Civil Engineering Reference
In-Depth Information
0.6% by mass of cement, see (Polder, 2009; Gehlen, 2000; Gaal et al., 2003;
Breit, 2001). This value applies only for concrete exposed to air, that is in
environmental classes XS1, XS3, XD1, XD3. In XS2, concrete permanently
submerged in sea water, the critical content is higher due to low availability
of oxygen, except for structures that are submerged in sea water on one side
and aerated on the other side (so-called hollow leg).
15.3.3 Time dependency of chloride penetration coefficient D(t)
The rate at which chloride ions penetrate into concrete decreases with
time. This is due to hydration of the binder, which causes narrowing of
capillary pores (especially in binders with slag or fly ash); and drying, which
reduces the amount of liquid in the pores available for chloride transport.
A decreasing rate of chloride penetration can be described with a time-
dependent diffusion coefficient (Maage et al., 1996):
n
t
⎛
⎞
(15.3)
D
(
t
)
=
D
0
⎜
⎝
⎟
⎠
0
t
where
D
0
is the
D
RCM
-value at reference time
t
0
(usually 28 days) and
n
is the ageing coefficient (0 <
n
< 1). The value of
n
for a particular binder
in a particular environment depends on the rate of hydration and on the
extent of drying. Ageing coefficients for different binders without the effect
of drying were determined from
D
RCM
-values that had been wet (immersed)
cured at 20
C for different periods from 28 days up to 3 years. In those
experiments, slag and in particular fly ash binders showed higher
n
-values
than Portland cement. This corresponds to results of chloride profiles from
cores taken in the field (Gehlen, 2000). Structures in the field will dry out
to some extent and hydration may occur more slowly than under water. In
°
of environmental classes
Environmental classes
Underground, splash
zone
Above ground, marine
atmospheric
Type of binder
XD2, XS2, XS3
XD1, XD3, XS1
CEM I
0.40
0.60
CEM I + 25-50% slag, II/B-S;
III/A, < 50% slag
0.45
0.65
CEM III, 50-80% slag
0.50
0.70
CEM I with 21-30 % fly ash or
CEM II B/V
0.70
0.80
CEM V/A with c. 25% slag and
25% fly ash
0.60
0.70
