Corrosion of steel bridge girder anchor bolts : prepared for Office of Materials and Research, Georgia Department of Transportation

School of Civil and Environmental Engineering
Structural Engineering, Mechanics and Materials Research Report No. 09-1
Corrosion of Steel Bridge Girder Anchor Bolts
Final Report
Prepared for Office of Materials and Research Georgia Department of Transportation GDOT Research Project No. 07-16
Task Order No. 02-42
by Lisa Lindquist, Lawrence Kahn, Preet Singh,
Kimberly Kurtis, and Robert Moser
January 2009

Contract Research Task Order No. 02-42 GDOT Research Project No. 07-16
Final Report:
Corrosion of Steel Bridge Girder Anchor Bolts
Prepared for Office of Materials and Research Georgia Department of Transportation
by
Lisa Lindquist, Lawrence Kahn, Preet Singh, Kimberly Kurtis, and Robert Moser
January 2009
The contents of this report reflect the views of the authors who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views
or policies of the Georgia Department of Transportation. This report does not constitute a standard, specification or regulation.

EXECUTIVE SUMMARY
The research objectives for this project were to explicitly define the anchor bolt corrosion in steel girder highway bridges in the State of Georgia, research solutions to the corrosion problem, and recommend action to the Georgia Department of Transportation. Inspection report data revealed that anchor bolt corrosion was ubiquitous for all environments in Georgia; the problem was reported for 27% of the steel girder bridges throughout the state. Based on a synthesis of the field investigations, bolt failure analyses, laboratory experimental testing, and review of GDOT inspection report surveys, the corrosion of carbon steel anchor bolts is caused universally by concentration cell corrosion. Other corrosion mechanisms of concern are galvanic and crevice corrosion, which are both enhanced by the current bearing design.
Corrosion protection provided through zinc galvanization cannot sufficiently protect the carbon steel bolt for its entire service life. Corrosion potential and cyclic polarization data confirmed that ASTM Type 304, Type 316, Type 2101, and Type 2205 were protected from concentration cell and localized corrosion in the simulated bearing environment. Therefore, it is recommended that the stainless steel anchor bolts of these types be used in future designs and that the bolts should be electrically separated from all dissimilar metals using a Nylon or Teflon washer to prevent preferential corrosion of carbon steel. It is further recommended that the bronze lube plate should be eliminated entirely and that the bearing type should be a reinforced elastomeric bearing. Maintenance of existing sliding plate bearings should include regular cleaning by brushing away debris from the bearing surfaces, and bridges with carbon steel anchor bolts should be retrofitted to provide additional lateral restraint according to current maintenance procedures.
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ACKNOWLEDGMENTS
The Georgia Department of Transportation provided the financial support for this project through Research Project No. 07-16, Task Order No. 02-42. The opinions and conclusions expressed herein are those of the author and do not represent the opinions, conclusions, policies, standards or specifications of the Georgia Department of Transportation or of other cooperating organizations.
Many GDOT employees were very helpful and provided information regarding bearing design, maintenance, and inspection. We specifically acknowledge Paul Liles, Melissa Harper, Lyn Clements, Bill Duvall, Ben Rabun, David Crim, Kerry Wood, and Steve St. John, and we sincerely thank them for their time and expertise.
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TABLE OF CONTENTS
EXECUTIVE SUMMARY................................................................................ii
ACKOWLEDGMENTS...................................................................................iii
CHAPTER 1: INTRODUCTION.........................................................................1 1.1 Problem statement and research objectives..................................................1 1.2 Research approach.............................................................................1 1.3 Significance of research.......................................................................1
CHAPTER 2: STEEL GIRDER BEARING DESIGN, MATERIALS, AND MAINTENANCE IN GEORGIA.....................................................................4 2.1 Bridge bearings.................................................................................4 2.2 Safety issues....................................................................................8 2.3 Bearing materials...............................................................................9 2.4 Maintenance procedures.....................................................................11
CHAPTER 3: CORROSION BACKGROUND....................................................14 3.1 Basics of corrosion...........................................................................14 3.2 Corrosion mechanisms......................................................................29 3.3 Corrosion measurements....................................................................39
CHAPTER 4: LITERATURE REVIEW................................................................47 4.1 Concepts in bearing design..................................................................47 4.2 Corrosion of bridge bearings................................................................59
CHAPTER 5: GEORGIA STATEWIDE CONDITION ASSESSMENT......................64 5.1 Inspection report survey process...........................................................64 5.2 Extent of anchor bolt corrosion in Georgia................................................65 5.3 Bridge age statistics...........................................................................74
CHAPTER 6: FIELD INVESTIGATION REPORTS............................................83 6.1 Inspection methods..........................................................................83 6.2 Bridges in Metro Atlanta area, Georgia DOT District 7................................84 6.3 Bridges in south Georgia, GDOT Districts 4 and 5.....................................93 6.4 Bridges in north Georgia, GDOT Districts 1 and 6......................................99
CHAPTER 7: FAILURE ANALYSIS OF FIELD SPECIMENS...........................101 7.1 Visual analysis..............................................................................101 7.2 Microscopy of failure surface............................................................106 7.3 Analysis of bolt scale......................................................................112
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CHAPTER 8: LABORATORY EXPERIMENTAL TESTING.............................115 8.1 Experimental method................................................................115 8.2 Experimental set-up .................................................................118 8.3 Experimental procedures............................................................128 8.4 Results.................................................................................130
CHAPTER 9: DISCUSSION.....................................................................154 9.1 Causes of anchor bolt corrosion....................................................154 9.2 Role of bearing design in corrosion.................................................158 9.3 Role of maintenance procedures in corrosion....................................159 9.4 Correlation to inspection report data...............................................160
CHAPTER 10: CONCLUSIONS AND RECOMMENDATIONS.........................162 10.1 Summary ............................................................................162 10.2 Conclusions..........................................................................164 10.3 Recommendations...................................................................165
REFERENCES....................................................................................168
VOLUME 2, APPENDICES
APPENDIX A: BRIDGES FROM QUERY OF INSPECTION REPORT DATA.......172
APPENDIX B: ADDITIONAL FIELD INVESTIGATION PICTURES....................280 B.1 Old Dixie Highway over Central of Georgia Railroad, District 7............281 B.2 Lawrenceville Highway over I-285, District 7.............................................298 B.3 Memorial Drive over I-285, District 7..............................................305 B.4 State Route 92 over I-20, District 7...............................................313 B.5 State Route 122 over Little River, District 4....................................320 B.6 US 1 over Satilla River, District 5.................................................331 B.7 State Route 121 over Fishing Creek, District 5.................................335 B.8 State Route 144 over Watermelon Creek, District 5...........................342 B.9 Northern Georgia bridges..........................................................343
APPENDIX C: SPECIMENS OBTAINED FROM BRIDGE DEMOLITION...........346
APPENDIX D: ADDITIONAL DATA FROM EXPERIMENTAL TESTING............356
APPENDIX E: BEARING DESIGN RECOMMENDATION.............................363
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CHAPTER 1: INTRODUCTION
1.1 Problem statement and research objectives Inspections of steel girder bridges throughout the state of Georgia have revealed a
widespread problem: corrosion in the steel superstructure's bearing areas that threaten the structural integrity of many bridges. The predominant component of the bearing assemblies found to be corroding is the anchor bolts fastening the steel girders to the concrete pier caps. The corrosion of these anchor bolts can result in two serious problems: reduction of lateral load bearing capacity of the bridge and increased stresses in bridge components due to immovable bearings. In both cases the safety of the structure is jeopardized under normal and seismic loading. While anchor bolt corrosion is known to exist in Georgia, the extent and specific cause of the problem is still unknown.
The research objectives for this project were to explicitly define the anchor bolt corrosion problem in the state of Georgia, research solutions to the problem, and recommend action to the Georgia Department of Transportation. Specifically, the research aimed to define the extent and cause of anchor bolt corrosion and to gain further knowledge from literature review. The final objective was to recommend bearing design procedure and maintenance actions to abate anchor bolt corrosion in Georgia.
1.2 Research approach The first objective of this research was to explicitly define the nature of the
anchor bolt corrosion problem in the state of Georgia. As a first step, the researcher interviewed key personnel within the state Department of Transportation to understand
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the key bearing design principles, maintenance procedures, and the history of both. The researcher compiled and interpreted inspection data from throughout the state. An analysis of these data provides answers concerning the extent of the problem throughout the state.
To determine the cause of the corrosion, eight key bridges were inspected by the researcher and were used as case studies representing anchor bolt corrosion throughout the state. Metallurgical samples obtained from these bridges were analyzed in the laboratory. Corrosion products from the samples analyzed by x-ray diffraction provided information about the corrosive environment of the bolts, while bolt analysis performed with optical and scanning electron microscopes was used to determine the mode of corrosion per bolt.
Additionally, laboratory experiments were conducted on metallurgical samples from new anchor bolts to analyze the corrosion behavior of the materials. Corrosion potential, cyclic polarization and polarization resistance data of samples in simulated environmental bearing conditions were used to evaluate the corrosion resistance of the current anchor bolt materials and several alternative alloys.
The second objective of this project was to research solutions to abate anchor bolt corrosion in Georgia. An in-depth literature review was performed to determine concepts of bearing design, maintenance, and corrosion. Additionally, the review covered the properties of alternative corrosion resistant alloys considered for use as bolt material.
Finally, the third objective was to recommend best practice solutions for bearing design and maintenance. The recommendations include alternatives in new anchor bolt construction which provide greater corrosion resistance, and include suggestions for
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improved maintenance procedures to address corrosion in existing bridges. A synthesis of the results from the first two objectives was the basis of the recommendations.
1.3 Significance of research The proposed research investigates the nature of anchor bolt corrosion in the state
of Georgia. In the process of the investigation, specific causes and modes of corrosion in bearing assemblies were explored. As a result, specific corrosion mechanisms in anchor bolts of steel girder bridges were determined. The intellectual merit of this research is that in understanding the corrosion mechanisms, engineers will be able to make informed decisions regarding bridge bearing design and material specifications.
The advent of corrosion resistant anchor bolts in bridge bearing assemblies has broad impacts. First, bridges will become overall safer structures when corrosion is eliminated from bearings. Bearings will be able to respond adequately to normal thermal stresses, protecting other bridge components from unnecessary high stresses. Additionally, bridges will maintain lateral load resisting capacity.
Second, corrosion resistant bearing assemblies will ultimately provide longer-lasting structures, which is an economic benefit to the state and its residents. As a result of this research, anchor bolts will be designed for a service life of over 100 years, reducing the need for costly repairs or reconstruction in the bridge's lifetime.
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CHAPTER 2: STEEL GIRDER BEARING DESIGN, MATERIALS, AND MAINTENANCE IN GEORGIA
2.1 Bridge bearings Bridge bearings assemblies function to anchor the bridge superstructure to the
piers and to allow for normal expansion and contraction of the bridge superstructure. Accordingly, the bearing assembly provides resistance against lateral loading transverse to the structure, such as wind, earthquake or vehicle impact loads. At the expansion end of a bridge span, the bearing contains a mechanism for rotational and/or sliding motion to accommodate the thermal movements of the superstructure.
Various types of bearing assemblies are commonly used in Georgia. This research project focused on the corrosion of the anchor bolts in the plate bearing assemblies. A typical current design for a plate bearing is shown in Figures 2.1 and 2.2. The bearing assembly at the fixed end of the span typically consists of two plates: the base plate, which is positioned on top of the pier cap, and the sole plate, which is welded to the underside of the girder flange. An anchor bolt that is embedded in a three-inch diameter hole in the pier cap with non-shrink grout passes through a 1 7/16-inch hole in each plate in the bearing assembly and the flange of the girder, where it is fastened with a washer and nut. At the expansion end of the span, the bearing assembly contains three plates: the base plate and sole plate, and a self lubricating, or "lube" plate. In the expansion bearing the bolt passes through 1 7/16-inch by 2 to 3-inch slotted holes in the lube and sole plates and flange. As the bridge superstructure undergoes thermal expansion or contraction, the girder and sole plate slide on the lube plate, and the anchor bolt's
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position in the slotted holes changes. Figures 2.3 and 2.4 show an expansion and a fixed bearing in good condition.
Figure 2.1: Typical fixed bearing design 5

Figure 2.2: Typical expansion bearing design.
Figure 2.3: Typical fixed bearing in good condition 6

Figure 2.4: Typical expansion bearing in good condition
Under the bridge superstructure, the bridge bearing is exposed to unfavorable environmental conditions. In addition to the moisture in the atmosphere, bearings may be exposed to excess moisture that improperly drains from the bridge deck through deck joints. As moisture drains from the deck it carries aggressive agents, such as chlorides and sulfates, to the bearings. Dirt and debris that collect on the surface of the pier cap around the bearings provide a medium for the highly conductive solution. Often, the moist dirt is packed around the anchor bolt in the slotted hole of the expansion bearing. Figure 2.5 shows the environmental condition of a bearing under a deck joint. Under these conditions, the bearings and anchor bolts are prone to corrosive attack.
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Figure 2.5: Debris around bearing at deck joint.
2.2 Safety issues Products of corrosion can cause bearings to become fixed in one position.
Bearings are designed to allow expansion and contraction of the bridge girders according to thermal conditions. When the girders are restrained from natural movement, high stresses form in the steel girder and the concrete pier cap, and failure in these bridge components may result.
Second, when anchor bolts corrode, the cross-sectional area of the bolt shaft is reduced. Consequently, the strength of the bolt is reduced. In severe cases of anchor bolt corrosion, the bolt may be easily broken off by hand. Figure 2.6 shows an example of an anchor bolt with significantly reduced cross sectional area due to corrosion. The lateral load resisting mechanism of a bridge structure depends on the strength of the anchor bolts; an unsafe structure results when the bolts are corroded. In the event of lateral
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vehicular impact to the bridge, high wind load, or an earthquake, a bridge with a reduced lateral load capacity is more prone to failure.
Figure 2.6: Corroded anchor bolt
2.3 Bearing materials In the bearing design an important factor contributing to the corrosion resistance
of the bearing is the material used for all components. Eleven bridge plans provided by the Georgia Department of Transportation were representative of the construction of steel girder bridges in different eras ranging from the 1930s to present day. A review of these plans provided insight into the historical development of the materials used in the bearing design.
The material composition and mechanical properties of steels used in current plate bearing designs and steels considered for use in future designs are shown in Tables 2.1 and 2.2. Cells in the table in which "..." appears indicates that there are no requirements for that category; cells in which "xx" appears indicates that the category does not apply to the material. The material properties of the structural carbon steel (CS) conform to ASTM A709: "Standard Specification for Structural Steel for Bridges", and the material
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properties of the stainless steel (SS) alloys conform to ASTM 276: "Standard Specification for Stainless Steel Bars and Shapes".

Table 2.1: Material composition of steel alloys used in bearings (ASTM 2006; ASTM 2007).

Steel

Material composition in %

Designation C Mn P

S

Si

Cr Ni Mo N

CS: Grade 36 0.26 ... 0.04 0.05 0.40 xx xx xx xx

18.0- 8.0-

SS: 304

0.08 2.00 0.045 0.030 1.00 20.0 11.0 ... ...

16.0- 10.0- 2.00-

SS: 316

0.08 2.00 0.045 0.030 1.00 18.0 14.0 3.00 ...

4.0-

21.0- 1.35- 0.10- 0.20-

SS: 2101

0.040 6.0 0.040 0.030 1.00 22.0 1.70 0.80 0.25

22.0- 4.5- 3.0- 0.14-

SS: 2205

0.030 2.00 0.030 0.020 1.00 23.0 6.5 3.5 0.20

Table 2.2: Mechanical properties of steel alloys used in bearings (ASTM 2006; ASTM

2007).

Mechanical properties

Steel

Yield Strength, Tensile Strength, Elongation, Brinell Hardness

Designation ksi

ksi

%

number

CS: Grade 36

36

58-80

23

...

SS: 304

30

75

40

...

SS: 316

30

75

40

...

SS: 2101

65

94

30

290

SS: 2205

65

95

25

290

Traditionally, the base plates and the sole plates of the bearing assembly conform to ASTM A709 Grade 36 carbon steel a practice which has been continued in current design. Prior to 1960, the self-lubricating bronze plate was not a part of the plate bearing design. With a few exceptions, a cast bronze lube plate conforming to ASTM B 22 was introduced in bearing designs in the mid-1960s. In current practice lube plates are cast bronze with lubricating oil machined onto them.
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In the design of the bearing, the anchor bolt material has undergone the most significant change. Before 1990, anchor bolts were ASTM Grade 36 carbon steel. Due to the high frequency of anchor bolt corrosion problems, the Georgia Department of Transportation started requiring the use of stainless steel conforming to ASTM A276 Type 304 anchor bolts in the early 1990s. Type 304 stainless steel was chosen because it was the most economical corrosion resistant material available at the time. Aside from the material specification, the other physical properties of the bolts remained the same. The bolts are generally 1 -inch in diameter and 18 inches long. Both the previous carbon steel bolts and newer stainless steel bolts used are swedged, or dimpled, where they are grouted into the concrete, and are threaded at the top 4 -inches, where they pass through the bearing plates. The mechanical properties of the stainless steel anchor bolt were determined to be acceptable by the Georgia Department of Transportation (GDOT). Since the switch to using stainless steel bolts, no attempt has been made to electrically separate the stainless steel nut and washer from the carbon steel girder flange.
Lastly, according to officials at the Georgia Department of Transportation, the majority of anchor bolt corrosion was observed to occur at the interface where the bolt protrudes from the concrete pier cap. In response to this observation the bearing design began to require a neoprene pad to be placed between the concrete and the base plate.
2.4 Maintenance procedures The DOT maintenance department reports that deicers are rarely used, limiting
the threat of chloride contamination of the bearings. However, the maintenance department admits that debris is not regularly cleaned out from around the bearing.
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Debris which is left around the bearings can trap any chlorides and other ions at the site, creating a corrosive environment.
The previous State Bridge Maintenance Engineer, Ben Rabun, informed the researcher on May 22, 2007 that routine bridge inspections have revealed a high number of loose anchor bolts, which is a sign of anchor bolt corrosion. Bearings in which the anchor bolts are no longer functional are repaired by "sleeving". Sleeving a bearing is a process in which steel angles or Z channels are bolted to either side of the bearing to prevent lateral movement. Stainless steel bolts are used for the repair. The durability of this repair has not been quantified by GDOT. Figure 2.7 shows a sleeved bearing in the field, which has been subject to corrosion attack.
Figure 2.7: A sleeved bearing, a repair process in which steel angles are bolted to both sides of the bearing to prevent lateral movement, which has been subjected to corrosion attack.
Other measures are also being taken to replace existing carbon steel anchor bolts with stainless steel bolts. For example, the Georgia DOT is now requiring that for any
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bridge widening or jacking projects, the anchor bolts on the existing structure must be replaced with stainless steel bolts. Also, a bolt replacement method of coring out old anchor bolts through the flange and into the concrete has been proposed by one contractor, but has not yet been attempted.
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CHAPTER 3: CORROSION BACKGROUND
3.1 Basics of corrosion Corrosion is the degradation of material. For a majority of metals and metallic
alloys, degradation is a result of electrochemical reactions between the material and the environment (Davis 2000). In this chapter, the concepts of electrochemical corrosion are presented along with a summary of several specific forms of corrosion and ways of measuring corrosion in metals.
Electrochemical corrosion is the reaction of metals in an aqueous environment involving the transfer of electrons. For this process to occur, a corrosion cell must have four components: an anode and a cathode, and an ionic current path and an electronic path between them (Singh 2008). Electrochemical corrosion proceeds when electrons are transferred from the anode to the cathode along the electronic path, enabling the dissolution of metal ions.
The anode of a corrosion cell is the location on the metal surface where the metal ions undergo dissolution. As the metal goes into solution, electrons are generated. This reaction is called the anodic or oxidation reaction, because the metal species is oxidized in the process according to the following general equation:
M Mn+ + ne- (Jones 1996). The electrons that are released in the oxidation reaction are consumed at the cathode by the cathodic, or reduction reaction. A typical cathodic reaction in neutral solutions involves the reduction of oxygen:
O2 + 2H2O + 4e- 4OH- (Jones 1996).
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Separately, the anodic and cathodic reactions are called half-cell reactions. The electronic path of the corrosion cell through which the electrons flow is
within the metal itself, or through an electrical contact between metals. Meanwhile, the ionic current path of the corrosion cell is through the solution, or aqueous environment, with which the metal is in contact. Current flow between the anode and the cathode through the ionic path is created by the movement of charged ions in the solution; positive ions are attracted to the cathode, while negative ions move towards the anode. (Davis 2000). In laboratory settings a salt bridge connecting the anodic and cathodic half cells replicates the electrolytic solution that naturally connects the half cells in the field. The salt bridge allows the transfer of charged ions between the anode and the cathode.
When all four components of an electrochemical corrosion cell are present corrosion can occur. Yet, whether corrosion will occur or not is dependent on the thermodynamics of the cell.
3.1.1 Thermodynamics The thermodynamics of a corrosion cell are the energy changes the cell
experiences as it undergoes electrochemical corrosion. The free energy changes incurred during electrochemical reactions are the driving force of corrosion, because the cell reacts in order to achieve its lowest free energy state (Davis 2000). By reflecting the free energy changes available in electrochemical reactions, thermodynamics predicts whether corrosion occurs spontaneously and the direction of electrochemical reactions, which determines which half-cell reaction is anodic and which is cathodic. Any reaction which
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produces a negative free energy change, G, is thermodynamically favored (Jones 1996).

However, kinetics of reaction may be controlled by other factors.

When the corrosion cell reaches its lowest free energy state and the reactions

produce no net change in free energy, the system has reached equilibrium (Davis 2000). The equilibrium potential of a reaction, Eo, is related to the free energy change of that

reaction by

Go = -nFEo,

where F is Faraday's constant and n is the number of electrons participating in the

reaction (Jones 1996). The equilibrium potential of a reaction is the characteristic

thermodynamic measurement of a corrosion reaction, because it indicates the free energy

and equilibrium conditions of the reaction (Singh 2008). The equilibrium potential of an electrochemical reaction, Eo, is the sum of the

equilibrium potentials of each half-cell reaction, also known as the single electrode

potentials or half-cell potentials. Half-cell electrode potentials, however, cannot be

measured independently; potential values are in fact potential differences measured with

respect to a specified reference electrode (Jones 1996). A listing of possible half cell

reactions and their associated potentials at standard state with respect to a standard

hydrogen electrode is called the Electromotive Force (emf) Series. A standard emf Series

is presented in Figure 3.1. By convention, all the reactions listed in the emf series are

written as reduction reactions (Jones 1996). Reactions with more negative potentials are

considered active, while reactions with more positive potentials are noble.

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Figure 3.1: A standard Electromotive Force Series with respect to the standard hydrogen electrode. Reactions with more negative potentials are considered active, while reactions with more positive potentials are noble (Jones 1996).
The emf Series is a tool that can be used to predict the direction of electrochemical reactions based on the half-cell electrode potentials. Since electrochemical reactions proceed spontaneously when the free energy is negative, it follows that corrosion occurs when the sum of the half-cell electrode potentials, E, is positive. Therefore, "[t]he half-cell reaction with the more active (negative) half-cell
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potential always proceeds as an oxidation, and the one with the more noble half-cell

potential always proceeds as a reduction in the spontaneous reaction produced by the

pair." (Jones 1996).

The emf Series lists the half-cell electrode potential measurements taken at a

standard temperature and pressure. Since actual environments may not conform to the

standard state, the half-cell potential may be adjusted for non-standard conditions using

the Nernst equation, shown for reaction aA + mH + + ne- bB + dH 2O below:

E

=

Eo

-

RT nF

ln

[B]b[H 2O]d [ A]a [H + ]m



where Eo is the standard state electrode potential, R is the gas constant, T is the absolute temperature, F is Faraday's constant, n is the number of electrons transferred, and [X]x terms in the natural logarithm represent the concentration (mol/L) of reactants and products

[X] raised to the power of their reaction coefficients x (Jones 1996). From the Nernst

equation, it is observed that if the concentration, or activity, of the metal ions in solution

is higher, the measured half-cell potential is more noble. Also, as the concentration of the oxidizer, H+, increases, the potential increases. As a result, potential characterizes the

oxidizing power of the solution of the corrosion cell.

Electrode potentials, as the characteristic thermodynamic property of

electrochemical corrosion cells, provide insight into the driving force of corrosion,

direction of electrochemical reactions, and the oxidizing power of the electrolytic

solution. However, thermodynamics cannot predict the rate of corrosion, which is

dependent on the kinetics of the electrochemical cells.

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3.1.2 Kinetics Thermodynamic properties of an electrochemical corrosion cell are useful in
understanding the corrosion behavior of a material, but understanding the kinetic properties of the corrosion cell is more practical from an engineering standpoint. From the kinetics of the corrosion cell, the rate of corrosion and mass loss can be derived, the effects of polarization of a metal can be observed, and the active-passive behavior of metals can be examined.
As discussed previously, corrosion of metals often occurs electrochemically, meaning that electron flow is fundamental to the process. Therefore, a measure of the electron flow as current, I, in a corrosion cell is one measure of the corrosion rate. According to Faraday's Law for general or uniform corrosion, the amount of material dissolution or plating in solution is directly proportional to the electric current flowing through it (Jones 1996). Thus, a measure of the mass reacted is another measure of the corrosion rate. Faraday's Laws and uniform corrosion are discussed further in Sections 3.2 and 3.3.
3.1.2.1 Exchange current density When the electrochemical cell is at equilibrium, the half-cell reactions
thermodynamically produce zero net free energy. Similarly, zero net current is produced by the reactions. At equilibrium the rate of the oxidation reaction is equal to the rate of the reduction reaction. The current at which electrons are consumed at the same rate that they are generated is called the exchange current of the cell (Davis 2000). The magnitude of the exchange current density, or exchange current divided by the surface area, of a
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corrosion cell is highly dependent on the surface on which the reactions are occurring, which in turn affects corrosion rates (Davis 2000).
Figure 3.2 illustrates the effect of electrode surface on exchange current density for hydrogen half-cell reactions. In Figure 3.2 three Evan's diagrams are shown; electrode potentials are plotted on the y-axis versus the log of the current density on the x-axis. Lines with negative slopes correspond to cathodic reactions in this case the reaction of hydrogen ions to produce hydrogen gas. The lines with positive slopes correspond to anodic reaction in this case the degradation of hydrogen gas into hydrogen ions and electrons. Equilibrium of the two half-cell reactions occurs where the two line intersect. From Figure 3.2 it can be observed that regardless of the metallic surface on which the electrochemical reaction of hydrogen evolution occurs, the point of intersection remains at the same equilibrium potential. In contrast, the exchange current density of the reaction varies by nine orders of magnitude depending on the electrode surface.
Figure 3.2: Exchange-current densities for hydrogen evolution on lead, iron, copper, and platinum surfaces. The surface of the electrode affects the magnitude of the exchange current density (Davis 2000).
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3.1.2.2 Electrochemical polarization Polarization of an electrochemical cell occurs when it is not at equilibrium. A
corrosion cell is polarized when the potential is changed from the equilibrium potential by a net current at the material surface (Jones 1996). The net current induces energy changes of the electrode; electrons removed from the surface induce positive, or anodic, polarization, and electrons supplied to the surface induce negative, or cathodic, polarization (Singh 2008). Because polarization is a forced change in electrode potential, it is also referred to as overpotential. To study the corrosion behavior of a material, polarization may be induced in the laboratory by externally removing or supplying electrons at the electrode surface, as discussed in Section 3.3.
Net current at the electrode surface is induced naturally in corrosive environments in several ways. The first cause occurs when a limiting, or slow step, reaction rate affects the rate of charge transfer. Charge transfer may also be affected by a limited availability of reactants for a particular half-cell reaction at the surface of the electrode. Lastly, electrons may be supplied or consumed by a second electrochemical reaction, which eventually coexists with the first (Singh 2008).
3.1.3 Mixed potential theory The coexistence of multiple electrochemical reactions is the basis for mixed
potential theory. The Evan's diagrams in Figures 3.3 and 3.4 illustrate mixed potential theory with the dissolution of zinc in acid, where the anodic reaction is Zn Zn2+ + 2eand the cathodic reaction is 2H+ + 2e- H2. As previously discussed, each half-cell reaction has its own equilibrium potential and exchange current density, as shown in
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Figure 3.3. As the two reactions interact, they polarize each other to an intermediate value, as shown in Figure 3.4. The half-cell with the lower potential, zinc in this case, is polarized in the anodic direction, while the half-cell with the higher potential, hydrogen, is polarized cathodically (Singh 2008). The point where the polarized anodic and cathodic curves intersect defines the equilibrium of the electrochemical corrosion of zinc in hydrogen. The potential at the intersection is the corrosion potential, Ecorr, of the electrochemical cell, and the current density is the corrosion current density, icorr, of the cell. According to mixed potential theory, Ecorr will always occur at an intermediate value between the half-cell equilibrium potentials (Singh 2008).
Figure 3.3: Evan's diagram of hydrogen and zinc half-cell reactions at equilibrium. The curves with negative slopes are the cathodic half-cell reactions, and the curves with positive slopes are the anodic half-cell reactions.(Singh 2008)
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Figure 3.4: Evan's diagram demonstrating mixed potential theory. Zinc and hydrogen electrochemical reactions polarize to reach an intermediate equilibrium value (Singh 2008).
A few key factors that influence the location of the intersection of the anodic and cathodic curves, and thus influence the corrosion behavior, include the magnitude of the exchange current density and the addition of oxidizers to the solution. The effect of the exchange current density is illustrated in Figure 3.5 and the effect of an added oxider is shown in Figure 3.6.
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Figure 3.5: The effect of exchange current density on corrosion behavior. Higher exchange current densities lead to higher corrosion rates (Jones 1996).
Figure 3.5 compares the corrosion potential and corrosion current density of iron dissolution in acid with those for zinc dissolution in acid. The polarized anodic and cathodic curves for the corrosion of zinc are shown as dashed lines, and the behavior of the iron is shown with solid lines. For both reactions, the half-cell potential of the hydrogen reduction is the same. The half-cell reaction of the zinc is much more active (negative) than that for the iron, signifying a greater driving force for corrosion. However, the corrosion current density of iron dissolution is higher than that for zinc dissolution, because the exchange current density of the hydrogen half-cell is higher on the iron surface than on the zinc surface (Jones 1996). In this manner, mixed potential theory confirms that corrosion rate is dependent on the kinetics of the cell rather than the thermodynamics.
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Figure 3.6: The effect of added oxidizers on corrosion behavior. Additional oxidizers increase the potential and current density of the electrochemical reaction (Fontana 1986).
The addition of oxidizers creates multiple cathodic reactions for a given anodic reaction. Figure 3.6 demonstrates the effect of the addition of the oxidizer Fe3+ on the corrosion potential and corrosion current density of the generic metal, M. Oxidation and reduction curves occurring at the same potentials added together to form the total rate are represented by the dashed lines in the figure. The intersection of the dashed line with the anodic curve for M occurs at a more noble potential and a higher corrosion current density than the intersection of the hydrogen cathodic curve with the anodic curve for M. Thus, it can be concluded that the addition of an oxidizer increases the driving force and the rate of the electrochemical reaction (Jones 1996).
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3.1.4 Passivity The thermodynamic and kinetic concepts presented in this chapter generally
define the electrochemical corrosion behavior of metals. In some alloys, however, an additional property of corrosion, passivity, seemingly contradicts thermodynamics and alters the kinetics of a corrosion reaction. Passivity is "a condition of corrosion resistance due to formation of thin surface films under oxidizing conditions with high anodic polarization." (Jones 1996).
Previously, it was assumed that as a material underwent anodic polarization, the current densities constantly increased and the material actively corroded. Materials that demonstrate passive behavior, on the other hand, experience a significant decrease in current density as the material is polarized in the anodic direction. Rather than the straight line representation of anodic polarization used in Figures 3.4 to 3.6, an anodic polarization curve for active-passive materials is generalized in Figure 3.7. The material initially exhibits active corrosion as it is polarized. The point at which current densities decrease occurs at the critical current density, ic, and the passivation potential, Ep (Revie 2000).
26

Figure 3.7: General polarization curve for an active-passive material. Current densities decrease at a specific passivation potential, and the metal becomes resistant to active corrosion. (Revie 2000)
Passivity is generally attributed to the formation of a film nanometers thick on the surface of the electrode that acts as a barrier to further oxidation of the metal. Several theories exist regarding the nature and properties of the passive film, and it is still an area of controversy among researchers (Revie 2000). It is accepted that high pH solutions aid passivation of iron based alloys (Landolt unpublished work). Typical materials that exhibit passive behavior are nickel, chromium, titanium, among others (Revie 2000). The passive behavior of stainless steels is attributed to the nickel and chromium alloying elements.
A material that forms a passive film is not guaranteed to resist all corrosion. At a sufficiently high enough potential, the passive film breaks down and the material may actively corrode again, in the transpassive region (Revie 2000). Regardless of the shape of the anodic polarization curve, corrosion of a material occurs where the anodic and cathodic curves intersect. Figure 3.8 demonstrates that the resistance of a metal to corrosion is also dependent on the cathodic reaction. Additionally, local breakdown of
27

passivity results in localized forms of corrosion, which are discussed further in Section 3.2.
Figure 3.8: The intersection of three possible cathodic curves with the anodic curve of a passive metal. Corrosion rates are dependent on the current density at the intersection of the curves.(Revie 2000)
In Figure 3.8 three possible cathodic curves intersect the anodic curve with a passive region. In conditions where the cathodic curve intersects in the passive region of the anodic curve, such as curve A, the material is protected from corrosion. If the cathodic curve intersects in the active region, however, such as curve C, the material will actively corrode. When the cathodic curve intersects both the active and passive regions of the anodic curve, such as curve B, unstable passivity is formed, and corrosion rates fluctuate between high, active rates and low, passive rates (Revie 2000).
28

3.2 Corrosion mechanisms The following section reviews several of the major corrosion mechanisms. In
preference to a comprehensive discussion of all corrosion mechanisms, only the mechanisms pertinent to the GDOT steel bridge plate bearing assembly are presented. 3.2.1 General corrosion
Also known as uniform corrosion, general corrosion affects the entire exposed surface of a metal. Both the cathodic and anodic electrochemical reactions occur on the metal surface, in agreement with mixed potential theory. Due to microscopic nonhomogeneities, such as grain boundaries or dislocations, the metal surface itself is divided into cathodic and anodic regions, as shown in Figure 3.9. Slight changes in the environment can induce cathodes to switch to anodes and vice versa, yielding a uniform metal dissolution along the metal surface (Singh 2008).
Figure 3.9: Anodes and cathodes occur on same metal surface in general corrosion. (Singh 2008).
29

Since the corrosion rates throughout the surface are similar and the metal looses mass uniformly along the part, Faraday's Laws relating mass loss to corrosion rates can be applied. Corrosion rate calculations from Faraday's Laws are discussed further in Section 3.3. 3.2.2 Galvanic corrosion
Electrical contact between dissimilar metals in the presence of electrolyte initiates galvanic corrosion. The potential difference between the two metals is the driving force for corrosion, and galvanic current flows between the two metals (Davis 2000). When coupled the metals polarize each other, and the most active metal becomes the anode. From mixed potential theory, shown graphically in Figure 3.10, the equilibrium potential of the couple occurs at the intersection of the total oxidation and total reduction curves, and is always in between the potentials of the uncoupled metals. Thus, the corrosion rate of the anodic metal increases, and the corrosion rate of the cathodic metal decreases (Jones 1996).
30

Figure 3.10: Generic Evan's diagram for galvanic corrosion. Coupled metals polarize each other, and the equilibrium potential at the intersection of the total oxidation and total reduction curves falls in between the potentials of the uncoupled metals. (Jones 1996).
Since the potential difference between the two metals is the driving force for galvanic corrosion, knowledge of the magnitude of the potential difference is necessary to determine whether galvanic corrosion will or will not occur. A comparison of the potentials of the uncoupled metals at equilibrium provides this knowledge. Keeping in mind that the equilibrium potentials of metals are dependent on environmental conditions, a galvanic series of metals can be created by measuring the potentials of several metals in a specific environment (Davis 2000). Figure 3.11 illustrates a galvanic series for typical construction materials in a seawater environment. The dark boxes in the figure correlate with active behavior for active-passive alloys. According to thermodynamic principles, metals which have a large difference in potentials in the same
31

environment have a high risk of developing galvanic corrosion if electrically coupled. The potentials versus a saturated calomel reference electrode for Type 2101 and 2205 stainless steels are slightly higher than the potentials shown for stainless steel Type 316.
Figure 3.11: Galvanic series for seawater. According to thermodynamic principles, metals which have a large difference in potentials in the same environment have a high risk of developing galvanic corrosion if electrically coupled. Dark boxes correlate with active behavior for active-passive alloys. (Jones 1996).
32

A significant factor which affects the rate of galvanic corrosion is the ratio of the cathode area to the anode area. At equilibrium, the current flow at the anode and cathode is equal. When the area of the anode is decreased, the current density is increased, intensifying the corrosion at that area. Therefore a large cathode to anode area ratio creates severely accelerated corrosion on the anode (Davis 2000). On the other hand, a large anode to cathode area ratio is not detrimental because corrosion over a larger anodic surface reduces the rate of corrosion penetration (Jones 1996).
3.2.3 Concentration cell corrosion The concentration cell corrosion mechanism is nearly the same as galvanic
corrosion. Aspects of the two corrosion mechanisms that are the same are: (1) corrosion is initiated when materials at differing equilibrium potentials are electronically and ionically coupled; (2) polarization of the two materials yields an equilibrium potential in between the original uncoupled potentials; (3) the more active material becomes the anode and is preferentially corroded at an increased corrosion rate; and (4) cathode to anode area ratios affect the corrosion rate of the anode. The only difference of concentration cell corrosion from galvanic corrosion is that instead of a couple of two dissimilar metals with a potential difference, the couple exists between areas of differing potential for the same metal. Potential differences on the same material are caused by the exposure of the metal to dissimilar environments (Jones 1996).
A common example of concentration cell corrosion is the differential aeration cell. In differential aeration cells, or oxygen cells, varying concentrations of dissolved oxygen on a single metal surface creates the potential difference that drives corrosion
33

(Jones 1996). The electrochemical reactions at the anode and cathode of the metal then propagate the corrosion mechanism. At the cathode, the reduction of available dissolved oxygen increases the local pH, promoting greater passivity, while the oxidation of the metal at the anode produces M+ (Landolt unpublished work). The local separation of the anode and the cathode in concentration cells allows the differential pH concentrations to build up (Landolt unpublished work). Typical places where differential aeration cells may form are illustrated in Figure 3.12.
Figure 3.12: Typical locations where differential aeration cells are formed in application.(Landolt unpublished work). 3.2.4 Crevice corrosion
Concentration cells that form at localized areas on the material, such as at an opening between the material and another material, is known as crevice corrosion. The formation of the concentration cell is aided by the geometry of the crevice; small open
34

areas between two surfaces in which the opening width is smaller than the length of the open area creates areas of differential aeration (Jones 1996). Liquid electrolyte that is able to enter the crevice becomes trapped against the metal, because the crevice restricts the solution convection and the electrolyte does not wash out (Singh 2008). Figure 3.13 portrays a typical crevice geometry and illustrates the crevice corrosion mechanism.
Figure 3.13: Typical crevice geometry and basic crevice corrosion mechanism. (Davis 2000)
In the shielded crevice area, the electrochemical reduction reactions deplete the dissolved oxygen, and an excess of positive metal ions collects. Halides, typically chlorides, migrate to the crevice to electrostatically neutralize the positive charge. The metal halides hydrolyze in water, and they produce hydrogen ions which lower the pH. The hydrogen and halide ions in the crevice accelerate metal dissolution, attracting more halides and increasing hydrolysis. In this manner, the propagation of crevice corrosion is autocatalytic in nature (Jones 1996). As the solution progresses to a lower pH, the metal within the crevice is the anode of the concentration cell, while outside the crevice, the oxygen rich solution on the metal surface acts as the cathode.
35

Passive materials may be protected from crevice corrosion, but as the metals are polarized to higher potentials, crevice corrosion initiates at a specific potential where passivity breaks down (Jones 1996). Crevice corrosion is dependent on many factors which are geometric, environmental, electrochemical, or metallurgical in nature (Davis 2000). Specifically, the variability in crevice geometry affects the individual breakdown potential for each application, and thus the initiation of crevice corrosion cannot be predicted for a particular alloy (Jones 1996). Once initiated, crevice corrosion attack is severe for passive alloys. The breakdown of the passive layer in the local area of the crevice yields a high cathode to anode ratio and corrosion penetration is intensified due to rapid metal loss in that small area (Davis 2000).
3.2.5 Pitting corrosion Pitting corrosion is another form of localized corrosion that shares a similar
corrosion mechanism as crevice corrosion. Inside actively corroding pits, oxygen is depleted and the same autocatalytic process of corrosion propagation exists as was described for crevice corrosion (Jones 1996). As with crevice corrosion, pitting corrosion is destructive because it can penetrate deeply into the metal at the anode, or pit.
Pits occur randomly along the surface of the metal, and thus, both active and passive areas exist simultaneously on the surface of the metal (Revie 2000). Generally, passive metals are more susceptible to pitting corrosion; active metals void of passive behavior typically corrode uniformly, instead. Additionally, among passive metals, some alloying elements in metals are more resistant to pitting than others. For example,
36

chromium and molybdenum have been found to strengthen passive layers of alloys and provide greater resistance to pitting (Revie 2000).
Several theories exist on the mechanism of pit initiation, and are still a controversial subject. The majority of pitting has been found to occur in the presence of halides, typically chloride (Revie 2000). As a metal surface is positively polarized, the negative chloride ions are electrostatically attracted and become more concentrated at the surface of the metal. Therefore, most theories attribute pit initiation to dissolution of metal halides that are formed when the passive layer is compromised (Singh 2008).
Pitting has been found to occur electrochemically when a metal reaches a critical polarization potential, Epit. Above this potential, pitting initiates; below this potential existing pits may grow, but new pits do not initiate. Additionally, the potential at which existing pits repassivate is called the protection potential. Below the protection potential, pits do not grow or initiate (Jones 1996). The critical pitting potentials and protection potentials are dependent on the material, temperature, and chloride concentration.
The critical pitting potential for a particular alloy at a certain temperature and chloride concentration is typically higher than the break down potential required to initiate crevice corrosion. The geometry of the existing crevice at the metal surface provides an environment favorable to initiation of corrosion compared to pitting corrosion (Jones 1996). In crevice corrosion, the depletion of oxygen in the crevice initiates corrosion through a concentration cell effect, while pitting corrosion is initiated when passivity is broken and metal ions dissolve, under the aid of chloride attack (McCafferty 1974).
37

3.2.6 Stress corrosion cracking The presence of corroding pits in a metallic member subject to tensile stresses can
initiate cracking in the metal called stress corrosion cracking (Jones 1996). Stress corrosion cracking is a major concern because cracks cause brittle failure of otherwise ductile materials. Also, stress corrosion cracking affects materials in which corrosion is usually minimal in the absence of stress (Singh 2008). For a given set of environmental conditions and material composition, a threshold stress value can be found, only above which failure from cracking will occur (Revie 2000).
Once a crack has initiated, several mechanisms may be responsible for crack propagation. One theory proposes that cracks propagate as metal ions are preferentially dissolved at the crack tip; other crack propagation models follow theories of fracture mechanics (Jones 1996). A complete review of stress corrosion cracking mechanisms is omitted from this chapter, but the reader is directed to D.A. Jones' book, Principles and Prevention of Corrosion, for a more complete discussion.
3.2.7 Fretting corrosion Fretting corrosion develops where movements between a metal and another solid
occur when loaded (Jones 1996). As a form of mechanical and corrosive wear of a metal, fretting corrosion may be classified as either wear-oxidation or oxidation-wear. In wearoxidation fretting, the two surfaces in contact with each other are not previously oxidized. An example of wear-oxidation fretting is an over-tightened bolt and nut which is subject to some form of mechanical vibration. As the surfaces wear on each other, small metallic particles break off, which then oxidize secondarily (Singh 2008).
38

In contrast, oxidation-wear fretting occurs where two previously oxidized surfaces contact each other. As the solids are subjected to movements, the oxidized layers of the solids are broken off, revealing clean material beneath. Depending on the environment, the clean metal surface may subsequently re-oxidize (Jones 1996). As shown in the schematic in Figure 3.14, the oxide film debris becomes trapped in between the solids, and functions as an abrasive agent during movement (Davis 2000).
Figure 3.14: Fretting corrosion. As two metal surfaces wear against each other, oxide layers on the surface are broken and reformed. (Davis 2000).
3.3 Corrosion measurements In this section, ways to quantify and interpret corrosion behavior are presented.
The following methods are used in laboratory corrosion experiments. Specific application and results of each technique are presented in Chapter 8.
3.3.1 Corrosion potential readings The first measurement that is useful in understanding the corrosion behavior of a
material is the measurement of corrosion potential of a metal in oxidizing solution. As 39

discussed in Section 3.1, corrosion potential is the characteristic thermodynamic measurement of a corrosion reaction, because it indicates the free energy and equilibrium conditions of the reaction (Singh 2008).
The corrosion potential of a material cannot be measured directly, but must be measured as a potential difference with respect to a reference electrode. The most common reference electrode is the standard hydrogen electrode (SHE). A schematic representation of the SHE device is shown in Figure 3.15. The SHE establishes a zero point from which all other reference electrodes and potential measurements are based (Jones 1996). The zero point for half-cell electrode potentials is the hydrogen reduction reaction.
Figure 3.15: Schematic of the standard hydrogen electrode (SHE) device. The SHE establishes a zero point from which all other reference electrodes and potential measurements are based (Jones 1996).
40

A commonly used reference electrode in laboratory settings is the saturated calomel electrode (SCE). A schematic representation of the SCE device is shown in Figure 3.16. The SCE employs the reduction reaction of mercury in contact with a saturated potassium chloride solution. Potential differences measured with respect to the SCE are 0.241 volts more positive than those measured against the SHE (Jones 1996).
Figure 3.16: Schematic of the saturated calomel electrode (SCE) device. The SCE is commonly used in the laboratory. (Jones 1996)
Potential differences are measured by electrochemically connecting the reference electrode to the corroding metal. The electric connection is provided by wire leads and
41

includes a connection to a voltmeter, and the ionic connection is provided through a salt bridge between the reference and the metal. Because the half-cell potential of the reference is known, the corrosion potential of the metal is determined as the voltage generated between the two electrodes is read from the voltmeter.
3.3.2 Electrochemical polarization Electrochemical polarization curves provide insight into the kinetics of corrosion
for a particular material. To study the corrosion behavior of a material, polarization may be induced in the laboratory by externally removing or supplying electrons at the electrode surface by a potentiostat. A plot of the log of the current density versus the potential during the polarization reveals how a material will behave under real applications. For example, a steadily increasing current density with respect to potential indicates that the metal is actively corroding, while a drop in current density at a certain potential indicates the formation of a passive layer at the passivation potential, as shown in Figure 3.7.
The potential sweep applied to the metal can be either potentiostatic or potentiodynamic. In a potentiostatic sweep, the current density of the working electrode, or metal being tested, comes to a steady value before potential is forced to change. In a potentiodynamic sweep, the potential of the working electrode is continuously changed at a certain rate, regardless of the stability of the current density (Jones 1996). A potentiostatic sweep generally gives the most accurate polarization curve, but a potentiodynamic sweep is useful in the laboratory where curves can be generated and analyzed quickly.
42

3.3.3 Cyclic polarization Cyclic polarization is a form of electrochemical polarization used to evaluate the
pitting and protection potentials of metals. During cyclic polarization, the potential is raised into the transpassive region for the material and then reversed. The behavior of the material, determined by the current density, during the reverse sweep serves as evidence of whether pitting or localized corrosion has occurred. If the current densities for each potential are equivalent to those at the same potential on the forward scan, then the passive layer has not been affected. However, if the current densities for each potential are higher than those during the forward scan, then it is apparent that the passive layer has been compromised and localized corrosion has initiated.
Figure 3.17 illustrates a general curve generated by cyclic polarization in the presence of chlorides. The point at which the passivity is broken and local corrosion initiates in the transpassive region is the pitting potential, Epit. The point where the curve of the reverse sweep returns to intersect the curve of the forward sweep is the protection potential, Eprot (Jones 1996). As discussed in Section 3.2, pitting is only initiated at potentials above Epit, existing pits will grow at potentials in between Eprot and Epit, and the metal is protected from pit growth and initiation at potentials below Eprot.
43

Figure 3.17: Typical cyclic polarization curve demonstrating Epit, above which pitting initiates, and Eprot, below which the material is protected from pit growth and initiation (Jones 1996).
3.3.4 Gravimetric calculation of corrosion rate Perhaps the simplest way to measure corrosion rate of a material is through the
measure of weight loss over time. According to Faraday's Law for general or uniform corrosion, the amount of material reacting in solution is directly proportional to the electric current flowing through it (Jones 1996). The relation between weight lost, W, and corrosion rate is defined by:
W = An I t ne F
where An is the total atomic weight of the material, I is the current, t is time, ne is the number of electrons involved in the reactions, and F is Faraday's constant (Jones 1996). Rearranging for rate and dividing by density produces an equation for thickness lost, or
44

penetration. In a neater form accounting for constants and conversion factors, the corrosion rate in mils/year, or 0.001 inch per year, is
Corrosion rate = 534 W D AT
and the corrosion rate in mm/year is Corrosion rate = 87.6 W D AT
where W is the weight change of the material in milligrams, D is the density of the material in grams/cm3, A is the area in in2, and T is time in hours.
It is important to note that Faraday's Laws relating mass loss to corrosion rate only apply to calculations of uniform corrosion rates. Gravimetric calculations cannot be used to measure local corrosion rates, because crevice or pit penetration is not a function of total weight loss.

3.3.5 Polarization resistance

Polarization resistance has been proven to be inversely proportional to corrosion

rate, and thus is commonly used to determine corrosion rates. The polarization resistance

of a sample is the slope of the polarization curve for that sample at its origin (Jones

1996). To measure the polarization resistance of a metal, a potentiostat is used to apply

overvoltages within a few millivolts of the materials' equilibrium potential and the

change in overvoltage, , over the change in applied current, iapp, is measured as the

overvoltage approaches zero, or

R p

=

d diapp

0

(Jones 1996).

45

The polarization resistance slope is related to the current density, and thus, the

corrosion rate is given by

Rp

=

a c 2.3 icorr ( a

+ c)

where a and c are Tafel constants, which define the linear slopes on a semi-log graph of

the half-cell reactions (Jones 1996).

Thus, the corrosion rate of the metal can be calculated using appropriately

selected values for Tafel constants and a plot of the overvoltages versus the current

(Jones 1996). Polarization resistance is a common method of determining corrosion rates

quickly in the laboratory.

46

CHAPTER 4: LITERATURE REVIEW
4.1 Concepts in bearing design
4.1.1 Bearing loads and movements Bearings are designed to support superstructure loading and movement. The
purpose of the bridge bearing is to transfer loads from the superstructure to the substructure, while accommodating the movement of the superstructure in relation to the supports (Lee 1994).
Bearings are subjected to both vertical and horizontal loads. Horizontal loads in the longitudinal and transverse planes originate from external sources or are induced by restrained movements at the bearing (Lee 1994). Bearing restraint exists at all bearings due to frictional effects, but in some cases bearing restraint is fundamental to the bridge design. For example, end rotations are free to occur in simply supported spans, but the redundancies in continuously supported spans limit motion at interior bents (Lee 1994). Also, restraint in the transverse horizontal direction may be provided to prevent the girders from sliding off the bearings. The moments produced by this restraint must be accounted for in the bridge and bearing design.
External horizontal loads include wind loads, traffic braking and impact loads, and earthquake loads; and vertical loads arise from the bridge dead and live loads. Generally, the vertical bridge loads are distributed among the bearings (AASHTO 2007). Overall, the distribution of the loads on the bearings is dependent on bridge members' stiffnesses and the range of fabrication and construction inaccuracies (AASHTO 2007).
47

Superstructure movements relative to the substructure are three dimensional translational and rotational movements. The movements are caused by loading, deformation, and displacement conditions, and may be internal or external to the bridge. Internal sources of movement arise from the bridge material's response to the environment and loading. Bridges physically deform in response to: variations in temperature and humidity; material tendencies of creep, shrinkage, and fatigue; axial and flexural strains due to bridge loads or prestressing; and dynamic traffic or braking loads, among others (Lee 1994). External sources of movement include displacements caused by: earthquakes; impact loads; foundation settlements; and the construction procedure and tolerances (Lee 1994). In bearing design rotational movement about two horizontal axes and a vertical axis must be addressed in addition to the longitudinal and transverse movements (AASHTO 2007).
To design an appropriate bearing system, one must predict the expected loading and movement at the bearings. Several aspects affect the estimation of these design factors: 1) for curved or skewed bridges, translational movement affects both the longitudinal and transverse directions, 2) misalignments during installation causes initial rotations that may exceed calculated rotations due to loading, 3) bearing restraints induce forces, 4) the "worst-case" combination of loads and movements must be used for bearing design, considering both initial and long-term conditions (AASHTO 2007). Specific load calculations for bearing design are determined according to the limit states and load factors prescribed in Section 3 of AASTHO LRFD Bridge Design Specifications.
48

Uniform thermal changes along the cross section of a girder induce translational movement, which can be calculated using the equations in Section 3 of the AASHTO specifications. However, temperature usually varies differentially through the cross section. These temperature gradients induce thermal bending of the girders, which in turn produces rotational movement in addition to translational movement (Lee 1994). Calculations to account for thermal gradients are provided in Section 4 of the AASHTO specifications.
Typically, factored displacements and rotations are used for bearing design. In some cases a tolerance as high as 0.005 radians is added to the calculated expected rotations to account for inaccuracies in construction, and an additional 0.005 radians is added to account for other uncertainties (AASHTO 2007). Improperly designed bearings can result in increased stresses in bridge members, damaging the structure. According to AASHTO specifications, "[n]o damage due to ... bearing movement shall be permitted at the service limit state, and no irreparable damage shall occur at the strength limit or extreme event states." (AASHTO 2007).
Movements due to creep and shrinkage and elastic shortening due to prestressing are not addressed in this chapter because these issues apply primarily to concrete girder bridges, while the scope of this research focuses on steel girder bridges.
4.1.2 Bearing selection The bearing system for a particular bridge design is selected from the types of
bearings available for use that satisfy the design considerations for that bridge. While AASHTO provides guidance on design considerations for bearings, in-depth resources on
49

bridge bearing design are not readily available. David Lee provides an excellent discussion of bridge bearing types and specific design considerations in his book, Bridge Bearings and Expansion Joints (1994). 4.1.2.1 Bearing types
In this section, the examination of bearing types is limited to simple or continuous span bearings at concrete supports. A graphic summary of the bearing types presented in this section is shown in Figure 4.1 (Lee 1994).
50

Figure 4.1: Graphic summary of bearing types. (Lee 1994). 51

Roller bearings that are comprised of one or more steel cylinders sandwiched between horizontal upper and lower steel plates allow longitudinal translation and rotation about the transverse horizontal axis. Guides are provided to maintain the positioning of the roller for its entire service life (Lee 1994). The curved surfaces at the bearing must have sufficient hardness to withstand deformation due to high bearing loads. Consequently, a single roller of a high tensile strength alloy may be used, or the load may be distributed among multiple mild steel rollers. In the application of multiple rollers, another bearing element, such as a knuckle or rocker, must be provided to accommodate for rotations (Lee 1994). For this reason, multiple roller bearings are no longer commonly used. Important design considerations specific to roller bearings are the selection of the radius and its effect on the stability of the bearing, and the mechanism employed to ensure proper alignment of the roller (Lee 1994).
In rocker bearings a curved surface is in contact with another surface that is either curved or flat. This type of bearing allows rotational but not translational movement (Lee 1994). For this reason, this bearing type is used at the fixed end of a span. A cylindrical curved surface permits rotation about the transverse horizontal axis, while a spherical curved surface enables rotation about multiple horizontal axes (Lee 1994). Similar to roller bearings, the curved surface of the rocker bearing must be hard enough to withstand deformation under bearing loads. A mechanism to prevent the horizontal movement of the two surfaces relative to each other is provided usually in the form of a dowel, or bolt connecting the two surfaces (Lee 1994).
In knuckle bearings an encapsulated pin provides rotational motion as the curved surfaces of the upper and lower support slide about the transverse axis. Translational
52

movement is restrained (Lee 1994). Knuckle bearings are difficult to maintain, and sliding surfaces tend to corrode, restraining all movement. Thus, knuckle bearings are not used anymore (Lee 1994).
Leaf bearings are a type of knuckle bearing where the pin passes through multiple plates which are alternately fixed to upper and lower steel plates. The same difficulties are present in leaf bearing designs as in traditional knuckle bearing design, and are also rarely used (Lee 1994).
Pot bearings are a type of bearing that are used in cases where the bearing is required to support large vertical load and a high degree of rotation. At the base of the bearing, a hollow metal cylinder contains an unreinforced elastomeric, neoprene pad. A piston fixed to the upper bearing plate transfers load from the bridge through the pad, which allows multidirectional rotation in the process (Lee 1994). The pot bearing itself does not allow translational motion. Present day pot bearings have replaced a heavy steel rocker bearing design and are typically used in long span or curved steel girder bridges.
Sliding bearings consist of two surfaces that are meant to slide against one another. The surfaces may be of either similar or dissimilar metals and may be either flat or curved. Flat surfaces allow translational but not rotational movements, while curved surfaces allow rotational but not translational movement (Lee 1994). Traditionally, one of the curved surfaces is a lubricated bronze plate. According to Lee, the metal plate sliding bearings are
"...very simple and cheap, but even when freshly and evenly lubricated the coefficient of friction is of the order of 20%. It was also found that, even with regular maintenance, the coefficient of friction increased to unacceptable limits. However, at a time when it appeared that sliding bearings were to become a historical curiosity, they were revitalized by the introduction of polytetrafluoroethylene (PTFE)." (Lee 1994).
53

PTFE is more commonly known as Teflon, and is well known for its chemical resistance and low coefficient of friction (AASHTO 2007). The application of PTFE to sliding mating surfaces improves the performance of sliding bearings. It is important, however, to protect the PTFE surface from debris, which could scratch the surface and increase the friction in the bearing. Equally important is that the PTFE is bonded to a corrosion resistant and hard metal plate, to prevent wearing of the Teflon surface. Such wearing would also increase the friction of the bearing. (Lee 1994). Thus, frequent maintenance is required for sliding bearings with or without the use of PTFE.
The final type of bearing is the elastomeric bearing. An elastomeric bearing is made of either natural or synthetic rubber that is stiff enough to support vertical loads and flexible enough to sustain translational and rotational movements. The bearing provides for multidirectional translation in the shearing capacity of the rubber as the surfaces of the rubber are able to move relative to each other. Rotational capacity is provided as the material exhibits a varying compressive strain across the cross section (Lee 1994). Stiffness is increased in some elastomeric bearings by incorporating steel plates within the rubber. Elastomeric bearings made with holes, to allow for anchorage and alignment, should be designed to be thicker, because the holes reduce the stiffness of the bearing and the ultimate load capacity (Lee 1994). An advantage of elastomeric bearings is that they require little maintenance because there are no mating surfaces to get filled with debris and no exposed metal to corrode (Lee 1994).
54

4.1.2.2 Other design considerations When selecting a bearing system for a bridge, one must consider that the types of
bearing elements discussed in the previous section can be combined to provide a mechanism for all necessary bridge movements. In addition to designing for bearing loads and movements, other factors can affect the selection of bearing type for a particular application.
First, the design life of the bearing may be an important factor in the selection of bearing type. Metallic bearings require constant maintenance to ensure proper functioning and a longer lifespan (Lee 1994). On the other hand, elastomeric bearings requiring no maintenance claim to have an equally long, or longer lifespan, as traditional bearings. While the lifetime of the alternate materials are promising, these claims have not yet been sufficiently proven in the field (Lee 1994). All bridge bearings suffer from material degradation eventually. To avoid premature bearing failure, Lee advises that "[b]earings should be detailed without crevices and recesses that can trap moisture and dirt. .... It is important to ensure that dissimilar materials that can give rise to corrosive currents are not used together" (1994).
Second, bearings should be selected that can be accurately installed in the available space. Accurate location of the bearings is important to avoid eccentric loadings or movements that were not accounted for in the design calculations (Lee 1994).
Third, the restraint provided by the bearing must be addressed. A bearing system may be selected according to the predicted frictional properties of the bearing. In many applications bearing restraints are necessary to prevent total or partial translational
55

movement. For example, the use of bolting or bedding adhesive may be required to prevent slipping of the bearing.
The use of "hold-down bolts", or anchor bolts, is a common practice to provide stability in the transverse horizontal direction (Lee 1994). Over-tightening of the bolts should be avoided to prevent excessive compressive stresses from being introduced into the bearing. (Lee 1994). Also, "[s]ince bearing replacement may be required during the life of a structure, the provision of a restraint (e.g. dowels) through the bearings may cause difficulties, and an alternative location of the restraints should be considered." (Lee 1994). A bearing system should be selected that can provide the necessary restraints in a convenient manner. In general, the base plate of most bearing types is anchored to the concrete pier cap, and thus the problem of anchor bolt corrosion is universal to all these bearings.
4.1.3 Bearing and anchor bolt materials The following section provides an investigation into the compositions and
properties of the metals typically used in metallic bearings. Table 4.1 and Table 4.2 show the material composition and mechanical properties of materials in use or considered for use in sliding plate bearings in Georgia. Cells in the table in which "..." appears indicates that there are no requirements for that category; cells in which "xx" appears indicates that the category does not apply to the material. The material properties of the structural carbon steel (CS) conform to ASTM A709: "Standard Specification for Structural Steel for Bridges", and the material properties of the candidate stainless steel (SS) alloys conform to ASTM 276: "Standard Specification for Stainless Steel Bars and Shapes".
56

Table 4.1: Material composition of steel alloys used in bearings (ASTM 2006; ASTM 2007).

Steel

Material composition in %

Designation C Mn P

S

Si

Cr Ni Mo N

CS: Grade 36 0.26 ... 0.04 0.05 0.40 xx xx xx xx

18.0- 8.0-

SS: 304

0.08 2.00 0.045 0.030 1.00 20.0 11.0 ... ...

16.0- 10.0- 2.00-

SS: 316

0.08 2.00 0.045 0.030 1.00 18.0 14.0 3.00 ...

4.0-

21.0- 1.35- 0.10- 0.20-

SS: 2101

0.040 6.0 0.040 0.030 1.00 22.0 1.70 0.80 0.25

22.0- 4.5- 3.0- 0.14-

SS: 2205

0.030 2.00 0.030 0.020 1.00 23.0 6.5 3.5 0.20

Table 4.2: Mechanical properties of steel alloys used in bearings (ASTM 2006; ASTM

2007).

Mechanical properties

Steel

Yield Strength, Tensile Strength, Elongation, Brinell Hardness

Designation ksi

ksi

%

number

CS: Grade 36

36

58-80

23

...

SS: 304

30

75

40

...

SS: 316

30

75

40

...

SS: 2101

65

94

30

290

SS: 2205

65

95

25

290

According to officials at GDOT, structural carbon steel is used for the base and sole bearing plates and was used for anchor bolts in construction prior to 1990 (Duvall et al. 2007). Currently, austenitic Type 304 stainless steel is specified by the Georgia Department of Transportation Bridge and Structures Design Policy Manual for use as anchor bolts (2007). This specification for anchor bolts was revised in the early 1990s, because stainless steel is known to be more corrosion resistant (Duvall et al. 2007). The mechanical properties of stainless steels have been found to be acceptable for anchor bolt application by GDOT.

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Stainless steels are divided into categories by compositional type. Austenitic stainless steels are the most common, containing both chromium and nickel alloying elements in quantities that preserve the non-magnetic austenitic phase of the steel (SSINA 2005). Type 304 and Type 316 stainless steels are both austenitic stainless steels. Chromium is the main alloying element in the magnetic ferritic stainless steels, whose carbon content is low (SSINA 2005). Austenitic-ferritic grades of stainless steels, or duplex stainless steels, combine properties of both austenitic and ferritic grades, containing high amounts of both chromium and nickel. Duplex stainless steels are highly corrosion resistant and stronger than either austenitic or ferritic grades alone (SSINA 2005). Type 2101 and Type 2205 stainless steels are both duplex stainless steels. The last grade of stainless steels is martensitic stainless steel. Martensitic stainless steels are magnetic alloys that contain moderate to low levels of chromium, a moderate level of carbon, and a very low quantity of nickel (SSINA 2005). In general, martensitic stainless steels have a higher strength than austenitic stainless steels, but a lower toughness. Also, due to the lower levels of chromium in the martensitic stainless steel, it is more susceptible to corrosion in the presence of chlorides and low pH compared to austenitic or duplex grades of stainless steel. The stainless steel types included in Tables 4.1 and 4.2 are candidate alloys to be considered for anchor bolt use.
The metal specified for the self-lubricating plate in a Georgia plate bearing is bronze conforming to the ASTM B 22, Alloy UNS 91100 standard (GDOT 2007). According to bearing manufacturers, bronze lube plates are impressed with a mastic lubricant, which consists of graphite, a mix of metal oxides, and a binder (Dabkowski 2007) This lubricant is compressed into recesses and grooves in the bearing plate under
58

hydraulic pressure; the target-like recesses in the plate are approximately 1/8 inch deep and cover 25 to 30 percent of the plate surface (Dabkowski 2007). The plate and lubricant are designed to survive the lifespan of the bearing.
4.2 Corrosion of bridge bearings The following two sections provide a brief overview of the literature concerning
corrosion of anchor bolt materials separately in the two environments to which concrete anchorages are exposed. The final section explores the combination of the environments, which is the realistic environment for anchor bolt application, and demonstrates the need for research in this area.
4.2.1 Steels embedded in concrete
4.2.1.1 Carbon steel reinforcements The corrosion of carbon steel in concrete is an important issue concerning the
durability of reinforced concrete structures and is well researched and documented phenomenon. In general, carbon steel is protected from corrosion when it is embedded in concrete because the high alkalinity of the concrete pore solution promotes steel passivation. The passive layer on the steel surface is broken by excess chloride ingress or carbonation of the concrete, which lowers the environmental pH. Once the passive layer is broken, corrosion of the reinforcing steel propagates according to the mechanisms discussed in Chapter 3. A detailed account of the corrosion of carbon reinforcing steel in concrete can be found in a number of textbooks, journal articles, and reports (ACI 2001;
59

Berke et al. 1993; Bohni 2005; Broomfield 2007; Hausmann 1967; Mehta and Monteiro 2006). Additionally, a significant quantity of research has been conducted concerning solely the initiation of corrosion, such as the chloride threshold level before passivity is broken, or the rate of concrete carbonation (Alonso et al. 2000; Glass and Buenfeld 1997; Gonzalez and Andrade 1982; Papadakis et al. 1991).
4.2.1.2 Stainless steel reinforcements Due to the high number of incidences of carbon steel reinforcing bar corrosion,
interest was generated in the use of stainless steel for concrete reinforcement. Therefore, numerous studies have been conducted on the corrosion behavior of stainless steels embedded in concrete. In general, it has been proven that the corrosion resistances of austenitic and duplex stainless steel grades are superior to that of carbon steel embedded in concrete in the presence of chlorides (Bertolini et al. 1996; Castro et al. 2003; GarciaAlonso et al. 2007; Gu et al. 1996; Hartt et al. 2007). . 4.2.1.3 Galvanic couplings of different steels in concrete
Since stainless steel reinforcements were introduced into reinforced concrete design, concern has arisen regarding the possibility of accelerated corrosion due to galvanic effects between the stainless and carbon steel reinforcements. According to theory presented in Chapter 3, both carbon and stainless steels are passivated in the alkaline concrete environment; thus, no significant potential difference exists between the two to thermodynamically trigger corrosion.
60

The results of several studies confirm that the corrosion rates of steel in concrete are not increased when passive stainless steel is electrically coupled with passive carbon steel (Abreu et al. 2002; Bertolini and Pedeferri 2002; Qian et al. 2006). Additionally, when carbon steel is actively corroding in concrete, the galvanic effect of passive stainless steel bars coupled to the active carbon steel bars is not noticeably greater than the effect of passive carbon steel bars coupled to active carbon steel (Bertolini and Pedeferri 2002; Qian et al. 2006). Thus, it can be concluded that "no significant risk of galvanic corrosion exists when carbon steel and stainless steel are electrically coupled in reinforced concrete structures." (Abreu et al. 2002).
4.2.2 Bearing materials in atmospheric conditions
4.2.2.1 Corrosion of uncoupled steels Carbon steel is known to corrode in general atmospheric conditions unless
protected. For bridges and bearings, the traditional method of protection is by painting all exposed surfaces.
Stainless steel corrosion generally does not occur in normal atmospheric conditions, but localized corrosive attack may initiate depending on specific environmental conditions. For example, in the presence of chlorides, stainless steels may experience crevice or pitting corrosion. In one study, Johns and Shemwell explored the susceptibility of several stainless steel alloys to crevice corrosion when used as a fastener (Johns and Shemwell 1997). Crevice corrosion initiation is highly dependent on the crevice geometry, and thus specific investigations into this mechanism are not
61

particularly useful. However, a case can be made that the initiation of pitting corrosion can be measured as a critical pitting potential for each individual material and is a practical method for predicting the susceptibility of the material to pitting corrosion (Leckie 1970). Environmental factors that affect the pitting potential of a material are the temperature, chloride concentration, competitive anion concentrations, and the pH of the conductive solution (Frankel 1998; Leckie and Uhlig 1966).
4.2.2.2 Galvanic couplings of different materials The mechanism of galvanic corrosion that was presented in Chapter 3 applies to
cases in which carbon steel and stainless steels are in electrical contact in normal atmospheric conditions. A galvanic series for a typical marine environment shows that the potential for stainless steel is more noble than the potential for carbon steel, creating a thermodynamic drive for preferential corrosion of the carbon steel (Jones 1996). Similarly, most bronze alloys also have higher potentials than carbon steel (Jones 1996). Accordingly, the AISC Steel Construction Manual reports that the use of an austenitic stainless steel fastener on either carbon steel or bronze base materials will increase the corrosion rate of the base material (AISC 2005).
4.2.3 Corrosion behavior of anchorages in concrete While an understanding of the corrosion behavior of anchor bolt materials both
embedded in concrete and in atmospheric conditions is important, the environmental conditions cannot be addressed separately. As discussed in Chapter 3, the variation in
62

environmental conditions along a material can initiate corrosion, as well. This corrosion behavior of concrete anchorages is not widely studied.
In one study, Type 316 stainless steel rods were partially embedded in concrete specimens which were exposed to marine conditions (Flint and Cox 1988). Two exposure conditions were employed full immersion and tidal immersion for up to twelve and a half years. The results of this study showed that the austenitic Type 316 stainless steel was remarkably resistant to corrosion, even at the interface where the rod protruded from the concrete block (Flint and Cox 1988). However, because this test stands alone, there is no repeatable evidence or corresponding studies with other candidate alloys to confirm the results.
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CHAPTER 5: GEORGIA STATEWIDE CONDITION ASSESSMENT
5.1 Inspection report survey process Access to the inspection reports for all state owned steel girder bridges were
provided to the researcher by the Georgia Department of Transportation. Pertinent information queried from the inspection report database included the bridge serial and location code numbers, superstructure evaluation notes, year constructed, and superstructure condition code. The number of inspection reports reviewed was narrowed down by maintenance item request number. Only the inspection reports of bridges with a superstructure maintenance item request number were reviewed by the researcher, since corrosion of bearings falls under the superstructure maintenance category. Additionally, the dates at which the maintenance item was submitted and at which the request was completed was recorded. The earliest report with a superstructure maintenance item request was dated 1984.
In summary, a query of the inspection report database supplied the researcher with a list of state owned steel girder bridges in Georgia in which bearing and anchor bolt corrosion possibly existed. A thorough review of all the inspection reports from this list, covering 1984 to present day, provided the data presented in this chapter. The researcher read the superstructure evaluation notes to find reports indicative of bearing and anchor bolt corrosion.
In addition to specific reports of anchor bolt corrosion, reports of loose, pushed up, sheared off, or missing anchor bolts and reports of bearing corrosion implicitly denoted anchor bolt corrosion, as well. For example, loose, sheared, and missing anchor
64

bolts are symptoms of anchor bolt diameter reduction by corrosion losses. Anchor bolts that are pushed up imply that corrosion products have built up inside the bearing plates, forcing the bolt upwards. Considering these assumptions, a survey of the inspection reports allowed the researcher to quantify the extent of anchor bolt corrosion in Georgia.
5.2 Extent of anchor bolt corrosion in Georgia The query of the inspection report database revealed that there are 1500 steel
girder bridges in Georgia. From the ensuing assessment of the inspection reports, 411 steel girder bridges, or 27% of the bridges, were judged to be experiencing anchor bolt corrosion. All of the bridges with reported anchor bolt corrosion were built before stainless steel anchor bolts were used in steel girder bridge bearings in Georgia, thus all reported anchor bolt corrosion pertains to carbon steel bolts. Possible trends relating anchor bolt corrosion to environmental conditions were examined by grouping bridges with anchor bolt corrosion by county, by region and by span type.
5.2.1 Bridges with anchor bolt corrosion grouped by county Figures 5.1 and 5.2 illustrate the distribution of bridges experiencing anchor bolt
corrosion throughout the state by county. In Figure 5.1 each county in the state is labeled with the number of bridges with corroding anchor bolts in that county. Counties with no labels do not contain any bridges with corroding anchor bolts. Subsequently, the percentages of the total number of steel girder bridges in each county that anchor bolt corrosion affects is shown by the county labels in Figure 5.2.
65

Counties in which a relatively high occurrence of anchor bolt corrosion was observed are highlighted. Counties in which the number of bridges with anchor bolt corrosion exceeded 10 are highlighted with yellow, whereas counties in which the percentage of bridges affected by anchor bolt corrosion exceeded 30% are highlighted with blue. Finally, counties in which the number of anchor bolt corrosion incidence exceeded 10 and the percentage exceeds 30% are highlighted in green.
66

Figure 5.1: Georgia counties labeled by number of bridges with anchor bolt corrosion in the state.
67

Figure 5.2: Georgia counties labeled by percentage of the total number of steel girder bridges affected by anchor bolt corrosion.
68

From Figures 5.1 and 5.2 it is observed that anchor bolt corrosion occurs throughout the whole state. Areas of a high number and a high percentage of anchor bolt corrosion incidences, highlighted in green, were further examined for environmental or regional trends.
Four of the eight counties highlighted green correspond to metropolitan areas. The areas are Macon in Bibb County, Columbus in Muscogee County, Savannah in Chatham County, and Athens in Clarke County. In contrast, the counties within the metropolitan area of Atlanta, including Fulton, DeKalb, Gwinnett, Clayton, Douglas, and Cobb counties, do not have a high percentage of bridges with anchor bolt corrosion. Assuming that bridges in Atlanta are inspected and maintained the same way as bridges in other metropolitan areas, it cannot be concluded that an increased percentage of steel girder bridges in an urban environments develops anchor bolt corrosion.
The other four counties that are highlighted in green are Bartow, Forsyth, Hall, and Franklin counties. These counties are not densely populated, but are all geographically situated in the northern half of the state. A preliminary hypothesis may predict that a greater percentage of bridges located in the northern half of the state experience anchor bolt corrosion due to the effect that deicing chemicals have on the bearing environment. However, Figure 5.2 shows that counties in the southern half of Georgia, where chemical deicing agents are not used, experience similar amounts of anchor bolt corrosion. Therefore, it can be concluded from Figure 5.2 that the occurrence of anchor bolt corrosion is not proportional to the use of chemical deicers.
69

5.2.2 Bridges grouped by region While grouping bridges with anchor bolt corrosion by county provided an
illustration of the distribution of anchor bolt corrosion throughout the state, it could not provide conclusive evidence that anchor bolt corrosion could be related to certain environmental conditions. In the following sections the extent of anchor bolt corrosion is examined according to environmental regions to clearly investigate the effect of environment on the occurrence of anchor bolt corrosion. Regions were chosen that are either geographic or demographic in nature.
5.2.2.1 Geographic regions The mutually exclusive geographic regions chosen were northern, southern, and
coastal Georgia. In Figure 5.3 the three regions are separated by color; northern Georgia is in blue, southern Georgia in red, and coastal Georgia in green.
Differentiating northern Georgia from the other two regions is the use of chemical deicing agents on bridges. Chloride ions from the deicers, carried to the bearing by moisture leaking through deck joints, are aggressive agents known to initiate and accelerate corrosion. According to Ben Rabun, the previous State Bridge Maintenance Engineer, chemical deicers are used in GDOT Districts 1, 6, and 7, and in the northern counties of District 2. Thus, the northern Georgia region is loosely defined by Interstate 20, which passes through the southern edge of the Districts named.
Bridges in coastal Georgia are exposed to a constant moist environment, due to the high humidity in the region, and bridges spanning sea water may also have bearings
70

exposed to chloride ions. These conditions are favorable to the initiation and propagation of corrosion. Interstate 95 was selected as a simple boundary for the coastal region.
Unlike the northern and coastal Georgia regions, bridges in the southern Georgia region are not exposed to aggressively corrosive agents like chlorides. This region, south of Interstate 20 and west of Interstate 95, is subjected to higher temperatures year round, which may increase corrosion rates.
Figure 5.3: Three geographic regions in Georgia with different environmental conditions, percentage of the total number of steel girder bridges in Georgia in each region, and percentage of bridges with anchor bolt corrosion in each region.
71

Figure 5.3 shows the division of the three geographic regions along with the percentage of the total number steel girder bridges in Georgia contained in each region and the percentage of bridges with anchor bolt corrosion in each region. Despite the differences in environmental conditions in each region, Figure 5.3 shows that approximately 30% of the bridges in each region experienced anchor bolt corrosion. Since the percentage of bridges with anchor bolt corrosion in each region is nearly the same, it can be concluded that the differences in the geographic environments in Georgia do not significantly affect on the occurrence of anchor bolt corrosion.
5.2.2.2 Demographic regions The demographic regions explored were metropolitan areas versus rural areas.
Metropolitan areas were defined by the researcher as regions in which the population density was greater than or equal to 2,500 people per square mile. In these regions of higher population density, the daily traffic on the bridges is higher, subjecting the bridge and its bearings to increased fatigue loading. Also, the bridges in urban regions are environmentally exposed to a greater variety and quantity of pollutants, which may affect the corrosion process. By exploring the percentages of bridges with anchor bolt corrosion in metropolitan and rural regions, the effects of the urban environment on occurrence of anchor bolt corrosion were observed.
Table 5.1 shows that less than half of all the steel girder bridges in Georgia are located in metropolitan areas. Despite the number of bridges in each region, however, Table 5.1 also shows that nearly the same percentage of bridges in metropolitan and rural regions were found to experience anchor bolt corrosion. In both regions approximately 27% of the bridges exhibited anchor bolt corrosion similar to the percentages of bridges
72

with anchor bolt corrosion in the geographic regions and the percentage calculated for the entire state. Therefore, it can be concluded that the differences in the environmental conditions throughout the state of Georgia do not affect the occurrence of anchor bolt corrosion in steel girder bridges.

Table 5.1: Percentage of the total number of steel girder bridges in Georgia in

metropolitan and rural regions, and percentage of bridges with anchor bolt corrosion in

each region.

Regional Areas

Percentages

Metro

Rural

% of steel girder bridges

37.1%

62.9%

% with anchor bolt corrosion

26.9%

27.7%

5.2.3 Bridges grouped by span type The span types designated by the researcher were Interstate bridges, Interstate
underpasses, or other spans. Bridges which are a part of the Interstate road system, or Interstate bridges, may span other roads as overpasses or may span water or lowlands. Interstate underpasses are bridges that carry other roads over the Interstate. Any bridges which are not a part of the Interstate system and do not span any Interstate roads fall under the classification of other spans. Other spans include bridges that span other roads, water, or low land. In general, Interstate bridges are subjected to heavier loads and more traffic compared to Interstate underpasses and other spans which are subjected to local traffic conditions.
In Table 5.2, the percentage of the total number of steel girder bridges for each span type is listed along with the percentage of bridges of each span type that are experiencing anchor bolt corrosion. From Table 5.2 it can be seen that slightly over one
73

quarter of the steel girder bridges in Georgia are part of Interstate roads, while over half of the steel girder bridges in Georgia are bridges that are not related to the Interstate system. Of those bridges away from Interstate traffic, approximately one quarter have reported anchor bolt corrosion, while reported anchor bolt corrosion for Interstate bridges approaches one third. The percentage of Interstate underpasses with anchor bolt corrosion falls in between one quarter and one third. These percentages suggest that the occurrence of anchor bolt corrosion may be related to loading conditions of the bridge.

Table 5.2: Percentage of the total number of steel girder bridges in Georgia that are

Interstate bridges, Interstate underpasses, and other spans, and percentage of bridges of

each span type with anchor bolt corrosion.

Bridge Type

Interstate

Percentages

Interstate bridge

underpass

Other

% of steel girder bridges

26.5%

21.2%

52.3%

% with anchor bolt corrosion

32.7%

27.4%

24.7%

5.3 Bridge age statistics To further assess the condition of anchor bolt corrosion in Georgia, the ages of the
bridges with anchor bolt corrosion were examined. The age of each bridge at the time when anchor bolt corrosion was first reported was collected and a histogram was created to present the data, as shown in Figure 5.4. Each bar in the chart on the horizontal axis represents a ten year age range. The percentage of bridges with anchor bolt corrosion whose ages fit within a specific ten year age range is read from the vertical axis. The ages of the bridges are not representative of present day ages, but rather of the individual bridge ages when anchor bolt corrosion was first reported for each bridge.

74

State of Georgia

50
40
% of bridges in the state 30 of Georgia 20

Median age = 38 yrs Mean = 39 yrs St. Dev. = 11.0 yrs

10

0 0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89
Age range in years when anchor bolt corrosion is first reported

Figure 5.4: Distribution of bridge ages when anchor bolt corrosion is first reported for the whole State of Georgia.

From Figure 5.4 it can be seen that the greatest percentage of bridges with anchor bolt corrosion are first reported when they are between 30 and 40 years old. Overall, the age distribution is positively skewed, with more anchor bolt corrosion reported for bridges over 40 years old than reported for bridges less than 30 years old. The median and mean ages at which anchor bolt corrosion was reported in Georgia are 38 and 39 years, respectively.
In addition to the age distribution of bridges with anchor bolt corrosion for the entire state of Georgia, age distributions for each region defined in Section 5.2 are presented in Figures 5.5 through 5.12. By comparing the age distributions for each region the effect of environment on the initiation and/or propagation of anchor bolt corrosion can be observed.

75

50
40
% of bridges in 30 Northern 20 Georgia
10

Northern Georgia
Median age = 37 yrs. Mean age = 38 yrs. St. Dev. = 10.60 yrs.

0 0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89

Age range in years when anchor bolt corrosion is first reported

Figure 5.5: Distribution of bridge ages when anchor bolt corrosion is first reported for northern Georgia.

Southern Georgia

40
% of 30 bridges in
20
Southern Georgia 10

Median age = 41 yrs. Mean age = 43 yrs. St. Dev. = 11.45 yrs.

0 0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89

Age range in years when anchor bolt corrosion is first reported

Figure 5.6: Distribution of bridge ages when anchor bolt corrosion is first reported for southern Georgia.

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70
60
% of 50 bridges 40 in Coastal 30 Georgia 20
10
0

Coastal Georgia Median age = 34 yrs. Mean age = 34 yrs. St. Dev. = 7.25 yrs.
0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89

Age range in years when anchor bolt corrosion is first reported

Figure 5.7: Distribution of bridge ages when anchor bolt corrosion is first reported for coastal Georgia.

Comparing Figure 5.5 to Figure 5.4, the age distribution for northern Georgia has a similar shape to the age distribution for the whole state. For all three geographic regions, the greatest percentage of anchor bolt corrosion is reported 30 to 39 years after the bridges are built. In the coastal Georgia region, shown in Figure 5.7, little variance in the distribution is observed, as approximately 65% of bridges with anchor bolt corrosion were reported in this 30-39 year age range. Conversely, in southern Georgia the age distribution is more positively skewed, and the mean and median ages for this region are 4 to 9 years higher than the other geographic regions.

77

60
50
% of
40
bridges in
30
Metropolitan
20
Georgia
10
0

Metropolitan Georgia Median age = 35 yrs. Mean age = 35 yrs. St. Dev. = 9.19 yrs.
0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89
Age range in years when anchor bolt corrosion is first reported

Figure 5.8: Distribution of bridge ages when anchor bolt corrosion is first reported for metropolitan areas in Georgia.

40
30
% of bridges in 20
Rural Georgia 10

Rural Georgia

Median age = 40 yrs. Mean age = 42 yrs. St. Dev. = 11.13 yrs.

0 0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89
Age range in years when anchor bolt corrosion is first reported
Figure 5.9: Distribution of bridge ages when anchor bolt corrosion is first reported for rural areas in Georgia.

The age distribution for bridges with anchor bolt corrosion in metropolitan areas of Georgia, shown in Figure 5.8, is symmetric about the 30 to 39 year age range; approximately 50% of the bridges with anchor bolt corrosion in the metropolitan areas
78

were reported in the 30 to 39 year age range. In contrast, the age distribution for bridges with anchor bolt corrosion in rural areas in Georgia is positively skewed and shifted toward higher ages. The 30 to 39 year age range contains 37% of the bridges in the region with anchor bolt corrosion; on either side of this range, the 40-49 year age range contains 31% while the 20 to 29 year age range contains just 9%. Additionally, the median and mean ages of bridges in rural areas are 5 to 7 years higher than those in metropolitan areas.
Anchor bolt corrosion in bridges associated with the Interstate system, either as a part of the Interstate roads or as an underpass, is most often reported three and a half decades after they are built. For both Interstate bridges and Interstate underpasses, the age distribution is symmetrically centered at the 30 to 39 year age range with narrow variance. However, the age distribution for the third span type shows that bridges that are not associated with the Interstate system tend to be older before anchor bolt corrosion is reported. Their age distribution is symmetrically centered at the 40 to 49 year age range, but has a wider variance.
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70
60
% of 50 Interstate 40 bridges in 30 Georgia 20
10
0

Interstate Bridges in Georgia Median age = 34 yrs. Mean age = 34 yrs. St. Dev. = 6.29 yrs.
0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89
Age range in years when anchor bolt corrosion is first reported

Figure 5.10: Distribution of bridge ages when anchor bolt corrosion is first reported for Interstate bridges in Georgia.

Interstate underpasses in Georgia

70
60
% of
50
Interstate
40
underpass
30
bridges 20 in Georgia 10

Median age = 36 yrs. Mean age = 35 yrs. St. Dev. = 6.55 yrs.

0 0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89

Age range in years when

anchor bolt corrosion is first reported

Figure 5.11: Distribution of bridge ages when anchor bolt corrosion is first reported for Interstate underpass bridges in Georgia.

80

40
% of bridges 30 unrelated to
20
Interstates in Georgia 10

Bridges unrelated to Interstates
Median age = 45 yrs. Mean age = 45 yrs. St. Dev. = 12.42 yrs.

0 0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89
Age range in years when anchor bolt corrosion is first reported
Figure 5.12: Distribution of bridge ages when anchor bolt corrosion is first reported for bridges not associated with the Interstate system in Georgia.

Table 5.3 concisely compares the age distributions of all the regions by reporting the median age, mean age, variance, and standard deviation of the age at which anchor bolt corrosion is first reported for each region.

Table 5.3: Characteristic statistics of the age distributions of reported anchor bolt

corrosion for the state of Georgia and for the environmental regions.

Age distribution statistics
Median age (yrs) Mean age (yrs)

whole state
38
39.4

north GA
37
38.1

south GA
41
42.5

Regional areas coast GA City Rural
34 35 40
33.9 34.9 42.0

Interstate Interstate bridge underpass

34

36

34.1

35.0

Other 45 44.9

Variance Standard deviation

121.1 113.2 131.1 52.6 84.5 123.9 39.6 11.00 10.64 11.45 7.25 9.19 11.13 6.29

42.9 154.3 6.55 12.42

Key factors which may affect the age at which reported anchor bolt corrosion is first reported include the regularity and quality of inspections performed. For example,

81

anchor bolt corrosion may be detected at an earlier stage for bridges that are part of the Interstate system because these bridges may be more carefully or more regularly inspected compared to the non-Interstate bridges. If anchor bolt corrosion is not reported for non-Interstate bridges until the corrosion has progressed into later stages, then a higher age until anchor bolt corrosion incidence is misleading. A similar analogy may be drawn for bridges in metropolitan and rural regions.
Assuming equivalence in the inspection quality and regularity among all environmental regions, the differences in the age distributions imply that the environmental conditions do affect the rate at which corrosion initiates and/or propagates. It may be concluded that the rate of the incidence of anchor bolt corrosion is higher for bridges in coastal and northern Georgia compared to southern Georgia, is higher for bridges in metropolitan areas compared to rural areas, and is higher for bridges associated with the Interstate system compared to non-Interstate bridges. Further discussion of these results is provided in Chapter 9.
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CHAPTER 6: FIELD INVESTIGATION REPORTS
6.1 Inspection Methods Several bridges throughout the state of Georgia were selected to be inspected
specifically for anchor bolt corrosion by the researcher. At each bridge attempts were made to inspect the anchor bolts at both fixed bearings and expansion bearings to evaluate the effect of bearing type on corrosion. However, for several of the highway bridges, access to the interior supports was not available, and in these cases only the bearings at the abutments were inspected.
The bearings were inspected by visual and physical methods. Visual signs of corrosion included rust staining, section loss in anchor bolts, and evidence of corrosion product by a "swelling" of bearing components. Physical methods of inspection included twisting or wiggling the bolt by hand, and hammering on the side of the bolt and nut. Corroded anchor bolts were found to be loose and would move upon contact, and paint would easily chip off a corroded anchor bolt or nut.
Additionally, soil samples from the bearings were collected from several bridges to further investigate the bearing environment in a laboratory analysis. The result of soil analysis is given in Chapter 8.
The following sections present the field inspection reports by region. Comparisons drawn from the reports for each region can be used to examine climate and local environmental effects on bearing corrosion.
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6.2 Bridges in Metro Atlanta area, Georgia DOT District 7 The following four bridges in the Metro Atlanta area were chosen to be inspected
because each one was in the process of being replaced at the time of the inspections. These bridges were not previously singled out by the Maintenance Department as bridges with reported anchor bolt corrosion.
6.2.1 Old Dixie Highway (State Route 3/US 19) over Central of Georgia Railroad. The Old Dixie Highway bridge is located southeast of Atlanta in Clayton County,
0.35 miles south of I-285. Designed in 1937, the eight span steel girder bridge extends over the railroad tracks of the Central of Georgia Railroad. Numbering from west to east, spans one through three and six through eight consisted of ten simply supported steel girders. A new five span prestressed concrete girder bridge was built just to the south of the old bridge, and an embankment in place for the construction of the new bridge allowed inspection of the southernmost edge beam at Bent 7 and Bent 8, in addition to inspections at both abutments.
The bearings for the simple spans were one steel bearing plate approximately three inches thick placed between the concrete pier cap and steel flange. The anchor bolt passed through a slotted hole in the bearing plate and flange at the expansion end and a round hole at the fixed end. A typical fixed bearing at Bent 9 is shown in Figure 6.1.
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Figure 6.1: Typical fixed bearing at Bent 9. In general the anchor bolts and nuts at the abutments appeared to be in good condition.
Upon general inspection the bolts and nuts in the bearing assemblies at Bents 1 and 9 appeared to be in good condition, with exception to a few anchor bolt nuts with flaking corrosion products. At the interior Bents 7 and 8, rust stains were noted on the bearing plates, beam flanges, and washers, despite the apparent sturdy condition of the anchor bolts, as shown in Figure 6.2. It was also noted that the interior bents collected more debris from spalling concrete diaphragms. Expansion slots were filled with dirt and oxides, as shown in Figure 6.3.
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Figure 6.2: Rust stains on bearing plates, beam flanges, and washers at interior bents, despite the apparent sturdy condition of the anchor bolts.
Figure 6.3: Expansion slot filled with dirt and oxides at interior Bent 7. In general, the anchor bolts of the Old Dixie Highway (State Route 3/US 19)
bridge over Central of Georgia Railroad were not visibly damaged by corrosion. Instead, 95% of the corrosion observed at this site occurred in the girder flanges originating from crevice corrosion at the interface between the bearing plate and the girder flange. Before demolition of the bridge it was not apparent how this corrosion may have affected the
86

bolts inside the bearing assembly. No movements were observed when the nuts and bolts were twisted or struck with the hammer and the paint did not readily chip. 6.2.2 Lawrenceville Highway (State Route 10) over I-285
Lawrenceville Highway is located east of Atlanta, and the bridge that was inspected spanned eight lanes of I-285. An eight girder four span bridge, the middle two spans are continuous and the end spans are simply supported. This bridge was designed in 1967 and employed the typical plate bearing design presented in Chapter Two.
At the time of the inspection, the bridge appeared newly painted and cleaned. No obvious corrosion was visible and little debris was found at all of the bents. Slight crevice corrosion was found between the base and sole plate of the fixed bearing for the edge beam at Bent 1, which is shown in Figure 6.4. The anchor bolts of the Lawrenceville Highway (State Route 10) bridge over I-285 were not visibly or obviously physically damaged by corrosion. The bolts and nuts did not move and the paint did not chip when struck with the hammer.
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Figure 6.4: Corrosion on bearing plates of edge beam bearing at Bent 1. The anchor bolts were not obviously visibly or physically damaged by corrosion.
6.2.3 Memorial Drive over I-285 Memorial Drive, just south of Lawrenceville Highway, is also east of Atlanta and
spans six lanes of I-285. The Memorial Drive bridge was designed in 1965 two years before the Lawrenceville Highway bridge. As a continuous four span bridge, rocker bearings are used at all of the supports except for the middle one.
Unlike the Lawrenceville Highway bridge, the Memorial Drive bridge had open deck joints above the abutments. For this reason, significantly more debris was found at the abutments of the Memorial Drive bridge compared to the Lawrenceville Highway bridge. The amount of debris notably affected the corrosion of the bearings.
Figures 6.5 and 6.6 show bearings from the Memorial Drive bridge without and with debris, respectively. At the bearing without significant debris, shown in Figure 6.5, a spot of corrosion scale approximately the size of a quarter was found on the base plate of the bearing and the other components appeared unaffected. In contrast, at the bearing surrounded by dirt and debris, shown in Figure 6.6, the entire base plate was enveloped in
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corrosion scale along with the bottom of the rocker and the anchor bolt. While specific anchor bolt corrosion problems were not found, the inspection of the Memorial Drive bridge over I-285 clearly illustrated the detrimental effect that debris at bearings has on the corrosion behavior of those bearings.
Figure 6.5: Bearing at abutment without debris. Base plate has a quarter sized spot of corrosion scale.
Figure 6.6: Bearing at abutment with debris. The base plate, anchor bolt, and bottom of the rocker that are enveloped in corrosion scale demonstrate the detrimental effect of debris at bearings.
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6.2.4 State Route 92 over I-20 State Route 92 crosses over six lanes of I-20 west of Atlanta in Douglas county.
The nine girder four span bridge is simply supported at all spans. Designed in 1960, the bearing omits a lube plate and consists of a base plate and a sole plate, with slotted holes for the expansion bearing and round holes for the fixed bearing.
Due to limited access to the interior bents, only the abutment bearings were inspected for this bridge. Open joints at both abutments allowed the ingress of debris on the abutment pier caps. Significant bearing corrosion was observed for each of the nine girders, and anchor bolts were missing in 15 of 36 locations at the abutment bearings. At locations in which anchor bolts were present, the material and bolt strength were tested by striking the anchor bolt and nut with a hammer. At all bearings the paint on the bolts and nuts was found to chip easily when struck, revealing dark oxides beneath.
One third of the remaining bolts were found to be very loose when struck. Little force was needed to wiggle the bolts within the holes, and the diameters of the bolts were observed to have been reduced to less than inch. In two locations the anchor bolts broke off completely at the juncture of the beam flange and sole plate. Figure 6.7 shows one of these locations prior to the bolt breaking off. This figure reveals that the bolt had been pushed up approximately 1 inches, presumably from the accumulation of oxides in the bearing hole. Although previous corrosion damage had been painted over, it can be seen that the diameter of the bolt started to decrease approximately one inch below the nut, indicating that the corrosion initiated within the bearing between the base and sole plate.
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Figure 6.7: Anchor bolt which was easily broken. The base of the nut is approximately 1 -in above the top of the flange indicating that the bolt has been pushed up that distance by corrosion products. A steel angle had been previously bolted to the abutment to restrain lateral movement.
At four other locations bolts were also found to be pushed up, but remained tight within the bearing hole. In these instances the bolts' diameters appeared to be swelled with corrosion product which inhibited any bolt movement within the bearing, as shown in Figure 6.8. Similarly, at 80 percent of the anchor bolt locations, the anchor bolt nuts appeared swelled with oxides and painted over. At one site, the corrosion product accumulation grew so large that it cracked the nut apart, as shown in Figure 6.9.
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Figure 6.8: Anchor bolt diameter swelled with corrosion product inhibits bolt movement within the bearing and restricts the thermal movements of the bridge.
Figure 6.9: Corrosion product accumulation breaks anchor bolt nut. In summary, all of the existing anchor bolts and/or nuts at the abutment bearings
of the State Route 92 bridge over I-20 were found to be significantly affected by anchor bolt corrosion.
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6.3 Bridges in south Georgia, GDOT Districts 4 and 5 Four bridges scheduled for replacement were inspected in south Georgia. These
bridges were near Valdosta and Waycross and were not near a coastal area.
6.3.1 State Route 122 over Little River, District 4__ The State Route 122 bridge is approximately 40 simply supported spans
extending over the Little River and surrounding lowlands for nearly mile. Even after periods of heavy rain, only the middle quarter of the bridge spanned water. A new pre-stressed concrete girder bridge was in construction to the south of the existing bridge.
In this inspection, a random sample of the bearings on the west side of the river was examined using the bridge contractor's bucket lift. The bearings consisted of a 1 inch base plate on the concrete pier cap and a inch flat sole plate beneath the girder flange. Expansion bearings had slotted holes and fixed bearings had round holes. Generally, the pier caps were clean of debris or bird droppings.
Of the anchor bolts inspected, approximately 40 percent displayed corrosion losses. Corrosion of the anchor bolts was observed most readily in the expansion bearings, as the slotted holes allowed greater access to the bolt shaft beneath the washer and girder flange. Signs of corrosion were most prevalent beneath leaking deck joints and in bearings closer to the river.
All corrosion losses observed were of a similar type. The anchor bolts appeared in good condition above the girder flange, but closer visual inspection of the bolt underneath the washer revealed gradual section loss of the bolt shaft. As shown in Figure 6.10, the diameter of the bolt began to decrease just below the washer as it passed through the
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flange, reaching its smallest diameter approximately one to 1 inches lower at the interface where the bolt protrudes from the concrete. In many instances the corrosion loss had been painted over. Figure 6.11 portrays the same type of corrosion loss that has not been painted over. In general, the diameter of the bolts at their smallest point was observed to be approximately to inches, which was roughly 50 % of their original diameter and between 84% and 64% loss of cross section. Although in one case, shown in Figure 6.12, 100 percent section loss was observed.
Figure 6.10: Typical anchor bolt corrosion of S.R. 122 bridge. The diameter of the bolts decreased just below the washer as they passed through the flange, reaching their smallest diameter approximately one to 1 inches lower at the concrete interface.
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Figure 6.11: Anchor bolt cross section loss that has not been painted.
Figure 6.12: Complete section loss in anchor bolt. The anchor bolt and washer appear corrosion free above the top of the flange. 6.3.2 US Route 1 over Satilla River, District 5
The US Route 1 bridge over the Satilla River was built in 1949 and is nearly one mile long. However, only the four spans over the river use steel girders, as shown in Figure 6.13. Access to the steel girder bearings was difficult, and the inspection was limited.
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Figure 6.13: US Route 1 over Satilla River. Only the four middle spans were steel girders and inspection of the bearings was limited.
Aside from a few visible spots of general corrosion on the bearing plates and nuts, the bearings of the US Route 1 bridge appeared to be in good condition. Figure 6.14 shows general corrosion on the anchor bolt nut at the fixed end of the end span. Detailed investigation was not feasible to determine the condition of the bolts below the washer or flange at any of the bearings. The bolts and nuts did not move and the paint did not chip when struck with a hammer.
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Figure 6.14: General corrosion on anchor bolt nut at a fixed bearing. The anchor bolt and nut did not move and the paint did not chip when struck with a hammer.
6.3.3 State Route 121 over Fishing Creek, District 5 Built in 1950, the State Route 121 bridge over Fishing Creek is a simply
supported bridge. The bearing design employed a inch flat sole plate beneath the girder on top of a one inch flat base plate on the concrete pier. No debris was present around the bearings.
At the fixed ends of the spans, no anchor bolt corrosion was visible. However, at the bridge expansion joints, corrosion was observed under the bolt washers. Similar to the findings at the State Route 122 bridge, the bolts were discovered to be decreasing in diameter from where the bolt first passed through the flange to where they entered the concrete pier cap. Figures 6.15 and 6.16 provide two examples of this behavior, where dirt was cleared away from beneath the washer to allow visual inspection of the bolt shaft within the expansion slot.
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Figure 6.15: Anchor bolt reduction in diameter due to corrosion in expansion slot. Dirt was cleared away to allow visual inspection within the expansion slot.
Figure 6.16: Anchor bolt corrosion below washer. Corrosion loss occurred in the area between the concrete interface and the top of the beam flange. 6.3.4 State Route 144 over Watermelon Creek, District 5
Spanning a small waterway, the State Route 144 bridge is a three span simply supported bridge that was built in 1948. Similar to the previous bridges inspected in the south Georgia region, the bearing design contained two flat plates. Also, no debris was near any of the bearings.
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At the fixed end of the spans, no corrosion was observed by either visual or physical methods on the anchor bolts or bearing plates. Limited access to the expansion ends of the spans prevented a thorough investigation of the bolt condition below the washer and beam flange. However, general corrosion was observed on the anchor bolt nut and base plate, as shown in Figure 6.17. Additionally, corrosion product was scraped from the bottom of the washer.
Figure 6.17: General corrosion on the anchor bolt nut and base plate at the expansion bearing. Limited access prevented a thorough investigation of the bolt condition below the washer and beam flange.
6.4 Bridges in north Georgia, GDOT Districts 1 and 6 Three bridges in northern Georgia were visited to investigate the bearing and
anchor bolt corrosion in this region. The bridges that were visited were: (1) US Route 76 over a creek in Blue Ridge, Fannin County, Georgia; (2) State Route 515 over Georgia Northeast Railroad in Blue Ridge, Fannin County, Georgia; and (3) US Route 19 over US Route 76 in Blairsville, Union County, Georgia. Access at each of these bridges was limited, preventing useful inspection of the bearing assemblies. Observations of bearing
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corrosion could not be recorded. However, soil samples accessible near the bearings were collected and were analyzed to determine typical Georgia bearing environments, as described in Chapter 8.
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CHAPTER 7: FAILURE ANALYSIS OF FIELD SPECIMENS 7.1 Visual analysis 7.1.1 Bolts broken in the field
During the field investigations, a few broken bolt specimens were obtained from corroding bearings. Two broken bolts, shown in Figures 7.1 and 7.2, were taken from the State Route 92 bridge over I-20 in Douglas County, constructed in 1962. One broken bolt, shown in Figure 7.3, was taken from the State Route 122 bridge over Little River in between Brooks and Lowndes counties, constructed in 1941.
Figure 7.1: Bolt 1 from northern Georgia at the SR 92 bridge over I-20. When this bolt was found, it was already bent and broken.
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Figure 7.2: Bolt 2 from northern Georgia at the SR 92 bridge over I-20. When this bolt was found, it was very loose and was easily broken by hand.
Figure7.3: Bolt from southern Georgia at the SR 122 bridge over Little River. The image on the left is a view of the bolt from the side, and the image on the right is a view of the bolt from directly underneath the washer. This bolt was already broken when it was found.
Both of the bolts in Figures 7.2 and 7.3 exhibit significant and rapid cross section loss. In areas where corrosion scale was present, it seemed to be uniform; the diameter of the bolt in Figure 7.3 was uniformly reduced. From the visual analysis of these two bolts,
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it can be hypothesized that this uniform corrosion may be locally accelerated in the trapped solution in the surrounding cavity, forming a concentration cell.
The bolt in Figure 7.1 does not exhibit a rapid cross-sectional loss to the point where it is broken. Instead, it appears that the entire cross-section of the bolt beneath the washer is marginally decreased due to general, uniform corrosion and that the bolt was bent and broken by a mechanical force. 7.1.2 Bolts and bearing plates from bridge demolition
In addition to the broken specimens obtained during field investigations, whole anchor bolts and bearing plates were acquired from the demolition of the Old Dixie Highway bridge in Clayton county. A variety of bearing plates and anchor bolts were acquired from fixed and expansion bearings at edge and interior girders. Figures 7.4 and 7.5 show a typical fixed and expansion bearing.
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Figure 7.4: Bearing plate and anchor bolts from an interior girder expansion bearing. The expansion slots are not visible because they are completely filled with debris and corrosion products.
Figure 7.5: Bearing plate and anchor bolts from a fixed bearing. 104

Originally, these samples were intended to be used for additional failure analysis. Unfortunately, difficulties in removing the anchor bolts from either expansion or fixed bearing plates prohibited further analysis. The anchor bolts had adhered to the bearing plates with corrosion products in the bearing holes acting as a weld. The shear strength of the adhesion is demonstrated in Figure 7.6. Even under the direct impact loads, the anchor bolt-bearing plate assembly would not dislodge.
Figure 7.6: Build up of corrosion products prevented the anchor bolts from dislodging from the bearing plate.
A commercially available corrosion debonding product was used in an attempt to disintegrate the corrosion scale. While the anchor bolts were still not freed from the bearing plate, enough corrosion scale was removed to allow the visual examination of the interface of the bolt and the bearing plate, as shown in Figure 7.7.
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Figure 7.7: Corrosion build-up visible at the interface of the bolt and bearing plate.
From a visual analysis of these anchor bolts and bearing plates, it is clear that in most instances, the diameter of the anchor bolt is necked down near the bearing plate. The accumulation of corrosion product from the anchor bolt hides the extent of bolt cross-section loss. Accumulation of corrosion product in the bearing hole was so voluminous that it expanded outwards from the hole. In conclusion, these samples proved that corrosion of anchor bolts is not only detrimental to the bolt but the corrosion build up can be responsible for bearing seizure. Further failure analysis was subsequently performed on the broken bolts obtained during the field investigations.
7.2 Microscopy of failure surface The bolts obtained during the field investigations were microscopically analyzed
to gain insight into the mode of corrosion and bolt failure. Prior to microscopic analysis of the failure surface, the corrosion scale was cleaned from the bolt surface using
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Clarke's solution, a hydrochloric acid solution with corrosion inhibitors conforming to ASTM G1: Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test Specimens. To ensure that the cleaning solution would not induce corrosion, dummy samples of carbon steel were immersed in the solution. The solution was accepted when the dummy sample experienced no weight loss after immersion. The bolt samples were submerged in the solution in five-minute intervals, allowing the corrosion scale to dissolve in the acid without corroding the remaining carbon steel (ASTM 2003).
7.2.1 Optical microscopy Optical microscopy of the bolt surfaces are shown in Figures 7.8 through 7.11.
Since the corrosion morphology of galvanic and concentration cell corrosion is the same as general corrosion accelerated in a specific area, the surface appearance of areas exposed to these forms of corrosion appears similar to areas exposed to general corrosion. In a few of the following figures, an oxide film is shown to exist on the surface. Since the field specimens were cleaned of all corrosion scale prior to the surface analysis, any apparent oxides are due to general atmospheric corrosion during the analysis and are not related to the field conditions.
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Figure 7.8: Surface of bolt shown in Figure 7.2 at 6.3x magnification. The surface of the bolt is uniformly rough in the necked region.
Figure 7.9: Localized corrosion at paint defects on the surface of the bolt in Figure 7.2 at 12.5 x magnification.
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Figure 7.10: Edge of washer hole on washer found with bolt in Figure 7.1 at 6.3 x magnification. The crack may have originated at a crevice and was propagated by mechanical means.
Figure 7.11: Corrosion product build up in the crevice of the bolt shaft and washer hole of the bolt shown in Figure 7.3 at 6.3 x magnification. The corrosion product fused the remaining bolt shaft to the washer.
In Figure 7.8, the surface of the bolt is analyzed at the region where the diameter of the bolt necks due to corrosion. The surface appears uniformly rough, indicating general
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corrosion. No variation in the surface waves are evident that would indicate local pitting or crevice corrosion. Local corrosion is apparent, however, in Figure 7.9 where defects in the paint surface lead to pitting and crevice corrosion.
Figure 7.10 shows a surface with similar general corrosion characteristics to the surface in Figure 7.8, but with a noticeable crack. The image was taken from the washer found with the bolt in Figure 7.1 This crack may have been induced at a local pit or crevice at the edge of the washer hole, and was propagated by mechanical means and is not related to corrosion phenonmenon.
In Figure 7.11 the crevice between the bolt and the washer hole is shown to be filled with layers of corrosion product, fusing what is left of the bolt shaft to the washer.
7.2.2 Scanning electron microscopy Scanning electron microscopy was used to achieve a higher magnification at the
bolt failure surfaces. Figures 7.12, 7.13, and 7.14 present the typical surface condition in the necked region of the field specimen shown in Figure 7.2 at 100, 300, and 500 times magnification.
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Figure 7.12: Surface of bolt in Figure 7.2 at 100 x magnification. Wavy surface indicates general corrosion.
Figure 7.13: Surface of bolt in Figure 7.2 at 300 x magnification. Ridges in the surface were created during ductile failure of the bolt.
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Figure 7.14: Surface of bolt in Figure 7.2 at 500 x magnification. Voids on surface, corresponding to dark spots in the image, were opened during ductile failure.
In Figures 7.12 through 7.14, the wavy surface indicates that general corrosion morphology is prominent for these specimens with a ductile fracture mode. Fracture surface showed a typical ductile fracture morphology with microvoids, which corresponds to the dark spots in the images.
7.3 Analysis of bolt scale X-ray diffraction under CuK radiation was used to analyze the chemical
composition of the bolt scale from the bolts obtained in the field. The chemical composition of the scale provides insight into the corrosive environment surrounding the bolt. The peaks in the diffraction patterns correspond to peaks produced by known
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chemical compounds. By matching up the peaks from the bolt scale to peaks of known compounds, the chemical composition of the scale can be deduced. Evidence of iron chloride products, for instance, would indicate the presence and participation of chlorides in the corrosion process. X-ray diffraction was performed on bolt scale from a sample in northern Georgia and southern Georgia to compare the different environments.
Figure 7.15 displays the diffraction pattern for the scale from the bolt in Figure 7.3.

SiO2

SiO2

Fe3O4

Fe FeSi Fe3O4

Fe2O3 Fe3O4

Fe2O3

Fe FeSi

Figure 7.15: X-ray diffraction pattern for the bolt scale taken from the specimen shown in Figure 7.3. The peaks in the pattern correlate with iron, iron oxides, and iron silicates.
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The diffraction pattern for the sample from southern Georgia correlates with iron, iron oxides and iron silicates, as shown in Figure 7.15. For this specimen from southern Georgia, no correlations with corrosive ions existed. Similarly, the diffraction pattern for the specimen from northern Georgia does not show any chlorides or presence of other corrosive compounds in the surface scale as the scales were mostly oxides of iron. The role of this environment in the anchor bolt corrosion mechanism is unclear because the chemical composition of the bolt scale is indistinct. Thus, the mechanism for corrosion of anchor bolts in southern and northern Georgia is not positively associated with chlorides from deicing salts or any other corrosive ions in the environment, according to analysis by x-ray diffraction technique. As discussed in Chapter 9, the lack of chloride-containing compounds in the bolt scale, as determined by x-ray diffraction analysis, does not eliminate the possibility that chlorides aided in the initiation or acceleration of corrosion.
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CHAPTER 8: LABORATORY EXPERIMENTAL TESTING
8.1 Experimental method
8.1.1 Laboratory tests conducted Several laboratory experimental tests were conducted to determine the corrosion
behavior of the anchor bolt materials currently in use and candidate alloy materials to be considered for future use in the State of Georgia.
In a long term test ASTM Grade 36 carbon steel and ASTM Type 304 stainless steel samples from new anchor bolts were exposed to simulated environmental conditions for approximately two months. Their corrosion potentials were regularly monitored to determine their corrosion behavior. Polarization resistance method and weight loss measurements on these samples provided their corrosion rates.
Additionally, electrochemical polarization curves were created for the same materials. The polarization curves provided insight into the corrosion behavior of the materials tested and served to isolate the specific environmental conditions of concern.
Cyclic polarization was performed on the Type 304 stainless steel samples from new anchor bolts and on commercially available coupons of other candidate alloy materials to compare the pitting corrosion potentials of these materials in the given environment.
Finally, potentials and corrosion rates were measured for several combinations of electrically coupled materials simulating galvanic or concentration cell conditions in the plate bearing assembly. These combinations included: (1) carbon steel coupled with
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stainless steel, simulating carbon steel bearing plates in contact with stainless steel bolts; (2) carbon steel coupled with bronze, simulating carbon steel bearing plates in contact with the bronze lubrication plate; (3) stainless steel coupled with bronze, simulating the stainless steel bolt in contact with the bronze lubrication plate; (4) carbon steel, stainless steel, and bronze all coupled with one another simulating the bearing plates and bolt in contact with one another; (5) carbon steel in the alkaline environment coupled with carbon steel in the neutral environment, simulating the partial embedment of the bolt in concrete; and (6) stainless steel in the alkaline environment coupled with stainless steel in the neutral environment, simulating the partial embedment of the bolt in concrete.
8.1.2 Experimental testing environments The testing environments chosen were based on existing conditions at bridge
bearings. A solution simulating the environment found at the State Route 92 bridge over I-20 in GDOT District 7 was used in the experimental testing because this bridge displayed the worst case conditions. Of the bridges involved in the field investigations, the State Route 92 bridge had the most bearing corrosion damage. Age distribution data given in Chapter 5 suggest that bearing corrosion initiates and/or propagates faster in northern, metropolitan, and Interstate-related bridges; the State Route 92 bridge matches all of these classifications.
Soil samples were taken from debris found within the expansion bearings of the bridge. The soil samples were made into a solution of 10 grams of soil in 25 mL of de-ionized water, which was then analyzed for cations and anions. Table 8.1 shows the
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results of the analysis which was used as the basis for the solutions used throughout the laboratory experimental testing.
Soil samples from several other existing bridges involved in the field investigation were taken to verify the validity of the environment chosen. The bearing environment for southern Georgia was determined from soil samples from bridges in Districts 4 and 5, and the environmental conditions for northern Georgia bridges was investigated through soil samples from bridges in Districts 1 and 6. These results can be seen in Appendix D. From these results, it was determined that the solution based on conditions in District 7 is representative for the state. Ionic concentrations in the District 7 solution were typically higher than those in southern Georgia, and with few exceptions, similar to those in northern Georgia.

Table 8.1: Soil solution analysis from S.R. 92 bridge over I-20 in District 7 used as basis

for solutions in laboratory experimental testing.

Ion Na+ Ca2+ K+

Concentration (mg/L) 59.5 30.5 12.9

Cl-

40.1

SO42-

65.3

NO3-

1.71

CO3 2pH

56.7 7.65

Experimental tests were conducted in four variations of the chosen solution. The first was a "normal solution" of ion concentrations matching that shown in Table 8.1. The normal solution simulated a wet bearing environment where abundant water could easily

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mix with the bearing soil. The soil was damp at the time it was retrieved. A second testing solution, the "concentrated solution", contained ion concentrations that were ten times higher than the normal solution. The concentrated solution simulated expected conditions at the bearing as ion concentrations build up over time and/or when less moisture would be available, such as after the area had dried. The concentrated solution also accounted for exceptions where normal ion concentrations were higher than those in the chosen solution, such as some northern bridges that are exposed to deicing salts more often.
Both the normal and concentrated solutions were used at two different pH values, pH 7.5 and pH 13. The solutions at pH 7.5 replicated the pH found at the bearing in atmospheric conditions, while the solutions at pH 13 represented the typical pH within new concrete. In addition to the normal and concentrated ionic solutions at pH 7 and pH 13, a concrete pore solution was the fifth testing environment used. The pore solution did not contain the ions found at the bearings, but rather represented the solution typically found in clean concrete.
8.2 Experimental set-up
8.2.1 Sample preparation New anchor bolts were ordered from Highway Materials, a contractor supply
company located in Forest Park, Georgia, and which commonly supplies anchor bolts to bridge constructors. Both galvanized Grade 36 carbon steel and Type 304 stainless steel 1 -inch diameter bolts were supplied conforming to the GDOT previous and current
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material specifications, respectively. Thus, the materials tested were the same as those used in bridge bearing applications.
To conduct the laboratory experiments, the bolts were cut into coupons. The corrosion coupons were a 3/8-inch thick slice of the 1 -inch diameter bolt. The carbon steel coupons cut from the galvanized bolt retained the zinc coating along the 3/8-inch edge. The cut faces of the coupons were polished on the grinding wheel with 120 grit paper. For crevice corrosion and waterline corrosion tests, as a part of the long term experiment, 9.6 mm holes were drilled through the center of the sample, to allow the building of the racks as described in Section 8.2.3.
For the galvanic corrosion series of tests, a new cast bronze plate conforming to ASTM B 22 Alloy 911 was cut into square samples that were 2 -inches by 2 -inches and 3/8-inch thick. The cast bronze was donated by Lubrite Technologies, the company that manufactures and supplies the bronze lubrication plates to Georgia bearing manufacturers.
8.2.2 Solution preparation Solid chemicals were dissolved in de-ionized water in proportion to create the
concentrations of ions described in Section 8.1.2. To reach the desired pH value for the solutions, trace amounts of sulfuric acid were added to decrease the pH, and sodium hydroxide was added to increase the pH.
The simulated concrete pore solution was based on a recommended pore solution in ACI 440R-96. The solution was a saturated calcium hydroxide solution containing trace amounts of potassium hydroxide and sodium hydroxide. The pore solution was
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created by dissolving the prescribed sodium hydroxide and potassium hydroxide in deionized water, followed by the addition of calcium hydroxide to saturation. 8.2.3 Equipment set-up
Each test conducted required a special set-up, as described in the following sections. Common to all the tests, however, was the saturated calomel reference electrode and salt bridge, shown in Figure 8.1. On the left side of the picture, where the salt bridge is held by a clamp, the reference electrode is securely inserted into the salt bridge with a stopper. The solution in the salt bridge and around the electrode is a highly conductive potassium chloride solution. The tip of the salt bridge, which is placed near the corrosion coupon for measurements, is on the right in the figure entering a beaker of potassium chloride solution.
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Figure 8.1: Saturated calomel reference electrode and salt bridge used in all experimental laboratory tests. 8.2.3.1 Long term tests
For the long term experimental testing, carbon steel and stainless steel corrosion coupons were placed directly in the solution. One five-liter capacity container was used for each solution type for a total of five containers. Three different corrosion coupons of each steel type were monitored in each solution. Figure 8.2 shows the complete set-up of one solution container, with the general, crevice, and waterline coupons.
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Figure 8.2: Example of long term test set-up. One solution container with carbon steel and stainless steel with the general, crevice, and waterline corrosion coupons.
The general corrosion coupons were completely submerged in the solution, resting on the bottom of the container on a 3/8-inch thick side, so that both polished faces of 1 -inch diameter were exposed to solution. Crevice corrosion and waterline corrosion coupons were placed in the solution as part of a rack assembly. A close-up view of a waterline rack assembly is shown in Figure 8.3.
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Figure 8.3: A waterline rack assembly with plastic washers electrically separating the stainless steel (left) and carbon steel (right) corrosion coupons.
The racks were composed of threaded rod supported by plastic end stands through which the rod passed. Stiff plastic tubing with an outside diameter of 916 mm was placed around the rod to prevent electrical contact between the corrosion coupons and the rod; the rod and tubing was fit through the holes drilled in the center of the coupons. Plastic washers, either crevice or flat, were placed in between the coupons to create a desired exposure surface on the coupon and for electrical separation between the carbon and stainless steel coupons. Nuts on the outside of the plastic end stands were used to tighten the assembly and hold it together. A torque wrench was used to consistently tighten the racks at a torque of 75 in-lb in accordance with ASTM G78: Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments (ASTM 2001).
For the crevice corrosion coupons, crevice washers were positioned on both sides of the steel coupons. The uneven surface of the plastic crevice washers created a surface on the coupon in which the solution was in contact with the metal at small regular
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locations where the washer material was cut away. The crevice corrosion coupons and racks were completely submerged in the solution.
For the waterline corrosion coupons, a flat washer surface in contact with the coupon was used to avoid crevice effects. The rack was placed in the solution such that the bottom half of the coupons were submerged while the top half remained outside of the solution.
Three additional corrosion coupons were set up in the container of concentrated solution at pH 7.5 carbon steel coupons of each type with the no zinc edge were included to provide a comparison to the coupons with the galvanized edge.
8.2.3.2 Electrochemical polarization tests The equipment set-up used for the electrochemical polarization of the carbon steel
and stainless steel was also used for the cyclic polarization of the stainless steel. The general set-up of the electrochemical polarization cell can be seen in Figure 8.4. The electrochemical polarization cell was a custom designed. As seen in Figure 8.4 the main component of the cell is a beaker of approximately 300 mL capacity with a spout to the side. The corrosion coupon was clamped at the opening of the spout, with a rubber O-ring sealing the contact of the specimen and the glass.
As the solution filled the beaker and came into contact with the face of the coupon, the O-ring formed a slight crevice on the polished metal surface. The possibility of crevice corrosion during the polarizations was recognized as it had an effect on the polarization results. In real life application, however, the bolt is also subjected to
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significant crevice effects in the bearing assembly, and thus the conditions at the surface of the material during the polarizations simulated the anchor bolt environment.
Other components of the electrochemical polarization cell included the reference and counter electrodes. The reference electrode salt bridge entered the cell through the top opening and was clamped to a stand to secure the tip position next to the corrosion coupon. A platinum foil was introduced into the cell as a counter electrode, or cathode, during the polarization. The reference electrode, counter electrode, and corrosion coupon, or working electrode, were all electrically connected to the potentiostat and data were collected by the computer, as shown in Figure 8.5.
Figure 8.4: Experimental set-up for electrochemical polarization and cyclic polarization. The coupon that is being tested is clamped to the spout of the beaker.
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Figure 8.5: Computer software collects data from the potentiostat, which is electrically connected to the electrodes in the electrochemical polarization cell.
8.2.3.3 Galvanic and concentration coupling tests Several combinations of galvanically coupled metals exist in plate bearings. In
newer bridges, carbon steel plates and flanges are in contact with stainless steel anchor bolts, and both carbon steel plates and stainless steel anchor bolts are in contact with bronze plates. In this series of experiments, all these combinations are reproduced in both normal and concentrated solution at a pH of 7.5.
Additionally, a concentration cell exists along the bolt itself. As part of the bolt is protected within high pH concrete, the rest is actively corroding outside of the concrete. To model this environment, concentration cells were also set-up experimentally between the pH 7.5 solutions and pH 13 solutions.
The corrosion coupons that were coupled had stainless steel wire spot welded to them. The face of the coupon to which the wire was welded and the wire itself were lacquered to prevent them from affecting the corrosion potentials. Wires from two
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specimens were connected to provide the necessary electrical connection for corrosion. In all carbon steel and stainless steel connections, lacquer was used to set the anode and cathode areas equal. Similarly, the cast bronze plate that had stainless steel wire spot welded to it, was lacquered, and was connected to either carbon steel, stainless steel, or both.
The same containers of solution as the long term tests were used for galvanic couples within the same solution. For concentration cell couples across two different solutions, the solution from the original containers was transferred to smaller containers so that a salt bridge could be placed in between the two to provide the necessary electrolytic connection, as shown in Figure 8.6.
Figure 8.6: Concentration cell coupling of similar metals in solutions with differing pH values. Stainless steel in solution with pH 13 is coupled with stainless steel in solution with pH 7.5, and carbon steel in solution with pH 13 is coupled with carbon steel in solution with pH 7.5.
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8.3 Experimental procedures
8.3.1 Potential readings Corrosion potential of exposed steel samples versus the saturated calomel
reference electrode was measured using the salt bridge described in Section 8.2.3, a stainless steel wire probe, and a voltmeter. The voltmeter leads were connected to the reference electrode and the stainless steel wire probe. The salt bridge tip was placed within a centimeter of the corrosion coupon for which the reading was taken. When the wire probe firmly touched the coupon, an electrical potential difference reading between the coupon and the reference electrode was recorded. Care was taken to isolate all but the tip of the stainless steel wire probe to avoid creating a mixed potential reading of the corrosion coupon and the wire probe.
8.3.2 Electrochemical polarization tests As described in Section 8.2.3, the reference electrode, counter electrode, and
working electrode in the polarization cell were all connected electrically to a potentiostat. In the electrochemical polarization tests potentiodynamic potentials were applied to the specimen and the corresponding currents were recorded. As explained in Chapter 3, a plot of the voltages and corresponding current densities on log scale generates the polarization curve, which can be used to provide insight to the material corrosion behavior.
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8.3.3 Cyclic polarization Cyclic polarization applies the same concepts as electrochemical polarization
described in Chapter 3 and the experimental procedure as described in Section 8.3.2. The test receives its name, however, from a cycling of the applied voltage. After the applied voltage has reached a set peak, it is subsequently decreased and the corresponding behavior of the current density is examined for evidence of localized corrosion.
8.3.4 Polarization resistance The polarization resistance method as described in Chapter 3 was used for
determining active corrosion rates on samples in both the long term test and the galvanic and concentration cell coupling tests. To measure the polarization resistance, the potentiostat was electrically connected to the reference electrode, platinum counter electrode, and working electrode. The polarization resistance curve was generated by plotting the overvoltages, within a few millivolts of the corrosion coupons' stable corrosion potential, versus the current. From these curves, the corrosion rates of the corrosion coupons were extracted according to the equations presented in Chapter 3, and using assumed Tafel constants equal to 0.12 volts..
129

8.4 Results 8.4.1 Long term tests 8.4.1.1 Corrosion potential readings
The following graphs, Figures 8.7 through 8.11, are representative of the long term corrosion potential measurements. The graphs shown are for the steel samples exposed to the concentrated test solution with pH 7.5 and pH 13. No significant difference in the corrosion potential readings was observed between steel samples exposed to the normal and concentrated solutions. A complete set of graphs for the normal solution and pore solution environments is given in Appendix D. In these graphs more negative potential difference values in the range of -0.9 to -0.6 volts indicates active corrosion, while more positive potential difference values in the range of -0.4 to 0.0 volts indicate passive behavior of the material.
130

Potential Difference (V) vs. SCE

0.00 -0.10 -0.20 -0.30 -0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00
0.00

Potential Difference vs. Time
Galvanized carbon steel in concentrated solution pH 7.5
C - Crevice C - General C - Waterline

10.00

20.00

30.00

40.00

50.00

Exposure Time (days)

60.00

70.00

80.00

Figure 8.7: Long term corrosion potential readings of galvanized carbon steel corrosion coupons in the concentrated solution at pH 7.5. The galvanized carbon steel displays unstable passivity with potentials undulating between active and passive values.

131

Potential Difference (V) vs. SCE

0.00 -0.10 -0.20 -0.30 -0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00
0.00

Potential Difference vs. Time
Stainless steel in concentrated solution pH 7.5
S - Crevice S - General S - Waterline

10.00

20.00

30.00

40.00

50.00

Exposure Time (days)

60.00

70.00

80.00

Figure 8.8: Long term corrosion potential readings of stainless steel corrosion coupons in the concentrated solution at pH 7.5. The stainless steel coupons have passive corrosion potentials.

132

Potential Difference (V) vs. SCE

0.00 -0.10 -0.20 -0.30 -0.40 -0.50 -0.60 -0.70 -0.80 -0.90 -1.00
0.00

Potential Difference vs. Time
Carbon steel without zinc in concentrated solution pH 7.5
C - Crevice w /o Zn C - General w /o Zn C - Waterline w /o Zn

10.00

20.00

30.00

40.00

50.00

Exposure Time (days)

60.00

70.00

80.00

Figure 8.9: Long term corrosion potential readings of carbon steel corrosion coupons without zinc in the concentrated solution at pH 7.5. The carbon steel coupons without zinc have active corrosion potentials.

133

Potential Difference vs. Time
Galvanized carbon steel in concentrated solution pH 13
0.00

-0.20

Potential Difference (V) vs. SCE

-0.40

-0.60

-0.80 -1.00 -1.20

C - Crevice C - General C - Waterline

-1.40 0.00

10.00

20.00

30.00

40.00

50.00

Exposure Time (days)

60.00

70.00

80.00

Figure 8.10: Long term corrosion potential readings of galvanized carbon steel corrosion coupons in the concentrated solution at pH 13. The carbon steel coupons achieve passive potentials.

134

Potential Difference vs. Time
Stainless steel in concentrated solution pH 13
0.00

-0.20

Potential Difference (V) vs. SCE

-0.40

-0.60

-0.80 -1.00 -1.20

S - Crevice S - General S - Waterline

-1.40 0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

Exposure Time (days)
Figure 8.11: Long term corrosion potential readings of stainless steel corrosion coupons in the concentrated solution at pH 13. The stainless steel coupons have passive potentials.

From these graphs several key observations are revealed. The first is that in solutions of both pH 7.5 and pH 13, and likewise in all solution types, the stainless steel demonstrated passive behavior with corrosion potential values in the range of -0.1 to -0.2 volts versus the saturated calomel reference electrode. Similarly, the carbon steel coupons displayed passive behavior in the range of -0.2 to -0.4 volts in solutions with high pH values. However, at the lower pH value, the carbon steel did not exhibit stable passivity. In fact, the corrosion potentials of the carbon steel coupons without the zinc edge remained constantly active with no indication of passivity.
In both the normal and concentrated solutions at pH 7.5 the galvanized carbon steel coupons sustained active corrosion potentials for the first 20 to 30 days. As shown
135

in Figures 8.7 (with zinc) and 8.9 (without zinc), the galvanized coupons were initially more active than the coupons with no zinc, with potentials in the range of -0.9 to -0.8 compared to -0.7. The zinc had effectively lowered the corrosion potential difference by preferentially corroding at a lower potential than the carbon steel. The corrosion potentials of the galvanized coupons universally increased at 30 days (Figure 8.7), which can be interpreted as the time when the zinc coating became ineffective. As the zinc protection dissolved, the corrosion potentials of the galvanized coupons undulated between active and passive values, signifying unstable passive behavior of the carbon steel. The dissolution of the zinc layer and the unstable passivity of the carbon steel proved that the zinc protection on galvanized carbon steel bolts can not be effectively sustained for the entire bridge service life.
8.4.1.2 Corrosion rates After the final corrosion potential readings, the corrosion rates of the general and
crevice coupons were found using two methods, polarization resistance and gravimetric calculation. Table 8.2 provides the corrosion rates found by both methods for select coupons. The complete table of results is included in Appendix D.
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Table 8.2: Corrosion rates calculated by polarization resistance and gravimetric methods for select corrosion coupons.

Time

Initial Final

exposed Coupon Exposure weight weight

ID (days) type solution/pH (grams) (grams)

Final Initial (mg)

Corrosion Rate

wt.

Polar.

loss resistance

(mpy)

(mpy)

C5

68 general conc / 13 65.5903 65.5764 13.90 0.147

0.043

C7

68

general
(no zinc)

conc / 7.5

65.0630 64.8046 258.40

6.470

3.129

C10 69

crevice norm / 7.5 60.4340 60.3313 102.70 1.197

1.926

C12 68 crevice conc / 7.5 59.4293 59.3004 128.90 1.525

C21

68

crevice
(no zinc)

conc / 7.5

58.8939 58.6487 245.20

6.139

S3 68 general conc / 7.5 65.6472 65.6464 0.80 0.008

2.307 4.574 0.015

S7

69

crevice norm / 7.5 57.9295 57.9278 1.70 0.020

0.001

In general, the corrosion rates by gravimetric method were found to correspond to measurements by the polarization resistance method, and the corrosion rates correlated with the corrosion behavior predicted by the corrosion potential measurements. The stainless steel coupons and carbon steel coupons in high pH solution, in which stable passive potential values were measured as shown in Figures 8.8, 8.10 and 8.11, were found to have corrosion rates of less than one mil per year (mpy); corrosion rates less than one mpy, or 0.001 inch penetration per year, are considered negligible. In contrast, the carbon steel coupons with no zinc, in which constant active potential values were measured as shown in Figure 8.9, were found to have the highest corrosion rates. The galvanized carbon steel crevice corrosion coupons in solutions of pH 7.5 were found to have corrosion rates greater than one mpy, and are expected to have higher corrosion

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rates similar to the carbon steel samples without zinc, as the passivity of the samples breaks down following the dissolution of zinc. 8.4.2 Electrochemical polarization curves
Electrochemical polarization curves for each tested material in each solution type further characterize material corrosion patterns. The following polarization curves of potential versus the log of current density, shown in Figures 8.12 through 8.15, are representative of the polarization behaviors of carbon steel and stainless steel in solutions of pH 7.5 and pH 13. For each experimental polarization curve, three tests were conducted on polished coupons to ensure reproducible and accurate results.

Potential (V) vs. SCE

1.5E+00

Polarization curves for carbon steel and stainless steel in concentrated solution pH 7.5

c46-1

1.0E+00

c46-2

c46-3

5.0E-01

s49-1 s49-2

0.0E+00

s49-3

-5.0E-01
Equilibrium
corrosion potential
-1.0E+00
1.0E-09 1.0E-08 1.0E-07

1.0E-06 1.0E-05
log i (A/cm^2)

Anodic polarization
Cathodic polarization
1.0E-04 1.0E-03 1.0E-02

Figure 8.12: Electrochemical polarization curves for stainless steel and carbon steel in concentrated solution with pH 7.5. The stainless steel displayed passivity, while the carbon steel actively corroded at significantly higher current densities.

138

Potential (V) vs. SCE

1.5E+00 1.0E+00 5.0E-01 0.0E+00

Polarization curves for carbon steel and stainless steel in normal solution pH 7.5
c60-1 c60-2 c60-3 s47-1 s47-2 s47-3

-5.0E-01

-1.0E+00 1.0E-08

1.0E-07

1.0E-06 1.0E-05 1.0E-04
log i (A/cm^2)

1.0E-03

1.0E-02

Figure 8.13: Electrochemical polarization curves for stainless steel and carbon steel in normal solution with pH 7.5. The stainless steel displayed passivity similar to stainless steel in concentrated solution, while the carbon steel actively corroded at current densities lower than current densities for carbon steel in concentrated solution.

139

1.5E+00 1.0E+00 5.0E-01 0.0E+00

Polarization curves for carbon steel and stainless stee l in concentrated solution pH 13
c47-1 c47-2 c47-3 s55-1 s55-2 s55-3

Potential (V) vs. SCE

-5.0E-01

-1.0E+00 1.0E-10 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02
log i (A/cm^2)
Figure 8.14: Electrochemical polarization curves for stainless steel and carbon steel in concentrated solution with pH 13. The stainless steel and carbon steel displayed similar passive behavior.

Potential (V) vs. SCE

1.50E+00 1.00E+00 5.00E-01 0.00E+00

Polarization curves for carbon steel and stainless steel in normal solution pH 13
c52-1 c52-2 c52-3 s52-1 s52-2 s52-3

-5.00E-01

-1.00E+00
1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02
log i (A/cm^2)
Figure 8.15: Electrochemical polarization curves for stainless steel and carbon steel in normal solution with pH 13. The stainless steel and carbon steel displayed similar passive behavior to each other and to the steels in concentrated solution at pH 13.

140

The horizontal portion of the curve corresponds to the open circuit, or equilibrium, corrosion potential of the coupon; the portion of the curve below the horizontal curve is the cathodic polarization of the sample and the portion above the horizontal curve is the anodic polarization, as labeled in Figure 8.12. The corrosion behavior of the material is interpreted from the anodic portion of the curve. In the curves shown in Figures 8.12 through 8.15, a nearly vertical slope in the curve indicates passive behavior of metal in that solution. Due to the fast potential scan rate used in these tests, 2 mV/s, a lower current density does not correspond to the passive region because the steel specimen did not come to a steady state at each point along the curve before the voltage was increased. If the steel sample had come to a steady state at each point before the voltage was increased, a decrease in the current density would have been observed along the vertical slope.
From the polarization curves, previous observations from the long term test were confirmed and new observations were made. First, the polarization curves verified the long term corrosion potential readings in high pH solutions. The stainless steel and carbon steel exhibited nearly the same stable passivity in both the solutions with pH 13. Second, in the solutions with pH 7.5, the polarization curves for stainless steel contained a region with a vertical slope, confirming that the stainless steel passivated, while the polarization curves for carbon steel did not, indicating active corrosion with no tendency for the carbon steel to passivate under these conditions.
In addition to insight on the active or passive behavior of the material, the polarization curves offer perspective on the material's corrosion rate. As shown in Figure 8.12, the current densities of the carbon steel are several orders of magnitude higher than
141

those for stainless steel at the same potential. This difference could translate into corrosion rates that are up to 100 times higher for carbon steel than stainless steel under environmental conditions representing that potential. Furthermore, the current densities as a function of the potential are nearly the same for stainless steel in concentrated solution compared to stainless steel in normal solution, as shown by comparing Figures 8.12 and 8.13, but the current densities versus potential for carbon steel vary with respect to the solution concentration. Although less than one order of magnitude, the current densities versus potential of carbon steel are higher in the concentrated solution in Figure 8.12 compared to the carbon steel polarization curve in normal solution in Figure 8.13, indicating that corrosion rates for carbon steel will be up to 10 times higher in the concentrated solution.
8.4.3 Cyclic polarization curves Cyclic polarization curves are used to evaluate a material's resistance to localized
corrosion attack. As the applied voltages are decreased, the response of the current densities indicates whether localized corrosion can occur during the process of the polarization. If the plot of the current density as a function of the voltage during the reverse scan of potential follows the same curve that was created during the forward scan of voltage polarization, then no localized corrosion is expected to occur. However, if the current densities are higher during the reverse scan than they were during forward scan at the same voltages, then the curve forms a hysteresis. This hysteretic curve is an indication that localized corrosion has occurred, because it shows that the passive layer has been broken and cannot be reformed. The area contained by the hysteretic loop correlates to
142

the voltages at which localized corrosion occurs, and the passive layer cannot repair itself.
Therefore the point at which the hysteretic curve crosses over the curve of the initial polarization is of special interest. If the hysteresis crosses the polarization curve within the passive region above the open circuit potential, then a margin of applied voltage exists in which localized corrosion will not occur. This point is called the protection potential. For example, in Figure 8.16 the hysteretic curve for sample s30-1 crosses the original polarization curve at 0.0 volts. Since the open circuit potential of that material was found to be -0.3 volts, localized corrosion will not be induced when that sample is at its equilibrium potential, and a range of 0.3 volts exists in which the material can be polarized without inducing localized corrosion. However, if the material is polarized beyond the protection potential of 0.0 volts, or more than the 0.3 volt range, then localized corrosion will occur.
The cyclic polarization curves shown in Figures 8.16, 8.19, and 8.20 were created in concentrated solution at pH 7.5 with the Type 304 stainless steel anchor bolt material and other candidate alloys that may be considered for anchor bolt use. The cyclic polarization of steel samples exposed to concentrated solutions at pH 7.5 is more critical than cyclic polarization of steel samples in normal solution at pH 7.5 because protection potentials are lower in concentrated solutions. Cyclic polarization curves for candidate steel alloys in normal solution can be found in Appendix D.
The curves in Figure 8.16 were produced using a corrosion coupon from the Type 304 stainless steel anchor bolt and the same equipment set-up as described in Section 8.2.3 for the polarization tests. As can be seen in Figure 8.16, the 304 stainless steel is
143

susceptible to localized corrosion at high potentials and has a relatively low protection potential. Figures 8.17 and 8.18 show images at 32 x magnification of the crevices and pits formed on the surface of the coupon during the test, respectively. However, all the potential measurements of the 304 stainless steel at equilibrium in the long term test fell below the protection potential, indicating that for typical conditions at a bridge bearing the Type 304 stainless steel anchor bolt will be protected from localized corrosion.

2.5E+00

Cyclic polarization curves for Type 304 stainless steel in concentrated solution pH 7.5

Potential (V) vs. SCE

2.0E+00 1.5E+00 1.0E+00 5.0E-01

s56-1 s30-1

0.0E+00 -5.0E-01

-1.0E+00
1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03
log i (A/cm^2)

Figure 8.16: Cyclic polarization curves for Type 304 stainless steel subjected to localized corrosion. The protection potential is higher than the potential readings taken during the long term test of the same material, indicating that this material is protected from localized corrosion in this environment.

144

Figure 8.17: Crevice formed on stainless steel corrosion coupon during cyclic polarization at 32 x magnification. The O-ring sealing the coupon to the glass opening in the polarization set-up created the crevice. Anchor bolts are also exposed to crevice conditions in the bearing.
Figure 8.18: Pitting on the surface of the stainless steel corrosion coupon during cyclic polarization at 32 x magnification. The typical environment at a bridge bearing will not induce pitting corrosion in Type 304 stainless steel anchor bolts.
145

The cyclic polarization curves in Figures 8.19 and 8.20 were created from manufactured corrosion coupons of candidate alloys which may be considered for anchor bolt use. These alloys were Type 304 stainless steel, Type 316 stainless steel, and the duplex stainless steel alloys 2101 and 2205. The equipment set up for these tests required that the coupons be dipped into the solution, rather than clamped to the spout on the beaker. The specimens used to create the curves in Figure 8.19 were not subjected to crevice conditions, and the figure presents cyclic polarization data in which the samples were only susceptible to pitting corrosion. The samples used to create the curves in Figure 8.20 were lacquered to control the exposed surface area. By lacquering the samples, crevices were created at holidays in the seal, and Figure 8.20 presents cyclic polarization data in which the samples were susceptible to pitting and crevice corrosion.
146

Cylic polarization curves for candidate stainless steel alloys

in concentrated solution pH 7.5

2

1.75

304 CS

304 CSNC

1.5

316 CS

2101 CS

1.25

2205 CS

316

1
2205

0.75

E (V) vs. SCE

0.5
2101

0.25
304 without

0

crevice

-0.25

-0.5

304 with crevice

-0.75 1.E-10

1.E-09

1.E-08

1.E-07 1.E-06
log(i) (A/sq.cm)

1.E-05

1.E-04

1.E-03

Figure 8.19: Cyclic polarization of candidate alloys which may be considered for anchor bolt use tested in concentrated pH 7.5 solution. Type 316, Type 2101, and 2205 stainless steel alloys are considered acceptable alternative materials to Type 304 stainless steel.

The results shown in Figure 8.19 clearly indicate that 316, 2101, and 2205 stainless steels did not show any susceptibility to pitting corrosion, even when tested in the concentrated pH 7.5 solution with a crevice condition. Only Type 304 stainless steel showed pitting and crevice corrosion when tested in the crevice condition. Further tests on Type 304 stainless steel in the no crevice condition (indicated "NC" in Figure 8.19) showed low susceptibility to pitting corrosion with no hysteresis loop. Regardless, the protection potentials indicated in Figures 8.19 are sufficiently high that localized corrosion will not be induced in Type 304, 316, 2102, or 2205 stainless steel alloys in concentrated solution at pH 7.5.
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Cylic polarization curves for candidate stainless steel alloys

in normal solution pH 7.5

2

1.75

304 NS 304 NSNC

1.5

316 NS

2101 NS

1.25

2205 NS

2205

2101 316

1

E (V) vs. SCE

0.75

0.5 0.25
0 -0.25

304 without crevice

304 with crevice

-0.5

-0.75 1.E-10

1.E-09

1.E-08 1.E-07 1.E-06
log(i) (A/sq.cm)

1.E-05

1.E-04

1.E-03

Figure 8.20: Cyclic polarization of candidate alloys which may be considered for anchor bolt use tested in normal pH 7.5 solution.

Shown in Figure 8.20, cyclic potential polarization curves were also generated for Types 304, 316, 2101, and 2205 stainless steels exposed to the less aggressive normal pH 7.5 solution. In the normal solution, corrosion behavior was similar to that observed in the concentrated solution.

148

8.4.4 Galvanic and concentration cell coupling
8.4.4.1 Galvanic coupling between dissimilar metals Galvanic corrosion cells between dissimilar metals that exist within a typical plate
bearing were simulated in the laboratory in both the normal and concentrated solutions of pH 7.5. These galvanic couples are electrical connections of: (1) stainless steel and carbon steel, (2) stainless steel and bronze, (3) carbon steel and bronze, and (4) stainless steel and carbon steel and bronze.
Once the mixed corrosion potential reached equilibrium for the couple, the corrosion rate of the anode was measured using the polarization resistance method described in Section 8.3.4. Table 8.3 compares the equilibrium corrosion potential of the couple to the corrosion potential of the individual materials in the same solution.

Table 8.3: Comparison of equilibrium corrosion potentials of galvanically coupled

coupons to corrosion potentials of non-coupled coupons in the same solution. When

coupled, the materials polarized each other.

Galvanic couple

Anodic metal

Equilibrium potential (V)

Uncoupled coupon

Corrosion potential
(V)

Notes

cs to ss

cs

-0.670

cs corrosion potential

C1

-0.163

is more active

Normal Solution pH 7.5

cs to bronze ss to bronze

cs

-0.615

ss

-0.085

cs corrosion potential

C1

-0.163

is more active

potential change for ss

S1

-0.120

is negligible

ss to cs to

cs corrosion potential

bronze

cs

-0.543

C1

-0.163

is more active

cs corrosion potential

cs to ss

cs

-0.714

C3

-0.141

is more active

Conc. Solution pH 7.5

cs to bronze ss to bronze

cs

-0.640

ss

-0.104

cs corrosion potential

C3

-0.141

is more active

potential change for ss

S3

-0.079

is negligible

ss to cs to

bronze

cs

-0.601

cs corrosion potential

C3

-0.141

is more active

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The differences in the equilibrium potentials of the coupled specimens compared to the corrosion potentials of the individual specimens shows that the materials polarize each other when electrically connected, and the equilibrium potential for the anodic material becomes more active. Table 8.4 qualitatively compares the corrosion rates of the anodic specimen in the couple to the corresponding corrosion rate measured gravimetrically in the long term test for a coupon of the same material in the same solution that was not galvanically coupled. For the comparison the corrosion rates based on an equal cathode to anode ratio was calculated.

Table 8.4: Qualitative comparison of corrosion rates of galvanically coupled coupons to corrosion rates of non-coupled coupons in the same solution. Corrosion rates increased for the carbon steel when coupled with the more noble stainless steel or bronze.

Galvanic couple

Normal Solution pH 7.5

cs to ss cs to bronze ss to bronze ss to cs to bronze

Conc. Solution pH 7.5

cs to ss cs to bronze ss to bronze ss to cs to bronze

Anodic Corrosion metal rate (mpy)

cs

10.27

cs

6.07

ss

0.06

cs

2.01

cs

7.66

cs

5.42

ss

0.07

cs

2.50

Uncoupled coupon C1 C1 S1 C1 C3 C3 S3 C3

Corrosion rate (mpy)
1.074 1.074 0.013 1.074 0.678 0.678 0.008 0.678

Notes
carbon steel corrosion rate increased ~10 times
carbon steel corrosion rate increased ~ 5 times
Corrosion rates are negligible
carbon steel corrosion rate increased ~ 2 times carbon steel corrosion rate increased ~ 10 times carbon steel corrosion rate increased ~ 5 times
Corrosion rates are negligible
carbon steel corrosion rate increased ~ 2 times

For an equal cathode to anode ratio, the corrosion rates of carbon steel increased by approximately two to ten times when galvanically coupled to more noble metals, as compared to the already high corrosion rate of carbon steel alone.
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8.4.4.2 Concentration cell coupling on same metal Concentration cells between two different environments to which anchor bolts are
simultaneously exposed were simulated in the laboratory. The part of an anchor bolt embedded in the concrete support is exposed to a high pH solution, while the part protruding from the concrete is exposed to a solution closer to a neutral pH, like the solution presented in Table 8.1. Samples of carbon steel and stainless steel were electrically connected such that: (1) carbon steel in normal solution at pH 7.5 was connected to carbon steel in normal solution at pH 13, (2) stainless steel in normal solution at pH 7.5 was connected to stainless steel in normal solution at pH 13, (3) carbon steel in concentrated solution at pH 7.5 was connected to carbon steel in concentrated solution at pH 13, and (4) stainless steel in concentrated solution at pH 7.5 was connected to stainless steel in concentrated solution at pH 13.
Once the mixed corrosion potential reached equilibrium for the couple, the corrosion rate of the anode was measured using the polarization resistance method described in Section 8.3.4. Table 8.5 compares the equilibrium corrosion potential of the couple to the corrosion potential of the individual materials in the same solution.
151

Table 8.5: Comparison of equilibrium corrosion potentials of coupons coupled in

concentration cells to corrosion potentials of non-coupled coupons in the same solution.

When coupled, the materials polarized each other.

Anodic Concentration cell couple metal

Equilibrium potential (V)

Uncoupled coupon

Corrosion potential
(V)

Notes

cs pH 7.5 to

cs

the coupled material has

Normal cs pH 13

pH 7.5 -0.542*

C1

-0.163

a more active potential

Solution ss pH 7.5 to

ss

the coupled material has

ss pH 13

pH 7.5 -0.418*

S1

-0.089

a more active potential

Conc. Solution

cs pH 7.5 to cs pH 13 ss pH 7.5 to ss pH 13

cs pH 7.5
ss pH 7.5

-0.627 -0.359

the coupled material has

C3

-0.141

a more active potential

the coupled material has

S3

-0.079

a more active potential

*Note: Corrosion potentials for couples in normal solution were not at equilibrium at the

time of corrosion measurements.

The differences in the equilibrium potentials of the coupled specimens compared to the corrosion potentials of the individual specimens proves that the materials polarize each other when electrically connected. Even though the couples in the normal solution had not completely polarized each other to the point of equilibrium after six weeks of electrical connection, the effects of the connection were apparent in the difference in corrosion potential and corrosion rate compared to uncoupled coupons, nonetheless. The anodic material is polarized to a more active equilibrium potential.
Table 8.6 qualitatively compares the corrosion rates of the anodic specimen in the couple to the corresponding corrosion rate measured gravimetrically in the long term test for a coupon of the same material in the same solution that was not coupled in a concentration cell. For the comparison the corrosion rates based on an equal cathode to anode ratio were calculated.

152

Table 8.6: Qualitative comparison of corrosion rates of coupons coupled in concentration cells to corrosion rates of non-coupled coupons in the same solution. Corrosion rates increased for the carbon steel coupled coupons.

Anodic

Concentration cell couple metal

cs pH 7.5 to

cs

Normal cs pH 13

pH 7.5

Solution ss pH 7.5 to

ss

ss pH 13

pH 7.5

cs pH 7.5 to

cs

Conc. cs pH 13

pH 7.5

Solution ss pH 7.5 to

ss

ss pH 13

pH 7.5

Corrosion rate (mpy)
4.677 1.146 4.226 330.2x10-3

Uncoupled coupon
C1 S1 C3 S3

Corrosion rate (mpy)
1.074 0.013 0.678 0.008

Notes
carbon steel corrosion rate increased ~4 times
Corrosion rates are negligible
carbon steel corrosion rate increased ~6 times
Corrosion rates are negligible

For an equal cathode to anode ratio, the corrosion rates of carbon steel in pH 7.5 solution increased by approximately four to six times, as compared to the already high corrosion rate of carbon steel alone, when coupled in a concentration cell to passive carbon steel in pH 13 solution. On the other hand, the corrosion rates of the stainless steel were not affected by the concentration cell coupling, because the stainless steel was passive in both environments.

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CHAPTER 9: DISCUSSION
9.1 Causes of anchor bolt corrosion Corrosion of bridge bearings and anchor bolts was observed throughout the state
of Georgia; corrosion affected the bolt shaft, nuts, washers, and bearing plates. By comparing what was observed in the field with laboratory test results and corrosion theory, several corrosion mechanisms were identified as the main contributing factors of anchor bolt corrosion in Georgia.
The first form of corrosion identified was concentration cell corrosion which affects the anchor bolt shaft and can be attributed to the fact that the bolts are partially embedded in concrete. The corrosion of the bolts' nuts and washers and the corrosion of the bearing plates are independent of concentration cell corrosion and are caused by the environmental conditions at the bearing. Both the corrosion of the materials in the bearing environment and the effects of partial embedment are relevant to anchor bolt corrosion in Georgia.
The three corrosion mechanisms presented in this section were determined to influence the corrosion behavior of the bearing assembly the most. However, anchor bolt and bearing corrosion is not exclusively defined by these three corrosion mechanisms.
9.1.1 Concentration cell corrosion As first discussed in Chapter 3, high pH is known to aid the passivation of steel;
the typical solution inside concrete is approximately pH 13. Therefore, the part of the anchor bolt embedded in the concrete pier cap is presumably passive and is protected
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from corrosion. On the other hand, the typical bearing environment on top of the concrete support is pH 7.5. Theoretically, the difference in the environments creates a differing corrosion potential for the carbon steel in each environment.
Laboratory testing in simulated field environments, presented in Chapter 8, verified that the equilibrium potential for carbon steel in the pH 13 environment is more noble than the equilibrium potential for carbon steel in the pH 7.5 environment. Corrosion potentials measured in the laboratory also proved that galvanized carbon steel is not sufficiently protected throughout the anchor bolts' expected service life. Thus, concentration cell corrosion is thermodynamically favorable in the carbon steel bolt in the presence of electrolyte. Laboratory tests also revealed that the corrosion rate of carbon steel electrochemically connected in a concentration cell is higher than the corrosion rate of uncoupled carbon steel.
Analysis of carbon steel bolts from the field confirmed the presence of concentration cell corrosion. The corrosion that was visually observed on carbon steel anchor bolt shafts, described in Chapters 6 and 7, indicated that general corrosion was accelerated near the interface of the bolt and the concrete. The accelerated corrosion caused a neck in the bolt shaft in the areas near the concrete, and the corrosion in the areas of the bolt farther from this interface was not as severe. The cone shape of the corroded carbon steel bolts indicated that a concentration cell had formed between the steel embedded in the concrete and the steel exposed to the environment.
Failure analysis of the carbon steel bolts gave further evidence supporting concentration cell corrosion. Microscopy of the necked surface of the bolts revealed that the corrosion had a uniform morphology the failure surface did not indicate any
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localized pitting, crevice, or corrosion cracking. From the nature of the corrosion mechanisms, it is suggested that galvanic and concentration cell corrosion morphology is the same as general corrosion, but accelerated in a specific area. Therefore, the microscopic images showing uniform corrosion of the carbon steel bolt in the necked area confirms that corrosion was accelerated in the part of the bolt that was not embedded in concrete.
Alternatively, laboratory testing in simulated field environments for the Type 304 stainless steel bolt material revealed that the equilibrium potential for the stainless steel was the same for the pH 7.5 and pH 13 solutions. Therefore, concentration cell corrosion would not initiate on stainless steel anchor bolts, according to corrosion thermodynamics. Corrosion rate measurements of stainless steels from each environment that were electrochemically coupled were found to be negligible.
9.1.2 Galvanic corrosion Laboratory testing in solutions simulating the bearing environment proved that an
electrochemical couple between carbon steel and a more noble alloy, such as stainless steel or bronze, increases the corrosion rate of the carbon steel, as predicted by theory presented in Chapter 3. Dissimilar metals in bearings should be electrochemically separated. Dissimilar metals may be electrochemically separated by encasing the anchor bolt in a non-conducting material and using a non-conducting washer to eliminate contact between the stainless steel anchor bolt and carbon steel bearing plates and flange.
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9.1.3 Localized corrosion effects Localized corrosion, such as pitting or crevice corrosion, must be considered
when using active-passive stainless steel alloys. Localized attack is not a concern for carbon steel, because in the pH 7.5 solution the equilibrium potential of carbon steel indicated active, general corrosion. Passive films are susceptible to local breakdown by chloride attack, depending on the chloride concentration in the environment. Soil samples from the bearing environment were used to determine ion concentrations at a typical bearing in Georgia, and the chemical composition of corrosion scale was analyzed by x-ray diffraction to determine the effect of the ions in the environment on the corrosion behavior of the bolt. From the soil analysis it was determined that chlorides, carbides, and sulfates were present in the bearing environment, but the chemical composition of the bolt scale revealed that these corrosive ions were not present in the corrosion products. Thus, the role of the ions in the environment on the corrosion behavior of the material could not be positively identified by x-ray diffraction technique.
Cyclic polarization tests in solutions simulating the bearing environment were used to determine the protection potential of the Type 304 stainless steel along with other candidate alloys. It was found that the protection potential of the Type 304 alloy was more noble than the equilibrium potential of the stainless steel in solution. This implies that Type 304 stainless steel passivates and remains protected from corrosion in the typical bearing environment. Other candidate alloys for which the protection potential was higher than the equilibrium potential are Type 316, Type 2101, and Type 2205 stainless steels. When crevice effects were included, however, cyclic polarization showed a decrease in the protection potentials for all the alloys, confirming the theory presented
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in Chapter 3 that the protection potential for pitting corrosion is higher than that for crevice corrosion. The introduction of crevice conditions on any of the alloys makes the material more susceptible to localized corrosion.
Currently, the most expensive alloying element in stainless steel is nickel. Since the composition of Type 2101 duplex stainless steel has significantly less nickel, the present day cost of this alloy is comparable to the cost of Type 304 austenitic stainless steel. The superior corrosion resistance of Type 2101 stainless steel makes it an attractive alternative to the current design. Yet, the corrosion resistance of Type 2205 stainless steel is even better. While the cost of this 2205 alloy is still considerably higher than the other two, it may also be accepted as a suitable design alternative. Further testing should be completed to verify the cyclic polarization data presented in Chapter 8.
9.2 Role of bearing design in corrosion The current design for steel girder bridge bearings in the state of Georgia
advances bearing corrosion. In the current design, stainless steel anchor bolts and bronze lube plates are in direct contact with carbon steel bearing plates and flanges; this contact promotes galvanic corrosion of the carbon steel components. The mechanism for movement of Georgia's bronze plate bearing design corresponds to the sliding bearing design described in Lee's book, Bridge Bearings and Expansion Joints, and is not a recommended type of bearing (Lee 1994). Sliding plates have a high coefficient of friction that only increases throughout the design life of the bearing as debris, moisture, and corrosion products are trapped in the crevices between the plates.
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As corrosion products and debris build up inside the sliding plates, the bearing is at risk of becoming frozen. Frozen bearings restrict movements that the bearings are designed to accommodate, and forces are induced for which the original bridge is not designed. The effects of corrosion build up were demonstrated in Chapter 7, where corrosion products had essentially fused the anchor bolts to the bearing plates in fixed and expansion bearing holes.
Given that the inspection quality and regularity is the same for all types of bridges, it was reported in Chapter 5 that the rate at which corrosion initiates and propagates in bearings is higher for metropolitan and Interstate-related bridges, compared to rural and non-Interstate bridges. Based on this finding, it may be hypothesized that bearing corrosion is related to the loading conditions of the bridge. To some extent, bridge movements are dependent on the loading conditions, with greater movement occurring in the bridges with higher loads. As bridges are repetitively loaded in high traffic areas, the bearings may become subject to fretting corrosion in addition to the major corrosion mechanisms proposed in the previous section, leading to the acceleration of the overall corrosion of the bearing.
9.3 Role of maintenance procedures in corrosion Proper bearing maintenance is key to reducing bearing corrosion. The build up of
debris at the bearings traps moisture and corrosive agents at the bearing surface. Since electrolyte is necessary for all corrosion processes, this build up of moisture enables all the corrosion mechanisms described previously.
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In the retrofit of existing bearings by "sleeving", steel angles are bolted to the sides of the bearings to provide the lateral restraint that was lost when the anchor bolt corroded. The bolts used for the retrofit are subjected to the same environment that corroded the original carbon steel anchor bolt. Use of stainless steel bolts in the retrofit would protect against the same corrosion failure.
9.4 Correlation to inspection report data Inspection report data enabled a statewide assessment of bearing corrosion in
Georgia, which was presented in Chapter 5. The distribution of bearing corrosion incidences was found to be uniform across the state, but the mean ages at which corrosion was reported implied that corrosion initiated and propagated faster in northern and coastal regions, metropolitan areas, and Interstate-related bridges.
Bearing corrosion reported at an earlier mean age for bridges in the northern and coastal geographic regions of the state may be attributed to a more corrosive bearing environment in these regions compared to southern Georgia. Soil analysis from bearings in southern and northern Georgia did reveal that chloride concentrations in northern Georgia are higher than the concentrations in southern Georgia. Despite the lack of chloride-containing compounds in the bolt scale as determined by x-ray diffraction analysis, chlorides may still be responsible for accelerated corrosion rates and accelerated corrosion initiation by functioning to break down local passivity of the bolt.
The report of bearing corrosion in metropolitan and Interstate-related bridges at an earlier age than their rural and non-Interstate counterparts may also be attributed to the environmental conditions specific to these classifications. As discussed in Section 9.2,
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fretting corrosion may be caused by the heavier traffic loading for these areas. Bridges that fall into the metropolitan and Interstate-related categories are also possibly exposed to higher amounts of pollutants, or collect more debris than bridges in other categories, both of which increase the rate of corrosion initiation and propagation.
Regardless of the bearing environment and apparent rate of corrosion initiation and propagation based on the age of bridges when anchor bolt corrosion was first reported, the occurrence of anchor bolt corrosion was found to be ubiquitous to all regions in Georgia. Concentration cell corrosion of the anchor bolts is not dependent on the geographic, demographic, or span type classifications of bridges, because it will occur anywhere that carbon steel is partially embedded in concrete. While the bearing environment may affect the rate of corrosion, the critical mechanism responsible for corrosion of carbon steel bolts is the concentration cell. Hence, the number of bearing corrosion occurrences is uniform for all bridge classifications.
The author notes that the inspection of anchor bolts within the plate bearing assembly is difficult, which leads to unknown variability in the inspection report data that are presented.
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CHAPTER 10: CONCLUSIONS AND RECOMMENDATIONS
10.1 Summary The research objectives for this project were to explicitly define the anchor bolt
corrosion problem in the state of Georgia, research solutions to the problem, and recommend action to the Georgia Department of Transportation. As a first step, the researcher interviewed key personnel within the state Department of Transportation to understand the steel girder bearing design, maintenance procedures, and the history of both. The main bearing assemblies of concern in Georgia are the plate bearing assembly, in which carbon steel base and sole plates slide on a bronze lubrication plate at the expansion end of the girder, and the plate bearing assembly, in which carbon steel base and sole plate support the girder at the fixed end of the girder. The bolts used to anchor the girder and the bearing to the concrete support were ASTM Grade 36 carbon steel bolts if the bridge was constructed prior to 1990, and Type 304 stainless steel bolts if constructed after 1990. Existing bearings in which the anchor bolts had corroded were retrofitted by "sleeving", in which steel angles were bolted to both sides of the bearing to provide lateral restraint.
To define the extent of anchor bolt corrosion in Georgia, inspection data from throughout the state was compiled and interpreted. The inspection report files were complete and reliable; the inspectors and the inspection process are commended. The occurrence of anchor bolt corrosion was found to be ubiquitous for all regional and demographic environments in Georgia, with 411 of 1500, or 27%, of steel girder bridges throughout the state reported to experience anchor bolt corrosion. Field investigations at
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eight key bridges representing anchor bolt corrosion throughout the state confirmed the inspection report data that anchor bolt corrosion is universal in older steel girder bridges. Based on the bridge ages when anchor bolt corrosion was first reported, the rate of initiation and/or propagation of anchor bolt corrosion was higher for bridges in northern, coastal, and metropolitan environments as well as for Interstate-related bridges. Bearing corrosion for bridges in these categories was reported at a median age in the range of 34 to 37 years, while the median age of bridges in southern, rural, and non-Interstate categories when anchor bolt corrosion was reported was in the range of 40 to 45 years.
To define the causes of corrosion specific to anchor bolts in Georgia, failure analyses on bolt specimens obtained from the field investigations were conducted and laboratory testing on anchor bolt materials in simulated environments were performed. Visual analysis of the field specimens indicated that concentration cell corrosion was prevalent, and surface analysis revealed general corrosion morphology in the necked regions of the bolts. Laboratory testing proved that corrosion potentials were favorable for concentration cell corrosion for carbon steel partially embedded in concrete, but not for stainless steel materials. Corrosion potentials of carbon steel and stainless steel also indicated that when galvanically coupled, the carbon steel preferentially corrodes. Carbon steel electrically coupled with stainless steel in a galvanic cell or passive carbon steel in a concentration cell corroded two to ten times faster than uncoupled carbon steel. Data from cyclic polarization tests confirmed that Type 304 and other candidate stainless steels were protected from localized corrosion in the simulated bearing environment.
To research solutions to abate anchor bolt corrosion in Georgia, literature addressing the corrosion of steel alloys and bridge bearing design was explored. A review
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of literature in this field revealed that extensive research has been conducted on the corrosion of reinforcing steels completely embedded in concrete, but in general, there is a need for research on the corrosion of anchorages in concrete. Regarding bearing design, the bearing type and material must be selected to accommodate for the loads, movements, and environment specific to the bridge.
10.2 Conclusions Based on a synthesis of the results of the field investigations, bolt failure analyses,
laboratory experimental testing, and inspection report survey, the corrosion of carbon steel anchor bolts is caused universally by concentration cell corrosion that is created by the partial embedment of the bolt. Inside the concrete, the anchor bolt develops a noble corrosion potential due to the alkaline environment, but outside of the concrete the bolt has an active potential in the neutral electrolytic solution at the bearing. Corrosion protection of the carbon steel bolt provided through zinc galvanization cannot sufficiently protect the carbon steel bolt for its entire service life.
Other corrosion mechanisms affecting steel girder bridge bearings are galvanic and crevice corrosion, which are both enhanced by the current bearing design. Carbon steel that is in contact with the more noble bronze plate or a stainless steel bolt is preferentially corroded in the galvanic cell. Meanwhile, the sliding plates of the bearing form crevices that trap moisture and corrosive agents. Additional factors that contribute to the initiation and propagation of corrosion are the debris found at the bearings, the ion concentrations in the solution at the bearings, and bridge loading that causes repetitive movements in the bearings.
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The use of stainless steel anchor bolts instead of carbon steel eliminates the concentration cell effect on the bolt and, thus, eliminates the predominant cause of anchor bolt corrosion. However, stainless steel is susceptible to crevice corrosion and the risk of galvanic corrosion is heightened where carbon steel is in contact with the stainless steel.
Most of the bridges included in this study were designed for a 50 year lifespan. The survey of the inspection reports revealed that bearing corrosion was first reported to occur approximately 30 to 40 years after the bridge was built for over 40% of steel girder bridges in Georgia. From this statistic, it is apparent that an improved bearing design is necessary. New construction is expected to have a service life of 100 years, and the new bearing design must be resistant to all forms of corrosion to accommodate this lengthy service life.
10.3 Recommendations
10.3.1 Recommendations for design The ASTM 276 Type 304 stainless steel anchor bolt that is specified in the current
GDOT design manual is an acceptable anchor bolt material. The stainless steel is resistant to concentration cell effects because it is passive both inside and outside of the concrete. This alloy is also resistant to localized corrosion because the protection potential of Type 304 stainless steel is more noble than the equilibrium potential in the typical bearing environment. ASTM 276 Type 316, Type 2101, and Type 2205 stainless steels are acceptable alternatives to the Type 304. Due to the cost of nickel, these duplex stainless
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steels may be economically similar to Type 304 stainless steel. Further cyclic polarization tests should be conducted to confirm the results of this investigation.
The stainless steel anchor bolt should be electrically separated from all dissimilar metals. To prevent preferential corrosion of carbon steel, the carbon steel flanges and bearing plates should be separated from the stainless steel anchor bolts and from the bronze lubrication plate. The bolt should be electrochemically separated by a non-conducting encasement and a non-conducting washer.
Short descriptions of several types of special non-conducting washers can be obtained from a supplier, Superior Washer and Gasket, Corp, with a plant in South Carolina (www.superiorwasher.com). The author recommends a polytetrafluoroethylene (Teflon) washer to replace the current stainless steel washer, because the low coefficient of friction of the Teflon material is desirable for expansion bearing applications. A local supplier, Metro Bolt & Supply Co, Inc., only supplies round nylon non-conducting washers with inside diameters of up to inch (http://metrobolt.com). Superior Washer and Gasket Corp supplies round Teflon washers with inside diameters of up to 1 1/8 inch (www.superiorwasher.com).
In the current design, the carbon steel plates are electrically separated from the bronze bearing plate by the mastic lubricant, but for the most corrosion resistant bearing design, the bronze lube plate should be eliminated entirely.
The recommended bearing type is the reinforced elastomeric bearing. Reinforced elastomeric bearings should be designed with appropriate thickness to account for the effect of holes for anchor bolts, and electrical separation must be kept between the
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reinforcing plates and the stainless steel anchor bolts. Appendix E presents details of the recommended bearing design. 10.3.2 Maintenance recommendations
The regular bearing maintenance program should be continued. Sliding plate bearings should be regularly cleaned by brushing away dirt and debris from the bearing surfaces so that moisture and corrosive agents will not collect in the bearing. Chemical cleaning agents should not be used under any circumstances. Additionally, the paint on the bearings should be maintained to prevent general atmospheric corrosion of the carbon steel components.
Existing bridges with carbon steel anchor bolts should be "sleeved" to provide lateral restraint, according to current maintenance procedures. Carbon steel bolts are known to be subjected to accelerated corrosion due to concentration cell effects, even though it may be hard to observe the anchor bolt corrosion within the bearing and near the concrete surface. Sleeving should be performed with stainless steel bolts.
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