Efeitos da Realkalização Eletroquímica

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RESEARCH ARTICLE
Long-term effects of electrochemical realkalization on
carbonated concrete
Peng ZHUa,b, Ji ZHANGa, Wenjun QUa*
a College of Civil Engineering, Tongji University, Shanghai 200092, China
b Key Laboratory of Performance Evolution and Control for Engineering Structures, Shanghai 200092, China
*Corresponding author. E-mail: quwenjun.tj@tongji.edu.cn
© Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
ABSTRACT The long-term effects of electrochemical realkalization on carbonated reinforced concrete with a W/C
ratio of 0.65 were studied. Fourteen out of 16 carbonated specimens had been subjected to realkalization seven years ago,
and the alkalinity of the concrete, the electrochemical characters (corrosion current density and potential) of the specimens
and the corrosion conditions of the steel bars were examined. Results of different specimens and also at different time (4,
10, 13 months and 7 years after realkalization) were compared. According to the phenolphthalein and pH meter test, the
alkalinity of the concrete had disappeared after seven years. Based on the potentiodynamic polarization test, various
corrosion conditions had developed on the steel bars, which was verified by visual observation. All bars were in the
depassivated state, and their corrosion current densities increased significantly after seven years. Cracks developed in
some of the specimens, and the diverse compactness of concrete and excessive current of realkalization were considered
to be possible causes. The effects of the realkalization treatment vanished after seven years.
KEYWORDS realkalization, concrete, carbonation, polarization curve, corrosion
1 Introduction
The durability failure of concrete induced by the corrosion
that results from carbonation has created problems world-
wide. To prolong the service life of carbonated concrete
and achieve the sustainable development of concrete, an
electrochemical realkalization technique was developed
with the aim of reestablishing the initial alkaline environ-
ment of concrete and reducing the corrosion rate of the
reinforcements [1–3]. Studies on the long-term effects of
this technique are quite necessary, as they determine the
application prospects and future research directions of
realkalization. Several experimental studies [4–6] were
carried out to examine this issue, and these studies mainly
concentrated on two aspects: the trends in restoring an
alkaline environment in the concrete and the corrosion
conditions of the reinforcements over time.
The pH value near a reinforcement decreased to 10.3
from the initial value of 12.0. 45 days after treatment and
then tended to remain stable as time passed, as reported by
Andrade et al. [7]. The author proposed that only a small
amount of carbon dioxide participated in the second
carbonation process and that the presence of the CO3
2–/
HCO3
– buffer pair was also beneficial for maintaining the
alkalinity of the concrete. Other experiments have also
revealed the same trend; for instance, the pH decreased
from over 14 to 12–13 after several months in an
experiment by Mietz [8], from 15 to 11.1 after 180 days
in a study by Qu et al. [9], and from 11 to above 9 after 900
days in an experiment by Tong et al. [10]. Based on the
available results, the pH value near a reinforcement was
proven to decrease gradually and eventually stabilize at a
certain value, which is induced by the reverse diffusion of
alkaline ions rather than a second carbonation process. The
speed of alkalinity decay is related to the realkalization
parameters, such as the electric charge, the water-cement
ratio of the concrete and the hydroxide contents at the end
of realkalization. Although a relatively constant value
without drastic fluctuations might be ultimately obtained in
a few days, this value is probably still not sufficient to
provide stable protection of the rebar.
In terms of the corrosion current density, Ribeiro et al.Article history: Received Jul 10, 2018; Accepted Oct 2, 2018
Front. Struct. Civ. Eng. 2020, 14(1): 127–137
https://doi.org/10.1007/s11709-019-0583-x
[11] observed that the corrosion potential varied linearly
with time, with values of – 250 mV/CSE one month after
treatment and – 200 to –250 mV/CSE six months later
with a further decrease 12 months later. In the studies of
Andrade et al. [7,12] and Tong et al. [10], the current
density decreased to over 0.2 μA=cm2 60 days after
treatment and 1.0 μA=cm2 360 days after treatment,
respectively, which meant that the steel bars were not
passivated but instead remained in an active state. Mean-
while, Tong et al. [10] reported that 720 days later, the
corrosion rate increased and then returned to the level
before treatment after another 900 days. Mietz [3] drew a
similar conclusion in which even though the corrosion
current density decreased to a negligible value when
realkalization was completed, just three months later, it
increased again to 43 μA=cm2. Realkalization clearly
reduces the corrosion current density of steel bars, but
this property increases again just several months or years
later whether the steel bars are repassivated after treatment
or not. In addition, the corrosion severity before realk-
alization is a major factor that influences the durability of
realkalization. According to the experiment of Redaelli and
Bertolini [13], the corrosion current density of the non pre-
corroded specimens exhibited a decreasing trend over the
400 days following realkalization, while that of the bars
that had already corroded before treatment began to
increase only 200 days after realkalization.
This paper presents the results of pH tests of the concrete
and corrosion condition tests of the steel bars in carbonated
reinforced concrete specimens subjected to realkalization
seven years ago with the aim of investigating the long-term
effects of this technique.
2 Experimental program
2.1 Specimen preparation
Several concrete cylinder specimens with plain steel bars
(diameter of 10 mm, height of 280 mm) placed in the
center along the axis were fabricated, as shown in Fig. 1.
The diameter of the specimens was 70 mm, and the height
was 250 mm. Wires were welded to one side of the steel
bar for the following treatment. Both ends of the specimen
were isolated with epoxy resin. In view of that realkaliza-
tion has a better effect on concrete materials with good
permeability, a W/C ratio of 0.65 was chosen for the
concrete mix design. The concrete mix consisted of
ordinary Portland cement 32.5, river sand, and coarse
aggregate (gravel) with maximum size of 15 mm, in which
cement, water, sand, and coarse aggregate were mixed in a
proportion of 1:0.65:2.25:3.68 by weight [14], as shown in
Table 1. According to GB/T50081 [15], the 28-day
compressive strength was determined to be 25.6 MPa
based on the averages of three cube specimens.
The specimens were cured at 20°C�3°C and a relative
humidity above 90% for 28 days. Then, the specimens
were placed in a drying oven at a temperature of 60°C for
48 h. After drying, the specimens were placed in a
carbonation chamber with a temperature of 20°C�3°C,
humidity of 70%�5% and carbon dioxide concentration of
20%�3% until carbonation was complete, as monitored
through phenolphthalein test.
2.2 Realkalization treatment
After carbonation was complete, the specimens were
immersed in deionized water until weight measurement
Table 1 Concrete mix design
component content (kg/m3)
Portland cement 32.5 323
water 210
sand 727
aggregate (with maximum size of 15
mm)
1189
W/C ratio 0.65
Fig. 1 Specimendesign (unit: mm). The specimens were concrete cylinders (diameter: 70 mm; height: 250 mm) with F10 mm plain
steel bars in the center. The top and bottom were all isolated with epoxy resin and the surrounding area was exposed to the air. Wires were
welded to one side of the steel bar for treatment and measurement.
128 Front. Struct. Civ. Eng. 2020, 14(1): 127–137
proved that there was no obvious changein the weight of
the concrete, which indicated that there was no extra
concrete pore space for deionized water and the specimens
had achieved saturated surface dry condition. Then, the
realkalization treatment was started. An electrical field was
generated between the steel bar (the cathode) and the
external steel mesh (the anode) using a DC power supply,
and a 1 M sodium carbonate solution was adopted as the
electrolyte [13]. After treatment, all the specimens were
stored in the laboratory (natural indoor environment) at a
temperature of 0–30°C and a humidity of 30%–80% for
seven years.
The treatment parameters of the 14 realkalized speci-
mens and 2 carbonated specimens can be obtained from
their names, as shown in Table 2. In the specimen names,
the first one or two letters (CR or C) before the digit
indicate the treatment the specimens were subjected to,
where CR represents carbonation and realkalization and C
represents carbonation. The first two numbers in the CR
specimen names are the current (A/m2) and duration time
(days), respectively, used in the realkalization treatment,
and the last number is the specimen number if multiple
specimens were tested with the same parameters. The letter
X in the names of the specimens (CRX-1 and CRX-2)
indicates that the detailed realkalization parameters of
these specimens are unavailable.
2.3 Measurement procedures
The measurements were conducted according to the
procedures below:
1) Cracking condition observation
During the seven-year period, cracks developed in some
specimens. Thus, the cracking conditions of 14 realkalized
specimens were first recorded.
2) Electrochemical measurements
Twelve realkalized specimens out of 14 are applied
electrochemical tests and the electrochemical characters of
the other two specimens (CRX-1, CRX-2) were not tested
due to their extensive cracking. The test specimens were
immersed in a 0.5 M sodium carbonate solution for 3 days
to increase the conductivity before the electrochemical
measurements. The corrosion conditions of the rebars were
studied by means of potentiodynamic polarization test,
which was measured with a CHI660B electrochemical
workstation. The saturated calomel electrode was used as
reference electrode, and a platinum electrode was used as
the auxiliary electrode. The scan rate was 2 mV/s, and the
scan range was �0.25 V vs the open-circuit potential.
3) Alkalinity measurements
After the electrochemical measurements, four specimens
CR3&14-2, CR3&14-1, CRX-1, and CRX-2 were cut, and
a 1% phenolphthalein solution was sprayed onto the
freshly cut surfaces as a pH indicator. The regions with a
pH value of 8 or above appeared red, while the color of the
regions with a pH value less than 8 remained unchanged,
and thus, the alkaline regions of the concrete could be
determined. To obtain a more accurate result, pH tests were
carried out via a pH meter. Concrete powders (depth of
0–5 mm from the steel bar) were drilled and sieved through
a 0.08 mm square hole sieve and dried to constant weight
at a temperature of 105°C. A total of 4 g of the powder and
16 g of deionized water were mixed and oscillated at a
frequency of 120 times/min for 24 h. Then, the pH of the
supernatant was measured by the pH meter (Leici PHB-4)
after filtration.
4) Corrosion condition observation of the steel bars
Finally, 6 realkalized and 2 carbonated steel bars were
removed from the concrete, and the corrosion conditions
were observed directly.
3 Results and discussions
3.1 Cracking of the specimens
New cracks that were not present in the previous tests
developed in some specimens seven years after realkaliza-
tion. Generally, for most realkalized specimens (9 out of
14), no cracks developed on the surface. However,
longitudinal cracks developed in specimens CR3&14-1,
CR10&14-2, CR10&28-2, while extensive cracks devel-
oped in specimens CRX-1 and CRX-2 and the concrete
was easily crushed when tapped with a hammer, as shown
in Table 3.
The cracking of the specimens after the realkalization
treatment had not been reported before and was unex-
pected. First, no rust was observed inside the cracks or on
the surfaces of the specimens, and the amount of corrosion
products was not sufficient to cause cracking. In addition,
all specimens were stored in the same environment, and the
Table 2 Realkalization parameters
treatment realkalization specimen name
current (A/m2) duration (days)
carbonation and
realkalization
3 14 CR3&14-1
CR3&14-2
CR3&14-3
3 28 CR3&28-1
CR3&28-2
5 14 CR5&14-1
CR5&14-2
10 14 CR10&14-1
CR10&14-2
CR10&14-3
10 28 CR10&28-1
CR10&28-2
3/5/10 14/28 CRX-1, CRX-2
carbonation with-
out realkalization – –
C-1, C-2
Peng ZHU et al. Long-term effects of electrochemical realkalization on carbonated concrete 129
corrosion degree of the rebars with same parameters was
assumed to be the same. However, only a small part of the
samples cracked. Therefore, the possible causes of
cracking are the occurrence of an alkali-aggregate reaction
(AAR), the diverse compactness of the concrete and the
excessive current of realkalization.
Some researchers doubt that the realkalization treatment
could cause an AAR to occur, as sodium carbonate was
used as the electrolyte [16], but no cases were reported
until now. In this experiment, no alkali-silicate gels were
found in the cracks, and the specimens were kept in the
laboratory during the entire seven-year period and were not
exposed to the natural environment, which decreased the
potential for the reaction to occur [17]. The specimens
were stored in the room with a humidity that was always
less than 80%, and the amount of moisture was not
sufficient to initiate the AAR. Additionally, the restored
alkalinity in the concrete induced by realkalization was not
as high as the initial pH developed during curing. No AAR
and crack formation occurred during curing, carbonation
and realkalization, which proved that no reactive aggre-
gates were present, even in a more alkaline and moist
environment. Initiation of the AAR became less likely as
the alkalinity and moisture decreased over time [18].
Another possible cause of the crack formation is the
diverse compactness of the concrete. The long and thin
shape of the specimens and the presence of a central bar
increased the difficulty of the casting process, and parts of
the specimens were not sufficiently vibrated during
casting, which led to the poor compactness of the concrete.
The compactness of concrete is closely connected to the
concrete strength, and the compressive strength of cubic
specimens of the concrete was only 25.6 MPa (described in
Section 2.1). Dry shrinkage would occur when the
moisture of the concrete decreased, which might produce
a tensile force. For the specimens with poor compactness,
this tensile force might exceed their tensile strength and
cause cracking. In addition to this process, during the
process of realkalization, chemical reactions and ion and
water exchange have an influence on the microstructure
and may increase the quantity of fine pores in the vicinity
of the steel bar [19], which may increase the possibility of
cracking.
The number of alkaline ions and the efficiency in
reducing corrosion strongly depend on the charge of the
realkalization treatment. To achieve a better realkalization
effect, higher charges (1008–6720 Ah/m2) and electric
currents (3, 5, and 10 A/m2) were adopted in this
experiment, and no side effects (excessive charge and
current of realkalization) were reported at the beginning of
the experiment. However, CEN/TS 14038-1 [20] and
NACE SP0107 [21] suggested that the electric charge
during realkalization should exceed 200 Ah/m2 but did not
specify a maximum amount of charge. COST [22]
recommended a charge between 200 and 450 Ah/m2.
Furthermore, the current used was also larger than that
recommended. A current between 0.8 and 2 A/m2 and less
than 4 A/m2 was recommended by Mietz [3] and CEN/TS
14038-1 [20], respectively. Yeih and Chang [2] reported
that the compressive strength andmodulus of elasticity of
realkalized concrete decreased with the increase of electric
charge, which made the concrete with excessive charge
easier to crack.
During realkalization, cathodic electrolysis is conducted
in two ways, as described in Reactions 1 and 2.
1=2O2 þ H2Oþ 2e – ↕ ↓2OH – , (1)
2H2Oþ 2e – ↕ ↓H2 þ 2OH – : (2)
Reaction 1 dominates when sufficient oxygen is present;
Fig. 2 Corrosion current density and potential 7 years after realkalization. The names below the columns indicate the specific specimens
and the digits above indicate the values of (a) corrosion current density and (b) corrosion potential.
130 Front. Struct. Civ. Eng. 2020, 14(1): 127–137
otherwise, Reaction 2 dominates. All the specimens were
immersed in deionized water until they reached the
saturated surface dry condition, after which they were
immersed in a sodium carbonate solution during realk-
alization, which guaranteed a sufficient supply of water but
a shortage of oxygen. Thus, Reaction 2 occurred. The
cathode gained a large number of electrons, as the current
was excessively high in this study. A large amount of
hydrogen was produced and may not have diffused
immediately. The pressure caused by the accumulation of
hydrogen may have caused the concrete to crack. A current
of 10 A/m2 was applied to two of the cracked specimens in
this study. However, no cracking had occurred immedi-
ately after realkalization was completed. Since concrete is
a porous material, its content of hydrogen will decrease
over time, which means that the formation of cracks in the
concrete becomes increasingly more difficult. Cracks
induced by hydrogen may have developed in the vicinity
of the rebars first and then propagated to the surface very
slowly over seven years. Further researches on the reasons
for cracking are planned to be performed because it may be
potentially a limiting factor for the use of the technique.
Table 3 Crack development in the realkalized specimens
cracking images
no cracks
CR3&14-2 CR3&14-3
CR3&28-1 CR3&28-2
CR5&14-1 CR5&14-2
CR10&14-1 CR10&14-3
CR10&28-1
longitudinal cracks
CR3&14-1 CR10&14-2
CR10&28-2
extensive longitudinal cracks;
part of the concrete fell out;
easily crushed when tapped with a hammer
CRX-1 CRX-2
Peng ZHU et al. Long-term effects of electrochemical realkalization on carbonated concrete 131
3.2 Corrosion current densities and potentials of the steel
bars determined by electrochemical measurements
The corrosion current densities and potentials of 12 steel
bars measured 7 years after realkalization were calculated
with the software CView, as shown in Fig. 2 and the
polarization curves are presented in Fig. 3.
The corrosion potentials of the various bars ranged
between – 560.4 and – 268.5 mV with corrosion current
densities ranging between 2.17 and 48.36 μA=cm2 7 years
after realkalization. According to ASTM C876 [23] and
Chinese standard GB/T 50344 [24], the steel bar is
depassivated when the corrosion current density is greater
than 0.2 μA=cm2.
The results of the corrosion current density and potential
measurements show that the maximum current is much
higher than the minimum. The corrosion current density is
dispersive even between specimens treated under the same
realkalization conditions, such as CR3&14-1–CR3&14-3
and CR10&14-1–CR10&14-3. The occurrence of cracking
is proposed to be one of the causes of the large variations,
as shown in Table 3. Several cracks developed in
CR3&14-1 and CR10&14-2 over 7 years, and the current
densities of these specimens were dramatically higher than
those of the other specimens. The diverse distribution and
width of the cracks in the different specimens can cause
variations in the diffusion coefficients of oxygen and vapor
and thus different corrosion rates [25,26]. Even in
complete specimens, the conditions of crack propagation
vary, although this variation cannot be observed directly.
Furthermore, the presence of different pore structures and
propagated cracks cause differences in the water content in
the concrete following immersion in the sodium carbonate
solution before measurement, which may influence the
results of the electrochemical measurements [27,28].
In addition, excluding the cracked specimens, the
corrosion current densities of CR3&14, CR3&28, and
CR5&14 were similar and of the same order of magnitude,
while those of CR10&14 were much higher, and those of
CR10&28 were the highest. Thus, excessive current and
charge can be inferred to have the opposite effect and
increase the corrosion.
For comparison, the polarization curves measured
previously at 0, 4, 10, 13 months were present in Fig. 3.
The polarization curve test was destructive, and each
specimen could be tested only once. Therefore, the
polarization curves measured at 0, 4, 10, 13 months and
7 years after realkalization were obtained from different
specimens subjected to the same realkalization parameters,
and the tests were conducted with the same parameters and
data processing method.
According to the polarization curves, there was little
difference between the Tafel slope of anodic curves and
cathodic curves before and 0 months after realkalization
while the Tafel slope of anodic curves of 4 months
increased and the Tafel slope of cathodic curves decreased
slightly. The polarization curves of 4, 10, 13 months
showed similar trend and the increased Tafel slope of
anodic curves indicated that the anodic reaction was
restrained. Reactions in the process of the corrosion of the
steel bar are present below.
Anodic reaction : Fe
↕ ↓Fe2þ þ 2e – : (3)
After realkalization, the pH of concrete around the steel
bar increased, which caused the formation of the passiva-
tion film on the surface of the steel bar and decreased the
rate of anodic reaction. As for the results of 7 years, the
corrosion current densities of CR3&14-2,3, CR3&28-1,2,
CR5&14-1,2, and CR 10&14-2 were relatively lower and
the Tafel slope of their anodic curves decreased, which
indicated the steel bars surface became active and the
corrosion was enhanced. The effects of realkalization were
weakened. However, the anodic curves of CR3&14-1,
CR10&14-1,3 and CR10&28-1,2 with higher corrosion
rates still showed high slope, which may result from the
high corrosion degree of the steel bars. The rebars had
depassivated and been corroded for a period of time and
most of the surface was covered with corrosion products
(iron oxides). The existence of rusts reduced the dissolu-
tion rate of iron and restrained the anodic reaction.
The changes in the corrosion current density and
potential over seven years are shown in Fig. 4.
Based on Figs. 3 and 4, both the corrosion current
density and corrosion potential of all the specimens
increased after realkalization due to polarization and then
gradually decreased over time. Little difference was
observed between the data from 4, 10, and 13 months,
and the values became stable 13 months after realkaliza-
tion. However, after 7 years, the variations in the data from
the different treatment conditions are larger than those in
the data measured at earlier time points. Based on just these
data, no direct relationship existed between the corrosion
current density, corrosion potential, and realkalization
parameters.
Compared with the results after 13 months, considerable
increases in the corrosion current density and potential of
varying degrees were observed after 7 years. The corrosion
current density of the carbonated specimen before
realkalization was 1.86 μA=cm2. Seven years later, the
corrosion current densities of all the specimens increased
and became even higher than those of the samples before
treatment, which was probably due to the excessive current
and occurrence of cracking.
An excessive electric current may influence the effects
of realkalization. A certain amount of hydrogen may be
produced during realkalization under oxygen-deficient
conditions, as described before. Wang et al. [14] found
that the corrosion potential was maintained at a highlevel
in the presence of hydrogen, which was not beneficial for
the passivation of the bars.
132 Front. Struct. Civ. Eng. 2020, 14(1): 127–137
The longer period in the active state caused by the higher
electric current may have accelerated the corrosion of the
steel bars. The potential takes even longer to decrease, and
therefore, the potentials of the specimens after realkaliza-
Fig. 3 Polarization curves before and 0, 4, 10, 13 months [14] and 7 years after realkalization. (a) Before and 0 months after
realkalization; (b) 0 and 4 months after realkalization; (c) 4 and 10 months after realkalization; (d) 10 and 13 months after realkalization;
(e) 13 months and 7 years after realkalization; (f) 7 years after realkalization (10 A/m2). The change rules of polarization curves of
specimens under the realkalization condition 3/5 A/m2, 14/28 days measured 0, 4, 10, 13 months and 7 years after realkalization are
present in Fig. 3(a)–3(e), and the specimens under the realkalization condition measured 10 A/m2, 14/28 days 7 years after realkalization
are shown in Fig. 3(f).
Peng ZHU et al. Long-term effects of electrochemical realkalization on carbonated concrete 133
tion were higher than those not subjected to realkalization
and those treated by realkalization at a lower electric
current.
In addition, the formation of cracks in the concrete will
accelerate the diffusion of oxygen, vapor, and other
negative substance into the concrete, which is unfavorable
for the protection of the steel bar. Thus, corrosion will be
accelerated to some extent.
3.3 Alkalinity measurements of the concrete
Since some of the specimens were cracked, we were afraid
that the complete specimens would also crack during
cutting or drilling, which would prevent us from conduct-
ing the electrochemical measurements. We preferred to
examine the corrosion rates and test the alkalinity after
realkalization because under normal circumstances, 3 days
of immersion will not influence the pH around the steel bar.
Specimens CR3&14-2, CR3&14-1, CRX-1, and CRX-2
were chosen as representatives for the alkalinity measure-
ments due to their different surface conditions. The results
of phenolphthalein test and pH meter test are presented in
Fig. 5 and red regions can be visually observed in the
images.
For specimen CR3&14-2 with an unbroken surface and
CR3&14-1 with longitudinal cracks, the external concrete
but not the concrete around the rebar appeared red, as
shown in Figs. 5(a) and 5(b). The pH of concrete powders
around the rebar were 7.94 and 7.86 which were not high
enough for the passivation of the steel bars. The specimens
were immersed in a sodium carbonate solution before the
electrochemical measurements, and the carbonate ions
penetrated the concrete by diffusion and capillary forces,
leading to an increase in the pH of the external concrete.
Therefore, the red color did not result from the realkaliza-
tion treatment performed seven years ago. The same
phenomenon was observed for specimen CRX-1 and
CRX-2, as shown in Figs. 5(c) and 5(d). These two
specimens were not immersed in the alkaline solution, and
the electrochemical measurements were not performed due
to the extensive cracking of the samples. The color of the
cut surface did not appear red and the pH were 7.78 and
7.42, which also indicated that the alkalinity produced in
the realkalization treatment had disappeared completely
over seven years. Since the complete and broken samples
exhibited similar results, the results for the other samples
can be inferred to be the same.
For comparison, the pH values measured at 4, 10, and 13
months after realkalization are presented in Fig. 6.
Whether the concrete still has an alkaline environment
depends on the reverse diffusion rate of the alkaline ions,
which is related to the compactness of the concrete. The
lower the compactness of the concrete is, the higher the
reverse diffusion rate is. Realkalization is most effective
for concrete materials with good permeability, so a water-
cement ratio of 0.65 was utilized for the specimens in this
experiment to guarantee a high porosity. According to this
design, the compactness of the concrete was low at the
beginning. In addition, crack propagation may aggravate
diffusion, even for those specimens without obvious cracks
on the surface.
3.4 Corrosion conditions of the steel bars determined by
visual observation
After all the tests, some of the steel bars were removed
from the concrete specimens, and their surface corrosion
conditions were observed, as shown in Fig. 7. The above
six specimens are realkalized specimens CR3&14-2,
CR10&14-3, CR3&14-1, CR10&28-2, CRX-1, and
CRX-2 and the other two are carbonated specimens C-1
and C-2.
Fig. 4 Corrosion current density and potential over 7 years. The corrosion current density and potential calculated from the polarization
curves are present. The x-axis indicates the time (months) after realkalization and -5 indicates carbonated specimen before treatment.
(a) Corrosion current density; (b) corrosion potential.
134 Front. Struct. Civ. Eng. 2020, 14(1): 127–137
Most of the rebar surface in specimens CR3&14-2 and
CR10&14-3 maintained a metallic luster, and a small
region was covered with yellowish-brown corrosion
products. In contrast, for the rebars in specimens
CR3&14-1 and CR10&28-2, most surface of the bar lost
its metallic luster and was covered by sepia rust. As for
CRX-1 and CRX-2, almost the whole surfaces were
covered by corrosion products. The corrosion current
densities of specimens CR3&14-2, CR10&14-3,
CR3&14-1, and CR10&28-2 were 3.47, 15.34, 35.37, and
43.15 μA=cm2, respectively. Thus, the electrochemical
results were consistent with the actual corrosion condi-
tions.
The degree of crack propagation in specimens
CR3&14-2 (CR10&14-3), CR3&14-1(CR10&28-2), and
CRX-1(CRX-2) increased in the given order, and the
observed corrosion also became more extensive. This
result is consistent with the finding by Montes [29] that
cracking plays an important role in the corrosion process.
Specimen C-1 and C-2 are carbonated specimens that were
not subjected to realkalization. No obvious cracks were
observed on their surfaces, and the bars retained a metallic
luster. In other words, when cracks were not sufficiently
developed, minimal amounts of rust were observed in the
specimens seven years later, even in the carbonated
concrete.
4 Conclusions
1) Realkalization had vanished completely seven years
after the treatment for a concrete with a W/C ratio of 0.65.
2) Multiple cracks developed in some of the specimens,
and the crack propagation differed between specimens.
The possible causes of cracking may be the diverse
compactness of the concrete and the excessive electric
current of the realkalization treatment.
3) The differences between the corrosion conditions of
the different steel bars seven years after treatment were
significant, as indicated by the potentiodynamic polariza-
tion measurement. The differences probably resulted from
Fig. 5 Results of the phenolphthalein tests for specimens CR3&14-2, CR3&14-1, CRX-1, and CRX-2 after seven years. A 1%
phenolphthalein solution was sprayed onto the freshly cut surfaces as a pH indicator. The region with pH value not less than 8 appeared
red, while the region with pH value less than 8 remained unchanged. The pH was tested by concrete powders (depth of 0–5 mm from the
steel bar) and the values were present in the subheadings. (a) CR3&14-2 (pH = 7.94); (b) CR3&14-1 (pH = 7.86); (c) CRX-1 (pH = 7.78);
(d) CRX-2 (pH = 7.42).
Fig. 6 pH around the steel bar 4, 10, and 13 months after
realkalization. The x-axis indicates the time (months) after
realkalization and the pH was tested by concrete powders (depth
of 0–5 mm from the steel bar).The pH around the steel bar 4, 10,
and 13 months after realkalization was directly related to the
charge. The higher the charge was, the higher the pH was. In the
period from 4 to 13 months, the pH around the steel bar dropped
over time in all the treatedspecimens. Based on the pH test, the pH
decreased to below 8.0 after 7 years from the value of more than
12.0 immediately following realkalization and 11.56–12.19 13
months later.
Peng ZHU et al. Long-term effects of electrochemical realkalization on carbonated concrete 135
the diversity in the extent of cracking. The corrosion
potential of the steel bars was between – 560.4 and
– 268.5 mV, while the corrosion current density was in the
range of 2.17 to 48.36 μA=cm2. Based on both the
potential and the current, the steel bars were concluded to
have returned to the depassivated state, and the corrosion
current was even higher than that of the carbonated
concrete before realkalization.
4) Based on the above, the effects of the realkalization
treatment were concluded to have disappeared after seven
years, and the long-term effects of this technique were
unsatisfactory. The protection provided by the realkaliza-
tion treatment was temporary, and a simple sodium
carbonate solution or alkaline solution was not sufficiently
effective to spontaneously form a stable passivation film
on the rebars. However, the data in this study are limited
and discrete. More data and investigations of the durability
of the realkalization technique are required. Because the
protection provided by the realkalization treatment for high
W/C ratio concretes is not durable, further research to
improve the existing realkalization technique (such as to
decrease the permeability of concrete cover immediately
after realkalization) should be performed.
Acknowledgements This study was funded by the National Key Research
and Development Program of China (No. 2017YFC0703000) and the
National Natural Science Foundation of China (Grant No. 51678430).
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