71
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
by CO
2
in atmosphere, the chloride-induced corrosion of reinforcement steel by the chloride
in sea water, the physical and chemical attack of salts (SO
4
2-
, Mg
2+
) in sea water and soils.
Moreover, the alkali-aggregate reaction and the delayed-ettringite formation (DEF) should be
avoided given the crucial function of concrete elements. The global philosophy of durability
design with respect to these deterioration processes is to formulate the requirements on both
material and structural levels, combining the material design with the structural design to
achieve a working life of 120 years. For the raw materials of structural concretes, the mass
ratio of calcium aluminates (C
3
A) in cement is controlled to 5%-8%; the SO
3
content is
controlled to below 4% of binder; the aggregates are required to be not alkali-reactive and the
alkali content of concrete is controlled to 3kg/m
3
. By these specifications, the risks of salt
attacks and internal expansion reactions are regarded low enough to be acceptable.
Accordingly, the carbonation-induced corrosion and chloride-induced corrosion remain as the
most critical processes for durability design.
The design working life of whole project is 120 years. Aiming at this target, the durability
design at structural level should firstly determine the working lives
for structural elements on
the basis of their structural importance and technical feasibility. The principal elements adopt
the same working life as whole project, 120 years, and the secondary or replaceable elements
can adopt shorter lives. For these elements, the maintenance and replacement schemes should
be specified in design phase. Durability limits states (DLS) are needed for quantitative
durability design [2]. These states are specified as the minimum acceptable performance
levels for different durability processes [3]. For carbonation-induced and chloride-induced
corrosion processes, two DLS can be defined: (a) corrosion initiation; (b) corrosion to an
acceptable extent. In the project, PC elements, principal RC elements and RC elements with
high maintenance difficulty should adopt DLS (a) while secondary RC elements can adopt
DLS (b).
2.2 Model-based design for chloride ingress
The design model for chloride induced corrosion is adapted from the widely used analytical
model of Fick’s second law [4,5]. With the DLS specified as the corrosion initiation, i.e. DLS
(a), the design equation in partial factor format writes,
cr
1
0
Cl
G
1 erf
0
2
nom
d
d
s
s
c
D
SL
C
x
x
C
D
t
(1)
Here, C
cr,s
stand for the threshold chloride concentration for steel corrosion and the concrete
surface chloride concentration (%binder) respectively; x
d
nom
and
x
d
are the concrete cover
nominal thickness and its construction error (m);
D
Cl
0
is the chloride diffusivity of concrete at
a given age (m
2
/s); is the ageing factor of concrete chloride diffusivity (-); t
SL
is the design
service life of structural elements; erf is the mathematical error function. The terms,
c,s,D
,
are the partial factors for the corresponding parameters. The ageing factor describes the
gradual decrease of concrete chloride diffusivity with time,
72
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
0
Cl
0
0
Cl
,
n
t
D
t
t t
D
t
with
0
0
,
30 years
,
30 years
t t
t t
(2)
Here, the term n is the exponential coefficient for the ageing law and D
Cl
0
the concrete
diffusivity at age
t
0
. Since it is not rational to assume this densification is to develop
infinitively, this decrease law is truncated at
t = 30 years to ensure the conservative design.
To apply this design model, the characteristic values for the parameters and their
corresponding partial factors should be determined for the concrete
structures in HZM
project. To this purpose, the statistical properties are regressed for the design parameters, C
cr
,
C
s
, x
d
, D
Cl
0
and n, from the long-term exposure tests on a similar exposure site (Zhanjiang
Exposure Station) and structural inspections during the recent 30 years. The detailed
statistical properties and the regression details were reported in [6,7]. On the basis of the
obtained statistical properties, the partial factors and characteristic values for design
parameters are calibrated through a fully probabilistic scheme for a target reliability index of
=1.3 [7].
2.3 Model-based design for carbonation
The carbonation model is an extended form of the widely used square time law for
carbonation depth x
c
[4,5], and the design equation is expressed in partial factor format as,
-1
2
SL
ACC,0 R
CO2
SL
RH
G
2
0
d
c
d
t
e
c
t
t
h
x
x t
x
W
k
k k R
C
t
(3)
Here,
W
t
is the weather function expressing the influence of atmospheric precipitation on
concrete carbonation process (-);
k
e
is the environmental factor of humidity with detailed
expression in (fib 2006) (-);
k
c
is the curing factor defining the influence of curing time during
concrete hardening (-); (
k
t
R
ACC
,
0
-1
+
t
) is the carbonation resistance of concrete in natural
environment; R
ACC,0
-1
is the concrete carbonation resistance in accelerated test conditions
(20ºC, RH=65%), in (mm
2
/year)/(kg/m
3
); k
t
is the regression parameter between the two
resistances;
t
is the error term; C
CO2
is the CO
2
concentration in atmosphere (kg/m
3
) and t
SL
is the design life (year). The terms
RH,R
are partial factors for relative humidity and concrete
carbonation resistance. The equation (3) takes the corrosion initiation as DLS.
The design value of cover thickness x
d
refers to x
d
nom
-
x
d
. The statistical properties for the
parameters needed for partial factor calibration can be found in [5]. Four representative cases
are considered: interior surface and exterior surface of concrete elements for service lives of
50 years and 120 years. Compared to interior surface, exterior surfaces have elevated
humidity and lower weather function due to rain exposure conditions. For CO
2
concentration,
a high concentration of 32.8×10
-4
, four times of design value, is also retained to consider the
possible CO
2
accumulation in the tunnel by automobile exhaust. The characteristic values of
concrete cover thickness
x
d
nom
and the carbonation resistance
R
ACC,0
-1
are retained as design
parameters.