Civil Engineering Reference
In-Depth Information
model value given above, depending on concrete composition and external
conditions. In principle it can be determined for the structure, but presently
a procedure that has been proven in practice is not available. In the ideal
case (excluding significant carbonation), a good picture is obtained of
chloride penetration up to the date of the inspection from core sampling.
Chloride penetration profiles should be fitted to a time-independent
diffusion equation of the form:
⎛
⎞
⎛
x
⎞
⎜
⎟
C
(
x
,
t
)
=
C
1
−
erf
⎜
⎟
(15.4)
S
⋅
2
Dt
⎝
⎠
⎝
⎠
The chloride surface content
C
S
obtained in this way may be considered
representative for the prevailing environment for a structure at least
10 to 20 years of age (Polder and Rooij, 2005; Rooij and Polder, 2005).
Similarly, the accumulated (apparent) diffusion coefficient at the point in
time of the inspection can be established. Both, by the way, will have the
character of statistically distributed parameters. For predictive purposes, two
possible approaches to the future development of the diffusion coefficient
D
exist. The more conservative approach is to use the best-fitting diffusion
coefficient as a fixed value and extrapolate towards future penetration.
A less conservative procedure would be to assume a decreasing diffusion
coefficient using equation (15.3) and the appropriate exponential factor for
the cement type used from
Table 15.1.
The difference between these two
approaches may be small, however, in particular if corrosion is soon to be
expected. In cases where some corrosion is already present, this method can
be used to predict its spread to wider areas of the structure, which may be
relevant for choosing repair options.
15.6 Concluding remarks
In many cases obtaining a long service life for civil engineering concrete
structures (bridges, harbour quays, tunnels, parking garages) is predominantly
a matter of postponing the onset of rebar corrosion due to chloride penetration.
Noted exceptions are structures with 'exotic' preventative measures such
as stainless steel reinforcement or cathodic prevention. The approach does
apply in principle, however, to cases where coatings or hydrophobic treatment
are used (although the model parameters should be modified to include
their effects). The most important parameters in the presented model-based
approach for concrete with ordinary steel reinforcement are:
•
chloride load from sea water or de-icing salt environments
•
chloride transport by diffusion
•
time-dependent diffusion coefficients
•
coefficients taking into account environmental, curing and materials
(binder type) influences
