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Electrochemical Corrosion Monitoring of Steel in RC Member
Bonded with Conductive Strengthening Layers
論文
Phanuphan PIBOONSAK*1, Takashi YAMAMOTO*2, Atsushi HATTORI*3 and Toyoaki MIYAGAWA*4
導電性補強材を接着した鉄筋コンクリート中の鉄筋腐食モニタリング
PIBOONSAK Phanuphan*1, 山本 貴士*2，服部 篤史*3，宮川
豊章*4
ABSTRACT: Half-cell potential (HCP) and polarization resistance (PR) measurement were applied to
estimate the corrosion of steel in RC specimen bonded with conductive strengthening layer. Results
obtained using 400 mm long specimen showed HCP readings were quite the same both in tendency and
magnitude on every surface layer, but PR reading are in the same tendency but different in magnitude.
1200mm long RC specimens were employed in order to investigate the HCP readings distribution on
different mix proportions. Results showed HCP readings on CFRP seemed to be uniform in all
specimens at the age of 120 days.
Keywords: Conductive strengthening layer, Steel corrosion, Half-cell potential, Polarization resistance
1. INTRODUCTION
The corrosion of reinforced steel is a major factor
in the deterioration of existing reinforced concrete
(RC) structure. Therefore, steel corrosion monitoring
technique is a necessary field that should be
emphasized in order to know the condition of
reinforced steel and also predict the life of structure.
The half-cell potential (HCP) and polarization
resistance (PR) are commonly used nowadays.
At present, the use of fiber reinforced polymer
(FRP) increases both in the construction work and in
the repair and strengthening work. FRP sheet is
commonly used to increase the ultimate strength
and/or the ductility of concrete members. Then, in
case of structural members wrapped or bonded with
FRP sheet, the problem arises how we will know the
reinforced steel corrosion’s condition inside concrete
member under the FRP sheet. Therefore, FRP sheet
with conductivity may solve the problem.
This research aimed at studying the effect of
conductive epoxy resin used for FRP sheet on the
electrochemical monitoring, half-cell potential and
polarization resistance, by measurement directly on
the surface of the conductive strengthening materials.
2. EXPERIMENT OUTLINE
In this research, the experiment can be divided into
two parts. The first part is the measurement of HCP
and PR on short concrete specimen, while the second
part is done after the first part experimental result
came out by investigating the HCP on long concrete
specimen.
2.1 Short Specimens
(1) Specimens
The concrete specimens, measured 400 x 100 x 50
mm, were cast with placing 300 mm-long D10 steel at
the center of the specimen, as shown in Fig.1. The
experiment factors and concrete mix-proportions are
shown in Table 1 and Table 2, respectively. The
strengthening material was attached on the top surface
of concrete specimen while the bottom surface was
left as non-attaching (concrete) surface. In some
specimens, the 60 x 60 x 2 mm size of plastic plate
was embedded at the center on top surface in order to
make the artificial gap. The artificial gap was
introduced in order to investigate that the gap can be
detectable by the corrosion monitoring or not. Hence,
if the gap can be detectable, the value reading above
the artificial gap may show some difference from
those obtaining on the other. All specimens were kept
in the plastic bag for 28 days for curing.
Electrical lead
wire
Conductive layer
Concrete
SIDE
Steel
NaCl solution
at top of steel
TOP
50
300
25 25
400
50
CROSS SECTION
Fig.1 Short specimen (unit in mm.)
*1
*2
*3
*4
Dept.
Dept.
Dept.
Dept.
of Civil & Earth Resources Eng., Kyoto Univ., DC
of Civil & Earth Resources Eng., Kyoto Univ., Research Associate
of Civil & Earth Resources Eng., Kyoto Univ., Associate Professor
of Civil & Earth Resources Eng., Kyoto Univ., Professor
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Table 3 Measurement methods
Table 1 Experimental factors
W/C
0.45, 0.65
Surface Type Con: No attaching
CER: Coating with conductive epoxy resin
CF: Conductive CFRP sheet
AF: Conductive AFRP sheet
ST: Steel plate
STG: Steel plate with art. gap
CFG:Conductive CFRP sheet with art. gap
2 holes in specimen no.1 and a hole in
Hole
specimen no.2 (see Fig.2)
Table 2 Concrete mix proportions
w/c
W/C
W
C
S
G
NaCl AE WRA
Unit (kg/m3)
Unit (cc/m3)
0.45
170
378
834
916
5
2520 944
0.65
170
262
901
967
5
1744 654
Specimen no.1
1) HCP
2) PR by AC Impedacne method, Guard-ON
3) PR by AC Impedacne method, Guard-OFF
4) PR by Rectangular pulse method
Note:
3 points
both
Note:The
themeasurement
measurementwas
wasdone
doneatat
3 points
bothtop
top
(conductvie
layer
)
and
bottom
(concrete
surface
)
(conductive layer) and bottom (concrete surface)
Table 4 Concrete mix proportions
W
C
S
C oncre te Mixe s
G
NaC l AE W RA
3
3
Unit (kg/m )
Unit (cc/m )
-
188
314
897
889
-
1256 785
-
188
314
897
889
5
1256 785
Cl free concrete
Cl mixed concrete
600mm
600mm
TOP
100mm
Measurement point
Conductive
Layer
50mm
Cl- free concrete Cl- mixed concrete
Steel
SIDE
FRONT
Specimen no.2
Fig.3 Long specimen
-100
0
100 mm
O: 6mm-diameter drilled hole on conductive layer to
concrete surface, X: No hole
Fig.2 Drilled hole of short specimens in series
CER, CF, AF, and ST (For STG and CFG
the central hole was not drilled)
(2) Application of conductive layers
The specimens were prepared for strengthening by
removing the weak surface by scrubbing with sand
paper.
Once the surface was cleaned, the
strengthening layer was applied on the concrete. The
epoxy resin, used in this research, is conductive by
additional mixing with 8% of carbon black powder
and its resistivity is 5x10-3 K Ω .cm. For CER
specimens, 350 g/m2 of conductive epoxy resin was
applied on the concrete surface. For conductive
carbon fiber reinforced polymer (CFRP) sheet and
conductive armid fiber reinforced polymer (AFRP)
sheet, 150 g/m2 and 200 g/m2 of conductive epoxy
resin were applied below and above the strengthening
material, respectively. In case of steel plate, only
150g/m2 of conductive epoxy resin was applied below
the plate. The electrical lead wire was connected to
the conductive layer, because the conductive layer also
was used as the counter electrode in PR measurement
(by double rectangular pulse method). After 1 week of
curing, the holes were drilled on conductive layer, as
demonstrated in Fig.2, in order to provide the direct
contact between reference electrode and concrete
surface.
(3) Exposure condition
In order to accelerate corrosion, 3 kg/m3 of Cl- was
mixed in the concrete at casting. Moreover, the wetdry cycling (3 days-wet / 4 days-dry) at 20˚C was
applied, by submerging the specimens into 3% NaCl
solution. The height of 3% NaCl solution was just at
the top surface of reinforcing steel inside concrete
specimen (3 cm from bottom of specimen), as shown
in Fig.1.
(4) Measurements
Two
electrochemical
corrosion-monitoring
methods, HCP by using silver silver-chloride saturated
reference electrode ((Ag/AgCl)KCl sat.) and PR, were
employed, as written in Table 3. The measurements
were done three points as shown in Fig.2, represented
by O and X. HCP was automatically obtained when
PR was measured by AC impedance method. PR was
measured by 2 methods, which are double rectangular
pulse method and AC impedance method. In case of
AC impedance method, the effect of guard counter
electrode was taken into account in order to
investigate its effect of restricting working area on PR
reading on conductive layers. Then both turning guard
on and turning guard off were used (hereafter, call
guard-on and guard-off, respectively).
The
measurement at hole was done by putting the small
wet cotton into the hole, and placing the reference
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100
0.65 CF/CFG
Con
CF
CF-hole
CF-gap
-100
-200
Rp (K cm 2)
HCP vs SSCE (mV)
0
-300
-400
-500
-600
-700
HCP vs SSCE (mV)
10
0.65
20
30
40
50
60
ST/STG Time (weeks)
70
80
Con
ST
ST-hole
ST-gap
-100
-200
-300
-400
-500
-600
-700
0
0.65 STG
10
20
30
40
50
Time (weeks)
60
70
80
Fig.4 HCP reading
electrode right at the hole’s position. PR by AC
impedance method was obtained on the first day of
dry period while PR by double rectangular pulse
method was obtained on the second day of dry period.
Three points measurement was done on both top and
bottom surface.
2.2 Long Specimens
(1) Specimens
The same of three specimens with 1200 x 100 x 50
mm dimension placing with 1100 mm-long D10 steel
at the center were cast, as shown in Fig.3. The factor
that was considered here is chloride content in
concrete mix-proportion. Then, two types of concrete,
which were Cl- free concrete and 3 kg/m3 Cl- mixed
concrete, were used. Table 4 shows concrete mix
proportion used in this experiment.
(2) Application of conductive layer
Only conductive CFRP sheet was chosen for long
specimen following 2.1 (3).
(3) Exposure condition
5% NaCl solution was sprayed on the Cl- mixed
concrete region from concrete surface for corrosion
acceleration and making the potential different
between Cl- free and Cl- mixed region. The exposure
temperature was room temperature.
(4) Measurement
HCP reading was obtained by using silver silverchloride saturated reference electrode ((Ag/AgCl)KCl
sat.). The measurement was done every 10 cm as
shown in Fig.3.
3. RESULTS AND DISSCUSSIONS
3.1 Short Specimens
(1) Visual Observation
There were no cracks showed up on concrete
surface of any specimens until the age of 78 weeks
Con
CF
CF-hole
CF-gap
0.65 CF/CFG
250
200
150
100
50
0
100 0
20
40
90 of 65 CFG
OFF PR
0.65 ST/STGTime (weeks)
80
70
60
Rp (K cm 2)
100 0
0.65 CFG
0
500
450
400
350
300
60
80
Con
ST
ST-hole
ST-gap
50
40
30
20
10
0
0
10
OFF PR of 65 STG
20
30
40
50
60
70
80
Time (weeks)
Fig.5 PR reading by AC impedance method
(Guard-on)
(about 1.5 year). White crystal of NaCl came out at
age of 1 year on concrete surface. The attached steel
plate got severely corroded since age of 6 months
while the others had the white substance, which might
be the crystal of NaCl, on their layers at age of 1 year.
(2) Moisture Content
Moisture content was measured just before PR
measurement was done by using moisture measure
device, model HI 520, that was set for reading
moisture of concrete material at depth 2.5 cm by autotemperature mode, by just placing it on concrete
surface. Then the results were in a range 5-9 % for
0.45 w/c and larger than 12% for 0.65 w/c.
(3) HCP
The values obtained on conductive strengthening
layers in every condition (hole, no-hole, art. gap) of all
types were close to those obtained on concrete surface,
as demonstrated in Fig.4.
It may be said that the HCP measurement may be
able to be successfully obtained through the
conductive layers. Moreover, the condition that was
done (hole, no-hole, artificial gap) had no effect to the
HCP reading. Then, this lead to the possibility of the
application of conductive epoxy resin into the real RC
structure, and be able to measure HCP without
window opening on the strengthening layer.
From the results of HCP reading, the question
arises what the obtained HCP represents. It represents
the value of local HCP, or the value of HCP average
over the whole specimen’s surface. Therefore, this
reason leaded to the second experiment that
investigate the effect of conductive layer on HCP
reading on 2 region that have different potential in the
same concrete specimen.
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70
25
50
40
2
30
20
15
5
Rp (KΩ.cm 2)
30
40
50
60
ST/STGTime (weeks)
70
Con
ST
ST-hole
ST-Gap
50
40
0
35 0
80
20
30
50
40
Time (weeks)
60
70
80
50
40
Time (weeks)
60
70
80
0.65 ST/STG
Con
ST
ST-hole
ST-gap
Actual
25
30
20
10
30
2
763560
PR of 65 CFG
0.65
20
Corrosion loss (g/cm )
10
0.65 CF/CFG
10
10
0
70 0
Con
CF
CF-hole
CF-gap
Actual
20
Corrosion loss (g/cm )
0.65 CF/CFG
60
Rp (KΩ.cm 2)
Con
CF
CF-hole
CF-gap
15
20
10
10
5
0
0
0
10
7635 PR of 65 STG
20
30
40
50
60
70
80
10
0
30
20
Time (weeks)
Fig.6 PR reading by double rectangular pulse
method
(4) PR by AC impedance method
The results of PR reading by AC impedance
method were quite similar both obtained by guard-on
and guard-off. Therefore, the experimental results of
guard-on are only demonstrated in this paper.
The readings obtained on all types of conductive
strengthening layers (CER, CF, and AF, but only the
result of CF was shown here, because the results of
CER and AF were similar to one of CF.) without hole
had more fluctuation than one with hole in almost
cases, as shown in Fig.5. The magnitudes were larger
than one of concrete surface at the beginning, and then
gradually decreased close to the one of concrete
surface with time. But in case of steel plate, the
reading on steel plate in all conditions were close to
each other. Gap did not lead to obvious difference
from one without gap.
It shows that PR measurement by AC impedance
on high conductivity material (steel plate) give closer
reading than the lower conductive material (CER, CF
and AF). And PR measurement by AC impedance
method cannot detect the gap.
(5) PR by double rectangular pulse method
As shown in Fig.6, PR obtained on conductive
strengthening layers with hole and without hole was
steady and were in the same tendency with PR
obtained on concrete surface. PR of one with hole
was closer to one of concrete than one without hole.
But PR obtained on the steel plate was in the same
tendency and magnitude in all conditions. Here again,
gap did not show obvious difference from the layer
without hole.
In this case, it obviously shows that the PR reading
on conductive layer with hole (CER, CF and AF) is
closer to one of concrete surface than one without
Fig.7 Corrosion loss
hole. Therefore, it may be said that hole has the effect
to PR measurement by double rectangular pulse
method. That is different from PR by AC impedance
method that hole does not play a role on the
measurement. At the present, the reason to explain the
different between the PR readings from those two
measurements is not clear, but one thing that
obviously seen is the difference of counter electrode
used in each measurement. Gap also cannot be
detected by this method. The conductive layer can be
used as the counter electrode for PR measurement by
double rectangular pulse method.
(6) Corrosion loss (Cd)
As shown in Fig.7, corrosion loss was calculated
from the PR reading by AC impedance method by
equation (1) below.
Cd (t ) 
where Cd (t)
m
z
Icorr
F
K
Rp
A
t
m
zF
t
 I corr dt 
0
m
zF
t
K
dt (1)
pA
R
0
Corrosion loss at time t (g/cm2)
Atomic mass of iron (=55.8g)
Valence electron of iron (=2)
Corrosion current density (A/cm2)
Faraday constant (96,500A.s)
Constant (general value = 0.026V)
Polarization resistance (Ω)
Measured area of reinforcing steel
(47.12 cm2)
Time (s)
From the equation, it is straightforward from the
equation that Cd is related to Icorr and Icorr is inversely
proportional to PR. Hence the calculated results of Cd
would be also inversely proportional to the obtained
PR. Therefore, the calculated Cd from PR obtained on
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HCP vs. SSCE (mV)
100
0
-100
-200
-300
-400
100
0
-100
-200
-300
-400
10 20 30 40 50 60 70 80 90 100 110 120
Position (cm)
Fig.8 Experimental Results of 7days
Con
CF
200
100
200 0
HCP vs SSCE (mV)
0
HCP vs . SSCE (mV)
Con
CF
200
HCP vs SSCE (mV)
Con
CF
200
10
20
30
40
50
60
70
80
Con
90 100 110 120
CF
70
80
90 100 110 120
Position (cm)
100
0
-100
-200
-300
0
-400
0
-100
10
20
30
40
50
60
Position (cm)
-200
-300
Fig.11 Experimental Results of 120days
-400
0
10
20
30
40
50
60
70
80
90 100 110 120
Position (cm)
Fig.9 Experimental Results of 30days
Con
CF
HCP vs . SSCE (mV)
200
100
0
-100
-200
-300
-400
0
10 20
30
40 50
60
70 80
90 100 110 120
Position (cm)
Fig.10 Experimental Results of 60days
the layer is lower than one calculated from PR
obtained on concrete surface, as shown in Fig.7.
In case of PR value on the layers, the new
coefficient (K) should be introduced in order to adjust
the obtained PR value to be equal to PR value from
concrete surface.
Moreover, the overestimate of the calculated value
of corrosion weight loss may be come from the effect
of high Cl- content in the concrete sample on the
corrosion measurement as mentioned in previous
studies by many researchers1).
3.2 Long Specimens
The HCP reading between two concrete mix
proportions just did not show the difference at 7days,
but it started to be in the step function from age of 30
days, as demonstrated in Fig.8 to Fig.11.
At 7 days, HCP reading in Cl- free region and in Clmixed region were in the same range both obtained on
concrete surface and conductive CFRP layer, as shown
in Fig.8.
At age 30 and 60 days, the reading on concrete
surface were in the step shape, but the reading on
conductive CFRP layer, some were gradually
decreased from left to right, or some seemed to be in
the horizontal line (Fig.9 and Fig.10).
In case of 120 days, the position of 5%NaCl
spraying for specimen no.2 and 3 was changed to
spray from the side (45 mm of concrete cover) from
age of 60 days for making the same moisture content
of top surface and bottom surface. Therefore, the
reading from concrete surface showed the obviously
different severity between them that was more severe
in specimen no.1 than specimen no.2 and specimen
no.3, as demonstrated in Fig.11. Moreover, the
reading on conductive CFRP layer seemed to be
constant in all specimens.
From the experimental results of HCP reading on
short and long specimen, HCP reading on conductive
layers attached on short specimen was quite equal to
one obtained from concrete surface while the HCP
reading on conductive CFRP layer attached on long
specimen was uniform and its value was in the middle
between the HCP reading of Cl- free concrete and Clmixed concrete (at age of 120 days). It may say that
the HCP reading obtained on the conductive layer
represent the average value of the HCP reading over
the whole concrete specimen.
4. ANALYTICAL INVESTIGATION
(FOR LONG SPECIMEN)
4.1 Analytical model
The commercial program for first order triangle
element in 2D FEA for steady DC conduction analysis,
which bases on Laplace’s equation as shown in
equation (2), was used in the analysis.
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50mm
-50mV
CER Layer
200
600mm
Cl- mixed concrete
-130mV
Con
CER
100
Steel
Fig.12 Analytical section and boundary condition
HCP (mV)
600mm
Cl- free concrete
0
-100
-200
-300
-400
0
Table 5 Boundary conditions.
Outward flow of current is equal to zero on the
surface, Jn = 0
Electric resistivity (ρ): Assume ρx = ρy
ρConcrete 40 and 20 Ω.m for Cl- free and Cl- mixed
concrete, respectively.
ρCER
5x10-5 Ω.m
ρSteel
9.71x10-8 Ω.m
where U
ρx
ρy
20
30
40
50
60
70
80
90
100 110 120
Position (cm)
Steel bar’s potential: The HCP reading on concrete
surface at age of 60 days was used as the steel bar’s
potential in the analysis that are -50 mV in Cl- free
concrete and -130 mV in Cl- mixed concrete.
  1 U    1 U 


0
x   x x  y   y y 
10
(2)
Potential (V)
Electric resistivity in x-direction (Ω.m)
Electric resistivity in y-direction (Ω.m)
Analytical section and boundary condition are
described in Fig.12 and Table 5. The value of HCP
used in the analysis is from the Fig.10, the
experimental results of specimen no.1 at 60 days.
Only the resistivity of CER is an available one.
Therefore, CER was chosen to use in the analysis, and
also its behavior was much closer to conductive CFRP
sheet than steel. The analysis was done to simulate the
HCP distribution of the specimen at age of 60 days.
Therefore, the potential boundary condition was
described as in Table 5 as same as one of the
experimental result in Fig.10.
4.2 Analytical Results
Fig.13 shows the potential distribution of the
analytically longitudinal section. The HCP reading on
the concrete surface is in the steps function that is –50
mV in Cl- free concrete, and then changed to –130 mV
in Cl- mixed concrete at the middle of the section.
While the HCP reading on CER layer seems to be a
straight line in between –50 mV and –130 mV.
Therefore, it shows that the conductive layer has the
effect on HCP reading by giving the average value of
HCP reading of the whole steel, not the local value.
By comparison the experimental results and the
analytical results, the HCP reading obtained on
conductive CFRP sheet at 60 days was quite uniform
Fig.13 Analytical result of HCP
but nobler than one of concrete surface that is different
from the analytical result.
5. CONCLUSION
(1) HCP
From two experimental results and the analytical
results, it can be concluded that HCP reading on the
conductive strengthening layer represent the average
value over the whole surface of concrete specimen.
(2) PR
PR reading by AC impedance method that obtained
on conductive strengthening layer is bigger than one
obtained on concrete surface both in case of the layer
with hole and the layer without hole. While PR
reading by double rectangular pulse method that
obtained on conductive strengthening layer is also
bigger than one obtained on concrete surface, but the
layer with hole give the reading closer to one obtained
on concrete surface than one of the layer without hole.
The artificial gap does not show any obvious effect.
Therefore, the application of conductive
strengthening layer may be used in the real situation to
measure the HCP to some extents. For example,
 In case of constant condition such as moisture and
corrosion,
 For the preliminary classification of the corrosion
state.
In case of PR reading, more experiments need to be
investigated in order to get more information.
REFERENCES:
1) For example, Baweja, D., Roper, H., Sirivivatnanon,
V.: Improved Electrochemical Determinations of
Chloride-Induced Steel Corrosion in Concrete, ACI
Materials Journal, pp 228-238, 2003
2) Piboonsak, P., Yamamoto, T., Hattori, A. and
Miyagawa, T.: Electrochemical Corrosion Monitoring
of Steel in Reinforced Concrete Member bonded with
Conductive Layer, Proceeding of JSCE Conference, V096, pp 189-190, 2004
3) Piboonsak, P., Yamamoto, T., Hattori, A. Miyagawa,
T.: Preliminary Test for Application of Half-Cell
Potential on Long Concrete Specimen Attached with
Conductive CFRP Layer, Proceeding of Kansai
Division of JSCE Conference, V-32, 2005