CONCRETE LIBRARY OF JSCE NO. 34, DECEMBER 1999
A CORROSION NIECHNISM OF CONCRETE IN A SEWAGE TREATMENT FACILITY
USING AN OXYGEN AERATION ACTIVATED SLUDGE PROCESS
(Translation from JOURNAL OF MATERIALS, CONCRETE STRUCTURES AND PAVEMENTS
of JSCE, No.599/V-40, August 1998)
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Manabu FUJII
In order to study the corrosion and deterioration mechanisms of concrete, concrete prism specimens were
immersed in the fmal sedimentation tank and troughs of an oxygen-aeration activated sludge facility at a
sewage treatment plant. Over the years, changes to the roughened surfaces, mass loss and a decreasing
relative dynamic modulus of elasticity were observed. After 10 years, the bending performance of
conventional concrete specimens immersed without a lining was lower in comparison to specimens with
a lining. Microstiucture and chemical analyses showed that surface samples underwent more obvious
changes than interior ones, with the degree of change depending on the concrete mixture and whether or
not the surface had been coated with a lining. The production of erosive flee carbon dioxide was
influenced by the quality of the sewage water, while the activity of bacteria and protozoa affected
corrosion and deterioration of concrete more than sulfuric acid in sewage treatment water. Corrosion
and deterioration of concrete progressed through chemical reaction and ion transport in concrete and
sewage treatment water flows.
A method for estimating the corrosion/deterioration of concrete in this
sewage facility is proposed from the results and a calculation of free carbon dioxide.
Kev Words: corrosion of concrete, sewage treatment facility, fi'ee carbon dioxide, mass loss,
dynamic modulus of elasticity, pore distribution, chemical analysis
Tatsuo KAWAHIGASHI is an assistant in the Institute for Environmental Science at Kinki University,
Higashi-Osaka, Japan. He received his M.Eng. from Kinki University in 1979. His research interests
relate to the corrosion/deterioration mechanism of concrete and the estimation of durability of concrete.
He is a member of the JCI, JSMS, and JSCE.
i
Hironobu SUZUKI is a section chief in the lst Technology Section at Chuken Consultant Corporation,
Osaka, Japan. He received his M.Eng. from Ritsumei University in 1986. His research interests relate to
the deterioration of concrete and the examination of concrete structure tmder various environment. He is
a member of JCI and JSCE.
Toyoaki MIYAGAWA is a Professor in the Department of Civil Engineering at Kyoto University,
Kyoto, Japan. He received his D.Eng. from -Kyoto University in 1985. He is the author of a number of
papers dealing with the durability, maintenance, and repair of reinforced concrete structures. He is a
member of the AC1, RILEM, fib, JSMS, JCI, and JSCE.
Manabu FUJII was a Professor in the Department of Civil Engineering at Kyoto University, Kyoto,
Japan. He received his D.Eng. from Kyoto University in 1972. His research activities were focused on
the optimal design of prestressed concrete cable-stayed bridges, structural safety evaluations, and the
durability of concrete structures. He has written some 50 papers on these subjects. He was a member of
the ACI, JSMS, JCI, and JSCE.
g
1. INTRODUCTION
The corrosion of concrete in sewage facilities
varies with sewage quality, the type and
quantity of microbes present, the sewage treatment environment, and other factors.
The main cause of sewage-pipe corrosion is thought to be acid (sulfuric
acid) corrosion,
and this is affected by the activity of bacteria such as sulfate reduction bacteria in the
liquid phase and sulfur oxidation bacteria in the gas phase. Recent work has gradually
clarified
that such corrosion occurs as a result of bacterial
activity
and chemical
reactions[1],[2],[3].
Thus, to protect sewage facilities
from corrosion, the mechanisms
at work must be clarified
[4],[5].
Examination of corrosion occurring early on concrete
surfaces in sewage treatment facilities
has been traced to erosive carbon dioxide
through analysis of the water quality and the corroded concrete [6],[7].
Thus, the
mechanism was different from the sulfuric acid corrosion of sewage pipes. To further
examine such mechanisms, experiments have been done on bacterial metabolism and
concrete deterioration
[8],[9].
With the increasing
demand for qualitative
and
quantitative
improvements in the water supply after wastewater treatment, there is a
great need to obtain basic information
on which to base countermeasures against
corrosion in concrete sewage facilities.
In this ten-year study, 13 types of concrete test specimen with different
mix
proportions
were placed at three locations in a final sewage treatment facility for an
examination of corrosion mechanisms. Over the 10 years, measurements were
conducted to evaluate changes in the specimens. This report presents our findings with
respect to concrete corrosion and deterioration
mechanisms. The main focus of this
study was to nondestructively
measure the degree of deterioration
under the differing
conditions
of each immersion location. Over the 10 years, the test specimens were
subjected
to visual inspections
and also to measurements of changes in mass and
dynamic modulus of elasticity.
After this 10-year period, some of the test specimens
were examined physically
for bending performance, water absorption,
carbonation
depth, and pore distribution
and were also analyzed chemically
by electron probe
microanalysis,
X-ray fluorescence analysis,
and X-ray powder diffraction.
2
.CAUSE OF EROSIVE CARBON DIOXIDE
The material balance in a sewage treatment facility,
as in all environments where
natural
reactions
take place,
depends
on various cyclic
processes
such as
decomposition,
linkage, and immobilization,
and there are complex interactions
among
them. Various organic and inorganic materials
are present in the water, composing
elements such as hydrogen, oxygen, carbon, nitrogen, sulfur, and phosphorus. Another
factor affecting
decomposition
and immobilization
in the material
balance is the
energy metabolism
of the bacteria
in sewage, and the growth/decline
of their
populations
as kept in balance by the predatory mechanism of protozoa.
The activated
sludge process is commonly used for aerobic and anaerobic treatment,
and the quantity of aerobic bacteria,
anaerobic bacteria,
or facultative
anaerobes in
each system depends on the concentration
of organic substances,
the amount of
dissolved
oxygen, and other conditions.
In aerobic treatment, organic substances are
54-
decomposed to carbon dioxide, water, nitric acid, phosphoric
acid, sulfuric acid, and
other
inorganic
substances
using
the oxidative
characteristic
of bacteria,
metabolism[10].
In anaerobic
treatment,
organic substances
are decomposed to
methane and carbon dioxide depending on the metabolism mechanism of the bacteria.
Both treatment methods were examined, with the oxygen aeration activated
sludge
process using pure oxygen, termed the "oxygen process",
and the conventional
activated sludge process with air, termed the "air process".
In the first sedimentation
tank of both systems, the occurrence of hydrogen sulfide and corrosion were minimal in
the process leading
up to a final product, since this stage of the process does not
involve holding the sludge, although anaerobic activity is observed. With aeration, the
activity
and proliferation
of facultative
anaerobes cause nitrification,
denitrification
and phosphorus
removal, so there is little
possibility
of the formation of hydrogen
sulfide and sulfuric acid (Fig.l).
first sedimentation tank
aeration system
Iflo w
I oi
sh o rt -term
slu d g e resid en c
(little an aero b ic
a ctiv ity )
CH4 t CO2
i o z b low in g p u r e o x yg en
.^^C
vT tr^ouj¥
-sg .h se dim e ntatio
fi n al n tan
, k
w a te r fro m
v C Ch C Ch trea ted
se d im e n tatio n
-= i
w a ter
i a
i o *
a ero b ic-a n a erob ic a ctiv ity
C O by
: gab sacter
relei ased
a
and
(m tnp fih osph
catio on ru-des nitri
re m fic
o v al)
ation
m e ta b olism
¥7
treated w ater
activ ity o f bacteri
an d p ro to zo a
(sim ilar to ae ratio
sy stem )
o u tfl ow
fap seu
cu ltad otivm oe n an
as a gero
ro up
b es etc . ',- /¥ th
greanater
aeropro
-anliferation
aerob ic
b acteria
Fig.1
Outline
of oxygen activity
sludge
treatment
system
Identifying
the main corrosion
among nitric acid, carbonic acid, and others,
pre d atory factor
I
a tory sulfuric
actipv rotoz
ity b oa
y acid, r- -¥/is pb re
ala
d nc
atory
e o fby because many factors are at
including hydrogen sulfide p redand
difficult
V ort icella
, E p istyaslis found
gro up e tc.
flo ck o f btreatment
acteria
work in a complicated
system
such
in sewage
facility.
For instance,
the corrosion mechanism taking place in sewage-pipe is influenced
primarily
by the
process of sulfuric
acid formation.
However, other contributing
factors
include
hydrogen sulfide and sulfuric acid. Carbon dioxide occurs in gas and liquid phases at
aeration points, according to the results of existing
measurements [7].
As for the
occurrence of carbon dioxide in the oxygen process, corrosion is more obvious than in
the air process. However, there are no significant
differences
with respect to other
corrosion factors between the oxygen and air processes. Therefore, at the immersion
locations
in the facility
used for these experiments,
erosive free carbonic acid from
carbonic acid in the liquid phase, is likely to be the primary cause of corrosion. The
carbon dioxide is formed by the bacterial
metabolism and the conditions
of its growth
or decline are affected
by the oxygen demand. If the concentration
of organic
substances in the sewage is high, the sewage load becomes high, and much carbon
dioxide is formed in the liquid phase. The sewage load in the oxygen process is higher
than in the air process, and the concentration
of carbon dioxide rises to a high level in
the liquid phase.
Also, the carbon dioxide concentration
in the liquid phase increases
between the aeration system and the final sedimentation
tank, as carbon dioxide
diffuses to the gas phase from the liquid phase and dissolves into the liquid phase.
-55
All the dissolved
carbonic acid in the liquid phase is called the "free carbonic acid",
referring
to the carbon dioxide remaining after linkage of carbonic acid with calcium,
magnesium, and other cations.
Of the free carbonic acid, the equilibrium
level of
dissolved
carbonic acid is called the "dependent free carbonic acid", and the remainder
is called the "erosive free carbonic acid" [11] (Fig.2).
Although the carbon dioxide
content of water is affected by the partial pressure and temperature of the water, after
dissolution,
the carbonic acid assumes various forms such as C02, H2C03, HC03" and
C032", which vary in proportion
with changes in pH,
Free carbonic acid, and especially
the erosive free carbonic acid, is affected
most by
changes in the inflow load, bacteria
metabolic
activity,
alkalinity,
and pH of the liquid
phase or the system environment. In the oxygen process in large treatment facilities,
concrete corrosion may be largely due to the formation of carbon dioxide rather than of
hydrogen sulfide and sulfuric acid, because of greater bacterial
metabolic
activity than
in a sewage-pipe.
carbonic acid
Table 1 Materials
Ca
linkageLWitlr~
tLCOj
HCCV
CO)2'
free form
i
C
link age
C
free
I
aCO) , MgCCh
a(HCO,).
M a t e ria l
substances
n liquid
N ote
C em e nt
phase
O .P .C
A g g re g a t e
C o a rs e
C ru s h e d st o n e
A d m ix t u r e
F in e
W a te r- r e d u c in g a g e n t
L ig m n s u lf o n ic a c id t y p e
ect.
, Mg(HCO3):
etc
H ig h w at e r- re d u c in g a g e n
H ig h c o n d e n s a tio n t n a z m e ty p e
COi (equilibrium)
A ii- e n t ra m in g a g e n t
N a tu ra l re s in a c id t y p e
COi (errosive)
P o ly m e r
S B R a te x
E p o x y r e s in
C o n d e n s at io n c o m p o u n d
o f b is p h e n o l A a n d e p ic h lo ro h y d rin
T a r d e n a tu r e d c o m p o n e n t
o f e p o x y re s in
L in in g
action with cement substance
corroding concrete
Fig.2
B each sand
"^
T a i- e p o x y re s in
Formation and action of
carbonic acid
T
able 2 Mix proportion
N o n - lin in g c o n c re t e
C o n v e n t io n a l c o n c re t e
D
S p e c im e n N o .
W a t e r- c e m e n t r a t io I
S a n d p e rc e n t a g e (s / a ,
U n it c e m e n t w e ig h t (k g / m )
50
2
3
4
5
6
55
44
3 00
60
50
55
60
L ru n g c o n c r e t e
E p o x y (r e s n
T a r- e p o x y
(r e s in ) lin in g
lin in g
M ortar
lin in g
c o nc rete
c o n c re t e
7 (1 r
7 0 )*2 8 0 r
8 (3 f
c o nc rete
9
3 1 TmTTifirfiinn
N o n - lin in g c o n c re t e
F lo w
P o ly m e r
c o n c re t e
10
55
42
33 0
(1)*
(3)*
3. EXPERIMENTAL
factors
48
ce m e n t
c o n c re t e
ED
51
42
: one coating (lining thickness
of 400-600
ju m)
: three coatings
(lining thickness
of 600-1000
n m)
OUTLINE
Conditions
The main location selected for immersion of the test specimens was the trough of the
final sedimentation
tank in the oxygen treatment process. The water depth in the
troughwas 13~32 cm. The flow speed was about 96 cm/sec at a depth of4.5 cm. For
comparison, other immersion locations,
such as the final sedimentation
tank of the
oxygen process and the trough in the air treatment process, were also selected. The
water level in the trough of the air treatment process system was 14~21 cm and the
flow speed at a depth of6 cm in the center of the trough was about 80 cm/sec [12].
-56-
3
.2 Test Specimen Factors
The materials
and mix proportion factors are shown in Table 1 and Table 2. Ordinary
Portland cement was used with sea sand (specific
gravity: 2.56; F.M.: 2.79) and crushed
stone (specific
gravity:
2.71; F.M.: 6.92) as aggregates.
The water-cement ratio was set
at the largest amount of water acceptable
for AE concrete based on durability
as given
in the sections on "water-tightness
concrete" and "mix proportion" of the Japan Society
of Civil Engineers
Standards
at the time of experiment planning.
As a result, three
water-cement ratios of 50, 55,.and 60 %, and two unit cement weights of 300 and 330
kg/m8 were chosen ("conventional
concrete" ; specimen number in Table 2 : 1~6). The
slump and air volume of all concrete specimens were, respectively,
8±1 cm and 4±
0.5 %. In addition
to these six types of concrete, specimens with different
coating
("lining")
specifications
("lining
concrete";
specimen number: 7(1) and 7(3) ("epoxy
lining concrete"),
8(1) and 8(3) ("tar-epoxy
lining concrete"),
and 9 ("mortar lining
concrete") ), flow (high slump; "flow concrete"; specimen number: 10) and polymercement concrete ("polymer-cement
concrete"; specimen number: ll) were also prepared.
The basic mix proportions
of the base concrete for the lining specification,
unit cement
weight and water-cement ratio, were respectively
330 kg/m3 and 55 %. The slump of
flow concrete was 18± 1 cm. The polymer cement ratio of the polymer-cement concrete
was set at 10 % and the water-cementratio
at51 %. For the lining, two kinds of resin
and one kind of mortar were used. Each epoxy resin and tar epoxy resin were used as
resin for lining.
The epoxy resin was combined with paint or other materials to form
condensation compounds of bisphenol A and epichlorohydrin.
The denatured aliphatic
polyamine was used as a hardener component. Also, the tar-epoxy resin was made with
a denatured component of tar, which was mixed with bitumen material to form epoxy
resin. The thickness
of the resin lining applied
to the concrete was 400-600 Mm or
600-1000
Mm.The thickness of 400-600 Mmrepresents one coating (specimen number :
7(1) and 8(1)) and 600-1000
Mmrepresents three coatings (specimen number: 7(3) and
8(3)). The mortar for the mortar lining concrete was made with a 34 % water-cement
ratio ; sand : cement =2 : 1 ; and a flow value of 183. The lining quantitywas
measured
from the difference
in test specimen mass before and after the lining was applied. The
lining thickness
was calculated
as the mean value by using the unit mass of the lining
material and the test specimen surface area. Furthermore, there was a possibility
that
a lining primer would impregnate
the concrete and change its physical
properties.
Therefore, no primer was used.
The concrete materials
were mixed using a 50-L tilting
mixer. The concrete was cast
in accordance with JIS A 1132.
All test specimens were cured in water for two weeks
and then under atmospheric conditions
for two weeks. Test specimens were subjected
to an immersion test after.which
the,concrete
surface was coated with the lining or
subjected
to other forms of processing.
3.3 Test Specimens
and Tests
Test specimens measuring 10X 10x40 cm were immersed in the sedimentation
tank
while those in the trough measured 5X 10X40 cm. Thinner test specimens were used
for the trough tests to allow easy installation,
to minimize disruption
to the flow, and
to prevent the loss of the test specimen due to flooding or other reasons. The test
specimens were placed in a stainless
steel net and immersed in the sedimentation
tank.
-57-
There was a possibility
that test specimens immersed in the trough could be washed
away by the water flow. Therefore, they were attached by metal fittings to a stainless
steel chain stretched in the flow direction of the trough. Test specimens were measured
after being raised from their location and washed. Three test specimens were prepared
for each factor.
Each test specimen was also inspected
visually
in its immersion
location
(roughness
of surface, swelling,
peelings
and cracks in the lining layer),
change in mass and measurement of the resonance number of vibrations
(JIS A 1127)
by the deflection-vibration
method. One test specimen for each factor was subjected to
a bending performance test (JIS A 1106).
After the test specimen had been ruptured by the bending performance test, it was
subjected
to visual
section inspection
and measurement of carbonation
depth
(phenolphthalein
method), water absorption rate, pore distribution
(mercury intrusion
method) as well as electron probe microanalysis,
X-ray powder diffraction
analysis,
and X-ray fluorescence analysis.
Multiple
test specimens were examined in cases
where the resin lining showed significant
swelling and peeling.
An overall outline of
this research is shown in Fig.3.
thirteen kinds of specim en
D
0
sedi
!¥_itank
nmIent
oxygen;_
ation;I di
____
f erence
｣1 __
of process
_
I in
troouxy
ghgen ^* mm, i^^B I
nu
nH
n
Hn
d
if erence ofD environm en adH
un
Hn
H
1 trough
I in air
i
; ｫ N4o+
starting testperiod
exp osu re
start of exposure test
visua l insp ection
roughness of surface
sw elling,peeling of lining
exam ining re sin w ith SEM
m ech a n icalp erf orm an ce
bending perform ance
ch an ge in com p o sitio n
carbonation depth
du rab ility
dynam ic m odulus
of elasticity
co rrosio n
m ass loss
p hy sica l ch an g es
w ater absorption
pore distrib ution
D
D
D
N¥ /
Braa
aIS
t
ch em ica l ch an g es
electron probe m icroanalysis
X -ray pow der dif ra ction
fluorescence X -ray an alysis
corrosion f acto rs
calculation offr ee carton dioxide
Fig.3
¥
Outline
esti m ation
corrosion m ech anism
p red ictio n
d eteriorati o n
y
nn
ng
of study
4. RESULTS AND CONSIDERATION
4.1 Change in Specimens with Exposure Period
a) Visual inspection
All test specimens without surface lining treatment (conventional
concrete) showed a
change in color (oxygen process)
at an early stage (3 months after immersion), while
those with a 60 % water-cement ratio immersed in the trough showed a loss of corners.
Erosion proceeded gradually.
After more than 10 years, the surface was very rough,
with exposure of coarse aggregate as shown in Photo 1. For specimens with a cement
ratio of 50 %, some mortar remained among the coarse aggregate
on the surface.
However, for specimens with 55 % and 60 % water-cement ratios, the surface mortar
-58-
had almost all eroded
often lost.
Therefore,
the water-cement ratio
erosion occurring at the
away and only coarse aggregates
were seen. Even this was
we concluded that immersion has a stronger erosive effect as
increases, with more mortar falling away from the surface and
interface between coarse aggregate and mortar.
For 3 months after immersion had begun, specimens with lining showed no color
change or other changes except in one case where swelling of the surface occurred.
As
the exposure period increased,
the lining of some test specimens began to swell and
peel.
Almost all tar epoxy resin linings peeled from the concrete surface leaving a
rough concrete finish.
With the epoxy resin linings, swelling occurred over almost the
entire surface with cracking and peeling of the single-application
resin layer (400-600
Mm). In cases where the lining had been applied three times, swelling was observed
in some specimens. In the case of the mortar lining, long-term immersion caused loss
of corners and exposure of the fine aggregate.
However, the concrete did not become
exposed, because the mortar was I cm thick and had a low water-cement ratio.
With
specimens, the flow concrete and the polymer-cement concrete showed the same degree
of roughness on the surface as the conventional concrete.
No advantage of the flow
concrete and the polymer-cement concrete in comparison to the conventional concrete
was observed visually.
Environmental differences
had a greater effect on erosion in the oxygen processing
system than in the air processing system. The degree of erosion differed with both the
processing
system and the specimen location. The most severe erosion occurred with
specimens placed in the trough of the oxygen process.
This is because these test
specimens were influenced by the difference in the content of the immersion solution
as well as the mechanical action of the processing water flow. In all environments,
the damage with the mortar lining and the three-layer
epoxy lining was less than with
a one-layer epoxy lining and a one- or three-layer
tar-epoxy lining.
No.3
""
No.6
Photo 1 Results
No.ll
of visual
inspection
-59-
(trough
in oxygen process)
b) Corrosion of resin
To examine the resin lining, it was peeled off from test specimens and observed with an
electron microscope.
Photo 2 shows representative
results. These samples were the
three-layer
epoxy and tar-epoxy linings from specimens which had been immersed in
the troughs of the oxygen process for about 13 years. Low-magnification
images of the
cross section show many bubbles of about 100 fJ>min the epoxy, but few in the tar-epoxy
lining.
However, high-magnification
images show the epoxy lining to be almost
uniform while the tar-epoxy lining is not. Both types of lining were coated with matter
which had been floating in the processing water. The high-magnification
images of the
surface also revealed that there were almost no faults in the epoxy lining but pinholes
of 1~10 Mmover the whole of the tar-epoxy lining.
The difference between the two
linings was evident. Examination of the back surface of the lining showed clear traces
of cement hydrates in the case of the epoxy. This indicated
that the epoxy resin had
adhered well to the concrete.
However, in the case of the tar-epoxy, swelling/peeling
was observed in many places, and the hydrates of cement or the resin itself had eroded.
(1) Low-magnification
(2) High-magnification
(1) Low-magnification
(2) High-magnification
(a) Epoxy resin lining
(b) Tar-epoxy resin lining
Photo 2 SEM micrographs
of resin lining (trough in oxygen process,
I : section,
II: surface,
HI: reverse)
c) Nondestructive testing
Changes in the relative
dynamic modulus of elasticity
during the exposure period are
shown in Fig.4.
In the case of the sedimentation
tank in the oxygen process, the
relative dynamic modulus of elasticity
was high until 1 year in all cases.
However,
the value decreased for some resin-lining
test specimens and for most non-lining
specimens immersed in the trough of oxygen process.
Most non-lining specimens
aside from the polymer concrete one immersed in the troughs of the air process showed
values above 100 % with a peak at 3 months.
On the other hand, for some resin-
-60-
140
140
120
C
3100
ensto m sedimentation tank
120
onventional concrete in trough
O
xygon process
Oxygen process
3| 00
-A-
I 80
Q
60
I»
1
540
|40
..à"O-.4
*20
à"à"à"D--5
---A--6
-O.-4
--D--5
i <r
---A-
20
40
20
0
4
6
8
10
12
4
Exposure period Cyoar)
6
8
10
12
Exposure period (year)
140
Oxygon process
120
^>.
|
100
^^5=^
|80
Specimen No
-0-7(1)
-0-
-*-9
-I-10
7(3)
-X-8(1)
---X--- 8(3)
-t-ll
8
t period
Lining,
.1
60
1
tc.
40
20
10
oretain trough
-0-7(1)
12
-0-- 7(3)
-X-8(1)
.1
i40
-X- 8(3) <r
6
8
10
12
Exposure period (yaar)
(year)
E
* Rolative
Fig.4
60
20
4
Lining, flow ind polymar-a
oonorote in aadimantation
Unk
Specimen No.
-0-7(1)
-à"-<>à"à"
7(3) -*-8(1)
-X-8(3)
-*-9
-I-10
-H-ll
gao
flow and polymai-o
Change in relative
D.M.E. : Rolativs
xposure period (ymr)
dynamic
modulus of elasticity!*)
dynamic modulus of elasticity
lining test specimens, the value fell below 100 %. The results for the three
environments differed
as a result of the variations
in processing
water quality,
mechanical action of water, and the period of hydration of the immersed concrete.
For
example, a comparison of specimens from the sedimentation
tank of the oxygen process
and the trough of the air process shows that the former had a more detrimental
effect
on the concrete.
However, in the sedimentation
tank of the oxygen process almost no
erosion due to mechanical action occurs, in contrast with the trough of the air process.
Also, the differences
in the degree of hydration
among the test specimens themselves
led to differences.
Based on these findings, the differences
were considered to arise from the environment
and depend on the moisture
condition,
caused by immersion (non-lining
test
specimens),
the environment during the drying before lining,
and the lining (the
resin-lining
test specimens).
Therefore, test specimens immersed for 1 year were
considered to have undergone a complex process including improvement of the physical
properties
due to the progression
of hydration,
which was then offset by the chemical
action of the processing water or the physical action of the trough water flow. The
relative dynamic modulus of elasticity
of conventional concrete in the trough of the
oxygen process decreasedby
around 5 % between 1 year and 3 years. However, except
for the tar-epoxy lining
specimens,
most others showed values above 100 %.
Differences
in relative dynamic modulus of elasticity
were also found to arise due to
variations
in the unit weight of cement and the water-cement ratio.
The rate of
decrease in relativedynamic elasticity
modulus of the conventional,
flow and
polymer-cement concrete in each environment were comparatively
large as the
exposure period increased, but very small for the resin-lining
test specimens.
The conventional,
flow and polymer-cement concrete showed decreases in relative
dynamic modulus of elasticity
of 10-30 % at the end of the exposure period in the air
process.
Some of the lining specimens showed about a 10 % decrease, but most
showed almost no decrease even after long exposure.
Many specimens immersed in
-61~
the oxygen process could not be measured due to the roughness of the specimen
surfaces.
Thus, it is difficult
to compare the effects of the sedimentation
tank and the
trough.
However, most of the lining specimens in the sedimentation
tank indicated
no decrease, except for the 5 % decrease found for the mortar lining.
For the trough
specimens, the decrease was about 5~20 %. The value of non-lining specimens in the
sedimentation
tank decreased about 20 % (flow and polymer-cement concrete)~25%
(conventional
concrete), and the value of non-lining specimens in the trough decreased
about 35 % (flow and polymer-cement concrete) and also about 25~40 % (conventional
concrete).
Therefore,
the influence of the trough in the oxygen process was greatest
among the three environments.
In general,
decreases in the relative
dynamic
modulus of elasticity
of the conventional,
flow and polymer-cement concrete specimens
were more obvious than for the lining concrete.
Comparison of the environments
shows that the decrease in the air process was of the same degree or larger than in the
sedimentation
tank in the oxygen process, and was largest in the trough in the oxygen
process.
The decrease began within a year for the conventional,
flow and polymercement concrete.
With the lining concrete, almost no decrease was noted under these
exposure environments for 10 years or more. This led us to conclude that lining
concrete can offer protection for a long period.
d) Mass change
The change in mass over the exposure period is shown in Fig.5.
Almost all specimens
showed loss of corners and other changes after 3 months of immersion. Nevertheless,
their mass increased due to absorption
because the initial
mass value was the dry
value before immersion.
The specimens were affected by erosion in the immersion
liquid,
mechanical
erosion from the water flow and other causes of mass loss.
However, the mass loss was small until 1 year because absorption
and hydration
exceeded the influence of erosion.
Mass loss of specimens in the air process and in the
sedimentation
tank of the oxygen process was not observed until 1 year. On the other
hand, all specimens of lining concrete in the trough of the oxygen process had
decreased in mass after 1 year. This result shows that the trough environment in the
C
onvolitional
C
oonorato in trough
Air process
onventional oonorata in trough
Oxygen process
Speoknan
E
xposureperiod (y«r)
No.
E
-2
-6-3à"
à"5
--A--6
xposureperiod (yo«r)
Lining,
-7(1)
à"à"-<>"
E
7(3)
E
xposureperiod (year)
E
xposureperiod (yair)
flow i
sts in trough
-
-0-7(1)
à"--<>à"à"7(3)
-X-8(0
-*-9
-I-10
---t---1l
4
6
8
10
Exposure period (yo.r)
xposureperiod (yaar)
Fig.5
Changes
à"à"-X--8(3)
12
14
in mass in each system
-62-
oxygen process system is severer than the other environments.
Until the third year,
the non-lining specimen immersed in the air process decreased in mass about 3 %, and
there was a 3~7 % mass loss after that period.
Ahigher unit weight of cement gave
some protection
against mass loss.
The mass loss of specimens with a water-cement
ratio of 50 % was less than that of 55 % or 60 % specimens.
Therefore, in non-lining
concrete, low water-cement ratio and high unit-cement weight was useful for corrosion
resistance.
Although
mass loss was not observed for most lining specimens, the
mortar lining specimen showed a few percent mass loss. The flow and polymer-cement
concrete was in the loss range of conventional
concrete and decreased by about the
same degree.
The ultimate mass loss at the end of the exposure period was as follows. In the air
process, non-lining concrete specimens decreased by 6~10 % and the lining ones 0~~
2 % (resin) and 4 % (mortar).
Also, in the sedimentation
tank of the oxygen process,
the mass losses of non-lining ones were 6~8 % and those of lining ones were 0 %
(resin)~3.5
% (mortar).
Furthermore, in the trough of the oxygen process, the mass
loss of the non-lining specimens was 9~16 % and that of the lining ones was 0~5 %
(resin) and 8 % (mortar).
The relative surface area of the test specimens in the trough
of the oxygen process was large, about 1.4 times that in the sedimentation
tank of the
oxygen process.
Even if this is considered,
the mass loss of the non-lining specimens
in the tough of the oxygen process was the largest.
There was no obvious difference in
mass loss with unit weight of cement, though there was influence with the watercement ratio in mass loss. These results show the need for a low water-cement ratio
and a high unit weight of cement to ensure concrete durability.
The mass loss of
non-lining specimens in the trough of the oxygen process was 15 % or more, and the
mass loss of the lining specimens was near that of the non-lining specimens in the air
process and in the sedimentation
tank of the oxygen process.
From these mass
change results, again the trough of the oxygen process is shown to have the severest
environment.
4.2
Physical
and Chemical
Properties
a) Bending strength
Bending strength tests were done in conformity with JIS A 1106. The bending section
of the test specimens in the trough had a width of 10 cm and a bending height of 5 cm.
The bending test specimens were divided into three equal 30-cm spans at loading.
Measurements were done for one conventional,
flow and polymer-cement concrete
specimen without the resin lining and mortar lining for each factor.
For the lining
concrete specimens in the trough of the air process and the oxygen process, all
specimens with a single epoxy coating and a single or triple tar-epoxy coating showed
obvious swelling/peeling
on the surface, and all the lining specimens except those with
a mortar lining in the sedimentation
tank of the oxygen process were measured
individually.
Loading in the bending test, was done in the casting direction.
Because the erosion of
the test specimen
surface
was severe, accurate
comparison of cross section
measurements could not be done. Instead of bending strength,
the maximum load
results were compared. These results are shown in Fig.6.
As noted earlier, the section
-63-
K g liM J
m
x 800
S
o
?
T3
5
_QI
I
Environm ent
D Trough (air process)
H Trough (oxygen process)
Q Sedim entation tank (oxygen process)
600
400
I
200
o
sp
1 i
mI
m1
a
i
m
&
71
7(3
8(1
M a x im u m b en d in g lo a d
F ig .6
E
1v_｣ -0Mx 1T02
A
b
m
8 3)
A
HIp一I+*
*?
-Trough (air process)
-Trough (oxygen process)
- A - Sedim entation tank (oxygen process)
J3
> 0A
Specimen
No.1
Fig.7
2
Effective
3
4
5
section height
6
7(1)
7(3)
by estimating
8(1)
8(3)
9
10
ll
'
maximumbending load
height of the test specimens in the sedimentation
tank of the oxygen process was twice
that of the test specimens in other environments.
Considering
this, the measured
values of bending strength for test specimens in the sedimentation
tank of the oxygen
process would be 4 times greater.
Therefore, these maximumloads are multiplied
by
a factor of 1/4 in Fig.6.
Since most of the measured values are from a single test
specimen only, a considerable
variance is conceivable.
However, as a whole, the
maximumload determined from bending tests on test specimens in the trough of the
oxygen process is lower than that in the trough of the air process.
Therefore, the
results clearly show the differences
between the influence of the environment in the
trough of the air and oxygen process. Thus, these test results suggest that a test
specimen in a particular
environment is influenced by the unit cement weight, watercement ratio, and lining on the surface.
Therefore, durability
increases with a high
unit cement weight, low water-cement ratio, and a lining.
All the test specimens were
subjected to destruction
within the bending span in the bending test.
The maximum
bending load depend on mix proportion factors and the system environment.
The effective section was estimated using the calculation
formula fb = PL/bd2 for the
bending strength based on the compression test results at the start of exposure, where
is P is the maximumload on the test specimen, L is the span, b is the width offailed
section, and d is the height of the failed section.
Estimation
of the effective section
was based on the following relationship.
The specimen exposed in the sedimentation
tank of the oxygen process showed the relation ofd ^ b and also the relation ofd =
b/2 in the trough of the oxygen and air process.
The fb used was 1/7 of the value of the
28-day compressive strength at the time the test specimen was cast. The value of d
estimated using this assumption is shown in Fig.7.
Comparison of the estimated
values showed the effective section of the non-lining test specimens to decrease more
than the epoxy resin lining ones. Also, looking at the difference in environments, the
effective section of the oxygen process specimens decreased more than that of the air
process specimens.
If the effective section of the resin lining specimen estimated to
be larger than the standard value before immersion was taken as equal to the standard
value, the ratio of the effective
section of each test specimen would be as shown in
Fig.8.
However, the estimated
effective
section was calculated
from the standard
value of the bending strength of the concrete based on the compressive strength for the
64-
E l ¥.｣.
1
id
｣m 0.8
2 0.6
> 0.4
+3
-5 0.2
ce
n
Specimen
Fig.8
*3
c
iS I
A
A
H
w
A
A
"Q
D T rough (air proc ess)
O T rough (oxygen process)
A S edim entation tank (oxygen process)
'"
10
8
6
4
2
n
o
o
0
No.
Relative height ratio
resin lining concrete:
6
7(1)
7(3)
(effective
1)
8(1)
8(3)
9
height
10
ll
Mean value ofcarbonation
A
8(1)
2
4
Carbonation
Fig.
of
IZ
10
8
46
P T ro u gh ( a ir p r o c e s s )
OAJ TSI ro
yy g6 en<3Mnt pa(Jnro
eO s.,^
e
2
eIUduUimgh
gMe (nV.UotaxAtio
fOk cO(o
xys )g e n p r o c e s s ) I
m
o
n
SpecimenNo.1
2
3
4
5
6
7(1)
7(3)
Fig.9
J
5
8-n
ｫ
I 1
"g
t
zo
r .o
6
8
10
12
depth (mm)
10 Relationship
between
carbonation depth and
non-effective depth
Q ...
8(3)
depth
10
ll
Fig. ll Geometric section of
specimen at cutting
compressive test specimen cured for 28 days.
Therefore,
the estimated
effective
section may have been too large.
Also, the d value of the lining test specimens shows
a larger value in Fig.7 than the standard value, may have been possible the influence
by the lining.
Further consideration
of this influence is needed.
b) Carbonation
A 1% of phenolphthalein
solution was sprayed on the section tested for bending
performance and the depth of color change was observed.
The specimen was observed
to have three regions from the outside to inward. The outside periphery ofa few to 10
mm
wasa faint brown, the next about 1 mmwas white, and the innermost section was
gray. Spraying with phenolphthalein,
the innermost gray portion changed to red
while the outer brown and white sections did not change.
Therefore, the outer brown
and white regions had undergone carbonation while the inner section had not. The
inner-gray region may not have been chemically
affected by erosion.
It was difficult
to judge whether the white region between the outermost and innermost regions was
due to direct chemical action of the immersion liquid or the secondary action of
equilibrium
reactions maintaining
the chemical balance between outside and inside.
The carbonation depth can be compared if the specimen is not damaged by chipping.
However, the degree of erosion of the cross section differs with test specimen and it is
difficult
to compare carbonation
depths.
Carbonation
depth was estimated to save
extent from the initial face of specimen before immersion by assuming that even the
part eroded and washed away passes through the process of carbonation
->à"
dissolution
->à"
outflow. All conventional,
flow and polymer-cement concrete specimens
showed carbonation depths of about 4 mmto 10 mm, while most the lining specimens
showed no carbonation,
as can be seen from Fig.9.
Also, differences
were apparent
according to the type of concrete and the environment.
Therefore, the non-effective
depth was taken to be half the height of the section (the standard section) before
corrosion minus the height of the estimated effective section in Fig.7.
Comparing the
non-effective
depth and carbonation
depth showed the latter to be larger, as can be
seen in Fig.10.
Thus, the part of carbonation
region contributes
to the mechanical
performance.
However, as mentioned above, if the effective section of the resin lining
specimen is assumed to be the basic effective section, the section of each non-lining
test specimen decreases, and non-effective depth approaches the carbonation
depth.
-65-
c) Water absorption and pore distribution
The test specimen used for the bending performance test were examined for absorption
and pore distribution
characteristics.
The test specimens were those from the trough
in the oxygen process and which were thought to have been exposed to the most severe
corrosion.
To eliminate influence from the edge face, the specimen was cut about 100
mm
along the long axis, and about 50 mm along the short axis and at about 10 mm
sections from the surface to the depth shown in Fig.ll.
The specimens were then
divided into samples for pore distribution,
chemical analysis,
and absorption
studies.
Before measurement, all samples were dried in a vacuum and then immediately
subjected to measurements.
Water absorption depends on the physical properties
of the hardening substance and
the absorption time. However, it was assumed that water absorption does not depend
on time and that the moisture distribution
gradient within the sample was zero in this
case because of its thinness.
Fig.12 shows the relation between water absorption rate
and time. This is compared with absorption over 24 hours as the standard value (the
final value).
Water absorption in the early period approaches the final value. This
tendency is especially
evident at the surface region of sample. In most samples, the
water absorption
rate near the surface at 10 minutes was about 7/10~8/10
of the
standard value. On the other hand, when there was a resin lining (except for the
single coating with tar-epoxy), the interior samples had a the 10-minute absorption of
about 3/10~5/10
of the standard value. Therefore, the physical
properties
of the
surface without a lining differ from those of the interior.
The water that comes into contact with concrete reaches close to saturation
in the
pores near the surface.
Water penetration
can be thought to advance inside via
smaller pores. The pore distribution
near the surface can be considered to change as
the pore size increases through contact with the eroding liquid.
Therefore, the pore
distribution
structure near the surface changes from the original pore distribution,
which is depend on the water- cement ratio and the unit weight of cement.
0 .2
.S
°-15
0 .15
o
ja(0
0.1
I
0.05
N
o.1
-O-No.3
A-
No.5
-A-No.2
- O-
No.4
- O-
No.6
0
500
Absorption
1 000
1 500
500
period (min)
Absorption
1 000
1500
2000
period (min)
N o.7(1)
- O- No.7(3)
N o.8(1)
- A- No.8(3)
-ONo.9 -No.10
- O- No.ll
500
Absorption
( a)
Surface
1 000
1 500
2000
500
period (min)
1 000
1 500
Absorption period (min)
region
Fig.12
Changes
in water absorption
-66-
(b) Inside
2000
Pore size of 6 nm to 200 Mm was measured by the mercury intrusion method.
The
total pore volume of the surface region and the interior is shown in Fig.13 (a) and the
ratio of each to the total pore volume in Fig.13 (b) (surface)
and (c) (interior).
The
total pore volume of the surface region is larger than that of the interior.
With the
exception of samples with a resin lining, the difference in total pore volume between
the surface region and interior is large.
These total pore volumes differ with mix
proportion factors, unit weight of cement and water-cement ratio.
The total surface
pore volume of the mortar lining sample is larger than that of other samples because
the surface mortar region contains many fine aggregate particles
with a large relative
surface area. Therefore, the inter facial zone attacked by the eroding liquid is greater
in the mortar than in the concrete [13].
In the resin lining samples, the difference in
total pore volume between the surface region and the interior may be caused by the dry
curing carried out prior to lining.
0 .2
-=
IV U
5ｫ
Is 8 0
-+->o 6 0
(a) T otalpo re volum e
0.15
a
a
a
jｧ
9
E
n
,«
0.1
-^
i-i
e
ua
D T otalvolu m e of su rface region (co/ cm 3)
T otalvolum e of inte rior (cc /cm 3)
0 .05
Specimen
u
一
6
No. 1
7(1)7(3)8(1)8(3)
9
10
(a
o
m 40
CvJ
20
8c n
ll
0
20
Absorption
40
60
80
ratio (1 0min/24hr,
1 00
K)
0 .2
0
10
Specimon No. 1
ll
£
1
nr~Kr~KrBrra"
0.8
( o)
o
.15
0.1
0
.05
Pore volume ratio (interior)
0.6
0.05
0.1
0.1
5
0.2
0.4
0.2
W
m6n-50nm
n50n-100nm
n100n-2um
~i»n»n!:riri»i.
Absorption
D10urn-
i»i. i»i. Emrgntr:
6
Fig.13
02u-10um
7(1)
7(3)
8(1)
Fig.14
8(3)
9
10
ll
Total pore volume and pore volume ratio
C
(24 hr)
Relationship
between
water absorption and
pore volume
omparing the various pore volumes in the surface region shows that pores of 2 Mm~
10 Mmare present in conventional concrete, although they were thought not to exist.
The surface region is thought to be more influenced by corrosion because pores of2 Mm
~10 Mmare rare in interior.
The pore distribution
was divided into small (below 20
nm~30 nm) and large (over 20 nm~30 nm). Fig.14 (a) shows a correlation
between
absorption over 10 minutes and the absorption
over 24 hours as well as between the
ratio of pore volume over 30 nm to total pore volume. There was also a correlation
between the ratio of absorption
over 24 hours and the total pore volume (Fig.14 (b)).
In the early period, absorption
is thought to be influenced
largely by the capillary
pores, including
those in the inter facial zone between aggregate and cement paste.
Also, although
the final absorption
ratio exceeded the total pore volume, this
difference
was large for interior of the specimens.
This result is mainly influenced by
gel pores less than 6 nm and shows that the gel pores of interior were not attacked by
the eroding liquid.
-67-
d) Electron probe microanalysis
(EPMA)
The surface of a vertical section taken toward the axis of the sample were subjected to
elemental analysis.
The main measurement conditions
were as follows: acceleration
voltage,
15 kV; absorption
current, 2.5X10"7 A; and probe diameter, 10~200 Mm.
Point analysis
of neighboring
corroded parts was also done. Fig.15 shows about a
20-mmwidth section from the middle of a cross section subjected
to the bending test
(Photo 3) with a probe diameter of200 Mm(Fig.15 (a)).
The elements in the corroded
part included carbon, calcium, silicon,
and even sulfur.
There were many cases in
which the carbon was concentrated mainly at the corroded surface and decreased
toward the inside, while there was little calcium at the surface but much further inside.
These results show that carbon (main corrosion factor) and calcium (main component
of cement) show concentration
gradients
which change with specimen depth.
No calcium concentration
gradient was observed in the sample with the resin lining,
though gradients
including one for calcium were confirmed in conventional,
flow and
polymer-cement concrete.
Thus, the coating with resin lining appears to protect
against or reduce the effect of the erosion factors at the surface.
Therefore, the
possibility
is great that erosive carbon dioxide originates
from carbon, although sulfur
can also be suspected.
tec.
0
'
i'
]
i
»
i
5
vol.
13.0
BcVl
<)
3
t)
1J
W-rf TAP
CH-1Of
CH-2 PET
OM »BTE
"
Photo 3 EPMAarea
?tu^
?S?4>g
t2£y£?1««£
i&»ii"& fjf
à"*«?*'*TOW
i
w*?*;-\.**ie£
-wt-
r+!smp--sv - *f**t
serr.
M
Fig.15
|-|
85.000
S.MO à"'
E3.WO
Ca
Si
(b) Point analysis (the arrow
(a) Surface analysis
point in Ca, result in (a) )
Typical results ofEPMA (No.5 sample in trough of oxygen process)
e) Fluorescence X-ray and powder X-ray diffraction
Each sample described
in 4.2 c) was analyzed by X-ray fluorescence
and diffraction
at
the three points: the softening part, the surface, and the inside.
Each sample was
prepared by removing the coarse and fine aggregates
with a sieve. The measurement
conditions
for X-ray fluorescence
analysis
were as follows: anticathode,
Rh; applied
voltage-current,
50 kV-50 mA; analyzer crystal, RX-60, TAP, GE, LiF; and detector, PC,
SC. Also, the powder samples of constant mass were molded into thin disks of less
-68-
than 1 mm to decrease
hand, the measurement
applied voltage-current,
Standard samples were
Standards)
and Standard
measurement errors by Compton scattering.
On the other
conditions for X-ray powder diffraction
were: target, Cu-Ka ;
40 kV-80 mA; and slit system, 1 degree-1 degree -0.15 mm.
several types of Portland cement of NBS (National Bureau of
Reference Material.
The principal
component elements of cement and carbon were analyzed by X-ray
fluorescence.
However, the concrete-attaching
carbon was judged to be difficult
to
obtain with good precision in comparison with other elements because it is a light
element and gave 10 or fewer measuring counts even in the softening part and surface
portion.
Also, every effort was made to prevent contamination of the test specimen
even by fine aggregate,
but the major elements were not considered and the focus was
directed mainly at calcium and magnesium. Fig.16 shows the count ratios of calcium
or magnesium in the softening part, surface, and inside of each sample. The count
ratio of calcium is about 0.8 to 1.0 in the inside, although the ratio fluctuates.
The
ratio in the softening part was shown to be about 0.2 irrespective
of mix proportion
factors, although this value is very low in comparison with the inside.
The ratio for
conventional,
flow and polymer-cement concrete is lower than that for lining concrete,
although
the ratio of the lining
series is almost 1.0, and this result shows a
characteristic
different
from the softening
part and the inside.
This result
demonstrates that calcium decreased due to dissolution
and ionization
in the surface
region in comparison with the inside, where there is much calcium hydrate from the
cement, and shows that little calcium is present in the softening part.
The lack of
calcium may be due to its dissolving
out of the sample.
This agrees with the
concentration
gradient of calcium determined from surface analysis.
On the other
hand, for magnesium, the relative concentration
is large inside each sample compared
with the surface and the softening
part.
The difference
between the amount of
calcium and magnesium may arise due to the calcium concentration
before corrosion
being greater than that of magnesium or due to the magnesium not dissolving because
of its higher solubility
product.
The sulfuric acid, nitric acid, carbonic acid, and other
components suggested as corrosion factors are present in sewage treatment water.
'
3
"6
( a)
.-A
Ca
0.8
A -
"A-
-A
.A.
"A
"A
7(1 )
7(3)
*14
212
tank(Oxygen),..
«ð
No.
Trou^
%., ,(Air)No.5
(b) Mg
- - -A - 一 S o ft e n i n g p a r t
- O - -S u rfa c e
7(1 )
-
7(3}
Specimen
Fig.16
teaimentation
8(1 )
Specimen
Count ratio
(No.7(3)
specimen
-69-
O -
8(1 )
n n e r
8(3)
tank (Oxygen)(Air)
No.5
No.
count:
1) by fluorescence
X-ray analysis
Acid-attacking
hydration
products of cement, such as calcium hydroxide,
ettringite,
and calcium silicate hydrate, react with acid and decompose, although the extent of the
reaction differs with the acid.
Calcium hydroxide was not found in the softened part
and the surface of samples without lining, and even in the lining samples the amount
of calcium hydroxide was small. On the other hand, calcium hydroxide clearly existed
in the interior,
which remained in better condition
as it was not affected as much by
corrosion in comparison with the surface.
Although sulfur was detected at the surface
by surface analysis,
no gypsum was found in the softening part and at the surface by
X-ray diffraction
[14].
Therefore,
it is likely that almost no corrosion due to sulfuric
acid occurred.
Identification
of the components was done by referring to a report on a
similar deterioration
case [6].
The main peak of calcium hydroxide
and calcium
carbonate and the peaks of the quartz and feldspar groups included in the aggregate
and other materials were identified.
Magnesium compounds were also identified
but
at a very low level in comparison with the other components. .However, they could not
be ignored in forming a hypothesis.
The results
are presented
in Fig.17.
The
headings used in the figure are those given by JCPDS (Joint Committee on Powder
Diffraction
Standards)
and thus some may not have been present in the actual
samples.
Aggregate component
M: mica , Q: quartz, FG: feldspar group
g Cement hydrate or reaction substances30
i) o u r ia utj r tjg｣iuuO
L
I
CSH:
C SH : CaO-SiCh- HaO type hydrate
P:P: cal
calsium hydroxide, Ett: ettringite
CC aASoH:
aA S
monosulfate
C aCC
I
30
No.l
O xy gen
｣ 2&20
n
o ou.
mo
ok
n. o
n
H ^^
s^
HH ^^M Sfi
n
Sd _
-A J _
- 3_
Tank
Trough
A¥ir_ 5
0
I
m s m m
HJ .
20
60
26C)
Fig.17
5
Results
of powder X-ray diffraction
. EXPLAINING THE CORROSION MECHANISMANDPREDICTING DETERIORATION
5.1
Explaining
the Corrosion Mechanism
The main factor causing corrosion from the outside toward the inside of the test
specimen is, according to the results of this study, erosive carbon. The erosive carbon
present in sewage process water includes free carbonic acid which is released
on
reaction with the calcium ions in concrete. Quantitative
analysis of the interaction
between this carbonic acid and cement compounds or hydration
products is difficult
due to the effects of changing water quality, water flow, and the other factors related
sewage process water.
However, in general,
the erosion mechanism due to free
carbonic acid is thought to be as follows. Sewage process water quickly penetrates
into
the concrete, with the degree of penetration
and the resulting
corrosion depending on
-70-
the composition of the concrete.
The erosive free carbonic acid links with calcium ions
at the surface or penetrates
into the concrete through the inter facial zone between
aggregate particles
and cement. As it penetrates,
it reacts with hydration products,
such as calcium hydroxide,
deposited
at the and forms calcium carbonate.
By penetrating
into capillary
pores, it reacts with hydration products or unhydrated
cement compounds. In this process, carbonation
occurs and causes corrosion of the
concrete, which is then affected by water flow and other conditions.
The degree of
corrosion differs with water-cement ratio and unit cement weight, because the reaction
rate of the penetrating
process water is influenced by pore volume and reaction area
and is controlled
by the pore structure, including interface area. For instance, even if
the total pore volume is equal, differences
can occur in the area of the wall that the
pore forms due to the pore diameter.
The total pore surface area becomes large if
there are many micropores.
Therefore, with a constant process water quality, the
reaction is greater for concrete with small water-cement ratio. The reaction rate
decreases rapidly toward the inside of the sample. This decrease also becomes more
significant
as the total reaction area increases.
When there is much free erosive carbonic acid in the sewage process water, calcium
carbonate is the main product at the reaction site.
Although the pores become
temporarily less porous, calcium carbonate dissolves on attack by free erosive carbonic
acid.
The calcium ionized in the process water again becomes linked with free erosive
carbonic acid. As a result, the calcium dissolves andis consumed. In such areaction
process, there is a boundary phase of alkali type compounds such as calcium or
magnesium compounds between the corroded portion and the non-corroded portion,
and this can be observed from the discoloration
described in section 4.1 b). This type
of reaction is repeated near the surface of the eroding concrete.
The concentration
gradient of calcium ions between the surface and the inside was confirmed even by
surface analysis in section 4.2 d). This mechanism shows that calcium ions move to
the surface due to the concentration
gradient and may cause gradual changes in the
structure of the intact portion.
Thus, the following may occur : secondary corrosion
-»erosion-^deterioration
of structure further inside.
5.2
Predicting
Deterioration
Deterioration
of the system under consideration
is mainly controlled
by the corrosion
rate of concrete.
However, a decline in the performance of reinforced concrete may
occur if corrosion occurs in the concrete cover and the reinforcing
rods. Therefore,
corrosion may invade more deeply than the index indicates.
Thus, the progress of
carbonation
and decrease of cover concrete become the most important indices of
deterioration.
We have reported that the existence of free erosive carbonic acid
differs over the years with fluctuations
in alkalinity
and other factors of the process
water [7],[15].
As described
in Section 2, the free carbonic acid in the process water
can be divided into subordinative
and erosive carbon dioxide, with both considered
related to alkalinity
[16].
Calculating
the free carbon dioxide using a previously reported method [17] from the
pH and alkalinity
values, which have been regular test items in water quality tests
over the past 20 years, the change in free carbon dioxide over time at the facility
is
-71-
shown in Fig.18.
For the calculation,
the mean value for one year was adopted, with
means found for each month. Also, the relation between the free carbon dioxide and
erosive free carbon dioxide was calculated
and is shown in Fig.19, which presents the
correlation
between erosive free carbon dioxide and free carbon dioxide.
The increase
in free carbon dioxide suggests the possibility
of an increase in the amounts of erosive
free carbon dioxide.
50
60
- -A- -Inflow
40
S
edimentation
50
Processing
"x 30
20
40
3
10
it
0
à"=
10
12
14
16
18
20
22
30
-g
Period (year)
^50
g
>fc
20
«
10
D.
^40
CD
à"D
'S
30
o 20
-e
0
10
10
Period
Fig.
20
Free carbonic
18
Changes
.12
Trier
14
16
(year)
in free C02 in each system
18
20
22
IP
«= à"
40
acid
60
80
(C02 ppm)
RplfltinnQViin
hptwppn
Jf
free CO2 and
erosive free C02
Comparison of the free carbon dioxide in each system shown in Fig.18 indicates
little
fluctuation
among locations,
such as the inflow, processing,
and sedimentation
processes, in the air process.
However, with the oxygen treatment process, with some
exceptions, there was a large amount offree carbon dioxide in the process water. This
shows that a large amount of carbon dioxide is discharged
when organic substances are
decomposed by bacterial
activity with aeration.
Also, the mean values for the first
10 years slightly
exceed those for the latter 10 years.
In water supply facility
design,
there is a general possibility
of a large amount of erosive free carbon dioxide being
present when the free carbon dioxide
level exceeds 20 mg/1.
Although aeration
processing or alkali processing
has been recommended previously [11], the calculated
free carbon dioxide value exceeds 20 mg/1. According to previous work, severe erosion
mayoccur when the level of erosive free carbon dioxide reaches 60 mg/1 or more [18].
This value does not exceed the calculated
value of the erosive free carbon dioxide.
However, even if each value is below 60 mg/1, the degree of the erosion may be high if
the hardness of the water is low. Fig.20 shows the relation between the calculated
free carbonic acid in the process water and the mean value of the mass loss of
conventional
concrete in the trough of the oxygen process.
The value of free carbon
dioxide
was calculated
from values accumulated until the year prior to mass
measurement. The mass loss shows close correlation
to the accumulated value offree
carbon dioxide.
Therefore, the mass loss of conventional
concrete is considered
to be
influenced by the free carbon dioxide.
The relation between the mass loss of the test
specimen in the trough environment and the accumulated value of free carbonic acid
over one year, shows the state on the same regression curve in all values of the air and
-72-
20
20
O Trough (air)
Regression
15
Tank(oxygen)
0.973
0.971
0.971
0.965
0.981
0.980
s
O
FT
curve
Y=0.80l
V~X-3.433
Y=0.946V
X-2.906
à" Y=1.131V~X-4.386
à"Y=0.695V
X-3.207
à" Y=0.864V
X-2.343
Y=1.031
V~X-4.963
à" Trough (oxygen)
°
10
10
Conventional concrete in trough
(Oxygen process)
0
100
Accumulated
Fig.20
200
value of carbon dioxide
300
' 400'
0
(ppm-year)
1 00
X: Accumulated
Relationship
between
free C02 and erosive C02
Fig.21
200
value of carbon dioxide
300
400
(pprrryear)
Prediction
(regression
curve accumulated
C02 against mass loss)
oxygen process.
In the test specimen in the sedimentation
tank of the oxygen process,
a different
tendency was observed in the regression curve than for the result in the
trough of the oxygen process in the same free carbon dioxide
environment. These
results show that the erosion rate is affected not only by the water flow but also by the
water quality in the processing system environment, and that the action of the flow in
the trough promotes physical erosion in addition to chemical erosion.
Fig.21 shows
the regression
curve applied to the relation between the accumulated value of free
carbon dioxide and the mass loss of conventional concrete in the trough of the oxygen
process.
Mass loss is related
to the square root of the accumulated value of free
carbon dioxide as follows:
Y
=aV~ X+jS
(1)
where, Y: mass loss (%); X: mean value of accumulated free carbon dioxide until the
year prior to mass measurement (ppmà"year);
a , j8 : regression
coefficient
and
constant of regression curve.
Each regression
curve based on expression (1) showed extremely good correlation.
As described
above in predicting
deterioration,
the most important factors are mass
loss, which is an estimate of the decrease in the amount of cover concrete, and the
progression
of the carbonation
depth, or erosion depth, which are closely related to
mass loss. The relations
between mass loss, carbonation depth, and erosion depth in
the trough are shown in Fig.22.
Variations
were observed in the linear regression
results but the mass loss correlated
with the depth ofcarbonation
and erosion.
Thus,
carbonation
depth and erosion depth can be expressed
in terms of the annual
accumulated value of free carbon dioxide in a similar way to expression (1).
(2)
Y '=a'/~X+j3'
where, Y': carbonation
depth or erosion depth (mm); X: annual accumulated free
carbon dioxide (ppm-year);
a', j8 ': coefficient
and constant.
The annual variation was predicted from the variation in free carbon dioxide over the
past 20 years in Fig.18.
Little
fluctuation
in the level of free carbon dioxide is
expected over the long term. About 30 ppm deterioration
can be expected annually as
the mean value from the movement average over time. For example, the largest mass
-73~
14
" -O
o
cb
.2
CD
o o
o
2
oo
oo
cP
10
X: Mass loss
15
20
(X)
5
Y = 0 .25 3 3X
(R 7: 0.8 33 4 )
10
15
20
X: Mass loss (X)
(a) Relationship
between mass
(b) Relationship
between mass
loss and carbonation depth
loss and erosion depth
Fig.22
Relationship
between mass loss due to carbonation depth or erosion
loss of 20-25 % over 20 years is predicted
for concrete in the trough of the oxygen
process, which is the severest environment.
Calculating
the test specimen area based
on a mass decrease of20% gives an erosion depth of about 3 mm. On the other hand,
an erosion depth of about 5 mm is obtained from the relation with mass loss based on
expression (2).
The difference
in erosion depth is nearly twofold.
Changes in erosion
depth or carbonation
depth might be estimated
from measured mass loss data and
calculated
free carbon dioxide.
This method can be used to predict deterioration
and
suitable
repair times.
However, some structures may require repair earlier than
predicted
due to invasion by corrosion greater than the deterioration
index and for
other reasons.
Further detailed
studies
are needed, because the corrosion and
deterioration
of concrete is influenced
by the water quality, flow, and other changes in
the environment.
Continuous control of water quality
is necessary
based on
consideration
of bacterial
and protozoa activity, the water flow, and other factors.
6
. CONCLUSION
The following conclusions
were reached from the results of exposure tests conducted
over 10 years in the end processing system of a sewage treatment facility.
(1) Changes in appearance,
decreases in mass, and changes in relative
dynamic
modulus of elasticity
were observed with exposure time for various test specimens.
Differences in environment led to variations
in the degree of corrosion, with the most
severe suffered by specimens in the trough of the oxygen system. Looking at mix
proportion factors in the case of non-lining concrete, the mass loss at a water-cement
ratio of55 % and 60 % was larger than thatat
50 %. Therefore,
a low water-cement
ratio is one effective way to control corrosion.
Also effective
is to increase the unit
cement weight.
Still another is treating
the concrete surface with a lining.
Almost
no corrosion was observed for test specimens with epoxy-resin lining even after more
than 10 years.
(2) The decrease
in maximumbending
load was large after exposure for more than 10
-74
years, with particularly
serious decreases in specimens with high water-cement ratios
and without lining in the trough of the oxygen treatment process. Therefore, even the
bending strength decreased in the case of high water-cement ratio or without lining.
In contrast, corrosion can be prevented by adapting a high unit cement weight and low
water-cement ratio, and also by lining the concrete.
(3) The decline of the physical properties
of the concrete as seen in the pore structure
was obvious in the surface region. Therefore, sewage process water was judged to
have a great influence on the pore structure.
Change in the pore structure clearly
differed
with or without lining.
Therefore,
the use of a lining gives effective
protection
against long-term attack by process water, as in this facility.
(4) In this processing
system, corrosion is considered
to have been mainly caused by
erosive free carbonic acid. The carbonates decompose cement hydration products of
concrete and finally
reduce them to ions.
The structure is gradually
destroyed
through the mechanism of ion dissolution.
(5) The mass loss, carbonation
depth, and erosion depth were judged to be related to
changes in the amount of free carbon dioxide, based on calculations
offree and erosive
free carbon dioxide and various experimental results.
Applying a regression curve to
the above relation
showed a good correlation,
especially
between the free carbon
dioxide and mass loss. This allows the deterioration
to be estimated and the repair
time predicted.
Control of the water quality can reduce the progress of deterioration,
which is influenced
by changes in bacterial
and protozoa activity or water flow and
other factors.
Acknowledgements
Wewould like to thank the manager of the sewage treatment facility, who allowed us to
conduct this study at that facility
and offered us long-term support. Also, we
appreciate
the support of the our colleagues
who helped us in carrying out this
research.
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