Civil Engineering Reference
In-Depth Information
Consequently, the Guideline aims to cover service-life design of concrete
structures in marine (exposure class XS) and de-icing salt environments
(XD) for a wide range of binders. Chloride migration test results are used
in a semi-probabilistic concept, which was condensed into a set of tables
specifying maximum values for chloride diffusion coefficients depending on
cover depth, cement type, exposure class and required service life.
15.3 A probability-based predictive model
The basic components of the present concept are a transport model, a chloride
transport coefficient, several model parameters and a semi-probabilistic
approach. For the description of chloride ion transport into concrete a
modification of the error function solution to Fick's second law of diffusion
is adopted. Using Fick's second law and the term diffusion coefficient
does not exclude, however, that other mechanisms may contribute to the
transport of chloride ions into concrete. For the present modelling, all non-
diffusion contributions are assumed to be small and sufficiently dealt with by
parameters in the model.
15.3.1 Chloride penetration (diffusion) coefficient
Since the diffusion model for chloride ingress in concrete was introduced
by Collepardi et al. (Collepardi et al., 1972), several methods have been
proposed for determining the resistance of concrete against chloride
penetration. In the 1990s, two methods were standardised in the Nordic
countries:
1 An immersion (pure diffusion) test, NT Build 443;
2 An accelerated (migration) test, NT Build 492, the rapid chloride
migration test (RCM).
The immersion test may be seen as a realistic representation of the natural
diffusion process. A drawback is that it requires seven weeks exposure of
specimens and involves chloride analysis of many samples. The migration
test involves a different transport mechanism (acceleration by an electrical
field), but has a short execution time and is less laborious. In European
research project Chlortest (Chlortest, 2005) both methods were studied
and compared in a round-robin test. A good linear correlation was found
between chloride diffusion coefficients from diffusion experiments and
chloride migration coefficients from RCM tests. It was considered justified,
therefore, to use the less time and labour consuming RCM test instead of the
diffusion test (Breugel et al., 2008).
In the past few years, RCM testing has been applied to many concrete
mixtures in association with service-life design of large infrastructural
projects in the Netherlands. For the guideline, a total of 500 RCM-values
