Civil Engineering Reference
In-Depth Information
15.4 Service-life design in practice - examples
Following the method described above, including a safety margin to the
cover depth of 20 mm for reinforcing steel or 30 mm for prestressing
steel, combinations of required cover depth and maximum
D
RCM
values (at
reference time 28 days) were calculated for service lives of 80, 100 or 200
years. Wet curing was assumed to last for three days (XD) or seven days
(XS). Values for 100 years are presented in
Table 15.2.
The following two
examples illustrate using it for service-life design.
Example I
concerns a reinforced concrete structure in XD1-3/XS1
environment. For the type of cement a CEM III/B with 70% slag was chosen.
The required service life is 100 years. From
Table 15.2
it can be seen that with
a cover depth of 45 mm (reinforcing steel), a maximum
D
RCM,28
is required
of 6.0
×
10
-12
m
2
/s. With this cement and a
w
/
b
of 0.45, a
D
RCM
-value of 4.0
×
×
10
-12
m
2
/s the cover depth
could be reduced to 40 mm.
Example II
concerns the same structure as Example I. The cover depth
is 45 mm, but now CEM I is used. For CEM I and a cover depth of 45 mm,
10
-12
m
2
/s. Even though
D
RCM
is
higher than in Example I, such a value might be hard to achieve with CEM
I (see
Figure 15.2).
It would require quite a low
w
/
b
, probably below 0.4,
which may cause workability problems. Increasing the cover to 50 mm will
allow an increase of
D
RCM,28
to 12
×
×
10
-12
m
2
/s, which can be readily achieved
with a
w
/
b
of about 0.45.
Navigating through all possible options in the tables, the designer can find
the economic optimum, while it can be demonstrated to the client that the
required service life is achieved.
15.5 Application to existing structures
For existing structures, the situation is obviously quite different from that
for design of a new structure. In a design situation, assumptions have to
be made for material properties, cover depth and environmental load;
the possibilities for obtaining information from testing in the intended
environment are limited. For an existing structure, the material composition
and cover depth cannot be changed. However, testing can reveal cover
depths, material composition and other properties (diffusion coefficient,
electrical resistivity) and the structure's response to the prevailing
environmental load (chloride profiles, carbonation depths). This requires
a detailed inspection including sampling and laboratory testing. It should
be noted that considerable numbers of samples may be required to obtain
a sufficiently representative picture, as scatter tends to be high (Polder and
Rooij, 2005; Rooij and Polder, 2005). It is also important to realise that
the critical chloride content for corrosion initiation may deviate from the
