation. The durability of these polymeric materials 

should be considered as described in Chapters 4 and 5. 

2.5 MARGINAL FILLS 

In some areas of the country, it is becoming more and more difficult to locate fill that meets 

the AASHTO electrochemical requirements. In addition, there is also a national effort to 

allow for increases in the amount of fines used for reinforced fill due to the rising cost of 

select borrow materials. There are fewer data available to document performance and 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 29 November 2009
 
formulate appropriate metal loss models for design in the case of marginal wall fill soils that 

do not meet the stringent AASHTO specifications for electrochemical properties. 

Based on the results from limited field studies, Caltrans (Jackura et al., 1987) has proposed 

design guidance for a wider range of reinforced fill conditions than those considered by 

AASHTO. Higher rates of metal loss are specified for computing sacrificial steel 

requirements when reinforced fills that are more aggressive relative to corrosion are 

considered during design. These metal loss rates are based on limited data collected from 

MSE wall sites in California (Jackura et al., 1987), and using data available from the earlier 

NBS studies. Interim design guidance considered corrosive conditions with minimum 

resistivity less than 1000 -cm. However, current specifications used by Caltrans do not 

allow use of reinforced fill with minimum resistivity less than 2000 -cm. 

Specifically, Caltrans current specification allows for backfill with a resistivity of greater 

than 2000 -cm, a pH between 5.5 and 10, and maximum chloride and sulfate concentrations 

250 ppm and 500 ppm, respectively. California considers these conditions by using a higher 

rate of metal loss in determining sacrificial steel and reducing the design life of the MSE wall 

to 50 years. Caltrans assumes that the zinc coating provides 10 years of service life for the 

specified minimum coating thickness of 2 oz.ft2 (85 μm per side). This is less than the 16 

years of zinc life inherent to the AASHTO metal loss model. A corrosion rate of 1.10 mils/yr 

(28 μm/yr) is considered to affect the base steel after the zinc has been consumed, and used 

to compute the sacrificial steel requirements. These corrosion rates account for the potential 

for localized corrosion and pitting; i.e. a factor of two relating the loss of tensile strength to 

idealized uniform corrosion rates is included. 

Caltrans specifications provide incentives to use select granular fill, which is a better quality 

fill with less than 5% fines and with PI <6. Caltrans reduces the steel corrosion rate to 0.50 

mils/yr (12.7 μm/yr) for backfill meeting additional requirements for select granular fill. For 

select granular fill, lower resistivity and higher salt concentrations are allowed, but the 

allowable fines content is less compared to current AASHTO requirements. 

2.6 TEMPORARY WALLS 

Temporary structures have a design service life of less than 3-years. The temporary 

reinforcements should be galvanized if contact between reinforcements from temporary 

structures and those from permanent structures with galvanized reinforcements is possible. 

Otherwise, the presence of the plain steel could have an adverse effect on the durability of 

the permanent galvanized reinforcements. If plain steel reinforcements are used, and 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 30 November 2009
 
reinforced fill is not corrosive or only mildly aggressive, a corrosion rate of 28 m/yr should 

be applied to assess sacrificial steel requirements for temporary walls (i.e., design life of 3 

years or less). This includes a factor of 2 to consider the effects of nonuniform corrosion. 

Higher corrosion rates need to be considered for reinforced fills that are moderately 

aggressive or corrosive, and a corrosion specialist should be consulted to assess the sacrificial 

steel requirements or other possible corrosion protection measures. 

– see section on Current Practice – sacrificial thickness requirements 

– see section on Current Practice – use of welded wire steel facing 

FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 31 November 2009
 
 

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FHWA NHI-09-087 2 – Corrosion 

Corrosion/Degradation 2 – 32 November 2009
 
CHAPTER 3 

MONITORING METHODS, METALLIC REINFORCEMENTS
 

The primary objectives of monitoring techniques are to assess corrosion rates as an adjunct to 

design and to provide data to assess the integrity of MSE structures constructed with steel or 

galvanized reinforcements. It is important to consider the performance of these systems 

within the context of Transportation Asset Management, as slopes and retaining walls are 

important components of the highway system. The test techniques and procedures described 

in this chapter have been researched and developed over the past several decades. They are 

mature technologies, and important tools for collecting performance data that is an integral 

part of asset management. Two techniques have been used to monitor corrosion rates of 

buried metallic elements for these purposes: 

C Retrieval of buried coupons with measurement of weight loss and section thickness, 

and/or loss of tensile strength at each retrieval interval. This is a destructive test method 

that requires excavation, or extraction through the wall face, for each retrieval. Further, 

the number of assessments is limited by the number of coupons buried and is restricted to 

the measurement of corrosion at the coupon only. Coupon corrosion is often not 

representative of the corrosion of full sized elements, which are affected by the variations 

of density, moisture content and dissolved salts inherent in contact with fill soils, which 

are never totally homogeneous. 

C Remote electrochemical methods using potential and polarization resistance 

measurements to obtain instantaneous, average, in-situ corrosion rates of reinforcement 

elements while in service, i.e., in their stressed state. This method is nondestructive and 

measurements can be taken at any time (e.g. different seasons, age) to more closely 

monitor performance. 

Given the advantages, utilization of remote electrochemical methods is highly recommended 

with at least some coupons buried for retrievals to confirm results. These techniques can be 

implemented on both existing structures and during construction and equipment to perform 

these measurements is commercially available. This chapter will develop the theoretical 

background for the use of potential and polarization resistance measurements, recommend 

protocols for implementation on both existing and new construction, and review data 

obtained since 1988 from corrosion monitoring programs that have been implemented by 

many state DOT’s and from research sponsored by FHWA and NCHRP. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 1 November 2009
 
 

3.1 CORROSION MONITORING FUNDAMENTALS 

Corrosion is an electrochemical process. In underground corrosion of steel, the 

electrochemical reaction responsible for corrosion is the oxidation of iron from the steel; 

2 e Fe  Fe  2 (3-1) 

Because it is an electrochemical process, there is an associated current (flow of electrons). 

Numerous methods exist to measure the current or the rate of corrosion (Tait, 1994). If the 

current is properly measured and the area of the specimen involved in the reaction is known, 

the local corrosion rate can be calculated directly. The following equivalent circuit idealizes 

the specimen surface involved in the corrosion process (Lawson et al., 1993): 

where Rp is the polarization resistance, which is the resistance of the surface to the corrosion 

process and is inversely proportional to the corrosion rate; C represents the capacitance of the 

surface. The electrolyte or soil resistance is represented by Rs on the circuit. 

If the electrochemical potential of the specimen is shifted, a slight amount (less than 0.02V) 

from its rest (or corrosion) potential by the use of a power supply and a remote electrode, 

then the current necessary to cause the shift can be measured and the value of Rp per unit area 

can be calculated. When normalized for area, which involves multiplying the measured 

polarization resistance by the electrode area, the polarization resistance of the material (Rp) 

results and has the units of ohm-cm2

. 

The classic description of the relation between Rp and corrosion rate was derived by Stern 

and Geary (1957) and is a modification of the fundamental equation for electrochemical 

kinetics, the Butler-Volmer Equation. The classic equation of Stern and Geary is: 

BA BC iCORR  (3-2) 2.3(BA  BC ) Rp 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 2 November 2009
 
 

where iCORR is the corrosion current density of the surface of the specimen, and BA and BC 

are the anodic and the cathodic Tafel slopes, respectively. A Tafel slope is the rate of change 

of voltage (in volts) per decade of current as the voltage of the specimen is shifted away from 

its rest or corrosion potential (Fontana, 1986). These Tafel slope values can be determined 

by experiments or estimated from literature data for soil corrosion. Further, because the 

expression involves the product of the two Tafel slopes divided by the sum, and because 

Tafel slopes are typically limited to a relatively small range, the calculation of corrosion rate 

(corrosion current density) is not highly effected by errors in the values of the Tafel slopes. 

The term 

BA BC
 

2.3(BA  BC )
 

typically varies between 0.08 and 0.02 volts and is hereafter referred to as the conversion 

constant. Stern and Weisert (1959) showed that the corrosion rate calculated from Rp 

correlated well with actual corrosion rates determined from weight loss measurements for a 

variety of steels in aqueous environments. 

Electrochemical Impedance Spectroscopy (EIS) or AC Impedance is another electrochemical 

technique that can be used to determine the values of Rp, Rs, and C in the equivalent circuit 

(Andrade et al, 1986; Scully, 2000). Although more information about the corrosion process 

may be obtained from the results of EIS, the test data are more difficult to interpret. The 

presence of factors such as metal phase (e.g. both zinc and steel present on surface) and 

passive film layers affect the results and needs for sophisticated techniques for data analysis 

and interpretation. 

Corrosion rates determined from polarization resistance measurements require that: 

C the exposed area of the component to be analyzed be known or estimated accurately. An 

accurate estimate can be made for reinforcing strips or grids in reinforced soil structures 

because the geometry is well defined. 

C the polarization resistance be determined independently of the ohmic resistance (soil 

resistance). This can be accomplished by using a spectrum of AC measurements, or can 

be neglected when considerable experience has been gained on actual structures that 

show it to be justified. Alternately, it can be measured separately by direct measurement 

with a soil resistance device and subtracted from the measured polarization resistance. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 3 November 2009
 
 

 

C the conversion constant necessary to convert the polarization resistance to corrosion rate 

be known. Estimates of this constant suffice in most applications. Data from Tokyo Gas 

suggest a value of 0.021 volts (V) for this constant represents the average of all corrosion 

systems. Pipeline companies routinely use a constant of 0.035 V for steel pipe. 

Montuelle and Jailloux (1979) suggest a conversion constant on the order of 0.050 for 

galvanized steel in numerous soils. Therefore, it can be assumed that conversion 

constants range between 0.020 V and 0.050 V. 

C the composition of the surface being analyzed be known. In the early life of MSE 

structures, it can clearly be assumed that the surface is galvanized. Corrosion potential 

monitoring can be used to determine subsequent metal phases as the reinforcement loses 

zinc, ultimately down to the carbon steel base. 

The use of polarization resistance measurements in general compliance with ASTM-G59, 

corrected for soil resistance by separate measurements, are applicable for determination of 

corrosion rates of reinforcing elements in MSE structures. While the conversion constants 

for steel and galvanized surfaces may vary (within a factor of 2), use of 0.035 V for steel 

and 0.050V for galvanized steel to obtain an average corrosion rate should be considered 

where exact data for a particular soil are not known. Potential measurements that will 

distinguish existing surface composition should be used as a basis for selecting the 

conversion constant. The basis for potential measurements is outlined below. 

The interpretation of potential measurements considers that four distinguishable layers of 

zinc coating are formed as a result of the hot dip process used to galvanize MSE 

reinforcements. The outside layer is nearly pure zinc, and the succeeding inner layers are 

essentially zinc-iron alloys. Progressively higher iron contents prevail as the interface with 

the base steel is approached. Therefore, as zinc consumption progresses towards the base 

steel interface, the half-cell potential is consistently shifted towards values inherent to iron. 

Ultimately, measurements of the half-cell potential reflect the presence of steel after all four 

layers of the zinc coating are exhausted and bare steel is exposed; at least in some areas. 

The primary purpose of potential measurements in MSE structures is to establish when 

significant portions of the reinforcements have lost zinc coverage and steel is exposed to the 

soil environment. The presence of rust does not necessarily indicate that zinc has been 

consumed. Rust color due to the presence of ferrous hydroxide may be visible on the surface 

during corrosion, however, some zinc is still present in the layer. Therefore, comparisons 

between the potential of the reinforcements and the potentials of buried zinc and carbon steel 

coupons are used to distinguish when a significant surface area of bare steel is exposed to the 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 4 November 2009
 
 

 

soil. Once bare steel is exposed to the soil environment, the corrosion rate of the sacrificial 

zinc (galvanization) at other locations on the surface may be accelerated due to the galvanic 

couple with the steel. 

If the potential of the reinforcing element is near that of the zinc coupon, the soil 

reinforcement remains well galvanized. As the potential of the reinforcement element 

becomes more positive and begins to approach that of the steel coupon, the galvanizing is 

being lost and more bare steel is being exposed. 

The corrosion potential is the voltage of a reinforcement element of interest measured with 

respect to some suitable reference electrode. The common reference electrode in 

underground corrosion studies is Copper/Copper Sulfate (Cu/Cu2SO4). For a given material 

in a given environment, the potential is an indication of the corrosion activity. The more 

positive the potential, the greater, in general, is the corrosion. Potential measurements are 

therefore only qualitative indications of corrosion activity and should only be used to 

determine the composition of the surface. 

Typical values for a galvanized reinforcement would be between -1.10 to -0.65 V and for 

carbon steel in the range of -0.60 to -0.20V. Transition stages would yield intermediate 

values. 

3.2 IMPLEMENTATION OF FIELD CORROSION MONITORING PROGRAMS 

a. Plan Development 

Corrosion monitoring programs and monitoring of in service reinforcements should be 

implemented with the following objectives: 

C Assess the validity of the design corrosion rates. 

C Evaluate how service life and sacrificial steel requirements may be affected by using 

reinforced fills that do not meet the minimum electrochemical criteria specified in current 

AASHTO specifications. Use of materials for reinforced fill that do not meet AASHTO 

electrochemical specifications is not recommended. If materials fall outside of the 

specified values, a long-term monitoring program is strongly recommended. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 5 November 2009
 
 

C Identify the impact of changing site conditions on service life and performance, e.g. if 

there is a suspicion that the groundwater regime established in the structure during 

service is acidic, highly alkaline, or contaminated with salts. 

C Provide a means to warn of impending failure in response to some visible distress in the 

MSE structure, and for structures of critical importance. 

C Identify how particular climatic conditions may affect service life, e.g. structures in or 

adjacent to warm marine environments. 

C Evaluate the effects from extreme events on service life, e.g. hurricane induced tidal 

surges may inundate structures constructed in estuary environments. 

The benefits of condition assessment and corrosion monitoring are to provide better estimates 

of expected service life, and to identify conditions where service life may be compromised. 

In this manner agencies can be proactive with respect to asset management and be better 

prepared to meet future obligations with respect to maintenance, rehabilitation, retrofit or 

replacement. 

Agencies need to develop an inventory of constructed MSE facilities as part of an asset 

management strategy. The inventory should include details of the construction, location and 

character of the reinforced fill. A list of priorities and a sampling plan can be developed from 

the inventory to establish a basis for performance monitoring. In general, approximately 

thirty sites should form the basis of the sampling plan. This basis should include a 

representative cross section of walls incorporating different wall height, function, wall 

supplier, contractor, fill sources, regions, and climates within an agency’s jurisdiction. The 

thirty-site sampling should be from conditions expected to correspond to good performance 

to serve as a baseline. Sites with special concerns can be added to check for the possibility of 

poor performance, based on issues that may be related to backfill quality, particularly adverse 

site conditions, or poor construction practice. Corrosion monitoring and performance data for 

asset management will be discussed in more detail in Section 3.4. 

The measurement concept recommended for monitoring MSE structures includes both 

potential measurements and polarization resistance (PR) measurements. Both measurement 

techniques must be performed on buried coupons, as well as the actual reinforcement 

members. The buried coupons will include coupons made from carbon steel, zinc, and 

galvanized steel. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 6 November 2009
 
b. Monitoring Programs 

The number of monitoring locations in each MSE structure is a function of the length and 

anticipated variability of the in-situ regime. As a rule of thumb, two locations spaced at least 

200 ft (60 m) apart should be considered for MSE structures 800 ft (250 m) or less in length 

and three locations for longer structures. At each location, corrosion should be monitored at 

a minimum of two depths from the surface or preferably at depth intervals of 10 to 13 ft (3 to 

4 m) because differences in oxygen content, moisture content, and salt concentration can 

produce different corrosion behavior. One critical location (center of structure) should be 

selected for establishing test locations at both shallow and deep positions. Higher oxygen 

and salt content are anticipated near the surface, and higher moisture contents or free water 

near the base of a structure. Prior field programs have indicated that where groundwater 

intrudes at the base of the structure, higher corrosion rates should be anticipated. Where this 

condition is not likely, representative estimates may be obtained from shallow-depth 

monitoring. The shallow depth stations should be approximately 5 ft (1.5 m) in depth, and the 

deep position should be approximately at one-fourth of the structure height from base level. 

Figure 3-1 shows the location of the coupons and instrumented reinforcement members. 

In general, more monitoring locations should be established for structures where poor 

performance is anticipated or known to exist (Withiam et al, 2003; Hegazy et al, 2003). 

Particular attention should be given to monitoring near drainage inlets or other areas that may 

be subject to fluctuations in moisture content, high moisture content, or inundation. However, 

monitoring at locations with “normal” conditions is still necessary to serve as a baseline, and 

to ensure that the sample statistics are not skewed. 

Ideally, three types of coupons should be placed at each location and depth; zinc, steel and 

galvanized. For monitoring, it is desirable to have one-zinc, one-steel and up to four 

galvanized coupons at each depth. The multiple galvanized coupons can provide 

opportunities for periodic removal. Coupons each have two leads to provide back-up in case 

one connection fails. 

At each location, access to the test leads should be housed within a water-tight lockable box. 

The front panel should have the capacity to provide isolated test lead connections from all 

depths. The total number of connections will depend on the number of coupons buried. All 

leads should be encased in conduit to prevent breakage. 

Similar location selection criteria apply for existing and new structures. However, it is 

realized that for existing structures access to deeper reinforcements may only be possible by 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 7 November 2009
 
advancing core holes through the wall face, and coupons may only be placed in proximity to 

the wall face. 

The rationale for measurements can be explained by viewing the corrosion process as having 

3 Stages as shown on Figure 3-2. 

Figure 3-1. Schematic diagram showing locations of coupons and instrumented 

reinforcement members. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 8 November 2009
 
Figure 3-2. Stages of galvanized structure performance. 

During Stage 1, the galvanizing is intact and either no steel is exposed to the soil or the steel 

is well polarized and completely protected. The potential of the structure is the same as that 

of zinc E = EZN. In Stage 2, increased steel is exposed as galvanization is lost, and the 

potential of the structure becomes more positive and approaches that of bare steel (E>EZN 

and E<EFE). In Stage 3, essentially all the galvanization has been lost and the potential of the 

structure is the same as that of carbon steel (E = EFE). During the different Stages, the 

monitoring procedures change, as summarized schematically in Figure 3-2. Real-world data 

depicting the trends described in Figure 3-2 are presented in Chapter 3.5 from the North 

Carolina Department of Transportation (NCDOT). NCDOT routinely monitors half-cell 

potentials of steel and zinc coupons and in-service reinforcements in an effort to evaluate 

zinc life for galvanized reinforcements. 

The details of the monitoring program differ from new construction to existing structures, as 

measurements for existing structures may begin in either Stage 1, 2 or 3, while for new 

construction, they always begin in Stage 1. 

c. New Structures 

For new structures, steel, zinc and galvanized coupons should be buried at each depth 

selected. At each monitoring station, sets of coupons should be placed both near the front 

and back of the reinforcements, i.e. close to the wall face and further back within the 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 9 November 2009
 
reinforced fill. This provides an opportunity to evaluate the spatial distribution of corrosion. 

Spatial variations along the reinforcement lengths are due to variations in moisture content, 

salt content and availability of oxygen. Also, many times a more open graded reinforced fill 

is placed within approximately 3 feet (1 m) from the wall face (i.e. less fines) compared to 

the backside of the reinforced fill to facilitate compaction. Coupons placed near the wall face 

may also be extracted through core holes cast into the wall face or cored during construction 

to facilitate removal of coupons for inspection without the need for more costly excavation. 

Direct physical observations of coupon condition are useful to verify and confirm results 

from nondestructive, electrochemical tests. 

In addition, actual reinforcement members must be instrumented for measurement purposes. 

The three stages of MSE structure life will be determined by comparing steel and zinc 

coupon potentials to those measured for the structure. During Stage 1 (reinforcement 

potential = zinc coupon) the following monitoring should be performed: 

C Potential measurements on each coupon and selected reinforcement members to establish 

the change from one stage of the corrosion process to another. 

C Polarization Resistance (PR) measurements on zinc and galvanized coupons and on the 

structure to estimate rate of zinc corrosion (zinc loss). 

C PR measurements on carbon steel coupons to characterize changes in the corrosion rate 

with time. 

Stage 2 is established when the potential of the reinforcement becomes more positive than 

the zinc coupon and approaches the potential of the steel coupon. During Stage 2, the 

following monitoring should be performed; 

C Potential measurements on each coupon and reinforcement to establish the change from 

one stage to another. 

C PR measurements on steel coupons to provide a conservative (high) estimate of the 

corrosion rate of the reinforcement. PR measurements on the reinforcement would yield 

mean values, as the actual exposed area is largely unknown, and the conversion constant 

is therefore uncertain or variable. 

C Begin retrieval of the galvanized coupons to quantitatively evaluate the condition of the 

structure. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 10 November 2009
 
Stage 3 is established when the potential of the structure becomes similar to the potential of 

the steel coupon. At this stage, little or no galvanized coating remains. For mildly corrosive 

conditions inherent to reinforced fill and site conditions meeting current AASHTO 

specifications, Stage 3 is not anticipated until at least 16 years of service and may not be 

reached before 30 or more years of service (Gladstone et al., 2006). During Stage 3, the 

following monitoring should be performed. 

C PR measurements on steel coupons and the reinforcement to estimate the corrosion rate 

(metal loss). 

C Potential measurements on galvanized coupons for comparison to the reinforcement, until 

the potentials of the steel and galvanized coupons are similar. 

C Continue periodical retrieval of the galvanized coupons to evaluate the condition of the 

reinforcement. 

Each reinforcement element must be isolated from the rest of the MSE structure to permit 

accurate polarization resistance measurements. Otherwise the polarized area is unknown and 

the corresponding corrosion rate calculation is erroneous. This excludes the possibility of 

monitoring individual in-service reinforcements for steel faced walls (e.g. wire facing), but 

coupons and “dummy” reinforcements may be used to infer performance of in service 

reinforcements for these systems. Reinforcements that are attached to precast concrete facing 

are generally isolated since current specifications require that reinforcement tie-strips 

embedded in the concrete be isolated from concrete reinforcing steel. The potential for 

electrical contact between reinforcements and other steel elements exists for MSE walls that 

serve as bridge abutments; near the corners of abutment facings and wing walls, and where 

steel piles protrude into the reinforced fill. Affected reinforcements may be isolated during 

construction by placing a dielectric (e.g. plastic wedge) to avoid electrical contact. 

Instrumentation of the reinforcement strips should be performed in pairs, such that two 

parallel strips are instrumented. Each member of the pair should have the same length (or the 

element serving as the counter electrode should be longer) to ensure that the entire surface 

area of the test element is polarized during the LPR test. On new structures, "dummy" pairs 

of reinforcement members may be instrumented and placed in the structure at the desired 

location to be sure that no contact is made to the working members on the structure or to the 

concrete panels. The instrumented members should be positioned such that 1 to 2 ft (0.3 to 

0.6 m) separation exists between members of the pair. The instrumented pairs will be 

exposed to the same conditions as the working members except that the stress profiles along 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 11 November 2009
 
 

the instrumented members will be somewhat different since the instrumented members will 

not be connected to the facing. 

The isolated strips will become test strips for making PR and potential measurements. The 

procedure for attaching test leads will be similar to that described for galvanizing coupons as 

described under Materials. The connection is shown schematically in Figure 3-3; red colored 

lead wires should be used to distinguish leads from the reinforcements and the steel coupons 

(black leads), zinc coupons (green leads) and galvanized coupons (white leads). Two 

connections will be made on each instrumented strip for redundancy. 

For MSE structures that have grid-type reinforcing members, the procedures for wiring the 

members will be similar to those previously described for reinforcement strips. If the 

connections to the front wall panels for grid systems are not tied to the rebar cage and the 

grid members are electrically isolated from each other, there is no need for "dummy" 

members to be placed in the wall. 

d. Existing Structures (Retrofit) 

Excavations can be made from the surface to reach upper level reinforcements to be 

monitored, and readings can be made at the top of the wall for those reinforcing elements. 

This method offers some advantages relative to data quality, but will be difficult to 

implement if pavement and impact barriers exist above the wall; and then only for shallow 

excavations. 

If excavation is performed from the top of the structure isolation should be accomplished by 

removing a 1 to 2 ft (0.3 to 0.6 m) section of the reinforcement strip or grid at a point 5 to 10 

ft (1.5 to 3 m) from either end. This removed section will also permit the condition of the 

reinforcement to be determined. The remaining section at either end of the reinforcement 

should be periodically excavated and 1-foot (0.3-m) sections cut and removed for evaluation. 

The 1-foot (0.3-m) sections removed from existing structures serve the same purpose as the 

retrievable galvanized coupons used for new structures. Care should be exercised because in 

many instances taking reinforcements out of service near the top of the wall may compromise 

the stability of the facing, e.g. considering truck impacts to parapet walls or Jersey barriers 

supported at the top of the MSE. 

If coupons are to be buried, they should be installed in the least disturbed soil along the side 

cuts of the excavation in order to obtain similar conditions to the existing fill. Care should 

also be taken to replace and compact the removed soil to a similar density as the original 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 12 November 2009
 
 

structure so as to avoid a differential condition that could potentially influence results as 

discussed in Chapter 3.2c. 

Instrumentation and lead connections on existing structures are made in a similar manner as 

described for the reinforcement strips and are shown on Figure 3-3. 

Alternately to minimize excavation, more limited but generally sufficient data can be 

obtained by accessing the reinforcements by drilling multiple 4-inch (100 mm) diameter 

holes thru the facing adjacent to reinforcement locations. Procedures using this alternate 

method have been developed and demonstrated (Berkovitz and Healy, 1997; CC 

Technologies, 1999; Wheeler, 2002; Beckham et al. 2005; Fishman et al., 2006). Figures 3-4 

and 3-5 illustrate the necessary field setup. 

Figure 3-3. Schematic diagram for connection to reinforcing members. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 13 November 2009
 
Electrical isolation of the reinforcements must be assured by testing prior to any potential or 

polarization measurements. For MSE walls that serve as bridge abutment, reinforcements 

may be in contact near the wing wall corners, and for reinforcements that are splayed around 

piles that are installed to support bridge seats. Older MSE walls, constructed before the early 

1980’s, may incorporate a U-shaped tie strip that is embedded into the precast facing such 

that the upper and lower reinforcing strips in a precast concrete panel are electrically 

connected through this connection. If electrical continuity exists, the working reinforcing 

element must be detached or isolated from the facing system, or another reinforcement 

should be selected for monitoring. 

Connections to in service reinforcements as shown in Figure 3-3 may not be possible. In this 

case connections may be achieved by soldering directly to the reinforcement after exposing 

bare steel, or attachments can be made with special c-clamps as described by Sagues et al. 

(1998). It is very important to ensure that connections are durable and made waterproof. The 

coupons may be inserted into the access holes, which also provide an opportunity to retrieve 

samples of reinforced fill. A smaller, 2-inch (50 mm) diameter hole is advanced near the 

center of the sample location, i.e. between the wired reinforcements, for contacting the 

reference electrode with the reinforced fill and subsequent potential and polarization 

measurements. 

Placement of the half-cell directly against the surface of the wall face will not provide 

sufficient contact with the reinforced fill and potential measurements are best achieved 

through additional core holes advanced through the wall face. A two-inch diameter core hole 

with a moist sponge at the base facilitates good electrical contact between the half-cell probe 

tip and the reinforced fill. The access hole should be located near the center of the sample 

location, i.e. in the middle of the reinforcements that are wired for monitoring. Another 

alternative depicted in Figure 3-4 is to place the half-cell in contact with soil at the base of 

the wall. 

e. Materials 

Carbon Steel Coupons 

Carbon steel coupons are used to estimate the rate of corrosion on the reinforcement 

members once the galvanized coating is lost (or partially lost) i.e. Stages 2 and 3. The carbon 

steel coupons will also provide the potential of steel for comparison to the potential of the 

reinforcement members in establishing when all galvanization has been lost (Stage 3). The 

carbon steel coupons should be made of similar materials as the reinforcement members. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 14 November 2009
 
Figure 3-4. A portable Copper/Copper Sulfate half-cell is hand held on the soil at the base 

of the wall as a reference electrode for multi-meter testing of electric potential. 

Exact duplication is not necessary since small concentration variations do not typically have 

a significant effect on general corrosion of carbon steel in soil. 

Coupon installation is described below and shown in Figures 3-6 and 3-7: 

C Coupon size is 100 by 100 by 10 mm minimum. 

C The top edge is drilled and tapped at two locations. 

C Coupon surfaces are finished to a 600 grit finish (320 to 600 is acceptable). 

C A 4-40 bolt is threaded into the top of the coupon and the head cut-off. 

C A No. 10 gauge type THNN coated copper wire test lead (black) is soldered to the 4-40 

bolt using a tension pin to provide support to the solder joint. 

C A solder joint should be sealed with Alpha FIT 300 shrink tubing and the ends coated 

with Carboline coal tar epoxy coating including a 25 mm area of coupon around the 

connection (use two coats of epoxy, with each coat increasing in area covered). 

Isolating the lead wire from the environment is critical for obtaining the desired life of the 

coupon. Manufacturers’ cleaning procedures for epoxy application should be followed 

closely. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 15 November 2009
 
Figure 3-5. PR Monitor evaluation of a test location. Note horizontal adjacent access 

holes for cross testing. 

Figure 3-6. Schematic diagram for coupons. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 16 November 2009 
Figure 3-7. Schematic diagram illustrating coupon connection. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 17 November 2009 
Zinc Coupons 

Zinc coupons are used to determine the rate of zinc loss, which is an estimate of the rate of 

galvanization removal from the in service reinforcements (Stage 1). The zinc coupons 

should also provide the potential of zinc for comparison to the potential of the reinforcement 

members for establishing the transition from Stage 1 to Stage 2. The zinc coupons should be 

made of solid zinc and should have a typical composition of zinc used for galvanizing. 

The configuration of the coupon should be similar to that of the carbon steel coupon, with the 

following exceptions: 

C The shape of the coupon may be round instead of square depending on the availability of 

the zinc. Size should be 23⁄4 to 43⁄4 in. (70 to 120 mm) in diameter. 

C The test leads are green. 

C The 4-40 bolt should be galvanized. 

Galvanized Coupons 

Galvanized coupons should be buried in new structures for periodic extraction to determine 

the condition of the galvanized coating and the steel substrate. The coupons should be made 

from reinforcement members; preferably from the supplier. The cut edges of the coupons 

should be re-dipped in a zinc bath to provide a galvanized coating at the cut edges, or else the 

cut edges should be coated with a dielectric (e.g., liquid tape and epoxy). Coupons should be 

both of the retrievable type and instrumented type. The retrievable type should be placed 

behind openings cut in the face panels and may be up to 4 ft (1.2 m) long. 

The instrumented coupons should have the following configurations: 

C Coupon size is 1 ft (0.3 m) long (thickness or diameter will be the same as the 

reinforcement element). 

C A single 4-40, 2-inch (50 mm) long bolt should be threaded through a drilled and tapped 

hole in the end of the coupon prior to regalvanizing the edges so that the rod can also be 

galvanized. 

C The No. 10 Gauge Type THNN coated copper wire test lead (white) is soldered to the 

bolt using a tension pin to provide support for the solder joint. 

C The connection and down to the top of the coupon is sealed with Alpha FIT 300 shrink 

tubing, and the ends and the solder connection is coated with the carboline epoxy coating 

(use two coats of epoxy with each coat increasing in area covered). 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 18 November 2009
 
Reference Electrode 

The reference electrode is used as the low end reference to probe reinforcing elements or 

coupons to measure the voltage corrosion potential. The purpose of the reference probe is to 

provide a stable, known, voltage serving as a datum. As previously indicated in this section, 

the common reference electrode in underground corrosion studies is Copper/Copper Sulfate 

(CSE). Several other types of reference electrodes are available (Fontana, 1986). However, 

the CSE is a popular choice for making measurements in soil because its accuracy and 

stability is adequate; it is relatively inexpensive and readily available. The disadvantage of 

the CSE compared to other reference electrodes is the need for maintenance at least once per 

month. Therefore, the CSE is not a good choice when a reference electrode must to be left in 

place for making measurements over an extended period. For the applications described 

herein, the CSE is only placed in contact with the soil for making discrete measurements, and 

there are opportunities for maintenance between monitoring activities. 

f. Measurement Procedures and Equipment 

After the installation of leads to the coupons, reinforcing element, and filling of at least one 

lift, initial measurements of potential and polarization resistance can be made. Figure 3-8 

shows a typical installation prior to backfilling. 

Potential measurements are relatively simple direct measurements that can be performed 

with a minimum of equipment, application time and experience. Recommended equipment 

is a high impedance voltmeter (10 M or greater) and a copper/copper sulfate (Cu/Cu2SO4) 

electrode. Multiple measurements (every 6+ ft {2± m}) may be made along the length of the 

buried reinforcement by moving the electrode and developing a profile of the potentials 

along the reinforcement. The details of the calibration and measurements procedures are 

fully discussed in FHWA RD 89-186 (Elias, 1989). 

Polarization resistance measurements require the application of a potential (current) to the 

specimen being monitored, with simultaneous measurement of the potential and current. The 

acquired data is processed to calculate the polarization resistance and to estimate the 

corrosion rate of the specimens. Standard methods for calibration of equipment and 

verification of the technique for performing polarization resistance are outlined in ASTM G59. Soil resistance measurement equipment is typically needed for correcting polarization 

resistance measurements for Rs. Equipment for performing polarization resistance 

measurements should be checked prior to making measurements in the field by applying a 

known resistance between the leads or with a “dummy” cell as described in Section 5 of 

ASTM G59. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 19 November 2009
 
Figure 3-8. Typical installation. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 20 November 2009 
Figure 3-9. Automated polarization resistance measurement equipment. 

Fully automatic equipment is available that can apply current, make polarization and soil 

resistance measurements, and integrate all data. (See FHWA RD 89-186 (Elias, 1989)). The 

use of this fully automated equipment is recommended. It consists of a self-contained unit in 

which the power source (battery) lead connector unit, and interactive computer are integrated 

as shown on Figure 3-9. 

The operation consists of connecting the leads from the coupons and reinforcements to the 

lead connector unit and activating the computer. The interactive programming then leads the 

operator through sequential steps in which the current is applied, measurements of 

polarization resistance and soil resistance made, and the instantaneous corrosion rate 

calculated and displayed. 

The following suggestions are offered to ensure the quality of the data and to identify 

measurements that may be subject to error. These suggestions are intended to supplement 

information included in the operating manual supplied with the LPR test equipment. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 21 November 2009
 
1. Maintain the copper-sulphate reference electrode, as described by ASTM C876, prior to 

the site visit. The porous tip needs to be damp, so after the electrode is maintained, let 

the tip soak in about 1 in. (25 mm) of water for a half an hour or so before the test. Be 

sure to replace the cap on the porous tip when not in use to prevent drying. 

2. After you arrive onsite connect the power supply and power-up the equipment for making 

LPR measurements. Most electronic equipment needs to warm-up for approximately 20 

minutes before taking readings. 

3. Place the reference electrode in the access hole, or other suitable location at a monitoring 

location. Direct contact with the concrete face will not achieve suitable contact with the 

reinforced fill. When contacting the reinforced fill through an access hole in the wall 

facing, place a moist piece of sponge into the hole and in contact with the reinforced fill, 

remove the plastic cap from the end of the reference electrode, and press the tip firmly 

against the sponge. Good contact must be maintained, which may be accomplished by 

wedging the half-cell into place (wooden shims work well). If the reference electrode is 

placed in contact with soil at the base or top of the wall, loosen and wet the soil before 

pushing the probe tip into place. If the ground surface is paved, core or chip a hole in the 

surface to allow direct contact with the underlying soil. Attach a short length of wire 

(#12 gage) to the exposed end of the reference electrode for making connections to a 

digital voltmeter. 

4. Record the half-cell potential of each element at the test station using a digital voltmeter. 

The voltmeter should have an impedance of at least 10M and a readout that includes at 

least four significant digits. Connect the common (black) lead from the voltmeter to the 

reference electrode, and the red lead from the voltmeter to the steel coupon. The voltage 

should be negative and in the range of –600 to –200 mV. Record the reading and then 

move the red lead from the voltmeter to the next element. The common lead remains 

attached to the reference electrode for each measurement. Record the reading and then 

read the remainder of the elements and record in a similar fashion. Zinc coupons 

normally read in the range from -1100 to –650 mV and in-service reinforcements and 

galvanized coupons will be in between the half-cell potentials recorded for the zinc and 

steel coupons; depending on age and condition. These reading also provide an 

opportunity to check that elements are electrically isolated from each other. Half-cell 

potentials that are within 5mV between elements may indicate electrical contact. As a 

second check, measure the resistance between different elements and a reading less than 

5 is indicative of contact (i.e. not electrically isolated). 

5. Connect the cable bundle supplied with the LPR test equipment. The cable bundle has 

three leads marked WE, CE (e.g., red lead) and Ref (e.g., black lead). Connect the black 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 22 November 2009
 
lead to the reference electrode, the CE lead to the counter electrode (usually this is the 

second dummy or in service element wired for monitoring), and the WE (working 

electrode) lead to the test element, which could be one of the coupons or the other inservice or dummy element. Use the same CE for monitoring all the other elements and, 

then switch this to the WE and use the other wired reinforcement or dummy element as 

the counter. 

6. Observe the initial half-cell potential recorded by the LPR equipment. This is the initial 

reading taken before current is impressed between the WE and CE. This measurement 

should be within  20 mV of the half-cell potentials measured with the voltmeter as 

described in item #4. If this is not the case, check the cable connections (this is usually 

the problem) and placement of the half-cell. In some instances problems may be due to 

the presence of stray currents or interference from nearby magnetic disturbances. These 

interferences affect the cable connections and it may be necessary to move the equipment 

or power sources to a new location to avoid interferences. 

7. Get an initial reading of soil resistance. This is often offered as a separate measurement 

with units specifically designed for making measurements in a high resistance electrolyte 

such as soil or concrete. For a given environment, the soil resistance is inversely 

proportional to the surface area of the test element; i.e. soil resistances surrounding inservice reinforcements should be less than those surrounding the relatively smaller 

coupons. If this is not the case there may be problems related to connections; e.g. the 

buried connections between the leads and the elements may not be sealed properly. If 

extremely low resistances are measured with respect to in service reinforcements, this 

may be an indication that more surface area is involved than anticipated; i.e. the element 

is not electrically isolated and is in contact with other metal elements such as piles or 

other reinforcements. 

8. Proceed to measurement of corrosion rate in accordance with the operating manual 

provided with the LPR equipment. Be sure to input the appropriate conversion constant, 

surface area, and metal type for the test element (i.e. zinc or steel). In some systems 

Tafel slopes rather than conversion constants are input and metal type is in terms of 

density and equivalent atomic weight. Other parameters include the potential shift, step 

size and step duration used to control the LPR test. For MSE reinforcements typical 

values are 20 mV, in 5mV steps, applied in a 15 second duration. 

9. Assign a file name for the test. A good naming scheme that allows data to be located in a 

search includes “state-day-month-year-locale-site name-wall name-WE#-CE#.” Also 

include fields of information that apply to weather and temperature as comments to the 

data record. 

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10. Run the LPR test. Check the applied step sizes during the test to see if it is consistent 

with the input controlled step size. If this is not the case there may be a problem with 

drift and possible interference. Check connections and look for sources of interference. 

11. Create and review a chart (software supplied with the LPR equipment will often produce 

this chart automatically) of applied current versus over potential (half-cell potential). The 

chart should be linear or at least include a linear portion in proximately to the open circuit 

potential. If this is not the case, repeat the test. If linearity cannot be achieved the data is 

suspect. 

12. Compare the measured soil resistance (Rs) to the polarization resistance (PR-corrected for 

soil resistance). Preferably the ratio (Rs/PR) is less than one, but ratios between 1 and 4 

are acceptable. If the ratio is greater than 4, move the half-cell to a position closer to the 

test element, and repeat the test. If a ratio less than 4 cannot be achieved, the measured 

corrosion rate is subject to a relatively high error. 

13. If previous measurements have been made at this site be sure to have them available for 

comparison. Discrepancies should be noted and these tests should be repeated and 

checked for consistency. 

14. LPR tests impart minimum disturbance to the system, and therefore a test may be 

repeated as a check on precision and repeatability. A wait period of approximately 10 

minutes between tests is advised to allow the half cell potential of the test element to 

return to the open circuit potential, and to be sure that any stray currents have been 

discharged. The LPR test is not accurate for measuring very high corrosion rates, and, in 

these cases, an average from three measurements should be achieved. 

. 

g. Frequency of Measurement 

Potential measurements of each coupon and instrumented reinforcement must be made at the 

time of installation to check lead connections and establish initial measurement values. 

Subsequent measurements are recommended monthly for the first 3 months, bi-monthly for 

the next 9 months (to determine seasonal variations, if any) and annually thereafter at 

approximately the same date. 

Polarization resistance measurements should be made at the same intervals and schedule. 

For long-term monitoring schemes, four galvanized coupons should be buried; the first 

coupon should be removed at the midrange of Stage 2, the second at the beginning of Stage 

3, and the remaining two at intervals established by the predicted metal loss from PR 

measurements. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 24 November 2009
 
For existing structures, shallow excavations can be made for observations of the 

reinforcement and thickness measurements. Reinforcements should be cleaned with a wire 

brush and all surface corrosion removed before measurements are made. For a more 

thorough evaluation, isolation of the reinforcement member can also be accomplished by 

removing a 1-foot (0.3-m) section at a point 5 to 10 ft (1.5 to 3 m) from either end of the 

reinforcement member. Of course, any reinforcements that are removed must be replaced 

and the excavation should be sufficiently large to accommodate this process. The sections 

removed during initial excavation are used for evaluations to establish the starting condition 

for monitoring. The remaining sections at either end can serve the same purpose as the 

galvanized coupons used for new structures and the middle section can be instrumented. For 

long term monitoring schemes, excavations can be performed and a 1-foot (0.3-m) section 

removed from the remaining sections at either end of the reinforcement members. These 

excavations should be performed at the midrange of Stage 2, at the beginning of Stage 3, and 

thereafter at intervals established by the predicted metal loss from PR measurements 

The above procedure is impractical when accessing reinforcements thru the facing. 

3.3 EVALUATION OF CORROSION MONITORING DATA 

The outlined concepts, methods, and equipment to determine corrosion rates on both new and 

existing structures have been evaluated at five existing and two new structures and reported 

in FHWA RD 89-186 "Durability/Corrosion of Soil Reinforced Structures" (Elias, 1989). 

The data obtained at Site 4, lower level of a newly constructed wall, for 26 months is shown 

on Table 3-1. At this site the reinforced fill soils were granular with minimum resistivities of 

15,000 ohm-cm, which would suggest a non-corrosive regime. Measurements for both the 

coupons and actual reinforcements (Reinforcement 1,2) are shown. 

The free corrosion potential measurements, Ecor, for the steel coupons, using a copper sulfate 

reference electrode, Cu/Cu2SO4, after an initial stabilization period are on the order of -0.4 v, 

which is within the range of potentials for carbon steel (from -0.75 v to -0.35 v depending on 

steel composition and activity). The corrosion rate as measured by polarization 

measurements is initially low at 2 to 3 μm/year, then increases to 10-11 μm/year, which is 

below the maximum predicted rate of 12 μm/year used to establish sacrificial steel thickness. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

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Table 3-1. Summary of Field Corrosion Data, Site 4, Lower Level. 

Test Specimens 

Initial 

Data 

Exposure Times 

2 

Months 

8 

Months 

15 

Months 

26 

Months 

Ecor (a), Steel, in Volts -0.548 -0.458 -0.400 -0.329 -0.351 

Corrosion Rate(b), Steel, in μm 6.5 3.2 1.9 10 11 

Ecor, Zn, in Volts -0.915 -1.000 -0.921 -0.830 -0.963 

Corrosion Rate, Zn, in μm 55 2.5 0.4 1.2 1.1 

Ecor, Galvanized, in Volts -0.938 -0.708 -0.775 -0.781 -1.014 

Corrosion Rate, Galvanized, in μm 43 1.0 0.4 1.6 2.7 

Ecor, Reinforcement level 1, in V. -0.976 -0.681 -0.870 -0.654 -0.710 

Corrosion Rate, Reinf. 1, in μm 7.6 1.2 0.9 1.8 0.9 

Ecor, Reinforcement level 2, in V. -1.005 -0.793 -0.920 -0.663 -0.756 

Corrosion Rate, Reinf. 2, in μm 4.5 0.8 0.6 1.3 0.6 

(a) Free-corrosion potential, V(Cu/CuSO4). 

(b) Corrosion rate estimated from polarization resistance corrected for soil resistance, μm/yr unless 

otherwise noted. 

(c) μm/25.4 = 1 mil 

The free potential for zinc coupons, galvanized coupons and reinforcing strips are all more 

negative than that for steel. They range from -0.75 to -1.1 v, which is typical for zinc or well 

galvanized steel. After an initial stabilization period, the corrosion rate as measured by 

polarization measurements, varies from approximately 0.5 to 2.7 μm/year, which is 

considerably less than the maximum design rate of 15 μm/year for the first 2 years for zinc or 

galvanized steel. This lower rate is reasonable when considering the resistivity of the fill at 

15,000 ohm-cm, which is considerably greater than the minimum required 3000 ohm-cm for 

MSE reinforced fills. This typical data confirms the suitability and practicality of the 

monitoring techniques using electrochemical principles. Complete confirmation of the 

suitability of the design corrosion rates previously recommended can only be made after at 

least a few years of measurements. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

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Additional confirmation of the suitability and practicality of techniques using potential 

monitoring have been made by a number of state transportation agencies through MSEW 

corrosion assessment programs as reviewed in Section 3.5. 

3.4 APPLICATION OF CORROSION MONITORING TO ASSET MANAGEMENT 

Asset management is an important issue facing highway operations, and forecasting the 

needs for maintenance, retrofit or replacement of existing facilities is an important 

component of Transportation Asset Management (TAM). MSE structures should be included 

in a TAM program along with pavements, bridges, ancillary structures, etc., to help ensure 

optimal usage of limited available funding (FHWA, 2008). Properly defining the existing 

inventory and development of a performance database are important components of asset 

management. Relatively rapid, non-intrusive, and nondestructive test techniques are needed 

to collect data necessary for corrosion monitoring and condition assessment of MSE. Results 

from condition assessment and corrosion monitoring indicate when, or if, accelerated 

corrosion is occurring, and can help transportation agencies decide on the most appropriate 

course of action when subsurface conditions are unfavorable and service life is uncertain. 

Agencies can also use these data to evaluate the variance associated with the performance of 

an inventory; this is valuable information for those with an interest in making reliabilitybased decisions. This chapter describes the framework of a performance database useful for 

asset management, test techniques and protocols that are being employed to collect 

performance data for earth reinforcements, data interpretation and preliminary information 

available from data that has been collected to date. 

a. Document Inventory and Prepare Performance Database 

As part of a TAM strategy, data are collected to document the performance of MSE 

structures including the condition of metallic reinforcements and corresponding rates of 

metal loss. A performance database is under development (NCHRP Project 24-28) wherein 

the attributes of the MSE inventory are documented, as are results from corrosion monitoring 

and condition assessment of metallic reinforcements. TAM programs are data driven and 

require databases that (a) identify and name components, (b) describe locations, c) define and 

describe data, and d) explain performance. Several existing databases developed by the: 

NYSDOT (Wheeler, 2002), Colorado Department of Transportation (CDOT [Hearn, et al., 

2004]), Association for Metallically Stabilized Earth (AMSE, 2006), KTRC (Beckham, et al., 

2005), and ODOT (Timmerman, 1990) were reviewed. In general, these databases follow a 

format and protocol consistent with that employed by the FHWA mandated Bridge 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

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Management System (Hearn, et al., 2004). These databases were considered and used as a 

basis to develop the framework for an MSE performance database that can be a useful 

resource for asset management. 

In general, the database is self-contained yet structured such that it can be ported to other 

existing databases. The database is formatted using MicroSoft Access, which is linked to a 

GIS (ArcView) platform to provide visual and spatial recognition of data. The organization 

and structure of the various tables and data fields are updated, as necessary, to accommodate 

different types of information that are identified from available data sets. For example, 

observations of reinforcement performance/condition may be based on nondestructive testing 

(NDT), direct physical measurement, or visual observations, and these data types are 

archived accordingly. Drop down lists and check boxes are implemented to facilitate 

mining/querying of the database. 

Information within the shell of the database is distributed amongst seven distinct tables 

comprising a total of 150 data fields. The tables are divided into categories of information 

similar to that employed in other databases that are based on the FHWA Bridge Management 

Inventory. The database includes the following tables: 

C Project 

C Walls/Structure 

C Reinforcements 

C Reinforced fill/Subsurface 

C Observation Points 

C NDT Results (e.g., potential and polarization resistance measurements) 

C Direct Observations (e.g., retrieval of buried coupons) 

Data forms were created to facilitate data entry. Tables are related using a one to many 

relationship using “project number” as a key parameter. Other relationships may also be 

created, but currently all other tables are considered to be a sub-form to the project form, 

which serves as the master form. Thus, a project may have a number of walls and/or 

reinforced fills. A wall may have numerous observation points; and a number of 

observations, including NDT or direct physical observations may be associated with each 

observation point. For example, the project in Las Vegas, Nevada described by Fishman et al 

(2006) includes three walls; Wall #1 has 15 monitoring locations, Wall #2 has six, and Wall 

#3 has four. Each monitoring location includes two in-service reinforcements wired for 

monitoring (NDT) and at least two steel coupons; one plain, and one galvanized. Also, direct 

physical measurement of section loss is performed on 18 samples retrieved from six of the 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

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monitoring stations (i.e., three reinforcements exhumed from six of the stations). These data 

are all organized into separate tables that are linked to the Las Vegas, Nevada entry from the 

Project Table. Relationships are also defined between reinforced fill, wall, reinforcements, 

monitoring stations and results tables. 

Each project is associated with a point that is displayed on a map within ArcView. ArcView 

mapped points are also linked to the Microsoft Access tables so pertinent information for 

each project can be displayed next to each point when selected by the user. In this way, the 

geographic distribution of performance data, as well as specific attributes for each site can be 

displayed within a GIS platform. Thus, the user may associate the data with geographic 

location and view all of the performance data and pertinent information associated with that 

point. 

The database provides an archive for information and can be used as a reference to compare 

measurements from sites that may represent similar conditions. As such the database is an 

asset and a valuable resource. Agencies implementing condition assessment and performance 

monitoring programs can use this information to help interpret their results within a broader 

context than a site-specific study may provide. Interpretations based on past experience can 

be useful for refining metal loss models, understanding conditions leading to end of service 

life, and gaining a better understanding of the various factors and their effects on corrosion 

rates. 

b. Data Needs and Data Analysis 

Several studies (e.g., Anderson and Sankey, 2002) involve exhuming reinforcements from inservice facilities, but the original dimensions of the elements are uncertain because corrosion 

monitoring was not anticipated during installation. In these cases, the original dimensions or 

weights of the elements must be estimated to compute metal loss from observations of 

remaining thickness or weight. Nominal dimensions may be used with confidence for the 

steel thickness, however the original thickness of the galvanized layer is more uncertain. A 

minimum thickness of zinc coating is specified, however the hot dipped galvanization 

process specified by ASTM A123 (ASTM, 2007) results in galvanized layer thickness that 

may exceed the minimum by a wide margin. (For example, Rossi (1996) reports thickness 

measurements as high as 280 m from elements with a specified minimum zinc coating 

thickness of 86 m.) 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

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Various estimates of initial zinc thickness are used as a basis to evaluate zinc loss of 

exhumed reinforcements in the absence of initial measurements including (1) observing the 

zinc thickness from a sample retrieved from the connection to the wall face where the 

reinforcement is sandwiched between two plates and assuming that zinc loss is negligible due 

to the fact the reinforcement sandwiched within the connection is not in direct contact with 

the reinforced fill, and (2) assuming that zinc oxide adhered to the surface is equivalent to the 

loss of zinc. Both of these assumptions can lead to large errors, and uncertainty with respect 

to initial conditions remains. However, often one or both of these methods may be used to 

obtain a conservative estimate of zinc loss, i.e., overestimate the initial zinc thickness. 

Alternatively, electrochemical corrosion monitoring test techniques may be applied for 

monitoring earth reinforcements as described in Section 3.1 and 3.2. With these techniques, 

a large number of frequent samples may be obtained, and the initial thickness of the zinc is 

not needed to interpret the measured corrosion rate. Because the tests are nondestructive, 

reinforcements are left intact and in service after testing, and available for future monitoring. 

As discussed in Section 3.2, when measurements are taken throughout the service life of a 

wall, these techniques can quantify the relationship between rate of metal loss and time. 

Ideally, protocols for condition assessment and corrosion monitoring should include both 

direct physical observations (i.e., weight loss, remaining thickness, pit depth) and 

electrochemical tests such as LPR and half-cell potential measurements. However, very few 

of the documented studies currently include such complete data (see Section 3.5 for examples 

of existing data). 

Collecting information on site conditions and reinforced fill character are very important 

components of data collection activates. Better and more consistent data on reinforced fill are 

needed. In particular, samples need to be taken in proximity to reinforcements that are being 

monitored, and at the time that measurements are being taken. Current data on reinforced fills 

are often from pre-construction samples taken from stockpiles that may not represent 

conditions prevailing during corrosion monitoring. 

The performance database includes thousands of measurements of element condition and 

corrosion rate from more than 150 sites distributed throughout the United States and Europe. 

The large sample domain allows evaluation of sample statistics, distributions of element 

conditions and corrosion rates, and corresponding probability based analyses. These issues 

are related to reliability of metal loss modeling, quantification of the effect of construction 

practice on performance, and understanding the cost benefits of using different materials. All 

of these are important components of asst management. For example the database can be 

used to: 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 30 November 2009
 
C Study the mean and variance of corrosion rates for data sets grouped according to 

different climate, site conditions, and reinforced fill conditions 

C Quantify performance for marginal reinforced fills 

C Evaluate the performance of different material, i.e. steel vs. zinc, other forms of 

metallization, and the use of polymeric coatings 

c. Estimate Future Needs for Maintenance, Rehabilitation and Replacement; or 

Extended Design Life 

Issues that can address future needs for maintenance, rehabilitation, retrofit or replacement 

include: 

C Spatial variations of element condition and corrosion rate; e.g. top vs. bottom of wall 

C Special areas that may deserve increased maintenance; e.g. in proximity to drainage inlets 

C Quantify effects of different climates, use of deicing agents, etc. 

Improved knowledge of spatial variations and special problems can lead to improved 

allocation of resources. For example, in some cases extended service life may be best 

achieved by retrofitting areas surrounding drainage inlets, or the benefits of improved 

maintenance of drainage inlets may be realized in terms of increased service life. In areas 

where deicing slats are used, corrosion monitoring can demonstrate the need to maintain 

pavements, improve drainage or install impervious barriers. 

d. Update Experience with Different Reinforced Fills 

An example of the experience gained from collecting and analyzing data relates to the use of 

reinforced fills that may or may not meet AASHTO specifications for electrochemical 

properties. The available database was divided into two primary groups including data from 

reinforced fill conforming to AASHTO criteria and from reinforced fill not conforming to 

AASHTO criteria. The AASHTO corrosion model was applied to estimate reinforcement 

corrosion rates and compare them to measured corrosion rates. The following observations 

were made from the existing database. These observations are preliminary and may be 

updated as more data become available. 

C For reinforced fills conforming to AASHTO criteria, the AASHTO corrosion model 

overestimates steel corrosion rates for 98% of the data. It should be noted that most of the 

data in this group are associated with reinforced fills that meet AASHTO requirements by 

a wide margin. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

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C For reinforced fills conforming to AASHTO criteria, marine environments have minor to 

no effect on measured corrosion rates of galvanized reinforcements, but marine 

environments accelerate corrosion rates of plain steel reinforcements. 

C For reinforced fills that do not satisfy AASHTO criteria, marine environments are 

associated with relatively high corrosion rates. 

C Reinforced fills that do not meet AASHTO criteria, i.e. soil resistivity values  < 3000 ­

cm and pH values < 5, can significantly affect steel corrosion rates, which tend to 

dramatically increase beyond rates estimated by the AASHTO corrosion model. 

C Based on available data, organics content, chlorides, sulfates and relatively high values of 

pH have much less effect on measured corrosion than do relatively low resistivity and 

pH. 

C Review of the latest research information confirms the safety of the electrochemical 

requirements for fill and associated metal loss rates in the current AASHTO standards. 

3.5 STATE CORROSION MONITORING PROGRAMS 

As of 2008 nine states including California, Florida, Georgia, Kentucky, North Carolina, 

New York, Nevada, Ohio and Oregon have implemented MSEW corrosion assessment 

programs. Corrosion monitoring practices vary including (1) installation of 

coupons/inspection elements for extraction and direct physical observations at planned 

intervals, (2) wiring of coupons and in service reinforcements for electrochemical testing and 

corrosion monitoring, (3) a combination of 1 and 2, and (4) monitoring reinforced fill 

conditions during service. Few agencies have comprehensive programs that include both 

direct physical observations and electrochemical testing. Table 3-2 is a summary of the 

programs, a brief description of the practices, and references that provide details of the 

program and results from corrosion monitoring. Detailed descriptions of corrosion 

monitoring activities in California, Florida, New York and North Carolina are presented to 

demonstrate different corrosion monitoring practices and implementation of corrosion 

monitoring programs. 

C Results presented from Caltrans demonstrate how climate and fill quality can impact 

performance; and the benefits of using inspection elements that can be probed by 

electrochemical techniques and later extracted for inspection. 

C Results presented from The Florida Department of Transportation (FDOT) describe how 

the impact from saltwater intrusion is evaluated with a corrosion-monitoring program. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 32 November 2009
 
FDOT implemented electrochemical techniques similar to those describe in this 

reference. 

C Results collected from two sites wired for monitoring by the The New York State 

Department of Transportation (NYSDOT) demonstrate how corrosion rates measured 

with the LPR technique can vary throughout the course of a year, and with respect to 

different seasons. NYSDOT also implemented electrochemical techniques similar to 

those describe in this reference. 

C Results presented from the North Carolina Department of Transportation (NCDOT) are 

used to demonstrate how half-cell potential measurements can be useful to assess zinc 

life for galvanized reinforcements. 

Details from other state corrosion monitoring programs are available from the references 

cited in Table 3-2 

Caltrans 

Based on the recommendations by Jackura et al. (1987), CalTrans began installing inspection 

elements within MSE walls constructed since 1987. Ten-foot long rods were inserted as 

inspection elements within walls using steel grid reinforcements; 10-foot (3 m) long strips 

were inserted within walls using strip-type reinforcements. Caltrans extracts the inspection 

rods to make visual examinations of surface pitting and loss of section, and laboratory testing 

to assess remaining zinc and loss of tensile strength ((Initial-Remaining)/Initial). Because 

initial tensile strength was not measured prior to installation, it is estimated from the intact 

sections of exhumed inspection elements. The inspection elements are accessible from the 

wall face and can also easily be wired for electrochemical testing. 

Caltrans collected data, in cooperation with NCHRP Project 24-28, including (1) fieldwork 

and LPR testing at 7 sites in the southern part of the state, (2) comparison of corrosion rates 

collected by different operators and equipment, and (3) laboratory testing and evaluation of 

inspection elements extracted from the field. Table 3-3 describes the sites. Eight walls were 

selected for monitoring (two walls were monitored at the site of Bridge #53-2819). All of 

these walls support highway embankments and are approximately 10 years old. The 

Riverside wall is located in a desert environment, two locations in San Bernardino are in a 

residential area and a Mediterranean climate, and the five walls in Long Beach are in a 

coastal/marine environment. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 33 November 2009
 
 

 

 

 

 

 

Table 3-2. Summary of State DOT MSEW Corrosion Assessment Programs. 

State Description References 

California Have been installing inspection elements with new construction since 

1987, and performing tensile strength tests on extracted elements. Some 

electrochemical testing of in service reinforcements and coupons has also 

been performed. LPR and EIS tests were performed on inspection 

elements at selected sites as part of NCHRP Project 24-28 and results 

compared with direct physical observations on extracted elements. 

Jackura et al. (1987), 

Elias (1990), Coats 

et al. (1990), Coats 

et al. (2003- Draft 

Report) 

Florida Program focused on evaluating the impact of salt-water intrusion, 

including laboratory testing and field studies. Coupons were installed and 

reinforcements were wired for electrochemical testing and corrosion 

monitoring at 10 MSE walls. Monitoring has continued since 1996. 

Sagues et al. (1998, 

1999, and 2000), 

Berke and Sagues 

(2009) 

Georgia Began evaluating MSE walls in 1979 in response to observations of poor 

performance at one site located in a very aggressive marine environment 

incorporating an early application of MSE technology. Exhumed 

reinforcement samples for visual examination and laboratory testing. 

Some in situ corrosion monitoring of in service reinforcements and 

coupons at twelve selected sites using electrochemical test techniques 

was also performed. 

McGee (1985), 

Deaver (1992) 

Kentucky Developed an inventory and performance database for MSE walls. 

Performed corrosion monitoring including electrochemical testing of in 

service reinforcements and coupons at five selected sites. 

Beckham et al. 

(2005) 

Nevada Condition assessment and corrosion monitoring of three walls at a site 

with aggressive reinforced fill and site conditions. Exhumed 

reinforcements for visual examination and laboratory testing; performed 

electrochemical testing on in service reinforcements and coupons. A total 

of 12 monitoring stations were dispersed throughout the site providing a 

very good sample distribution. 

Fishman et al. 

(2006) 

New York Screened inventory and established priorities for condition assessment 

and corrosion monitoring based on suspect reinforced fills. Two walls 

with reinforced fill known to meet department specifications for MSE 

construction are also included in program as a basis for comparison. 

Corrosion monitoring uses electrochemical tests on coupons and in 

service reinforcements. 

Wheeler (1999, 

2000, 2001, 2002a 

and 2002b) 

North 

Carolina 

Initiated a corrosion evaluation program for MSE structures in 1992. 

Screened inventory and six walls were selected for electrochemical 

testing including measurement of half-cell potential and LPR. This initial 

study included in service reinforcements but coupons were not installed. 

Subsequent to the initial study, NCDOT has installed coupons and wired 

in-service reinforcements for measurement of half-cell potential on MSE 

walls and embankments constructed since 1992. LPR testing was also 

performed at approximately 30 sites in cooperation with NCHRP Project 

24-28. 

Medford (1999) 

Ohio Concerned about the impact of their highway and bridge deicing 

programs on the service life of metal reinforcements. Performed 

laboratory testing on samples of reinforced fill but did not sample 

reinforcements or make insitu corrosion rate measurements 

Timmerman (1990) 

Oregon Preliminary study including 1) a review of methods for estimating and 

measuring deterioration of structural reinforcing elements, 2) a selected 

history of design specifications and utilization of metallic reinforcements 

and 3) listing of MSE walls that can be identified in the ODOT system. 

Raeburn et al. 

(2008) 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 34 November 2009
 
Table 3-3. Selected Sites in Southern California. 

Bridge # Location Intersection Reinforcement 

Type 

Year 

Built 

Height 

(ft) 

56-0794M Riverside I10E & Union Pacific RR Strip 1996 25 

54-1093M San 

Bernardino 30th St. & Pico Road Strip 1988 30 

54-1094M San 

Bernardino 30th St. & Miramonte Dr. Strip 1988 18 

53-2819M Long Beach Rt. 47 Naval & Marine Base Grid 1997 36 

53-2821M Long Beach Rt. 47 & Navy Way Grid 1997 38 

53-2823M Long Beach Rt. 47 & Navy Way Grid 1997 40 

53-2822M Long Beach Rt. 47 & Navy Way Grid 1997 28 

1 ft = 0.3 m 

Data collection focused on testing all available inspection elements with electrochemical test 

techniques including half-cell potential and linear polarization resistance measurements. In 

general, a cluster of 18 inspection elements was available for monitoring along each wall. 

The LPR test results were used as a guide to select two or three elements for extraction and 

physical observation. In this manner, elements suspected of a low level of corrosion, as well 

as those suspected to exhibit a higher level of corrosion, could be extracted and the utility of 

the LPR technique to identify moderate to severely corroded, or mildly corroded elements 

could be evaluated. This is a very important exercise because the LPR technique also 

provides an opportunity to study the spatial and temporal variability of corrosion rate. 

Caltrans prepared special spools of wire for making the electrical connections necessary for 

electrochemical testing of each element. Alligator clips were used to attach a spool of wire 

to each inspection element. The spools were used to extend the wires from each element to a 

junction platform with a set of holes to accommodate the end of each spool as shown in 

Figure 3-10. One person, aided by an aerial lift, could make spool connections to all 

elements in the cluster, which greatly facilitated production. 

LPR tests were performed with the 3-electrode configuration consisting of a working 

electrode, counter electrode and reference electrode. The element directly above or below 

each test element (working electrode) was used as a counter electrode. Reference cells 

(CSE) were placed in wetted soil at the base of the wall directly beneath each element, and 

connected to a spool of wire that also led to the spool junction. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 35 November 2009
 
 

8.4 1763 47 

8.1 1389 26 

 

9.1 2072 26 

 

8.8 1763 99 

9.1 2223 99 

 

 

 

 

 

 

Figure 3-10. Junction platform and wire spools used to organize connections to inspection 

elements for electrochemical testing. 

Table 3-4. Summary of Laboratory Data From Caltrans and 

Comparison with Field Observations. 

Inspection Element Reinforced fill Condition CR (m/yr) 

Bridge 

No. 

Locale Location, 

Type 

w 

% 

min 

(-cm) 

Ecorr 

(mV) 

Zinc 

(oz/ft2) 

Pitting Loss LPR 

53-2819 07-LA-47 R13, Rod - 1610 -793 1.5 N NO 0.4 

53-2821 07-LA-47 L14, Rod -474 - Y 40 

53-2821 07-LA-47 L16, Rod -484 - Y 32 

53-2822 07-LA-47 L13, Rod 8.5 3580 -740 1.4 N NO 0.9 

53-2822 07-LA-47 L14, Rod -537 - Y 104 

53-2822 07-LA-47 L15, Rod 10.7 5849 -714 1.5 N NO 1.0 

53-2823 07-LA-47 L17, Rod -511 Y 42 

53-2823 07-LA-47 L15, Rod -475 Y 25 

54-1093 08-SBD-30/215 L2, Strap 2.9 6223 -540 5.5 Y 9.51 0.7 

54-1093 08-SBD-30/215 L12, Strap 2.1 12705 -594 4.7 N NO 2.0 

54-1094 08-SBD-30/215 L8, Strap - - -581 4.8 Y 6.01 1.2 

54-1094 08-SBD-30/215 L14, Strap - - -610 1.3 Y 7.51 1.3 

56-0794 08-Riv-10 L7, Strap - - -356 Y break 80 

56-0794 08-Riv-10 L 11, Strap - - -567 Y 40 50 

56-0794 08-Riv-10 L 13, Strap 0.4 377 -550 5.2 Y 281 3.7 1 pit involves a small surface area on strap 

 High moisture content, low min, and corresponding higher corrosion 

μm/25.4 = 1 mil 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 36 November 2009 
 

Reinforced fill samples were extracted near inspection element locations and Caltrans 

completed lab testing on samples of reinforced fill and inspection rods. Table 3-4 is a 

summary comparing reinforced fill and inspection rod conditions, and corrosion rates 

measured via direct observation (measured pit depth) and from linear polarization resistance 

(LPR) measurements. Rod location is identified from a cluster at the left, L, or right, R, side 

of the wall; and from near the top (#1-#6), middle (#7-#12), or bottom (#13-#18), i.e. R13 is 

from the bottom row of the right cluster. 

Corrosion rates computed from observations of remaining diameter/pit depth shown in Table 

3-4 compare qualitatively with measurements from LPR. In cases where relatively high 

corrosion rates were measured via LPR (> 20m/yr), pitting and corresponding loss of 

section were also observed along the inspection rods. The corrosion rates at the point of 

maximum section loss may be four times higher than the average rates measured via LPR, 

which is consistent with expectations considering the geometry of the rod type inspections 

elements (Smith et al., 1996). In a couple of instances, corrosion rates measured via LPR are 

higher than direct observations, however, these LPR measurements are anomalous, and when 

repeated with different equipment such high values of corrosion rate are not consistently 

observed (Fishman and Reis, 2008). Pitting observed for strap type reinforcements covered 

small areas that did not have a significant impact on tensile strength, and relatively low 

corrosion rates are indicated via LPR. 

Correlations of corrosion rate and loss of zinc are particularly interesting because the 

reinforced fill samples were retrieved from the same locations as the inspection elements. 

This is not usually the case, and most often reinforced fill data is derived from samples taken 

at stockpiles or from random locations within the reinforced fill. Higher corrosion rates and 

lower remaining zinc (< 2 oz/ft2 {0.605 g/m2}) measurements are consistently correlated with 

reinforced fill samples that simultaneously exhibit relatively low minimum resistivity (min) 

and high moisture content. This trend is illustrated in Figures 3-11 and 3-12 where higher 

corrosion rates are not always observed in Figure 3-11 when min is low; or in Figure 3-12 

when moisture contents are higher. However, a comparison of points with CR > 20 m/yr 

reveals that both of these conditions are met in these instances. This comparison 

demonstrates the value of obtaining reinforced fill samples and corrosion rate measurements 

at the same location and at similar times. The data shown in Figures 3-11 and 3-12 help to 

explain why higher corrosion rates are not always observed from sites with poor quality 

reinforced fill (e.g., low min). 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 37 November 2009
 
 

0

50

100

150

0 5000 10000 15000

min (-cm) 

CR (m/yr) 

0

50

100

150

0 5 10 15

w% 

CR (m/yr) 

Figure 3-11. Minimum Resistivity (min) vs. Figure 3-12. Moisture content (w%) vs. 

corrosion rate (CR) for corresponding corrosion rate (CR) for corresponding 

reinforced fill and inspection rod locations. reinforced fill and inspection rod locations. 

Inspection elements that exhibited high corrosion rates appeared to break at a reduced cross 

section during extraction. Therefore, a lot of the data on remaining tensile strength is not 

with respect to the locations with the most severe section loss. Tensile strength data is useful 

to document the remaining strength of less corroded sections (that did not break upon 

extraction), and to study inherent variation of material strength. 

Data from LPR measurements are useful for screening sites relative to level of corrosion. 

Figure -13 shows the distribution of corrosion rate measurements from sites in Riverside, San 

Bernardino, and Long Beach/San Pedro (coastal). These data clearly indicate that corrosion 

is more severe at the coastal sites, and at the Riverside site, compared to San Bernardino. 

Florida 

In 1995, the FLDOT implemented a corrosion-monitoring program to assess the condition of 

in-service galvanized reinforcements used in the construction of MSEW. Due to the coastal 

locations of many of the MSEWs within the FLDOT inventory, the department placed 

particular emphasis on the effects that saltwater inundation may have on the performance of 

metallic reinforcements. Due to the occurrence of episodic flooding (e.g. hurricanes), 

saltwater inundation may occur for sites located in tidal regimes or estuaries. Laboratory 

tests were conducted at the University of South Florida (Sagues, et al., 1998) to evaluate rates 

of corrosion for controlled environments and include effects of saltwater inundation and 

development of macro cells. Field studies involved NDT (LPR and Ecorr) performed on a set 

of structures incorporating a range of ages and environments, and including installation of 

coupons, and wiring of in-service or active reinforcements. Visual observations and results 

from direct physical testing of exhumed components from selected sites were also 

documented. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 38 November 2009
 
CR - ALL SAN BERNARDINO 

0

5

10

15

0.5 1 1.5 2 2.5 More

m/yr 

Frequency 

a) San Bernardino – Foothills 

CR- All Navy 

0

10

20

30

40

5 25 45 65 85 More

m/yr 

Frequency 

b) Long Beach/San Pedro Area- Coastal 

CR - Riverside 

0

5

10

15

1.04 20.78 40.53 60.28 More

(m/yr) 

Frequency 

c) Riverside- Arid 

Figure 3-13. Distribution of corrosion rates from sites in Southern California. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 39 November 2009
 
The FLDOT-specified reinforced fill for MSE construction must meet a minimum set of 

electrochemical criteria similar to that described by AASHTO. The FLDOT reinforced fill 

selection guidelines relative to soil chemistry and particle-size distribution were met in 

virtually all of the test locations and structures tested. The mean resistivity and pH of 

reinforced fill data collected from all the Florida sites is 25,000 -cm (range 11,000 to 

60,000 -cm) and 8.2, respectively. Thus, in the absence of episodic salt water flooding, 

these reinforced fills are in the range of low aggressivity relative to corrosion. Two of the 

structures monitored by FLDOT were in a marine environment where the reinforced fill was 

susceptible to inundation with seawater. At one of these locations elevated chloride levels 

(457 ppm) and correspondingly low resistivity (445 -cm) were observed. However, these 

low resistivity levels were transient due to flushing of salts from the free draining fill. 

Ten walls were selected for corrosion monitoring as described by Sagues, et al. (1998). The 

walls were constructed between 1979 and 1996. Baseline readings were taken in 1996. At 

each site, the measured half-cell potentials (CSE) of the galvanized reinforcements were 

lower (more negative) than those of the embedded carbon steel samples. On average, the 

difference was approximately 200 mV. This indicates that a significant amount of zinc 

coating was still present on the reinforcements. Tables 3-5 to 3-8 summarize statistics of the 

observed corrosion rates, which are based on LPR measurements taken between 1995 and 

1998. The data are described in terms of (a) the two sites subjected tidal fluctuations (Table 

3-5); (b) the oldest site located in Tallahassee (Table 3-6), where the possibility exists for 

zinc to be consumed along some of the galvanized reinforcements that were monitored; (c) 

the newest sites, Brickell Ave, in Miami used crushed limestone reinforced fill rendering a 

unique mineralogy compared to the other sites, high in carbonates (Table 3-7); and (d) the 

remaining five sites considered typical coastal or inland locations (Table 3-8). Each of the 10 

walls incorporates between two and four clusters of measurements. A test cluster consists of 

electrical connections to two close, but electrically independent reinforcing strips or grids, 

and a reference electrode. A buried plain steel bar is usually included in the cluster. In 

general, the test cluster is identified by the number of precast panels with respect to the 

abutment or some other specified point or reference (i.e., Station R3 is the third panel from 

abutment), and usually the elevations are within approximately five feet from the base of the 

wall to facilitate access. Data are presented in terms of , COV and range of measurements 

from each monitoring station or measurement cluster. 

For the five walls that are considered “typical,” (Table 3-8) the mean corrosion rates range 

from 0.5 to 1.15 m/yr with a COV ranging from 21% to 38%. The mean corrosion rate for 

the oldest wall (Tallahassee, Table 6 is 0.9 m/yr, which is within the range for the typical 

walls, but the COV of 82% is much higher, which may reflect the variation inherent to loss 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 40 November 2009
 
of zinc coating. After the zinc is consumed, higher corrosion rates of the underlying steel are 

anticipated, and due to variation of zinc consumption between samples, the corrosion rate 

measurements are also variable. Similar statistics are realized for the new walls (Table 3-7) 

and the walls subject to tidal fluctuations (Table 3-5), whereby the mean corrosion rates are 

slightly higher, or within the range measured for typical walls, but the COV is much higher. 

These higher COV’s may be due to the variable effects of time on corrosion, and variable 

conditions within the reinforced fill subsequent to saltwater inundation. 

Table 3-5. Florida Corrosion Rate Statistics for Sites Subjected to Tidal Fluctuations. 

(Sagues et al., 1998) 

Site Built 

(Year) Station  

(m/yr) COV Range 

(m/yr) 

St. Lucie 1992 

R3 2.0 15% 1.7-2.4 

R7 2.6 46% 1.3-4.1 

Site 2.3 38% 

Palm City- NW 1991 

R3 1.2 65% 0.25-2.4 

R7 1.3 80% 0.48-2.7 

Site 1.25 73% 

Table 3-6. Florida Corrosion Rate Statistics for 17 Year Old Tallahassee Site. 

(Sagues et al., 1998) 

Station  

(m/yr) COV Range 

(m/yr) 

R17 1.6 50% 0.9-3.0 

R23 1.3 96% 0.4-3.1 

R44 0.4 35% 0.3-0.5 

R62 0.3 29% 0.2-0.3 

Site 0.9 82% 

Table 3-7. Florida Corrosion Rate Statistics for New Brickell Ave. Site. 

(Sagues et al., 1998) 

Station  

(m/yr) COV Range 

(m/yr) 

NW-Bottom 1.0 59% 0.4-2.3 

NW-Top 0.7 1.7% 0.69-0.71 

SE-Bottom 1.8 130% 0.01-9.8 

SE-Top 0.7 67% 0.6-0.7 

Site 1.1 112% 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 41 November 2009
 
(Sagues et al., 1998) 

Table 3-8. Florida Corrosion Rate Statistics for Typical Coastal and Land Sites. 

Site Built 

(Year) Station  

(m/yr) COV Range 

(m/yr) 

Howard Franklin 1992 

R7 0.7 33% 0.5-1.0 

R11 0.6 20% 0.4-0.7 

R15 0.4 37% 0.2-0.5 

R17 0.5 39% 0.1-0.7 

Site 0.5 36% 

Ocala 1983 

R6 0.3 20% 0.2-0.4 

R5 0.7 20% 0.6-0.8 

Site 0.5 21% 

Jacksonville 1990 

R9 0.5 40% 0.3-0.7 

R21 0.7 18% 0.6-0.9 

Site 0.6 28% 

Tampa 1997 

R16 1.2 20% 1.0-1.5 

R23 1.1 23% 0.8-1.3 

Site 1.15 21% 

Palm City-NE 1991 

R1 0.5 29% 0.3-0.8 

R5 0.7 29% 0.5-1.0 

R14 0.6 32% 0.4-0.8 

R28 0.6 54% 0.4-1.1 

Site 0.6 38% 

New York 

In 1998, The New York State Department of Transportation established corrosion monitoring 

stations at two walls in Amherst, NY as part of its MSE wall corrosion assessment program. 

The walls included embankment walls for the Lockport Expressway (I-990) over Sweet 

Home Road and I-990 over Dodge Road. These MSE walls were constructed in 1980 and 

1981 and were filled with a mixture of lightweight fill containing blast furnace slag and 

cinder ash. Due to the nature of the reinforced fill and the fact that other MSE walls 

backfilled with a lightweight industrial waste product in the area had to be replaced due to 

severely corroded metal reinforcing straps, the occurrence of accelerated corrosion was 

suspected at these sites. Results from chemical testing indicated that the reinforced fill 

provided an aggressive corrosive environment. Table 3-9 presents a summary of the 

chemical properties of the slag/cinder ash reinforced fill. All of the reinforced fill parameters 

described in Table 3-9 are outside the limits recommended by AASHTO. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 42 November 2009
 
Table 3-9. Electrochemical Properties of Slag/Cinder Ash Reinforced Fill at I990 

over Dodge and Sweethome Road Sites in Buffalo, NY 

Test Results AASHTO 

Specified Limits 

Resistivity 426 to 963 -cm > 3000 -cm 

pH 10.9 to 11.4 5 to 10 

Chlorides 6 to 499 ppm with 4 of 8 

samples > 100ppm < 100 ppm 

Sulfates 523 to 742 ppm < 200 ppm 

Each site includes four monitoring stations that are at least 100 ft (30 m) apart. Each station 

includes measurements from five in service reinforcements that are within 2 to 3 feet (0.6 to 

1 m) of each other, and from steel, zinc and galvanized coupons. NYSDOT collected 

corrosion monitoring data from these sites subsequent to installation of corrosion monitoring 

stations in 1998 (Wheeler, 2002); and then in May 2002 and November 2005. Corrosion 

monitoring includes measurements of half-cell potential and corrosion rates from LPR. 

Observations from coupons and reinforcements support the following conclusions: 

1. The corrosion rates of the zinc coupons are significantly higher than those of the steel and 

galvanized coupons and of the in-service galvanized reinforcements. This may be due to 

the fact that the reinforced fill at this site includes slag and cinder ash that is relatively 

alkaline (pH > 11), and the corrosion rate of zinc is adversely affected by high alkalinity. 

2. In general, the observed temporal variations of corrosion rates are negatively correlated 

to the temporal variation in measured soil resistance. 

3. In general, the half-cell potentials of the steel and galvanized coupons and the in-service 

reinforcements are in the same range of -400 mV to –500 mV, but the zinc coupons 

exhibit a distinctly different range of –800 mV to –900 mV. This indicates that the zinc 

is nearly consumed on the galvanized coupons and in-service reinforcements. 

4. In general, corrosion rates observed for galvanized coupons and in-service 

reinforcements are similar, and less than the corrosion rates for the plain steel coupons. 

As part of NCHRP 24-28 readings were taken at approximately monthly intervals between 

June 2007 and December 2008 in an effort to document the seasonal variations of corrosion 

rate. Results from these monthly measurements are summarized in Figure 3-14 including a 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 43 November 2009
 
comparison between corrosion rates and monthly precipitation; and measurements of soil 

resistance. The data in Figure 3-14 represent the means of the measured corrosion rates from 

reinforcements at the respective sites. These means are associated with coefficients of 

variance between 10 and 40 percent. These data indicate that corrosion rates appear to 

correlate well with monthly precipitation and corrosion rate measurements at this site vary by 

a factor of 1.5 considering seasonal fluctuations. 

North Carolina 

In 1990, the North Carolina DOT (NCDOT) implemented a corrosion-monitoring program 

for MSE walls, and proceeded to monitor half-cell potentials of reinforcements and coupons 

at regular intervals (Medford, 1999). Corrosion monitoring stations were established during 

wall construction including zinc and steel coupons, and in service reinforcements were wired 

for monitoring. Initial half-cell potential measurements were taken immediately after wall 

construction, with subsequent readings taken approximately once a year thereafter. 

Figure 3-15 depicts the distribution of measurements obtained from 48 monitoring stations 

distributed among 18 project locations and 12 counties within North Carolina. All of the 

MSE walls in Figure 3-15 incorporate grid-type reinforcement, galvanized in accordance 

with ASTM A123 (ASTM, 2002), corresponding to at least 2 oz/ft2

 (0.605 g/m2) of zinc. 

Walls are backfilled with material meeting the requirements of North Carolina designation 

No. 57 stone, with resistivity greater than 5000 -cm, pH between 4.5 and 9.0, and less than 

1% organics; i.e. these reinforced fills are noncorrosive. Walls included in Figure 3-15 were 

constructed between 1992 and 1999, and records of half-cell measurements are available 

from the date of wall construction. 

The data in Figure 3-15 are useful to evaluate how the potential of the galvanized 

reinforcements approach those of steel coupons; presumably as the sacrificial zinc coating is 

consumed by corrosion. These data present a real-world example of the concepts described 

in Section 3.2 and Figure 3-2. Initially, the half-cell potentials of the reinforcements and zinc 

coupons are relatively close. After ten years the half-cell potentials of the galvanized 

reinforcements have shifted slightly towards those of the steel coupons and away from those 

of the zinc coupons. However, in general, they are still closer to zinc than steel. This 

supports the conclusion that much of the zinc coating remains after ten years, which is to be 

expected given the noncorrosive nature of the reinforced fills included in the study. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 44 November 2009
 
Rs () r (

um/yr) or Precip. (in/month) 7.00

6.00

5.00

4.00

3.00

2.00

1.00

SHR 

DR 

PRECIP. 

Jul-07 Aug-07 Oct-07 Dec-07 Feb-08 Apr-08 Jun-08 Aug-08 Oct-08 Dec-08 Feb-09

Date 

a) Mean Monthly Corrosion Rates Compared to Monthly Precipitation 

160

140

120

100

80

60

40

20

0

SHR 

DR 

Jul-07 Aug-07 Oct-07 Dec-07 Feb-08 Apr-08 Jun-08 Aug-08 Oct-08 Dec-08 Feb-09

Date 

b) Mean Monthly Soil Resistance 

Figure 3-14. Transient response of reinforcements at Dodge and Sweet Home Road sites. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 45 November 2009
 
NC Sites - t= 0 years 

100

80

60

EZn E EFe

Probability of

Nonexceedance

40

20

0

-1500 -1200 -900 -600 -300 0

Ecorr (mV) 

a) initial measurements (taken within several months after construction) 

NC Sites t = 10 years 

100

EZn E EFe

Probability of

Nonexceedance

80

60

40

20

0

-1500 -1200 -900 -600 -300 0

Ecorr (mV) 

b) measurements taken 10 years after construction 

Figure 3-15. Half-cell potentials from coupons and in service reinforcements from 

corrosion monitoring of MSE walls constructed in North Carolina. 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 46 November 2009
 
We can obtain a another “picture” of the performance of the galvanized reinforcements if we 

define the relative change of half-cell potential in terms of the zinc index (ZI), which is 

computed at a given time (age of reinforcement) as: 

E E  EFe ZI  	 (3-3) E E Zn Zinc	  EFe 

where E and EZn are the differences, compared to the half cell potential of the steel coupon 

(EFe), for the reinforcement (E) and zinc coupon (EZn), respectively, at a given time. The 

value of the zinc index ranges from one to zero. A value of one corresponds to 100% zinc 

remaining, and a value of zero corresponds to 100% zinc consumption. Figure 3-16 shows 

the evolution of ZI as a function of time for the North Carolina data. 

Figure 3-16 graphically illustrates that the rate of zinc consumption (decreasing zinc index) is 

greatest during the first two years; i.e. the zinc index decreases more in the first two years 

compared to the five-year time interval between years five and ten. 

NC Sites 

100
 

80
 

60
 

40
 

20
 

0

10 years 

0

25 

time 

0 0.2 0.4 0.6 0.8 1

Zinc Index 

Figure 3-16. Zinc Index (ZI) distributions at increasing reinforcement age from corrosion 

monitoring of MSE walls constructed in North Carolina. Probability of Nonexceedance

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 47 November 2009
 
 

[ BLANK ] 

FHWA NHI-09-087 3 – Metallic Monitoring Methods 

Corrosion/Degradation 3 – 48 November 2009
 
 

 

 

CHAPTER 4 

DURABILITY OF GEOSYNTHETIC REINFORCEMENTS
 

4.1 INTRODUCTION 

Due to their economic advantages and relatively inert state, the use of polymeric 

reinforcements in MSE walls and RSS embankments has significantly increased over the past 

three decades. Because of their relatively short period in use, there are some uncertainties as 

to their durability with respect to maintaining tensile strength properties after exposure to 

construction stresses and during exposure to an in-soil environment over the anticipated 

design life. Potential degradation of polymeric reinforcements with time (aging) will depend 

on the specific polymer, configuration of the reinforcements, the environment to which they 

are exposed, and the level of stress to which they are subjected. 

The current design approach to account for construction damage and long-term degradation 

strength losses is to apply to the ultimate tensile strength (Tult) reduction factors. The 

nominal strength, Tal is then obtained from: 

Tult Tult Tal   (4-1) RFCR xRFID x RFD RF 

where: 

RFCR = Creep reduction factor 

RFID = Installation damage reduction factor 

RFD = Durability reduction factor 

RF = The product of all applicable reduction factors, dimensionless 

Tult = Ultimate geosynthetic tensile strength, lb/ft (kN/m) 

This chapter is intended to provide a background on geosynthetic materials for reinforcement 

applications. It addresses their structure and manufacturing process as it may affect 

durability, identify degradation mechanisms and environments, and provide the basis for 

selecting appropriate polymers. It also provides the basis for selecting aging (RFD) and 

installation damage (RFID) reduction factors, consistent with the in-ground regime and the 

corresponding nominal strength used for design, in lieu of using an overall default reduction 

factor. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 1 November 2009
 
 

a. Overview of Available Products 

The main polymers currently used for MSEW and RSS structures include the polyolyfins 

(i.e., polypropylene and polyethylene) and polyester. The final form of the polymer and its 

corresponding reaction to its environment may vary considerably, depending on the polymer 

formulation, additives used in the composition, and the methods of processing the polymer 

into its final form (fibers, filaments, and subsequent fabric for geotextiles or joined drawn 

strands in the case of geogrids). The method of manufacture for geosynthetics may be a 

factor for short-term construction durability. 

Based on 2003 market information (Koerner, 2005), the total worldwide geosynthetics 

market was over 1500 million square yards. Estimates by the authors indicating the U.S. 

market exceeds 600 million square yards for geotextiles and geogrids alone. Elias (2001) 

found that the U.S. market was divided on a raw polymer material basis as shown in Table 4­

1. The U.S. market as further divided on a geosynthetic structure basis is shown in Table 4­

2. 

The U.S. geosynthetic market based on an application basis, is shown in Table 4-3. 

Geogrids, woven geotextile, and nonwoven geotextiles, are used extensively for steepened 

slopes, but most retaining wall applications use geogrid reinforcement. 

Table 4-1. Raw Material in Geotextile and Geogrid Production. (Elias, 2001) 

Raw Material % of Market 

Polypropylene 80 

Polyester 14 

Polyethylene 6 

Other < 1 

Table 4-2. Geotextile and Geogrid Structure. (Elias, 2001) 

Structure % of Market 

Nonwoven Geotextile 77 

Woven Geotextile 16 

Geogrid 6 

Other 1 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 2 November 2009
 
 

 

Table 4-3. Geosynthetic Applications. (Elias, 2001) 

Application % of Market 

Asphalt Overlay 22 

Separation/Stabilization 28 

Reinforcement for Subgrade, Wall & Slope 9 

Drainage 12 

Lining Systems 17 

Erosion Control 6 

Silt Fence 6 

For polyolefins, polypropylenes (PP) are manufactured from general purpose homopolymers 

(group 1, class 1, ASTM D4101), which may differ in grade (Melt Flow) slightly, based on 

the manufacturing process used. Grade 4 appears to be universally used. 

The major suppliers of PP reinforcement geosynthetics and their properties as furnished by 

the manufacturers and listed in their catalogs are listed in Table 4-4. 

The manufacturing of polypropylene geosynthetics requires that process stabilizers (e.g. 

antioxidants) and ultra violet (UV) inhibitors be used to maintain the required end-properties 

of the polymeric materials. The type and quantity of additives used is considered proprietary 

by manufacturers and can be expected to vary slightly between production runs. 

A variation in additive concentration may result from the amount of "regrind" material used. 

Regrind generally results from material which has been manufactured but does not meet the 

quality assurance standards set for that particular product. For example, the leading portion 

of a nonwoven geotextile in a production run may have inconsistent density, or a density too 

low for the product style being manufactured. This is not an unusual occurrence at the 

beginning of a nonwoven geotextile production run. Because there is nothing inherently 

wrong with the resin used in the material, it may be reground, melted, and added to the resin 

used for other production runs. Most manufacturers add resin obtained from polypropylene 

regrind to "virgin" batches of resin which do not yet contain stabilization additives. The 

entire batch is then treated as if it was comprised entirely of virgin resin and the 

corresponding amount of additives is then introduced. Therefore, the amount of stabilization 

additive contained in a batch of resin which includes previously used resin will have a higher 

concentration of additives than batches which are truly virgin. This procedure is employed to 

ensure that a minimum standard of geosynthetic stability is consistently adhered to. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 3 November 2009
 
 

 

Only one high density polyethylene (HDPE) product, a geogrid, is currently being 

manufactured and distributed in the U.S. and it is widely used in reinforcement applications. 

Its composition is shown in Table 4-5. 

The major product groups and the properties of polyester products (PET) that are produced 

for geogrids and geotextiles are listed in Table 4-6. PET geogrids are coated with a 

polyvinyl chloride (PVC) or an acrylic polymer to provide some protection from construction 

induced damage and to ensure dimensional stability during manufacturing. 

From Table 4-6, in which all products in this group are listed, high tenacity polyester fibers 

used in grids and high-strength woven geotextiles are characterized by molecular weights 

(Mn) in the 25,000-30,000 range and Carboxyl End Group (CEG) numbers of 12 to 29. The 

non-woven geotextiles are typically manufactured using PET with a somewhat lower 

molecular weight and substantially greater CEG number. No additives are used other than a 

surface finish on the high tenacity fibers. It should be noted that high tenacity fibers 

classified by the producers and/or manufacturers as "first quality" only, will have carefully 

controlled physical and chemical properties. Manufactured woven products may be 

produced from “industrial grade” PET fibers, which are composed of material not meeting 

the requirements for the end use for which they have been formulated and manufactured. 

Accordingly, physical and chemical properties may vary widely from production run to 

production run. 

b. Materials Structure and Manufacture 

The engineering properties of a geosynthetic, i.e., its functionality in terms of tensile 

strength, water permeability, and filtration efficiency are significantly influenced by their 

composition and structure. The most commonly used geosynthetics for transportation 

applications are: 

C Woven geotextiles: C Geogrids: 

o monofilaments o extruded uniaxial 

o multifilaments o extruded biaxial 

o slit tape o woven coated fiber 

o combinations o welded 

C Nonwoven geotextiles: C Geocomposites 

o needlepunched o geogrid/nonwoven geotextiles 

o heat bonded o inline filament knitted to 

o resin bonded nonwoven geotextiles 

o combinations o geonet with nonwoven geotextile 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 4 November 2009
 
Table 4-4. Major PP Product Groups. (2000) 

Product Name Structure Manufacturing Process 

Polymer 

Class & Grade 

ASTM D4101 

Melt Flow 

g/10 min. 

ASTM 

D1238 

Antioxidants Process/ 

End Use Additives 

1. Tensar 

 "BX" Grid Extruded Sheet, punched & stretched Grade 2 Group-1, Class-1 0.3 -1.0 Irganox 1010 Carbon Black 

2. Tenax 

 "MS" Grid Extruded & stretched Multi layer Grade 2 Group-1 Class-1 0.3 - 1.0 Carbon Black 

3. a) TenCate Mirafi 

Construction Products 

Woven Fibrillated yarns woven 

Grade 4 

Group-1 

 Class-1 

4 1% Carbon Black 

3. b) TenCate Mirafi 

Construction Products Woven Monofilament yarns Grade 4 Group-1 

Class-1 

3 

2% Total of 

36% Carbon Black 

63% LLDPE 

4. a) LINQ 

 "Typar” Non Woven Thermally spun bonded 

e 

Grade 4 

Group-1, Class-1 4.5 

HALS 

Carbon Black 

4. b) LINQ "GTF" Woven Extruded tap Grade 4 Group 1, Class 1 8 N/A 

5. a) Propex Fabrics Inc. Woven Monofilament Group 1, Class 1 0.1-12 Carbon Black 

5. b) Propex Fabrics Inc. Woven Slit film tape fibrillated tape Grade 4 

Group 1, Class 1 

N/A N/A 

5. c) Propex Fabrics Inc. Non Woven Needle punched, staple fiber N/A N/A 

7. Tensar TG Non Woven Needle punched continuous filament Grade 6 Group 1, Class 1 < 22 HALS 

N/A - Not Available 

FHWA NHI-09-087 4 – Durability of Geosynthetics Corrosion/Degradation 4 – 5 November 2009 
Table 4-6. Major PET Product Groups. (2000) 

Table 4-5. HDPE Product Group. (2000) 

Product Name Structure Manufacturing Process Polymer Class & Grade ASTM D1248 Melt Flow ASTM D1238 (g/10 min) Antioxidants Process/End Use Additives Weigh

(g/m

Tensar "UX" Grid Extruded Sheet Punched & Stretched Type III, Class A, Grade E-5, Category 5 < 0.4 Irganox 1010 Carbon Black 360-

Product Name Structure 

Geogrid 

Coating 

(Grid Only) 

Fiber or Polymer 

Source, Type 

Molecular Wt. 

g/mol 

 or Viscosity 

CEG 

meq/kg. Additives 

We

Ra

g/

1. STRATA 

Systems, Inc.

 "Stratagrid" 

Grid 

knitted PVC dipped N/A 27,000 24 

none for PET 

PVC- fungicide, 

UV inhibitor 

200 

2. Reco 

"Matrex" 

Grid PVC dipped T-811 Hoechst/Celanese I.V. = 0.89 17 none 330 - 

3. TenCate Mirafi 

Construction Products "Miragrid" 

Grid PVC Coating T-811 Hoechst/Celanese 30,000 (I.V. = 0.91) 26 – 29 none 200 - 

4. Huesker "FORTRAC" Grid PVC dipped & cured AKZO 30,000 (I.V. = 0.91) 20 none 170 - 5. TenCate Mirafi 

Construction Products HS Series 

Woven 

filament 

yarn 

Hoechst/Celanese T-800, 100, 236 

I.V. = 0.88 

I.V. = 0.905 I.V. = 0.910 

17 28 12 

none 290 - 

6. LINQ 

"GTF" 

Woven 

filament 

yarn 

T-800 Hoechst/Celanese I.V. = 0.88 17 none N

7. Huesker 

"COMTRAC" 

Woven 

filament 

yarn 

AKZO Allied Fibers I.V. = 0.91 17 none 250 - 

N/A - Not available 

FHWA NHI-09-087 4 – Durability of Geosynthetics Corrosion/Degradation 4 – 6 November 2009 
 

In manufacturing geotextiles, elements such as fibers or yarns are combined into planar 

textile structures. The fibers can be continuous filaments, which are very long thin strands of 

a polymer, or staple fibers, which are short filaments, typically 3⁄4 to 2 in. (20 to 50 mm) long. 

The fibers may also be produced by slitting an extruded plastic sheet or film to form thin flat 

tapes. In both filaments and slit films, the extrusion or drawing process elongates the 

polymers in the direction of the draw and increases the filament strength. After the drawing 

process, filaments and tapes may also be fibrillated, a process in which the filaments are split 

into finer filaments by crimping, twisting, cutting or nipped with a pinned roller. This 

process provides pliable, multifilament yarns with a more open structure that are easier to 

weave. 

Geotextile type is determined by the method used to combine the filaments or tapes into the 

planar structure. The vast majority of geotextiles are either woven or nonwoven. Woven 

geotextiles are made of monofilament, multifilament or fibrillated yarns, or of slit films and 

tapes. The weaving process derives from textile cloth-making. Nonwoven textile 

manufacture is a process by which synthetic polymer fibers or filaments are laid onto a 

moving belt. Then the mass of filaments or fibers are either needlepunched, in which the 

filaments are mechanically entangled by a series of small needles, or heat bonded, in which 

the fibers are welded together by heat and/or pressure at their points of contact in the 

nonwoven mass. 

Geogrids with integral junctions are manufactured by extruding and drawing polyolefin 

(polyethylene or polypropylene) sheets. These types of geogrids are often called extruded or 

integral geogrids. Geogrids may also be manufactured of multifilament polyester yarns, 

joined at the crossover points by a knitting or weaving process, and then encased with a 

polymer-based coating. These types of geogrids are often called woven or flexible geogrids. 

A third type, a welded geogrid is manufactured by welding polymeric strips (e.g., strapping 

material) together at their cross over points. All these manufacturing techniques allow 

geogrids to be oriented such that the principal strength is in one direction, called uniaxial 

geogrids, or in both directions (but not necessarily the same strength), called biaxial 

geogrids. 

4.2 FUNDAMENTALS OF POLYMER DEGRADATION 

Polymers principally consist of long chains of carbon atoms with various branches and side 

groups. Under certain conditions, this structure can be attacked by oxidation promoted 

thermally, catalytically, or by ultraviolet light; by other forms of chemical attack including 

hydrolysis; by the combined effect of chemicals and mechanical load; or, by microorganisms. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 7 November 2009
 
 

Most polymers used in geosynthetics contain additives and stabilizers that improve the 

resistance of the basic polymer; however, these additives themselves can be susceptible to 

leaching or to biological attack, ultimately leaving the polymer unprotected. In addition, the 

structure can be damaged during compaction or by subsequent abrasion. The principal 

results of these degradative mechanisms are loss of mechanical strength and changes in 

elongation properties. This section outlines the potential degradation mechanisms and the 

available testing methods to quantify tensile strength losses and identifies the role of 

additives/antioxidants in enhancing long-term, in-ground durability. 

a. Oxidation of Polyolefins (PP and HDPE) 

The predominant degradation mechanism for most polymeric materials is chain scission, 

which is a polymeric reaction that breaks a bond on the backbone of a polymer chain, 

reducing the chain length and thereby reducing molecular weight (Koerner et al., 1986). This 

in turn significantly changes the polymeric structure and material properties, chiefly strength 

and elongation. The oxidation process is initiated by heat, light (UV radiation), mechanical 

stress, catalyst residue from manufacturing remaining in the geosynthetic, or reaction with 

impurities. 

Antioxidants are additives that interrupt the degradation process in different ways, depending 

on their structure. The two major classifications are: 

C chain terminating primary antioxidants and 

C hydroperoxide decomposing secondary antioxidants. 

Primary antioxidants are often sterically hindered phenols. They react rapidly to terminate 

chain scission and protect the polymer chain. Secondary antioxidants are most effective at 

elevated temperatures, as during manufacture processing, and effectively protect both the 

polymer and the primary antioxidant. They would include but not be limited to phosphite/ 

phosphonite compounds. A new class of UV stabilizers, sterically hindered amines (HALS), 

is very effective in imparting stability at the lower temperatures consistent with in-ground 

use. Often, the protection obtained against oxidation by using a mixture of primary and 

secondary antioxidants in certain proportions is stronger than the sum of the protection 

effects obtained with individual compounds used separately. These synergistic mixtures are 

known as "master batch" and are proprietary to each producer. They can be varied to satisfy 

the intended usage and use regime. 

For long-term protection against oxidation induced strength losses, the geosynthetic 

should be produced with primary antioxidants that are not consumed during the 

manufacturing process. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 8 November 2009
 
 

b. Hydrolysis of Polyester (PET) 

Hydrolysis is the reverse reaction of the mixing (synthesis) of terephthalic acid and ethylene 

glycol, which forms polyethylene terephthalate (PET) and water. Since this is an equilibrium 

reaction, it is reversible. Therefore, it is possible for the PET to react with water and to 

revert back to acid and glycol, which is a non-reversible process. In neutral environments 

(pH = 7), the reaction is initiated by the carboxyl end group (CEG) of the macro molecule of 

PET and is relatively slow. In alkaline environments, the reaction is more rapid due to the 

presence of the more reactive OH- ion compared to the water molecules as reagents in neutral 

(pH = 7) reactions. The effect of these reactions is a decrease in the molecular weight 

(Mn) with a corresponding decrease in strength. 

The rate of hydrolysis is primarily affected by: 

C Carboxyl End Group (CEG) Concentration– These end groups are situated at the end of 

the molecular chains. The amount of carboxyl end groups in a particular PET product is 

dependent on the polymerization process used. Typically, the high tenacity fiber 

produced for geogrid and high strength woven products have lower CEG numbers 

compared to fiber produced for nonwoven geotextiles. Research has indicated that the 

hydrolysis rate of PET with higher CEG numbers proceeds faster under equivalent 

conditions. (Koerner et al., 1986) 

C Molecular Weight – Molecular weight directly affects the CEG concentration under the 

same polymerization conditions. Therefore, PET polymers with a higher molecular 

weight contain less CEG than those with lower molecular weight and are less susceptible 

to hydrolysis under equivalent conditions. 

C Temperature – As with oxidation, hydrolysis proceeds at a faster rate with increasing 

temperature. 

C pH Level – High levels of environmental alkalinity will cause fiber dissolution in 

addition to hydrolytic reactions. 

C Relative Humidity – The rate of hydrolysis increases as relative humidity increases. 

For long term usage, PET products of high molecular weight (Mn) and low CEG will be 

least susceptible to strength losses due to hydrolysis. PET should not be used in highly 

alkaline environments characterized by pH greater than 9 without significant test data to 

document suitability. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 9 November 2009
 
 

c. Stress Cracking 

Semi-crystalline polymers such as high density polyethylene (HDPE) have a potential for 

stress cracking, which is a material failure caused by tensile stresses less than the short term 

mechanical strength. The failure is characteristically brittle, with no elongation adjacent to 

the failure. This phenomenon has two phases: crack initiation and crack growth. 

Environmental stress cracking (ESC) is the rupture of a polymer in a stressed state when 

exposed to a chemical environment. ESC tests are, therefore, differentiated from chemical 

resistance tests, in general, by the fact that the test specimens are exposed to a chemical 

environment while under stress. 

Experience in the plastic pipe industry has shown that certain grades of polyethylene (PE) 

can experience stress cracking under certain conditions, and recent data has suggested a 

potentially similar behavior for some grades of PE used in geomembranes. It follows, 

therefore, that a possibility of stress cracking in geogrids fabricated of PE exists. Under 

"low" stresses at ambient temperatures, PE could fracture by slow crack growth given 

sufficient time. This mode of failure may limit the lifetime of and/or stress levels on PE used 

for critical load-bearing applications such as pressurized pipes, landfill linings, and 

reinforcement applications. 

The stress-crack resistance of HDPE geogrids has been studied in FHWA RD-97-142 and a 

preliminary testing protocol was developed. For the presently available geogrid it was 

concluded that stress cracking is a less stringent than or equal consideration to creep for 

intact geogrids. For severely damaged geogrids, stress cracking may occur at load levels 

lower than inferred by the material creep limit. (Elias et al., 1997) 

The maximum stress level under either mode of failure determines a nominal stress basis. 

Therefore, if the backfill maximum size is limited to 3⁄4-in. (20 mm), ensuring a low level of 

construction damage, no consideration for stress cracking appears warranted for the geogrid 

detailed in Table 4-5. 

d. UV Degradation 

UV degradation occurs when geosynthetics are exposed to the influence of sunlight, rain, 

temperature, and oxygen. This type of degradation is caused primarily by the UV content of 

sunlight, which initiates the photo-oxidation process. The rate of degradation depends on the 

intensity of the relevant wave length and such additional factors as temperature, the presence 

of water and of certain atmospheric components such as ozone, nitrous oxides, hydrocarbons, 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 10 November 2009
 
 

 

etc. Other factors are the material structure and the rate at which degraded layers are 

removed by rain and wind and new surfaces are exposed to UV radiation. 

For polyolefin geosynthetics, significant resistance to UV radiation is obtained with the 

addition of antioxidants such as phenolics, hindered amine light stabilizers (HALS) and 

carbon black. These act as a screen to harmful portions of the light spectra. Once the 

geosynthetic is buried, the UV light-induced degradative process ceases because exposure to 

the UV source is terminated. Polyester is less affected by UV radiation because of the 

resistance of ester bonds to breakage. 

Recent research has shown that the outdoor degradation process is a synergistic one in which 

both photo-oxidation caused by UV radiation and oxidation caused by elevated temperatures 

have an effect on the rate of degradation (Husan et al., 1994). The data has indicated that 

the major effect is photo-oxidation, and, therefore, consideration of annual average energy 

incidence alone at a site may be sufficient to evaluate the effects of UV exposure. 

The resistance to UV degradation is measured in the laboratory by exposure to xenon arc 

light for a duration of up to 500 hours in accordance with ASTM D4355, or outdoors by 

direct exposure to sunlight and weather conditions under ASTM D5970. 

e. Biological Degradation 

Microorganisms causing deterioration are found in a wide range of environmental conditions. 

These microorganisms require a source of carbon for growth and obtain it from reactions 

degrading organic-based materials such as some of the polymers and additives potentially 

used in geosynthetics. Environmental factors controlling biodeterioration are temperature, 

humidity, pH, etc. Microorganisms of importance in biodeterioration are bacteria, fungi, 

actinomycetes, algae, and yeast. In general, elevated temperatures, high humidity, and the 

absence of UV light are required conditions. (Bright, 1993) 

To grow, microorganisms excrete enzymes into the surrounding medium. The enzymes 

degrade the host material by breaking down its large molecular units into much smaller units 

that serve as food for the microorganism. The net effect is a reduction in molecular weight, 

with ensuing deterioration of physical properties such as weight, strength and elongation. 

High-molecular-weight, high-density polymers used for geosynthetics, do not appear to be 

susceptible to direct enzymatic degradation by microorganisms such as fungi and bacteria. 

Several biodegradability studies have shown little loss in strength of any typical polymers 

used in geosynthetics when exposed to biologically active environments (e.g., mildew) for 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 11 November 2009
 
 

 

periods of 1 year or more. There is some indication that very low molecular weight polymers 

can be consumed especially in the presence of nutrient fillers such as starch. (Potts, 1973) 

No completely relevant test to measure the resistance of geosynthetics to biological effects 

in unstressed states is presently available. ASTM D3083 has been used and can be 

adopted on an interim basis. Statistically, significant strength losses measured from this test 

should disqualify a candidate geosynthetic for long-term in-ground applications. 

f. General Chemical Dissolution 

Exposure of polymeric materials to extremely aggressive chemicals may accelerate the 

oxidation/hydrolysis processes in conjunction with a process of dissolution which is a 

separation into component molecules by solution. Such regimes are not likely to be found 

naturally in soils, but may be encountered in hazardous waste sites. 

With regard to specific chemicals that may affect polymers, numerous chemical 

compatibility tables have been published by geosynthetic manufacturers such as the Plastic 

Pipe and Wire Insulation Institute and others in the polymer industry. There are several 

considerations if using such tables. Test conditions, including the exposure time (always 

short, less than 1 year), temperature, chemical concentration (usually very high), and strength 

evaluation methods, vary between the tables. For any specific polymer, the plastic 

formulations may vary considerably, especially between industries. Also, the form of the 

material evaluated (e.g., strap, fiber, block) and the material additives will have an effect. 

Therefore, these tables of compatibility are only useful in identifying specific regimes that 

are aggressive and therefore incompatible with specific geosynthetics. 

The resistance of geosynthetics to chemical effects in unstressed states can be measured in 

accordance with: 

C ASTM D5322 "Practice for Immersion Procedures for Evaluating the Chemical 

Resistance of Geosynthetics to Liquids". This is a relatively short-term test (120 days) 

that should be modified for longer durations. A minimum of 9 months is recommended. 

The selection of immersion liquids is not specified. 

C EPA 9090, "Compatibility Tests for Wastes and Membrane Liners" is a similar test at 

higher than ambient temperatures (122 F {50 C}), conducted with specific chemicals 

considered present at the investigated site. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 12 November 2009
 
 

Statistically significant strength losses measured from these short-term tests should disqualify 

a candidate geosynthetic for long-term, in-ground applications where the chemical condition 

is anticipated. Neither test, however, provides a sound basis for determining a reduction 

factor for strength. 

g. Summary 

The principal mechanisms of polymeric degradation have been outlined, all of which 

decrease tensile strength and change elongation characteristics. Geosynthetics are seldom 

degraded by a single environmental condition or mechanism, but often by a combination of 

synergistic actions or events. A list of commonly identifiable degradation mechanisms, their 

source, effect and test procedures to identify and quantify (by short-term laboratory tests) 

their consequence on long-term strength is presented in Table 4-7. 

A general approach to quantifying geosynthetic durability and making lifetime predictions 

requires that the following objectives be achieved: 

C Identify the nature of potential degradation mechanisms within a particular site and 

functional use, by examining the mechanisms and sources listed on Table 4-7. 

C Identify the nature of the physical and chemical effects that these mechanisms have on 

candidate geosynthetics and their properties. 

C Identify the type of test data necessary. 

C Evaluate the degradation from available test data. 

This process is illustrated in an example of a permanent geosynthetic-reinforced retaining 

wall. The wall is to be built in Arizona along a stream bank with a wrapped facing using 

local gravel as wall fill. The soil is determined to have a pH value of 8.5. Based on this 

information, the probable aging mechanisms can be identified as oxidation, hydrolysis, 

stress-cracking, UV degradation, and installation damage. Therefore, the design engineer 

should require the following for the geosynthetics being considered for use: 

1. Polypropylene and polyethylene materials contain an antioxidant package to inhibit 

oxidation. 

2. Polyester materials have suitable high molecular weight and low CEG numbers to inhibit 

hydrolysis. 

3. Polyethylene materials manufactured from stress-crack resistant grade polymer. 

4. The material is UV stabilized and is handled in a manner which minimizes exposure to 

sunlight on the project site. Further, a UV-resistant coating, such as bitumen or shotcrete, 

should be applied to the wall face or a wood or concrete panel facing can be constructed. 

5. Installation damage testing be available for each candidate material consistent with the 

available gravel fill. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 13 November 2009
 
 

Table 4-7. Commonly Identifiable Degradation Mechanisms. (Horrocks, 1990) 

Mechanism Source Effect Variables Test Procedures 

PHYSICAL 

Stress/ 

pressure 

Installation/in use Rupture, creep, 

stress cracking 

Stress level, 

Backfill grain size 

ASTM D5262 & 

ASTM D6992 

Stress Rupture Tests 

ASTM D5818 

Installation Damage 

Water Installation/in use Leaching of 

additives and 

plasticisers, 

hydrolysis 

Temperature, 

pH 

ASTM D5496 

Immersion Testing 

Solvents/ 

hydrocarbons 

Installation: 

diesel 

 mineral oils 

 hot bitumen 

In use: bitumen 

Leaching of 

additives, swelling 

and embrittlement, 

plasticization 

Temperature, 

chemical 

concentration 

ASTM D5322, EPA 

9090 and Leaching 

tests. 

Biological Installation/in use: 

birds, animals, 

insects 

Localized damage Soil type and 

density 

Not Available 

CHEMICAL 

Heat 

(+ oxygen) 

Installation: 

 hot bitumen 

In use: 

 ambient 

 environment 

 temperature 

Chain scission and 

oxidation; Loss in 

tensile properties 

Temperature 

Oxygen Content 

Transition metals 

Oven Aging Tests at 

multiple 

temperatures 

Light 

(+ oxygen) 

Installation: 

UV exposure 

Chain scission and 

oxidation; Loss in 

tensile properties 

Radiation 

Intensity, 

Temperature, 

Humidity 

ASTM D4335 

Water (pH) In-use: hydrolysis 

 in acid, neutral 

 and alkaline soils 

Chain scission; 

Loss in tensile 

properties 

Temperature, pH 

concentration, 

acid and alkali 

exposure 

Hydrolysis testing at 

multiple 

temperatures 

General 

chemicals 

In use: exposure to 

natural soils and 

waste deposits 

Degradation of 

polymer structure 

via oxidative/ 

hydrolytic chain 

scission 

Temperature, 

Concentration 

Immersion tests 

EPA 9090, 

ASTM D5322 

Microorganisms In use: bacterial 

and fungal attack 

in soils 

Polymer chain 

degradation; Loss 

in tensile properties 

Temperature, pH 

Soil type, 

Organism type 

ASTM D3083 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 14 November 2009
 
 

4.3 IDENTIFICATION OF SOIL ENVIRONMENTS WHICH ACCELERATE 

DEGRADATION 

Soil environments that could accelerate degradation can be identified by their geological 

origins and composition. The physical regime (temperature and groundwater) can accelerate 

the degradation rate. Since not all polymers are subject to accelerated degradation in the 

same environments, it follows that an appropriate geosynthetic (polymer type) material can 

be chosen. 

a. Background 

Soil contains both inorganic and organic chemicals, with the inorganic material derived 

largely from the weathering of rocks and minerals and the organic materials from plants, 

animals, and microorganisms. In a large majority of soils, inorganic substances constitute the 

bulk of the soil material. In addition, the inorganic fraction contains acids and alkalies. 

Organic matter normally varies from less than 1 to 10 percent in soils that may be considered 

as highway construction fills, although in separation or stabilization applications, the 

geosynthetic may be placed directly over highly organic soils that may contain as much as 95 

percent organic matter. 

The bulk of most soils is made up of inorganic matter that ranges from 60 to 99 percent of 

the total weight, averaging 95 percent. About 47 percent is oxygen, the most abundant 

element, with oxides being the most prevalent form. 

The physical soil environment, which includes such factors as temperature and moisture, 

varies widely. In-ground temperatures at a depth of 5 ft (1.5 m) vary from 32 to 75 F (0

C to 24 C) in the continental U.S. Surface temperatures vary between below freezing to 

110 F (43 C). 

The annual rainfall in the continental United States varies from nearly zero in the western 

desert to 60 in. (1500 mm) of rain in the Southeast and Pacific Northwest. Highway fills 

where reinforcement may be used are generally compacted near optimum moisture, which 

for many of the soils used would mean saturation percentages in excess of 65 percent and 

often near 95 percent. Geosynthetics used in separation/stabilization functions are often 

found at or below the piezometric water levels and therefore under fully saturated conditions. 

Inorganic chemicals that are believed to affect buried geosynthetics comprise mineral acids, 

alkalies, salts, certain bivalent metals, gases and water. The organic compounds in soils 

affecting durability of geosynthetics are understood to be organic acids and solvents. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 15 November 2009
 
 

Certain natural soil environments can contain significant amounts of chemical substances 

which are degradable. The following natural processes have been identified as sources of 

chemicals, with water, oxygen and water, or heat being the catalyst: 

C Sulfur Transformation - producing sulfuric acid, sulphur dioxide, hydrogen sulphide and 

water. 

C Ammonification - producing ammonia in gaseous and aqueous state, ammonia bearing 

salts. 

C Nitrification and denitrification - producing nitrates, nitric acid, nitrogen dioxide and 

nitrous oxide. 

C Ferralitisation - producing hydroxides/oxides and ionized forms of iron and aluminum. 

C Phosphorous Transformation - producing phosphate and phosphoric acid. 

These and other processes form aggressive soils such as acid sulphate soils, organic soils, 

saline-alkali soils and calcareous soils. Other chemically reactive soils are ferruginous soils, 

which are high in iron content, and soils containing metals of manganese, copper, cobalt, and 

chromium (transition metals), as well as modified soils that may contain cement, limes, or 

de-icing salts. Cinders or slags may contain significant amounts of iron or other metals and 

sulfur. 

The composition of some of the major natural soil groups identified as being potentially 

aggressive are further discussed below: 

b. Salt-affected Soils 

Salt-affected soils are generally found in arid and semiarid regions where there are high 

evaporation and transpiration rates. In the U.S. they primarily occur in 17 western states. 

Sodic soils, a sub group of salt-affected soils, are characterized by a low permeability and 

thus restricted water flow. The pH of these soils is high, usually >9 or 9.5, and the clay and 

organic fractions are dispersed because of the high levels of monovalent sodium and OHions. They are the most alkaline environments found in the U.S. 

c. Acid-Sulphate Soils 

Acid-sulphate soils are extremely acidic with pHs of < 3.5 and even lower. Such low pH 

levels are indicative of the presence of strong acids in the soils and thus hydrogen is the main 

acidic culprit. The origin of these strong acids is often the oxidation of pyrite (iron sulfide), 

which is oxidized to sulfuric acid. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 16 November 2009
 
 

Generally, rock containing pyritic sulfur in excess of 0.5 percent and containing little or no 

alkaline minerals will produce pHs of less than 4.5, which has considerable potential to 

produce sulfuric acid. These soils or rock are identified by the presence of noticeable yellow 

mottles attributable to pyrite oxidation. Typically, acid sulphate soils contain soluble levels 

of iron, manganese, copper, zinc, aluminum, and chlorides, although levels vary greatly and 

are abundant in the Appalachian regions. When excavated and in the presence of 

groundwater, these soils produce sulfuric acid in significant quantity. 

d. Calcareous Soils 

Calcareous soils are those that contain large quantities of carbonate such as calcite (calcium 

carbonate), dolomite (calcium-magnesium carbonate), and sodium carbonates and sulfates 

such as gypsum. These soils are characterized by alkaline pHs but are not saline. Calcareous 

soils are widespread and occur in Florida, Texas, New Mexico, and many of the western 

states. Under certain conditions, they are characterized by pH in the range of 9 to 10. 

e. Organic Soils 

Organic soils are referred to as bogs, peats, mucks, moors, organic silts, and organic clays. 

Most of them are water-saturated for most of the year unless they are drained. They contain 

organic soil materials to a great depth. The major concentrations are found in the Everglades 

of Florida and in the bog regions of Michigan and Minnesota. They are, however, 

widespread throughout the U.S. 

The major organic components are fulvic, humic, and humin materials. Organic acids are 

generally negligible. Biological degradation of geosynthetics in these environments is 

possibly due to the presence of nutrients for bacteria and microorganisms. 

f. Soils Containing Transition Metals 

The literature has indicated adverse effects on polyolefin oxidation rates when transition 

metals such as copper, iron, chromium, manganese, and cobalt are present (Koerner et al., 

1986). These metals are generally not found in the free state but rather as sulfides and 

oxides. Iron, the most abundant metal in the earth's crust, is not generally found in a free 

state but rather as sulfides such as pyrite (FeS2) or ferrous silicates (Mg Fe)2SiO4 or from 

weathering in the form of oxides such as ferric oxide, hydrous oxide, ferrous carbonate and 

ferrousferric oxide, which characterize the "red earth" ferrugineous soils. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 17 November 2009
 
 

The rest of these metals are rarely found in nature other than in spoil areas developed from 

mining operations or fills constructed from these spoils. Their presence, therefore, would 

indicate the potential for accelerated degradation by oxidation of any polyolefin geosynthetic 

(PP, HDPE). 

g. Modified Soils 

Modified soils such as cement or lime-treated fills can be quite alkaline depending on the soil 

type and the quantity of additive. Sandy soils of low plasticity treated with cement are often 

characterized by a pH greater than 10. Lime modification (1-2 percent lime) of sodic soils is 

also likely to increase the pH to 10 or more. Lime stabilization (5-10 percent lime) will 

always raise any soil pH above 12. 

4.4 IDENTIFICATION OF POLYMER CHARACTERISTICS/ADDITIVES TO 

MITIGATE DEGRADATION AND TESTING METHODS 

Each of the degradation mechanisms and specific environments will have a varying degree of 

effect, as the soil environment is quite diverse and changeable. Certain aggressive 

environments have been identified, but the level of in-ground aggressiveness for each 

polymer type is a function of such variables as oxygen availability, relative humidity 

(saturation level), concentration of aggressive elements (pH, transition metals, etc.), and 

temperature. The resistance of each polymer type is a function of its molecular structure 

and/or the additives (antioxidants) used to enhance its resistance to a specific degradation 

mechanism and environment. 

The relative resistance of polymers and, therefore, their potential uses on specific soil 

environments identified as aggressive is indicated in Table 4-8. Certain polymers should not 

be considered without site-specific testing for their long-term durability or specific knowledge 

of their additives or molecular structure as indicated in Table 4-8. It should be noted that 

polymers identified as "Exposure Tests Required", may perform satisfactorily if formulated 

with specific antioxidants or additives to prevent degradation in that specific environment. 

The durability of geosynthetic products can be significantly increased by the addition of 

antioxidants for polyolefin products (PP, HDPE) and longer polymerization to achieve 

high molecular weight (Mn) and low Carboxyl End Groups (CEG) for PET. An 

understanding of these issues provides the background for discussion with manufacturers in 

selecting an appropriate product and/or site-specific testing for products listed as “Exposure 

Tests Required” in the potentially aggressive environments identified in Table 4-8. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 18 November 2009
 
 

 

 

 

Table 4-8. Anticipated Resistance of Polymers to Specific Soil Environments. 

Soil Environment Polymer 

PET PE PP 

Acid Sulphate Soils NE ETR ETR 

Organic Soils NE NE NE 

Saline Soils pH < 9 NE NE NE 

Ferruiginous NE ETR ETR 

Calcareous Soils ETR NE NE 

Modified Soils/Lime, Cement ETR NE NE 

Sodic Soils, pH > 9 ETR NE NE 

Soils with Transition Metals NE ETR ETR 

NE =No Effect 

ETR =Exposure Tests Required 

a. Polyolefins (PP and HDPE) 

Resistance to Oxidation 

The relatively poor thermal stability of unstabilized polyolefin requires the addition of 

stabilizers during the heat processing of the raw polymer and its conversion to filaments, 

tapes, or sheets. Antioxidants are always added during melt processing to minimize the 

degree of thermal degradation on extrudates, and they may be varied in both character and 

concentration where long-term thermal stability is required based on ultimate functional use. 

The antioxidant package and its quantity (loading) are often proprietary to each 

manufacturer. Polyolefin geosynthetics are often stabilized through the use of a hindered 

phenol as a primary antioxidant in combination with a phosphite. Selection of the hindered 

phenol depends on performance requirements such as thermal stability and on 

extraction/chemical resistance. In this case, the phosphite can be used to sacrificially 

stabilize the polymer, thus preserving the primary antioxidant for later use as a long-term 

thermal stabilizer. It has been further reported that the use of hindered amine light stabilizers 

(HALS) provides additional thermal stability when used with certain primary hindered 

phenol stabilizers (Gray, 1990). Carbon black, usually added to provide UV stability, is also 

believed to have some slight antioxidant property. 

One measure of antioxidant effectiveness is resistance to leaching and chemical attack. 

While polyolefins are resistant to hydrolytic and other chemical attack, additives within them 

may not be and may leach out during exposure to water or other liquids. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 19 November 2009
 
 

The quantity and composition of antioxidants used for stabilization are often proprietary. 

The usual quantity of phenolic antioxidants is on the order of less than 1.0 percent by weight, 

somewhat less for hindered amine light stabilizers (HALS). Carbon black varies from 0.5 to 

2.0 percent. The level of metal deactivators, where used, is unknown and proprietary. 

A measure of the effectiveness of the antioxidant package or relative effectiveness to other 

formulations can be obtained from oven aging tests. In such tests, samples are incubated at a 

given elevated temperature for a specified period of time and their residual strength is 

measured. The relevance of these tests to long-term oxidative resistance is discussed in 

FHWA RD-97-144 (Elias et al., 1997a). 

ASTM D-4355 is usually employed to measure effectiveness against UV radiation. This test 

incubates specimens in a weatherometer chamber, with retained strength measured after 

specific exposure times. Many current specifications require a minimum retained strength 

of 70 percent after 150 hours, although a higher level of resistance such as 70 percent at 

500 hours would be more appropriate for critical applications of permanent soil 

reinforcement in MSE walls and RSS slopes, especially in Southern areas with higher 

solar intensity. This higher level of resistance is obtained by increasing the quantity of 

antioxidant which can increase product costs 10 to 15 percent, as the cost of the antioxidant 

by weight is roughly 100 times that of the polymer. 

Stress Crack Resistance 

High-density polyethylene materials are susceptible to stress cracking; therefore the final 

product for soil-reinforcement use should be formulated with additives to increase its stresscracking resistance. Stress crack resistance of polymers as defined by ASTM D1248 is a 

function of its grade with eleven grades out of twenty being designated as having 

environmental stress-crack resistance. These are, E-4, E-5, E-8, E-9, E-10, E-11, J-3, J-4, J5, P-24 and P-34. 

Stress-crack resistance is measured by constant stress methods which quantify resistance and 

generate data for design purposes. Because of the long testing times required, the notched 

constant stress method ASTM D5397 has been introduced. This test is capable of 

determining relative ESC resistance among products. Testing to obtain design nominal 

strength based on stress-cracking limits is detailed in FHWA RD-97-142 (Elias et al. 1997b). 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 20 November 2009
 
 

 

b. Polyesters (PET) 

The chief degradation mechanism for PET is hydrolysis in any aqueous solution, especially 

in inorganic acids, halogenated organic acids, inorganic and organic bases, benzyl alcohol, 

and halogenated bases. In acid media, degradation in laboratory tests has been observed at a 

pH of 1 or less, a condition never found in nature (Elias et al., 1997). In highly alkaline 

media, (pH >9) a dual degradative process of dissolution and hydrolysis has been reported in 

laboratory tests (Davis, 1988; Elias et al., 1997). 

In an acid sulphate soil environment, sulfuric acid is generated to pH levels of as low as 1.5, 

but usually in the pH range of 2 to 3. In an alkaline environment, soils can exhibit pH greater 

than 10 with monovalent OH- ions present. Therefore, sodium hydroxide immersions in the 

pH range of 10-12 could model extreme in-soil regimes in some sodic alkaline areas as found 

in the West and Southwest. 

For hydrolysis to occur as modeled in immersion tests, the soil would have to be saturated, 

or sufficient moisture present for the PET to absorb some of the moisture and maintain 

high levels of humidity during its functional period of use. 

Calcareous soils are also alkaline and generally contain calcium (Ca) salts, although their pH 

is generally less than 9. These soils, including dolomitic soils, are widespread in the United 

States. Therefore, calcium hydroxide immersion tests in the pH range below 10 could model 

these extreme in-soil conditions. 

The resistance to hydrolysis of PET geosynthetics is impacted during the polymerization 

process and is primarily a function of the molecular weight (Mn) and carboxyl end group 

(CEG) obtained from it. It has been reported that hydrolysis reactions can be slowed by the 

addition of certain stabilizers such as carbox di (ionides) or ethylene oxides. The function of 

these stabilizers is to convert the carboxyl end group to a non-acidic component. (Koerner et 

al., 1986) 

Based on a survey of available products at present, it appears that high-strength PET 

geosynthetics (primarily woven and grid products) are produced with a molecular weight 

(Mn) generally, in excess of 25,000 and a CEG of less than 30. In contrast, nonwoven 

products are produced with molecular weights (Mn) of about or less than 20,000 and CEGs 

upward to 50. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 21 November 2009
 
 

Therefore, the (PET) geosynthetics produced with higher molecular weight are anticipated to 

be more resistant to hydrolysis, potentially by a factor of 1.5, which is roughly the ratio of the 

square root of their CEG concentrations (Koerner et al., 1986). 

The presence of coatings (PVC or acrylic) over the load-carrying fibers of PET grids should 

not affect the potential rate of hydrolysis, as these coatings primarily function to protect the 

fibers from construction damage and provide dimensional stability during manufacture. 

During installation these coatings are pierced and provide an entry for the aqueous 

environment necessary to support the hydrolytic reaction. 

In the selection process for reinforcement applications, the use of high molecular weight 

(Mn > 25,000) and low Carboxyl End Group (CEG < 30) PET geosynthetics should be 

considered as most applicable. In alkaline soils (pH > 9) where the geosynthetic may 

become saturated either because of its position below the water table or from rainfall 

infiltration, PET geosynthetics should be considered only if long-term immersion testing 

has been conducted in an aqueous media with the salts present in the proposed backfill 

soils. 

4.5 EVALUATION OF INSTALLATION DAMAGE 

Significant loss of tensile strength has been attributed to geosynthetic damage during 

construction, in soil reinforcement applications. This damage is not time dependent since it 

occurs during the backfill placement and compaction operations. Installation damage could 

be a significant reduction factor in the determination of an allowable design strength for 

permanent applications. Assessment of the damage can be made by any of the following: 

• Conducting field installation damage testing for each candidate geosynthetic and the 

proposed backfill in accordance with the procedures outlined in Chapter 5, Installation 

Damage Testing of this manual which are modifications to ASTM D5818 "Construction 

Damage Practice for Obtaining Samples of Geosynthetics from a Test Section for 

Assessment of Installation Damage". 

• Use of estimates of the damage based on past testing, summarized below. 

• Use of default values. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 22 November 2009
 
 

a. Summary of Available Installation Damage Results 

The level of damage for each geosynthetic is a variable and a function of: 

C Weight and type of construction equipment used for fill spreading. 

C Weight and type of compaction equipment. 

C Weight and type of geosynthetic. 

C Lift thickness of backfill material. 

C Gradation and angularity of backfill. 

Significant data have been generated to assess this important performance characteristic and 

provide quantitative data useful for design (Elias, 1989; Bush and Swan, 1986; Rainey and 

Barksdale, 1993). The range of strength loss reported for a wide range of geosynthetics 

varies between 10 and 77 percent. Current data strongly suggests that extreme damage is
 

associated with coarse angular backfills spread in relatively thin lifts, compacted with heavy
 

compaction equipment, and heavy construction traffic (e.g., loaded dump trucks) traversing 

the thin lift. The most important variables affecting the level of damage appear to be 

angularity, average backfill size used (D50), weight and type of construction equipment used 

for fill spreading and weight or thickness of the geosynthetic.
 

The results further suggest that: 

C Slit film geotextiles are most subject to damage. 

C Damage decreases substantially with increasing geotextile weight. 

C Minimum geotextile weights of 8 oz/yd2 (270 g/m2) should be considered based on 

construction damage survivability. This recommendation is consistent for gravelly sandy 

fills often used for MSE construction. 

Extensive construction damage testing has been reported on HDPE geogrids (Bush and 

Swan, 1986). The variables examined were: 

C Geogrid thickness. 

C Compactive effort and lift thickness. 

C Grain size distribution of backfill. 

The results indicated the following: 

C Damage and resulting loss of initial strength increased with decreasing geogrid thickness 

and weight. 

C Damage and resulting loss of initial strength increased logarithmically with increasing 

maximum backfill size as denoted by the D50 size. Backfills with D50 sizes greater than 1 

in. (25 mm) significantly increased the level of damage with correspondingly greater 

losses of strength. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 23 November 2009
 
 

C Varying compacted lift thicknesses between 6 and 9 in. (150 and 230 mm) had very little 

effect on the loss of strength. 

C Varying compactive effort from four to more than eight passes with a heavy vibratory 

compactor had only a minor effect on the resulting damage and loss of strength. 

The effects of installation damage on geosynthetic reinforcement strength should be 

determined for each product from results of full scale installation damage testing. Test 

results from damaged specimens should be compared to ASTM D4595 test results obtained 

from undamaged (i.e., not exposed to installation conditions) specimens taken from the same 

lot, and preferably the same roll, of material as the damaged specimens. The installation 

damage reduction factor for ultimate limit state design is then determined as follows: 

Tundam RFID  (4-2) Tdam 

where, Tundam is the lot specific tensile strength before installation, and Tdam is the lot specific 

tensile strength after subjection to installation. In no case should RFID be less than 1.10. 

To select an appropriate reduction factor for design, the project site installation conditions 

must be related to the installation test conditions. To relate the installation damage test 

conditions to the actual site conditions, one should primarily consider the backfill 

characteristics (mean particle size, potential for oversize material, particle angularity, and 

gradation), and to a lesser degree type of compaction equipment and initial backfill lift 

thickness over the geosynthetic, provided that the initial lift thickness is 6 in. (150 mm) or 

more. 

Note that engineering judgment is required to characterize the site conditions, as there are 

many combinations of conditions which can occur. This creates some uncertainty when 

relating the test conditions to the project site conditions, if the test was not conducted at the 

project site. If the anticipated installation conditions are poorly defined or unknown and the 

installation damage data is not site specific, a conservative interpretation of the available 

installation damage data may be warranted. It is best to obtain installation damage test data 

at the actual project site so that relating test conditions to site conditions is unnecessary. 

Even in the case of testing at the project site, however, the measured average geosynthetic 

strength loss for a sample due to installation damage may vary within a given geosynthetic 

structure even though the installation conditions appear to be the same at each point, which 

may justify exhuming and testing at more than one sample location. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 24 November 2009
 
 

 

Based on published results of construction damage for a wide range of geosynthetic 

reinforcements, a preliminary estimate of the range of reduction factors (partial factors of 

safety) for construction damage has been prepared and is shown on Figure 4-12 (based on 

Elias, 1989; Bush and Swan, 1986; Rainey and Barksdale, 1993; Sandri et al., 1993). The 

partial factor of safety is defined as the inverse of the retained strength which is defined by 

wide-width tensile strength of the material before and after the construction damage trials as 

discussed above. The lower-bound damage level is generally associated with the products 

having the highest mass per unit area. 

The wide range indicated in Figure 4-12 suggests that geosynthetic and backfill specific 

testing is necessary to evaluate the minimum installation damage strength loss. Figure 4-12 

should not be used directly to obtain this reduction factor, but rather as a check of 

manufacturers supplied data. 

b. Summary 

To account for installation damage losses of strength where full-scale product-specific testing 

is not available, Table 4-9 can be used with consideration of the project specified backfill 

characteristics. 

Table 4-9. Installation Damage Reduction Factors. 

Reduction Factor, RFID 

No. Geosynthetic 

Type 1 Backfill 

Max. Size 4 in. 

(100 mm) 

D50 about 11⁄4 

in.(30 mm) 

Type 2 Backfill 

Max. Size 3⁄4 in. 

(20 mm) 

D50 about #30 

(0.7 mm) 

1 HDPE uniaxial geogrid 1.20 - 1.45 1.10 - 1.20 

2 PP biaxial geogrid 1.20 - 1.45 1.10 - 1.20 

3 PVC coated PET geogrid 1.30 - 1.85 1.10 - 1.30 

4 Acrylic coated PET geogrid 1.30 - 2.05 1.20 - 1.40 

5 Woven geotextiles (PP&PET)(1) 1.40 - 2.20 1.10 - 1.40 

6 Non woven geotextiles (PP&PET)(1) 1.40 - 2.50 1.10 - 1.40 

7 Slit film woven PP geotextile (1) 1.60 - 3.00 1.10 - 2.00 

(1) Minimum weight 8 oz/yd2

 (270 g/m2) 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 25 November 2009
 
 

Figure 4-1. Installation damage vs. backfill size. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 26 November 2009 
 

4.6 AGING REDUCTION FACTORS 

Laboratory methods to predict aging strength loss for geosynthetics for in-ground use 

(reinforcement, stabilization) have been developed and are outlined in FHWA RD-97-144 

(Elias et al. 1997b). Conceptually, there are two approaches to provide data usable in 

determining reduction factors against aging (degradation), namely: 

C Excavation and retrieval of geosynthetics from construction projects with subsequent 

relevant testing to isolate the changes in mechanical properties attributable to a 

degradation mechanism; and 

C Use of accelerated laboratory testing in selected environments to model the degradation 

phenomena and to measure the changes in mechanical properties with time. 

The relevance and validity of both methods and recommendations for implementation are 

further discussed below: 

a. Field Retrievals 

This apparently direct method is fraught with practical and technical difficulties, namely: 

C Geosynthetics have been used extensively for MSEW and RSS applications for about 25 

years. Therefore, the potentially available sample age is considerably less than the 

usually required 50 to 100 year life span for the product. 

C The composition (polymer grade, additives) of products manufactured 15 to 25 years ago 

is quite different from those available today, and further it is likely to be quite different 

again, 10 to 25 years from now. 

C The major change in mechanical properties occurring during construction is due to 

installation damage. Unless this was measured separately during the initial construction, 

it is almost impossible to separate initial damage from the long-term time-dependent 

damage attributable to aging degradation mechanisms. 

C Archive samples or complete mechanical strength data and polymer composition index 

data must be available for comparison. This is seldom, if ever, the case. 

C Sufficient samples must be recovered to obtain a statistically significant analysis. 

Given the above, very few sites qualify for this type of investigation, and the reported data in 

the literature is indeed sparse on this subject. Alternately temperature accelerated laboratory 

testing provides a potential method of obtaining an order of magnitude for degradation 

losses, in a relatively short time (1 to 3 years), for some products. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 27 November 2009
 
 

 

 

b. Accelerated Laboratory Testing 

The predictive technique most widely used by industry for polymer degradation is based on a 

time-temperature superposition principle referred to as "Arrhenius modeling". It uses hightemperature incubation of the polymeric material, followed by laboratory testing to define 

physical or chemical properties in order to extrapolate the experimental behavior to a sitespecific and lower temperature.(Koerner et al., 1992) 

Aging losses for polyolefin products (PP and HDPE) are initiated by oxidation (availability 

of oxygen) and can be thermally accelerated. Therefore, aging tests are conducted in ovens 

at various temperatures and controlled oxygen content to model in-soil behavior, where 

oxygen concentration can vary from atmospheric (21 percent) to a fraction thereof. Note that 

these testing protocols are only applicable to products which do not exhibit crazing or 

cracking in their as manufactured state. 

Aging losses for polyester (PET) products are principally initiated by hydrolysis (availability 

of water or moisture) and can be thermally accelerated. Therefore, aging tests are conducted 

in aqueous solutions at various temperatures, where the solution pH is controlled to model a 

given in-soil regime. 

All rates of chemical degradation are temperature-sensitive according to the exponential term 

of the Arrhenius rate equation: 

 E / RT K  Ae (4-3) 

where: 

K = the rate constant at temperature T (Kelvin), 

A = the pre-exponential factor (often termed the collision factor), 

E = the activation energy (J mol-1) and 

R = the gas constant (= 8.3136 J mol-1K-1). 

This equation, strictly speaking, may only be applied to homogeneous chemical reactions and 

degradation mechanisms of polymers below their melting point. Degradation reactions of 

most polymers may not be simple and are usually heterogenous, which would involve both 

simultaneous and competing reactions that themselves may give rise to secondary reactions. 

Therefore, standard methods of determining the pre-exponential factor, A, and more 

importantly, the activation energy, E, only yield apparent values which must be constant for 

the same material within the temperature range studied, to be useful, using this model. The 

validity of this general approach has been demonstrated in FHWA RD-97-144 (Elias et al., 

1997a). 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 28 November 2009
 
 

Testing temperatures must be below the melting or phase-transition temperatures for each 

polymer and as close as possible to the temperature under which the product will be 

ultimately used. In practical terms to achieve significant and measurable property changes, 

incubation temperatures of 105 to 160 F (41 to 71 C) are used for PET incubations, 85

to 175 F (29 to 79 C) for PP, and 160 to 195 F (71 to 91 C) for HDPE. At these 

temperatures, testing times are on the order of 2 to 4 years. Typical laboratory equipment is 

shown on Figure 4-13. 

The usual method of analyzing laboratory incubation data is to plot the log of a Reaction 

Rate or the inverse of a Reaction Time for a preselected property against the inverse of 

temperature as shown on Figure 4-14. For geosynthetic durability studies a typical reaction 

rate would be the degradation rate in tensile strength with time at a given laboratory elevated 

temperature. From this type of data plot, reactions at lower temperatures can be predicted 

from higher-temperature experimental data. 

A wide range of physical and chemical properties may be extrapolated using this technique 

as extensively discussed by Koerner et al. (1992), provided that: 

C The logarithm of the reaction rate vs. the inverse of temperature is linear. 

C The investigative temperatures are spaced reasonably apart and are below any transitionphase temperatures. 

In oxidation studies where the logarithm of the reaction rate vs. the inverse of temperature is 

not linear or where no strength loss is measured initially (induction period) at a given 

temperature, similar analysis techniques consistent with the well developed basic 

autoxidation scheme (BAS), as outlined in FHWA RD-97-144 (Elias et al., 1997a), must be 

applied (Kelen, 1982). 

Of primary interest to design engineers are mechanical properties such as tensile strength and 

elongation. For example, if the time to reduce tensile strength by 50 percent of its unaged 

value is required for a site ambient temperature (e.g., 68 F {20 C}), multiple samples of the 

product are aged at three elevated temperatures (e.g., 122, 149, 176 F {50, 65, 80 C}) 

as shown on Figure 4-14, High Temperature. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 29 November 2009
 
 

Figure 4-2. Laboratory aging equipment setup. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 30 November 2009 
FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 31 November 2009 

 

Figure 4-3. Generalized Arrhenius plot used for low-temperature predictions form hightemperature experimental data. 
 

Based on the tensile strength of samples retrieved at various time intervals from the ovens, a 

linear regression analysis of retained strength versus exposure time is used to compute the 

rate constant of tensile strength degradation for each temperature tested. The natural 

logarithm of degradation rate at each temperature is plotted versus the reciprocal of 

temperature (Kelvin) to obtain a relationship which then can be extrapolated to any other 

lower temperature such as ambient. This process, known as Arrhenius modeling, is 

illustrated in Figure 4-14. 

It should be noted that for stabilized polyolefin geosynthetics, the initial stages of oven aging 

at most elevated temperatures will yield no strength loss. This induction period defined as 

aging time with no strength loss is a direct measure of antioxidant effectiveness. After all of 

the antioxidant is consumed, strength loss of the now unstabilized polyolefin will begin. 

The strength retained, TD, at the end of the desired design life at the site temperature can 

then be calculated by considering no strength loss during the induction period, followed by a 

loss calculated directly from the linear Arrhenius equation. Once the tensile strength at a 

given design life has been estimated from the test data, RFD is determined as follows: 

Tultlot RFD  (4-4) TD 

where: 

Tultlot = the average lot specific ultimate tensile strength for the unaged lot of material 

used for the durability testing, and 

TD = the extrapolated lot specific tensile strength after degradation based on the 

laboratory aging tests. 

In no case should RFD be less than 1.1. 

For better understanding of the chemical degradation process, chemical properties may also 

be extrapolated. Research is presently under way using these techniques to completely 

define testing methods and protocols, and develop design data to yield lifetime predictions 

for certain preselected, possibly typical geosynthetics. 

c. Summary 

The outlined framework of procedures summarizes the present state of the art in developing 

credible long-term degradation strength losses for polymeric reinforcement. Interpretation of 

the test data is still somewhat subjective, and subject to revision based on future research 

efforts. Table 4-10 summarizes specific studies needed. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 32 November 2009
 
 

 

Table 4-10. Summary of Product-Specific Studies Needed to Evaluate 

the Durability of Geosynthetic Reinforcement. 

Environmental Factor Polymer Resin Type for 

Which Studies are Needed 

Geosynthetic 

Reinforcement Studies 

Needed 

Mechanical Damage All geosynthetic reinforcements Full Scale Installation 

Damage Tests 

Stress Levels All geosynthetic reinforcements Creep tests 

Stress Levels All geosynthetic reinforcements Stress crack evaluation: 

long-term stress rupture tests 

at ambient and elevated 

temperatures 

Chemical Exposure 

(oxygen) 

Polyolefins (e.g., polypropylenes 

and polyethylenes) 

Long-term oxidation studies 

Chemical Exposure 

(water, pH) 

Polyesters and polyamides (and 

any polymer coatings present such 

as PVC or acrylic) 

Long-term hydrolysis studies 

and short-term effects due to 

plasticization 

Other potentially 

reactive chemicals 

present in the in-situ 

environment 

Resin types which are potentially 

susceptible depend on the specific 

chemical present 

Other specialized chemical 

tests 

Microbiological Attack Most resins used for geosynthetic 

reinforcements are generally not 

susceptible to this but should at 

least be checked 

ASTM D3083 

Table 4-11. Aging Reduction Factors, PET. 

No. Product* 

Reduction factor, RFD 

5 ≤ pH ≤ 8 3 ≤ pH ≤ 5 

8 ≤ pH ≤ 9 

1 Geotextiles 

Mn < 20,000, 40 < CEG < 50 

1.6 2.0 

2 Coated geogrids 

Mn > 25,000, CEG < 30 

1.15 1.3 

* Use of materials outside the indicated pH or molecular property range requires 

specific product testing. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 33 November 2009
 
 

With respect to aging degradation, current research results suggest the following: 

Polyester geosynthetics
 

PET geosynthetics are recommended for use in environments characterized by 3 < pH < 9, 

only. The following reduction factors for PET aging (RFD) are presently indicated for a 100­

year design life in the absence of product specific testing: 

Polyolefin geosynthetics 

To mitigate thermal and oxidative degradative processes, polyolefin products are stabilized 

by the addition of antioxidants for both processing stability and long term functional stability. 

These antioxidant packages are proprietary to each manufacturer and their type, quantity and 

effectiveness varies. Without residual antioxidant protection (after processing), PPs are 

vulnerable to oxidation and significant strength loss within a projected 75 to 100 year design 

life at 68 F (20 C). Current data suggests that unstabilized PP has a half-life of less than 50 

years. Therefore the anticipated functional life of a PP geosynthetic is to a great extent a 

function of the type and remaining antioxidant levels, and the rate of subsequent antioxidant 

consumption. Antioxidant consumption is related to the oxygen content in the ground, which 

is only slightly less than atmospheric. 

Heat aging protocols for PP products, at full or reduced atmospheric oxygen, with subsequent 

numerical analysis are available for PP products which exhibit no initial cracks or crazes in 

their as manufactured state, typically monofilaments (Elias et al., 1997a). For PP products 

with initial crazes or cracks, typically tape products, or HDPE, heat aging testing protocols 

may change the nature of the product and therefore may lead to erroneous results. Alternate 

testing protocols using oxygen pressure as a time accelerator have also been developed 

(ASTM D5885). 

Since each product has a unique and proprietary blend of antioxidants, product specific 

testing is required to determine the effective life span of protection at the in-ground oxygen 

content. Limited data suggests that certain antioxidants are effective for up to 100 years in 

maintaining strength for in-ground use. 

A rough measure of antioxidant effectiveness for PP products formulated without significant 

carbon black is resistance to UV degradation measured in accordance with ASTM D4355. A 

retained strength of 90% at 500 hours or more generally indicates an effective antioxidant 

blend and potentially a reduction factor as low as 1.1 at 68 F (20 C) and 100 years. For 

HDPE geogrids presently available (Tensar UX Series), current research data indicates a 

Reduction Factor of 1.1 for use at 68 F (20 C) and 100 years. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 34 November 2009
 
 

Note that for products meeting the minimum requirements of Table 4-5 in "Mechanically 

Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines" a 

total default reduction factor RF of 7 has been recommended. This total reduction factor 

includes a reduction factor for creep RFCR not discussed in Chapter 4. 

4.7 USE OF DURABILITY DATA FROM "SIMILAR" PRODUCTS 

Long-term chemical/biological durability data obtained from tests performed on older 

product lines, or other products within the same product line, may be applied to new product 

lines, or a similar product within the same product line, if one or both of the following 

conditions are met: 

C The chemical and physical characteristics of tested products and proposed products are 

shown to be similar. Research data, though not necessarily developed by the product 

manufacturer, should be provided which shows that the minor differences between the 

tested and the untested products will result in equal or greater chemical/biological 

degradation resistance for the untested products. 

C A limited testing program is conducted on the new or similar product in question and 

compared with the results of the previously conducted full testing program. 

For polyolefins, similarity could be judged based on molecular weight and structure of the 

main polymer (i.e., is the polymer branched or crosslinked, is it a homopolymer or a blend, 

percent crystallinity, etc.), percentage of material reprocessed, tenacity of the fibers and 

processing history, and polymer additives used (i.e., type and quantity of antioxidants or 

other additives used). For polyesters and polyamides, similarity could be judged based on 

molecular weight or intrinsic viscosity of the main polymer, carboxyl end group content, 

percent crystallinity, or other molecular structure variables, tenacity of the fibers and 

processing history, percentage of material reprocessed or recycled, and polymer additives 

used (e.g., pigments, etc.). The untested products should also have a similar macrostructure 

(i.e., woven, nonwoven, extruded grid, yarn structure, etc.), relative to the tested products. 

FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 35 November 2009
 
 

 

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FHWA NHI-09-087 4 – Durability of Geosynthetics 

Corrosion/Degradation 4 – 36 November 2009
 
CHAPTER 5 

MONITORING METHODS, GEOSYNTHETIC REINFORCEMENTS
 

The principal degradation mechanisms for geosynthetics discussed in Chapter 4 fall in the
 

following two categories: 

C Construction damage degradation that is a short-term phenomenon. 

C Polymer degradation due to oxidation, hydrolysis, or UV radiation that is a long-term
 

phenomena. 

For MSE walls or slopes construction, the results of these degradative mechanisms are a loss 

of tensile strength and potential change of elongation properties. Monitoring schemes 

designed to assess these mechanisms are, therefore, quite different in nature and duration, 

although in all cases they would principally assess the change in tensile and elongation 

properties. 

Short-term construction damage determinations lend themselves to model field testing 

programs with retrievals tailored either to evaluate a single geosynthetic with one or several 

backfills or multiple geosynthetics with a single or with multiple fills. 

Polymer degradation monitoring is a long-term endeavor characterized by multiple retrievals 

spaced over a decade or two. The protocols for each are summarized below. 

5.1 INSTALLATION DAMAGE TESTING 

Where previous knowledge does not exist to assess the strength loss caused by fill 

construction and compaction on a specific geosynthetic and reinforced fill, field testing is 

required. The use of the ASTM D5818 Standard Practice for Exposure and Retrevial of 

Sample to Evaluate Installation Damage of Geosynthetics methodology as modified/ 

expanded by the following procedure, is recommended: 

1. Preliminary laboratory characterization testing of geosynthetics focused on geosynthetic 

strength properties, using the wide-width tensile strength test (ASTM D4595) or multi-rib 

geogrid tensile strength test (ASMT D6637, Method B), in both directions. Single rib 

strength, junction strength, grab strength, puncture strength, tear strength and burst 

strength may also be performed to relate performance to typically available index tests. 

The minimum number of samples should be in accordance with the requirements of 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 1 November 2009
 
ASTM D4595 and ASTM D6637 to ensure that the coefficient of variation is less than 5 

percent from true average values. A minimum of five samples is required. 

2. Field placement of geosynthetics as follows: 

C Place and compact 12 in. (300 mm) of soil (same as soil used to cover the 

geosynthetic) on a flat, level, relatively incompressible subgrade. 

C Place the geosynthetic with the machine direction perpendicular to the face of a wall 

or embankment. Geosynthetics should be pulled taut with no wrinkles or folds. 

Pinning at the corners should be considered to maintain the position. Each adjacent 

sheet of geosynthetic should be overlapped a minimum of 6 in. (150 mm) with the 

upper sheet placed in the direction of soil placement. A total sample size of 16 by 10 

ft (5 by 3 m) should be used as a minimum. 

C Place 8- to 10-inch (200-to 250-mm) compacted thickness of backfill using a frontend loader or a D-4 to D-7 dozer. 

C Compact the backfill using a 10,000 to 30,000-lb (4500 to 13,600 kg) vibratory 

smooth-drum roller with a set number of passes. The minimum number of passes 

should insure compaction equal to at least 95 percent of (Modified Protor) AASHTO 

T 180 density. As a maximum, 10 passes are recommended. 

C Traverse the complete area with construction equipment used to place subsequent lifts 

(e.g., loaded dump truck). The minimum number of passes should follow anticipated 

construction conditions (e.g., two passes for end dump placement and five passes for 

pass through traffic). 

C Carefully remove the backfill by hand and document any observable geosynthetic 

damage, including photographs of entire area and visual survey of puncture holes per 

square yard (m). 

C Select specimens of the geosynthetic for testing. Specimen selection should be 

guided by the placement of a primary template 24 by 32 in. (600 by 800 mm) shown 

in Figure 5-1 on the center of the sample. 

Cut and number twelve adjacent specimens 8 in. (200 mm) by nominally 8 in. (200 mm) as 

shown in Figure 5-1. Specimens having areas marked with damage from the retrieval 

process, if any, should not be tested. A minimum of nine specimens with consecutive 

numbers should be initially tested. For high-strength polyester geotextiles and geogrids, the 

length of the specimens should be increased to accommodate the roller grips, which require 

up to 5 ft (1.5 m) long test specimens. For geogrids, the specimen size should be such to 

contain at least three ribs in the machine direction and five ribs in the cross direction. 

Additional cross direction ribs may be necessary to accommodate roller clamps. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 2 November 2009
 
1

6 in. (150 mm) 

6  ft (1.8  m) 

1 2 3 

4 5 6 

7 8 9 

10 11 12 

Machine Direction 

Note: 

1) Primary template 

specimens 1 to 12 

2) Specimens are 8 in. 

by 8 in. (200 mm by 

200 mm) except for 

geogrids & high 

strength geotextiles, 

where longer 

specimens are 

required 

3) Secondary template 

specimens 13 to 21 

13 14 15 

16 17 18 

19 20 21 

3  ft (0.9  m) 

Figure 5-1. Scheme for sampling test specimens. 

FHWA NHI-09-087 

Corrosion/Degradation 5 – 3 

5 – Geosynthetic Monitoring Methods 

November 2009 
Evaluation 

Wide-width or multi-rib tensile strength obtained from these 9 primary tests should be 

analyzed to determine the coefficient of variation as outlined in ASTM D4595 or ASTM 

D6637 with an allowable 5 percent variation from true average values at a 95 percent 

confidence level. 

If the coefficient of variation for the 9 primary samples is greater than 5 percent, the required 

number of samples should be recomputed using the one sided student t variation (Table 1, 

ASTM D4959). If more than 9 specimens are required, then additional specimens as 

available should be secured from a secondary template location also shown on Figure 5-1. 

No more than a total of 18 specimens per sample should be tested for wide-width or multi-rib 

strength. 

Prior to wide-width or multi-rib tensile testing, mass-per-unit area should be determined from 

specimens marked 9 through 12 in accordance with ASTM D5261. 

C If other index tests are desired or warranted for correlations to performance, then 

additional templates should be placed centered on the machine direction and samples 

obtained. These additional tests should be performed in accordance with the following 

standards: 

o For Geotextiles 

 Grab Tensile Strength – ASTM D4632 

 Puncture Strength – ASTM D6241 

 Trapezoidal Tear Strength – ASTM D4533 

o For Geogrids 

 Single Rib Strength – ASTM D6637 

 Junction Strength – GRI:GG2 

C Evaluate the retained strength on the basis of the average results obtained from the widewidth or multi-rib tensile test. 

C Determine the reduction factor (partial factor of safety) for construction damage by 

dividing the initial tensile strength by the retained tensile strength. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 4 November 2009
 
5.2 POLYMER DEGRADATION MONITORING 

A protocol for exhuming and testing a geosynthetic for resistance to long-term degradation is 

characterized by multiple retrievals over a long period of time and identification of the 

soil/water environment in which the product is placed. 

The essential steps are as follows: 

C Identify site conditions. 

C Select location and depths for future retrievals. 

C Test control samples. 

C Perform construction damage testing immediately after completion of construction. 

C Develop subsequent retrieval schedule. 

C Successive retrieval of samples. 

C Test retrieved samples for both mechanical and chemical properties. 

C Evaluate and extrapolate data. 

It should be recognized that each retrieval requires a significant physical sample size and 

therefore the design of a test section must consider accessibility after the facility has entered 

its intended use. This task may not be easy to accomplish, especially where retrieved 

samples are significantly below the finished grade, which may require braced excavations to 

reach intended locations. 

The developed protocol recommended for use is as follows: 

a. Identification of Site Conditions and Structure Description 

Each site should be described as to the functional use of the geosynthetic mapped for 

retrieval. Such description shall include a location plan indicating the location, plan, and 

elevation of the geosynthetic in the structure; and an estimate of the loading conditions, 

including piezometric surfaces; design service life; and any other pertinent observations 

as to the functionality of the structure after construction. Available performance data 

such as deflection measurements and/or strain gauge data should be included where 

available. The information should be summarized on a Project Information Sheet. 

Retrieval intervals at the approximate same location should be planned at about 5-to-7 

year intervals for a minimum of four retrievals or 1/3 the expected life of the facility. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 5 November 2009
 
b. Testing of Control Samples and Retrieved Samples 

Testing methods for all required physical, endurance and chemical tests Applications 

(Bright, 1993) have been developed under FHWA Durability of Geosynthetics for 

Highway project. They are described as follows: 

C The principal test method to characterize residual strength should be the wide width 

tensile test performed in accordance with ASTM D4595 or ASTM D6637 in the 

machine direction. Where strength in previous retrievals has been characterized by 

the grab tensile test performed in accordance with ASTM D4632, it should be 

considered the principal test for that specific site. 

C Physical property tests on retrieved and control product specimens 

o Mass-per-unit area should be measured for geotextiles in accordance with 

ASTM D3776, Option C, using at least one specimen retrieved. Thickness 

may be measured on polyolefin grid products in accordance with ASTM 

D5199. 

o Density/Specific Gravity should be measured in accordance with ASTM 

D792. 

o An assessment of the number of holes per unit area of retrieved specimen for 

geotextiles should be made by placing the specimen on a light table and 

counting the number of holes visible. The level of abrasion, if visible, should 

be noted. For geogrids rib cuts, abrasions and percent of ribs severed across 

the specimen width should be noted. This assessment is subjective and no 

standard exists. Photographs should be taken and special conditions shown in 

photographs noted. 

C Chemical tests on product specimens 

For PET (polyester products) 

o The principal tests performed on geotextiles and fibers of coated geogrid 

products are molecular weight determinations (Mn) using gel permeation 

chromatography and determination of carboxyl end groups (CEG number) 

using ASTM D7409. A minimum of three tests should be performed. 

o Where product control data for molecular weight has been expressed in terms 

of intrinsic viscosity or where reagents and test temperatures are known, they 

may be substituted from molecular weight (Mn) determinations. Intrinsic 

viscosity may be determined in accordance with ASTM D4603. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 6 November 2009
 
 

C 

o Thermogravimetric Analysis (TGA) in general accordance with ASTM 

E1131, to determine transition temperatures and any changes from archive 

samples. 

o Scanning Electron Microscopy (SEM) at a magnification of at least 1000x, to 

determine any fiber diameter changes from archive samples. 

For polyolefin products (PP and HDPE), the following tests are recommended: 

o Melt Flow index, ASTM D1238 to establish changes from archive samples. 

o Differential Scanning Calorimetry (DSC) to obtain oxidation induction time 

(OIT). Three specimens should be tested per retrieval sample. Methods 

outlined in FHWA "Durability of Geosynthetics for Highway Applications" 

should be used. 

o Thermogravimetric Analysis (TGA) to obtain transition temperatures. Three 

specimens should be tested per retrieval sample ASTM E1131. 

o High Performance Liquid Chromatography (HPLC) to determine the level of 

antioxidants present. Three specimens should be tested using industry 

standards. This test should only be performed if the type of antioxidant in the 

product is known. 

o Scanning Electron Microscopy (SEM) at a magnification of at least 1000x to 

determine the presence of longitudinal or circumferential cracking. 

Performance Tests 

In isolation creep tests in general accordance with ASTM D5262 for a 1,000 hour 

maximum duration may be conducted on selected retrieval specimens only if control 

data for this particular product is available for comparison or if the site will be 

available for future retrievals and the functional use of the geosynthetic on the project 

is for reinforcement/stabilization. 

Where conducted, two specimens shall be tested at each of three load levels. The first 

load level shall be approximately equal to the specimen creep load limit as defined by 

the virgin creep limit (as a fraction of ultimate strength) multiplied by the ultimate 

tensile strength retained after damage; the second load level at the design load used 

for the project but no higher than the estimated limit state reinforcement tensile load 

as defined in Article 11.10.6.4 AASHTO (2007); and the third at a high load level 

approximately equal to 1.5 times the virgin creep limit of the virgin specimen. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 7 November 2009
 
 

 

c. Retrieval Methods 

Sampling must be performed carefully in order to avoid damage to the product. The 

excavation operation may begin with power equipment, but such excavation methods 

must terminate within 6 to 8 in. (0.15 to 0.2 m) of the geosynthetic. Excavation must 

then continue manually with a hand trowel, hand rake, and broom to remove the 

remaining soil gently over a minimum 3 by 6 ft (1 by 2 m) surface area. It is 

recommended that the 6-ft (2-m) length be parallel to the machine direction. High 

strength polyester geotextile and woven geogrid products requiring roller grips for testing 

will necessitate a longer total sample (e.g., 10 to 15 ft). Damage during this excavation 

phase should be noted and marked on the product. The site should be photographed and 

the visual appearance of the product be noted with emphasis on existing holes, tears, 

folds, root penetration, presence of water, and uniformity of backfill. 

The sample is then cut along the sides, lifted carefully, excess soil shaken off and placed 

in black polyethylene bags, sealed and marked with appropriate identification. The 

sample must be identified with respect to machine and cross direction which must be 

noted on the location sketch. 

Concurrently, a soil sample must be secured adjacent to the product sample retrieval. 

Separate samples are required if the soils above and below differ visually in composition. 

Sample preparation for testing 

• Total sample 

Prior to specimen selection, the retrieved sample should be prepared by removing 

any soil by gently shaking the sample. 

The full sample should then be hand washed gently under tap water, removing 

only any adhering surface soil cake that had formed. No attempt should be made 

to remove any soil that does not easily wash away. 

The washed sample should then be laid out horizontally in a darkened room and 

allowed to dry under ambient temperature. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 8 November 2009
 
 

• Specimens for mechanical testing 

To avoid bias, selection of specimens from the recovered product sample should 

be made in accordance with the following structured random process. 

A primary template 24 by 32 in. (600 by 800 mm) should be laid out in the 

machine direction on the recovered sample by locating it 6 in. (150 mm) from the 

top edge and centering it along the 3 ft (0.9 m) recovered width. The primary 

template length should be increased for high strength polyester geotextiles and 

geogrids tested with roller grips, which require up to 5 ft (1.5 m) long test 

specimens (in the machine direction). 

Twelve adjacent specimens 8 by 8 in. (200 by 200 mm) should be cut and 

numbered in accordance with Figure 5-1. Specimens having areas marked with 

damage from the retrieval process should not be tested. A minimum of nine 

specimens with consecutive marked numbers should be initially tested. For highstrength polyester geogrids and geotextiles, the length should be increased to 

accommodate the roller-grip requirement. For geogrids, the specimen size should 

contain a minimum of three ribs in the machine direction and five ribs in the cross 

direction. Additional cross direction ribs may be necessary to accommodate roller 

clamps. 

Prior to wide-width tensile testing, mass-per-unit area should be determined from 

specimens marked 9 through 12 in accordance with ASTM D5261. Specimens 

tested for strength should be reserved for chemical testing, utilizing principally 

those portions of the product held in the jaws of the testing apparatus. 

Wide-width tensile strength test (ASTM D4595 or ASTM D6637, Method B) data 

obtained from these nine primary tests should be analyzed to determine the 

coefficient of variation as outlined in ASTM D4595 or ASTM D6637, with an 

allowable 5 percent variation from true average values. Based on the obtained 

actual coefficient of variation, the required number of specimens should be 

recomputed. If greater than nine, additional specimens as available should be 

secured from a secondary template location also shown on Figure 5-1. No more 

than a total of 18 specimens per recovered sample should be tested for wide width 

strength. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 9 November 2009
 
 

• Specimens for chemical testing 

Chemical testing of specimens not sufficiently cleaned of colloidal soil 

particles may be ultrasonically cleaned using water and/or a mild wetting 

agent at room temperature. A 2 percent solution of Micro Cleaner has been 

successfully used in 2-to 5-minute cycles in a stainless steel basket. 

d. Soil Tests 

The ambient soil regime shall be characterized by performing the following tests on one 

sample of the representative soils (10 lb {15 kg}) obtained above or below the retrieved 

geosynthetic specimen: 

• Grain size distribution (ASTM D854) 

• Atterberg Limits for fine grained soils (ASTM D4318) 

• pH (AASHTO T-289) 

• Electrical conductivity or resistivity (AASHTO T-288) 

• Organic content (AASHTO T-267) 

• Transition metals (EPA SW 6010) 

• Chloride, sulfate, carbonate (ASTM D4327) 

• Calcium, sodium (EPA, SW 6010) 

5.3 EVALUATION OF GEOSYNTHETIC DEGRADATION MONITORING DATA 

The principal method of evaluating aging degradation is to determine loss of tensile strength. 

The short-term loss is totally due to construction damage. Therefore, potential aging 

polymer degradation is measured from the reduced average strength obtained after a new 

baseline initial strength is calculated by subtracting the construction damage from the initial 

control sample strength data. 

Subsequent retrievals would therefore indicate the rate of aging degradation, which can be 

extrapolated to the design life of the product in a straight-line fashion. The measured 

chemical-property data at each retrieval may in some cases provide a confirmation of the 

measured rates and an indication that the process is linear or autocatalytic. The significance 

of changes in chemical properties varies with each base polymer additive combination and is 

under current research. Some preliminary conclusions can be drawn at this time as follows: 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 10 November 2009
 
a. Polyester (PET) 

The principal long-term, in-ground degradation mechanism of PET in acidic, neutral and 

slightly alkaline conditions characterized by pHs greater than three and less than nine, is 

hydrolysis. As a result of hydrolysis, molecular weight (Mn) decreases apparently almost 

linearly with loss of strength, to at least a point equal to a 50 percent strength loss. 

Correspondingly, CEG number increases although its relationship to molecular weight 

loss has not been established to date, but could be linear as well, as the two properties are 

directly related. Loss of strength as evidenced by tensile tests in successive retrievals can 

therefore be confirmed by an equal percentage decrease in molecular weight (Mn) up to a 

50 percent strength decrease. 

For degradation in more strongly alkaline media, an additional mechanism may cause 

losses of strength termed "outer hydrolysis" which is a physical dissolution of polyester 

material at the outer surface of each fiber. The effect of such loss of fiber mass on its 

circumference can only be measured by comparing fiber diameter under an SEM or more 

indirectly inferred by weight-per-unit-area measurements of the retrieved sample. This 

latter procedure, although simpler, is fraught with experimental problems, as it may be 

extremely difficult to completely clean the retrieved samples of embedded colloidal soil 

particles, especially for non-woven geotextile reinforcements. 

Figure 5-2 shows the diameter change of a PET geotextile fiber subjected to laboratory 

immersion on a strongly alkaline solution. The geosynthetic is characterized by an initial 

number molecular weight (Mn) of 19,000 and a Carboxyl End Group of 47. 

b. Polyolefins (PP and HDPE ) 

The principal long-term, in-ground degradation mechanism of polyolefin polymers is 

oxidation. The major resistance to oxidation is impacted by the antioxidant package 

included during the manufacturing process. It would therefore appear that confirmation 

of strength losses measured (due to aging) can only be obtained from successive 

retrievals and may be confirmed by measurable loss of antioxidants, measurable loss of 

their effectiveness, or both. 

If the type of antioxidant is known, the level present at anytime can be measured by High 

Performance Liquid Chromatography (HPLC) with a ±20 percent accuracy. The 

Oxidation Induction Time (OIT) is only a measure of an antioxidant's relative 

effectiveness within each product as a thermal or oxidative stabilizer and therefore is a 

poor indicator. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 11 November 2009
 
Based on our present knowledge, the decrease of the antioxidant level or OIT time is not 

evidence of strength loss, but can serve as a confirmation of either leaching or 

consumption of antioxidants. The complete loss of antioxidants as evidenced by HPLC, 

or decreases of OIT time greater than 90 percent, indicates active oxidation with strength 

loss accelerating. 

A simpler qualitative determination of significant past oxidation activity is to examine 

archive and aged samples under an SEM. Oxidation activity may be reflected in 

circumferential cracking of the fibers as shown on Figure 5-3. 

Note that some PP geotextiles, chiefly tape or staple products may exhibit initial 

longitudinal or transverse cracks or crazing in their virgin as manufactured state. For 

these products an increase in crack length or frequency is an indicator of oxidation 

activity. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 12 November 2009
 
a) Virgin fibers 

b) Fibers after alkaline hydrolysis 

Figure 5-2. Scanning Electron Microscopy (SEM), polyester fibers. 

FHWA NHI-09-087 5 – Geosynthetic Monitoring Methods 

Corrosion/Degradation 5 – 13 November 2009 




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Frondistou-Yannis, S. (1985). Corrosion Susceptibility of Internally Reinforced Soil 

Retaining Structures, Report No. FHWA/RD-83/105, National Technical Information 

Service, Springfield, VA. 

Gladstone, R.A., Anderson, P.L., Fishman, K.L., and Withiam, J.L. (2006). “Durability of 

Galvanized Soil reinforcement: 30 Years + of Experience with MSE,” Journal of the 

Transportation Research Board, No. 1975, Transportation Research Board, National 

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U. S. Department of Transportation Publication No. FHWA-NHI-09-087
 Federal Highway Administration November 2009
 

NHI Courses No. 132042 and 132043 

CORROSION/DEGRADATION OF 

SOIL REINFORCEMENTS FOR 

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AND REINFORCED SOIL SLOPES 
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