Chloride penetration model capable of correctly predict the peak shaped chloride profiles

Title: Chloride penetration model capable of correctly predict the peak shaped chloride profiles
Author(s): B. Johannesson, K. Yamada, Y. Hosokawa
Paper category : conference
Book title: 2nd International RILEM Symposium on Advances in Concrete through Science and Engineering
Editor(s): J. Marchand, B. Bissonnette, R. Gagné, M. Jolin and F. Paradis
Print-ISBN: 2-35158-003-6
e-ISBN: 2351580028
Publisher: RILEM Publications SARL
Publication year: 2006
Nb references: 6
Language: English

Abstract: Different possible explanations to the existence of the so-called ‘peak’ in the chloride profile in concrete are discussed. Experiments on concrete using constant chloride exposure concentration, almost always gives results in which the ‘peak’ is present in the chloride profile. Most probably the active mechanisms contributing to the occurrence of the ‘peak’ behaviour affects the overall chloride penetration to a great extent. Hence, in order to obtain reliable chloride penetration models also the physical background to the existence of the ‘peak’ behaviour must be understood.
Different simulations using transient solutions to coupled differential equations describing multi-species diffusion and chemical effects are performed. In these simulations different constitutive assumptions and values of included material constants are tested. Indeed, some choices of constitution of model give the ‘peak’ in the calculated chloride profile. The main important issue is to correctly account for the mass exchange between ions in the pore solution and ions incorporated in solids. Results from simulation strongly suggest that leached calcium ions from cement hydrates causes the chlorides to accumulate according to the ‘peak’ shape. Another, theoretical result is that when assuming much lower diffusion coefficients for cations compared to anions the simulated chloride profiles also becomes peak shaped. The motivation to use lower diffusion coefficients for cations is that significant amounts are located in or close to (negatively) charged surfaces where ions can be seen as ‘loosely’ bound. In this context the effect of charged surfaces on the free diffusion are also investigated by assigning a fixed charge into the Gauss’ law.
The Finite Element Method allowed for development of consistent numerical models capable of solving the proposed set of coupled equations. The main findings from making simulations are increased knowledge about possible mechanisms behind the development of the peak behaviour of chloride profiles and the effect of charged surfaces on the free diffusion of ionic species. Illustrative theoretical examples are presented for different cases of multi-species diffusion in porous materials with charged or uncharged surfaces.

Keywords: Chlorides, Ionic transport, Modelling, Concrete

Online publication: 2006-08-02
Classification: 3.1 Theme 1: Numerical Models: from Microstructure to Transport Properties and Durability
Publication type : full_text
Public price (Euros): 0.00
doi: 10.1617/2351580028.028