Civil Engineering Reference
In-Depth Information
(Polder and Rooij, 2005; Polder, 1997; Bertolini et al., 2004; Bamforth and
Chapman-Andrews, 1994; Rooij et al., 2007).
Design for 100 years or more requires some form of mathematical
modelling that extrapolates degradation processes in concrete structures
beyond our modern experience. Various models have been developed
(Collepardi et al., 1972; Maage et al, 1996; DuraCrete, 2000; Siemes et
al., 2000) and in the early 2000s significant experience was gained with the
DuraCrete model in some major construction projects in the Netherlands.
In 2003 a Dutch industry-wide committee, CUR VC81, set out to develop
a practical probability-based guideline for service-life design of structural
concrete with service lives up to 200 years, based on DuraCrete. Due to
limited experience with this method, it was agreed that the requirements
of the prevailing Dutch concrete standards should be taken as upper limits,
which corresponded to international regulations (e.g. EN 206). This implied
the usual maximum water-to-cement ratios and minimum cement contents
depending on environmental class, with additional requirements to the
minimum length of the curing period. Under these conditions chloride-
induced rebar corrosion is likely to be the dominant mechanism determining
the service life, whereas carbonation-induced corrosion can be ruled out
safely. The Guideline was published in Dutch in 2009 (CUR, 2009) and is
described in (Polder et al., 2010). It should be noted here that the Dutch
climate is relatively mild and wet, resulting in moderate de-icing salt loads
and limited carbonation in concrete with at least a reasonable quality. In
other climates, the chloride load could be more severe (e.g. for marine
conditions in warmer climates or for de-icing salt exposure in colder
climates) or carbonation could be more important (e.g. for climates with
longer dry periods). Carbonation modelling is discussed in (CEB, 1997), see
also (FIB, 2006).
The basis for a probabilistic performance-based methodology for service-
life design (SLD) was conceived in the 1980s by Siemes et al. (Siemes et al.,
1985) and developed in detail in the 1990s in European research project
DuraCrete (DuraCrete, 2000; Siemes et al., 2000). It follows structural limit
state design philosophy by stating that the service life is the period in which
the structure's
resistance
R
(
t
) can withstand the environmental
load
S
(
t
).
R
(
t
)
and
S
(
t
) are time-dependent, statistically distributed variables. A particular
(specified)
performance
is predicted with a predetermined maximum
probability of failure at the end of the design life, as shown schematically in
Figure 15.1.
Scatter, variation and uncertainties, e.g. in material properties,
geometries (concrete cover) and model parameters, are to be taken into
account by probabilistic considerations.
The limit state is assumed to be initiation of reinforcement corrosion
due to ingress of chloride ions. When the chloride content at the surface
of the reinforcing bars exceeds the critical chloride threshold, the structure
is considered to fail. The
load
is represented by the chloride content at
the steel surface that increases with time due to chloride ions building
