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Time-Dependent Diffusivities: Possible Misinterpretation Due to Spatial Dependence.

Time-Dependent Diffusivities: Possible Misinterpretation
Due to Spatial Dependence.
(583 K)

Bentz, D. P.; Feng, X.; Hooton, R. D.

Testing and Modeling the Chloride Ingress Into Concrete.
Proceedings. 2nd International RILEM Workshop.
September 11-12, 2000, Paris, France, 225-233 pp, 2000.

### Keywords:

concretes; diffusion; chloride ion

### Abstract:

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To accurately predict the service life of a reinforced
concrete structure exposed to chloride ions, it is
necessary to estimate the temporal dependence of the
diffusivity coefficient. Due to continued hydration of
the cement, etc., the diffusivity of the concrete can
vary considerably with time. Conventionally, a power-law
type function is used to describe the relationship
between diffusivity and time. Care must be taken in
using this approach, because often, the fitted power law
coefficient results in diffusion coefficients which
become extremely small at long times and may result in
predicted service lives far larger than thse actually
achieveable by the field concrete. One possible reason
for these "high" power law coefficients may be a
misinterpretation of the field concrete cores' chloride
profiles. The obtained profiles are typically fitted to
a function determined by Fick's second law, where D, the
determined diffusion coefficient is assumed to be only a
function of time. In fact, for most field concrete, D is
spatially dependent as well, with the exposed surface of
the concrete having a diffusion coefficient
substantially different from that of its interior. In
this paper, it will be demonstrated that when the
diffusivity of the surface concrete exceeds that of the
interior concrete, the conventional analysis may predict
a power law type behavior for D vs. time, when in fact,
none may exist.
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