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


pdf icon 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:

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.