1. TECHNOLOGY FOR DURABLE REINFORCED CONCRETE
STRUCTURES USING CR-BEARING CORROSION
RESISTANT REBAR
S.H.Tae and T.Noguchi
Tokyo University, Japan
T.Ujiro and O.Furukimi
JFE Steel Corporation, Japan
Abstract
This study was investigated to corrosion resistance of Cr-bearing corrosion resistant
rebars in simulated concrete pore solutions and concrete specimens having chloride ion
contents. Simulated concrete pore solutions were saturated Ca(OH)2 solutions
containing 0.27%, 1.07%, and 21.4% NaCl. The pH of the test solutions were
adjusted at 12.5, 11, 10, and 9 by HCl. Pitting potentials of the steels in the solutions
were investigated. The results of the study showed that 5Cr and 9Cr steels showed
good corrosion resistance in 1.07% NaCl solutions at ph12.5 and ph10, respectively.
In the case of concrete specimens having chloride ion contents, the test specimens were
made by installing 8 types of rebars into concrete specimens having chloride ion
contents of 0.3, 0.6, 1.2, 2.4 and 24kg/m3 each. The corrosion resistance of the Cr-
bearing corrosion resistant rebar was examined by measuring corrosion losses and
average corrosion rates up to 155 cycles of corrosion accelerated curing. The results
of the study showed that the Cr-bearing corrosion resistant rebar containing a Cr
content greater than 5% and 9% exhibited corrosion resistance when the chloride ion
contents were 1.2 and 2.4kg/m3 respectively.
1. Introduction
Recently the durability of steel reinforced concrete structures has drawn large public
attention. The lifetime of the structures has been considered to be semi permanent up
to the present. For this reason, many studies have been performed to discover the best
method of preventing corrosion in reinforcing bars. However, most of them have bee
n disproportionately concentrated on the improvement of concrete quality such as the
increase in concrete cover thickness, optimization of water-to-cement ratio or the
addition of corrosion resistant materials. In America and Europe, stainless steel rebars
with high corrosion resistance have already been applied to many concrete structures,
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2. such as road bridges, elevated highways, tunnels, parking garages, residential buildings,
and port facilities [1]. Stainless steel rebars have excellent properties as corrosion
resistant rebars, but their higher cost prevents from coming into wide use. If alloy
content (Cr, Ni, Mo, etc.) can be reduced preserving appropriate corrosion resistance,
new corrosion resistant rebars (hereinafter referred to as Cr-bearing corrosion resistant
rebar) that have good properties and cost performance should be designed.
As a fundamental study on the Cr-bearing corrosion resistant rebar with the necessary
corrosion resistance for use in steel reinforced concrete structures under corrosive
environments, This study was investigated to corrosion resistance of Cr-bearing
corrosion resistant rebars in simulated concrete pore solutions and concrete specimens
having chloride ion contents.
2. Corrosion resistance in simulated concrete pore solutions
2.1 Outline of the experiments and Methods of testing
2.1.1 Steels
Chemical compositions of steels are given in Table1. The steels containing Cr from 0
% to 16% were used to change the corrosion resistance in simulated concrete pore solut
ions. SD345 steel that is a typical carbon steel rebar and SUS304 steel that is a typic
al austenitic stainless steel were also used to compare the corrosion resistance to the
Cr-bearing corrosion resistant rebar. All steels were produced from 100kg vacuum-m
elted ingots. These ingots were hot forged and hot drawn at 1100-1150℃ to 13m
mφ wires. These wires except SUS304 steel were annealed at 700℃for 2hours.
SUS304 wires were annealed at 1100℃ for 5minutes. All wires were pickled to
remove surface oxides. The wires of the Cr-bearing corrosion resistant rebar and
SD345 steel had ferritic microstructure. The wire of SUS304 steel had austenitic
microstructure. These wires were sliced to 2mm thick plates for corrosion test
specimens.
Table1:Chemical compositions of steels used, mass%.
Steel C Si Mn P S N Cr Ni
SD345 0.2280 0.31 1.34 0.029 0.020 0.005 0.084 0.04
0Cr 0.0120 0.32 0.50 0.031 0.006 0.010 0.005 0.01
5Cr 0.0150 0.28 0.53 0.027 0.006 0.006 5.020 0.01
9Cr 0.0107 0.28 0.53 0.028 0.006 0.010 9.140 0.01
11Cr 0.0117 0.28 0.53 0.028 0.004 0.010 11.00 0.01
13Cr 0.0117 0.28 0.53 0.028 0.004 0.008 13.050 0.01
16Cr 0.0113 0.29 0.53 0.027 0.004 0.009 15.980 0.01
SUS304 0.0630 0.31 1.01 0.026 0.006 0.046 18.360 8.28
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3. 2.1.2 Pitting potentials
Pitting potentials were measured to evaluate the resistance to corrosion initiation. The
plate specimens were polished to #800 finish and degreased with acetone. These
specimens were coated by silicon sealant except a measuring area (10 mm in diameter).
Simulated concrete pore solutions were saturated Ca(OH)2 solutions containing 0.27%,
1.07%, and 21.4% NaCl. These NaCl concentrations correspond to the Cl-
concentrations in our research work [2] that investigated the corrosion resistance of Cr-
bearing corrosion resistant rebars in concrete with high chloride content. In the
research work, the concrete contained chloride ion (0.3, 1.2, and 24 kg/m3), water (185
kg/m3), and other stuffs (cement, sand, coarse aggregate, and air entraining agent).
0.27%, 1.07%, and 21.4% NaCl correspond to the concentration of 0.3, 1.2, and 24
kg/m3 of Cl- in 185 kg/m3 of water, respectively. The pH of the test solutions were
adjusted at 12.5, 11, 10, and 9 by HCl. Pitting potentials in deaerated these solutions
at 30℃ were measured using a potentiostat. Specimens were polarized in an anodic
direction at the rate of 20 mV/min after 10 min immersion in the solutions. Pitting
potentials were determined as the most noble potential at which the anodic current was
over 100µA.
2.2 Results and discussion
Figure1 shows the effect of pH on the pitting potential. In the case of 1.07% NaCl
solutions, pitting potentials decreased largely between pH 12.5 and 11. The decrease
of pitting potentials with pH drop was almost saturated in the range of less than pH 11.
5Cr steel showed considerably better pitting potential than that of 0Cr steel at pH 12.5.
However, its advantage became small at pH 11 as long as corrosion initiation like
pitting potentials was concerned. The steels containing 9%Cr and more showed good
corrosion resistance even in the range of less than pH 11. In the case of 21.4% NaCl
solutions, pitting potentials of all steels decreased gradually with the drop in pH.
16Cr steel showed good corrosion resistance even in 21.4% NaCl solutions at
8 0 0 8 0 0
(a) 1.0 7 % N a C l (b ) 2 1.4 % N a C l
6 0 0 6 0 0
Pitting potential (mV vs Ag/AgCl)
Pitting potential (mV vs Ag/AgCl)
4 0 0 4 0 0
1 6 C r
2 0 0 2 0 0
1 6 C r
0 5 C r
0
9 C r
9 C r
- 2 0 0 - 2 0 0
0 C r 5 C r
- 4 0 0 - 4 0 0
0 C r
- 6 0 0 - 6 0 0
8 9 1 0 1 1 1 2 1 3 8 9 1 0 1 1 1 2 1 3
p H p H
Figure1:Effect of pH on the pitting potentials of 0Cr, 5Cr, 9Cr,
and 16Cr steels in simulated concrete pore solutions containing.
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4. 1 6
neutralized condition. In our research work [2],
1 4
open circuit potentials of rebars in concrete were
measured. It was showed that the potentials could 1 2
rise up to 0V Ag/AgCl (~ -0.1 V CSE). Therefore, 1 0
S afe ar e a
if the pitting potentials of the steel are nobler than
Cr (%)
8
0V Ag/AgCl, the steel has few risks of corrosion. 6
Figure2 shows a safe area and a risk area in Cr 4 Ris k ar e a
content vs. NaCl concentration map judging from
2
pitting potentials in pH 12.5 solutions. Corrosion
0
resistance of rebars should be discussed in terms of 0.1 1 1 0 1 0 2
corrosion initiation and corrosion rate. Figure2 is N a C l (%)
just a guideline for corrosion initiation. However, if
corrosion does not initiate in a corrosion condition Figure2:Guideline map of Cr-
of steels, the steels are safe for use in the condition. bearing corrosion resistant re
The steels containing few percent of Cr have bars in simulated concrete
potential as corrosion resistant rebars in salt- pore solutions at pH 12.5.
damage environments.
3.Corrosion resistance in concrete having chloride ion contents
3.1 Outline of the experiments and Methods of testing
Table1 indicates the basic components of the reinforcing bars. Table2 shows the mix
proportion of concrete. Five levels of chloride ion contents, being 0.3, 0.6, 1.2, 2.4
and 24 kg/m3 of concrete were applied, which were adjusted using NaCl (first class
reagent). In this study, test specimens were made to form micro cell corrosion by
pouring concrete with particular chloride ion content after laying each rebar with
20mm concrete cover thickness as shown in Figure3. In the specimen, two types of
rebars were arranged at the left and right sides in two layers, with one type of rebar in
each layer. The specimen was sealed and cured for 7days and then with the forms
removed, it was cured in the air for one week in an area having constant temperature
and constant humidity (20±3℃, 50±5%). Following the above-mentioned curing
process, the acceleration of corrosion for each specimen was achieved by repeated high
and low temperature curing as well as wet and dry curing. Each curing cycle
consisted of one-day high temperature/humidity (temp. 60, humidity 95) curing and
one-day low temperature/humidity (temp. 30, humidity 50) curing.
The measurement of the weight loss by corrosion was performed in accordance with
the “method and criteria for testing concrete structures; and evaluation of corrosion of
rebars in concrete” published by the Japan Concrete Engineering Association [3].
Rust was removed from the rebars taken from the specimen concrete by dipping SD345,
0Cr and 5Cr rebars in a 10% aqueous solution of diammonium citrate and the rebar
with Cr contents exceeding 5% in 30% nitric acid, at which point their weights were
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5. measured in grams up to the second decimal place (0.01g) using an electronic balance.
After that, the weight loss was calculated using an equation (1).
∆W =
(Wo − W ) − Ws × 100 (1)
Wo
Where： ∆W = Weight Loss (%)
Wo = Reinforcing bar mass before the rust removing process (g)
W = Reinforcing bar mass after the rust removing process (g)
Ws = The non-corroded part of the rebar dissolution (g)
Table2:Mix proportion of concrete.
W/C Water Cement Sand Coarse s/a Air entraining Cl-
Aggregate agent
(%) (kg/m3) (kg/m3) (kg/m3) (kg/m3) (%) (g/m3) (kg/m3)
0.3, 0.6 , 1.2 ,
60 185 308 810 970 46 20
2.4, 24
Placing Epoxy Resin
a
20
100
Chloride addition concrete
20
20 20 10 a’ 10
100 370
a-a’ Cross section *Unit；mm
Figure3:Details of test specimen.
3.2 Results and discussion
3.2.1 Weight Losses
Figure4 shows the change in weight losses of various types of rebars by chloride ion co
ntent with an elapse of time. According to Figure4, as the age of corrosion accelerate
d curing increased, the corrosion loss showed a tendency to increase for all types
of rebars, and when the chloride ion content was the same, the corrosion loss of the
rebar shows a tendency to decrease as the Cr content of the rebar increased. In addit
ion, regardless of the rebar type, the corrosion loss increased as the chloride ion conte
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6. nt increased. Such tendency was conspicuous in the SD345 rebar and 0Cr rebar.
And the corrosion loss, which was less than 3% when the chloride ion contents were
0.3kg/m3 and 0.6kg/m3, increased rapidly as the chloride ion content increased. Such
phenomenon became conspicuous as the cycle of the corrosion accelerated curing incr
eased. In the case of the concrete having a chloride ion content of 1.2kg/m3, which
is the limit of chloride content for corrosion of the carbon steel rebar set out by the
Japan Society of Civil Engineers [4], while the corrosion loss ratio of the SD345 reb
ar exceeded 3% at the 155cycle of the corrosion acceleration curing, that of the 5C
r rebar was approximately 0.3%, which is very minute. When the chloride ion conte
nt is 2.4kg/m3, even for the 5Cr rebar the corrosion loss showed a tendency to slowl
y increase as the cycle of corrosion accelerated curing increased. However, for
the Cr-bearing corrosion resistant rebar with a Cr content of more than 9%, the co
rrosion loss measured was very small. In addition, when the chloride ion content
is 2.4kg/m3, for the Cr-bearing corrosion resistant rebar with a Cr content of more
than 11%, the corrosion loss measured was less than 1%.
9 9
Cl-:0.3kg/m3 Cl-:0.6kg/m3
Weight Loss （％）
Weight Loss （％）
6 6
3 3
0 0
0 50 155 0 50 65 155
9 9
Cl-:1.2kg/m3 Cl-:2.4kg/m3
Weight Loss （％）
Weight Loss （％）
6 6
3 3
0 0
0 50 65 155 0 50 65 155
9 Cycles of Wetting & Drying
Cl-:24kg/m3
Weight Loss （％）
6
SD345 0Cr
5Cr 9Cr
3
11Cr 13Cr
16Cr SUS304
0
0 50 65 155
Cycles of Wetting & Drying
Figure4:Change in Weight Loss.
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7. 3.2.2 Average Corrosion Rates
Figure5 shows the relationship between the chloride ion content and the average
corrosion rate by corrosion for different types of rebars. The average corrosion rate is
the average value during 50, 65 and 155 cycles of corrosion accelerated curing.
According to Figure5, the average corrosion rate increased along with the increase of
chloride ion content irrelevant to the types of rebars. Such tendency was conspicuous
in the SD345 rebar and 0Cr rebar having no Cr content. And the average corrosion
rate occurred to the SD345 and 0Cr rebar in the concrete containing a chloride ion
content of 0.3kg/m3. As well, the weight loss by corrosion did not increase
remarkably while the chloride ion content showed an increase up to 1.2kg/m3.
However, rapid increase in the average corrosion rates was observed when the chloride
ion content had increased to 24kg/m3 from 1.2kg/m3, and the measured average
corrosion rates for the SD345 rebar, which contains much carbon, was as high as that of
0Cr rebar. On the other hand, very minute average corrosion rate occurred to the 5Cr
rebar when the chloride ion content of the concrete was 1.2kg/m3, and the average
corrosion rate showed a tendency to slowly increase as the chloride ion content had
increased to 24kg/m3. However, for the rebar with Cr content exceeding 9%,
infinitesimal average corrosion rates was observed even in the concrete with chloride
ion content of 24kg/m3. This is due to the strong passive film that had formed on the
rebar surface by the reaction between chromium (Cr) and oxygen (O2) [5]. Generally,
it is known that Cr alloy having Cr content greater than 10% has a higher corrosion
resistance than carbon steel [6]. Moreover, Figure6 shows the effect of the chloride
ion content and Cr-content to the average corrosion rate of a rebar. According to
Figure6, the higher the Cr content, the less the average corrosion rate regardless of the
chloride ion content.
Therefore, according to the results of above weight losses and average corrosion rates,
it can be considered that the rebar with Cr contents above 5% and 9% have corrosion
resistant characteristics against concrete with chloride ion contents of 1.2kg/m3 and
2.4kg/m3, respectively.
10 16
4
Corrosion Rate （mg/cm /month）
SD345 0Cr 5Cr 9Cr
Average Corrosion Rate
Average Corrosion Rate
8 11Cr 13Cr 16Cr SUS304 13
(mg/cm2/month)
(mg/cm /month)
3
2
Cr content (%)
6 11
2
2
4 9
1
2 5
0
0 0
0.1
0.1 1.0
1.0 10.0
10.0 100.0
100.0
0.3 0.6 1.2 2.4 24.0
Concentration of Chloride Ion （kg/m3 ） 3)
Concentration of Chloride Ion (kg/m Concentration of Chloride Ion (kg/m3)
Figure5:Change in Average Corrosion Rate. Figure6:Effect of the chloride ion content
and Cr content to the Average Corrosion Rate.
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8. 4. Conclusion
For the purpose of developing a Cr-bearing corrosion resistant rebar that can be used
under corrosive environments, experiments were conducted on the corrosion resistance
of Cr-bearing corrosion resistant rebars in simulated concrete pore solutions and
concrete specimens having chloride ion contents.
The following is the knowledge obstained from these experiments:
1. Corrosion resistance of the steels in simulated concrete pore solutions was increased
with Cr content.
2. Increase of chloride content and drop in pH of the solutions deteriorated the pitting
potentials of the steels. Decrease of pitting potentials in the solutions with the drop
in pH was large between pH 12.5 and 11.
3. 5Cr and 9Cr steels showed good corrosion resistance in 1.07% NaCl solutions at pH
12.5 and pH 10, respectively. Corrosion rate of 5Cr steel was smaller than that of
0Cr steel in 1.07% NaCl solutions at pH 10.
4. 16Cr steel showed as good corrosion resistance as SUS304 steel even in 21.4% NaCl
solutions at pH 10.
5. Irrelevant of the chloride content, the average corrosion rate was low when the Cr
content was high. From this it is considered that the corrosion resistance against
the chloride has improved due to the formation of passive film by chromium.
6. For the chloride ion contents, the Cr contents required for corrosion prevention are
as follows:
Cl-（kg/m3） Chloride ion contents（%more）
1.2 5
2.4 9
24 11
5.Reference
1. F. N. Smith and M. Tullmin, “Materials Performance”, May, 72, 1999.
2. S. H. Tae, T. Noguchi, M. Kanematsu and T. Ujiro, “FIB2002 Osaka Congress”, Sess
ion 8, p.155, 2002.
3. Japan Concrete Institute, “Examination method and criterion about corrosion, corrosi
on resistance of a concrete construction (plan), JCI - SC1 -corrosion evaluation
method of steel materials in concrete-”, pp. 1-2.
4. Japan Society of Civil Engineers, “Concrete standard specification (construction), -du
rability collation type-”, 2000.
5. Stainless steel association, “Basics knowledge of stainless steel”, p.2, 1984.
6. H.H. Uhlig, “Corrosion Handbook”, p.150, 1953.
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