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Assessing Corrosion of MSE Wall Reinforcement
for I-15, Salt Lake County, UT
Daniel A. Billings
A project submitted to the faculty of
Brigham Young University
in partial fulfillment of the requirements for the degree of
Master of Science
Travis M. Gerber, Chair
Kyle M. Rollins
Norman L. Jones
Department of Civil and Environmental Engineering
Brigham Young University
April 2011
Copyright © 2011 Daniel A. Billings
All Rights Reserved
ABSTRACT
Assessing Corrosion of MSE Wall Reinforcement
for I-15, Salt Lake County, UT
Daniel A. Billings
Department of Civil and Environmental Engineering
Master of Science
Mechanically stabilized earth (MSE) retaining walls are versatile and cost-effective to
construct, leading to their increasingly widespread use. MSE walls are designed as a system of
interdependent components – chiefly a retention face and soil reinforcement. The MSE panel
walls owned by Utah Department of Transportation (UDOT) in the Salt Lake Valley are
typically constructed with galvanized steel soil reinforcement. This project assesses the
corrosion conditions of the reinforcement.
The walls covered in this study are approximately 12 years old and have removable
corrosion coupons placed in the backfill and made accessible from the face of the wall for
successive removal over the life of the wall. This study represents the first extraction of any of
these coupons from UDOT walls.
Twenty-two coupons were removed from 19 MSE walls including 13 one-stage and six
two-stage walls. The coupons were stripped with acid to determine the remaining zinc
galvanization and the measurements were analyzed to identify trends in corrosion rates. The
results were compared against the manufacturer’s specifications and the AASHTO corrosion
model.
All coupons were found to be in good condition with minimal corrosion. The existing
coating thicknesses were in excess of specified values for initial installation thickness. A few
instances of damaged galvanization were found and attributed to installation. The exposed plain
steel core in these areas showed minimal red rust corrosion.
Pullout resistance during coupon extraction was significantly less for two-stage walls
than for their one-stage counterparts. This was attributed to differing degrees of relative
compaction. The one-stage walls seemed to exhibit somewhat greater zinc loss than the two-
stage walls, with the latter experiencing more loss toward the interior of the fill than nearer the
face. This effect is also believed to be a result of differences in relative compaction.
Keywords: Daniel Billings, MSE wall, corrosion, galvanization, steel reinforcement, pullout,
two-stage, one-stage, AASHTO, I-15, Salt Lake City, Utah
ACKNOWLEDGMENTS
I would like to thank my advisor, Dr. Travis Gerber for his help and direction with this
project and for coordinating the financial and administrative aspects. I express my gratitude to
the Utah Department of Transportation who sponsored and funded this research project. Thank
you also to Drs. Gerber, Rollins, and Jones for their helpful reviews. Thanks are due to David
Anderson, Rodney Mayo, Ben Holdaway, and Mitch Shurtliff for their many good ideas, strong
backs, and willing hands.
Finally, I would like to thank my wife Wendy and our children for their support and
encouragement and for putting up with my long hours away, late nights writing, and an extra
heavy load while I have finished this project.
v
TABLE OF CONTENTS
LIST OF TABLES ....................................................................................................................... ix
LIST OF FIGURES ..................................................................................................................... xi
1 Introduction ........................................................................................................................... 1
2 Background ........................................................................................................................... 3
2.1 MSE Wall Construction Basics ...................................................................................... 3
2.1.1 One-Stage Walls ......................................................................................................... 3
2.1.2 Two-Stage Walls ......................................................................................................... 6
2.2 Corrosion Basics ............................................................................................................. 8
2.2.1 Gradation of Structural MSE Fill .............................................................................. 10
2.2.2 Moisture Content & Resistivity ................................................................................ 10
2.2.3 Dissolved Salts Content ............................................................................................ 11
2.2.4 pH .............................................................................................................................. 11
2.2.5 Galvanization ............................................................................................................ 12
2.2.6 Design Life ................................................................................................................ 13
2.2.7 Corrosion Models ...................................................................................................... 13
vi
2.2.8 AASHTO Design Corrosion Rates ........................................................................... 15
2.2.9 Atmospheric Corrosion Rates ................................................................................... 17
2.3 MSE Wall Corrosion Studies by Others ....................................................................... 17
3 Field Data Collection .......................................................................................................... 19
3.1 Extractable Coupons ..................................................................................................... 19
3.1.1 Specifications ............................................................................................................ 22
3.2 Design of Extraction Device ......................................................................................... 22
3.3 Calibration .................................................................................................................... 25
3.4 Coupon Extraction Procedure ....................................................................................... 26
4 Coupon Pullout Resistance ................................................................................................. 31
4.1 Pullout Resistance versus Coupon Length, Wall Height, and Wall Type .................... 31
4.2 Pullout Force versus Distance Pulled ........................................................................... 36
5 Laboratory Assessment of Coupon Properties ................................................................. 41
5.1 Sample Preparation ....................................................................................................... 42
5.2 Initial Measurements ..................................................................................................... 45
5.3 Acid Stripping Procedure .............................................................................................. 46
5.4 Secondary Measurements—Magnetic Thickness Gauge ............................................. 48
vii
5.5 Thickness Results ......................................................................................................... 50
5.6 Spot Measurement of Steel Corrosion (―Special Measurements‖) ............................... 52
6 Analysis and interpretation of Field and laboratory data .............................................. 55
6.1 Thickness versus Spatial Location (Coupon Segments A, B, and C) ........................... 57
6.2 Thickness versus Overburden Height ........................................................................... 58
6.3 Thickness versus Wall Type ......................................................................................... 59
6.4 Thickness versus Pullout Resistance ............................................................................ 61
6.5 Comparison to Design Coating Thickness .................................................................... 64
7 Conclusion ........................................................................................................................... 67
REFERENCES ............................................................................................................................ 71
Appendix A .................................................................................................................................. 73
viii
ix
LIST OF TABLES
Table 3-1: Summary table of extracted wire coupons. .................................................................23
Table 4-1: Summary of coupon lengths and pullout resistances. .................................................32
Table 5-1: Variation in uncoated steel wire. .................................................................................47
Table 5-2: Summary of coating thickness measurements as determined by weight. ...................49
Table 5-3: Summary of coating thickness measurements by method. ..........................................51
Table 5-4: Special measurements of localized red rust. ...............................................................52
x
xi
LIST OF FIGURES
Figure 2-1: Typical one-stage wall detail. ......................................................................................5
Figure 2-2: Typical two-stage wall detail. ......................................................................................7
Figure 2-3: Conceptual model of oxidation of steel in soil contact. ...............................................9
Figure 2-4: Corrosion failure of an Idaho MSE wall. (Armour, et al., 2004) ..............................10
Figure 2-5: Aggregation of Romanoff's (1957) zinc and galvanized steel samples. ....................15
Figure 2-6: AASHTO corrosion rates assuming spec coating thicknesses of 86 μm (3.4 mil,
or 2.0 oz/ft2). .............................................................................................................16
Figure 3-1: Typical extraction site. Note close-up of coupon in place in access hole. .................21
Figure 3-2: Wire corrosion coupon extraction device. .................................................................24
Figure 3-3: Extraction device force calibration. ............................................................................26
Figure 3-4: Extraction process. .....................................................................................................28
Figure 4-1: Peak pullout force normalized by coupon embedded length by wall type. ...............34
Figure 4-2: Pullout resistance normalized against embedded length and surcharge height. .........34
Figure 4-3: Coupon pullout resistance versus displacement.........................................................37
Figure 5-1: Observed coupon conditions. Top: Mechanical damage. Center: Heavy zinc
oxidation. Bottom: Little to no zinc oxidation. .......................................................43
Figure 5-2: Observed coupon conditions. Top: mechanical damage and spalling due to
installation of coupon. Center: spalling and heavy zinc oxidation. Bottom:
typical light, even oxidation of zinc. ........................................................................44
Figure 5-3: Coupon segmentation. ................................................................................................45
Figure 5-4: Localized red rust (before zinc stripping) mostly due to mechanical damage of
coupons. This image represents a little more than half of the observed red rust
locations. The complete photos including wall identification numbers can be
found in the Appendix. .............................................................................................53
Figure 6-1: Coating thicknesses of all samples and mean coating thickness for each coupon. ....56
xii
Figure 6-2: Influence of overburden height on coating thickness. ...............................................58
Figure 6-3: Overburden height versus coating thickness neglecting outlying data point. ............59
Figure 6-4: Normalized coating thickness histogram comparing one and two-stage walls. ........60
Figure 6-5: Normalized peak pullout force versus mean coating thickness. ................................62
Figure 6-6: Coating thickness versus pullout resistance. Note distinction between one and
two-stage walls. The trend line and correlation coefficient are for the one-stage
walls only. ................................................................................................................63
1
1 INTRODUCTION
Mechanically stabilized earth (MSE) retaining walls are in use in many locations
throughout the state of Utah, frequently in freeway or other transportation applications. They are
commonly used alongside approach ramps to overpasses or at other locations where an elevation
differential exists between a roadway surface and the surrounding surface grade. The use of this
type of wall allows vertical soil retention to great heights, exceeding 100 ft in some applications.
Additionally, the walls are often modular allowing great flexibility in design.
MSE walls are built as a system with three main components acting in concert to provide
structural support to an otherwise unstable or excessively steep slope. Although differences in
wall systems exist, this report will focus on the types of MSE panel (i.e. not modular block) wall
systems used by the Utah Department of Transportation (UDOT). The three main components
are: a structural face which may be pre-cast concrete panels (in the case of one-stage walls) or a
geo-textile face supported by welded wire mesh (in the case of two-stage walls); structural fill,
carefully selected, prepared, and placed; and soil reinforcement (galvanized steel welded wire
mesh for all walls in this study – metal straps or strips are also used) buried in the structural fill,
connecting the structural face to the fill. The integrity of the wall system depends wholly on the
integrity of the reinforcement.
The greatest threat to steel soil reinforcement is corrosion. For this reason, the
characteristics of the structural fill as well as the reinforcement itself are carefully controlled.
However, while numerous models have been developed to predict corrosion in galvanized steel
2
subject to earth contact, there is no local model or data specific enough to accurately predict the
long-term performance of the soil reinforcement in use in the MSE walls in UDOT’s inventory.
The purpose of this project was to assess the current condition of integral reinforcement
of UDOT’s MSE walls and to determine actual corrosion rates. This objective was
accomplished by retrieving, visually inspecting, and carefully measuring extractable corrosion
coupons embedded in the structural fill of MSE retaining walls located throughout Salt Lake
County, Utah. These walls were constructed in 1998 and 1999 during the expansion of I-15 in
Salt Lake County which occurred prior to the 2002 Olympic Games, therefore all extracted
coupons are similar in age. The American Association of State Highway Traffic Officials
(AASHTO) specifies corrosion rates for MSE walls in transportation applications which are used
in design to size corrodible elements. By comparing actual corrosion rates and AASHTO design
corrosion rates, one can validate the design life of the walls.
Additionally, since all coupons were removed from walls substantially similar in age and
construction methods, this report can serve as a comparative baseline that may be used in
conjunction with future extractions and measurements of neighboring coupons from the same
walls to further substantiate a locally applicable corrosion predictive model.
In addition to providing corrosion rate information, another outcome of this study was the
development of a process for the extraction of corrosion coupons from two-stage walls.
3
2 BACKGROUND
2.1 MSE Wall Construction Basics
MSE walls are built as a system of several discrete but interdependent components. At
the most basic level, the system consists of modular panels restrained by reinforcement
embedded in compacted structural fill material. Two basic wall types will be considered in this
study: one-stage and two-stage walls.
2.1.1 One-Stage Walls
A one-stage MSE wall system is a simple system with few components and a simple
construction sequence. One-stage walls are quick to build due their simplicity but have limited
tolerance for significant differential settlement (design limits typically specified as about 1%, or
1 foot in 100 feet) (Elias, 2001). This makes this type of wall unsuitable for applications where
appreciable post-construction consolidation is likely due to the surcharge retained by the wall.
The first component of a one-stage MSE wall consists of a leveling foundation. This acts
not so much as a structural support, as there is little gravity load, but as a placement guide for the
panels that will follow. A proper foundation locates the panels in the correct alignment for the
wall and assures they are square and level to each other.
4
The panels form the cosmetic face of the wall and are the primary method of soil
retention in one-stage walls. The panels are typically pre-cast concrete (typical of the walls in
this study) and modular, allowing placement in a variety of configurations. They are placed in a
pattern as specified by the wall manufacturer. Typically, the panels interlock via tongue and
groove profiles on their edges. They are set and securely braced one course at a time until
backfill is fully placed and compacted.
The panels are usually capped by a concrete element called a coping. This provides a
structural edge which locks the panels together at the top of the wall and also presents a pleasing
finish to the wall.
The next component is the reinforcement. Its purpose is to couple the panels to the soil,
unifying the whole system. This component is made of either galvanized steel or some type of
geo-synthetic. All walls evaluated in this study have a W-11 (3/8‖ diameter) galvanized steel
welded wire mesh for reinforcement. The reinforcement is attached to the back side of the
panels with a loop and pin connection. The reinforcement extends back into the soil, anchoring
the panels by means of soil friction and/or cohesion. The reinforcement is placed concurrently
with the soil and unifies the entire system.
The final component is the structural fill. It is carefully prepared to meet gradation,
moisture, and chemical properties (a more complete discussion of the specific structural fill
requirements follows later in this report). The soil is placed and carefully compacted in layers or
lifts with reinforcement interspersed at specified intervals, both horizontal and vertical. Panels
are not placed in succeeding courses until the soil is backfilled to nearly the top of the preceding
course. Figure 2-1 shows a typical cross-section of a one-stage MSE wall.
5
Figure 2-1: Typical one-stage wall detail.
Care must be taken when constructing one-stage walls. Although they are more tolerant
of settlement than cast-in-place concrete walls, they can experience cracking or buckling with
large differential settlement. The subsurface below the foundation must be carefully prepared to
ensure panels remain in place and properly aligned throughout the life of the wall.
6
2.1.2 Two-Stage Walls
Two-stage walls are somewhat more complicated and difficult to construct than their one-
stage counterparts. Their primary advantage is that they are not as sensitive to differential
settlement as one-stage walls.
Two-stage walls are constructed somewhat differently than one-stage walls. A two-stage
wall has two façades – the modular concrete panel face visible upon completion of the wall and a
second, inner face that forms the actual soil retention layer. The erection of this inner face (the
first stage) – (consisting of a geo-textile fabric supported by a W-11 (3/8‖ diameter) galvanized
steel welded wire mesh similar to that of the horizontal reinforcement within the soil mass) —
proceeds in much the same fashion as the one-stage wall construction previously described. Soil
is placed and compacted in lifts and the fabric and mesh are successively installed, keeping pace
with the soil. Only once the first stage is completed will the outer façade be placed on its
foundation and tied back to the first stage face.
There is an air space between the inner structural wall and the outer cosmetic wall. This
space was observed to be on the order of two feet in the two stage walls evaluated in this study.
Figure 2-2 shows a typical cross-section of a two-stage MSE wall.
It is significant that the flexible first stage face cannot be braced as securely as the
concrete panels of one-stage walls. This flexibility presents a challenge to compaction
equipment operating near the face. For this reason the backfill of two-stage walls cannot be as
highly compacted near the face.
The value of this type of wall system is that the first stage can be installed over
foundation materials subject to significant settlement or consolidation without failure of the
7
Figure 2-2: Typical two-stage wall detail.
system. The physical flexibility of the two-stage system allows more settlement to occur without
the damage which would occur in one-stage systems. The deformability of the soil-mesh unit
allows the retained soil to be used as surcharge to consolidate the underlying strata. The system
can tolerate significant differential settlement as well as total settlement.
8
Both types of MSE walls act as unified systems, with soil, reinforcement, and panel
acting in concert. If any element fails so too fails the wall. This report is concerned with the
failure of the reinforcement and of the first stage mesh of the two-stage walls.
2.2 Corrosion Basics
Oxidation of metallic elements such as zinc or steel in soil contact is an electro-chemical
process influenced by several factors including soil type and condition, metal type and condition,
soil moisture content, differential compaction, and the presence of salts in the soil. The chemical
reaction transfers electrons, water and oxygen from the electrolytic soil into the embedded basic
metal reinforcement, creating oxides.
As explained by Beavers and Durr (1998) and shown in Figure 2-3, iron atoms in the
steel are oxidized in an anodic reaction, thus losing electrons. Components of the soil (i.e.,
minerals, water, air, and possibly organic material) are reduced in a cathodic reaction, thus
gaining the previously lost electrons. The resulting current flow in the steel (defined as being
from positive to negative charge, opposite the direction of electron flow) is from the cathode to
anode, and an equilibrating current flow from anode to cathode is established in the soil, thus
forming a corrosion cell. The iron ions produced by oxidation of the steel will react with
components in the soil to form corrosion products, including red rust). Stray electrical currents
can increase the corrosion rate. Corrosion is most likely to occur at or just above the water table
in disturbed, non-uniform soils having low resistivity and/or high soluble salt content. It is
important to note that the generated oxides tend to bond to the surface metal, creating a residual
protective coating that resists further oxidation; hence, corrosion rates vary significantly with
time as oxidation occurs.
9
Figure 2-3: Conceptual model of oxidation of steel in soil contact.
Loss of section due to excessive oxidation is a primary failure mode of MSE wall
systems with metallic reinforcement. It can be mitigated by careful preparation of the backfill
prior to placement and proper erection of the system. Recent failures of MSE wall systems in
Nevada highlight this need for due vigilance in inspection and control in the construction of
these walls (Thornley, et al., 2010).
Figure 2-4 shows an MSE wall failure caused by corrosion of the steel reinforcement.
This particular failure in Idaho, involving one of the earliest MSE walls constructed in the United
States, was determined to be caused by corrosive backfill not meeting AASHTO requirements
(Armour, et al., 2004).
10
Figure 2-4: Corrosion failure of an Idaho MSE wall. (Armour, et al., 2004)
2.2.1 Gradation of Structural MSE Fill
MSE walls are created using placed fill with a carefully controlled grain size distribution.
For many highway projects in the United States, gradation is to comply with AASHTO T-27
which specifies no particles larger than four inches, less than 60% passing no. 4 sieve, and less
than 15% passing no. 200 sieve. Additionally, the plasticity index shall not exceed 6. The MSE
design guide put out the Federal Highway Administration (FHWA) recommends 100% passing
the ¾‖ sieve (Elias, et al., 2001). This assures an easily compactable soil that is relatively free-
draining and stable during freeze-thaw cycling.
2.2.2 Moisture Content & Resistivity
Water acts as an electrolyte increasing conductivity by facilitating the transfer of
electrons between the metal and the soil. All else being equal, soils with moisture contents of
between 60 and 85 percent exhibit the highest rates of corrosion. This is quantified by
measuring the opposite of conductivity, resistivity. AASHTO requires resistivity values of
11
greater than 3000 ohm/cm with this value considered as being ―mildly corrosive‖ (Elias, 2000).
Ideally, resistivity will be higher than 3000 ohm/cm. Lower values have been shown to increase
rates of corrosion (Elias, 2000).
2.2.3 Dissolved Salts Content
The presence of certain dissolved salts within the soil enhance its electrolytic
conductivity and hence increase corrosivity. Research published by the FHWA has identified
sulfates and chlorides as being chief among these. This research cites limits for these chemicals
as follows: sulfates, 200 ppm; and chlorides, 100 ppm. Soils with high sulfate concentrations
tend to be very acidic while high concentrations of chlorides attend alkaline soils. (Elias, 2000)
2.2.4 pH
Highly acidic and highly alkaline soils exhibit higher corrosivity than soils of more
neutral pH. This is due to the electrochemical nature of corrosion – soils with high electro-
negativity as well as high electro-positivity facilitate electron exchange with materials of lower
electro-potential.
For this reason, AASHTO T-289 specifies an acceptable pH range from 5 to 10 for
structural MSE wall fill. Fill materials with pH lower than 5 are considered acidic and
inordinately corrosive to metallic elements. Materials with pH higher than 10 are alkaline and it
is believed that zinc galvanization will oxidize at an elevated rate (Elias, 2000).
12
2.2.5 Galvanization
Coatings of various types have been used to mitigate the high rate of oxidation of bare
steel in soil contact. These include galvanic coatings (chiefly zinc and zinc alloys), epoxies, and
other non-metallic coatings.
Galvanic coatings protect steel in several different ways. Zinc has a low rate of oxidation
relative to bare steel in standard MSE backfill. This protects the structural steel section by
providing a slowly eroding barrier to corrosion. Because of the electrochemical properties of the
material, zinc corrodes preferentially to steel. This is known as cathodic protection. Because of
the cathodic protection exposed steel will oxidize at a much slower rate in contact with zinc than
it would alone. The byproducts of zinc oxidation form a tough protective layer on the steel even
after the zinc is consumed, providing further protection against corrosion.
Non-metallic coatings provide barrier protection against corrosion. This is the only
protection they can provide and this protection depends wholly on complete integrity of the
coating. Even small breaches in the coating render that area unprotected and can allow
significant localized corrosion or pitting to occur. Mere contact with the gravel present in
approved backfill would likely prove sufficiently damaging to this kind of coating to breach its
integrity. Another disadvantage to epoxy coatings is their relative smoothness. Since the
purpose of MSE reinforcement is to provide frictional pullout resistance this type of coating is
generally inappropriate and rarely used.
All the walls in this report use zinc galvanized reinforcement. Documents produced by at
least one of the wall manufacturers supplying the walls studied in this report specified a
galvanizing coating thickness of 2.0 oz/ft2, which equates to 86 μm or 3.4 mil per surface (VSL
13
Corp. unpublished manufacturer’s specifications). This is consistent with the requirements of the
FHWA MSE Wall design manual (Elias, et al., 2001).
2.2.6 Design Life
The walls in this study have a design life of 75 years. Some metal loss is anticipated over
the life of the wall. The design of the reinforcement assumes this and the design section is
initially oversized beyond what is required to develop its full capacity to allow loss in diameter
and a corresponding loss in strength without failure in the future.
It is critical that these calculations are sufficiently conservative to prevent catastrophic
failure of the reinforcement and the wall itself. This is the motivation behind the corrosion
models which have been created. At the same time, the reinforcement needs to do its job
efficiently without excessive material to reduce costs.
2.2.7 Corrosion Models
Melvin Romanoff (1957) produced the pioneering treatise on underground corrosion,
published in 1957 by the National Bureau of Standards. His work has been the basis for many of
the existing corrosion models in use today, including the one used by AASHTO for MSE wall
design.
Romanoff proposed an exponential formula to predict either thickness or weight loss of
various metals in soil contact. This formula is shown as Equation 2-1:
nX kt (2-1)
14
where X is the amount of material (weight or thickness) lost, k and n are constants dependant on
soil condition and metal type, and t is time with units of years, representing the duration of
contact between soil and metal.
Romanoff calculated n values of 0.5 to 0.6 and k values of 150 to 180 μm (4.8 to 5.9 mil,
or 2.8 to 3.5 oz/ft2) for bare steel samples. He did not determine constants for pure zinc or
galvanized steel although he did report on tests of both materials. I have aggregated some of his
results for zinc and galvanized steel samples into Figure 2-5. The galvanized samples
represented in the figure are galvanized steel pipe samples with an average zinc coating thickness
of 2.82 oz/ft2 (4.8 mil, or 120 μm). This data set is represented because the coating thickness is
representative of the reinforcement coatings in this study. The data points in excess of this value
are assumed to include steel loss in addition to all zinc.
Romanoff’s tests were performed primarily on pipe specimens in a wide range of soil
conditions from relatively clean sand with good drainage to loam to muck. A large majority of
the soils he tested would be considered marginal to poor for MSE wall construction based on
AASHTO standards.
Others have proposed constants appropriate to galvanized steel in normal MSE wall
backfill. As reported by Elias (2009), Darbin, et al. (1986) suggest n value of 0.60 for
galvanized steel and k values between 3 and 50 μm (0.07 to 1.16 oz/ft2, or 0.1 to 2.0 mil). The n
value becomes 0.65 to 1 after depletion of the zinc coating.
15
Figure 2-5: Aggregation of Romanoff's (1957) zinc and galvanized steel samples.
2.2.8 AASHTO Design Corrosion Rates
AASHTO’s design corrosion rates are based on Romanoff’s (1957) work with buried
metallic samples. They are liberal by comparison to the observed corrosion rates from those
studies but AASHTO’s rates consider the quality of the backfill used to create MSE walls.
AASHTO further specifies the qualities affecting the corrosivity of acceptable backfill as
described previously in Sections 2.2.1 through 2.2.4.
The AASHTO metal loss model is a linearized adaptation of Romanoff’s exponential
model. The design rates assume high initial zinc loss until the surface is fully coated in a
protective zinc oxide layer. This initial loss rate of 15 μm (0.59 mil, or 0.35 oz/ft2) per year is
16
assumed to occur during the first two years. Following this high initial loss, the rates decline
significantly to a rate of 4 μm (0.16 mil, or 0.09 oz/ft2) per year until all zinc is consumed.
After all zinc is consumed, steel begins to oxidize. Steel is more reactive with oxygen
than zinc and its predicted corrosion rate is higher but constant. AASHTO predicts steel loss at a
rate of 12 μm (0.47 mil, or 0.31 oz/ft2) per year.
These rates are placed together in Figure 2-6. The results plotted in this figure assume
the AASHTO specified initial zinc coating thickness of 86 μm (3.4 mil, or 2.0 oz/ft2) which is
depleted in 16 years. In the case of the twelve year average that the coupons considered in this
0
100
200
300
400
500
600
700
800
900
0 10 20 30 40 50 60 70 80
Me
tal L
oss
(μ
m)
Time in Service (yrs)
Figure 2-6: AASHTO corrosion rates assuming spec coating thicknesses of 86 μm (3.4 mil, or 2.0 oz/ft2).
study have been in place, there should be a nominal zinc loss of 70 μm (2.8 mil, or 1.6 oz/ft2) and
corrosion on the underlying steel should just be starting. The figure shows that the total design
thickness loss (zinc plus sacrificial steel) should be about 794 μm (31.3 mil) per surface (i.e.
radially, not diametrically, in the case of a round wire) over the 75-year design life of the wall.
17
2.2.9 Atmospheric Corrosion Rates
Two-stage walls have a void space between the first and second stages. The portion of
the reinforcement in the air space should only be subject to atmospheric corrosion and rates
which might be different than those cited in the previous section, however, Elias (2000) suggests
that corrosion rates in free draining soils approximate atmospheric rates. This assessment
assumes that the reinforcement in the air space is not exposed to runoff and/or salt intrusion.
Atmospheric corrosion rates vary widely depending on a variety of conditions. A study
published by the American Galvanizers Association (2000) indicates that long-term atmospheric
corrosion rates in suburban to moderately industrial areas typically range from about 0.04 to
0.06 oz/ft2 (1.7 to 2.6 μm, or 0.07 to 0.10 mil) per year. This is far less than the 15 μm (0.59 mil,
or 0.35 oz/ft2) per year initial zinc loss rate prescribed by AASHTO and, at the upper bound,
approaches the reduced rate of 4 μm (0.16 mil, or 0.09 oz/ft2) per year used in the AASHTO
model after the first two years.
2.3 MSE Wall Corrosion Studies by Others
MSE walls are in widespread use throughout the United States. As such, studies similar
to this one have been performed by a few departments of transportation (DOT’s) where MSE
walls are in service. For example, the Kentucky DOT published a survey of corrosion of MSE
reinforcement similar in scope to this one (Beckham et al., 2005). These studies included
installation and monitoring of corrosion coupons as well as electronic testing. They also tested
samples of reinforcement from demolished walls that had been in service up to twenty years. In
all cases corrosion was significantly less than predicted by the AASHTO model.
18
Of particular note are studies by the Nevada DOT because the climate is so similar to that
of Utah. Thornley, et al. (2010) reported on several recent premature failures and near-failures
of MSE walls attributed to corrosion. These failures and accelerated corrosion rates were
determined to be caused by backfill not meeting AASHTO requirements. The walls
experiencing failure were reinforced with bare steel reinforcement.
In a report sponsored by the Indiana Department of Transportation, Bobet (2002)
reported on performance of a wide range of retaining walls in use in Indiana. His assessment
was that corrosion was not a problem in the walls considered in his report.
An investigation into the long-term performance of MSE reinforcement in Florida (Berke
et al., 2008) determined the average coating thickness and corrosion rates for 10 sites ranging in
age from 10 to 23 years old. Similar to the samples in this study all samples had intact
galvanization measuring at least 2.0 mil (1.2 oz/ft2, or 51 μm). Red rust was described as absent.
In some instances the zinc had nearly fully converted to oxidation products.
The Association of Metallically Reinforced Earth (ASME, 2006) has aggregated a
number of corrosion studies similar to the ones mentioned previously (with some overlap) to
develop a revised corrosion model that is less conservative than the AASHTO model for MSE
walls. Their conclusion is that given properly prepared backfill meeting AASHTO requirements
for gradation, resistivity, and chemical constituents actual corrosion rates when zinc is present
and intact should be less than 1.0 μm/yr (0.02 oz/ft2/yr, or 0.04 mil/yr).
19
3 FIELD DATA COLLECTION
3.1 Extractable Coupons
As a corrodible metallic element in contact with soil, galvanized steel MSE wall
reinforcement is subject to degradation over time. Periodic inspection and assessment are
required to assure continued integrity against unanticipated loss of design section. Since the
reinforcement is concealed behind the wall and beneath the soil, non-destructive visual
inspection and sampling is not possible. In addition, removal of any in-service reinforcement
would compromise the integrity of the system. Therefore removable wire coupons are often
placed in the structural fill of the walls during construction.
These coupons are not integral to the structure of the walls and were made to be
removable from the exterior of the wall system with no damage to the integrity of the wall. They
are of necessity and by design made from the same type and size wire as that found in the actual
reinforcement.
In this study, 22 corrosion coupons were extracted from 19 different walls along or near
I-15 in Salt Lake County, Utah. One-stage and two-stage walls are represented with 15 and
seven samples, respectively. The walls sampled vary in exposed height (actual height including
burial was not discernable without construction documents or exposing the leveling pad) from
less than five feet to over 35 feet.
20
The coupons were placed in groups of six at each location so that they could be
successively removed at periodic intervals during the life of the wall to assess the condition of
the reinforcement at that time and location. Some tall walls (in excess of 10 to 12 feet) have
multiple groups of six coupons each installed at differing heights. It is anticipated that future
extractions will compare the coupons removed to their neighboring coupons as a way to further
assess the rate of corrosion experienced by the wall reinforcement. The samples were located at
various heights from the grade at the bottom of the wall and beneath varying amounts of
surcharge. It was difficult to accurately measure the surcharge height at most of the sample
locations due to accessibility of the sites and sloping ground above the top of the walls so this
measurement was taken to be wall height above coupon extraction site – the rationale being that
although the surcharge height is not 100% accurately represented, a sufficiently valid
comparison of in-situ conditions between coupons is presented.
A summary of the coupon location, wall type, wall number, and height measurements, is
presented in Table 3-1. The coupons are listed in the table by the number of the wall from which
they were taken. This number is a UDOT designation. The coupon number was assigned to the
specific coupon for convenience in this study as a brief identification number corresponding to
the order the coupon was removed from its respective wall. The coupon group location is
identified in the table by the nearest traffic intersection and distance from the end of the wall at
the named intersection to the coupon grouping. The cardinal directions (e.g., NW) listed in the
table refer to the corner of the bridge abutment and/or side of the freeway nearest the coupons.
The coupon is identified within the set of six by the row of circles, the darkened circle
representing the extracted coupon as viewed facing the wall. The general orientation is indicated
21
by N, S, E, or W, representing the cardinal wall orientation. The height of overburden is also
represented here as the distance from the coupon to the top of the wall coping.
The coupons were made accessible for removal by means of access holes in the front of
the walls, plugged to seal out weather and to prevent tampering. A typical extraction site
showing all six plugged access holes is shown in Figure 3-1. One-stage walls have an inner plug
at the back face of the wall to prevent soil from collecting in the access hole (visible in inset).
Figure 3-1: Typical extraction site. Note close-up of coupon in place in access hole.
22
3.1.1 Specifications
By design, these wire coupons should be made of the same material, have the same
section, and receive the same anti-corrosion treatment as the actual soil reinforcement. In this
case, the design specification called for the coupons to be made of W11 steel wire (nominal 3/8‖
diameter), hot-dipped in zinc to a coating thickness of 2.0 oz/ft2 (3.4 mil, or 86 μm) (VSL Corp.
unpublished manufacturer’s specifications).
To be most meaningful, the coupons should have been carefully inspected and measured
at the time of installation, with the measurements recorded in a permanent and accessible
manner. This would allow easy comparison of existing conditions on pullout. Unfortunately, we
were not able to locate any such measurements during this study. The measurements in this
report may serve as a benchmark for future studies in the absence of initial condition data.
3.2 Design of Extraction Device
Extraction of the wire coupons tested in this project required the design and construction
of an apparatus capable of withdrawing the coupons from the structural backfill wherein they
were installed and buried. The device needed to be simple, portable, free of a tethering power
source, and capable of exerting sufficient force to pull the coupons with up to an anticipated 10
feet of embedment in the structural backfill. Further, the device would need to be transported up
to 3000 feet over uneven terrain without the use of heavy equipment. This necessary portability
was satisfied by building the unit such that it could be disassembled and reassembled on site.
The completed device consists of two main parts and is shown to scale in Figure 3-2 with
individual component callouts. The first part of the device is a force distributing frame
23
Table 3-1: Summary table of extracted wire coupons.
Wall #
Coupon
ID #
Intersection and Quadrant and Distance
from End of Wall (ft)
Wall
Stage
(1 or 2)
Position of
Extracted Coupon
Wall Height
Above
Coupon (ft) Year Built
R-343-7-A 6 7200S and I-15 SW (60) 1 N ● ○ ○ ○ ○ ○ S 35.0 1999
R-343-13-A 8 7200S and I-215 Ramp NW (100) 1 N ○ ○ ○ ○ ○ ● S 11.6 1999
R-343-37-A 7 7200S and I-215 Ramp SE (100) 1 S ○ ○ ○ ○ ○ ● N 12.4 1998
R-343-42-A 22 I-215 WB to I-15 SB Ramp NW (45) 1 N ● ○ ○ ○ ○ ○ S 9.5 1998
R-344-1-A 3 5900S and I-15 SW (250) 1 N ● ○ ○ ○ ○ ○ S 7.3 1998
R-344-1-B 4 5900S and I-15 SW (565) 1 N ● ○ ○ ○ ○ ○ S 6.7 1999
R-344-2-A 1 5900S and I-15 SE (240) 1 S ○ ○ ○ ○ ○ ● N 9.4 1999
R-344-2-B 2 5900S and I-15 SE (490) 1 S ● ○ ○ ○ ○ ○ N 13.2 1998
R-344-4-A 5 5900S and I-15 NW (260) 1 N ○ ○ ○ ○ ○ ● S 15.2 1998
R-344-7-A 15 5300S and I-15 SW (550) 1 N ○ ○ ○ ○ ○ ● S 6.7 1999
R-344-11-A 14 5300S and I-15 NE (200) 1 S ○ ○ ○ ● ○ ○ N 8.5 1998
R-346-8-A 19 3300S and I-15 NW (95) 1 N ● ○ ○ ○ ○ ○ S 11.0 1999
R-345-3-A 16 4500S and I-15 SW (45) 1 N ○ ○ ○ ○ ○ ● S 7.5 1998
R-345-4-A 17 4500S and I-15 NW (75) 1 S ● ○ ○ ○ ○ ○ N 11.9 1999
R-345-10-A 18 4500S and I-15 NE (45) 1 N ● ○ ○ ○ ○ ○ S 17.3 1998
R-346-1C-A 20 ~3500S and I-15 (behind Parts Plus) (175) 2 N ○ ● ○ ○ ○ ○ S 19.5 1999
R-351-9-A 9 I-15 and 400S (@765W) SE (150) 2 S ○ ○ ○ ○ ○ ● N 13.5 1998
R-351-9-B 10 I-15 and 400S (@765W) SE (150) 2 S ○ ○ ○ ○ ○ ● N 8.5 1998
R-351-26-A 13 N Temple and I-15 SE (190) 2 S ● ○ ○ ○ ○ ○ N 18.0 1998
R-351-30-A 12 Argyle Ct (300N) and I-15 NE (60) 2 S ○ ○ ○ ○ ○ ● N 13.1 1998
R-351-34-A 11 400S and UPRR S side (130) 2 W ○ ○ ○ ○ ○ ● E 8.7 1998
R-351-50-A 21 400S and UPRR N side (26) 2 E ● ○ ○ ○ ○ ○ W 13.3 1998
24
Figure 3-2: Wire corrosion coupon extraction device.
25
consisting of 4x4-inch lumber, built to spread the extraction force to a minimum of three other
panels beyond the one containing the sample. The frame was 10 feet tall and 40 inches wide. It
is shown cut away in the figure. It was wheeled at one end to allow rolling through brush and
over curbs.
The second piece, a steel frame housing two 10-ton, 12-inch stroke hydraulic rams, was
designed to attach to and detach from the reaction frame in the field. Attachment height varies
depending on the terrain and the height of the individual coupon to be extracted. Power to these
rams was supplied by a hydraulic hand pump which was removable for portability. The rams’
power was transferred to the wire coupon by means of a C4x12.5 structural steel beam fabricated
to attach to both the rams and to allow the coupon to pass through its center.
The coupons were installed with a 3/8-16 x 2‖ threaded end facing outward. This end,
which was recessed anywhere from 0 to 5 inches behind the wall face, was connected to the
extraction device by means of a series of couplers and threaded rod extensions.
3.3 Calibration
The extraction device was load-tested, gauged, and calibrated (gauge pressure versus
pullout force) in the lab up to 10 kips. Calibration resulted in a nearly linear load curve,
approximated by Equation 3-1 and shown in Figure 3-3.
Pullout Force = 0.0043 kips/psig (3-1)
This calibration facilitated the collection of an additional data set regarding pullout
resistance of the wire coupons.
26
Figure 3-3: Extraction device force calibration.
3.4 Coupon Extraction Procedure
Figure 3-4 shows the extraction procedure in various stages. The wooden reaction frame
was placed in a vertical position against the MSE wall with the hole containing the coupon to be
extracted at the center of one of the bays. The frame was oriented such that the other bay
spanned across the vertical joint between the panel containing the coupon and the adjacent panel.
This location and orientation allowed the load created by the extraction process to be distributed
to four or more panels. The frame was braced in this position to restrain it from movement
during and after the procedure. This was performed by attaching a 2 x 4 ―kicker‖ brace to the
upper portion of the frame, the other end attached to a steel stake or other anchor driven into the
ground.
The extraction device was carefully centered over end of the coupon. Centering the
device prior to extraction was necessary to avoid bending the coupon and to minimize
27
mechanical damage to the coupon from abrasion against the side of the hole in the panel or the
device itself.
A 24 inch extension rod was attached to the end of the coupon by means of a coupler.
The cross-bar was secured to the top of the jacks with the 24 inch rod protruding. Two plate
washers were added over the end of the 24 inch rod, past the cross-bar. These washers were
followed by a high strength nut, tightened by hand to secure the entire apparatus together to the
wall. Hydraulic hoses were attached to the fittings on the jacks to complete the assembly.
Once everything was assembled, a measurement was taken to relate the length of the rod
with the extension of the jacks. This measurement would then be correlated with the pullout
force as reported by the calibrated gauge attached to the pump.
Pumping was performed slowly and as uniformly as possible, avoiding sudden
application of load which would over-stress the coupon and/or over-report the pullout force.
Force readings were taken at least every 6 inches of displacement of the coupon. The peak
measurements (reported in Table 4-1 in Section 4.1) were taken at the maximum gauge reading,
typically occurring just prior to the initial coupon displacement.
On the two occasions when a coupon broke during this process, a peak measurement was
taken at the point of yield as well as at the point when the coupon began to move (both coupons
were extracted on a subsequent attempt after rethreading the coupler). Only the higher
measurement is used.
This set-up made it possible to withdraw a coupon approximately 12 inches. Once the
coupon was extracted to this point, the jacks were retracted and a 12 inch length of pipe was
placed over the 24 inch rod and the nut and washers resecured over the end of the pipe extension.
28
Figure 3-4: Extraction process.
This extension permitted another 12 inches of withdrawal. Subsequent force readings were
taken, again every six inches, as the jacks were extended their full travel.
At the end of the second cycle the coupon was withdrawn by about 24 inches. The jacks
were again retracted and the 24 inch threaded rod removed. The apparatus was reassembled with
the washers placed directly on the wire coupon and secured with the high strength nut. This third
cycle permitted another 12 inches of travel.
Usually by this time the coupons were loosened sufficiently to remove the rest of the way
by hand. If this was not the case, the nut and washers were removed and the pipe replaced over
the end of the coupon for a fourth cycle and another 12 inches.
29
Disassembly of the extraction device was roughly the assembly process in reverse.
Before the site was vacated, the outer plug was reinstalled to seal the hole and to prevent the
addition of atmospheric moisture to the soil. The coupon was marked to indicate which wall it
was taken from as well its assigned coupon number (simply a number indicating the order it was
retrieved).
30
31
4 COUPON PULLOUT RESISTANCE
Gauging the extraction device and monitoring the pressure in the hydraulic lines during
the withdrawal of the coupons provided a data set to use in conjunction with the lab results to
draw additional correlations and conclusions regarding the walls studied.
Only about two-thirds of the coupons were pulled with the extraction device and have
precise pullout resistance values. The remaining third were pulled by hand using a short length
of threaded rod and a piece of pipe as a make-shift ―slide hammer‖. The pullout resistance for
these coupons was qualitatively recorded as ―low,‖ ―moderate,‖ or ―heavy‖ manual effort. These
values were converted to quantitative equivalents by comparing the resistance measured with the
jacking device just as coupons were pulled the last distance by hand and the effort used for the
―low,‖ ―moderate,‖ or ―heavy‖ manual extraction.
4.1 Pullout Resistance versus Coupon Length, Wall Height, and Wall Type
A summary of the peak pullout resistances as measured with the extraction device and as
approximated in the case of manual extraction are shown in Table 4-1. This table also includes
the height of wall above the coupon, the lengths of the coupons, and as the embedded lengths as
calculated by subtracting the length of coupon beyond the retained soil face from the total length
of the coupon. Pullout resistances as a function of displacement for coupons extracted with the
extraction device are shown subsequently in Section 4.2.
32
Table 4-1: Summary of coupon lengths and pullout resistances.
Wall # Coupon ID #Wall Stage
(1 or 2)
Overall
Coupon length
(ft)
Embedded
Length (ft)
Wall Height
Above Coupon
(ft)
Peak Pullout
Force (kips)
R-343-7-A 6 1 6.52 6.02 35.0 2.80
R-343-13-A 8 1 6.51 6.01 11.6 5.38
R-343-37-A 7 1 6.51 6.43 12.4 6.45
R-343-42-A 22 1 10.01 9.57 9.5 2.15
R-344-1-A 3 1 10.31 9.83 7.3 7.10
R-344-1-B 4 1 10.20 9.70 6.7 6.45
R-344-2-A 1 1 6.52 6.08 9.4 0.43
R-344-2-B 2 1 10.06 9.60 13.2 5.38
R-344-4-A 5 1 6.50 6.00 15.2 2.04
R-344-7-A 15 1 10.32 9.82 6.7 2.15
R-344-11-A 14 1 9.93 9.45 8.5 4.09
R-346-8-A 19 1 6.52 6.13 11.0 4.62
R-345-3-A 16 1 7.99 7.61 7.5 0.50*
R-345-4-A 17 1 6.52 6.29 11.9 1.50*
R-345-10-A 18 1 7.88 7.38 17.3 1.00*
R-346-1C-A 20 2 8.00 6.13 19.5 0.65*
R-351-9-A 9 2 7.99 6.41 13.5 0.50*
R-351-9-B 10 2 8.01 6.28 8.5 0.50*
R-351-26-A 13 2 10.03 8.36 18.0 1.50*
R-351-30-A 12 2 10.00 8.58 13.1 0.65*
R-351-34-A 11 2 10.11 8.47 8.7 0.50*
R-351-50-A 21 2 10.30 8.78 13.3 0.54
Note: Extraction forces marked (*) are qualitative approximations attempting to quantify hand effort.
33
It was initially anticipated that, qualitatively, longer coupons (or those with longer
embedment lengths) would generally exhibit higher pullout resistance. However, the weight of
the overburden soil above the coupons also significantly affects the frictional resistance along the
coupon. Because of this, to fairly compare coupon pull-out resistances, one must normalize by
embedment length and height of the wall above the coupon level.
Figure 4-1 shows variation in peak pullout resistance normalized by embedded coupon
length for each coupon. When normalized in this manner, two-stage walls appear to exhibit
lower pullout resistance.
Figure 4-2 further normalizes Figure 4-1 against height of overburden. This chart takes
into account the varying length of embedment between one and two-stage walls as well as the
wide range of amounts of overburden. It shows scatter in the data, however the plot does clearly
show that there is disparity in pullout force between one and two-stage walls.
Three of the coupons with higher normalized pullout resistance (all from one-stage
walls), R-344-1-A, R-344-1-B and R-344-2-B, rubbed against the side of the panel upon
extraction giving artificially high readings. These readings should be somewhat discounted
when interpreting the data.
Additionally, coupons in one-stage walls were installed through a neoprene plug at the
back face of the panel to seal the access hole from soil contact. Because of the void space
present in two-stage walls, this inner plug was neither necessary nor present. This plug added
frictional resistance when the coupons were removed from the soil mass. We experimentally
estimated this additional force to be on the order of 20 to 70 lbs using plugs retrieved during
coupon extraction. Regardless of these factors, the one-stage walls exhibit significantly higher
pullout resistance than two-stage walls.
34
0
200
400
600
800
1000
1200
R-3
43
-7-A
R-3
43
-13
-A
R-3
43
-37
-A
R-3
43
-42
-A
R-3
44
-1-A
R-3
44
-1-B
R-3
44
-2-A
R-3
44
-2-B
R-3
44
-4-A
R-3
44
-7-A
R-3
44
-11
-A
R-3
45
-3-A
R-3
45
-4-A
R-3
45
-10
-A
R-3
46
-8-A
R-3
46
-1C
-A
R-3
51
-9-A
R-3
51
-9-B
R-3
51
-26
-A
R-3
51
-30
-A
R-3
51
-34
-A
R-3
51
-50
-APu
llo
ut
Fo
rce /
Em
bed
men
t L
en
gth
(lb
/ft)
1 Stage Walls
2 Stage Walls
Figure 4-1: Peak pullout force normalized by coupon embedded length by wall type.
0
10
20
30
40
50
60
70
80
90
100
110
R-3
43
-7-A
R-3
43
-13
-A
R-3
43
-37
-A
R-3
43
-42
-A
R-3
44
-1-A
R-3
44
-1-B
R-3
44
-2-A
R-3
44
-2-B
R-3
44
-4-A
R-3
44
-7-A
R-3
44
-11
-A
R-3
45
-3-A
R-3
45
-4-A
R-3
45
-10
-A
R-3
46
-8-A
R-3
46
-1C
-A
R-3
51
-9-A
R-3
51
-9-B
R-3
51
-26
-A
R-3
51
-30
-A
R-3
51
-34
-A
R-3
51
-50
-A
Pu
llo
ut
Fo
rce /
Overb
urd
en
Heig
ht
/
Em
bed
men
t L
en
gth
(lb
/ft/
ft)
1 Stage Walls
2 Stage Walls
Figure 4-2: Pullout resistance normalized against embedded length and surcharge height.
35
The pullout data seems to imply that neither overburden nor length of coupon was the
critical factor in defining pullout resistance, but rather wall type. The pullout force for the two-
stage walls is significantly less than that of the one-stage walls regardless of coupon length or
overburden height. After discounting those coupons which appeared to rub on the back/side of
the access hole during extraction, the average and median peak pullout resistance normalized by
embedded coupon length and overburden height for the one-stage walls is about 34 and
23 lb/ft/ft, respectively. For two-stage walls, the average and median values are about 7 and
6 lb/ft/ft, respectively. These values indicate that the mean pullout resistance for the coupons
extracted from the one-stage walls exceeds that of the two-stage walls by a factor of about four
to five.
This reduced resistance to pullout may be a limitation imposed on this type of wall by the
manner in which these walls are constructed. According to the FHWA MSE wall construction
manual, the structural fill of an MSE wall is to be compacted to between 95 and 100 percent of
maximum dry density (Elias, et al., 2001). The flexible nature of the first stage face in two-stage
walls presents a problem when the workers compact the backfill. Compaction equipment is
limited in how close to such a face it can be used. This limitation results in less effective
compaction near the wall, and thus less frictional resistance. Fortunately, this compaction
problem likely only exists in the outermost structural fill or that nearest the wall face. If we
assume that the poor compaction exists only in say the first three feet of structural fill, this
represents a high percentage of the embedded length of the sampled coupons. A ten foot coupon
with a two foot air space between first and second stages may have less than half of its length
embedded in fully compacted fill. This would result in significantly lower resistance to pullout
as seen in these tests. The reinforcement that the wall actually relies upon for stability is usually
36
much longer than the extracted coupons, with a significant portion of its length in fully
compacted fill, and should thus have adequate resistance to avoid pullout failure.
Additionally, the assumed lower compaction of the two-stage walls would perform
differently under the shear stresses imposed by coupon extraction. The looser fill will exhibit
contractive behavior (or loosening from a pullout standpoint) under shear stress as opposed to the
dilative (or tightening) behavior of the highly compacted one-stage walls. This would also
contribute to the very high pullout resistances seen in some of the one-stage walls as well as the
relatively low pullout resistance of the two-stage walls. However, given the importance of
pullout resistance on MSE wall performance, this issue should be studied more thoroughly.
4.2 Pullout Force versus Distance Pulled
Plots of pullout force versus displacement for 13 of the coupons are presented in Figure
4-3. These plots represent the pullout force data from the extraction process. The remaining
nine coupons were extracted fully by hand; therefore detailed pullout data could not be obtained.
Peak pullout force forms the initial data point for each plot. The second data point,
marked by an empty diamond was taken after an initial displacement of three to six inches.
Recall that force measurements were taken incrementally (at approximately six-inch intervals of
coupon displacement) rather than continuously. A finer graduated curve may have indicated a
steeper initial slope than shown.
Pullout measurements appeared to trend toward a nominal or ―residual‖ force of about
450 lbs after a variable amount of displacement. The accuracy of this measurement was limited
by the precision and sensitivity of the gauge and extraction device at very low values.
37
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-343-7-A-6 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-343-13-A-8 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-343-37-A-7 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-343-42-A-22 Displacement (ft)P
ullo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-1-A-3 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-1-B-4 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-2-A-1 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-2-B-2 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
Figure 4-3: Coupon pullout resistance versus displacement.
(continued on next page)
38
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-4-A-5 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-7-A-15 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-344-11-A-14 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-346-8-A-19 Displacement (ft)P
ullo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
0
2000
4000
6000
8000
0 1 2 3 4 5
Coupon R-351-50-A-21 Displacement (ft)
Pu
llo
ut
Fo
rce
(lb
)
Indicates Coupon Pulled
by Hand after this Point
Figure 4-3: Coupon pullout resistance versus displacement
This value at the end of the extraction was equal to a value anywhere from 10 and 100 percent of
the peak pullout force for a given coupon, with the 100 percent indicating that the was no
difference between the peak and residual values as was the case with those coupons which
rotated in their access holes prior to extraction when the coupler was attached (i.e. R-344-2-A-1
and R-351-50-A-21).
(continued from previous page)
39
These pullout forces are not directly representative of the pullout force used to calculate
reinforcement length in MSE wall design since the coupons differ from the actual soil
reinforcement. The pullout strength of the welded wire mesh soil reinforcement is due not only
to the frictional resistance of the longitudinal wires but also the transverse wires in the wire mesh
(which are not present on the coupons). Additionally, design equations are conservative –
anticipating worst-case conditions and therefore under-predicting capacity and hence pullout.
40
41
5 LABORATORY ASSESSMENT OF COUPON PROPERTIES
The galvanized coating on the coupons was measured by weight by stripping the
galvanization with dilute hydrochloric acid in general accordance with ASTM A90/A90M-95a.
This process allows for the precise measurement of average coating thickness, given accurate
dimensions of the item coated. In this process it is assumed that the coating is evenly distributed
on the coupon (an average is calculated). One cannot simply measure thicknesses before and
after stripping to determine the amount of galvanization present since zinc oxide expands and
therefore differs in density from pure zinc (this behavior was confirmed with manual
measurements made using digital calipers).
Equipment limitations required the coupons to be segmented prior to stripping. This
segmentation had the added benefit of distinguishing between the inner and outer ends of the
coupons for statistical analysis of the coating thickness and determining if a difference was
present in coating thickness depending on where the sample was located in the backfill (being
either nearer or farther from the free face). The coating thickness was also gauged with an
electronic magnetic thickness tester.
Figure 5-1 and Figure 5-2 show some of the conditions discovered on the extracted
coupons. Most coupons had light zinc oxidation as shown at the bottom of both figures. Several
coupons exhibited the heavy oxidation characteristic of the center image in Figure 5-2. A few
coupons showed signs of mechanical damage, apparently from construction or installation.
42
Examples of this can be seen in the top images. A full collection of images of all the coupons
analyzed in this report can be found on the DVD in the Appendix.
Qualitatively, all coupons were in relatively good condition with most or all of the zinc
galvanization intact. Some coupons exhibited localized exposed steel but even these areas had
little to no red rust characteristic of steel oxidation. This low oxidation can be attributed to a
combination of factors including the cathodic action of the zinc coating, the quality of the
backfill, and its free-draining characteristics.
The most extensive area of red rust is shown in the top of Figure 5-2. In this case
(coupon R-351-9-B-10), the coupon was apparently bent during installation and the
galvanization detached from the steel, leaving the steel exposed to the environment in the gap
between the two faces of this two-stage wall.
5.1 Sample Preparation
In order to analyze the difference in corrosion spatially within the structural fill, the
coupons were segmented as shown in Figure 5-3. Coupons were marked at the soil/air interface.
The interface was delineated by the inner plug in the one-stage walls and by the face of the first
stage mesh in the two-stage walls. Each coupon was cut at this mark. A second cut was made
12 inches from the first cut, creating sample ―A‖. A third cut was made 12 inches from the
second cut, creating sample ―B‖. The next cut was made 1-2 inches from the inner end of the
coupon, removing any abnormality in the galvanized coating from the end of the coupon. The
final cut to each coupon was made 24‖ from the previous cut. This created a third sample,
sample ―C‖. Note that samples A and B were made 12 inches long while sample C from the
43
other end of the coupon was cut to 24 inches. The minimum required length per the ASTM
standard is 12 inches.
Each sample was marked with a steel stamp to indicate on the inner end, which coupon it
was taken from. It was stamped on the outer end to indicate which sample it was, A, B, or C.
This careful marking had the added effect of maintaining the in-situ orientation of the coupon on
each sample.
Figure 5-1: Observed coupon conditions. Top: Mechanical damage. Center: Heavy zinc oxidation. Bottom:
Little to no zinc oxidation.
44
Figure 5-2: Observed coupon conditions. Top: mechanical damage and spalling due to installation of
coupon. Center: spalling and heavy zinc oxidation. Bottom: typical light, even oxidation of zinc.
45
Following segmentation, each sample was washed in xylol, which is a volatile organic
solvent, to remove paint, wax, light oxidation, etc. Where necessary, samples were scrubbed
with a terrycloth rag. Following the xylol bath, the samples were rinsed in denatured alcohol to
remove the xylol. The alcohol was allowed to evaporate from the samples prior to further
measurement or treatment.
Figure 5-3: Coupon segmentation.
5.2 Initial Measurements
Precise measurements were critical to accurately assessing the thickness of the remaining
zinc galvanization on the samples. Samples were weighed to a precision of 0.01g. The sample
length was recorded to the nearest 1/16 of an inch. The diameter of each sample was measured
at each end and center with a digital caliper to a precision of 0.0005 in. These diameter
measurements were taken five times at each location rotating the sample approximately 72° or
one-fifth of a revolution between measurements. This was done to identify and average
abnormalities in coating thickness.
46
As previously mentioned, some samples had significant variation in diameter depending
on where the caliper was placed with respect to rotation of the sample. This variation often
appeared to be due to sagging of the coating that developed during application and subsequent
cooling. Additionally, some samples had visible irregularities such as drips, runs, or chips.
Visible irregularities were noted and photographed. Such photographs are in the Appendix.
Following the acid stripping process, each sample was similarly remeasured for weight,
length, and diameter. The final diameter was subtracted from the initial diameter to determine
the coating thickness. Table 5-3 in Section 5.5 shows the galvanization thickness of each coupon
based on the caliper-based diametral difference, as well as the thickness based on the acid-
stripping test and measurements with a magnetic thickness gauge (discussed in the next section).
As expected, the thicknesses calculated by direct diameter difference measurements
showed significantly and unrealistically higher amounts of galvanization than those calculated by
weight. This was largely an effect of the caliper contacts measuring the peaks of the uneven
micro-texture along the galvanization’s surface and the measured thickness including zinc
corrosion products (i.e., zinc oxide) which typically exhibit a bulking factor of about 1.3.
Table 5-1 shows the variation in the bare steel wire. It can be seen that the average
diameter of steel wires is 0.3724 in, as compared to the nominal 0.374-inch diameter for W11
wire. However, several wires appear to have marginally less than the minimum diameter (0.370
in) per ASTM A 82, the specification regarding the fabrication of welded wire mesh.
5.3 Acid Stripping Procedure
In order to precisely determine the amount of galvanization on each sample, the
galvanization was stripped away in general accordance with ASTM A90/A90M–95a. In the
47
process, each sample was immersed in a 50% solution of hydrochloric acid and water for 3 to 5
minutes or until stripping was completed as evidenced by ceasing of bubbling from the chemical
reaction. Once the stripping was completed, the samples were immersed in a distilled water
Table 5-1: Variation in uncoated steel wire.
Outer
Sample (A)
Center
Sample (B)
Inner
Sample (C)
Coupon
Mean
R-343-7-A 0.3696 0.3692 0.3696 0.3695
R-343-13-A 0.3691 0.3690 0.3698 0.3694
R-343-37-A 0.3697 0.3696 0.3698 0.3697
R-343-42-A 0.3708 0.3707 0.3713 0.3710
R-344-1-A 0.3728 0.3725 0.3724 0.3725
R-344-1-B 0.3719 0.3719 0.3724 0.3721
R-344-2-A 0.3693 0.3696 0.3690 0.3692
R-344-2-B 0.3712 0.3706 0.3710 0.3709
R-344-4-A 0.3684 0.3694 0.3694 0.3692
R-344-7-A 0.3722 0.3718 0.3727 0.3723
R-344-11-A 0.3709 0.3707 0.3704 0.3706
R-345-3-A 0.3693 0.3691 0.3691 0.3691
R-345-4-A 0.3782 0.3776 0.3775 0.3777
R-345-10-A 0.3693 0.3686 0.3693 0.3691
R-346-8-A 0.3779 0.3782 0.3779 0.3780
R-346-1C-A 0.3782 0.3779 0.3778 0.3779
R-351-9-A 0.3772 0.3775 0.3785 0.3779
R-351-9-B 0.3777 0.3778 0.3776 0.3776
R-351-26-A 0.3711 0.3713 0.3710 0.3711
R-351-30-A 0.3728 0.3729 0.3723 0.3726
R-351-34-A 0.3728 0.3726 0.3725 0.3726
R-351-50-A 0.3726 0.3719 0.3728 0.3725
Mean: 0.3724 0.3723 0.3725 0.3724
Std Dev: 0.0033 0.0033 0.0032 0.0033
Median: 0.3715 0.3715 0.3718 0.3716
Wall/Coupon #
Bare Steel Wire Diameter (in)
48
rinse to remove most of the acid. They were rinsed in fresh tap water and toweled dry to remove
remaining acid.
The coating thickness was determined using the initial and final weight, the length, and
final diameter of the stripped sample. For this calculation, the mean final diameter was used but
there was little variation in final diameter measurements (see Table 5-1). The thickness in oz/ft2
was calculated using the following equation from ASTM A90/A90M-95a.
i f
f
W WC D M
W (5-1)
This is an empiric equation, therefore it is unit specific. C is the coating thickness in oz/ft2, Wi is
the initial weight, Wf is the final weight, D is the stripped diameter in inches, and M is a constant
equal to 163.
A summary of the thicknesses of all the samples as determined by weight is shown in
Table 5-2. Note that every sample has a mean coating thickness in excess of the specified
2.0 oz/ft2 (3.4 mil, or 86 μm). Note that since C segments are twice as long as A and B segments
they are given twice the weight in assessing the mean coupon coating thickness.
5.4 Secondary Measurements—Magnetic Thickness Gauge
A DeFelsko Positector 6000 digital magnetic thickness gauge was used to make
correlating measurements with the acid stripping weight measurements as shown in Table 5-3.
These measurements were compared to the other thickness calculation methods. They are used
in this report and during the stripping process as a sanity check to confirm the stripping
calculations.
49
Table 5-2: Summary of coating thickness measurements as determined by weight.
Segment A Segment B Segment C Coupon Mean
R-343-7-A 2.57 (4.4) 2.67 (4.5) 3.13 (5.3) 2.87 (4.9)
R-343-13-A 2.82 (4.8) 2.22 (3.8) 2.66 (4.5) 2.59 (4.4)
R-343-37-A 2.68 (4.6) 2.41 (4.1) 2.64 (4.5) 2.59 (4.4)
R-343-42-A 2.88 (4.9) 3.13 (5.3) 2.66 (4.5) 2.83 (4.8)
R-344-1-A 2.76 (4.7) 2.77 (4.7) 2.07 (3.5) 2.42 (4.1)
R-344-1-B 2.83 (4.8) 2.76 (4.7) 2.44 (4.1) 2.62 (4.5)
R-344-2-A 3.01 (5.1) 3.02 (5.1) 3.20 (5.4) 3.11 (5.3)
R-344-2-B 2.93 (5.0) 2.93 (5.0) 2.36 (4.0) 2.64 (4.5)
R-344-4-A 2.33 (4.0) 2.40 (4.1) 2.49 (4.2) 2.43 (4.1)
R-344-7-A 2.53 (4.3) 2.55 (4.3) 2.37 (4.0) 2.46 (4.2)
R-344-11-A 2.81 (4.8) 2.69 (4.6) 2.40 (4.1) 2.57 (4.4)
R-345-3-A 2.72 (4.6) 2.83 (4.8) 2.88 (4.9) 2.83 (4.8)
R-345-4-A 3.11 (5.3) 3.42 (5.8) 3.49 (5.9) 3.37 (5.7)
R-345-10-A 3.00 (5.1) 3.17 (5.4) 3.40 (5.8) 3.24 (5.5)
R-346-8-A 3.61 (6.1) 3.65 (6.2) 3.70 (6.3) 3.67 (6.2)
R-346-1C-A 4.00 (6.8) 4.20 (7.1) 3.56 (6.1) 3.83 (6.5)
R-351-9-A 4.06 (6.9) 4.01 (6.8) 3.53 (6.0) 3.78 (6.4)
R-351-9-B 3.91 (6.7) 3.72 (6.3) 3.42 (5.8) 3.62 (6.2)
R-351-26-A 3.35 (5.7) 3.18 (5.4) 3.49 (5.9) 3.38 (5.7)
R-351-30-A 2.63 (4.5) 2.34 (4.0) 2.84 (4.8) 2.66 (4.5)
R-351-34-A 2.75 (4.7) 2.61 (4.4) 2.43 (4.1) 2.55 (4.3)
R-351-50-A 3.06 (5.2) 3.05 (5.2) 3.12 (5.3) 3.08 (5.2)
Mean: 2.84 (4.8) 2.84 (4.8) 2.79 (4.7) 2.82 (4.8)
Std Dev: 0.29 (0.5) 0.39 (0.7) 0.48 (0.8) 0.41 (0.7)
Median 2.82 (4.8) 2.77 (4.7) 2.66 (4.5) 2.73 (4.6)
Mean: 3.39 (5.8) 3.30 (5.6) 3.20 (5.4) 3.27 (5.6)
Std Dev: 0.60 (1.0) 0.70 (1.2) 0.43 (0.7) 0.54 (0.9)
Median 3.35 (5.7) 3.18 (5.4) 3.42 (5.8) 3.34 (5.7)
Mean: 3.02 (5.1) 2.99 (5.1) 2.92 (5.0) 2.96 (5.0)
Std Dev: 0.48 (0.8) 0.54 (0.9) 0.50 (0.8) 0.47 (0.8)
Median 2.86 (4.9) 2.88 (4.9) 2.86 (4.9) 2.83 (4.8)
On
e-S
tag
eT
wo
-Sta
ge
All
Wa
lls
Coating Thickness by Weight [oz/ft2 (mil)]
Tw
o-S
tag
eO
ne-S
tag
eWall #
50
A magnetic thickness gauge utilizes the ferrous nature of the steel coupon and the non-
magnetic properties of the zinc coating. The tester detects changes in the magnetic field caused
by increased distance between the magnet and the magnetic substrate. The tester is unable to
distinguish between thick zinc layers and heavy oxidation or non-metallic substances between
the tester and the steel. Its accuracy is further hindered by the physical limitations of the probe
itself – the probe can only get as close to the substrate as the tallest bump of an uneven coating
such as hot-dipped galvanization.
The gauge is calibrated by testing an uncoated specimen multiple times and adjusting the
calibration by means of a button or other electronic control until the display consistently reads
zero or very nearly zero. The gauge can also be calibrated by means of spacers of known
thickness but appropriate spacers were not available for this procedure.
5.5 Thickness Results
A summary of the measurements by method is shown in Table 5-3. The table shows
statistically significant variation in the coating thickness measurement means by method. This is
due to a combination of factors. Physical limitations of the caliper only allow measurement of
the micro-peaks of the texture of the coating. Also, the bulking of the oxide inflates the
thickness measured.
The magnetic gauge is sensitive to coating thickness, coating type, and separation
distance between the magnetic core and the tester probe. In an ideal situation mean coating
thickness and separation distance are the same quantity however irregularity in the coating
surface will force the gauge to bridge the high points resulting in a somewhat artificially higher
reading.
51
These different measures are provided for comparison but the weight measurement from
acid stripping is deemed authoritative. This determination is consistent with the guidance given
in ASTM A 90/A 90 M. It will be used as the standard or correct measure for the remainder of
this report.
Table 5-3: Summary of coating thickness measurements by method.
By Weight
Magnetic
Measurement
Diameter
Difference
R-343-7-A 2.87 (4.9) 3.59 (6.1) 4.73 (8.0)
R-343-13-A 2.59 (4.4) 3.22 (5.5) 3.59 (6.1)
R-343-37-A 2.59 (4.4) 3.27 (5.6) 4.90 (8.3)
R-343-42-A 2.83 (4.8) 3.03 (5.2) 4.89 (8.3)
R-344-1-A 2.42 (4.1) 2.88 (4.9) 4.62 (7.8)
R-344-1-B 2.62 (4.5) 2.94 (5.0) 4.01 (6.8)
R-344-2-A 3.11 (5.3) 3.46 (5.9) 4.46 (7.6)
R-344-2-B 2.64 (4.5) 2.97 (5.1) 4.11 (7.0)
R-344-4-A 2.43 (4.1) 3.02 (5.1) 4.30 (7.3)
R-344-7-A 2.46 (4.2) 3.30 (5.6) 4.87 (8.3)
R-344-11-A 2.57 (4.4) 3.13 (5.3) 5.13 (8.7)
R-345-3-A 2.83 (4.8) 3.34 (5.7) 4.77 (8.1)
R-345-4-A 3.37 (5.7) 4.21 (7.2) 5.78 (9.8)
R-345-10-A 3.24 (5.5) 3.81 (6.5) 4.82 (8.2)
R-346-8-A 3.67 (6.2) 4.18 (7.1) 5.98 (10.2)
R-346-1C-A 3.83 (6.5) 4.06 (6.9) 5.77 (9.8)
R-351-9-A 3.78 (6.4) 4.22 (7.2) 5.72 (9.7)
R-351-9-B 3.62 (6.2) 5.67 (9.6)
R-351-26-A 3.38 (5.7) 3.71 (6.3) 5.24 (8.9)
R-351-30-A 2.66 (4.5) 2.97 (5.1) 4.89 (8.3)
R-351-34-A 2.55 (4.3) 2.71 (4.6) 3.96 (6.7)
R-351-50-A 3.08 (5.2) 3.27 (5.6) 5.04 (8.6)
Mean: 2.96 (5.0) 3.39 (5.8) 4.87 (8.3)
Std Dev: 0.47 (0.8) 0.47 (0.8) 0.65 (1.1)
Median: 2.83 (4.8) 3.27 (5.6) 4.88 (8.3)
Wall/Coupon #
Coating Thickness Measurements [oz/ft2 (mil)]
52
5.6 Spot Measurement of Steel Corrosion (“Special Measurements”)
Despite the relatively good condition of the zinc coating, there were a few instances
where the galvanization was breached, exposing the bare steel beneath which exhibited varying
degrees of red rust. Examples are shown in Figure 5-4. These localized areas of corrosion were
identified and measured to ascertain bare steel corrosion rates. Because of the small amounts of
metal loss and the inherent variability in the sample shape due to manufacturing tolerances (see
Section 5.2) the local minimum was subtracted from the adjacent stripped and uncorroded steel
measurement. Figure 5-4 shows most of the instances of localized red rust on the coupons.
Table 5-4 summarizes these measurements. Even areas apparently bare since installation exhibit
very little actual steel loss.
Table 5-4: Special measurements of localized red rust.
Wall #
Sample
#
Steel Loss
(mil)
R-343-42-A 22A 0.3
R-344-11-A 14C 0.6
R-344-1-A 3A 0.9
R-344-1-B 4A 0.3
R-344-7-A 15A 0.6
R-344-7-A 15B 0.6
R-345-4-A 17A 0.2
R-345-4-A 17C 0.4
R-346-8-A 19C 1.5
R-351-9-B 10X 1.0
53
Figure 5-4: Localized red rust (before zinc stripping) mostly due to mechanical damage of coupons. This
image represents a little more than half of the observed red rust locations. The complete photos including
wall identification numbers can be found in the Appendix.
54
55
6 ANALYSIS AND INTERPRETATION OF FIELD AND LABORATORY DATA
The thickness of the galvanization coating for each sample as determined by acid
stripping was analyzed to determine if any significant trends exist leading to conclusions
regarding the vulnerability of the steel reinforcement. In particular, the coating thickness was
plotted against its particular location in the backfill to determine if spatial variation affected
coating thickness. Coating thickness was also plotted against wall type to determine if the
variation in construction methods between wall types led to significant differences in oxidation
rates. Coating thickness was also plotted against pullout to see if there were any correlations
between factors influencing pullout resistance and rates of corrosion.
The weight-based zinc coating measurements shown previously in Table 5-2 (located in
Section 5.3) are shown graphically in Figure 6-1. The specified amount of galvanization for the
MSE reinforcement steel at the time of installation was 86 μm (3.4 mil or 2.0 oz/ft2) per surface,
and it can clearly be seen that in all instances the average amount of galvanization currently
present on each specimen exceeds this amount. Even after acknowledging that the AASHTO
design rates are conservative, it is still surprising that after eleven to twelve years so much
galvanization is present (even exceeding the initially specified amount). Unfortunately, because
the initial galvanization thickness is unknown, we are unable to estimate a reliable corrosion rate
prior to the present time. However, the data describing current conditions can be used as
baseline information going forward to compute corrosion rates in the future.
56
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
R-3
43-1
3-A
R-3
43-3
7-A
R-3
43-4
2-A
R-3
43-
7-A
R-3
44-1
1-A
R-3
44-
1-A
R-3
44-1
-B
R-3
44-
2-A
R-3
44-2
-B
R-3
44-
4-A
R-3
44-
7-A
R-3
45-1
0-A
R-3
45-
3-A
R-3
45-
4-A
R-3
46-
1C
-A
R-3
46-
8-A
R-3
51-2
6-A
R-3
51-3
0-A
R-3
51-3
4-A
R-3
51-5
0-A
R-3
51-
9-A
R-3
51-9
-B
Mea
n:
Zin
c C
oat
ing
Thic
kne
ss (o
z/ft
2 )
Wall Number
Inner Section (C)
Center Section (B)
Outer Section (A)
Mean by Wire
Figure 6-1: Coating thicknesses of all samples and mean coating thickness for each coupon.
To further assess these results, several predictions were made concerning the corrosion
rates. It was initially anticipated that corrosion rates might be higher nearest the top of the wall
because of the infiltration of roadway runoff with its attendant deicing salts and other chemicals
not initially present in the soil. We expected that we might see higher corrosion near the face of
the walls due to an anticipated higher moisture content and exposure to oxygen relative to the
main soil mass. We also expected that pullout resistance would have no relationship with
corrosion rates. Finally, it was anticipated that two-stage walls might exhibit lower corrosion
rates than one-stage walls due to the anticipated overall drying effect of the air space between
stages one and two; however, the first-stage wire mesh face might also be exposed to variable
57
wetting and drying cycles due to surface runoff from the overlying roadway which could lead to
increased corrosion. The following sections comprise an assessment of these assumptions.
6.1 Thickness versus Spatial Location (Coupon Segments A, B, and C)
Coupons were segmented to determine if corrosion rates differ depending on distance
from face of wall. They were cut into smaller sections nearest the air-soil interface giving a
higher resolution in the assumed critical area. Segment A is immediately behind the air-soil
interface, segment B follows immediately after segment A, and segment C is at the far end of the
coupon, deepest into the backfill. A summary of the coating thickness measurements broken out
by segment and wall type was presented in Table 5-2. When all the walls are aggregated
together there is no substantial difference between segment and no definitive trends.
Interestingly, if any trend does exist, the amount of coating seems to increase slightly as you
advance toward the face of the wall; however, the differences are within the range of
measurement error (caliper precision of 0.5 mil). This does not support the idea that the first
stage face material in two-stage MSE walls would undergo higher corrosion due to the
fluctuating moisture content anticipated in this region, nor that elevated moisture contents near
the face of one-stage walls negatively influences corrosion.
Table 5-2 presented previously breaks the data out by wall type and shows mean, median,
and standard deviation data for both types. It can be seen that the one-stage walls exhibit slightly
higher corrosion deeper into the fill although the trend is not significant. The trend is more
pronounced in the two-stage walls. Segment B shows a mean coating thickness approximately
one-tenth of an ounce less per square foot than segment A, and segment C shows an additional
tenth of an ounce per square foot less than segment B.
58
6.2 Thickness versus Overburden Height
Initially, we considered that there might be some difference in corrosion conditions and
hence coating thickness with differing amounts of overburden above the coupons. This was
anticipated since it is likely that moisture content could vary vertically within the profile. Also,
since moisture and deicing chemicals could percolate into the soil from the road surface above,
coupons with less overburden could be surrounded by soil with a higher moisture content and
higher dissolved salts content. The measured coating thickness strictly by overburden is shown
in Figure 6-2. The upward trend is very slight correlating to the assumptions but the large
amount of scatter makes inferring any definite trend impossible. Ideally the comparison should
be made using pairs of coupons from different heights at the same location, but this situation
only occurred once in this study (at Wall R-351-9) and unfortunately, one of the coupons was
damaged which prevented meaningful comparison.
R² = 0.0219
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 5 10 15 20 25 30 35 40
Me
an C
oat
ing
Thic
kne
ss (
oz/
ft2)
Total Overburden Above Coupon (ft)
Figure 6-2: Influence of overburden height on coating thickness.
59
The coupon embedded in wall number R-343-7-A is under nearly twice as much
overburden as any other coupon. This outlying data point severely biases the trend and disrupts
the correlation. Figure 6-3 neglects this outlier and shows a slightly stronger trend suggesting
that increased depth of overburden may positively influence corrosion rates; however the
correlation is quite weak.
R² = 0.1384
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 5 10 15 20 25
Me
an C
oat
ing
Thic
kne
ss (
oz/
ft2)
Total Overburden Above Coupon (ft)
Figure 6-3: Overburden height versus coating thickness neglecting outlying data point.
6.3 Thickness versus Wall Type
Initially, we considered that there might be some difference in zinc loss depending on
whether the MSE wall was one or two-stage. The air space between the first and second stages
in two-stage systems should have a drying effect on moisture content, and by extension, lessen
the rate of corrosion. However, on the other hand, the first-stage wire mesh face is directly
exposed to the atmosphere within the wall cavity and could experience multiple wetting and
60
drying cycles and salt exposure from overhead runoff, leading to increased corrosion similar to
that observed at the tops of steel piles in marine applications.
Figure 6-4 shows a histogram of coating thicknesses by wall type, normalized to account
for the differing number of walls of each type. It can readily be seen that the two-stage walls
tend to have thicker galvanization and by extension, lower corrosion rates. The mean coating
thickness of the two-stage walls is 3.27 oz/ft2 (5.7 mil, or 141 μm) versus 2.82 oz/ft
2 (4.9 mil, or
120 μm) for the one-stage walls. Considering only the ―A‖ samples nearest the face of the wall,
the average coating thicknesses are 2.84 oz/ft2 (5.0 mil, or 120 μm) and 3.39 oz/ft
2 (6.0 mil, or
146 μm) and for the one and two-stage MSE walls, respectively.
0
0.05
0.1
0.15
0.2
0.25
2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9
Pro
bab
ility
(n
= 4
5 fo
r o
ne
-sta
ge
and
n =
21
for
two
-sta
ge)
Coating Thickness (oz/ft2)
One-Stage Walls Two-Stage Walls
Figure 6-4: Normalized coating thickness histogram comparing one and two-stage walls.
Granted that the number of specimens is few (only seven for the two-stage walls) and that
the differences are not extreme, it does seem that more corrosion is occurring in the one-stage
walls. (Alternatively, the reinforcement placed in the two-stage walls could have initially had a
heavier coating, but there is no direct evidence of this). One possible explanation for this was
61
previously stated – that the space between the first and second stages in two-stage systems could
have a drying effect on moisture content, and by extension, lessen the rate of corrosion. Soils
further away from the wall face, as well as those soils concealed behind the first stage wall
concrete facing panel, could retain more moisture and experience more corrosion.
6.4 Thickness versus Pullout Resistance
In Section 4.1, some assessment of pullout resistance versus coupon length, wall height,
and wall type was offered. The pullout resistance will now be examined relative to the thickness
of the galvanization, with the idea that corrosion byproducts (of which there would be more for
the lesser thicknesses assuming that the thicknesses were all initially the same at installation)
form some type of bond with the reinforcement and increase pullout resistance. However, this
doesn’t seem likely and doesn’t appear to be documented in literature. Another possible
contributing factor to the relative difference in corrosion between one and two-stage walls lies
with the degree of soil compaction. As discussed previously in Chapter 4, the two-stage walls
had much lower pullout resistance which was inferred to mean lower relative compaction, at
least near the wall face.
Figure 6-5 shows coating thickness plotted against pullout normalized for length of
coupon and for height of overburden. This plot excludes the three coupons (R-344-1-A, R-344-
1-B, and R-344-2-B) identified in Section 4.1 as being misaligned relative to the access hole and
thereby exhibiting unrealistically high pullout resistance. It shows a loose but discernable
inverse correlation between the pullout and coating thickness. Figure 6-6 breaks the data out into
two sets, by wall type. If only the one-stage walls are considered, where the pullout data is more
62
reliable (larger sample population and fewer coupons extracted by hand), the trend is essentially
unchanged but our confidence in the trend is better.
If relative compaction is a factor in corrosion rate prediction, and if the two-stage walls
exhibit lower compaction near the face of the wall, a trend should manifest itself in coating
thickness trending toward lower thicknesses as the coupon goes further into the soil. This trend
should be more pronounced on the two-stage walls than on the one-stage walls. Referring back
to Table 5-2, there is essentially no trend in the one-stage wall samples with the innermost
sample (sample C) exhibiting only about 0.05 oz/ft2 (0.08 mil, or 2 μm) less coating than the
outermost sample (sample A). The coating thickness difference of 0.19 oz/ft2 (0.3 mil, or 8 μm)
is nearly four times that amount for the two-stage walls. This accounts for 10 percent of the total
specified thickness.
R² = 0.1963
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 20 40 60 80 100 120
Me
an C
oat
ing
Thic
kne
ss (
oz/
ft2)
Peak Pull-out Resistance per Foot of Coupon Length per Foot of Overburden (lb/ft/ft)
Figure 6-5: Normalized peak pullout force versus mean coating thickness.
63
R² = 0.1813
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 20 40 60 80 100
Me
an C
oat
ing
Thic
kne
ss (
oz/
ft2)
Peak Pull-out Resistance per Foot of Coupon Length per Foot of Overburden (lb/ft/ft)
Figure 6-6: Coating thickness versus pullout resistance. Note distinction between one and two-stage walls.
The trend line and correlation coefficient are for the one-stage walls only.
It is believed that this correlation is due to increased ―drainability‖ of the soil as
compaction decreases and void ratio increases. Elias (2000) notes that, ―Well-drained, granular
soils with moisture contents of less than 5 percent are non-aggressive, but drainage decreases
with increasing compaction, leading to marginal increases of corrosion. These theoretical
marginal differences have not been quantified to date.‖ And that as the soil becomes more free-
draining the corrosion model moves toward atmospheric rates. As reported in Section 2.2.9,
atmospheric corrosion rates vary widely depending on a variety of conditions but appear to be on
the order of 0.04 to 0.06 oz/ft2/yr (0.07 to 0.10 mil/yr, or 1.7 to 2.6 μm/yr) suggesting a total zinc
loss of 0.48 to 0.72 oz/ft2 (0.8 to 1.2 mil, or 20 to 31 μm) over the time the walls in this study
have been in use, assuming atmospheric rates apply. This is far less than the 1.64 oz/ft2
(2.87 mil, or 70 μm) total zinc loss rate predicted by the AASHTO model after the first 12 years,
although the AASHTO rate of 4 μm (0.16 mil, or 0.09 oz/ft2) per year after the first two years
does compare more favorably with the aforementioned rates.
64
None of the studies consulted in the compilation of this report discussed the effect of
compaction on corrosion of reinforcement in two-stage walls. Without any baseline data with
which to compare these results against it is difficult to definitively determine the relative
significance of the observed differential corrosion as it relates to the predictive model. It is
recommended that this issue be further studied.
6.5 Comparison to Design Coating Thickness
The coating thickness specified by AASHTO in the FHWA Design Manual for MSE wall
construction is 2.0 oz/ft2 (3.4 mil, or 86 μm). This is consistent with specifications from
documents produced by the wall manufacturer for UDOT’s I-15 Corridor Reconstruction Project.
In no instance was the sampled coating thickness equal to or less than the specified initial coating
thickness. Since the reinforcing and coupons have been in place for approximately 12 years, the
coating should been substantially thinner. According to the AASHTO linearized exponential
zinc loss model, which is considered conservative for less than mildly corrosive soils, the
existing coating should have reduced to 0.4 oz/ft2 (0.7 mil, or 17 μm).
Localized measurements on a coupon occasionally placed the coating thickness at less
than specified but the instances of this seemed to be due not to corrosion, but to spalling or
mechanical damage caused by construction or installation of the coupons themselves.
Without initial data to compare against the thicknesses measured in this project there is
not any real way to assess corrosion rates. However, the data obtained from this study is
important because later measurements may be used in conjunction with this data to begin to
establish definite corrosion rates and develop a local corrosion predictive model. In general, it
seems that the findings of this project are consistent with the findings of other such studies as
65
described in Section 2.3, and that zinc corrosion rates are relatively low in properly prepared
backfill with good drainage.
66
67
7 CONCLUSION
Based on the results of the work presented herein, the following conclusions are made
and recommendations presented:
1. There was little corrosion for all samples as compared to the predicted corrosion
rates and specified coating thickness.
All samples appear to exhibit very low corrosion. The thickness of the
coating exceeds manufacturer’s specifications in all but the smallest localized
areas. Surface oxidation was generally present and widespread but its effects
were primarily surficial. Other studies (Elias, 2009; ASME, 2006; etc.) indicate
that the zinc corrosion by-products form a very durable protective layer that will
degrade at very low rates.
2. Areas apparently bare since installation have little corrosion.
Some coupons were observed to have areas of spalled or mechanically
damaged coating, exposing the steel beneath. In most cases, these areas appeared
to have existed since installation. These bare steel regions exhibited very little
oxidation – on the order of 0.5 mil (12 μm) over twelve years, far less than
predicted by any of the corrosion models referenced in this report.
3. AASHTO corrosion rates seem overly conservative for these MSE walls.
68
The AASHTO model predicts complete zinc depletion at the end of 16
years of embedment. At that point the model assigns steel oxidation rates to
govern the section loss of the reinforcement. After twelve years of embedment
the reinforcement should have only 0.4 oz/ft2 (0.7 mil, or 17 μm) of zinc
remaining. This is far less than the nearly 3.0 oz/ft2 (5.0 mil, or 127 μm) found on
the coupons in this study. While this indicates that the galvanization was applied
thicker than specified by either the manufacturer or AASHTO, it also strongly
suggests that the zinc loss rate is much lower than anticipated by the model.
Since the AASHTO corrosion rates predict much higher zinc losses, it is
the opinion of this author that the AASHTO model is overly conservative for the
MSE walls in this report. This may not be true in differing soil conditions as
evidenced by Romanoff (1957) and Thornley, et al. (2010), but the select backfill
used on this series of walls, coupled with the arid climate where they are located,
is very favorable to the integrity of both the zinc coating and the steel core
material.
4. There was significant difference between the pullout resistance of coupons in one
and two-stage walls.
The pullout resistance data suggests that there is a wide discrepancy
between the compaction of one-stage and two-stage wall fill, at least in the first
few feet of fill. Whether this discrepancy exists beyond the end of the coupon is
unknown. This finding was unexpected and potentially significant to the design
procedures used to calculate the length and capacity of soil reinforcement. It may
suggest that two-stage wall reinforcement must extend further into the backfill to
69
develop the same capacity of a similarly placed piece of one-stage reinforcement.
Further tests should be conducted to determine if this assumed lesser compaction
in two-stage MSE walls is more widespread than just the first few feet of fill, and
also to determine if the strength reduction is retrogressive back into the fill.
5. Differential compaction affects corrosion rates.
The influence of differential compaction on corrosion rates may also prove
significant given more study. This study suggests that highly compacted material
induces higher rates of corrosion. Since the amount of zinc lost is still so low, it
is unlikely that any differences will affect the stability of the wall for many years
— likely not becoming a factor until after the end of the design life. Further study
in conjunction with the data in this report will enable this issue to be explored
further.
6. Data from this project will serve as a baseline for future corrosion assessment of
these MSE walls
The final result of this report is the creation of a baseline for the
development of a site or region specific corrosion model. Future extractions of
the remaining coupons can now more fully predict the service life of the
reinforcement and, by extension, the wall system.
These measurements are useful in their own right to present the current
condition of the reinforcement but they essentially represent a single data point.
Present trends and interpretations may change as time passes and more data is
collected.
70
71
REFERENCES
Allen, T., Christopher, B., Elias, V., and DeMaggio, J. (2001). "Development of the simplified
method for internal stability design of mechanically stabilized earth walls." Olympia,
WA: Washington State Department of Transportation.
Alzamora, P., Enrique, D., and Anderson, S.A. (2009). "Review of mechanically stabilized earth
wall performance issues." Transportation Research Board (TRB), 2009 Annual Meeting
CD-ROM.
American Galvanizers Association. (2000). "Hot dip galvanizing for corrosion protection of steel
products." Englewood, CO.
Armour, T., Bickford, J., and Pfister, T. (2004). ―Repair of Failing MSE Railroad Bridge
Abutment.‖ GeoSupport 2004: Drilled Shafts, Micropiling, Deep Mixing, Remedial
Methods, and Specialty Foundation Systems, ASCE, Orlando, FL., 380-394.
Association for Metallically Stabilized Earth, (ASME). (2006). "Reduced zinc loss rate for
design of MSE structures." White Paper: McClean, VA.
Beavers, J.A. and Durr, C.L. (1998). ―Corrosion of Steel Piling in Nonmarine Applications.‖
NCHRP Report 408. Transportation Research Board, National Academy Press,
Washington, D.C.
Beckham, T. L., Sun, L., Hopkins, T. C. (2005) "Corrosion evaluation of mechanically stabilized
earth walls." Lexington, KY: University of Kentucky.
Berke, B. S., Sagues, A.A., and Powers, R. G. (2008). ―Long term corrosion performance of soil
reinforcement in mechanically stabilized earth walls.‖ Paper 08319, NACE International.
72
Bobet, A. (2002) ―Guidelines for use and types of retaining devices.‖ Purdue University [Joint
Transportation Research Program]. Indiana Dept. of Transportation,
Elias, V. (2000) "Corrosion/degradation of soil reinforcements for mechanically stabilized earth
walls and reinforced soil slopes." Washington, D.C.: National Highway Institute: Federal
Highway Administration: U.S. Department of Transportation.
Elias, V., Christopher, B. R., and Berg, R. R. (2001) "Mechanically stabilized earth walls and
reinforced soil slopes design and construction guidelines." Washington, D.C.: National
Highway Institute: Federal Highway Administration: U.S. Department of Transportation.
Elias, V., Fishman, K. L., Christopher, B. R., and Berg, R. R. (2009) "Corrosion/degradation of
soil reinforcements for mechanically stabilized earth walls and reinforced soil slopes."
Washington D.C.: National Highway Institute: Federal Highway Administration: U.S.
Department of Transportation.
Higbee, J. B. (2002) "Geotechnical Issues with Large Design-Build Highway Projects,"
Transportation Research Record: Journal of the Transportation Research Board, Vol.
1808, No. -1, pp 144-152.
Romanoff, M. (1957) "Underground Corrosion," National Bureau of Standards. Washington
D.C.: U.S. Government Print Office.
Stuedlein, A. W., Bailey, M., Lindquist, D., Sankey, J., and Neely, W. J. (2010) "Design and
performance of a 46-m-high MSE wall," Journal of Geotechnical and Geoenvironmental
Engineering, Vol. 136, p 786.
Thornley, J. D., Siddharthan, R. V., Luke, B., and Salazar, J. M. (2010) "Investigation and
implications of mechanically stabilized earth wall corrosion in Nevada," Transportation
Research Record: Journal of the Transportation Research Board, Vol. 2186, No. 1, pp
154-160.
73
Appendix A
Photographs
The attached DVD contains photographs of every coupon segment, labeled with the wall
number from which it was obtained and its segment letter (A, B, or C for outer, middle, and inner
segment respectively). The DVD also contains site photos identified in photo name with wall
number. There are also some photos relative to the general process, not attached to a specific
wall. Photographs of anomalous coupon conditions are also represented including, notably, the
red rust areas referenced and reported on in Section 5.5.
