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INITIATION AND PROPAGATION OF CORROSION IN DRY-CAST REINFORCED
CONCRETE PIPES
by
Hariharan Balasubramanian
A Thesis Submitted to the Faculty of
The College of Engineering and Computer Science
in Partial Fulfillment of the Requirements for the Degree of
Master of Science
Florida Atlantic University
Boca Raton, FL
August, 2013
iii
ACKNOWLEDGEMENTS
The author is grateful to the staff of the FAU and especially Dr. Francisco
Presuel-Moreno for providing a student research contract to conduct the study. Also
without the tireless efforts of laboratory research assistants this endeavor would have
been impossible. Thanks again to all who contributed to this project.
iv
ABSTRACT
Author: Hariharan Balasubramanian
Title: Initiation and Propagation of Corrosion in Dry-Cast Reinforced Concrete
Pipes
Institution: Florida Atlantic University
Thesis Advisor: Dr. Francisco Presuel-Moreno
Degree: Master of Science
Year: 2013
This study investigates corrosion initiation and propagation in instrumented
specimens obtained from segments of Dry-cast reinforced concrete pipes. Potential, LPR
and EIS measurements were carried out. During the propagation stage in different
exposures, reinforcement eventually reached negative potentials values, which suggest
mass transfer limitations. So far these specimens show no visual signs of corrosion such
as cracks or corrosion products with one exception; where corrosion products have
reached the surface. Moreover, the apparent corrosion rate values obtained suggest high
corrosion rate. No crack appearance so far, could be explained by the high porosity of the
specimens; the corrosion products are filling these pores. It is speculated that although,
there might be mass transfer limitations present, the current demanded by the anode is
being balanced by a larger cathode area due to macrocell effects, since the high moisture
conditions likely reduced the concrete resistivity and increased the throwing power.
v
INITIATION AND PROPAGATION OF CORROSION IN DRY-CAST
REINFORCED CONCRETE PIPES
LIST OF FIGURES ....................................................................................................................... vii
LIST OF TABLES ........................................................................................................................... x
1 INTRODUCTION ................................................................................................................... 1
1.1 Research Background ...................................................................................................... 1
1.2 Research Objective .......................................................................................................... 3
1.3 Research Methodology .................................................................................................... 3
1.4 Thesis Structure ............................................................................................................... 4
2 LITERATURE REVIEW ........................................................................................................ 6
2.1 Corrosion ......................................................................................................................... 6
2.1.1 Corrosion process ..................................................................................................... 6
2.1.2 Corrosion of steel in concrete .................................................................................. 8
2.2 Reinforced Concrete Pipes ............................................................................................. 10
2.3 Existing Service Life Model .......................................................................................... 11
2.4 Field Performance Analysis ........................................................................................... 12
2.5 Interpretation of Mix potential value measured ............................................................. 16
2.6 Testing Standards & Methods ........................................................................................ 17
2.6.1 Accelerated Chloride Transport ............................................................................. 17
2.6.2 Corrosion monitoring methods .............................................................................. 18
3 APPROACH .......................................................................................................................... 22
3.1 Materials and Instrumentation of Specimens ................................................................. 22
3.1.1 Materials ................................................................................................................ 22
3.2 Concrete Characterization .............................................................................................. 28
3.2.1 Measurement of concrete cover ............................................................................. 28
3.2.2 Porosity .................................................................................................................. 28
3.2.3 Resistivity .............................................................................................................. 29
vi
3.2.4 Rapid Chloride Migration Test .............................................................................. 29
3.3 Experimental Set-ups for Accelerated chloride transport methods ................................ 30
3.3.1 Potentiostatic .......................................................................................................... 31
3.3.2 3.3.2 Galvanostatic ................................................................................................. 33
3.3.3 Migration cell ......................................................................................................... 34
3.3.4 Migration cell method under high humidity .......................................................... 36
3.4 Experiment Set-up for Study of Corrosion Propagation ................................................ 37
3.4.1 Environmental Chamber (High Humidity) ............................................................ 37
3.4.2 HDPE Horizontal (High Humidity) ....................................................................... 39
3.4.3 Fiber glass chamber (High Humidity) ................................................................... 40
3.4.4 Specimens covered with saturated sand ................................................................. 41
3.4.5 Immersed in water .................................................................................................. 46
3.6.1 Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS) ................................................................................................................ 49
3.6.2 Cathodic Polarization Scan .................................................................................... 49
3.6.3 Half-Cell Potentials ................................................................................................ 50
4 RESULTS .............................................................................................................................. 51
4.1 Concrete Characterization .............................................................................................. 51
4.1.1 Concrete cover ....................................................................................................... 51
4.1.2 Porosity .................................................................................................................. 55
4.1.3 Resistivity .............................................................................................................. 56
4.1.4 Diffusivity from RMT ............................................................................................ 58
4.2 Accelerated chloride transport and time to corrosion initiation ..................................... 60
4.3 Corrosion Propagation Stage: Environmental exposure ................................................ 64
4.4 Apparent corrosion rates look-up tables ........................................................................ 65
4.5 Typical plots of potential vs. time and plots of Rs, Rpapp Rs vs. time ...................... 66
5 DISCUSSION ........................................................................................................................ 88
5.1 Discussion for the corrosion propagation stage ............................................................. 88
Change in cathode area: ............................................................................................................. 91
6 CONCLUSIONS.................................................................................................................... 92
APPENDIX .................................................................................................................................... 94
REFERENCE ................................................................................................................................. 99
vii
LIST OF FIGURES
Figure 2.1 Schematic illustration of anodic dissolution reaction ........................................ 7
Figure 2.2 Schematic representation of the electrochemical reaction steel/concrete ......... 9
Figure 2.3 An Evans diagram with oxygen limiting factor. ............................................. 15
Figure 2.4 Anodic polarization curve of steel in non-carbonated concrete. ..................... 19
Figure 2.5 Schematic cathodic polarization curves in alkaline concrete: (a)
aerated and semi-dry: (b) wet or completely saturated with water, (c) de-
aerated solution. .............................................................................................. 21
Figure 3.1 Angles at which the test pieces were segmented ............................................. 24
Figure 3.2 Steps of instrumentation on specimens ........................................................... 26
Figure 3.3 Steps of reservoir construction on horizontal orientation specimens .............. 27
Figure 3.4 Reservoir constructions on vertical orientation specimens ............................. 27
Figure 3.5 Potentiostatic method set-ups .......................................................................... 32
Figure 3.6 Galvanostatic method ...................................................................................... 33
Figure 3.7 Migration cell .................................................................................................. 35
Figure 3.8 environmental chambers with specimens ........................................................ 38
Figure 3.9 specimens in HDPE Box ................................................................................. 39
Figure 3.10 Fiber glass 95-98% RH Chamber .................................................................. 41
Figure 3.11 In saturated soil inside HDPE Box ................................................................ 43
Figure 3.12 Specimens fully covered in saturated soil inside HDPE Box ........................ 44
Figure 3.13 Lower 1/3rd of length of each specimen is covered by saturated soil ............ 45
Figure 3.14 Fully immersed (horizontal) in water ............................................................ 47
Figure 3.15 Fully immersed (vertical) in water ................................................................ 48
Figure 4.1 Concrete cover distributions (Group Fg & Cg). L-Longitudinal, C-
Circumferential directions .............................................................................. 51
viii
Figure 4.2 Concrete cover distributions (Group Fg & Cg). L-Longitudinal, C-
Circumferential directions .............................................................................. 52
Figure 4.3 Concrete cover distributions (Group Fm & Cm). L-Longitudinal, C-
Circumferential directions .............................................................................. 52
Figure 4.4 Concrete cover distribution summaries L-Longitudinal,
C-Circumferential directions .......................................................................... 54
Figure 4.5 Resistivity plot for type F specimens .............................................................. 58
Figure 4.6 Resistivity plot for type C specimens .............................................................. 58
Figure 4.7 Chloride migration coefficients vs. resistivity ................................................. 60
Figure 4.8 F2 potential vs. time ........................................................................................ 67
Figure 4.9 F2 Rs, Rpapp Rs vs. time ............................................................................ 68
Figure 4.10 C23 potential vs. time .................................................................................... 69
Figure 4.11 C23 Rs, Rpapp Rs vs. time ........................................................................ 69
Figure 4.12 Plots of F1 potential vs. time ......................................................................... 71
Figure 4.13 Plots of F1 Rs, Rpapp Rs vs. time ............................................................. 71
Figure 4.14 Plots of C3 potential vs. time ........................................................................ 72
Figure 4.15 Plots of C3 Rs, Rpapp Rs vs. time ............................................................. 73
Figure 4.16 Plots of F10 potential vs. time ....................................................................... 74
Figure 4.17 Plots of F10 Rs, Rpapp Rs vs. time ........................................................... 75
Figure 4.18 Plots of C15 potential vs. time ...................................................................... 76
Figure 4.19 Plots of C15 Rs, Rpapp Rs vs. time ........................................................... 77
Figure 4.20 Plots of F21 potential vs. time ....................................................................... 78
Figure 4.21 Plots of F21 Rs, Rpapp Rs vs. time ........................................................... 79
Figure 4.22 Plots of C8 potential vs. time ........................................................................ 80
Figure 4.23 Plots of C8 Rs, Rpapp Rs vs. time ............................................................. 81
Figure 4.24 Plots of F9 potential vs. time ......................................................................... 82
Figure 4.25 Plots of F9 Rs, Rpapp Rs vs. time ............................................................. 83
Figure 4.26 Plots of C24 potential vs. time ...................................................................... 84
Figure 4.27 Plots of C24 Rs, Rpapp Rs vs. time ........................................................... 85
Figure 4.28 Cathodic potentiodynamic scans performed on F13 ..................................... 86
Figure 4.29 Cathodic potentiodynamic scans performed on C18 ..................................... 87
ix
Figure 5.1 Schematic representation of the Evans diagram: change in Cl-
concentration and oxygen concentration. ...................................................... 90
Figure 5.2 Schematic representation of the Evans diagram (change in anode area) ........ 91
x
LIST OF TABLES
Table 3.1 Mixture proportion of products ......................................................................... 23
Table 4.1 Mean and Standard Deviation on cumulative Concrete Cover distribution ..... 54
Table 4.2 Porosity of Concrete ......................................................................................... 56
Table 4.3 Time to Corrosion Initiation for Type F specimens ......................................... 61
Table 4.4 Time to Corrosion Initiation for type C specimens .......................................... 63
Table 4.5 Environmental Exposure during Corrosion Propagation stage for type F
specimens ......................................................................................................... 64
Table 4.6 Environmental Exposure during Corrosion Propagation stage for type C
specimens ......................................................................................................... 65
Table 4.7 Rc (i.e. Rapp – Rs) to Apparent Corrosion rate calculation table .................... 66
1
1 INTRODUCTION
1.1 Research Background
Dry-cast reinforced concrete pipes (D-C-RCP) have been used as drainage pipes by
the Florida Department of Transportation (FDOT) and other DOTs in United States. The
D-C-RCP is prepared with a low water to cementitious ratio (0.3 to 0.35). However, the
curing process of D-C-RCPs usually results in concrete with a high degree of absorption
(~9%). Some of these pipes are located in Florida areas where there is a low water table
that allows the soil to saturate (seasonal). Additionally, at sites close to the ocean, the soil
could contain a significant chloride concentration. The time to corrosion initiation period
could be short (a few years) if corrosion initiates due to chloride transport from both (1)
capillary absorption (if the concrete is exposed to wet/dry cycles) and diffusion. However
moderate or no corrosion has been observed on dry-cast reinforced concrete pipes placed
in soils containing high chloride concentration and high moisture conditions. Moreover,
the high moisture of the soil could result in low oxygen availability.
Dry-cast reinforced concrete pipe (D-C-RCP) is one of the most frequently used
drainage pipes. Its durability and service life in aggressive environments is of great
concern. Exposure to chlorides from marine environments can result in the premature
corrosion of reinforcing steel and failure of such structures. Several departments of
2
transportation including Florida, have developed models to estimate the service life of
various types of drainage pipes, including D-C-RCPs.
FDOT existing service life model for reinforced concrete pipes is based on the
assumption that the durability of reinforced concrete pipes in the Florida environment is
controlled by the time to corrosion initiation of the reinforcing steel. The equation in the
model was first developed by Cerlanek and later updated by Sagues, et al (2) (3). The
equation in the model is a function of the environmental sulfate content, environmental
chloride content, pH of the soil and/or water, cement content, cover thickness, and total
percentage of mix water. The correlations in the model are based on empirical
observations. For many reinforced concrete structures, the end of service life is
considered, when cracks occur in concrete due to the corrosion of reinforcing steel and/or
can’t carry the structural load. During the propagation stage, corrosion products built up.
After some time the accumulated volume of these products become constrained and can
produce significant tensile stress onto the concrete. These stresses could eventually lead
to cracks. However, the small-diameter steel stirrup and high porosity of D-C-RCP might
allow the corrosion products to move through the pore structure and not produce cracks
or produce only small cracks. Moreover, when the concrete is quite saturated it has been
observed that the corrosion product flow easier into the available pores structure.
Furthermore, it also has been reported that under saturated concrete, corrosion rate of the
reinforcing steel is also slower, than when wetting/drying cycles are taking place. It is
frequently assumed that this is due to oxygen concentration limitations. The propagation
stage of D-C-RCP might be longer than what is usually assumed for a typical reinforced
concrete structure.
3
1.2 Research Objective
The objectives of the study are as follows:
1. Develop methods that initiate corrosion in a short period of time.
2. To understand the mechanism of corrosion propagation in dry-cast reinforced
concrete pipes
3. To identify the factors that affects the corrosion propagation.
1.3 Research Methodology
Experiments were conducted on two different types of D-C-RCP. Type F contains
20% of fly ash as cement replacement and type C contains only ordinary Portland
cement. The D-C-RCP pipe sections for both types (provided by FDOT) were segmented,
instrumented and solution reservoirs installed. Two different specimen geometry
orientations were used: horizontal or vertical. In the horizontal specimens the longitudinal
steel rod under the reservoir was used as a working electrode.
Potentiostatic and galvanostatic methods were used to accelerate chloride transport
via modest polarization. In the vertical specimens a migration method was used.
Depolarization tests were performed periodically on all specimens to identify the time to
corrosion initiation, via the corrosion potential. Once the off-potential of the steel reached
a value more negative than -250 mVsce after 24 hrs, the specimen was considered active.
4
The activation potentials ranged from -250 mVsce to -550 mVsce. Specimens undergoing
active corrosion were transferred to 95~98% high humidity chamber, and/or to fully or
partially buried in simulated saturated soil, and/or immersed in water. During the
propagation stage potential measurements were taken every week. Linear polarization
tests were carried out to obtain Rpapp values. Electrical Impedance spectroscopy was
performed to obtain the solution resistance. Both tests were carried out every other week.
Some specimens eventually reached potentials values at or below -530 mVsce. So far
these specimens show no visual signs of corrosion such as cracks or corrosion products
with one exception; where corrosion products have reached the surface within the
reservoir. One reason could be that since both concrete types have high porosity the
products are moving through interconnected pore structure. From the test results gathered
so far no increase in Rpapp with time has been observed. Upon further exposure there
could be an increase in Rpapp.
1.4 Thesis Structure
This thesis consists of five chapters, including the introduction as chapter 1 where
the research background, objective and methodology were discussed briefly.
Chapter 2 gives the literature background necessary for this study. Such as previous
research and field reports carried out on Dry-Cast Reinforced Concrete Pipes (D-C-RCP).
D-C-RCP and Wet-Cast Reinforced Concrete Pipes (W-C-RCP) manufacturing methods.
W-C-RCP are only produced in large diameters pipes, smaller diameters were produced
from late 1800’s to late 1950’s. The details of the current service life modeling and its
5
drawbacks were briefly discussed in this chapter. (4) A brief description of experiments
and measurements to be used in this thesis were also included in chapter 2.
Chapter 3 describes the experiments and methods carried out to accelerate the
chloride transport. Chapter 3 also describes the various set-ups carried out in this study to
simulate the environmental conditions during the corrosion propagation stage.
Chapter 4 presents the results. Chapter 5 presents a brief discussion of what might
be taking place during the propagation stage on the tested specimens. Finally, Chapter 6
presents the conclusions of this study.
6
2 LITERATURE REVIEW
2.1 Corrosion
All metals and alloys tend to return back to its ore state, that is the deterioration of
materials properties due to its interaction with its environment. In general electrochemical
reactions on metals and alloys take place by involving both chemical reactions and
charge transfer reactions. One type of electrochemical reaction is corrosion. Corrosion
initiation to final state is governed by the principles of thermodynamics and reaction rate
theory (reaction kinetics). Reaction rate theory facilitates representation of the time
required for the change of state (4).
2.1.1 Corrosion process
The nature of current flow in an electrochemical or corrosion cell is more complex than
for conduction of electrons through solid materials. In an electrochemical cell, electrons
are the charge carriers through the metallic portions of the circuit. Ions are transported in
the electrolyte solution (5). Electrochemical reactions necessarily occur at the metal-
electrolyte interface. At the electrode of lesser electron affinity also termed the anode, an
7
oxidation reaction takes place and due to this atoms of the particular metal are ionized.
MA → MA+n
+ ne−
where MA refers to atoms of metal A and MA+n
to ions of metal A with charge +n. The
process is described schematically by Figure 2.1. From this diagram it is seen that
Figure 2.1 Schematic illustration of anodic dissolution reaction the positive metal ions enter the electrolyte and completes the ionic path, but the free
electrons contribute to an electrical current within the metal. Electrons and ions are the
products of any oxidation reaction. At the electrode of greater electron affinity, termed
the cathode, a reduction reaction occurs. The nature of this can vary depending upon the
particular environment at hand; however, reactions that have been found to be of
generalized importance are as listed below:
2H+ + 2e− → H2 (hydrogen reduction acid solution)
O2 + 2H2O + 4e → 4OH (in well aerated neutral or alkaline environments)
O2 + 2H2O + 4e → 2H2O (oxygen reduction in acid solution)
M+n + e− →M
+(n-1) (metal ion reduction)
M+n +ne− →M (metal ion deposition)
8
Irrespective of the solution, the rate of oxidation must equal the rate of reduction. Thus
for every positive charge generated during oxidation, similar charge must be consumed
during the reduction reaction. Thus the anodic and the cathodic reactions taking place
will be in the balanced state (4).
2.1.2 Corrosion of steel in concrete
Reinforced concrete is a widely used construction material. It is generally durable; as a
thin dense film called a passive layer forms on the surface of the reinforcing steel. The
formation of this passive layer at the steel surface is promoted by the concrete pore
solution high alkaline environment (pH = 12.5 – 13.5) (5). However, steel in concrete can
corrode due to the carbonation of concrete (lower pH) and chloride ions attack from
marine environment or de-icing salts. Once the chloride content at the surface of the
reinforcement reaches a critical chloride threshold value (CT), the passive film is broken
down and corrosion initiates. Corrosion of the steel reinforcement is an electrochemical
cell that is composed by four processes Figure 2.2.
9
Figure 2.2 Schematic representation of the electrochemical reaction steel/concrete
I. The oxidation of iron (anodic process) that liberate the electrons in the metallic
phase and gives rise to the formation of iron ions (Fe Fe2+ + 2 e-) whose
hydrolysis produce acidity (Fe2+ + 2H2O Fe(OH)2 + 2H+)
II. The reduction of oxygen (cathodic process) that consumes these electrons and
produces alkalinity O2 + 2H2O + 4 e- 4OH- (other cathodic process are also
possible)
III. The transport of electrons within the metal from the anodic regions where they
become available, to the cathodic regions where they are consumed.
IV. Finally, in order for the circuit to be complete the flow of current inside the
concrete from the anodic regions to the cathodic ones, transported by ions in the
concrete.
These four process needs to be present for the corrosion to takes place on steel embedded
in concrete.
10
2.2 Reinforced Concrete Pipes
Reinforced concrete pipes are classified as two major types according to their
manufacturing methods. One is wet cast reinforced pipes and another is dry cast
reinforced concrete pipes. For RCP prepared via wet cast method, the concrete is
prepared with an intermediate water to cement ratio.
The concrete is poured between the spaces inside concentric mold forms and
allowed to set. The mold forms are kept either horizontal or vertical. Depending upon the
mixture of concrete the forms are removed and the pipes are immersed in a water tank, or
covering with water saturated material, or by a system of perforated water tubes for
further (wet) curing process. The curing process takes about 2-4 weeks or less when
acceptance is based on passing the specified D-load as stated in ASTM C 76-08a. (6)
(7)These pipes are no longer being manufactured in small diameters,
Dry-cast reinforced concrete pipes are manufactured with low water to cement
ratio. The widely used manufacturing methods of D-C-RCP are a) Packer head method,
b) Petershaab method, c) Hawkeye method, and d) Sherman-Dixie Schlusselbauer Exact
2500 (8) (9). The selected method is based on preferred manufactured approach.
Regardless of the method, the steel reinforcement is placed in the space between the two
concentric cylinders and the space around the reinforcement is filled with the concrete. In
Packerhead manufacturing method, the inner cylinder is made to vibrate by electro
hydraulic method to gain the compactness of the concrete mix between the two cylinders.
The cylindrical forms are removed after achieving the compactness. These formed dry-
cast pipes are cured by steam in a temperature (65-80ºC) controlled room for 12 hours.
11
After curing the pipes are moved to the open yard and kept ready for use. The D-C-RCP
also needs to pass the specified D-load for the corresponding geometry. The Dry-cast
reinforced pipes will be investigated here (10).
2.3 Existing Service Life Model
The first model was prepared for California transportation by R. Stratfull and
defined end of the service life as the time when externally observable corrosion (9)
(2)occurred. The proposed equation (1) as shown below
trc = 1.011 ( 1.107C C0.717 S1.22 K-0.42 W-1.17 (1)
C = Sacks of cement per cubic yard
S = Concrete cover in inches
K = Chlorides in environment in ppm
W = Total mix water in % of concrete volume
The above equation lacked expressions for the presence of sulfate and low pH in
the environment. Later on, Dr. Sagues updated the model and included as part of the
service life model, the presence of sulfate and low pH factors.
The existing model is based on the assumption that the durability of reinforced
concrete pipes in the Florida environment is controlled by the corrosion initiation of the
reinforcing steel. As indicated above, it was first developed by Cerlanek and later
updated by Dr. A. A. Sagues, et al (3). The model is a function of the environmental
sulfate content, environmental chloride content, pH of the soil and/or water, cement
12
content, cover thickness, and total percentage of mix water in the concrete. Equation (2)
shows the equation currently in use.
SL = 103 * 1.107CC * CC 0.717 * DC
1.22 *(K+1)-0.37 * W-0.631-4.22 * 1010 * pH-14.1-2.94*10-3 *
S + 4.41 (2)
Where,
S = Environmental sulfate content in ppm
Cc = sacks of concrete in per cubic yard
Dc = concrete cover in inches
K = environmental chloride content in ppm
W = total percentage of water by volume
2.4 Field Performance Analysis
Factors affecting the corrosion propagation period include: concrete porosity,
concrete cover, concrete degree of saturation, availability of water, availability of
oxygen, resistivity (last three parameters both in the concrete and at the surrounding soil),
corrosion rate, and environmental conditions (e.g. humidity, temperature, etc) (11) . The
corrosion rate of steel in concrete can be limited by the concrete resistivity which is
controlled by w/c, moisture content, and presence of admixtures such as Fly Ash. Fly Ash
presence usually reduces the porosity with time as hydration and pozzolanic reaction
progresses. Some researchers claim that the corrosion rate is highest when the relative
humidity (RH) ranges from 70% to 95%, i.e. under conditions of plenty of oxygen and
water available and for a given modest to low intrinsic resistivity (12). It has been
reported that the corrosion rate of wet-cast reinforced concrete would decrease when the
13
environmental humidity increase from 95% to 100% (13), but it is not clear at what RH
the transition occurs or if additional parameters (e.g. porosity) would affect when this
transition occurs. It is widely understood that the corrosion rate of the submerged or fully
saturated reinforced concrete is low compared to atmospherically (70-95% RH) exposed
reinforced concrete. Previous studies have confirmed that the corrosion rate is reduced in
such conditions due to the low availability of oxygen at the steel depth (14) and lower
oxygen diffusivity. More recently, Hussain et. al., investigated that the coupled effect of
oxygen and moisture on the corrosion of reinforcing steel in concrete cast with chlorides
(1%, 5%, and 10%). The results reported by Hussain (14) revealed that the diffusion of
oxygen is a vital limiting factor for corrosion of the rebar only when the reinforced
concrete structure is either submerged or in a high RH environment (60%, 95% & 100%)
with a dense concrete cover and low w/c ratio. The concrete composition and preparation
used was different from the concrete used in the D-C-RCP. The porosity of dry-cast
reinforced concrete pipes is known to be high. Therefore, studies of D-C-RCP in marine
environments are required in order to obtain a new or improved service life model that
takes into account the propagation stage. Such that the model better reflects D-C-RCP
composition and exposure conditions, since the previous field reports shows modest or no
visual corrosion indications when fully buried.
The electrochemical process described above can be seen as an electrical model
represented as four resistors (15). One of them is the electrical resistance of steel which is
practically zero, by determining the other three resistances [anodic polarization
resistance, cathodic polarization resistance, ion transportation in electrolyte solution
resistance], the electric current flowing through within the electric circuit can be
14
calculated, which corresponds directly to the rate of iron removal at the anode and the
cathodic reaction by using the faraday`s law. If any of the above mentioned resistances is
extremely high; the corrosion process is very slow and almost stifled. If the steel surface
is not de-passivated by chlorides or carbonation, then the anodic polarization resistance
Ra the surface process or (i.e. electrochemical reaction taking place at a passive steel
surface) is significantly large and becomes the controlling factor. If the water to cement
ratio is low the electrolytic resistance Rs could become the controlling factor. Whereas if
there is no oxygen (or a low oxygen concentration) available, then the cathodic
polarization resistance Rc would become the controlling factor. M.Raupach (16)
investigated the influence of oxygen on corrosion of steel in concrete and illustrated the
cathodic process as a simple two resistance electrical circuit.
Rauapach proposed that the cathodic polarization resistance can be subdivided into
two resistance components:
RC = RC, C+RC, O2
RC, C = activation contribution of the cathodic reaction [ionic path is incomplete]
RC, O2 = oxygen diffusion resistance (mass controlled contribution), which controls the
cathodic reaction rate depends mainly on the ambient conditions. This resistance is high
when the limiting current density is low.
15
Figure 2.3 An Evans diagram with oxygen limiting factor.
Figure 2.3 shows an Evans diagram that is sometimes used to understand the
intersection between anodic and cathodic polarizations curves, in this case these taking
place at the reinforcing steel embedded in concrete. The x-axis shows the current density
in log-scale, and the y-axis the potential. As shown in Figure 2.3, curve A represents the
anodic polarization of passive steel (in concrete). Curve B represents the anodic
polarization of de-passivated steel (due to chlorides or carbonation). Curve C represents
the cathodic polarization of the steel in an abundant oxygen available environment (e.g.
atmospheric conditions). Curve D represents the cathodic polarization of the steel in a
low oxygen available environment. EAC is the mix potential value when curves A and C
16
intersect, it suggest a noble (positive) potential for passive steel. EBC is the corrosion
potential for the intersection of de-passivated steel (Curve B) in an oxygen available
environment, the potential is significantly more negative and the current density is
significantly large, suggesting a high corrosion rate (when the areas for anode and
cathode are the same). EBD is the corresponding corroding potential for the de-passivated
steel in a low oxygen available environment. We can observe that the potential at EBD is
even more negative than EBC, but the current density is lower due to lower oxygen
concentration which would suggest that the corrosion rate is lower. In real structures the
dimension of anode and cathode area are sometimes unknown, which might result on
steel corroding at a high rate if the size of the cathode is large enough to sustain the rate
that a small anode demands. This is known as a corrosion macrocell.
2.5 Interpretation of Mix potential value measured
Non corroding steel in concrete has a noble open circuit potential (EAC). Once corrosion
has initiated (i.e. per ASTM c876-09 (10) there is 95% probability of corrosion when
half-cell potential (vs. SCE) measured values range from –273mV to – 423mV under
atmospheric conditions), the anodic polarization curve A would shift to polarization
curve similar to that represented by the curve B in Figure 2.3 and the corresponding
potential will then be EBC. The location of the polarization curve once corrosion has
initiated is known to vary depending on the amount of chlorides surrounding the anode
(active corroding) site. Hence, curves parallel to curve B would represent situation in
which steel is exposed to a lower chloride concentration. D-RCP are usually covered by
soil where the diffusion of oxygen and the oxygen concentration there might be low,
17
particularly if fully saturated. Under these environmental conditions, oxygen reduction
reaction is the main cathodic process. Under mass-controlled (or mix control) scenarios
the cathodic polarization curve would shift from curve C to a polarization curve similar to
curve D. For active steel, the new intersection of curve D with curve B determines the
new potential of the steel as EBD and corresponding current density iBD. If these are the
conditions present then the corrosion would propagate at a significantly slower rate (15)
than that observed when the relevant polarization curves were B and C. But if the de-
passivated steel area (anodic) has access to a large cathode area, provided that the
electrical concrete resistivity is low, then a macro cell might be present that allows the
anode to continue to corrode at a high dissolution rate.
2.6 Testing Standards & Methods
2.6.1 Accelerated Chloride Transport
To investigate the propagation stage, it would be desirable to shorten the time to
corrosion initiation via an accelerated chloride transport method. Trejo, Castellote et. al.
(17)among others has proposed methods (18) to determine chloride threshold on freshly
prepared specimens (usually mortar) via a set-up that applies an electric field. This
accelerates the transport of chlorides. Migration tests (e.g. Rapid chloride permeability,
Nordtest Build 492 (19)) have also been developed to assess chloride permeability into
concrete after a short period of time. The accelerated transport of ions in solution under
an electric field is called migration. The velocity of ion movement is proportional to the
strength of electric field and the charge and size of the ion. Ui is the ion mobility.
18
𝐷𝐷𝑖𝑖 = 𝑅𝑅𝑅𝑅 𝑈𝑈𝑖𝑖 𝑍𝑍𝑖𝑖 𝐹𝐹
Where, R is the gas constant (j/(k . mole)), T the temperature (kelvin), F faraday`s
constant (96 490 C/mole) and zi the valence of ion i. Migration of ions (e.g. Cl-) into
concrete are tested as a measure of the concrete resistance to chloride penetration. The
name of the test is called non-steady-state migration. A constant potential difference is
applied at both sides of the specimen. The depth of penetration of chloride ions into is
measured and plugged into an equation to obtain the migration coefficient Dnss.
2.6.2 Corrosion monitoring methods
1] Half-cell potentials measurements are a non-destructive method (except for the
electrical connection to the reinforcement) for assessing the corrosion state of rebar in
concrete. The more negative the measured potential is; the higher the probability that the
rebar is corroding. Problems in interpreting the negative potentials occur, when dealing
with structures that are completely immersed in water or in water saturated conditions or
in any other condition where the oxygen concentration is low. Standard ASTM C876
provides the criteria for interpretation. In atmospherically exposed reinforced concrete in
chloride-induced conditions; there is 50% probability of corrosion when the half-cell
potential value ranges from -123mVsce to -273mVsce and there is 95% probability of
corrosion when the potential value measured ranges from –273mVsce to – 423mVsce. In
water saturated reinforced concrete conditions due to low concentration of oxygen, the
measured potential values are typically very negative potentials (lower than -423 mVsce).
2] Linear polarization resistance measurement (5)
19
Figure 2.4 Anodic polarization curve of steel in non-carbonated concrete.
The rate at which the anodic or cathodic process takes place depends on the
potential of the corroding metal (steel). The corrosion behavior of the reinforcement can
be described by means of polarization curves that relate the potential and the anodic or
the cathodic current densities. Determination of curves is much more complicated for
metals (steel) embedded in concrete than in aqueous solutions.
The polarization resistance method is based on the assumption that the
polarization curve close to the corrosion potential is linear (21). In Figure 2.4 the slope
ΔE/ΔI, is defined as polarization resistance, Rp, where ΔE is the step in potential, ΔI is
the resulting current (20).
The Rp value is related to the corrosion current icorr = B / Rp; where B is the slope of the
polarization curve. For an actively corroding reinforcing steel in concrete a B = 26mV is
usually assumed. icorr is related to the area of the reinforcement under test. The Rp is
measured in Ωm2 and the corrosion density is converted to mA/m2 or µA/m2.
20
3] Potentiodynamic cathodic polarization scan have been used to determine cathodic
limiting current density and from this value obtain the amount of O2 at the steel under
saturated conditions. (Assumes that the interrogated area is known) (16) . The kinetics of
oxygen reduction can be illustrated by the cathodic polarization curves a and b shown in
Figure 2.5. The rate of the cathodic process along each of these two polarization curves
depend to some degree on the oxygen availability at the surface of the steel: on the
activation control region there is almost not influence by O2 concentration, mix-control
region is affected by the O2 concentration and limiting current value is observed at more
negative potential values where the dominant process is the mass transport. The potential
of the reinforcing steel can shift to more negative values as a result of an increase in the
concrete moisture (since this would reduce the transport of oxygen (DO2 is low in
saturated concrete) and after sometime the O2 could be consumed or found at a
significantly lower concentration when compared to atmospheric conditions)
21
Figure 2.5 Schematic cathodic polarization curves in alkaline concrete: (a) aerated and semi-dry: (b)
wet or completely saturated with water, (c) de-aerated solution.
4] EIS (electrochemical impedance spectroscopy) has been used to monitor corrosion on
reinforced steel concrete structures. In here the solution resistance Rs will be determined
by the magnitude of the impedance at 54.51Hz. (5) The EIS test applies a small-
sinusoidal potential signal at a variety of frequencies. The impedance modulus is the
value assumed for Rs.
22
3 APPROACH
3.1 Materials and Instrumentation of Specimens
Materials, specimen’s set-up, experimental set-ups used to accelerate chloride transport
and the environmental exposures used during the corrosion propagation stage are
described below.
3.1.1 Materials
Dry cast reinforced concrete pipes sections (D-C-RCP) of 24 inches in diameter and 60
inches long, were supplied by FDOT. The D-C-RCPs sections were placed in the outdoor
yard for 3 years at SMO-FDOT after being produced. D-C-RCPs were transported to the
marine materials and corrosion laboratory at SeaTech FAU campus from FDOT-SMO in
Gainesville. D-C-RCPs were placed outdoors in the yard on a wooden base before they
were cut into smaller segments. Two different types of D-C-RCPs were supplied. One
type of D-C-RCPs contained fly ash and is identified here in as type F. The second type
contained only ordinary Portland cement as the cementitious material. This second type
of D-C-RCPs is referred here in as type C. Two 60 inches long sections of each D-C-
RCPs type were supplied.
23
One of the D-C-RCPs type C sections was found to have a crack throughout the length of
the pipe. The crack probably occurred during transport. The compositions of the two
different D-C-RCPs types are shown in Table 3.1. The compositions correspond to those
provided by the manufacturers.
Table 3.1 Mixture proportion of products
Mix Design 4000 PSI
Materials Type Materials Unit
Type F Type C Lb/yd3
Cement 391 590 Lb/yd3
Fly Ash 103 0 Lb/yd3
Sand 1689 1895 Lb/yd3
Stone 1773 1300 Lb/yd3
Water 16 29 Gal/yd3
Admixture 0 0 Oz
24
3.1.2 Segmentation
Each 60 inch long D-C-RCPs segment was marked before cutting. The D-C-RCPs was
first cut into 11 inch long slices (rings). After a slice (ring) had been cut from each D-C-
RCPs it was sectioned at approx. 30°, 45° and 60° angles as shown in Figure 3.1. Some
slices were left in the ring shape but will not be discussed in here.
Figure 3.1 Angles at which the test pieces were segmented
The test pieces were segmented in order to have at least 2 cuts for each arch-angle. The
reason for segmenting the test pieces at different angles were to end up with 1, 2 and 3
longitudinal reinforcement bars for the test specimens, i.e. at 30° there would consist of 1
bar, 45° would contain 2 bars, and 60° would contain 3 bars. The larger the arch-angle of
the cut surface the larger the reinforcement area will be. The various specimen sizes were
allowed to have different cathode to anode ratios. On each specimen; only a portion of
one of the longitudinal bars was subjected to chloride solution (see below). Each
25
specimen was labeled and grouped as the type of concrete and a number corresponding to
where the segment came from relative to the cut (labeled clockwise), i.e. the base
segment from the type F D-C-RCPs was labeled as Fp1 the subscript represent the
method of accelerated chloride transportation . A total of five full rings were sliced from
each D-C-RCPs provided. The segmentation was carried out in 14 rings. Five full rings of
each type of D-C-RCPs were subjected to experimental studies that are not discussed in
here. The exposed steel reinforcement bars of each specimen were checked for electrical
continuity with all other longitudinal and transverse (spiral) reinforcement segments. This
was verified by using a multi-meter set to the Ω (ohm) setting, then placing one needle
probe on a steel reinforcement bar and touching the rest of the reinforcement ends with
the other needle. For all specimens the reinforcements within each specimen were found
to be continuous.
3.1.3. Instrumentation/Electrical terminal installation
On each specimen, one of the exposed steel rods was selected to establish an electrical
connection. This was accomplished by threading #2 or #4 machine stainless steel screw
into the center of the selected reinforcement. This was achieved by locating the center of
reinforcement rod with a proper indentation by punch mark and then drilling a ~ 1.2 cm
deep hole into that reinforcement, as shown in Figure 3.2. Threads were created from
carefully turning a plug tap then finishing it off with a bottoming tap to ensure the screw
secured properly. Two nuts were placed on the screw with two washers on each side so
that an electrical wire could be secured in contact with the reinforcement.
26
Figure 3.2 Steps of instrumentation on specimens
3.1.4. Reservoir construction and installation
The reservoirs were installed on the specimens in the horizontal or vertical orientations.
On specimens with the reservoir in the horizontal orientation, the reservoir was made-up
of plexy-glass and was held together by a waterproof UV resistant marine sealant. The
nominal cross-section area for the horizontal reservoir was 2.5” x 6”. The reservoir was
glued on the inner curvature of the specimen right above the selected reinforcement,
where the electrical installation was made, as shown in Figure 3.3. The bottom edge of
the reservoir`s horizontal section was also cut in a concave shape so that it matched with
the inner curvature profile of the specimens. The height of the reservoir was 2”.
27
Figure 3.3 Steps of reservoir construction on horizontal orientation specimens
In specimens with the reservoir in the horizontal direction, two elliptical plastic
containers were first cut to the shape of the inner and outer diameter of the pipe. The
containers were then glued using marine grade silicon, and let to cure for 1 day as shown
in Figure 3.4. Each container reservoir has an area of 2.5” x 7” and the reservoirs were
filled with the solution up to 5” height.
Figure 3.4 Reservoir constructions on vertical orientation specimens
28
3.2 Concrete Characterization
3.2.1 Measurement of concrete cover
Concrete cover at each specimen`s reinforcement was measured and tallied to obtain
cover cumulative distribution for both longitudinal and circumferential directions. These
covers were measured from the internal diameter to the exposed steel rod. The covers
were measured by using a Vernier Caliper. Cumulative distributions were tallied and
recorded and grouped for each accelerated chloride transportation methods and for each
specimen type.
3.2.2 Porosity
Additional cuts were done on a selected D-C-RCPs segment to obtain prismatic concrete
specimens for porosity tests. Each small segment had a weight of approximately 800
grams. Since the specimens were obtained between the longitudinal and circumferential
reinforcements, the size of the specimens obtained was not always according to the
standard size. Modified ASTM C642-97 (21) porosity test was conducted on these
segments. In our test the maximum heating temperature was maintained at 67 degree
Celsius while in the oven to minimize pore structure changes.
29
3.2.3 Resistivity
Six cores of type F and nine cores of type C were obtained from selected segments. These
cores were obtained using a wet-drill coring device with a nominal 5.7 cm diameter drill
bit that produced 5 cm diameter cores. The cores were immersed in water. Resistivity
measurements were taken two days after immersing the cores in water. Five
measurements were performed within 12 days on these specimens. Resistivity readings
were made using a Werner four point apparatus with 2 cm spacing between probes. The
cores were surface dried before taking the measurements. Eight resistivity measurements
were taken on each cylinder (90 degrees apart, i.e., twice around). The average of the
recorded values were calculated and corrected for finite geometry. The temperature was
also measured. The K value used for type C and type F were 2.45 and 2.61 respectively.
Each time measurements were normalized to a reference temperature of 21º C during the
data processing (22). Additional readings were taken on subsequent days until the
resistivity values were found to be stable.
3.2.4 Rapid Chloride Migration Test
Segments from each D-C-RCP type were selected to obtain concrete cores for rapid
chloride migration test as per Nordtest Build 492 (19) on saturated concrete. These cores
were obtained using a wet-drill coring device with a nominal 5.7 cm diameter drill bit
that produced 5 cm diameter cores. The test was slightly modified to accommodate the
smaller diameter core. Four cores of type F and seven cores of type C samples were used
30
for the rapid migration test; resistivity test was also carried out on all the cores used for
this test.
3.3 Experimental Set-ups for Accelerated chloride transport methods
Three approaches were used to shorten the time to corrosion initiation on the dry-cast
reinforced concrete pipes instrumented specimens. Two of the accelerated chloride
transport method used the reinforcement directly as one of the electrodes with a second
electrode placed inside the solution reservoir. The latter electrode acted as the counter
electrode. In the third approach, the reinforcement was not connected directly. These
approaches were named here in: potentiostatic, galvanostatic, and migration cell
approaches and are described below. The three methods accelerate the chloride transport
(i.e. the chloride ions in the solution reservoir) towards the reinforcing steel. The chloride
transport is due to migration transport and to a smaller extent, to diffusion transport. All
the accelerated chloride transportation methods were carried out under laboratory
environment conditions, except for some samples subjected to the migration method,
which were first subjected to high humidity conditions for 14 days before applying the
electric field.
31
3.3.1 Potentiostatic
A dedicated potentiostat was used for each D-C-RCP instrumented specimen. An
activated titanium mix metal oxide (MMO) mesh was placed in the solution reservoir as a
counter electrode. For this experiment, 15% NaCl solution was used. A calomel reference
electrode was inserted through a lid covering the solution reservoir, and was connected to
the reference electrode terminal of a potentiostat. The activated titanium mesh was
connected to the counter electrode terminal of the potentiostat. A 100Ω resistor was
connected across the counter and the working electrode terminals of the potentiostat to
measure the current flow at any given time. The reinforcing steel contact (via the tapered
screw) was connected to the working electrode terminal of the potentiostat. Once the
connections were made, the potentiostat was then turned on and adjusted to the desired
potential. The potentiostat was set to a potential hold of +2 V for Group Fp specimens
and +3 V for Group Cp specimens. This potential was set between the reference electrode
and the reinforcing steel. The actual applied potential between the counter and the steel
was also measured and was typically a few mV larger. Figure 3.5 shows the potentiostatic
method set-up.
32
Figure 3.5 Potentiostatic method set-ups
To set the applied voltage, the potential difference between the reference electrode
(green tab, calomel electrode) and the working electrode (black tab, steel reinforcement)
was measured and adjusted if necessary. This polarity was the same for every
measurement. Periodically, Vapplied and Vresistor were measured. The Vresistor potential
values were measured to monitor the amount of current flowing through the titanium
MMO mesh and steel reinforcement by Ohm’s Law, Vresistor = IR.
The off potential, or V1off, was recorded about 5-10 seconds after disconnecting the
potentiostat. At least three more measurements of the off potential were done: 10, 30 and
60 minutes after disconnection (V2off, V3off and V4off). The potential hold was usually
reinstated after 60 minutes. In some cases additional measurements were performed at
120, 240 minutes, and 24 hrs. Once, the off potential of the specimen reached -250mVsce
(or a more negative potential value) after 24 hrs of being disconnected, the specimen was
declared active and the potential hold suspended. The specimens under this accelerated
chloride transport are named Group Fp specimens and Group Cp specimens. There were
33
four Fp specimens (F0, F1, F2 & F3) and six Cp specimens (C0, C1, C2, C3, C4 & C5)
subjected to this method.
3.3.2 3.3.2 Galvanostatic
A constant current was delivered using a six- multi-channel galvanostat as shown in
Figure 3.6 Initially; a current of 1 mA was applied to each specimen. The positive
terminal of one of the channels was connected to the reinforcement terminal post on the
specimen and the corresponding negative terminal in the galvanostat was connected to
the counter electrode activated titanium mesh). Group Fg specimens and Group Cg
specimens were polarized using the galvanostat method. Four Fg specimens (F4, F5, F6
& F7) and six Cg specimens (C6, C7, C8, C9, C10 & C11) were used for galvanostatic
method of chloride transportation.
Figure 3.6 Galvanostatic method
34
The solution reservoir was filled with 3,400 ppm of chloride ions (~120 gr of NaCl to 20
L). This lower chloride concentration (3,400 ppm) is within the range observed in Florida
soils. A second group of specimen`s (F12, F16, F17, F18, F19, F20, F21, F22, & F23)
was also subjected to this galvanostat approach. The reservoirs of these specimens were
filled with 10,150 ppm to reduce the time for corrosion initiation. The off potential, or
V1off, was recorded about 5-10 seconds after disconnecting the galvanostat. Three more
measurements of the resting potential were done at 10, 30 and 60 minutes after
disconnection (V2off, V3off and V4off). In some cases additional measurements were
performed at 120, 240 minutes, and 24 hrs. Once, the off potential of the specimen
measured was -250mV Vssce within (or more negative) 24 hrs, the specimen was declared
active and the applied current suspended. Early on the galvanostat was reconnected after
60 minutes; later on after 120 minutes up to 24 hours if the criteria previously explain
was not met.
3.3.3 Migration cell
On specimens subjected to the migration method, solution reservoirs were installed on
both sides of the selected longitudinal rebar. In this case, the longitudinal bars were in the
vertical direction as shown in Figure 3.7. The container at the inside curvature (typically
smaller cover) was filled with the chloride solution. The container glued to the outside
curvature of the specimen was filled with Ca(OH)2 solution. Stainless steel mesh
electrodes were placed in each container. The solution volume added at each side was
approximately 200 ml, and re-filled once a week or sooner when necessary. The positive
35
terminal of the power supply was connected to a stainless steel mesh placed inside the
calcium hydroxide solution (outer circumference side). The negative side of a power
supply was connected to the stainless steel mesh placed inside the chloride solution, with
a concentration of 10,150 ppm (via NaCl added). The increase in chloride concentration
with respect to the galvanostatic approach is two-fold: 1) try to reduce the time to
initiation on D-C-RCP specimens tested using this approach, and also to have other
values for surface concentration Cs representative of a higher concentration also observed
in Florida soils.
Figure 3.7 Migration cell The applied voltages were 30 volts and 15 volts for Group Fm specimens (F8, F9, F10,
F11, F13 & F15) and Group Cm specimens (C12, C13, C14, C15, C16, C17 & C18)
respectively. (One of the specimens of each D- RCPs type was subjected to a range of
applied potentials) Once, the 24-hr off potential of the specimen`s measured value was -
250mVsce (or more negative); the specimen was declared active and the electric field
36
suspended.
3.3.4 Migration cell method under high humidity
A set of specimens were prepared with the migration set-up. The inner reservoir was
filled with chloride solution of 10,150 ppm and the outside container was filled with
calcium hydroxide solution. These specimens (C21, C23, C24, C25 & C26) were exposed
with the solution to laboratory RH and temperature for about two weeks. The samples
were then transferred to a high humidity chamber (~95-98 RH), see section 3.4.3 for
description of the chamber. The specimens were inside the chamber for 3 days under high
moisture condition before applying the electric field (migration test). These specimens
were then connected to the power supply by wires through the access port on the wall of
the chamber. The measurements on these specimens were taken by momentarily opening
the chamber. Measurements were taken two times a week. The depolarization
measurements were recorded every time with Voff 3-4 seconds, 5, 15 and 30 minutes
from the power off time. Depending on the depolarization rate of the specimens,
additional measurements were made up to 24 hrs after turning-off the power. Once, the
off potential of the specimen`s measured value was -250mV Vssce (or more negative)
within 24 hrs: the specimen was declared active and the electric field suspended. The
samples then remained in the 95-98 RH as one of the exposures during the propagation
stage.
37
3.4 Experiment Set-up for Study of Corrosion Propagation
To study the corrosion propagation of the D-C-RCPs during the propagation stage, the
specimens would be exposed to various laboratory environmental conditions ranging
from specimens exposed to high humidity, fully or partially immersed in water and fully
or partially covered with simulated saturated soil (water saturated sand). These set-ups
were implemented to try and replicate limitation/depletion of oxygen (or exposed to a
lower O2 concentration) at the reinforcement. Under the above scenarios the reinforcing
steel would be surrounded by water saturated concrete, which reduces the oxygen
transport and a limited oxygen reduction reaction usually, develops. As corrosion
propagates (anodic reaction), it is unknown if the cathodic reaction would slow the
corrosion rate due to limited oxygen availability or if a macrocell would develop that
would allow a high corrosion rate to continue. These propagation experiments were
achieved by the following configurations.
3.4.1 Environmental Chamber (High Humidity)
A commercial environmental chamber that operates at 115v/ac, 60Hz, 25A was used in
monitoring selected specimens. The environmental chamber allows for temperature and
humidity to be controlled. The Specimens were placed on the stainless steel racks inside
the chamber as shown inFigure 3.7. The Chamber was maintained at 99% RH and 21ºC
initially. Later on due to some operating limitations of the chamber, the RH was
maintained at 95%.
38
Figure 3.8 environmental chambers with specimens
Specimens Cg9, Cg7, Fp3, and Fc6 were placed inside this environmental chamber. Each
specimen had its own dedicated reference electrode. Wirings were installed through the
cable port hole provided on the frame of the chamber. Half-cell potentials and linear
polarization measurement could be performed without opening the chamber door.
Initially, LPR and EIS measurements were performed once a week and the potentials
were measured every two days to monitor the changes during the first month. Later on
the measurements were taken every month. Due to some technical issues the
environmental chamber became inoperable for nearly two months, but the specimens
remained inside the chamber. Once the chamber was back in operation, the high
moisture exposure continued.
39
3.4.2 HDPE Horizontal (High Humidity)
A 36x36x24 inches container made of high density polyethylene was used as a high
humidity chamber. PVC cylinders two inches in height and four inches diameter were
placed at the bottom of the box. A plastic mesh was placed on top of these cylinders. The
high humidity condition was achieved by filling with water to a height of 1.5 inches.
Specimens Cp1, Cp3, Cg11, Fp1 and Fg5 were placed on this chamber.
Figure 3.9 specimens in HDPE Box Each specimen was installed with its individual reference electrode as shown in Figure
3.9. Wires of sufficient length were prepared and installed for each specimen to reach
outside the box. Initially, the linear polarization, impedance and half-cell potentials are
measured once a week. Water was sprayed manually inside the box twice a week. A hole
of 1/4” diameter was drilled on top cover of the box and fitted with a coupling. A
humidity sensor probe was inserted periodically to check the relative humidity inside the
box by removing the blank. The box cover is clamped down using six C clamps to
maintain the high humidity. After the initial set of measurements were processed, it was
40
determined that the corrosion rate remain the same. The measured relative humidity was
not higher than 90%. To improve the set-up after initial exposure one month, weather
tight strips were installed on the box top to improve the seal. The relative humidity
environment was now closer to 95% after improving the set-up. However, no decay in the
corrosion rate was observed after five months of exposure. The specimens were then
placed in fully or partially buried condition as explained in section 3.4.4
3.4.3 Fiber glass chamber (High Humidity)
A fiberglass tank was set-up as a high humidity chamber as shown in Figure 3.10. The
chamber bottom was laid with PVC cylinders of 2inches height and plastic grating placed
on top of the PVC cylinders. The bottom of the chamber was filled with water to 1 inch
height. The chamber was covered with half an inch thick Plexiglas sheets. Weather tight
strips were used to obtain the weather and air tightness of chamber, so that inside the
chamber the environment was 95-98% RH at room temperature (~21 oC)
41
Figure 3.10 Fiber glass 95-98% RH Chamber
The 95-98% RH fiber glass chamber was also used to monitor the propagation stage.
Selected specimens in which corrosion had initiated were place in this chamber. On each
specimen, the reinforcing steel, a reference electrode and a counter electrode were wired
to the outside of the chamber. This allows performing: linear polarization, impedance and
the half-cell potentials measurements (once a week). Two type F specimens (F10, F15)
and ten type C specimens (C15, C18, C22, C16, C24, C23, C17, C26, C21, & C25) were
exposed in this high humidity.
3.4.4 Specimens covered with saturated sand
The sand was acquired from a local quarry. The sand had been stored outdoors in a semi-
sealed container. It was decided to wash the sand by rinsing it with tap water thoroughly
(i.e. several rinses) to reduce the chloride concentration. Sand was surface saturated with
the tap water before it was used in the following experimental arrangements.
42
3.4.4.1 Horizontal (fully covered)
Horizontal samples that were subjected to High Humidity in HDPE were transferred to
fully cover with saturated sand exposure. A HDPE container of dimension 36x36x24 was
used for this set-up; this container was used for HDPE horizontal high humidity. The
container was emptied and the bottom two inches filled with the saturated sand. The
samples were then transferred into the HDPE container. The sand was spread evenly and
wooden wedges were placed to support the curvature of the specimen. The space around
the samples (C1, C3, C11, F5, F1) and below the curved profile was also filled with the
sand leaving the electrical connection area as shown in Figure 3.11. Sand covered the top
of the specimens except for the reservoir area. During the early measurements it was
observed the sand on the surface dried up. The sand was sprayed with the tap water to
keep the moisture. C8 and F4 samples were cover by sand in individually prepared
container for each specimen of dimension 26x15x7.
43
Figure 3.11 In saturated soil inside HDPE Box
Measurements were taken by momentarily opening the lid of the container. Every week
potential measurements were carried out. Initially the linear polarization measurements
were carried out sporadically. Once the reinforcement potential of the specimen went
below -500mVsce the polarization and solution resistance measurements were carried out
every alternate week.
3.4.4.2 Vertical (fully covered)
HDPE container of 36x36x18inches of dimension was used to accommodate nine
samples (C13, C21, C16, C25, C24, F8, F9, F11, & F13) of vertical orientation. The
bottom of the container was first filled with two inches of sand. The samples were then
placed in two rows on the evenly spread sand. The spaces around the samples were filled
with the saturated sand till upper edge of the reservoir on each sample as shown in Figure
3.12. Most of the upper parts of the samples were covered with saturated sand, leaving
44
not covered only the area above the reservoirs. Early during the exposure period, it was
observed that the sand on top was dry. Water was added to the drier sand by gently
pouring additional water from a small beaker. Latter the addition of water was stopped,
since the sand remained saturated.
Figure 3.12 Specimens fully covered in saturated soil inside HDPE Box
Every week potential measurements were carried out. The linear polarization
measurements were carried-out twice a month. Once the reinforcement potential of the
specimen went below -500mV the polarization and solution resistance measurements
were carried out every alternate week. Cathodic polarization scans were carried out on
selected specimens at 0.25mV rate.
45
3.4.4.3 Buried in sand 1/3 vertical
A semi-transparent plastic container of 15x24x12 inches size was used for this set-up.
The container was cleaned and rinsed with tap water. The container bottom was filled
evenly with saturated sand to a height of one inch. Three vertical orientation specimens
of type C (C22, C23, and C16) were chosen from those already exposed in the fiber glass
chamber. The container was placed in the fiber glass high humidity chamber. The
specimens were then moved into the plastic container (see Figure 3.13). The samples
were covered with the sand up to one third of its height.
Figure 3.13 Lower 1/3rd of length of each specimen is covered by saturated soil
Periodically the potentials, linear polarization and solution resistance were carried out by
momentarily opening the cover of fiberglass chamber.
46
3.4.5 Immersed in water
In the experimental arrangements described below specimens were immersed in water.
3.4.5.1 Horizontal
The electrical connection post would be immersed in water due to its location on
specimens to be immersed in water. An isolation chamber was installed around the
electrical post connecting the reinforcement on each selected specimen to prevent direct
contact with water. So that monitoring measurements could be performed. The isolation
chamber is a reinforced fiber glass trapezoidal shape enclosure (or triangular shape). The
bottom of the isolation chamber was sealed by an acrylic plate using a UV resistance
epoxy marine sealant. The top was left open for access during measurements. A hole of
8mm was made on the flush side of the isolation chamber. The electrical post on the rebar
was inserted in the 8mm hole of isolation chamber. The flush side of the isolation
chamber was then glued to the specimen. The glue was allowed to cure for at least 24
hours. The isolation chamber was checked for leaks and if none was present then the
specimen was immersed in water.
47
Figure 3.14 Fully immersed (horizontal) in water Linear polarization and solution resistance measurements were carried out on the selected
horizontal specimens before immersing them in water. After these measurements the
specimens were placed inside a plastic container of 26x15x7 inches size on top of a
plastic mesh. The container was filled with tap water until each specimen was in fully
immersed condition as shown in Figure 3.14. Three such containers were prepared and
each container had two samples one from each type (C0, C5, C8, F12, F21, F22). The
linear polarization and solution resistance measurements were carried out every alternate
week by momentarily removing the container cover.
3.4.5.2 Vertical
Plastic containers of dimensions 26x17x15 inches and 24x12x12inches were placed
inside the high humidity fiber glass chamber. One type C (C18) and two type F (F10,
F15) specimens of vertical orientation were placed inside the larger container. In the
smaller container one specimen of type C (C15) was placed. The containers were filled
48
with water till the top edge of the reservoirs fitted in the specimens as shown in Figure
3.15. Potential, linear polarization and the solution resistance measurements were carried
out every alternate week by momentarily removing the cover of the fiber glass chamber.
Figure 3.15 Fully immersed (vertical) in water
3.5 Preliminary Specimen in High Humidity
Before all the experimental arrangement for the propagation studies were carried
out, one of the type F specimens (F2) was subjected to a high humidity condition. A
plastic container of size 26x15x7 inches was prepared with the weather tight sealing
strips glued on top edge of the box. A plastic mesh was placed inside the container. The
bottom of the container was filled with water for a height of one inch. Specimen F2 was
placed on top of the plastic mesh. The humidity inside the box was increased by spraying
49
the water on the specimen and also on the inside walls of the container. The container
was kept sealed by keeping the lid in place all the time, except for when the
measurements were performed. The linear polarization and solution resistance
measurements were carried out periodically by momentarily removing the container
cover. After every set of measurements, water was sprayed on the walls of the box.
3.6 Electrochemical Measurements
3.6.1 Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS)
The solution resistance Rs was measured via EIS. The magnitude measured at a
frequency of approx. 60 Hz was assumed to be the Rs of the system.
The linear polarization value measured is labeled on the y-axis as “Rp_meas”. The
Rp_meas contains the solution resistance of the system. The linear polarization test was
performed from 15 mV below OCP to OCP at a scan rate of 0.1 mV/sec.
The LPR values shown in the results section are the Rp-meas minus Rs. An area of 1 cm
was assumed during the test, but the actual half surface area of the each specimen of the
both orientations were taken respectively from Table 4.7 to calculate the apparent
corrosion rate.
3.6.2 Cathodic Polarization Scan The cathodic polarization scans were carried out on the selected specimens. These
specimens reached potentials values of -500mV or more negative. The scan started at 10
50
mV above the corrosion potential value and finished 200 mV below the corrosion
potential or at -810 mVsce (if this was reached first). The scan rate used was 0.25mV/sec.
The evolution of the measurements conducted during the above-described experiments
are presented and discussed in the following chapters.
3.6.3 Half-Cell Potentials The half-cell potentials of the reinforcement steel were carried out using a high
impedance voltmeter. The measurements were made periodically during the time to
initiation and about every week during the propagation stage exposures. A saturated
calomel reference electrode (SCE) was connected to the black lead (COM) of the
voltmeter. The red lead of the voltmeter was connected to the electrical post installed on
the steel reinforcement on each specimen. All values will be reported in mV vs. the SCE
reference scale.
51
4 RESULTS
4.1 Concrete Characterization
4.1.1 Concrete cover
The cumulative concrete cover distance (longitudinal and circumferential
directions) on type F and type C specimens segmented grouped by accelerated transport
method for the potentiostatic method (Fp,Cp), galvanic method(Fg,Cg), and migration
method(Fm,Cm), are shown in Figures 4.1, 4.2, and 4.3, respectively. Similar
distributions were observed for same type of specimens.
Figure 4.1 Concrete cover distributions (Group Fg & Cg). L-Longitudinal, C-Circumferential directions
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50 60
Cum
mul
ativ
e (%
)
Concrete Cover (mm)
FpL
FpC
CpL
CpC
52
Figure 4.2 Concrete cover distributions (Group Fg & Cg). L-Longitudinal, C-Circumferential directions
Figure 4.3 Concrete cover distributions (Group Fm & Cm). L-Longitudinal, C-Circumferential directions
Figure 4.4 contains the overall cumulative distribution for all type F and type C
specimens segmented for this study. It was identified that the concrete covers of Type C
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 10 20 30 40 50 60
Cum
mul
ativ
e (%
)
Concrete Cover (mm)
FgL
FgC
CgL
CgC
0%10%20%30%40%50%60%70%80%90%
100%
0 10 20 30 40 50 60
Cum
mul
ativ
e (%
)
Concrete Cover (mm)
FmL
FmC
CmL
CmC
53
pipes in both longitudinal and circumferential direction were generally deeper than those
of Type F D-C-RCP. This was as expected. The concrete cover distance for both type
pipes are summarized in
Table 4.1. The standard deviation measured on type F D-C-RCP specimens was ~3 mm
whereas for C specimens the standard deviation was 7.5 along the longitudinal direction
and more than 10 mm along the circumferential orientation.
Along the circumferential direction, it was observed that the concrete cover of type C
specimens varied from 19 mm to 50 mm, with an average value of 33 mm. The average
concrete cover thickness reported by Dr. A. A. Sagues (23) for type C (it is believed that
FDOT provided USF with sections of the same D-C-RCP than those used here) was 30.4
mm, which was slightly smaller than the average value observed in this study. The
standard deviation for type C as reported by Dr. A. A. Sagues (2)was 7.2 mm, which was
also slightly smaller value than that was observed in this study.
The average and standard deviation of type F as reported by Dr. Sagues (23) were 16.5
mm and 4.1 mm respectively. The average and the standard deviation value obtained for
type F in this study were 19 mm and 3.1 mm respectively. The values obtained in this
study indicate that the mean cover thickness was slightly larger than those reported by
Dr. Sagues (2).
54
Figure 4.4 Concrete cover distribution summaries L-Longitudinal, C-Circumferential directions
Table 4.1 Mean and Standard Deviation on cumulative Concrete Cover distribution
Pipe Type Items Mean(mm) STD (mm)
F
Longitudinal(FL ) 17.78 2.93
Circumferential(FC
)
19.31 3.2
C
Longitudinal (CL) 31.91 7.549
Circumferential
(CC)
33.065 10.71
0%10%20%30%40%50%60%70%80%90%
100%
0 10 20 30 40 50 60
Cum
mul
ativ
e (%
)
Concrete Cover (mm)
FL
FC
CL
CC
55
4.1.2 Porosity
The porosity of each tested piece from Type F and Type C is listed in Table 4.2. It
illustrates that the average porosity of Type F pipes were generally somewhat lower than
those of Type C pipes by about 1%. The calculated percentage absorption values for
Type F and type C were 3.98% and 4.62%, respectively. These values are within the
standard of D-C-RCP requirement, which is <9%. However, these porosities were
significantly larger than those observed on properly prepared wet cured concrete with low
w/c ratio. The average porosity values reported by Dr. A.A. Sagues from an earlier D-C-
RCP study (D-C-RCP were from different batch of production line that used in this
study) (2) for similar type C was 11.2% and for type F was 13.5%. Thus, the porosity
values measured for this study on Type F were significantly lower than the values
reported by Dr. A. A. Sagues, (2) by approx. 4.5%. This difference in porosity on type F
D-C-RCP could be due to the pipes being from different batches (and/or producers).
Moreover, type F D-RCPs tested here were placed in an open yard for 3 years before they
were transported to the FAU laboratory, it is possible that the pozzolanic reaction of FA
and further hydration of the concrete mix took place causing the lower porosity. It is
possible that a combination of these two factors explain the difference in measured
porosity.
56
Table 4.2 Porosity of Concrete
Pipe Type Specimen Porosity
(%)
Average Porosity
(%)
F
Fs1 9.41
9.135
Fs2 8.90
Fs3 8.96
Fs4 9.16
Fs5 9.24
C
Cs1 9.12
10.385
Cs2 9.19
Cs3 9.63
Cs4 12.21
Cs5 11.78
4.1.3 Resistivity
Figure 4.5 shows the resistivity measured with time for type F specimens. Figure 4.6
shows the measured resistivity with time for type C specimens. It is clear that the
resistivity of these specimens show almost no change. The cores absorbed water from the
wet drilling and also during the first two days of exposure. Unfortunately, no
57
measurements were made before immersing the cores in solution. The average resistivity
for Type F specimens is ~22 KOhm-cm and the average resistivity for Type C specimens
is ~12 KOhm-cm. The resistivity value reported by Mr. Echevarria (24) measured on
specimens made of wet cured concrete with 20% fly ash and a w/c ratio of 0.37 at 18
months of age (wet cure method) was 40.32kΩ -cm, which is significantly larger than the
value measured on Type F cylinder. The resistivity value measured on three years old
concrete cylinders with 20% fly ash with a w/c of 0.41 reported by Dr. Liu (22) was 27.2
kΩ-cm, this value is still larger than that measured here for Type F. For concrete similar
to type C, i.e. with only Portland cement as the cementitious material and with water to
cement ratio (0.37), t six month old concrete specimens, the resistivity value reported by
Dr. Liu was 8.97 kΩcm, which is lower than the resistivity value observed in this study
for type C.
58
Figure 4.5 Resistivity plot for type F specimens
Figure 4.6 Resistivity plot for type C specimens
4.1.4 Diffusivity from RMT
Figure 4.7 shows the chloride migration coefficient and resistivity measured on saturated
concrete specimens. It was observed that the chloride migration coefficient (Dnss) for
0
5
10
15
20
25
30
2 4 6 8 10 12 14
Appa
rent
resi
stiv
ity (K
Ω-c
m)
Time (days)
F1
F2
F3
F4
F5
F6
0
5
10
15
20
25
30
2 4 6 8 10 12 14
Appa
rent
resi
stiv
ity (K
Ω-c
m)
Time (days)
C1
C2
C3
C4
C5
C6
C7
C8
C9
59
each of the Type F (with Fly Ash) specimens were generally less than that of the Type C
specimens, while the resistivity of the former was higher than that of the latter. The use of
fly ash and its corresponding pozzolanic reaction on the type F D-C-RCPs likely reduced
the pore size and produced a more tortuous path, which in turn, resulted in a somewhat
improved resistance to chloride penetration. Additionally, it can be observed from Figure
4.7 that, for Type C cores, the Dnss (~6x10-12 m2/s) of inner concrete (i.e. those cores in
which the inside curvature was exposed to chloride solution) was lower than the Dnss
(10x10-12 m2/s) measured on outer concrete cores (the outside curvature directly exposed
to chloride solution). This would agree with the visual observation that the outer portion
was more porous, and might be due to the manufacturing process that allows for a more
compact concrete on the inside curvature side of the pipe. The Dnss values measured on
Type F cores were generally similar independent of which surface (inner or outer) was
directly in contact with the chlorides. For a concrete similar to type F, Dr. Presuel
reported an apparent diffusivity coefficient value of 1.48 x10-12 m2/s. This Dnss was
measured on one year old concrete specimens with 20% FA, w/c of 0.37 and wet cured.
Overall, the average Dnss values measured in this study were almost three times than the
value reported by Presuel. For wet cured concrete with only Portland cement (0.37 w/c).
Dr. Liu (22)reported a value of Dnsss of 17.49 x10-12 m2/s on concrete approx. one year
old. This value is almost twice the Dnss value observed in this study for type C (dry cure)
outter side. The difference in diffusivity might be due in part to the larger aggregate size
used on the wet-cure specimens compared to the coarse aggregate used on D-C-RCP type
C.
60
Figure 4.7 Chloride migration coefficients vs. resistivity
4.2 Accelerated chloride transport and time to corrosion initiation
Table 4.3 and Table 4.4 present the average ½ surface area of the reinforcement steel
under the reservoir calculated by the formula shown in the appendix. The minimum
concrete cover under the ponding was the smaller concrete measured cover distance for
the reinforcements under the reservoir. These covers were measured from the inner side
of the D-C-RCP. Table 4.3 also shows how many type F specimens were subjected to the
different accelerated chloride transport methods. The specimen ID and the time required
to initiate corrosion.
In general, it took a longer time for the type F specimens with a cover thickness of 2 cm
or larger to initiate corrosion than for F specimens with a cover thickness of less than 2
cm. However, it took comparatively shorter time to initiate corrosion on specimens used
in potentiostatic and galvanostatic method (Fp & Fg) when compared to those tested with
the migration cell method. The reason for the shorter time to initiation could be chloride
0
2
4
6
8
10
12
0 5 10 15 20 25 30
Dnss
X10
-12
m2 /
s
Resistivity (KΩ-cm)
Type C OuterType C Inner
Type F Outer
61
diffusion transport was also contained by gravity (solution preferentially descends
downward within the reservoir) and the higher resistivity on type F specimens which
likely resulted in a larger electric field (even if the same current or potential was applied
between counter and the reinforcement) thus the chloride ions reached CT at the
reinforcement surface under the reservoir in a shorter time.
Table 4.3 Time to Corrosion Initiation for Type F specimens
Dry-Cast Pipe Type
Group (number of specimens)
Specimen ID
Minimum Concrete
Cover under
ponding (cm)
Days to Corrosion Initiation
24 Hours off
Potential vs.SCE (mV)
Average ½ Surface
Area Under
the Ponding
(cm2)
Method
F
Fp(4)
F0 1.75 3 -566
27.8 Potentiostat F1 1.86 7 -570 F2 2.06 3 -575 F3 2.05 4 -520
Fg(4)
F4 1.8 2 -302
23.5 Galvanostat
F5 2.1 10 -280 F6 1.9 9 -270 F7 1.65 7 -372
2nd Fg(9)
F12 2 5 -427 F16 1.5 5 -441 F17 1.4 5 -544 F18 1.7 5 -413 F19 1.9 4 -476 F20 1.6 5 -572 F21 1.4 5 -491 F22 2.2 17 -336 F23 2.3 37 -473
Fm(6)
F8 2 13 -417
22.9 Migration Cell
F9 1.6 44 -289 F10 1.4 72 -295 F11 1.5 13 -280 F13 2.7 78 -368 F15 2.2 27 -414
62
Polarizing the reinforcement anodically, might also have lowered the CT value needed to
initiate corrosion. Table 4.3 also shows the potential value measured one day after
corrosion was declared to have initiated. The potential upon corrosion initiation ranged
from -250 to -575 mVsce.
Similar to what was described for Table 4.3; Table 4.4 shows how many type C
specimens were subjected to the different accelerated chloride transport methods. (The
specimen ID and the time required to corrosion initiation as well as the potential
measured 24hr after declaring a specimen active). The potential upon corrosion initiation
ranged from -190 to -330 mVsce (not including specimens subjected to exposure Cm-
HH).
Table 4.3 and Table 4.4 show that, when specimens that were exposed to laboratory
relative humidity, and migration method, the moisture might have moved laterally
outside the area with solution (due to gravity and moisture differential). The additional
concrete volume with higher moisture content might have interacted in a detrimental
way, i.e., the electric field was longer confined within the two solution reservoirs.
The time required to initiate corrosion for specimens (Cm-HH) (C21, C23, C24, C25, &
C26- specimens exposed to high humidity before and during electric field application)
was shorter than that of the laboratory environment under the different accelerated
chloride transport for type C specimens. The potential upon corrosion initiation ranged
from -380 to -395 mVsce on specimens exposed to Cm-HH accelerated transport method.
These values are significantly more negative than those observed on specimens subjected
to Cm, Cp or Cg methods.
63
On a separate note, Specimen C22 was found to have a potential of -263mV (SCE)
without being subjected to the accelerated method. Specimen C22 was found to have a
hairline crack throughout, which likely allowed chloride ions to reach the reinforcement
upon exposure to the solution.
Table 4.4 Time to Corrosion Initiation for type C specimens
Dry-Cast Pipe Type
Group (number of specimens)
Specimen ID
Minimum Concrete
Cover Under
Ponding (cm)
Days to Corrosion Initiation
24 Hours off
Potential vs.SCE (mV)
Potential at
Apparent Repassiva
tion
Average ½
Surface Area
Under the
Ponding (cm2)
Method
C
Cp(6)
C0 3.8 91 -263
33.2
Potentiostat
C1 3.02 28 -215 -150, Yes C2 2.78 22 -308 C3 2.87 30 -205 C4 3.8 77 -273 C5 4.33 68 -193 -110, Yes
Cg(6)
C6 3.3 105 -250
Galvanostat
C7 2 11 -250 -150, Yes C8 4.3 163 -330 C9 3.5 163 -330
C10 2.5 60 -250 C11 4.2 58 -265
Cm(7)
C12 2.8 89 -188
23.5 Migration Cell
C13 3.1 55 -293 C14 3.3 7 -279 C15 3.3 78 -297 C16 3.9 84 -285 -105, Yes C17 1.6 27 -290 -160, Yes C18 4.2 79 -333
Cm-HH(5)
C21 2.9 7 -384 C23 2.9 7 -394 C24 4.3 7 -379 C25 3.7 7 -364 C26 3.7 7 -392
Crack Not Polarized C22 2.5
64
4.3 Corrosion Propagation Stage: Environmental exposure
Table 4.5 and Table 4.6 shows a detailed description of each specimen’s exposures
sequence (i.e., the time period in each exposure condition) during the corrosion
propagation stage of any given type. In several instances a specimen was exposed to
more than one exposure environment.
Table 4.5 Environmental Exposure during Corrosion Propagation stage for type F specimens
Dry-Cast Pipe Type
Group (number of specimens)
Specimen ID Amount of Time in Days as of 6/6/13 LAB RH High
Humidity Buried in
Sand Immersed in
Water
F
Fp(4)
F0 >400 X X X F1 181 147 230 X F2 71 >500 X X F3 143 220 X X
Fg(4)
F4 239 X 232 X F5 88 147 230 X F6 162 220 X X F7 384 X X X
2nd-Fg(9)
F12 181 X X 7 F16 188 X X X F17 188 X X X F18 188 X X X F19 183 X X X F20 188 X X X F21 129 X X 146 F22 143 X X 7 F23 150 X X X
Fm(6)
F8 134 X 223 X F9 144 X 223 X
F10 32 136 X 141 F11 58 X 223 X F13 134 X 223 X F15 90 136 X 141
65
Table 4.6 Environmental Exposure during Corrosion Propagation stage for type C specimens
4.4 Apparent corrosion rates look-up tables
As described in the experimental section the LPR measurements were conducted
assuming 1 cm2 area, as the actual area of corroding was unknown. The table below
provides a conversion table from Rc values (i.e. Rapp-Rs) to apparent corrosion rate. A
value of 26mV for B, usually assumed for corroding steel in concrete. A value of 52mV
assumed for B for the steel in passive state in concrete. Table 4.7 shows computed
apparent corrosion rates assuming both B values and assuming that half of the reinforcing
steel surface area under the reservoir. The following sections will only show Rc (Rapp-
Dry-Cast Pipe Type
Group (number of specimens)
Specimen ID Amount of Time in Days as of 6/6/13 LAB RH High
Humidity Buried in
Sand Immersed in
Water
C
Cp(6)
C0 377 X X 94 C1 24 147 317 X C2 302 X 232 X C3 28 147 230 X C4 110 X X X C5 328 X X 94
Cg(6)
C6 110 X X X C7 160 220 X X C8 83 X 110 122 C9 8 220 X X
C10 110 X X X C11 49 147 230 X
Cm(7)
C12 X 48 233* X C13 X X 223 X C14 X X X X C15 45 136 X 141 C16 X 182 223 X C17 X 174 X X C18 38 136 X 141
Cm-HH(5)
C21 X 46 223 X C23 X 36 233* X C24 X 46 223 X C25 X 46 223 X C26 X 274 X X
Crack Not Polarized C22 X X 233# X
66
Rs) values, some values will be discussed with respect to the apparent corrosion current
density. For Rc values not discussed below the reader can come back here and find the
conversion values for the different specimen geometries.
Table 4.7 Apparent corrosion rate based on Rc (i.e., Rapp – Rs)
4.5 Typical plots of potential vs. time and plots of Rs, Rpapp Rs vs. time
Figure 4.8 presents the potential evolution with time after corrosion initiation for
reinforcement embedded in specimen F2. Specimen F2 was kept in the laboratory
environment after corrosion initiated for about 7 days and then the specimen was
transferred to the high humidity exposure. The potential measured after corrosion
mV/cm2
kΩ cm2 cm2 cm2 cm2 cm2 cm2area1(H) area2(V) area3(H) area1(H) area2(V) area3(H)
33 27 23 33 27 2350 0.02 0.02 0.02 0.03 0.04 0.0530 0.03 0.03 0.04 0.05 0.06 0.0810 0.08 0.10 0.11 0.16 0.19 0.237 0.11 0.14 0.16 0.23 0.28 0.325 0.16 0.19 0.23 0.32 0.39 0.453 0.26 0.32 0.38 0.53 0.64 0.751 0.79 0.96 1.13 1.58 1.93 2.26
0.7 1.13 1.38 1.61 2.25 2.75 3.230.5 1.58 1.93 2.26 3.15 3.85 4.520.3 2.63 3.21 3.77 5.25 6.42 7.540.1 7.88 9.63 11.30 15.76 19.26 22.61
0.07 11.26 13.76 16.15 22.51 27.51 32.300.05 15.76 19.26 22.61 31.52 38.52 45.220.03 26.26 32.10 37.68 52.53 64.20 75.360.02 39.39 48.15 56.52 78.79 96.30 113.040.01 78.79 96.30 113.04 157.58 192.59 226.09
Rp - Rs
B 26 B 52Apparent corrosion rate in µA/cm2
67
initiated was -570 mV. The potential of the reinforcement remained around this value
during the laboratory exposure period and also during the prolonged high humidity
exposure of 490 days.
Figure 4.9 presents the Rs, Rpapp-Rs evolution vs. time once corrosion initiated on the
reinforcement embedded in F2 specimen. The Rpapp - Rs value remained almost the
same throughout the exposure periods, and the magnitude of (Rpapp-Rs) was very small
which likely suggests a high corrosion rate. Table 4.7 states that the corresponding
corrosion rate for the average value of 0.02 KΩ ( Rpapp-Rs) was 56.52 µA/cm2. This
extremely high corrosion rate, additional monitoring is planned.
Figure 4.8 F2 potential vs. time
68
Figure 4.9 F2 Rs, Rpapp Rs vs. time Figure 4.10 presents the reinforcement potential evolution measured for reinforcement
embedded in specimen C23. This specimen was exposed to a high humidity condition,
while the accelerated chloride transportation was implemented i.e., Cm-HH. After
corrosion had initiated, the specimen remained under the same environment for 25 days,
then it was transferred to an exposure in which the specimen was covered by sand up to
1/3rd of the specimen’s height. The specimen remained in the (fiber glass chamber) high
humidity environment. The potential one day after corrosion initiation was -394 mV. The
potential measured during the high humidity exposure remained close to the value
measured after the corrosion initiated. While the specimen was partially sand covered, the
reinforcement potential gradually drifted towards more negative values and by day 269
the potential of the reinforcement had reached a value of -453 mV. Figure 4.11 presents
the trends for Rs, and Rpapp Rs obtained on specimen C23 vs. time. The corrosion rate
is likely high as the average Rpapp Rs value was 0.15 KΩ while partially covered with
69
sand. The corresponding apparent corrosion rate for the average Rpapp-Rs of the
reinforce steel in this specimen (Table 4.7) was 6.2 µA/cm2.
Figure 4.10 C23 potential vs. time
Figure 4.11 C23 Rs, Rpapp Rs vs. time
70
Figure 4.12 presents the potential evolution for the reinforcement in specimen F1. The
F1Specimen was subjected to the potentiostatic method of accelerated chloride transport.
After 7 days it was declared that corrosion had initiated, and the reinforcement potential
upon corrosion initiation was -570 mV. After corrosion had initiated, this specimen was
exposed to the laboratory environment for 178 days, then transferred to the high humidity
environment in an HDPE box for 147 days and finally moved to an exposure in which the
specimen was covered with saturated sand until the present date. The potential of the
reinforcement gradually increased during the exposure to laboratory conditions from -570
mV to -519 mV by day 178. However, during the high humidity exposure, the potential
of the reinforcement decreased from -519 mV to -562 mV by day 328. While the
specimen was fully covered in a saturated sand condition, the reinforcement potentials
decreased to a lower value (-635 mV) by day 482. Figure 4.13 shows the measured
values of the Rpapp-Rs during all the exposure conditions. The Rpapp-Rs value remained
at a low value during the different exposure conditions, likely because of the steel
experiencing a high corrosion rate. While cover by saturated sand, the average Rpapp-Rs
value was 0.045 KΩ and the corresponding apparent corrosion rate from the Table 4.7
was 39.35 µA/cm2.
71
Figure 4.12 Plots of F1 potential vs. time
Figure 4.13 Plots of F1 Rs, Rpapp Rs vs. time
Figure 4.14 presents the potential evolution measured during three different exposures for
the reinforcing steel of specimen C3. The potetiostatic method was used to accelerate the
chloride transport in specimen C3. Corrosion initiation was declared after 30 days, when
the reinforcement potential had reached a value of -205mV. The specimen was exposed
to the laboratory environment for 29 days and then transferred to high humidity exposure
72
in the HDPE box for 147 days; the specimen was then covered with saturated sand until
the present date. The potential of the reinforcement during the laboratory exposure
quickly shifted to a more negative value of -342 mV and remained close to this value
during the lab humidity exposure. Once the specimen was transferred to the high
humidity exposure, the potential of the reinforcement decreased from -342 mV to -402
mV by day 176. Then, the specimen was transferred to an exposure that fully covered it
in the saturated sand condition. The potential of the reinforcements showed a gradual and
moderate decrease in value to -426 mV by day 298 of exposure. After day 320, the
reinforcement potential gradually increased to -320 mV by day 406 of exposure.
Figure 4.14 Plots of C3 potential vs. time
Figure 4.15 shows that the Rpapp-Rs value decreased from 1.25 kΩ to 0.97 kΩ by day 33
under lab RH exposure. The Rpapp-Rs value decreased further to 0.41 kΩ by day 176,
while exposed to high humidity. The specimen was then exposed to cover with saturated
73
sand; the Rpapp-Rs value remained almost unchanged over a long period under this
exposure. The average Rpapp-Rs observed while covered with saturated sand was 0.8
kΩ, and the corresponding apparent corrosion rate was 0.98 µA/cm2. However, a
moderate increase in Rpapp-Rs was observed by day 406 to an Rpapp-Rs value of 1.28
kΩ, additional monitoring would be required to confirm this as a trend or an oscillation.
Figure 4.15 Plots of C3 Rs, Rpapp Rs vs. time
Figure 4.16 presents the potential evolution measured during three different exposures for
the reinforcing steel of specimen F10. The accelerated chloride transport was achieved on
F10 using the migration cell method. After 72 days, the reinforcement in specimen F10
had initiated corrosion. Upon corrosion initiation, the reinforcement potential was -295
mV. This specimen was then exposed to the laboratory environment for 32 days and then
transferred to a high humidity environment inside the fiberglass chamber for 136 days; it
was then immersed in water in the fiberglass chamber for over 100 days. It was observed
74
that the potentials of the reinforcement, during the laboratory exposure condition
gradually increased from -435 mV to -345 mV by day 32. While in the high humidity
exposure, the reinforcement potential increased from -345 mV to -309 mV by day 168.
In the immersed water condition, the reinforcement potential decreased to -534 mV by
day 222 and remained stable around this value for 30 days, this was followed by a modest
increase in potential to a value of -493 mV by day 309.
Figure 4.16 Plots of F10 potential vs. time
Figure 4.17 presents a trend of Rs (solution resistance) and Rpapp (apparent anodic
polarization resistance) - Rs in the F10 specimen against time in days. It shows that the
Rpapp Rs measured value (while exposed in high humidity conditions) decreased from
0.435 kΩ to 0.345 kΩ by day 168. The specimen was then immersed in water and the
Rpapp - Rs value slowly decreased to a value of 0.099 kΩ by day 236, followed by a
gradual increase to 0.174 kΩ by day 309. The average Rpapp Rs value while immersed
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in water was 0.18 kΩ and the corresponding apparent corrosion rate was 6.25 µA/cm2
from the look-up Table 4.7.
Figure 4.17 Plots of F10 Rs, Rpapp Rs vs. time
Figure 4.18 presents the potential evolution measured during three different exposures for
the reinforcing steel of specimen C15. C15 was subjected to the migration cell method
for accelerated chloride transportation. The specimen started corroding after 78 days and
the corresponding potential upon corrosion initiation was -297 mV. After corrosion
initiated, the specimen was exposed to the laboratory condition for 45 days and then it
was exposed to the high humidity condition in the fiber glass chamber for another 136
days. Specimen C15 was then immersed in water inside the fiber glass chamber for more
than 100 days. The potential of the reinforcement showed almost no change during the
laboratory and high humidity exposures. While immersed in water, the reinforcement
potentials decreased at fast rate to a value of -312 mV by day 253 and followed by a
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gradual increase to a value -261 mV by day 309 of exposure. Figure 4.19 presents the
trend for Rs, and Rpapp - Rs measured on the reinforcement of C15 specimen against
time in days. Figure 4.19 shows that the Rpapp - Rs value decreased from 0.574 KΩ to
0.387 kΩ by day 168. While being immersed in water, the Rpapp Rs value oscillated
initially and reached a lower value of 0.176 kΩ by day 309. Additional monitoring of
Rpapp – Rs value is required to determine the corresponding apparent corrosion rate.
Figure 4.18 Plots of C15 potential vs. time
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Figure 4.19 Plots of C15 Rs, Rpapp Rs vs. time
Figure 4.20 presents the potential evolution measured during three different exposures for
the reinforcing steel of specimen F21 specimen. Specimen F21 was subjected to the
galvanostatic method for accelerated chloride transportation. The reinforcement potential
upon corrosion initiation was -491mV. After corrosion had initiated, the specimen was
exposed to laboratory conditions for 120 days and then immersed in water for more than
100 days. During the laboratory exposure, the potential values showed a moderate
increase to -461 mV but while immersed in water, the potential values decreased to a
more negative value of -633 mV by day 168, and then was followed by a gradual increase
to a value of-546 mV by day 266 of exposure.
78
Figure 4.20 Plots of F21 potential vs. time
Figure 4.21 represents the Rpapp - Rs value remains almost unchanged, while exposed to
laboratory atmosphere. While immersed in water, the Rpapp – Rs value initially
decreased from 0.493 kΩ to 0.245 kΩ by day 127 and gradually reached a lower value of
0.177 kΩ by day 168, then the Rpapp - Rs value showed an increasing trend and reached
a value of 0.2467 kΩ by day 266. While immersed in water the average Rpapp - Rs value
was 0.2 kΩ and the corresponding apparent corrosion rate from Table 4.7 was 5.65
µA/cm2.
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Figure 4.21 Plots of F21 Rs, Rpapp Rs vs. time
Figure 4.22 presents the potential evolution measured during three different exposures for
the reinforcing steel of specimen C8. C8 was subjected to the galvanostatic method for
accelerated chloride transportation. The specimen started corroding after 163 days, and
the corresponding potential was -330 mV. After corrosion had initiated, the specimen was
exposed to laboratory conditions for 83 days and then it was placed in the fully buried
sand condition for 110 days. C8 was then immersed in water for more than 60 days. The
potential was increasing gradually to -240 mV by day 83 while in laboratory conditions,
when exposed to the fully covered sand environment, the potential decreased gradually
and reached a similar value (-338 mV) to the corrosion initiation value by day 193. While
fully immersed in water, the potential values decreased to -538 mV by day 239 of
exposure and showed a gradual increasing trend to -481 mV by day 315.
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Figure 4.22 Plots of C8 potential vs. time
Figure 4.23 presents the trend of the Rs (solution resistance) and Rpapp (apparent anodic
polarization resistance) - Rs in the C8 specimen against time in days. It shows the Rpapp
- Rs value increased from1.2 kΩ to 3.063 kΩ by day 83 of laboratory exposure. While
covered with saturated sand exposure, the Rpapp - Rs value decreased from 3.063 kΩ to
0.762 kΩ by day 99. When immersed in water, it decreased to a lower value from 0.762
kΩ to 0.4593 kΩ gradually by day 315. The average Rpapp - Rs value, while immersed
in water condition was 0.4393 kΩ as stated in the Table 4.7 the apparent corrosion rate
was 2.4 µA/cm2.
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Figure 4.23 Plots of C8 Rs, Rpapp Rs vs. time
Figure 4.24 presents the half-cell potential trend for the F9 specimen. F9 was subjected to
the migration cell method for accelerated chloride transportation. The specimen was
declared corroding after 44 days, and had a corresponding potential of -289 mV. After
corrosion initiated, the specimen was exposed to laboratory conditions for 58 days, and
then buried in sand for more than 223 days. During laboratory exposure, the potential
values showed very little change, but when covered in saturated sand, the potential values
initially decreased at a faster rate from -360 mV to -555 mV by day 115 and then
gradually reached a lower value -592 mV by day 163. The last few measurements showed
a slight increase in the recorded potential.
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Figure 4.24 Plots of F9 potential vs. time
Figure 4.25 presents the trend for the Rs (solution resistance) and Rpapp Rs in the F9
specimen against time in days. While covered in saturated sand condition; the Rpapp
Rs value decreased drastically from 1.18 kΩ to 0.277 kΩ by day 58, then showed a
moderate increase in value to 0.419 kΩ by 98 days, and then gradually reached a lower
value to 0.248 kΩ at a slow rate. The corresponding apparent corrosion rate with respect
to the average value of 0.2 kΩ (during last 100 days of saturated sand exposure) from
Table 4.7 was approximately 1.26 µA/cm2.
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Figure 4.25 Plots of F9 Rs, Rpapp Rs vs. time Figure 4.26 presents the trend for the potential evolution of the reinforcement in
specimen C24. C24 was subjected to the migration cell method for accelerated chloride
transportation and was exposed to high humidity inside the fiber glass chamber. The
specimen was declared corroding after 7 days, with a corresponding potential of -379 mV.
After corrosion had initiated, the specimen was exposed to high humidity conditions for
136 days, and then covered with saturated sand (up to the level of the reservoir) for more
than 190 days. During the high humidity exposure, the potential values showed a
moderate increase (-336 mV), while buried in saturated sand, the potential values
gradually decreased from -336 mV to -435 mV by day 110 and stabilized at that value for
next 30 days, then decreased drastically to a lower value of -685 mV by day 157 and then
finally, showed a gradual increasing trend to -553 mV by day 249.
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Figure 4.26 Plots of C24 potential vs. time
Figure 4.27 presents the trend of the Rs (solution resistance) and Rpapp (apparent anodic
polarization resistance) - Rs in the C24 specimen against time in days. The Rpapp - Rs
value decreased shortly after placing them in saturated sand. The Rpapp - Rs value first
gradually dropped from 0.38 kΩ to 0.2 kΩ by day 89 and then a second drop occurred
between day 120 and 170 at a slow rate while being fully covered by saturated sand.
After day 170, the Rpapp - Rs remained relative stable at this low value. This low value
suggests that corrosion continued to take place at high corrosion rate even if O2
limitations were present, possibly due to the cathode to anode area ratio. This could be
seen from Table 4.7 where corresponding apparent corrosion rate had increased from the
5.56µA/cm2 (0.18 kΩ) to 18.84µA/cm2 (0.06 kΩ).
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Figure 4.27 Plots of C24 Rs, Rpapp Rs vs. time
Cathodic Polarization
The following two plots presents cathodic potential scans performed on the selected
specimens during the propagation stage exposure.The horizontal axis shows the current
density in log scale corrected for the assumed surface area. The vertical axis represents
the potentials in volts measured with respect to SCE (saturated calomel electrode). The
legend in the plots shows the date when the scans were performed.
The Figure 4.28 shows three potetiostatic scans performed on three different days on
specimen F13 while exposed to saturated sand environment (corrosion propagation
experiments). The right most curve was the earliest cathodic polarization scan performed
on F13 specimen, the open circuit potential of the steel was -550 mV and the current
density measured at -800 m Vsce was 7.35xE-6 A/cm2. During the second potentiostatic
scan (middle cure) its was observed that the half cell potential of the specimen was now -
580 mVsce. There was a small reduction in the current density (6.0E-6 A/cm2) observed
86
at a potential of -800 mV. In the latest scan performed after a long exposure, there was no
change in the open circuit potential, only a small reduction in the current density
measured was observed (i.e. at -800 mV the current density was 5.0E-6 A/cm2). The
three polarization scans suggest that at open circuit potential the cathodic reaction was
under mix control i.e. both activation potentials and mass transfer control. The recorded
current density measured at -600 mV had gradually decreased by an order of magnitude
from January to July. Hence, although the cathodic reaction was not under mass transport
only, it significantly influences the cathodic output per unit area. It was possible that the
area where the cathodic reaction took place increased with time, partially due to larger
throwing power and also as corrosion progresses it also increases the surface area. This
change was not reflectled in the curves shown below, and thus the corresponding current
densities for latter scans might be smaller than that reported in here.
Figure 4.28 Cathodic potentiodynamic scans performed on F13
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In the Figure 4.29 presents the two potentiodynamic scans performed on specimen C18
after immersion in water. The scan performed in January suggest that the open circuit
potenial was-440 mV and the current density was 5.7E-6 A/cm2for -700 mVsce. From the
cathodic polarization scan performed after a long exposure, it can be observed that the
open circuit potential decreased to -520 mV. At a potential of -800 mV the current
density was 8E-6 A/cm2. This current density was an indication of the limiting current
density, as indicated above it could be possible that the area correction was off and the
limiting current density could be lowered. When comparing the current density from both
curves at potential of -660 mV, there was a modest reduction (log scale) in the measured
current density to about 1/3rd.
Figure 4.29 Cathodic potentiodynamic scans performed on C18
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5 DISCUSSION
5.1 Discussion for the corrosion propagation stage
The potential and apparent corrosion evolution results, observed during the propagation
stage were influenced by a combination of changing reaction kinetics as corrosion of the
reinforcing steel progressed as the specimens were exposed to the different propagation
environments (i.e., high humidity, saturated sand (partial or fully covered) and immersed
in water (partial or full immersion). The dominant kinetics, both cathodic and anodic,
influences the rates of ongoing corrosion; hence, it could be that one or a combination of
the following scenarios might have occurred over time.
Change in chloride concentration:
Let’s consider a case in which the cathode and anode areas are the same, and that there
was no change in the sizes of the anode area and the cathode area. Figure 5.1 shows two
anodic polarization curves that represent two different chloride concentrations at the
anode for two different specimens. Let us consider that the cathodic polarization curve
farther to the right represents a situation with mix-control, but with higher O2
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concentration. The specimen with the lower chloride concentration at the anode would
have the more positive mix potential and somewhat lower corrosion current density than
another specimen with the higher chloride concentration at the steel surface. When the
specimens were transferred to a high moisture environment, then the cathodic
polarization curve in the middle might better represent the cathodic reaction. For both
specimens the new steel mix potential would be more negative and the corrosion current
density would be lower. Upon further O2 reduction, the curve in the left would represent
full saturation or immersed reinforced concrete. The mix potential would decrease to
even more negative values and the corrosion current density would be even smaller. The
reduction in potential was observed in many cases, upon exposure to high humidity, or
covered in saturated sand and even more negative potential when the specimen was
immersed in water. However, the corrosion current density decay was not observed. The
anode to cathode area ratios on most specimens are likely not the same as it was assumed
above.
If the chloride concentration at the steel surface increases on the same specimen, then the
polarization curve would shift from low chloride anodic curve to high chloride anodic
curve, and the potential would toward more negative potential values, when this occurs
there is little or no change in the corrosion current density.
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Figure 5.1 Schematic representation of the Evans diagram: change in Cl- concentration and oxygen
concentration.
Change in anode area:
Since the specimens during the propagation stage were exposed to high moisture
conditions, the specimens eventually reached full saturation condition. This increase in
moisture content likely reduce the concrete resistivity and hence allowed for a more
favorable macrocell, which eventually might resulted in larger anode and cathode areas.
Figure 5.2 shows an exaggerated increase in the anode area, a four-fold in area would
shift the anodic polarization curve from the solid red line to the dotted red line. The mix
potential value would shift to a more positive potential value; provided that cathode area
remains the same.
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Figure 5.2 Schematic representation of the Evans diagram (change in anode area)
Change in cathode area:
Assuming there is no change in anodic area. Due to the increase in moisture content in
the specimens, the resistivity of the concrete would be lower and the anode would be able
to interrogate a larger remote cathode. As indicated above the moisture content increase
likely shifts the cathodic polarization curve to the left. However, better connectivity
allows for a larger cathode which might compensate and still provide the required anodic
current, resulting in very little change on the mix potential and the corrosion current.
Based on the experimental observations it is likely that the macro cell situation is taking
place. Thus, the high apparent corrosion rates are being observed.
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6 CONCLUSIONS
Corrosion initiation:
o The time to allow the initiation of corrosion varied from a couple of days to several
months. The time range was not just as a function of the applied method, but also the
type of concrete (i.e. resistivity, Dnss, porosity) and concrete cover.
o All three accelerated transport methods were successful in reducing the time to
corrosion initiation.
o On specimens in which Ponentiostatic and Galvanostatic method were applied.
Although specimens from type F contained flyash and showed lower porosity and
Dnss. The higher resistivity and smaller concrete cover might have allowed the
reinforcing steel to initiate corrosion in a shorter period of time than specimens from
type C D-C- RCP.
Corrosion propagation:
o On specimens exposed to 95% R.H. - 98% R.H. did not experience a reduction in
corrosion rate even after a prolonged exposure (500 days). A potential drop during the
exposure to high humidity was observed on specimens that had a potential upon
corrosion initiation from -200 mVsce - to -350 mVsce.
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o There was no decrease in corrosion rate observed on specimens (both type F and C)
that were partially covered in super saturated sand (1/3 of the specimen height) while
store in a high humidity chamber after more than 200 days. A decrease in the
reinforcing steel potential was observed after 150 days of exposure for both types F &
C. The potentials ranged from -530mVsce to -685mVsce. Upon additional exposure
the potentials were found to drift toward slightly more positive values (-580mVsce).
o The specimens which were immersed in water with small cover thickness shows a
significant shift in potential (-538 mVsce to-633 mVsce) at a faster rate. However, no
change (reduction) in corrosion rate has been observed so far.
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APPENDIX
Typical plots of potential vs. time and plots of Rs, Rpapp Rs vs. time for additional specimens
Plots for C11 specimen during Lab – High Humidity – Saturated Sand exposures
Plots of C11 potential vs. time
Plots of C11 Rs, Rpapp Rs vs. time
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Plots for C21 specimen in Lab - saturated sand exposures
Plots of C21 potential vs. time
Plots of C21 Rs, Rpapp Rs vs. time
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Plots for F13 specimen in Lab - saturated sand exposures
Plots of F13 potential vs. time
Plots of F13 Rs, Rpapp Rs vs. time
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Plots for F15 specimen in Lab – High Humidity – Immersed in Water exposures
Plots of F15 potential vs. time
Plots of F15 Rs, Rpapp Rs vs. time
98
Plots for C18 specimen in Lab – High Humidity – Immersed in water exposures
Plots of C18 potential vs. time
Plots of C18 Rs, Rpapp Rs vs. time
99
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