Civil Engineering Reference
In-Depth Information
Distribution of R(t)
R(t)
R,S
P
f
S(t)
Distribution of S(t)
Mean service life
Time
Failure probability
P
f
Target service life
Service life distribution
DuraCrete, 2000, Siemes et al., 2000).
up at the concrete surface and their subsequent transport into the cover
concrete. The
resistance
is the critical corrosion initiating threshold
for chloride at the steel surface. This latter property cannot reliably be
influenced with present-day knowledge (Polder, 2009). As a practical
approach, the critical chloride content is assumed constant in time. The
concrete resistance to chloride penetration, expressed by its chloride
diffusion coefficient, is taken as the model variable that, together with
the chloride surface content, determines the load side of the service-life
(limit state) equation. The chloride diffusion coefficient changes with time
due to hydration, binding of ions and drying out, which complicates the
modelling. It is emphasised that the
performance
is considered in terms
of absence of corrosion initiation, which is not an ultimate limit state,
because no direct danger for human lives is at stake. It is a maintenance
limit state (MLS), because corrosion means the upcoming need to repair,
which is an economic threat rather than a safety issue, corresponding to
a serviceability limit state (SLS). Target probabilities of failure for SLS are
usually in the range of up to 10 per cent.
Chloride transport modelling in DuraCrete was based on literature and
exposure site data that mainly represented Portland cement (CEM I) concrete,
with the exception of one set of exposure data (Bamforth and Chapman-
Andrews, 1994). Additional information has since become available from
systematic exposure (Tang and Andersen, 2000). More data on blast furnace
slag cement (CEM III/B) concrete were provided by field work on six marine
structures in the Netherlands. Chloride profiles were taken at ages of 20
to 40 years (Polder and Rooij, 2005; Rooij and Polder, 2005) from which
modifications of model parameters were deduced. Additional data on CEM
III/A and fly ash containing binders were obtained from participants in the
CUR committee.
