Geology Reference
In-Depth Information
2. CORROSION INITIATION
AND PROPAGATION
(2000) suggest an idealized isotherm which
simulates the exposure conditions of marine
structures. This isotherm is called Freundlich
isotherm and its relationship can be expressed as:
Chloride-induced corrosion is identified as one
of the major causes for the structural deteriora-
tion of reinforced concrete bridges. This type of
corrosion is initiated by the ingress of chloride
ions into structural concrete members during the
concentration and diffusion cycles. The sources
of chloride ions are mainly air-borne sea-salts
in coastal areas and deicing salts used in winter
times. This chapter focuses on the corrosion
resulting from sea-salt particles floating in the
air and assumes that the diffusion process is the
dominant mode of chloride intrusion. In order to
study the diffusion process, it is essential to find
the changes in the chloride content at different
depths of the concrete member. The total chloride
content refers to the total acid-soluble chloride in
concrete, which is the summation of free chlorides
and bound chlorides. The relationship among the
total, C t , free, C f , and bound, C b , chloride content
in unsaturated concrete is as follows:
C
C
β
β
1
C
=
α
C
b
=
α β
C
(3)
F
F
b
F
f
F F
f
f
where α F and β F are the Freundlich binding con-
stants equal to 1.05 and 0.36, respectively. The
chloride diffusion coefficient in Equation 2, D Cl ,
is calculated by taking into account the effects
of major influential parameters, such as water
to cement ratio, ambient temperature, relative
humidity, age of the concrete, free chloride con-
tent, and chloride binding capacity. The chloride
diffusion coefficient can be determined from a
diffusion coefficient estimated for a reference
temperature and humidity, D Cl,ref , multiplied by
the modification factors as below:
( )
( )
( )
D
=
D
F T F h F t F C
(
)
(4)
Cl
Cl ref
,
1
2
3
e
4
f
C
=
C
+
w C
(1)
where F 1 ( T ) accounts for the dependence of
chloride diffusion coefficient on the ambient
temperature, F 2 ( h ) represents the influence of
relative humidity, F 3 ( t e ) denotes the influence
of concrete age, and F 4 ( C f ) considers the effects
of free chloride content. Table 1 summarizes the
mathematical expressions for the modification
factors used in Equation 4 (Bažant and Najjar,
1972, Saetta et al., 1993, Bamforth and Price,
1996, Xi and Bažant, 1999, Martin-Perez et al.,
2001, and Kong et al., 2002).
In Table 1, F 1 ( T ) is calculated using the tem-
perature data, T , gathered for a specific region
where the structure is located. In the current study,
the temperature data of the Los Angeles area dur-
ing last 15 years (from 1995 to 2009) has been
collected from the National Oceanic and Atmo-
spheric Administration (NOAA). It is evident
from this database that the temperature has a
periodic trend over the year and a sinusoidal func-
t
b
e
f
where w e is the evaporable water content (m 3 of
evaporable water per m 3 of concrete). According
to the Fick's second law which is based on the
mass conservation principle, the diffusion process
is expressed as the change in the free chloride
content over the time, t (Equation 2).
C
t
D
) (
)
f
= −
fdiv
Cl
C
(
f
+ (
)
1
1
/
w
C
/
C
e
b
f
(2)
where D Cl is the chloride diffusion coefficient and
(
)
b / is the binding capacity. The chloride
binding capacity characterizes the relationship
between the free and bound chloride ions in con-
crete at a constant temperature and it is also referred
to as the binding isotherm. Martin-Perez et al.
C
C
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