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Experimental studies on penetrating-type corrosion
inhibitor in reinforced concrete
R. G. Limaye, R. D. Angal, A. S. Radke
The paper presents experimental studies on a new
generation of corrosion inhibitor, which is based on bipolar
mechanism and which can penetrate even dense
concrete by virtue of its vapour pressure and affinity for
the embedded steel in concrete. The inhibitor is used both
as an admixture in fresh concrete and as a coating on
hardened concrete.
Steel is an essential part of reinforced and prestressed
concrete and the corrosion of steel is identified as the
single largest factor responsible for its deterioration. The
alkalinity of concrete, which protects the steel may be
affected either by carbonation of the concrete, or by
ingress of chloride ions. These may arise from sea salts or
environmental Corrosives.
The chlorides (Cl - ) are deadly and can depassivate steel
even under alkaline conditions. The rebar corrosion in
concrete leads to the cracking and eventual spalling of
concrete, because the rust formed has a greater volume
than the steel and hence exerts considerable pressure on
concrete which eventually leads to its disintegration.
Work on rebar corrosion problem in concrete goes back
more than 75 years. The severity of the problem in the
past few decades has prompted considerable acceleration
of research. A major factor causing corrosion of steel in
concrete has been attributed to the chloride ions and their
role in destroying the- passivity of steel. Many physical
methods for corrosion control have been in vogue which
include:
R. G. Limaye, Associate Professor, Department of Civil
Engineering, Indian Institute of Technology, Powai.
Mumbai 400 076
R. D. Angal, Associate Professor. Department of
corrosion science of Engineering, Indian Institute of
Technology, Powai, Mumbai 400 076
A. S. Radke, M.Tech student, Department of Civil
Engineering; Indian Institute of Technology, Powai,
Mumbai 400 076 .
rebar coatings, concrete coatings, use of admixtures,
methods for decreasing permeability, increased concrete
cover, cathodic protection, electrochemical method to
remove chlorides, etc. Many of these have shown promise
in extending the initiation of corrosion.
New generation of corrosion inhibitors
Concrete penetrating corrosion inhibitors (CPO) based on
bipolar inhibition mechanism is a new generation corrosion
inhibition. They can penetrate even dense concrete by
virtue of its vapour pressure and its natural affinity for the
embedded steel in concrete. Upon penetration CPO based
on bipolar inhibition mechanism (both cathodic and anodic
inhibition) inhibit the corrosion even at high chloride levels.
This differs from conventional corrosion inhibitors based
on calcium nitrite which are only anodic inhibitors in
mechanism. Calcium nitrite inhibitors are effective at low
chloride concentrations but may actually accelerate
corrosion at higher chloride levels in concrete. Further, the
new generation corrosion inhibitor (CPCI) is eco-friendly
and non-toxic. The paper presents the experimental studies
carried on CPCI which are being manufactured and
marketed indigenously.
Scope of present study
This project is intended to study CPCI and its effects on
different concrete properties and its effectiveness to prevent
corrosion. When a corrosion inhibiting admixture is used,
its effect on fresh concrete properties like workability,
compaction factor, initial and final setting time also needs
to be studied.
Corrosion inhibitors when used as admixture or concrete
surface coatings, their effect over the period of time needs
to be studied in order to ascertain the following.
( Reprinted from ICJ ( Indian Concrete Journal ) January 2000, pp. 22-26 )

Table 1: Specimen details
Specimen
no
Nomenclature Specimen details
1 Mx-8/P Plain mix Mx-8
2 Mx-4/P Plain mix Mx-4
3 Mx-8/A
Mix Mx-8 with corrosion
inhibiting admixture
4 Mx-4/A
Mix Mx-4 with corrosion
inhibiting admixture
5 Mx-8/C
Mix Mx-8 with corrosion
inhibitor coating
6 Mx-4/C
Mix Mx-4 with corrosion
inhibitor coating
• The effect on bond strength.
• Effectiveness of corrosion inhibitors after being
subjected to thermal cycles (essential in a tropical
country like India).
• Effect of environment on this concrete.
• Effect of corrosion inhibitors on fresh concrete
properties like setting time, workability,
compaction factor, etc.
• Effect on rate of corrosion.
• Effect on compressive strength and tensile strength
of concrete.
• Cost-effectiveness of the corrosion inhibitor.
Protection of life of corrosion affected structure.
Due to the electrochemical nature of the corrosion
mechanism, different electrochemical tests were planned
out in this study. The 'half-cell potential' (E corr )
measurement, indicating the probability of corrosion of
reinforcement, was the basic test followed for the
corrosion current measurement.
Failure of a reinforced concrete (RC) member due to
corrosion can be measured in terms of coulombs
(amp.hours). The extent of corrosion is given in terms of
corrosion-current density, I corr and time. With proper
protection method, the I corr for the given steel bar would
be less and hence would take more time for the failure.
The time to failure hence can be conveniently measured in
terms of potential as per ASTM method. In this project it
was decided to find out the number of coulombs required.
External DC voltage is imposed on the concrete specimen
in order to accelerate the process of corrosion. This
voltage, the driving force for corrosion, was maintained
constant throughout the study. With increase in the
corrosion activity, resistance of the specimen decreased
and the current increased. This current was monitored
regularly. Multiplication of the total current and the
duration for which it is flowing in the circuit gives the
total coulombs consumed on the specimen. Total
coulombs consumed on the specimen up to failure were
used for the prediction of life of the specimen.
Experimental work
In the literature it was observed that certain work has been
done on corrosion inhibitors used as reinforcement coatings,
concrete coatings and corrosion inhibiting admixture. With
the use of these inhibitors. new structures can be protected or
the process of the onset of corrosion can be delayed. But,
in case of existing structures little choice is available to
safeguard the structure, after the irreversible process of
corrosion has begun. Also in the case of corrosion-affected
existing structures it would be useful to know the magnitude
of damage caused by corrosion attack and the rate of
corrosion so that the time required to cause failure of
structures (life of a structures) can be predicted. The different
.electrochemical tests for the measurement of corrosion
current, corrosion rate and potential (indication of magnitude
of corrosion activity) were carried out in this study. For the
simulation of study specimen, as a corrosion affected
structure, it was essential to us a small-sized specimen for
accelerating the process and the specimen were placed in
saline medium giving them a constant DC Voltage of 7.5
volts by means of a rectifier, Fig 1.
Corrosion inhibitors
For corrosion inhibiting admixtures to be viable, they should
not only prevent or delay the onset of corrosion, but also must
not have any detrimental effects on the properties of the
concrete itself, such as strength, setting time, workability or
durability. For this study a penetrating corrosion inhibitor
(CPCI) was used as corrosion inhibitor admixture at one
percent by weight of cement. In an upcoming structure there
can be many methods to prevent corrosion, but in an existing
structure if corrosion starts much before the service life of
structure, a limited choice is then available to save the
structure. Protective coating on, concrete structure surface to
inhibit the ingress of aggressive elements is the only remedy.
This can be useful to new structures as well. CPCI is a
concrete-surface applied penetrating corrosion inhibitor and is
not in direct contact with the reinforcement. It is brush
applied after the specimen is cured for 28 days. Two coats
were applied with a gap of 4 hours between them, as
recommended by the manufacturer.

Table 2 : Effect on setting time and workability
Mix
Setting time , min
Workability
Initial final (flow, mm)
Cement 80 240 -
Cement + admixture 85 240 -
Mx-8P 150 270 120
Mx-8A 150 270 124
Mx-4/P 160 300 120
Mx-4A 170 300 125
An anti-corrosive protective coating system known as
epoxy-based interpenetrating polymer network (IPN) was
used for reinforcement bars. Portion of the reinforcement
bar under study is embedded and bare (uncoated). But
during the period of study, the outside portion of the bars
might get corroded and therefore is coated by epoxy-IPN
system.
Concrete mix
Quality of materials and concrete mix design can
influence the degree of protection offered by concrete to
the embedded steel. The final set of variables deals with
the way the structure/ concrete is designed, placed,
finished and cured. To evaluate the effect of corrosion
inhibitors on reinforcement bars, reinforced cement
concrete specimens were cast. Preliminary tests were
conducted on cement, aggregates and sand before starting
the main experimental programme.
For deciding the mixes, different mix proportions were
tried and keeping workability pivotal, two mixes, Mx-8,
having 28-day strength of about 12 N/mm2 and Mx- 4
with 28- day strength of about 30-N /mm2 were arrived at
through trial mixes. They were chosen so as to evaluate
corrosion in weaker and stronger concretes.
Preparation, mixing, casting and sampling of the fresh I
concretes were done in accordance with 15:1199-1959 1
(Methods of sampling and analysis of concrete). Cube size
adopted was 7.07 x 7.07 cm.
There were three types of specimen made from each mix.
• A control specimen
• Specimen with corrosion inhibitor admixture and
• Specimen with corrosion inhibitor coating over
concrete surface.
Table4: Effect on water absorption, bulk density and UPV
Specimen
Mix-8/P
Mix-8/A
Mix-4/P
Mix-4/A
28-
day
Absorption percent
28day +
90day
Natural
condition
28day + 90day
Temperature
cycle
Bulk
density
Kg/m 3
UPV
km/s
10.98 11.02 10.85 2176 3.190
10.73 10.72 11.08 2185 3.280
8.67 8.98 8.80 2286 3.544
8.55 8.65 8.26 2306 3.850
Sufficient number of specimens were cast since following
tests on hardened concrete were planned at 7 and 28 days
i. after keeping for 90-days in natural condition,
compressive strength, water absorption and ultrasonic
pulse velocity (UPV) measurements
ii. after giving the specimens 90 temperature cycles, each
60°C temperature at 6 hours, compressive strength, water
absorption and ultrasonic pulse velocity (UPV)
measurements
iii. pull out strength after 28 days.
Specimen preparation
Cylindrical concrete specimens were cast in a split mould of
40-mm diameter and 80-mm long, with a high yield strength
deformed (HYSD) steel bar of 8-mm diameter embedded
into it. The rebar was placed centrally such that 16 mm cover
was present on all sides, including bottom. Also 16 mm
length from the top of mould was insulated. Three specimens
were cast in each category.
Electrochemical studies
At the top, the protruding bar was protected with coating
except where DC voltage was connected. Each test specimen
was immersed in the saline media (3.5 percent NaCl) and
connected to the positive terminal of the rectifier. The salt
solution was changed periodically when the change in colour
was significant. Each specimen was surrounded by stainless
steel mesh, the cathode, connected to -ve pole of a rectifier.
Specimen
7day
28day
Compressive strength, N/mm 3
90day
(natural condition)
90day
(temperature cycle)
28day
Pull out force, N
90day
(natural condition)
90day
(temperature cycle)
Mix-8/P 8.93 13.4 14.71 14.28 5600 5646 5610
Mix-8/A 9.47 14.14 14.85 14.92 5860 5870 5884
Mix-4/P 21.87 31.34 32.09 32.68 8900 9010 8980
Mix-4/A 23.47 35.00 36.19 36.00 9233 9235 9242

Table5: Electrochemical and pull out test results
Table6: Electrochemical and pull out test results
Specimen
Pull out force
required initial,
N
Coulombs
consumed,
mA-hrs
Pull out force
required final,
N
Mx-8/P 5600.20 6628 846.80
Mx-8/C 5600.20 4820 1648.09
Mx-8/A 5860.00 4226 2008.80
Mx-4/P 8900.00 2640 5110.00
Mx-4/C 8900.00 1608 6345.17
Mx-4/A 9233.33 1333 7020.23
Notes:
i. Samples cured for 28 days and connected in circuit
ii. Mx-8: F c = 12 N/mm 2 , Mx-4: F c = 30 N/mm 2
iii. P – Plain mix, A – Mix with corrosion inhibiting admixture,
C- Plain mix with corrosion inhibiting coating
iv. Initial – before the electrochemical tests started
v. Final – at the end of electrochemical tests
Specimen
Pull out force
required
Initial, N
Coulombs
consumed,
mA-hrs
Pull out force
required
Final, N
Mx-8/P 5610.00 6872 588.00
Mx-8/C 5884.13 5182 1469.00
Mx-8/A 5884.13 4420 2104.20
Mx-4/P 8980.20 2824 4840.00
Mx-4/C 8980.20 1820 6405.30
Mx-4/A 9242.33 1530 7156.33
Notes:
vi. Samples cured for 28 days given 90 thermal cycles and connected in
circuit
vii. Mx-8: F c = 12 N/mm 2 , Mx-4: F c = 30 N/mm 2
viii. P – Plain mix, A – Mix with corrosion inhibiting admixture,
C- Plain mix with corrosion inhibiting coating
ix. Initial – before the electrochemical tests started
x. Final – at the end of electrochemical tests
Due to transport of anions through the concrete, corrosion
of deformed bar took place, reducing the bond between
steel and concrete. Since the applied potential of 7.5V was
constant, decreasing IR drop resulted in increasing the
corrosion current and hence the corrosion rate. An
ammeter in series measured the current for each corrosion
cell. Three samples of each specimen are connected in
series and all the six sets of specimens are connected in
parallel. The corrosion potential against copper sulphate
electrode was measured in the standard manner as
described in ASTM. From the above measurement of total
current and duration the number of coulombs were
calculated. Same number of specimens are given the
temperature cycles and then connected in the circuit.
Pull-out test
Evaluation of pull-out force is a most important part of
corrosion study. As the corrosion advances the bond
between rebar and concrete is significantly affected
reducing the pull-out force. Hence to evaluate corrosion
physically the electrochemically-tested specimens were
subjected to pull-out test to assess bond strength.
Results and discussion
Corrosion of steel embedded in concrete is guided by the
formation of potential difference between cathodic and
anodic sites which is responsible for the development of
corrosion current. Although the corrosion current gives the
corrosion rate, the extent of corrosion has to be measured
in terms of number of coulombs. In the present study
effectiveness of corrosion inhibitor was studied for both
migratory type coatings and admixture. The concrete
specimens chosen were of two types: weak and strong
grades namely mix 8 (F c : 12 N/mm 2 ) and mix 4 (F c : 30
N/mm 2 ). Tests on fresh and hardened concrete were carried
out first to rule out any deleterious effect of corrosion
inhibiting admixture. Further electrochemical tests were
carried out to assess their effectiveness in corrosion
protection.
Tests on fresh concrete
Setting time and workability
There was no significant variation in the values of initial as
well as final setting time with or without corrosion
inhibiting admixture. These were carried out on cement
using Vicat's apparatus in accordance with 15:269-1976
(Specifications for ordinary and low heat portland cements)
and on mixes using penetrometer as per 15:8142-1976
(Method of test for determining setting time for concrete by
penetration resistance).
Mixes were chosen with the criteria that they have the same
workability; but when corrosion inhibitor in the form of
admixture was used, it did not change the workability either.
No accelerating or retarding effect was observed, Table 2.
Test on hardened concrete
Compressive strength
Few earlier studies carried out by various authors had shown
that corrosion inhibiting admixtures have exhibited adverse
effects on the compressive strength. However in this study
on migratory corrosion inhibitor, it was observed that the
admixture does not exhibit any adverse effect on

Compressive strength. On the contrary, it shows slight
increase in the compressive strength values. Table 3 shows
compressive strength values of the samples cured for 7
days and 28 days and later subjected to 90 thermal cycles
(each of 60°C at 6 hours) also those after 28 days curing
placed in natural condition for 90 days.
Bond strength
Bond strength between reinforcement and concrete
interface is the single most vital factor for the reliable
performance of reinforced concrete. Due to corrosion of
rebar; bond strength is affected most and hence
performance of RC. From Tables 5 and 6 it is seen that
bond strength of control specimens was reduced to 10-15
percent of original strength after corrosion tests. This is a
clear indication of a bond failure. It is however restricted
between 40-50 percent in the corrosion inhibitor
admixtured samples. Even after 90 thermal cycles there is
no reduction in bond strength as compared to naturally
cured specimens.
Other properties like UPV, bulk density, water absorption,
etc have not shown any change due to incorporation of
corrosion inhibitor admixture, which is indicative of
increase in cohesiveness of concrete, Table4.
Effect on corrosion
The current was measured at regular intervals and the
cumulative number of coulombs consumed was found out
and tabulated in Table 5. The same procedure was repeated
for specimens after 90 thermal cycles, Table 6. The
potentials were measured at the interval of 150 hours. In
accordance with ASTM standards the potential of -780 mV
against CSE was considered as the criterion for total
failure. From this electrochemical study it was evident that
the coulombs consumed in corrosion inhibitor admixtured
sample was 60 percent in both weak and strong concrete as
compared to control. However, in the coated specimens it
was between 68 to 70 percent.
Conclusions
From the study undertaken on effectiveness of corrosion
inhibitors the following conclusions may be drawn.
i. The incorporation of penetrating corrosion inhibitor as
admixture to concrete does not impair any mechanical
properties. Physical properties like workability, water
absorption etc. do not show any change. On the other
hand, the properties like compressive strength, bond
etc. showed improvement at ambient temperature and
even at higher temperatures of 60°C.
ii. The findings of electrochemical studies reveal that
there is significant advantage in incorporating
penetrating type corrosion inhibitor as admixture in
concrete as it dramatically reduces the rate of
corrosion. The reduction in coulombs and increase of
bond strength in corrosion-inhibiting admixtured
sample as compared to control specimens by more than
300 percent is the most redeeming feature. However,
this effect is more pronounced in weak concrete as
compared to stronger concrete grades.
iii. The application of penetrating corrosion inhibitor
coating has reduced the corrosion rate, though it is less
as compared to the one containing corrosion inhibiting
admixture. However, it is significant to note that there
is certainly migration of inhibitor to steel since it is
reflected in reduction of coulombs and increase in
bond strength.
iv. The thermal cycles study on coated as well as
admixtured samples reveals that even at higher
temperature of 60ºC, the effectiveness is the same,
which is heartening. In tropical climate like India, it is
essential to evaluate these penetrating type of
inhibitors at higher temperatures (higher than 60°C)
since their effectiveness may reduce in view of their
volatile nature.
Acknowledgement
This work was carried out at the Indian Institute of
Technology ( IIT ) Mumbai as a part of the MTech
programme. The authors are grateful to the authorities of
IIT, Mumbai. We are also thankful to Krishna Conchem
Products Pvt. Ltd., for supplying penetrating corrosion
inhibitors for this study.
Reference
1 RADKE, A. S. Some studies on Corrosion Inhibitors in
Reinforced Concrete, MTech Dessertation, 1999, IIT Bombay
2 ANGAL, R. D. LIMAYE, R.G. and KAMTE, V. D. Comparative
studies on concrete coatings for corrosion protection. The
Indian Concrete Journal, May 1995, pp. 285-287