Civil Engineering Reference
In-Depth Information
revision of the earlier CEB-FIP Model Code 1990 (CEB-FIP,
1992) which focused mainly on the structural aspects associated
with the design of concrete structures. However,
ib
Model Code
2010 takes a holistic through-life perspective upon design, con-
struction and in-service care and management of concrete struc-
tures. Accordingly
ib
Model Code 2010 also addresses issues
concerned with matters such as sustainability, through-life care,
impacts and management, as well as providing detailed and
extensive guidance upon structural design issues. This broader
treatment of the wide range of issues involved is reflected in
a consistent manner in the recommendations made concerning
execution, the selection and use of materials and conservation,
as well as in the associated service life design concepts. It is
thought that
ib
Model Code 2010 is the first structural design
code to adopt this philosophical approach.
and the associated requirements for design supervision and
execution control.
Should a structure experience deterioration in service, con-
sideration would need to be given to the potential implica-
tions of the nature and rate of deterioration upon the strength
of a structure, as noted above. This would involve making a
prognosis of the change in condition and strength with time,
together with the length of time before any critical limit states
(e.g. cracking or spalling of concrete, critical reduction in
strength, etc.) will be reached.
Figure 5.16
presents a pictorial
representation of this, showing how these circumstances would
be addressed in the probabilistic service life design approach.
5.5.4 Approaches to service life design
Service life design approaches may be divided into two princi-
pal design strategies:
Service life design strategy A:
Avoiding deterioration by the
design-out approach.
Service life design strategy B:
Providing resistance to the de-
terioration mechanisms active
in the service environment.
The two principal service life design (SLD) strategies can
be subdivided into a number of different methodologies for
achieving the service life design objectives of the particular
design strategy. The approaches adopted for the various struc-
tural materials differ in terms of their application and practical-
ity in various circumstances.
It is perhaps easiest to illustrate the concepts in respect of one
structural material, and this will be done for structural concrete.
A classification of the various service life design strategies and
associated methodologies for structural concrete is given below.
SLD Strategy A: Avoiding deterioration by the design-out
approach
on the basis of:
A1.
Changing the service environment to remove the deteri-
oration mechanisms.
A2.
Using non-reactive materials to avoid potential deteri-
oration reactions.
A3.
Inhibiting the potential deterioration reactions.
SLD Strategy B: Providing resistance to the deterioration
mechanisms active in the service environment
on the basis of
adopting:
B1.
5.5.2 What is service life?
Box 5.4 discusses the issue of the required service life.
However, in practice there are numerous different definitions
of what constitutes the service life of a constructed asset,
depending on the type of performance being considered. Five
examples of other service life definitions are given below:
■
Technical service life:
the actual time in service until a defined
minimum acceptable performance (functional) state is reached,
perhaps due to deterioration associated with the service
environment.
■
Functional service life:
the actual time in service until the struc-
ture, component or system becomes obsolete due to changes in the
performance (functional) requirements, probably due to changed
operational requirements.
■
Economic service life:
the actual time in service until replace-
ment of the structure, component or system is economically more
advantageous than bearing the maintenance/intervention costs
associated with keeping them in service.
■
Extended service life:
the increased length of time the structure,
component or system is required to remain in service due to
changed performance (functional)/operational requirements; this
is longer than the length of service life anticipated at the time of
design and construction.
■
Achieved (or realised) service life:
the time that the structure,
component or system actually remains in service (and presum-
ably performing satisfactorily) until decommissioned, dismantled,
demolished or otherwise removed from service.
It should be noted that generally the definitions of the above
and related issues given in different guidance documents,
standards and codes of practice are not entirely consistent.
Deemed to satisfy (code) provisions.
B2.
A single or multi-stage (barrier) protection strategy:
B2.1
Basic resistance using a single protection strategy.
B2.2
Enhanced resistance using a multi-stage protec-
tion strategy.
5.5.3 Structural and service life design considerations
Structural and service life design needs to achieve appropri-
ate levels of safety, in addition to seeking to ensure that the
constructed asset is durable for the chosen design service life.
These issues are addressed by establishing the required level of
reliability for the structural and service life (durability) design
B3.
The factorial method (adapted from BS ISO 15686:
Part 8 (BSI, 2008)).
B4.
A reliability-based methodology:
