CHLORIDE ION CONCENTRATION ON THE DISCOLORATION
BOUNDARY PRODUCED BY SPRAYING AgNO3 SOLUTION
Yusuke Aoki*, Kisarazu National College of Technology, Japan
Keiji Shimano, Kisarazu National College of Technology, Japan
Masashi Suzuki, Toden-Kogyo Co., Ltd., Japan 35th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 27 August 2010, Singapore Article Online Id: 100035022
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CHLORIDE ION CONCENTRATION ON THE DISCOLORATION
BOUNDARY PRODUCED BY SPRAYING AgNO3 SOLUTION
Yusuke Aoki*, Kisarazu National College of Technology, Japan
Keiji Shimano, Kisarazu National College of Technology, Japan
Masashi Suzuki, Toden-Kogyo Co., Ltd., Japan
Abstract
Experimental study on the chloride ion concentration (Cbw) in concrete at the
boundary of the colored zone by spraying of AgNO3 solution was carried out. In
ranges of water-cement ratio (W/C) and sand-coarse aggregate ratio (s/a) of normal
concrete, it is found that Cbw strongly depends on changes in W/C but hardly depends
on changes in s/a. At each W/C, Cbw and the limit chloride ion concentration for
corrosion initiation of steel in concrete (Clim) was compared. Then, Cbw and Clim was
found to become about the same for concretes with W/C range of 0.50~0.55.
However, Cbw was found to become higher than Clim for concrete with W/C of about
0.4, and become lower than Clim for concrete with W/C of about 0.6.
Keywords: Chloride ion, AgNO3 solution spray method, Corrosion of steel
1. Introduction
For deterioration diagnosis of reinforced concrete members under salt attack environments, it is
important to predict the time of corrosion initiation of reinforcing bar induced by the chloride ions
migrated into the concrete. As a method to realize this, for example, JSCE-G573-2007, “Measurement
method for distribution of total chloride ion in concrete structures” has been standardized by Japan
Society of Civil Engineers [1]. In this method, a concrete core is taken or powder of concrete is taken
by drilling from actual structures. Total chloride ions contained in it is measured by chemical analysis.
From these measurement results, distribution of total chloride ions in the concrete cover is evaluated.
When total chloride ions at the location of reinforcing bar surface exceed the Clim, reinforcing bar
corrosion is judged to initiate. When these do not exceed Clim at the time of investigation, its progress in
future is predicted by the distribution of the total chloride ions. Then the time at which total chloride ions
at the depth equivalent to the reinforcing bar surface exceed Clim is calculated.
A drawback of this method is that the status of
chloride ion migration cannot be confirmed
immediately and visually. Another is that total
chloride ions in concrete at the depth equivalent to
the reinforcing bar surface must be compared with
Clim. Furthermore, for prediction of the status of
chloride ion migration after the investigation, a
complicated
calculation
based
on
physical
phenomena such as diffusion or advection must
be performed.
Otsuki et al. reported that when 0.1 mol/l
Photo-1 The surface of concrete specimen
after spraying AgNO3 solution
AgNO3 solution is sprayed on the surface of the
hardened cement containing chloride ions, white precipitate of AgCl is formed and the region
containing more than the specific concentration changes into white color [2]. Photo-1 shows its
appearance. In this report, Otsuki et al. pointed out that the amount of soluble chloride ions contained
in the hardened cement paste at the boundary of whitely colored zone became constant regardless of
difference in W/C or of existence of aggregates. He also found that this amount was 0.15% of the unit
cement mass, and it accidentally coincided with the Clim specified by ACI.
If the research results of Otsuki et al. are utilized, it is possible to propose a method of predicting
the time of corrosion initiation of the reinforcing bar without the drawback mentioned above.
First, a
concrete core is taken from the subject reinforced concrete member for investigation. Then, the core
taken is split in the manner of the split tensile test. 0.1 mol/l AgNO3 solution is sprayed on the split
surface. If appeared colored zone reaches the depth of the reinforcing bar surface, the reinforcing bar
is judged to be at the corrosion initiation. If the colored zone has not reached the reinforcing bar depth,
the depth of colored zone is predicted to advance at a rate proportional to the root of time, similar to the
method of predicting neutralization depth. Then time at which the colored zone reaches the cover
depth is calculated and this time is judged to be initiation of reinforcing bar corrosion. According to this
method, chemical analysis to measure chloride ion is not required. The way in which the Clim
approaches to the reinforcing bar surface can be confirmed immediately and visually. Comparison of
the chloride ions at the depth of reinforcing bar surface in concrete with Clim is also unnecessary.
Prediction of the chloride ion migration after the time of the investigation is realized by very simple
calculations.
In order to establish a procedure to predict the corrosion initiation time by this method, it is
necessary to confirm whether the chloride ion concentration contained at the boundary of the colored
zone, Cbw, agrees with the Clim specified or proposed by other than ACI. In this paper, this will be
verified experimentally [3]. That is, by using mortar specimens made at mix proportions in a range of
normal concrete, experiments of measuring Cbw are performed. Then, the measured Cbw is compared
with the Clim proposed by Horiguchi et. al. [4], and with Clim specified by EN206 [5].
2. Experimental Program
The specimens are made of mortar with 6 patterns of mix proportion.
2.1 Experimental Flow
Specimens are cured for 32 days.
The flow of the experiment to
measure Cbw is shown in Fig.-1.
Specimens are immersed into a 10% NaCl solution over 25 days.
Each stage of the work is as
Specimen is split. AgNO3 solution is sprayed on the sprit surface.
shown in Photo-2(a)-(f).
The boundary of the colored zone is marked.
2.2 Materials and Mix Proportion
Powder sample is taken from the marked location.
The specimens are made of
mortar with 6 patterns of mix
proportion shown in Table-1. The
Total chloride ion concentration in the powder sample is measured.
Fig.-1 Experimental flow to measure Cbw
mix proportions of these mortars
are determined respectively based
on the mix proportions of the
assumed
concrete
shown
in
Table-2. Mix proportions of the
mortars shown in Table-1 is made
by replacing the volume fraction of
coarse aggregate of the assumed
concrete shown in Table-2, with
water, cement, fine aggregate, and
air without changing proportion of
Photo.-2 Experimental work
each constituent. For example, the
mortar
with
m0.4-0.4
mix
proportion is the same quality as
the
mortar
contained
in
the
assumed concrete with c0.4-0.4
mix proportion. The first letter m
stands for mortar and letter c
stands
for
concrete.
The
proportion
and
the
right
number stands for the s/a of the
assumed concrete. Among the
mixes
number,
having
the
the
fine
same
right
aggregate
proportion is kept almost the same.
This is to clarify the relationship
between the Cbw and W/C.
m0.4-0.4
m0.4-0.5
m0.5-0.4
m0.5-0.5
m0.6-0.4
m0.6-0.5
W/C
s/a
0.4
0.5
0.6
1.0
Air
(%)
8.4
7.5
8.4
7.5
8.4
7.5
3
Unit weight　(kg/m )
C
S
Ad.
W
260 649 1176 2.44
233 584 1321 2.19
285 570 1176 2.14
256 512 1321 1.92
305 508 1175 1.90
275 459 1315 1.72
left
number stands for the W/C of the
mix
Table-1 Mix proportions of mortar
mix
propotion
Table-2 Mix proportions of assumed concrete
3
Air
Mix
Unit weight　(kg/m )
W/C
s/a
propotion
(%)
C
S
G
Ad.
W
0.4
155
388
702 1129 1.45
c0.4-0.4
0.4
0.5
155
388
877
941 1.45
c0.4-0.5
0.4
170
340
702 1129 1.275
c0.5-0.4
0.5
5.0
0.5
170
340
877
941 1.275
c0.5-0.5
0.4
182
303
701 1128 1.138
c0.6-0.4
0.6
0.5
183
305
875
938 1.138
c0.6-0.5
W ： tap water
3
C ： ordinary portland cement （dens. 3.15g/cm ）
3
S ： fine aggregate （pit sand，dens. 2.62g/cm ，F.M.2.63）
3
Ｇ ： coarse aggregate（crushed stone，dens. 2.80g/cm ，5-20mm）
Ad. ： air-entraining and water-reducing agent
2.3 Specimens
Three specimens are made for each mix proportion. The specimen is cylinder of 100 mm diameter
and of 200 mm high. After placing mortar into a plastic mold, the top surface of the mold is covered
tightly with a vinyl sheet. Specimens are cured in this condition at 20±1.0°C in a room for 32 days.
After the hardened specimen is removed, epoxy resin is provided at the top and bottom surfaces of the
specimen. This is to limit the direction in which chloride ions migrate into the specimen to the radial
direction. It can be understood important from Photo-2(f) showing a condition of sampling. After
finishing this procedure, the specimens are immersed into a 10% NaCl solution at room temperature.
The immersion period is set not less than 25 days.
2.4 Spraying of AgNO3 solution
As shown in Photo-2(c), the specimen finished with the immersion to salt water, is split at its center.
Then as shown in Photo-2(d), 0.1 mol/l AgNO3 solution is sprayed on the split surface of the specimen.
After that, if the split surface is dried by a dryer, the colored zone becomes clearer as shown in
Photo-2(e). When the colored zone becomes distinct, the boundary of the colored zone is marked.
The marked boundary of the colored zone is drilled by a fixed drilled as shown in Photo-2(f). Powder
sample used for the chloride ion analysis is taken from the drilled location. The diameter of the blade of
the drill to be used is 3 mm.
2.5 Measurement of Chloride Ion Concentration
Total chloride ion concentration contained in the powder sample taken is measured by the
potentiometric titration method with a chloride ion electrode which is one of the standardized methods
in JIS A 1154:2003, “hardened concrete” [6]. In this test method, total amount of chloride ions
contained in the powder sample is extracted with nitric acid and its mass rate to the sample is
measured.
3. Results and Discussions
3.1 Measurement Results of Cbw
The mass rates of chloride
Table-3 Measurement and conversion results of Cbw
Mix
-
Mix
3
ions into the sample taken from
propotion
Cl mass(%)
in powder sample
propotion
Cbw (kg/m )
in assumed concrete
the specimen of each mix are
m0.4-0.4
0.475
c0.4-0.4
5.60
m0.4-0.5
0.382
c0.4-0.5
5.17
m0.5-0.4
0.312
c0.5-0.4
3.50
As W/C in the mix becomes
m0.5-0.5
0.282
c0.5-0.5
3.65
higher, the rate of chloride ion
m0.6-0.4
0.204
c0.6-0.4
2.19
m0.6-0.5
0.202
c0.6-0.5
2.53
shown in the left side of Table-3.
becomes
lower.
Among
the
mixes of the same W/C, the rate
of chloride ion is lower for the mix with higher s/a.
In the right side of Table-3, Cbw in the assumed concrete mix described in Table-2 is shown. These
are calculated based on the measurement results in the left side of Table-3. This calculation method is
explained as follows. As mentioned above, the values shown in the left side of Table-3 are the mass
rates of chloride ions contained in the sample taken from the boundary line of the colored zone. The
mortar of each specimen is of the same quality as the mortar in the corresponding assumed concrete.
Therefore, if the values in the left side of Table-3
6.0
are multiplied by the unit mass of the mortar in
Cbw= -15.15(W/C)+11.35
5.0
ion contained in the mortar of the assumed
concrete is calculated. Then if no chloride ion is
assumed to be contained in coarse aggregate in
Cbw (kg/m 3)
the assumed concrete, unit mass of the chloride
4.0
3.0
2.0
s/a 0.4
the assumed concrete, the unit mass of chloride
ion calculated as explained above becomes Cbw
of the assumed concrete mix.
s/a 0.5
1.0
0.0
0.3
Relationship between Cbw and W/C in the
0.4
0.6
0.7
W/C
assumed concrete mix calculated by the
procedure above, is shown in Fig.-2. Cbw and
0.5
Fig.-2 Relationship between Cbw and W/C
W/C is almost drawn as a linear relationship. As
W/C becomes higher, Cbw becomes lower. As
relationship becomes less steep. As far as s/a
being in a range from 0.4 to 0.5 that is the mix
proportion of the assumed concrete, there
seems no significant difference between the
both. In this case, it is important to obtain a
regression line for all the data, like the straight
line in Fig.-2. Equation of the regression line is
Total Cl- concentration (kg/m 3)
s/a becomes higher, slope of the linear
6.0
Cbw obtained from eq.(1)
5.0
4.0
3.0
Clim-EN
2.0
1.0
0.0
0.3
shown as below.
C bw  15.15(W / C )  11.35
Clim-H
0.4
0.5
0.6
0.7
W/C
(1)
Fig.-3 Comparison between Cbw and Clim
By using equation (1), if it is like a concrete mix
shown in Table-2, Cbw can be calculated by the known W/C [3].
3.2 Comparison between Cbw and Clim
Fig.-3 shows comparison between Cbw and Clim. The Cbw here is a value obtained from equation (1).
Two values are indicated as Clim. Clim-H is that in the concrete of W/C 0.45, 0.55, and 0.65 (unit cement
content of 254kg/m3, 291kg/m3, and 362kg/m3), recently reported by a paper by Horiguchi et. al. [4].
Horiguchi et. al. also reported the maximum, minimum, and average of Clim measured from specimens
of each mix. Values indicated in Fig.-4 are the averages. On the other hand, Clim-EN is the Clim specified
in EN206 [5]. In the original EN206, Clim is regulated as 0.4% of cement mass. Here, average value of
unit cement content in c0.4-0.4 mix and c0.4-0.5 mix is calculated, and it is multiplied by 0.4% to obtain
Clim-EN for the case of W/C 0.4. The same procedure is applied for W/C0.5, and W/C0/6.
According to Fig.-3, in a range between W/C0.50 and W/C0.55, Cbw becomes nearly equal to the
Clim-H reported by Horiguchi et. al.. However, in the higher W/C range, Cbw becomes lower than Clim-H.
This signifies that reinforcing bar corrosion initiates when the colored zone boundary appeared by the
AgNO3 solution spraying passes over the cover depth for the reinforcing bar. In the lower W/C range on
the contrary, Cbw becomes higher than Clim-H. This signifies that reinforcing bar corrosion initiates
before the colored zone boundary reaching the cover depth of reinforcing bar. Cbw and Clim-EN becomes
close in the case of W/C0.6. But in cases with the lower W/C, Cbw largely exceeds Clim-EN.
4. Conclusions
Chloride ion concentrations (Cbw) contained in concrete at the boundary of the zone colored by
spraying AgNO3 solution are experimentally investigated. Especially, relationship between Cbw and the
limit chloride ion concentration for initiating steel corrosion in concrete (Clim) was intensively
investigated. The following findings were confirmed in this study.
(1) For concrete in a range of normal mixes, Cbw highly depends on the change in W/C but hardly
depends on the change in s/a.
(2) Cbw and Clim become almost the same in concrete with a range of W/C from 0.5 to 0.55.
However,
in concrete with W/C of around 0.4, Cbw becomes higher than Clim. In concrete with W/C of around
0.6, Cbw becomes lower than Clim.
(3) Understanding the characteristics mentioned above, this method can be used for prediction of
corrosion initiation time in the reinforced concrete member. The similar study will be carried out in
future for concrete made of blastfurnace slag cement or fly ash cement.
Acknowledgement
The authors would like to express special thanks to Ms. Atsuko Ono, Ms. Yuka Miyoshi, and Mr.
Hiroki Nishi, students of Kisarazu National College of Technology, for their assistance to the study, and
to Dr. Shin-ichi Miyazato for his precious advices.
References:
[1] Japan Society of Civil Engineers, "Standard Specifications for Concrete Structures -2007 Test
Methods and Specifications –JSCE STANDARDS-", JSCE, 2007, pp.289-296
[2] Otsuki et al., “Evaluation of AgNO3 Solution Spray Method for Measurement of Chloride Penetration
into Hardened Cementitious Matrix Materials” ,ACI Materials Journal,No.84, Nov.1992,pp.587-592
[3] Aoki et al., “Estimation of chloride ion penetration to hardened concrete by AgNO3 solution spray
method”, Proceedings of the Japan Concrete Institute, Vol.30, No.1, 2008, pp.759-764
[4] Horiguchi et al., “An Effect of Concrete Mix Proportion on Chloride Threshold Value”, Proceedings
of the Japan Concrete Institute, Vol.29, No.1, 2007, pp.1377-1382
[5] A.M.Neville, ”Properties of concrete Fourth and Final Edition “, Pearson Education, Jun 2004
[6] Japanese Industrial Standard A1154:2003, "Methods if test for chloride ion content in hardened
concrete," Japanese Standards Association, 2003