Civil Engineering Reference
In-Depth Information
•
an assumed critical chloride content for corrosion initiation
•
determining the rapid chloride migration coefficient of the intended
concrete.
This chapter presents a probability-based design procedure for determining
combinations of cover depth and 28-day chloride diffusion coefficients that
are required to guarantee a specified service life for the limit state 'corrosion
initiation due to chloride penetration'. Target probabilities of failure are
10% and 5%, respectively, for reinforcing steel and prestressing steel. Based
on a semi-probabilistic simplification, the required combinations of cement
type, cover depth and diffusion coefficient are brought together in design
tables. The tables give limiting values for chloride diffusion coefficients
obtained with the RCM test for service lives of 80 to 200 years in marine
(XS) or de-icing salt (XD) environments. From analysis of a large number of
test results, the dependency of the
D
RCM
-value on
w
/
b
and cement type was
determined and an indication was obtained in which values are practically
possible using present-day concrete technology. Distinguishing cement types
is considered important. Even the most recent international regulations do
not seem to make such distinctions (EN1992-1-1, 2004).
Similar tables as proposed here have recently been presented by Li et al.
(2008). In their tables, however, the compressive strength is still considered
one of the durability parameters. Instead of strength, here an explicit
transport parameter is chosen, i.e. the RCM-value, to indicate the concrete's
susceptibility to chloride ingress. A similar probability-based approach to
various degradation mechanisms has been presented by
fib
(FIB, 2006) using
a slightly different model for chloride-induced corrosion.
With this Guideline (CUR, 2009), the Dutch concrete industry now has
rules for practical service-life design for structural concrete. All parties
involved have agreed to collect their experience using the Guideline, with the
intention to evaluate it and if necessary, to improve it in the near future. At the
same time, however, it was realised that many items used in the calculations
still contain large uncertainties. Further research should contribute to
reducing them. Finally, the approach taken may contain elements that are
useful for determining the remaining life of existing structures.
Acknowledgement
This chapter is based on ideas and results from the DuraCrete consortium,
researchers at TNO, INTRON and TUDelft and CUR-committees B82
and VC81. The financial and in kind support of Rijkswaterstaat and other
participating organisations are gratefully acknowledged. Any errors are the
author's.
