CONCRETE LIBRARY OF JSCE NO. 34, DECEMBER 1999
AN EXPERIMENTAL STUDY ON PENETRATION OF CHLORIDE IONS INTO CONCRETE
AND CORROSION OF REINFORCING BARS IN VARIOUS MARINE ENVIRONMENTS
(Translation from Proceedings of JSCE,
No.599/V -40, August 1998)
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Takashi IDEMITSU
Exposure tests on concrete in marine environments were conducted over a period of 10 years to
establish a rational design method for concrete structures that takes account of the durability. In these
tests, the penetration of chloride ions, the corrosion of reinforcing bars, and changes in concrete quality
were investigated. The following findings were obtained: (1) Chloride ion distribution can be predicted
using the chloride ion diffusion coefficient (Dc) and the surface chloride ion concentration (Co)
obtained from exposure tests; (2) The rate of chloride ion penetration in marine environments is fastest
in the splash zone, followed by the underwater and atmospheric zones; (3) In marine environments,
properly treated construction joints and cracks under 0.1mm in width do not significantly advance
reinforcement corrosion for an exposure period of up to 10 years.
Key words: diffusion coefiicient, exposure test, marine environment, penetration of chloride ions,
reinforcing bar corrosion
Nobufumi Takeda is a deputy chief research engineer at the Technical Research Institute of Obayashi
Corporation, Tokyo, JAPAN. He obtained his D.Eng. from Kyushu Institute of Technology in 1999.
His research interests include the durability design and durability improvements of concrete structures.
He_i_s a member of the JSCE and J CI.
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Shigeyuki Sogo is a General manager at the Technical Research Institute of Obayashi Corporation,
Tokyo, JAPAN. He obtained his D.Eng. from Kyushu Institute of Technology in 1991. His research
activities relate to the development of special concreting methods such as underwater concrete, mass
concrete, and self-compacting concrete, and the durability design of concrete structures. He is a
member ef the J$CE.e11dlCI@.
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Shigemi Sakoda is a professor in the School of Marine Science and Technology of Tokai University,
Tokyo, JAPAN. He obtained his D.Eng. from Tokai University in 1992. He has been involved in
research on the properties of concrete with inferior quality aggregates and the durability of concrete in
marine environments. He is a member of the ACI JSCE and JCI.
Takashi Idemitsu is a professor at Kyushu Institute of Technology, Fukuoka, JAPAN. He obtained his
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D.Eng. from Kyushu Institute of Technology in 1992. His research interests include prestressed
concrete using reinforcing bars, self—compacting concrete, and the quality assurance of concrete
structures. He is a member oft_he,JSCE and JCI.
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1
. INTRODUCTION
In order to design concrete structures for a marine environment with a proper consideration
of durability,
it is vital to clarify the rate of chloride penetration into concrete and the rate of reinforcement corrosion.
There have been numerous reports on both these topics, mainly based on investigations
of structures
constructed in marine environments and exposure tests in such environments [1]. According to these
reports, chloride ion concentration, moisture content, and oxygen supply in the concrete all affect the
corrosion of reinforcing
steel, and the splash zone is generally
considered
to be the severest
environments with regard to concrete structure durability,
followed by the underwater and atmospheric
zones. It has also been reported that the chloride ion limit, at which reinforcement corrosion begins to
occur, lies between 1.2 and 2.5 kg/m3[2],[3].
Further, estimation of long-term chloride ion penetration
has been attempted by measuring chloride ion concentrations near surfaces and using the diffusion
coefficient
of chloride ions in Pick's diffusion equation [4],[5],[6].
However, in a report in which a number of investigations
of chloride penetration
into concrete in
various marine environments were analyzed, it is pointed out that chloride ion concentrations in marine
structures vary widely even in environments that may be classified into the same category [7],[8].
In
addition to these wide variations, the marine environment varies according to geographical
location,
position of the structure, and the arrangement of structure members, making it difficult to correlate their
influence on the rate of chloride penetration
and progress of reinforcement corrosion. This has
hampered the elucidation
of the effects of different marine environments and construction parameters,
such as cracks and construction joints, on chloride ion penetration in concrete and reinforcement
corrosion.
In this study, exposure tests on concrete specimens in different marine environments, i.e., the splash
zone, underwater, and the atmospheric zone, were conducted over a period of 10 years with the aim of
establishing
a rational design method for concrete structures in marine environments taking into account
durability.
This paper reports on the estimation of chloride penetration and rate of reinforcement
corrosion based on the results of these exposure tests, and the following issues are discussed:
(1) Influence of different environmental conditions on the quality of concrete, chloride ion penetration,
and reinforcement corrosion
(2) Influence of types of cement on concrete quality, chloride ion penetration,
and reinforcement
corrosion
(3) Influence of cracks and construction joints on the rate of reinforcement corrosion
2
. METHODSOF EXPERIMENT ANDANALYSIS
2.1 Outline of experiment
Three experiments were implemented with the following aims:
Experiment I: Experiment to elucidate the effects of different environmental conditions
Experiment II: Experiment to elucidate the effects of cement types
Experiment III: Experiment to elucidate the effects of cracks and construction joints
For each experiment, concrete specimens encasing reinforcing
steel were exposed to different marine
environments for 10 years. Changes in the quality
of concrete, chloride
ion penetration,
and
reinforcement corrosion over time were investigated
periodically.
Table 1 gives the combinations of
experimental conditions, such as exposure, type of cement, and presence of cracks and construction
joints. Four exposure conditions were selected: splash zone, atmospheric zone, underwater, and inland.
Three cements were used: ordinary portland cement, Type B blast-furnace slag cement, and sulfateresistant portland cement. Specimens with cracks or construction joints were also prepared using
ordinary portland cement and were exposed to the same four environmental conditions.
90-
Table 1 Combinations of Experimental
E xposure env ironm ents
Sp lash A tm ospheric underw ater Inland
zone zone
o
o
o
o
O rdinary Portlan d cem ent (O P)
S tand ard T ype B blast-furnace slag cem en t (B B )
o
o
b eam
o
S ulfate-resistantPo rtland cem ent (SR )
o
C rack ed O rdinary portland cem ent (O P)
o
o
o
o
b eam
Jointed O rdinary portland cem ent (O P)
o
o
o
o
b eam
T y pes o f
sp ecim en
nnnDeformed bar
900
/
j
Ba 3
(a ) S ta n d a rd b e a m
(b ) J o in te d b e a m
c
rn
fro
m
0,
W
[ ]
- - M
m
*
T y pes of cem ents
Table 2 Physical Properties and Chemical
Composition of Cements
P ro p e rtie s
T yp es o f
^
S p ec ific
-
g rav ity
O rd in a ry
,
(c) Crackedbeam
.
.
mm)
Us
v Jt-f
l .4 . u
Ur
1 4n1a
Uc
W e
bIoiSll aasi-iurnace
of chloride
S O 2 A 12O 3 F e2 O 3
C aO
M gO
SO 3
3 .1 5
3 ,3 0 0
0 .5
0 .3
2 1 .7
5 .3
3 .0
6 4 .7
1 .3
2 .2
3 .0 4
3 ,4 4 0
0 .9
0 .5
2 5 .2
7 .3
2 .2
5 7 .5
2 .8
1 .7
3 .1 8
3 ,3 7 0
0 .8
0 .1
2 2 .1
4 .1
4 .5
6 4 .3
1 .0
1 .9
slagcement
ion penetration)
P o rtla n d c e m e n t
Portland
cement
Fig.l Type, Shape and Size of Beam Specimens
Table 3 Properties
M a te ria ls
F in e a g g r e g ate
C o a rs e a g g re g a te
2
In so l.
s la g c e m e n t
S u lfa te -re sis ta n t
( 4" Direction
(c m /g ) Ig .lo ss
T yp e B
10 0
(unit.
C h e m ic a l c o m p o s itio n s ( % )
B la m e
tm e n es
c e m e n ts
P o rt la n d c e m e n t
IO C
Conditions
of Aggregates
P ro p e rtie s
L a nd sa n d , S p e cific g ra v ity : 2 .5 8
A b so rp tio n : 1 .6 4 % , F in en e ss m o du lu s : 2 .6 7
C ru sh e d ston e , G m ax : 15 m m
S p ec ific g rav ity : 2 .6 6 , A b so rp tio n : 1 .0 9 %
.2 Specimens
a) Composition, shape, and size
Three types of specimen were prepared for exposure: reinforced concrete beams for investigation
of
reinforcement corrosion (RC specimen), plain concrete cylinders
for compression tests, and plain
concrete cylinders for chloride penetration analysis. The reinforced concrete beams were 9 by 18 cm in
cross-sectional
size and 90 cm in length, as shown in Fig. 1. These beams had deformed reinforcing
bars embedded with a cover depth of 2 cm. These were SD 295/D19 bars with transverse ribs
conforming to JIS G 3112. Both sides and ends of the beams were coated with a thick-film epoxy resin
to allow chloride ion penetration
only from both edges. Whereas cylinders for the compression tests
were 10 cm in diameter and 20 cm in height, those for chloride penetration
analysis were 15 cm in
diameter and 15 cm in height. The curved surface and one end of each cylinder for chloride penetration
were coated with a thick-film epoxy resin to allow chloride ion penetration only from one end.
The reinforced specimens included some with a construction joint (jointed
beams) and some with
bending cracks (cracked beams) prepared for exposure in experiment III. Jointed beams had a
construction joint at mid-length.
At 7 days after placement of the first half, the joint surface was
wirebrushed to a depth of 0.5 mmor deeper to remove the laitance layer, and the second half with
identical mixture proportions was placed. Cracked beams, with four to five pre-formed cracks per beam,
were fastened into jigs to maintain the crack width at 0.05 to 0.1 mmat the surface.
-91~
Table 4 Mix Proportions
O P
B B
SR
G m ax w /c
(m m ) %
T yp es o f
ce m e nt
Sym b ol
O rd inary
p o rtlan d ce m e nt
T yp e B
b last-furnac e
slag cem en t
S u lfate-resistan t
P o rtlan d ce m e nt
Ad : air-entraining
and Properties
15
s/a
%
of Fresh Concrete
U nit w e igh t (k g/m )
w
c
s
G
1 6 6 3 3 2 8 4 7 9 3 5 1.0 4 1 2 .0
5 .0
5 0 .0 4 8 .0 1 6 2 3 2 4 8 5 1 9 3 5 1 .0 1 1 0 .5
4 .4
1 6 6 3 3 2 8 4 9 9 3 7 1.0 4 1 3 .0
4 .1
and water-redueing-agent
A g e (d a y ) 7
14
35
D
HD
(a ) S ta n d a rd b e a m ,
P la c in g C on structio n
jo in t
D
DD
(b ) J o in te d b e a m . A
D1 M ak in g crac ks
(c)
C
uring
method!
p
F resh c on crete
S lu m p A ir co% ntent
A d*
cm
50
E x p o su re
start
Cracked beam
*+à"*
M-*
M
Moist
I
Coating
| Dryingin
|
curin§
atmosphere
Fig.2 Flow of Specimen Preparation
b) Materials and mixture proportions
The following three types of cements were used: ordinary portland cement, Type B blast-furnace slag
cement, and sulfate-resistant
portland cement. Table 2 gives the physical properties
and chemical
compositions
of these cements. Land sand and crushed stone were used as the fine and coarse
aggregates, respectively,
in every experiment. Table 3 gives the properties
of the aggregates. An airentraining and water-reducing agent containing modified lignosulfonate
as the main component was
used as a chemical admixture.
Table 4 gives the concrete mixture proportions and the properties of the fresh concrete. Concretes made
using ordinary portland cement, Type B blast-furnace slag cement, and sulfate-resistant
portland cement
are hereafter referred to as "OP," "BB", and "SR," respectively.
Whereas OP, BB, and SR were used in experiments I and II, only OP was used in experiment III. All
concretes were proportioned to have a water cement ratio of 50%, a cement content of around 332 kg/m3,
a slump of 12.0+1.5 cm, and an air content of 4.5± 1.5% in consideration
of the maritime conditions.
c) Preparation of specimens
Figure 2 shows the flow of specimen preparation from concrete placing to the beginning of exposure, as
well as the curing methods. The specimens were wet-cloth-cured up to an age of 14 days. They were
then dried in preparation
for epoxy coating on the specified
surfaces, air-dried, and exposed to the
marine environment at an age of 50 days. Beams to be precracked were loaded at an age of 14 days such
that one of the permeable edges formed the tension edge, thus inducing bending cracks. After unloading,
pairs of cracked beams were fastened together with jigs as shown in Fig.l before being coated on both
sides.
2
.3 Exposure conditions
The exposure site was a breakwater in Shimizu Harbor, Shizuoka Prefecture.
location of the exposure site. Figure 4 and Table 5 indicate the exposure positions
-92-
Figure 3 shows the
of the specimens on
the breakwater and the conditions
to which each position was exposed, respectively.
The marine
environments were three: the splash zone between high and low tide (splash),
in the atmosphere and
subjected to salt spray only during the strong winds (atmospheric),
and in the sea at a depth of ll m
(underwater). An inland site was also selected at a point 30 km from the coastline with little influence
from air-borne chloride ions (inland).
Whereas specimens were exposed to all four environments
(splash-zone,
atmospheric, underwater, and inland) in experiments I and III, the environments for
experiment II were only atmospheric and underwater.
E138
Gauze-covered wood frames
30'
tiu-Mucm;\
/Specimens
Measurementpoint ol
Air-borne chloride
ions
Atmospheric zone
+5.0
0.5'
m
Splash zone
Breakwater
H.W.L
+1.7"
L.W.L
0.0
1"
Underwater
l
Direction
Fig.4 Exposure Positions
Fig.3 Location of Exposure Site
Table 5 Environmental Conditions
E n v iro n m e n t
E n v iro n m e n ta l c o n d itio n s
A tm o sp h e ric Z o n e 5 m h ig h e r th an L .W .L
B
A ffec ted b y s p in d ri ft a n d sea b re ez e
zo ne
D ep th o f s eaw ate r
:llm
S e aw ate r p rop erties :
A v e ra g e te m p e ra tu re
: 1 8 .4 C
c U n d e rw a te r
pH
: 8 .2 9
C h lo rid e io n c on c en tra tio n
: 1 8 .4 % o
Ins olub le o x yg en
: 7 .98 p p m
In la n d
of Specimens on Breakwater
of Exposure Positions
A S p la s h z o n e B e tw e en H .W .L (+ 1.7 m ) an d L .W .L (0 .O m )
D
-l.0m
of chloride ion penetration
L ev el 5 0 0 m o ff
S h im iz u p o rt
A v e rag e
tem p e ra tu re : 1 6 .0 C
P rec ip ita tio n
:2 3 60 m m /y ea r
3 0 km fro m c o astlin e (K iyo se , T o ky o )
A v e rag e te m p e ra tu re
: 1 5 .3 C
P rec ip itatio n
: 1 4 6 0 m m /y ea r
Also set up on the breakwater were a capture
vessel for air-borne chloride ions as proposed by
the Public
Works Research Institute
of the
Ministry
of Construction
[9], and four gauzecovered wooden frames. These were positioned
0.5 m above the top of the breakwater, so were in
the atmospheric zone. The amounts of air-borne
chlorides
(NaCl equivalent)
from four directions
were measured using these devices every month
from July
through
December
1995.
The
measurements are shown in Fig.5. The values
varied widely depending on direction,
with the
maximumand minimum of the averaged catch
over the period in one direction being 1.5 and 0.32
mg/dm2/day, respectively.
The average of airborne chloride ions from all directions
over the
period was 0.77 mg/dm2/day. The average of air-
-93
^ >
s
'3
10
ll
12
Month of measurement (1995)
Fig.5 Result of Measurements of Amount of
Air-borne Chloride Ions
borne chloride
2
ions from all directions
measured using the capture vessel was 0.22 mg/dm2/day.
.4 Measurements and methods
The measurements made and methods used are given in Table 6, and the times of each measurement are
given in Table 7. Compressive strength and progress of reinforcement corrosion were measured by
recovering specimens from the sites at 1, 3, 6, and 10 years of exposure. The chloride ion concentration
was quantitatively
analyzed in terms of total salts by potentiometric
titration with chloride ion-selective
electrodes, using concrete drill dust samples from each 2 cm in depth at 1 and 6 months and 1, 2, 3, 4, 6,
and 10 years of exposure. The chloride ion concentration was expressed as a percentage by weight of
concrete. Corrosion of reinforcing steel was evaluated by cleaving the beams and measuring the ratio of
corroded area to total reinforcement surface area.
Table 6 Measurement Items and Methods
Ite m s
M eth o d s
C o m p ressive
streng th
C o m p ressive stren gth test (JIS A 11 08 )
. S p ec im en s : diam eter lOc m , len g th 2 0c m
N eu traliz atio n
M easu re th e d ep th o f z on es un co lo red b y
p h eno lp hthalein so lu tion sp ray
C h lo rid e io n
c on ten t
Q u an titativ e d ete rm ina tio n of tota l salts b y p o te ntiom etn c titra tio n
w ith ch lo rid e io n-selec tiv e electro d es
S am p ling o f co n cre te d rill du st from each 2 cm in de pth
R e in forcin g
b ar co rrosio n
Sk e tc h co rro de d p arts a nd ev alu ate p erc en tage c orro d ed are a
to to tal reinforcem en t surface are a
C h em ical
c om p ositio n
P ow d er X -ray diffractio n an aly sis
P orosity
/v dp istrib
o re size
u tio n '¥
M ercury m strusio n m etho d b y p orosim eter
2
M ea su re m e nt ra ng e o f p ore d iam eter :3 .O X lO"
3.O X lO " m
Table 7 Measurement Period
Exposureterm
Measurement
item
1M 6M 1Y 2Y 3Y 4Y 6Y 10Y
Compressive
o
o o
o
strength
o
o
o o
Neutralization
Chlorideion o o o o o o o o
content
Reinforcing
o
o
o o
barcorrosion
Chemical
o
composition
o
Porosity
.5 Method of estimating chloride ion penetration
Movementof chloride ions into concrete dose not simply depend on diffusion of chloride ions, but is
knownto be macroscopically describable as diffusion. Many models have been proposed for chloride
ion movementinto concrete [5],[10],[11],[12],[13].
When the boundary condition is the chloride ion
concentration at the surface (Co) and is constant, the solution of Pick's diffusion equation can be
expressed as Eq. (1). When expressing the movemento f chloride ions into concrete in a marine
environmentusing this equation, the diffusion coefficient (Dc) in Eq. (1) takes account of such
phenomenaas concentration and fixation of chloride ions and suction of water, as these are included in
the diffusion phenomenonand are dealt with as macroscopic diffusion. For this reason, this diffusion
coefficient is referred to here as the "apparent diffusion coefficient."
In this paper, the movementof chloride ions by meansother than diffusion is included in the diffusion
phenomenonto simplify the movementof chloride ions as muchas possible, thereby grasping the trend
in long-term indices of chloride ion movement.T he changes in the chloride ion concentration at the
surface (Co) and apparent diffusion coefficient (Dc) in Eq. (1) were investigated for up to 10 years of
exposure under these assumptions. Co and Dc in Eq. (1) were calculated by the least squares method
from the chloride content distribution measured for each exposure period. However,if the penetrability
of chloride ions is assumed to change over time, then Dc changes accordingly, and the value of Dc
determined in this mannerwill indicate an average of changing diffusion coefficients over the period
from the beginning of exposure to the time of measurement.In other words, the value may reflect the
influence of penetrability of chloride ions prior to the time of measurement.
-94-
C
(1)
=Co \ 1-erf
where x =depth from the surface (cm)
t = elapsed time (s)
C =chloride ion concentration at depth x from the surface (%)
Co =chloride ion concentration at the surface (x = 0) (%)
DC=apparent diffusion coefficient of chloride ions (cm2/s )
2
erf:
error
function,
erf(Z)
z
2
= -j= -f e"r-dt
VrcJo
3
. INFLUENCE
OF ENVIRONMENTAL CONDITIONS
PROPERTIES OF CONCRETE IN MARINE ENVIRONMENTS
3.1 Influence
AND CEMENT
TYPE
ON
of exposure conditions
a) Appearance of specimens
Photograph
1 shows the appearance of
cylinder specimens exposed to the splashzone environment for 10 years. Organisms
such as shellfish
became attached to the
splash
and submerged specimens, and
particularly
on splash specimens. Cracks and
rust exudations
due to reinforcement
corrosion were observed on the splash and
submerged reinforced specimens, both those
with precracks and joints and others. No
cracks or rust exudations were observed on
atmospheric and inland reinforced specimens.
No sign of deterioration was observed in the
appearance of any plain concrete specimen in
any of the environments.
Photo. 1 Appearance of Cylinder Specimens at
10 Years of Exposure (Splash Zone)
b) Changes in compressive strength
The changes in compressive strength of concrete made using ordinary portland cement (OP) are shown
in Fig. 6. The compressive strength before exposure (at 50 days) was approximately 45 N/mm2.The
compressive strength of atmospheric cylinders took nearly the same course over time as standard-cured
cylinders,
increasing by about 10% after 10 years of exposure. The compressive strength of splash
cylinders increased in the first year by about 20% compared with the pre-exposure strength and leveled
off thereafter up to 10 years. The compressive strength of submerged cylinders was 10% higher at 3
years of exposure, but was 4% lower at 6 years, and eventually 8% lower at the end of the 10-year test
period. Inland cylinders showed few changes in their compressive strength over 10 years.
The compressive strengths of the cylinders in all marine environments except the inland environment
were higher than those of standard-cured specimens for up to 3 years. At 10 years of exposure, however,
the ratio of compressive strength to that of standard-cured specimens was 1.12, 1.02, 0.83, and 0.89 in
the splash, atmospheric,
underwater, and inland zones, respectively.
This suggests wide-ranging
compressive strength changes over time depending on the environment. In addition to the wetness of the
site, various other factors are thought to synergistically
influence the compressive strength of concrete
exposed to a marine environment, including the amount of ettringite
formed through reaction with
sulfates in the seawater and the amount of calcium leaching into the seawater.
-95-
40
S ym bol E nvironm ent
S plash zone
Sym bol E nvironm ent
Azone
tm ospheric - o
A
Inland
D
Standard curing
U nderw ater
0
u
I
2
£
S ym bolC em en t E nvironm ent
Inland
OP
o
A
B B A tm ospheric
zo ne
n
SR
3
10
6
1
3
Exposure term (years)
6
Exposure term (years)
Fig.6 Changes in Compressive Strength over Time
c) Progress of carbonation
Figure 7 gives the measurements of carbonation
depth. At 10 years of exposure, no carbonation
was observed in splash and submerged beams,
while the carbonation
depths of atmospheric
beams and inland beams were 1.2 mmand 2.0 mm,
respectively.
Progress
of carbonation
may
therefore be affected
by its wetness. In all
environments, alkalinity
was maintained at the
reinforcement depth (2 to 4 cm in depth).
d) Powder X-ray diffraction analysis
Figure 8 shows the results of powder X-ray
diffraction
analysis of OP specimens after 10
years of exposure in the four environments. A
comparison of the X-ray diffraction
intensities
of
calcium hydroxide (Ca(OH)2)
in the surface range
(0 to 2 cm in depth) shows that intensities
for
splash and underwater specimens are lower than
those for atmospheric and inland specimens. This
suggests leaching
of Ca(OH)2 from near the
surface of splash and underwater specimens into
the sea. When exposed to the underwater
environment, leaching
of Ca(OH)2
can be
expected to cause losses in compressive strength.
The tendencies
exhibited
by underwater
specimens were also observed in splash specimens,
but Friedel's
salt was detected near the surface
(within 2 cm of the exposed surface). Ettringite
was not clearly
detected
in any of the
environments.
Fig.7 Changes in Carbonation Depth over Time
Depth below surface :8~10cm Ca(OH)2
Splash zone -^^^^^^t
4
~6cm
Ca(OH) 2[
if
: fn
A|-M>v'w^Jl^Y^Jj|~-
5
/\iraospnenc
-^
- *- _ , à"
j-Lj-L-ii^.
10
20
_.
_Depth below surface:8~10cm
Ca(OH)2
zone
0
~2cm
|
Ca(OH)
^W^<lNvA>AA\K^-/l>li^ALjA^
j^^-
10
Underwater
Jl
15
Depth below surface:
A|'^s^rv~^/^v^?^--^-^^
4 ~,6cm
*X**UWw*A,~iJlA^L
20
8~10cm Ca(OH)2
2
J^,l^ Ca(OH)
^ l^ _ii>,..
\J<
10
D
Inland
1
15
epth below surface:
'^^^m^v^vX
20
8~10cm CapH)2
,~; 6cm
r^^v<>^v^Awaw^^^A.
_yx-
4
0-
-2cm
>*''o>HW-*N-Va-u.
'*HW"-wV\i-u-J-Js-i.j/'^->.
IA
10
28
Ca(OH)J
.
A
15
20
(deg)
Fig.8 Result of Powder X-ray Diffraction
(OP, after 10 Years of Exposure)
-96-
-k-.-.-r'A
15
Analysis
0 .8
1 .3
S .3
^t
CS (D
<u 43
>~*
i/i
<LJ
5 a
sH ｧS 0.4
1 .2
o <u
T-H
>
O u_,
o
o. 5"
C
S plash zo n e
U
nderwater
ea
* 0 .6
0.2
^ ,
¥ *m
*A ,>
,
ｰ ^
_
V
* x 5.
1 .1
X O
<D O
0 -I f o .4
1 .0
'C
o<o
1-4
p V
0 .2
PS &
Inl and
A tm o sph eric z o ne
^
J ^
'fr
1
3
5
^ m ^
0 .9
0.95
1.00
1.05
1.10
1.15
n
Ratio of surface (0 to 2 cm in depth) to
middle (8 to 10 cm in depth) total pore volume
7
9
1
3
5
7
9
Depth below surface (cm) Depth below surface (cm)
Fig.10 Changes in Chloride Ion Content
Distribution
Over 10 Years (OP)
Fig.9. Relationship
between Ratio of Surface to Middle
Total Pore Volume and Ratio of Post-exposure
to Pre-exposure Compressive Strength
e) Pore volume
Figure 9 shows the relationship
between surface (0 to 2 cm in depth) to middle (8 to 10 cm in depth)
total pore volume and the ratio of post-exposure (10 years) to pre-exposure compressive strength. The
total pore volume refers to the total volume of pores 30A to 30Mm in equivalent diameter. The total
pore volume near the surface tended to be larger than that in the middle for all cement types. This
tendency was more evident in underwater specimens than in atmospheric specimens. Submerged OP
specimens exhibited a total pore volume 13% larger near the surface than in the middle. In the case of
OP specimens in the splash and atmospheric zones, the differences were 3% and 5%, respectively.
This
may be because Ca(OH)2 in the concrete exposed to seawater leached into the seawater, increasing the
total pore volume near the surface. According to this X-ray diffraction
analysis, leaching of Ca(OH)2
from concrete also takes place in specimens in the splash zone, but the difference between the total pore
volume near the surface and that at 8-10 cmin depth is inappreciable.
This may be affected by the fact
that shellfish
and other organisms cling to concrete surfaces in the splash zone, filling relatively
large
pores of specimens.
The compressive strength ratio after 10 years of exposure decreased more significantly
as the surfaceto-middle total pore volume ratio increased. The pore volume measurements also suggest that Ca(OH)2
loss has an effect on the compressive strength loss of submerged specimens.
Cement : OP
f) Changes in chloride ion content over time
Environment
Underwater
The changes in the distribution
of chloride ion
R^
content (as a percentage of mass of concrete)
.3
over 10 years are compared among the four
\ Exposure terra
environments in Fig. 10. At any time during the
s1 ~3 years
period, the chloride ion content was the highest
bO
in the splash zone, followed by the underwater
3
and atmospheric
environments.
Figure
ll à"sg
0~1 year
~6 years
.1
ctf <u
x
shows the distribution
of increments in chloride
-y
-/
av&
L6 à"10 yea'rsion content with depth from the exposed surface
during four time spans: from the beginning to 1
3
5
7
year, from 1 to 3 years, from 3 to 6 years and
Depth below surface (cm)
from 6 to 10 years. Up to 3 year, the increments
near the surface (0-3cm) are relatively
high, but
Fig.ll Distribution
of Increase in Chloride Ion
the increments in the deeper range became
Content (OP)
relatively
large there after.
\
0 .4
0
_«>.G
\
3
o
0 .2
<D
Cfl
-97-
O
V-c
0
Figure 12 shows the surface chloride ion concentration (Co) calculated from the measurements using
Eq. (1) as well as approximation curves. In all environments, the surface chloride ion concentration
increases with time up to an exposure period of 3 years and levels off thereafter. When exposed to the
splash zone, the surface chloride ion concentration at 10 years was 0.70% by mass of concrete (around
16 kg/m3), whereas for underwater and atmospheric specimens the figures were 0.57% (around 14
kg/m3) and 0.22% (around 5 kg/m3), respectively.
The time-related changes in surface chloride ion concentration were approximated using Eq. (2). In this
approximation equation, the ultimate surface chloride ion concentration (Co*) and coefficient
a for
each environment are the values indicated in Fig. 12. The ultimate surface chloride ion concentration
(Co*) was highest in the splash zone, followed by the underwater and atmospheric zones. These results
are similar to those reported by Sugiyama et al [14]. The differences in coefficient
a with environment
were relatively
small. As in past reports [8],[15],[16],
these experiments also confirmed the strong
correlation between environment and surface chloride ion concentration.
exp
(2)
:)
Co(t)=
Co* (1-
where
Co(t) = surface chloride ion concentration after an exposure period of t years (%)
Co* =ultimate surface chloride ion concentration (%)
a =coefficient
t = exposure period (years)
Figure 13 shows the changes in apparent diffusion coefficient
(Dc) over time, as determined similarly
to the case of Co and the approximation
curves. The apparent diffusion coefficient
of chloride ions
diminishes
with time in all environments. The fall in the first 3 years tends to be large, but becomes
movemoderate thereafter. The apparent diffusion coefficients
after a 10-year exposure period were
7xlO~8 cm2/s, 4.3xlO"8 cm2/s, and 2.4xlO~8 cm2/s in the splash, underwater, and atmospheric zones,
respectively,
so the fastest rate of penetration of chloride ions is in the splash zone, followed by the
underwater and atmospheric zones.
X-ray diffraction
analysis and pore volume measurements revealed the leaching of Ca(OH)2 and a large
increase in total pore volume near the surface of underwater specimens, suggesting that the concrete
surface has a porous microstructure. However, the chloride ion penetration was found to be lower than
in the splash zone. This may be because more chloride ions penetrate into concrete in the splash zone
than in the seawater under the influence of effects other than diffusion,
such as cyclic drying and
wetting due tidal wetting and the impact of chloride ions against the concrete surface caused by waves.
This leads to a higher apparent diffusion coefficient in the splash zone.
E n v iro n m e n t
fe~(U
.-
£3
1.0
0 .8
II
C a lcu la te d
v a lu e
A p p ro x im a tio n c u rv e
S p la s h z o n e
C o (t) = 0 .7 0 ( l -e 'a t>11 )
A tm o s p h e ric
zone
U n d er w a te r
C o ( t) = 0 .2 2 ( 1 -e "" " ' )
A
100
"
J i
o
I
_
-
-
ｫ
A pp ro x im atio n cu rv e
D eft) = 18 .9 x lO
t
D c(t) = 9 .7 x lO'8 -t
D c(t) = 2 1 .3 x lO--- t
U nd erw a ter
C o (t) = 0 .6 3 ( l -e " * " )
0 .6
E nviro nm en C alculate
v alue
Sp lash zo ne
A tmneo spheric
zo
A
«
ｫ
S
x 10
BQ
m
OH
t
3
4
5
6
7
8
3
10
Changes in Surface Chloride Ion Concentration
over Time and Approximation Curves (OP)
5
6
7
-
10
Exposure term t (years)
Exposure term t (years)
Fig.12
4
Fig.13 Changes in Apparent Diffusion Coefficient
over Time and the Approximation Curves (OP)
98-
The changes in apparent diffusion coefficient
over time were approximated using Eq. (3). In this
approximation expressing time-related changes in apparent diffusion coefficient, coefficients
DC* and (3
for each environment are the values shown in Fig. 13. Over a long period, the apparent diffusion
coefficient approaches a limit value, and this is highest in the splash zone, followed by the underwater
and atmospheric zones. This suggests that the environment also affects apparent diffusion in concrete.
The tendency of the apparent diffusion coefficient to fall over time has been reported in past studies
[8],[14],[17],[18].
This may be attributed to the high penetration of chloride ions at an early stage in
marine environments and the increase in the denseness of the concrete microstructure over time.
(3)
Dc(t)=
DC* à" t
where
Dc(t) = apparent diffusion coefficient
DC*=coefficient
(3 = coefficient
t = exposure period (years)
after an exposure period of t years (cm2/s)
Accordingly,
the surface chloride ion concentration
and apparent diffusion coefficient
are strongly
affected by environmental conditions. Also, these values can be approximated by Equations (2) and (3),
and can be estimated to a certain extent if the exposure periods and environmental conditions are known.
For a more accurate estimation, the water-cement ratio and curing temperature should also be taken into
account.
g) Progress of reinforcement corrosion
Figure 14 shows the state of corrosion of the reinforcing steel in RC specimens without precracks or
construction joints after an exposure period of 10 years. The ratio of corroded area at 10 years was
highest in splash zone specimens, followed by underwater and atmospheric specimens. Corrosion was
limited to the surface of the reinforcement in all environments, and no pitting corrosion was observed.
The changes in reinforcement corrosion ratio over an exposure period of 10 years are shown in Fig. 15.
In the splash zone, corrosion was first observed after an exposure period of 1 year and it increased over
time until the corrosion ratio reached more than 25% after 10 years. In submerged specimens, corrosion
first appeared at 3 years but scarcely increased up to 10 years. In the atmospheric specimens, corrosion
was observed for the first time after a period of 10 years.
Underwater
Splash zone
Corroded
area ratio
: 27.5%
Atmospheric zone
Corroded
area ratio
Corroded
area ratio
: 5.1%
Corroded
area ratio : 0.4%
Inland
: 2.3%
Fig.14 State of Corrosion of Reinforcing
Steel after 10 Years of Exposure
Figure 16 shows changes in the chloride ion content at the depth
the surface). In all environments, the chloride ion content at this
3 years of exposure. Chloride contents at the reinforcement after
0.34% (around 8 kg/m3), and 0.46% (around 10 kg/m3) by
underwater, and splash zones, respectively.
of the reinforcement
(2 to 4 cm below
depth scarcely increased after the first
10 years were 0.06% (around 1 kg/m3),
mass of concrete in the atmospheric,
The chloride ion penetration rate and oxygen supply are said to affect the onset and progress of
corrosion. As shown in Fig.15, it was confirmed in this experiment that corrosion progresses more
rapidly in the splash zone, where both are high, than in the air and underwater. The corrosion ratio in
-99-
u
1
2
3
4
5
6
7
8
9
3
10
6
Exposure term (years)
Exposure term (years)
Fig.15 Changes in Corroded Area Ratio of
Reinforcement over Time
Fig.16 Changes in Chloride Ion Content at
Reinforcement Depth over Time
atmospheric specimens was as low as 2% after an exposure period of 10 years, though more than 1.2
kg/m3 of chloride ions had been present near the reinforcement after the first 3 years.
In the marine environments studied in these experiments, reinforcement corrosion was most intense in
the splash-zone specimens up to an exposure period of 10 years, followed by underwater and
atmospheric specimens. Reinforcement corrosion in submerged specimens started early due to the high
chloride ion content, but the low oxygen supply inhibited
its propagation.
The time to onset of
reinforcement corrosion in atmospheric specimens was long, but the high oxygen supply may lead to a
higher rate of corrosion than in submerged specimens for exposure periods over 10 years.
3
.2 Effects of cement type
a) Compressive strength
Figure 17 shows the time-related changes in compressive strength of concretes made using OP, SR, and
BB cements. When submerged, SR and BB specimens began to lose compressive strength after the first
3 years as in the case of OP specimens, but at 10 years, they were both 5% higher than the values before
exposure. The loss in compressive strength of submerged SR and BB specimens was less than that of
OP specimens.
(b) Underwater
3
o s p h e ric U n d e rw a te r
e
o
A
A
n
6
Exposure term (years)
Exposure term (years)
Fig.17 Time-related
C e m e n t A tm
zon
O P
BB
SR
Changes in Compressive Strength
-100-
of Concrete Made with Different
Cements
b) Pore volume measurement and powder X-ray diffraction
As shown in Fig. 9, the total pore volume of SR and BB specimens submerged for 10 years was higher
near the surface (2 cm from the surface) than in the middle (8 to 10 cm below the surface) by
approximately
10% and 5% in SR and BB specimens, respectively.
Whereas the total pore volume of
SR specimens showed a similar tendency to that of OP specimens, BB specimens exhibited small losses
in total pore volume, as their low initial Ca(OH)2 content limits the leaching of Ca(OH)2. This
presumably explains the smaller fall in compressive strength than with OP and SR specimens.
Powder X-diffraction
revealed that the diffraction
intensities
of Ca(OH)2 near the surface (0 to 2 cm in
depth) of SR and BB specimens submerged for 10 years were low, as with OP specimens, suggesting
that it leached into the sea.
^ -v U . O
^
<u
ｧg
o
o
o
<^
T <o3
ITg
n
｣ >
to o
o .6
\
\
ｧa
E n v ir o n m e n t
C e m e n t A tm o s p h e n U n d e
rw a te
zo n e
o
O P
B B
A
A
SR
D
0 .4
o .2
(D
o
a
c) Chloride ion penetration
Figure 18 shows the state of chloride
ion penetration in specimens made with
different cements and exposed to the
marine atmosphere
or seawater.
Similarly to OP specimens, the chloride
ion contents in SR and BB specimens
were higher in submerged specimens
than in specimens exposed to marine
air. In the sea, the chloride ion content
in BB specimens was higher than that
in OP specimens near the surface, but
lower beyond 3 cm below the surface.
The chloride ion distribution
in SR
specimens was similar to that in OP
specimens in all environments.
1
3
5
7
Depth below surface (cm)
Fig.18 Chloride Ion Penetration of Concretes
Made with Different Cements
Figures 19 and 20 show the time-related
surface chloride ion concentration
and apparent diffusion
coefficient
with approximation
curves, respectively,
determined from the distribution
of chloride ion
content using Eq. (1). All cements exhibited small changes in surface chloride ion concentration after
the first 3 years of exposure. When exposed to marine air, no marked differences
were observed
between cement types with regard to surface chloride ion concentration at 10 years. When submerged,
however, the surface chloride ion concentration
in BB specimens was 0.95% (around 21 kg/m3),
whereas that of OP and SR was 0.60% (around 14 kg/m3). The apparent diffusion coefficient
of all
cement types diminished over time. SR specimens exhibited apparent diffusion coefficients
similar to
OP specimens up to 10 years both in marine air and underwater. The apparent diffusion coefficients
of
BB specimens were similar to OP specimens in the air, but these were lower when submerged,
illustrating
BB's effect of inhibiting
chloride ion penetration throughout the exposure period.
d) Reinforcement corrosion
Figure 21 shows the time-related
changes
specimens containing different cements. The
content in atmospheric specimens. In the sea,
OP and SR specimens leveled off after the first
increase thereafter.
in chloride
cement type
the chloride
3 years, but
ion content at the reinforcement
depth in
had no appreciable
effect on chloride ion
ion concentration near the reinforcement in
that of BB specimens continued to gradually
Figure 22 shows the changes in ratio of corroded area over time in concretes made of different cements.
Corrosion in submerged BB specimens commencedlater than in OP specimens, and its area was found
to be smaller than in OP specimens after 10 years. The corrosion ratio of submerged SR specimens was
smaller than that of OP specimens up to 6 years, but converged at 10 years. In the atmospheric
environment, no appreciable
differences
in corrosion ratios by cement type were observed after
-101-
exposure for 10 years. In other words, within the range of experimental exposure, BB had a certain
inhibiting
effect on reinforcement corrosion in submerged specimens when compared with OP, whereas
SR was similar to OP in terms of corrosion-inhibiting
effect.
1 .2
( a ) A tm o s p h e ric z o n e
*｣
o
C e m e n t C a lc u la ted
v a lu e
o
OP
A
BB
1 .0
*
8
8 0 .8
'S >+-<
B =CTJK O I
5e SE O .6
o
ｧ
"
sw
h
a
c3
se
(
A p p ro x im a tio n c u rv e
C o (t) = 0 .2 2 (l-e -- -55 t)
C em ent C alculated
value
o
OP
100
IS
» I-I
C o (t) = 0 .2 2 ( l-e -1 -17 ')
n
SR
a ) Atmospheric zone
£j
x
S
Q
O -4
A
D c(t) = 9 .7x lO
A
BB
SR
C o (t) = 0 .1 7 (l -e -- -'70 t)
A pproxim atio n curv e
f
D c(t) = 6 .7x lO -I '
D c(t) = 12 .9 X lO f
n
I
0 .2
A
u
uo
o
1 .2
( b ) U n d e rw a te r
oocd ^" **
( b ) Underwater
1 .0
ｫｰ>
CO
ｧ
rca;
oc
S o .8
uo-c
8
d
1
- -6
&
0 .4
^
C e m e n tI B C S a ulcS u la t e d
O P
100
A
X
BA ,
A
A p p ro x im a tio n c u rv e
D c ( t ) = 2 1 .3 x l O
A
B B
D c ( t) = 8 .2 X l O
S R
/
t
-I"
D c (t) = 2 4 .8 X l O " - t "
*
/
D
//
0 .2
'
X
, i / l
'
A p p ro x im a tio n c u r v e
C o (t) = 0 .6 3 ( l-e " * " )
1
2
3
4
5
6
*-»
o
§
£
Q
C o (t) = 0 .9 6 ( l-e " " ')
C o (t) = 0 .5 9 ( l-e "* " )
SR
r "i
0
15
x
à"a ^
x ' -'
'
C e m en t C alc u la te d
v a lu e
OP
A
BB
7
8
9
0
10
1
2
Exposure term t (years)
Fig.19 Time-related Surface Chloride Ion Concentration
with Approximation Curves for Concretes
Made with Different Cements
0
3
4
5
6
7
8
9
10
Exposure term t (years)
Fig.20 Time-related Apparent Diffusion Coefficient
with Approximation Curves for Concretes
Made with Different Cements
.4 r
S«
8*
w
c
0 .3
°
5
813
I1
<DE
3^
cd
CD
S
'O
CD
TD
0.2
21-
g
n
0.1
6
0
a
lh
3
6
10
0
Exposure term (years)
2
4
6
8
Exposure term (years)
Fig.21 Time-related Changes in Chloride Ion
Content at Reinforcement Depth in
Specimens Containing Different Cements
10
0
2
4
6
8
Exposure term (years)
Fig.22 Changes in Corroded Area Ratio Over Time
in Concretes Made with Different Cements
102
10
4
. INVESTIGATION INTO METHODS OF ESTIMATING CHLORIDE TON CONTENT AND
REINFORCEMENT CORROSION
4.1 Estimation
of chloride
ion content
If the apparent diffusion coefficient
in Pick's diffusion equation were constant and independent of time,
then concrete's resistance to chloride penetration
would not change over time. However, the apparent
diffusion
coefficient
evidently
changes with time, as described
in the previous chapter. Concrete's
chloride penetration properties
are therefore variable with time. The apparent diffusion coefficient
calculated from the accumulation amount of chloride ions in the concrete after, say, exposure for 10
years does not represent a property of the concrete at the 10 year point, but is rather an averaged
property for the period. As noted in Chapter 2, the apparent diffusion coefficient
thus determined will
include the effect of chloride ion penetrability
prior to measuring the chloride ion content, including the
early stage during which chloride ions penetrate rapidly into the concrete.
Figure 23 shows the chloride ion content distribution
in OP specimens exposed to the underwater
environment for 10 years as estimated from the surface chloride ion concentration (Co) and the apparent
diffusion coefficient
(Dc) obtained at 1, 3, 6, and 10 years using Eq. (2) and (3), respectively.
Whereas
the estimated chloride ion distribution
differs widely from the measured values when using Co and DC
data obtained at 1 or 3 years, it agrees better with the measured values when using the data obtained at 6
years. This is because, when calculated using Eq. (2) and (3), Co is lower and DC is higher before 6
years than the values at 10 years, as shown in Figs. 12 and 13, but these values converge at around 6
years and thereafter.
Figure 24 shows the chloride ion distribution
at 1, 3, 6, and 10 years as estimated from Co and DCdata
obtained at 10 years. The small Co value at 1 year led to a large error between the estimated and
measured values. At 3 to 6 years, the estimated chloride ion content was lower than the measured values,
due to the DCvalues being smaller than the actual values.
These results indicate that the accuracy of estimation
can be improved by using Co and DC values
obtained after exposure for the longest possible time span. Conversely, DC obtained from a long-term
experiment may lead to underestimation
of the earlier chloride penetration.
0 .8
C o e ffic ie n t
E t e x r mp o s u r e C o
D c
C
0.7
ement : OP
Envi ron ment
: Underwater
(y ea rs)
D
0.6
E s t i m a te d
§£
O
o
v a lu e
0.5
6
1 0
M
0.4
B
3
e a su r e d v a lu e
(%
(X l O
c m
0 .2 2
2 1 .3
0 .4 6
9 .6 6
0 .5 8
5 .8 6
0 .8
C h lo r i d e
io n
/s) c o n te n t
4 .0 6
0.6 2
(a fte r 1 0 y e a rs )
0.7
ti
H
0.5
0.4
0.3
0.2
0.2
0.1
0.1
3
5
7
Depth below surface (cm)
Fig.23
Chloride Ion Distribution
at 10 Years
Estimated from the Co and DCData
Obtained at 1,2,3,6, and 10 Years
E x p o s u r e M e as u re d
te rm
v a lu e
(y e a rs)
D
n
A
3
6
n
n
0.6
0.3
1
Cement : OP
_
Environment : Underwater
1
3
5
E stim a te d
v a lu e
7
Depth below surface (cm)
Fig.24 Chloride Ion Distribution
at 1,3,6, and 10
Years Estimated from Co and DCData
Obtained at 10 Years
103
In actual practice, a surface chloride ion concentration and apparent diffusion coefficient obtained from
an experiment over 5 to 6 years appears to be adequate for estimating chloride ion penetration after a
relatively long exposure period of more than 10 years, as the values have nearly converged by this time
and the resulting estimate falls on the safe side.
Figure 25 shows the chloride ion distribution
in concrete made using OP and BB after exposure periods
of 50 and 100 years as estimated using the above-mentioned equations for estimating Co and DC. When
BB is used for a typical underwater marine structure in which the reinforcement cover is 75 to 100 mm,
it is estimated that the chloride ion content is held to a low level at near the reinforcement even after
100 years, and that the chloride content is lower,than with OP.
i
.o r
E nv iro nm en C em ent Y ears L ine
^
0 .8
|-
O P
U nd erw ater
BB
I80*8
50
Co
(% )
1 .2
D c
(X lO
0 .63
cm
/s)
TCL=
8*
1.2 7
«
:
100
0 .63
0.77 3
50
0 .9 6
0.5 73
'-^
ID
100
0 .9 6
0.35 8
1-1
-i->
u
n
cd
« o
o f
o 3
Ss
C S
c
Q
.2
0 .4
J2
0.8
0.6
>.
<D
3&
° 2
At:)
i.o
H
g8
0 .6
V/Ch+Clm
l(i=iv
2
"°
0.4
C
u 8
0 .2
0
5
10
15
Fig.25 Estimated Chloride Ion Distribution
after Exposure for 50 and 100 Years
.2 Estimation
1
Exposure term (years)
Depth below surface (cm)
4
0
Fig.26 Calculation Method for Total Chloride
Factor (TCL)
Ion
of corroded area of reinforcement
The ratio of reinforcement corroded area in concrete exposed to the splash-zone environment for 10
years was around 25%, and there was no marked profile loss. Nevertheless,
corrosion-induced
cracks
were observed. It follows that even a relatively small corroded area ratio can threaten the durability
of
reinforced concrete. The authors therefore attempted to estimate the reinforcement corrosion ratio as an
index of the durability
of reinforced concrete.
As shown in the previous chapter, reinforcement corrosion can propagate even while the chloride ion
content at reinforcement depth scarcely increases. This suggests that the degree of corrosion cannot
simply be estimated from the chloride ion content at a certain time. Though oxygen supply should be
taken into consideration,
the environmental conditions selected for this study were limited, and the
water-cement ratio was equalized to estimate the corroded area. As shown in Fig. 26, the sum of the
products of the mean chloride ion content near the reinforcement multiplied
by the exposure period
calculated at yearly measurements is defined as the total chloride ion factor (TCL), and is calculated
using
Eq. (4).
-104-
n-i/ri
TCL=?!
where
61
+n
'
2
\
i+1"AtJ
(4)
V
TCL = total chloride ion factor (%à"year)
Clj = measured chloride ion content at z'-th time increment (%)
At; = time interval between measurements at z-th and (z+l)th time increment (years)
n =numberof measurements of chloride ion content
Figure 27 shows the relationship
between total chloride ion factor and corroded area ratio for concretes
made with different cements. In the case of OP, the total chloride ion factor at the reinforcement depth
in the splash and underwater zones is 7 times and 5 times that in the atmospheric zone, respectively,
after an exposure period of 10 years. A linear relationship
is observed between total chloride ion factor
and corroded area ratio for each environment. As the total chloride ion factor increases, the corroded
area ratio exhibits an increasing tendency. Corroded area ratios of up to 30% can be estimated to a
degree from the total chloride ion factor, if the environmental conditions are known.
In the splash zone, the same total chloride ion factor leads to a higher corroded area ratio than
underwater. This suggests a strong effect of oxygen supply on corrosion propagation. Despite the high
oxygen supply, corrosion is retarded in the marine atmosphere due to the low total chloride ion factor.
30
U
nderwater
30 }-
a
E nv iron m ent O P B B S R
S p lash zon e
A tm ospheric A A A
zone
o c
U nde rw ater
C0
-g*------:o
\3ndetwatet
_.
--^r%SS)
1
Total chloride
ion factor TCL (% à"years)
Fig.27 Relationship between Total Chloride
Factor and Corroded Area Ratio
5
. EFFECTS
CORROSION
OF
3
6
10
1
Exposure term (years)
3
6
10
Exposure term (years)
Fig.28 Time-related Changes in Corroded Area Ratio
in Specimens with Cracks or Construction Joints
Ion
CRACKS AND CONSTRUCTION
JOINTS
ON REINFORCEMENT
Figure 28 shows the time-related
changes in corroded area ratio for specimens with induced cracks and
construction joints. The state of reinforcement corrosion in cracked specimens after exposure for 10
years is shown in Fig. 29. Corrosion in cracked beams in the splash and underwater zones began earlier
than in uncracked beams, but eventually led to corrosion ratios similar to those of cracked beams at 10
-105-
years. When the initial crack width was 0.05 to 0.1 mm,the cracks did not significantly
corrosion up to 10 years in any of the environments in this experiment.
accelerate
Figure 30 shows the state of reinforcement corrosion at 10 years in beams having a construction joint.
Corrosion tended to start at the joint in all environments and was concentrated near the joint up to 6
years. By 10 years, however, corrosion had propagated to other parts of the reinforcement. Nevertheless,
the corroded area ratio of beams with a construction joint was similar to that of monolithic beams at 10
years in all environments. Accordingly,
properly treated construction joints may not significantly
accelerate corrosion in marine environments.
Over a 10-year exposure period, the splash zone is the harshest environment for concrete with minute
cracks 0.05 to 0.1 mmin width or properly treated construction joints, followed by the underwater and
atmospheric environments. In this respect, cracked and jointed concrete is similar to plain reinforced
concrete.
Splash zone
Corroded
area ratio
[Lc<|
Underwater
: 20.9%
|u=a|
Corroded
poi]
area ratio
^ "
: 5.2%
pan
isr
TRT
4 .2%
Atmospheric zone
Corroded
area ratio
: 2.6%
[fag
Corroded
peg
area ratio
: 0.3%
!"="!
1
s
M
Inland
/
^
y
1
2 3
r>
o
l >
/
s
w** . . - t
be d
Fig.29
( )
¥
*l '
t*. f
ｫJ
- .,
0l n .= 3n l%
State of Reinforcement Corrosion in Cracked Beams at 10 Years
Splash
zone
Corroded
Constructionjoint
area ratio
Underwater
: 28.0%
Atmospheric zone
Corroded
area ratio : 5.1%
Inland
in
El
Corroded
Corroded
area ratio : 1.9%
area ratio
: 0.5%
Fig.30 State of Reinforcement Corrosion in Beams with Construction Joint at 10 Years
6
.CONCLUSION
Concretes made using different types of cement and also concrete samples with induced cracks and
construction joints, were exposed to various marine environments for 10 years. An investigation
of
-106-
these concrete revealed the following:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Whereas the compressive strength of concrete scarcely changed in the marine atmosphere over an
exposure period of 10 years, that of submerged concretes tended to decrease after the first 3 years.
In the marine environments selected for these experiments, the rate of chloride ion penetration was
highest in the splash zone, followed by the underwater and atmospheric zones.
In the marine environments selected for these experiments, the rate of reinforcement corrosion
propagation was highest in the splash zone, followed by the underwater and atmospheric zones.
Type B blast-furnace slag cement was found to inhibit chloride ion penetration when compared with
ordinary portland cement. When the same concentration of chloride ions was present in submerged
specimens, the propagation of reinforcement corrosion was slower in specimens made with Type B
blast-furnace slag cement than in specimens made with ordinary portland cement.
The surface chloride ion concentration in the diffusion equation tends to increase over time, while
the apparent diffusion coefficient
of chloride ions tends to decrease. However, these changes
become less significant
after the first 3 years. Both surface chloride ion concentration and apparent
diffusion coefficient
are strongly affected by environmental conditions.
Changes in the surface chloride ion concentration and apparent diffusion coefficient of chloride ions
can be predicted
using approximation
equations. Long-term chloride ion content is roughly
predictable
from the surface chloride ion concentration and the apparent diffusion coefficient
if the
type of marine environment and cement type are taken into account.
The ratio of reinforcement corrosion was found to correlate with the product of chloride ion content
and time in each environment. Corroded area ratios of up to 30% may be estimated to a certain
degree using this index.
It is considered that properly treated construction joints and cracks of up to 0.1 mmin width do not
significantly
accelerate corrosion of reinforcement in any marine environment.
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