Characterization of Steel Corrosion Products in Reinforced ...
Active Protection of Fiber-Reinforced Polymer Corrosion--libre
-
Upload
adel-chelba -
Category
Documents
-
view
218 -
download
0
Transcript of Active Protection of Fiber-Reinforced Polymer Corrosion--libre
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
1/11
CORROSION SCIENCE SECTION
CORROSIONVol. 67, No. 2 025002-1
Submitted for publication March 19, 2009; in revised form, June24, 2010.
Corresponding author. E-mail: [email protected]. * Department of Civil Engineering, Indian Institute of Technology
Bombay, Mumbai 400076, India.** Indian Institute of Technology Bombay, Mumbai 400076, India.
Present address: Thapar University, Patiala 147004, India.*** Department of Material Science and Metallurgical Engineering,
Indian Institute of Technology Bombay, Mumbai 400076, India.
Active Protection of Fiber-Reinforced Polymer-Wrapped Reinforced Concrete StructuresAgainst Corrosion
S. Gadve,,* A. Mukherjee,** and S.N. Malhotra***
ABSTRACT
Large numbers of reinforced concrete (RC) structures that have
been damaged from corrosion of steel reinforcements are reha-
bilitated with fiber-reinforced polymer (FRP) composites. This
paper investigates active protection of the steel embedded
in concrete that is treated with surface-bonded carbon FRP.
The electrically conductive carbon fiber is used as an anode
while the reinforcing bar is used as a cathode. Concrete cyl-
inder specimens with embedded steel bars are immersed in
salt water, and anodic current is passed through the reinforce-
ment to initiate cracking in concrete as a result of acceler-
ated corrosion of steel. Carbon FRP sheets have been bonded
adhesively to the cylinders. The adhesive has been modified
to impart electrical conductivity. Specimens were exposed to
a highly corrosive environment for a specified time. Pullout
strength, mass loss, potentiodynamic scans, and the half-cell
potential of steel are reported as metrics of performance of the
samples. The proposed technique has been very effective inretarding the corrosion of steel.
KEY WORDS: active protection, anodic current, corrosion, fiber-
reinforced polymer, pullout strength, reinforced concrete
INTRODUCTION
Reinforced concrete has been developed and applied
extensively for a century. However, there are many
instances of premature failure of reinforced con-crete components from corrosion of reinforcement.
The economic implications of such damage are enor-
mous. Overall, repair and maintenance of reinforced
concrete structures cost 1% to 2% of the yearly rate
of new construction. In a tropical country like India,
where approximately 80% of the annual rainfall takes
place in the two monsoon months, corrosion-related
problems are more alarming. India also has a very
long coastline where marine weather prevails. Typi-
cally, a building in the coastal region requires major
restoration work within 15 years of its construction.
Recent developments in the field of fiber-rein-
forced polymer (FRP) have resulted in highly efficient
construction materials. FRP are being used increas-
ingly to rehabilitate corrosion-affected structures. FRP
sheets are wrapped around beams and columns to
rehabilitate them from the loss of shear capacity and
confinement attributable to the corrosion of links or
stirrups. The efficiency of FRP in the enhancement of
bending,1shear capacities of flexure elements,2and
enhancement of concrete confinement in compres-
sion elements3is well established. An important spin-
off from the FRP treatment of RC structures can be
their resistance to corrosion. There have been vari-
ous attempts of passive protection of steel reinforce-ments with surface-bonded FRP, both in labs and
ISSN 0011-9312 (print), 1938-159X (online)11/000013/$5.00+$0.50/0 2011, NACE International
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
2/11
CORROSION SCIENCE SECTION
025002-2 CORROSIONFEBRUARY 2011
at sites.4FRP are unaffected by electromechanical/
electrochemical deterioration and can resist aggres-
sive corrosive effects of acids, alkalis, salts, and simi-
lar aggregates under a wide range of temperatures.
Therefore, unlike steel reinforcement, they are less
susceptible to environmental degradation. Arguably,
FRP wraps prevent the increase in the volume of rein-
forced concrete (RC) members from rusting by apply-
ing confinement pressure, thereby resisting dislodging
of the concrete cover.
The FRP wraps provide a barrier layer that should
impede further corrosion of the steel. Site applications
have been on structures that have been damaged by
corrosion.5FRP has been applied primarily to com-
pensate for the lost steel and to improve confinement
of the concrete. However, there is near unanimity that
FRP wraps have slowed down the rate of corrosion,
albeit in varying degrees. The experience of the pres-
ent authors on the performance of glass fiber-rein-
forced polymer (GFRP) wraps on RC in coastal Gujarathas been very good. The FRP wraps in the corrosion-
affected areas have not shown any sign of deteriora-
tion in six years.
The general procedure of laboratory experiments
has been to accelerate corrosion in steel embedded in
concrete and then apply the FRP to observe its effects
on corrosion.6-21Corrosion has been accelerated
through the application of impressed potential on the
reinforcements7,10,14,16or by simulating wet and dry
cycles in chloride-rich environments.22The main indi-
cators of performance are mass loss of reinforcement,
pullout strength, electric resistance, half-cell poten-tials, and potential scans. The suitability of perfor-
mance indicators depends on the method of imparting
corrosion. It is noted that the benefit of FRP depends
on several factors, such as adhesive, fiber, method of
application, and environment.
So far, researchers have used FRP as a passive
barrier layer that would impede ingress of moisture
and corrosive chemicals. One class of FRP, namely
carbon FRP (CFRP), is electrically conductive. There-
fore, they may be used in the active protection of RC
structures. The authors are unaware of any previ-
ous investigation on the active protection of the steel
using FRP. This paper investigates the use of FRP
wraps for active protection of steel in concrete. We
briefly introduce active protection.
ACTIVE PROTECTION
The corrosion of steel reinforcing bars is an elec-
trochemical process that requires a flow of electric
current and several chemical reactions. The three
essential components of a galvanic corrosion cell are
anode, cathode, and electrolyte. At the anode, iron is
oxidized to the ferrous state and releases electrons:
Fe e +e ++ 2 (1)
hese electrons migrate to the cathode where they
com ine with water and oxygen to orm hydroxyl ions:
2e O OO
+ +++ O+ O
(2)
he hydroxyl ions com ine with the errous ions to
orm errous hydroxide (Fe[OH]2):
e e++ 2( ) (3)
In the presence of water and oxygen, the Fe(OH)2is
oxidized urther to orm erric oxide (Fe2O3):
2
2 2 2
3 2 2
Fe O HH
2 2
H
2 2
F4Fe
Fe Fe O HH
2
H
2
O
HH )O H
HH
+ +
2 2
+
2 2
H+H
2 2
H
2 2
+
2 2
4H
2 2
O FF
2
2
e ee e
2
e
2
2
2e
2
HHHH
2
H
2
2
2H
2
(4)
Corrosion of steel in concrete in the presence of chlo-
ides, but with no oxygen (at the anode), takes placen several steps. At the anode, iron reacts with chlo-
ide ions to orm an intermediate solu le iron-chloride
complex:
Fe e+ +22 2 2 )Cl 2222+ 22e 2e 2 222 2 2 2 2 2 22e 2 22 2 22 2 2 (5)
When the iron-chloride complex diffuses away from
the ar to an area with higher pH and concentra-
tion o oxygen, it reacts with hydroxyl ions to orm
Fe(OH)2. This complex reacts with water to orm
Fe(OH)2
( Fe Cl
HO
Cl+ ++ + +++222
2 2
Cl 2l + 2+ 2+l +l 2l +l 2 222e F 2F 2O 2O 2H2H2 2 + +2 + 22222O 2O 2 2 2 22 222 222 2O 2O 2O 2O 22 2H2H2 H 2H22 2 2 2 F 2 (6)
he hydrogen ions then combine with electrons to
form hydrogen gas:
eee
e e e (7)
As in the case of corrosion of steel without chlorides,
the Fe(OH)2, in the presence o water and oxygen, is
oxidized urther to orm Fe O3:
2
2 2 2 3
3 2 2
Fe O HH
2 2
H
2 2
F4Fe
Fe Fe O HH
2
H
2
O
HH )O H
HH
+ +
2 2
+
2 2
H+H
2 2
H
2 2
+
2 2
4H
2 2
O FF
2
2
e ee e
2
e
2
2
2e
2
HHHH
2
H
2
2
2H
2
(8)
he principle of active protection lies in connecting
an external anode to the metal to e protected and
the passing o an electric direct current to make all
areas o the metal sur ace cathodic. There ore, the
onization illustrated in Equations (1) and (5) is pre-
vented. The external anode may e a sacri icial gal-
vanic anode, where the current is a result of the
otential difference between the two metals, or it may
e an impressed current anode, where the current is
mpressed rom an external direct current (DC) power
source. In this paper, we investigate the impressedcurrent anode system. In the impressed current
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
3/11
CORROSION SCIENCE SECTION
CORROSIONVol. 67, No. 2 025002-3
system, an inert (zero or low dissolution) anode is
used and an external source of DC power is used to
impress a current from an external anode onto the
cathode (reinforcing steel) surface. The issues on
active protection of RC structures are:
to create an anode around the structure
to maintain adequate electric potential
to avoid sympathetic damage as a result of
overprotection
There have been many alternative anode sys-
tems developed for active protection to RC structures,
such as highway bridge substructures, buildings, and
marine structures.23A variety of anodes, such as tita-
nium wire titanium strip,24titanium mesh,25and zinc
spray,26and conductive systems, such as conductive
concrete mortar27and carbon fiber-reinforced over-
lay,24have been attempted by the researchers. The
driving voltages for protection vary with the type of
anode and environmental conditions. Typical operat-
ing current densities range between 0.2 mA/m2and2.0 mA/m2for the cathodic prevention of new rein-
forced concrete structures and between 2 mA/m2and
20 mA/m2for cathodic prevention of existing salt-
contaminated structures.28A cathodic current density
of 5 mA/m2to 20 mA/m2to the steel reinforcement
reduces its corrosion rate to negligible values.27Over-
voltage or higher current densities for prolonged peri-
ods may lead to damages such as degradation of the
steel-concrete bond, which is associated with soften-
ing of the cement matrix in contact with the metal.29-30
There has been an enhanced risk of expansive alkali
silica reaction in cathodic regions of concrete with sili-ceous aggregates. It has been reported that the risk is
reduced considerably if the cathodic current density is
maintained uniformly and consistently at a level lower
than 20 mA/m2.26Although significant research has
been done on the active protection of RC, the applica-
tion of the technique has remained meager because
of the requirement of special anodes that are often
expensive.
In a previous investigation, the authors have
reported the efficacy of FRP wraps for passive protec-
tion of RC structures.31In addition to passive protec-
tion, the FRP materials that are electrically conductive
can be designed to offer active protection as well. This
has not been reported so far.32With the FRP wraps
acting as anode, no other anode would be necessary.
Therefore, the cost of active protection can be brought
down significantly. Present work investigates active
protection using surface-bonded CFRP sheets as the
anode.
Laboratory samples of RC specimens were pre-
pared. To initiate corrosion, the specimens were
exposed to accelerated corrosion by impressing anodic
current into the reinforcing bar. After a specific period
of exposure, the cracked RC specimens were treated
with surface-bonded FRP. The samples were activelyprotected while subjecting them to a specified envi-
ronment in a salt mist chamber for a specific period.
Results of actively protected samples have been com-
pared with that of unprotected and passively pro-
tected ones.
EXPERIMENTAL PROCEDURES
The experimental program was carried out in the
following steps:
Cylindrical reinforced concrete specimens were
cast.
Initial corrosion was induced into the reinforced
concrete specimens.
Precorroded specimens were wrapped with
CFRP and GFRP sheets.
The wrapped specimens were subjected to fur-
ther corrosion by exposing them to salt mist
while applying active protection.
Corrosion was monitored.
Destructive tests were carried out.
Preparation of Test SpecimensIn the present program, cylindrical specimens
with an embedded steel bar (Figure 1) were used. The
height and diameter of the specimen were 230 mm
and 100 mm, respectively, with an accuracy of 1 mm.
A standard reinforcing bar of 330 mm length and
12 mm nominal diameter of Fe 415 grade was used.
The bar was shot-blasted to Sa 2.5 (ISO 8501:1)33
sur-face and immediately dipped in oil. The white shining
FIGURE 1. Cylindrical-reinforced concrete specimen.
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
4/11
CORROSION SCIENCE SECTION
025002-4 CORROSIONFEBRUARY 2011
surface was maintained in the laboratory until it was
embedded in concrete. Before placing the bar in con-
crete, a 2-mm-diameter groove was drilled on one end
of the bar and a copper stud was fixed in the groove.
The plug was used for electrical connections. Polytet-
rafluoroethylene (PTFE) tape was wound around the
bar at two locationsthe bottom edge and at the
interface with the concrete top face. This served as a
bond breaker, and the embedded length of the bar in
concrete was maintained precisely at 152 mm. The
bar was placed in such a way that the transverse
clear cover was 45 mm and the bottom cover was
51 mm. Before casting of the test specimens, each
reinforcing bar was weighed to a 0.1-gm accuracy.
The protruded part of the steel bar was coated with
liquid epoxy resin for corrosion protection.
Ordinary Portland cement of nominal strength
(43 MPa), fine aggregate (medium-sized natural/
river sand), and crushed stone coarse aggregate with
a maximum size of 20 mm was used in the con-crete. The ratio of cement:sand:coarse aggregate
was 1:2.16:2.44. The water-cement ratio was 0.42and aggregate-cement ratio was 4.6. The resulting
strength of concrete was 40 MPa.
A special molding system (Figure 2) was fabri-
cated for casting the specimens. The system is able to
maintain accurately the position and inclination of the
bar with respect to that of the cylinder.
Inducing CorrosionThe objective of inducing corrosion to the bar is to
simulate corrosion-damaged concrete. The commonly
used methods of inducing corrosion in RC specimens
are salt mist,11,14,18
chloride diffusion,19-21
alternatedrying and wetting in salt water,14,19and impressing
anodic current.10,16
In this investigation, the impressed current tech-
nique has been used. Specimens were kept immersed
in 3.5% sodium chloride (NaCl) solution for 24 h to
ensure full saturation. A stainless steel mesh rolled
into a hollow, open cylinder was used as the cathode
(Figure 3). The cathode and the specimen were placed
in 3.5% NaCl solution. The level of NaCl solution was
3 cm below the top surface of the specimen to allevi-
ate corrosion at the steel-concrete interface. The
DC-regulated power supplier (DCRPS) used in the
present study could supply 500 mA DC at 60 V. The
reinforcing steel bar was connected to the positive
terminal of the external DC source and the negative
terminal was connected to the stainless steel mesh.
The 100-mA direct electrical constant current (CD =
1,740.67 A/cm2) was impressed between the rein-
forcing bar and the stainless steel mesh. It is more
common to maintain a constant voltage between the
cathode and the anode.6-7,19However, in this inves-
tigation, a constant current was preferred because
the goal of this investigation was to examine the
active protection through maintenance of constant
cathodic current. A total of 12 specimens was usedin this experiment, divided into three groups. Con-
FIGURE 2. Specimen casting system.
FIGURE 3. Schematic representation of the device for accelerated
corrosion.
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
5/11
CORROSION SCIENCE SECTION
CORROSIONVol. 67, No. 2 025002-5
stant current was impressed for 2 days, 4 days, and
8 days into four specimens of each group, respec-
tively. Anode-to-cathode voltage, corresponding to a
constant current of 100 mA, was monitored every day
for all specimens. Half-cell potential of the corroding
reinforcement bar was also noted every day with ref-
erence to a standard silver/silver chloride (Ag/AgCl)
electrode. Using this method, the concrete was found
to develop cracks within 2 days. There were brown
stains of rust on the concrete. The crack widths were
measured after 2 days, 4 days, and 8 days. Potentio-
dynamic electrochemical anodic polarization scans
were obtained for all the specimens after initial cor-
rosion. Cracks initiated at the surface of the cylinder
and ran along the direction of the reinforcement on
the sides of the cylinder (Figure 4). Voltage between
the reinforcing steel anode and stainless steel mesh
cathode decreased with time, indicating that the resis-
tance lessened with the progression of the crack.
Wrapping of Precorroded SpecimensTwo fiber materials are popular in the rehabilita-
tion of structures in Indiaglass and carbon. Because
of the electrical conductivity of carbon fiber, only CFRP
has been used for active protection. However, compar-
ison has been made between the active and the pas-
sive protection offered by both glass and carbon fiber
sheets. The fibers are applied in the form of unidirec-
tional sheets. Glass fiber sheets are thicker than the
carbon fiber sheets. In this investigation, two often
used, commercially available, unidirectional CFRP and
GFRP sheets and compatible epoxy adhesive are used.Properties of the sheets are presented in Table 1.
Samples were air-dried prior to the application of
FRP wraps. Manufacturers specification was followed
in the application of the wraps. Out of the four speci-
mens in a group, one was kept unwrapped. One layer
of either CFRP or GFRP sheet was wrapped around
the test specimens with the fiber along the circumfer-
ential direction of the cylinder. The entire length of the
cylinder was covered. A 25-mm overlap was provided
at the ends of the sheets. The remaining specimens
were used for active protection. These CFRP-wrapped
test specimens were provided with additionally adhe-
sively bonded 25-mm to 30-mm wide, vertically ori-
ented carbon sheet (Figure 5) to facilitate uniform
distribution of direct current throughout the speci-
men for effective active protection. The epoxy adhesive
used was modified to be conductive.32
Active ProtectionSince carbon is electrically conductive, an
attempt was made to use this property in apply-
ing active protection to the reinforced concrete sys-
tem without using any external anode. In this case,
the carbon fiber sheets that were wrapped around the
cylindrical reinforced concrete specimen themselves
were used as anodes and the reinforcing steel bar as
FIGURE 4. Specimens after initial exposure.
FIGURE 5. Schematic representation of cathodically protected
specimen.
TABLE 1Properties of Fibers Used in the Experiment
Thickness Tensile Tensile Ultimate Electrical
Material (mm) Strength (GPa) Modulus (GPa) Strain Conductivity (s/cm)
Carbon sheet (net fiber) (CS) 0.13 3.79 230 0.015 551
Glass sheet (net fiber) (GC) 0.35 2.30 76 0.018 Adhesive 15 4.3 0.02
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
6/11
CORROSION SCIENCE SECTION
025002-6 CORROSIONFEBRUARY 2011
cathode. To achieve this, the wrapping system had to
be modified in two ways.
One of the requirements of the system was to
make its electric conductivity uniform. A ribbon of
carbon fibers was stitched through the CFRP sheet in
the perpendicular direction of the fibers. The ribbon
was extended beyond the sheet by about 25 mm. It
was used as the anode terminal for supplying electric-
ity to the CFRP sheet. The ribbon pressed against the
fibers of the sheet and ensured proper contact and
uniform conductivity.32The only non-conductive part
in the system was the epoxy adhesive used to bond
carbon sheets onto the concrete. In the present study,
the epoxy was made conductive by mixing conductive
particulates into the epoxy. The conductive particu-
lates that were used are commercially available graph-
ite powder of particle size in the range from 0.1 to
10 . An experiment was carried out to find the opti-
mum amount of the particulate to be mixed into theepoxy, such that the epoxy became sufficiently con-
ductive without losing the required consistency for
proper coverage of the concrete surface. The workabil-
ity and conductivity were studied by adding 2% to
20% conductive particulates by weight of epoxy.32
An external DC power supply was used to
impress the constant current for active protection. The
positive terminal of the DC power supplier was con-
nected to the protruding ribbon of the carbon sheet
and the negative terminal was connected to the rein-
forcing bar, to be protected from corrosion. A constant
current of 50 mA (current density = 870 A/cm2) was
impressed between the carbon fiber, cathode and rein-
forced steel, anode. To simulate the practical condi-
tion of applying active protection to the reinforced
concrete structures in a corrosive environment, the
specimens were kept in a salt mist chamber, with all
necessary electrical connections for active protection,
for 60 days (Figure 6).
Exposure of Wrapped SpecimensTo simulate corrosion-damaged structures, prior
to the application of the wrap, an initial exposure
was applied. In practice, the FRP wraps are applied
on structures that are corroded to varying degrees.
Therefore, different exposure durations were cho-
sen prior to the application of the wrap. Three expo-
sure durations2, 4, and 8 dayswere applied. The
details of the samples are presented in Table 2. In two
days, the first crack appeared in all the samples. In
4 days and 8 days, the crack became wider and cor-
rosion products oozed out in larger volumes. The con-
trol samples were not wrapped. GPP and CPP indicatepassive protection with glass and carbon FRP wraps,
respectively. CAP indicates active protection that is
active protection applied to the RC specimens using
carbon FRP wraps.
All specimens with varying degrees of initial cor-
rosion, both control and treated, then were exposed to
a severe corrosive environment created in a salt mist
chamber. The salt mist chamber was designed accord-
ing to IS 1186434(Figure 6). The salt mist test was
carried out with 5% NaCl solution at 50C. Injection
of salt fog was put on for 8 h and put off for 16 h,
keeping the samples in the chamber. On switching
off, the temperature inside the chamber subsequently
would have come down to ambient temperature
(~27C). The total duration of exposure in the salt
mist chamber under the specified condition was
1,500 h, i.e., 60 days.
Corrosion MonitoringSeveral parameters have been monitored during
the entire process. Some of these are nondestructive,
such as half-cell potential, cell voltages, and potentio-
dynamic scans. These studies were carried out simul-
taneously on all specimens. Half-cell potential was
noted every day. Cell voltages were observed every dayduring induction of initial corrosion by impressing the
FIGURE 6. Passive protection and active protection applied to FRC-
wrapped specimens exposed to salt mist.
TABLE 2Test Specimens
Wrap Anodic Salt
Material Current Mist Protection Nomenclature
Unwrapped 2 60 No Control-24 60 No Control-48 60 No Control-8
Glass 2 60 Passive GPP-24 60 Passive GPP-48 60 Passive GPP-8
Carbon 2 60 Passive CPP-24 60 Passive CPP-48 60 Passive CPP-8
Carbon 2 60 Active CAP-24 60 Active CAP-48 60 Active CAP-8
Exposure in
Days
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
7/11
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
8/11
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
9/11
CORROSION SCIENCE SECTION
CORROSIONVol. 67, No. 2 025002-9
Mass Loss
TABLE 3Pullout Strength and Mass Loss
Percent Pullout
Difference Strength
Specimen Percentage (Equation [9]) (MPa)
Control-2 6.47 8.86Control-4 4.29 5.87Control-8 10.71 5.21GPP-2 2.86 55 12.99GPP-4 2.5 42 13.05GPP-8 8.57 20 11.15CPP-2 6.2 5 13CPP-4 4.6 7 10.32CPP-8 8.27 23 11.2CAP-2 1.65 75 7.81CAP-4 1.38 68 4.15CAP-8 2.87 73 5.63
The lack of corrosion products in actively protected
samples does not lead to the improvement in grip.However, this point should be examined before arriv-
ing at a firm conclusion. The authors are planning
another set of experiments by varying the protection
current density. The microstructure of the bar-con-
crete interface shall also be studied.
The variations of mass loss with pullout strength
for all the specimens are plotted in Figure 10. A lin-
ear fit though the data of different systems has been
carried out. Understandably, pullout strength varies
inversely with the mass loss, i.e., corrosion reduces
bond strength. The rate of loss of pullout strength in
CPP and GPP samples is the lowest. This illustratesthat even with the same level of corrosion, the pas-
sively protected specimens exhibit a better bond with
concrete. As a result of corrosion, the bars exert a
bursting pressure on the concrete. In absence of the
FRP wrap, tensile stresses develop in the concrete
that result in its cracking and spalling. The FRP-
wrapped samples resist the expansion pressure by
developing a hoop stress. As a result, concrete goes
in compression. This results in better grip of concrete
on steel. In the case of actively protected samples,
the corrosion products did not develop. Therefore, the
compressive force in concrete was also absent. The
ribs on the bars, on the other hand, were lost. This
results in the loss of bond strength. For the success
of the active protection technique, it is imperative that
an optimum protection current density is achieved
such that the loss of bond strength is avoided.
Along with the destructive tests that were car-
ried out at the end of the exposure, each sample was
non-destructively monitored. Potentiodynamic scans
were obtained after induction of initial corrosion and
then every 2 weeks during their exposure to salt mist.
The corrosion current (Icorr) was determined. Fig-
ure 11 shows Icorrfor different samples. Understand-
ably, the scatter in data was very large. A linear fit ofthe data was carried out to determine the trends. It
FIGURE 10. Variation of mass loss with pullout strength.
is clear from the figures that the control sample had
higher corrosion currents throughout the exposure
time. The corrosion current increased with exposure.
The trend was similar for all the samples, regardless
of the length of impressed current exposure prior to
salt mist exposure. The protected samples, both pas-
sive and active, exhibited much lower Icorr. This dem-
onstrates that FRP wraps are extremely effective in
protecting the corroding reinforcements in concrete.
Moreover, the current reduced with time. This indi-
cates passivation of steel. Therefore, the efficacy of the
proposed protection systems is established.The initial current was higher in passively pro-
tected systems. However, they passivated at a fast
rate with time. The active system exhibited a lower
Icorrthroughout the period of exposure. It is postulated
that the mechanisms of passivation in the active and
passive systems are different. In the passive systems,
the resistance offered by the FRP wrap to expansive
pressure from corrosion does not allow the corrosion
products to dislodge. As a result, the electrical resis-
tance at the steel-concrete interface goes up. Thus,
the passivation is created. In the active systems the
passivation is from the prevention of ionization of
iron. Therefore, the rate of decay in Icorrin the two sys-
tems was different. The Icorrdecayed faster in the pas-
sive systems.
CONCLUSIONS
v A novel system for the active protection of steel
bars in concrete is reported. The electrical conductiv-
ity of the carbon FRP is utilized in creating an anode
around the steel. Thus, the proposed system elimi-
nates any requirement of an external anode and so
has a very favorable impact on cost.
v The efficacy of the system was established througha set of experiments and performance metrics, both
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
10/11
-
8/10/2019 Active Protection of Fiber-Reinforced Polymer Corrosion--libre
11/11