Capillary suction and diffusion model for chloride ingress into concrete David Conciatori ⁎, Hamid Sadouki, Eugen Brühwiler EPFL-MCS, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland ABSTRACT ARTICLE INFO Article history: Received 19 March 2007 Accepted 26 June 2008 Keywords: Concrete transport capillary water model A numerical approach, named TransChlor, is proposed to simulate transport phenomena of various substances in concrete. This approach is a theoretical model based on finite elements and finite differences methods. The model consists of coupled nonlinear partial differential equations based on Fick's diffusion law and on kinematics equations. Simulation results from a parametrical study highlight the influence of microclimatic conditions, exposure to deicing salts and concrete cover permeability and thickness on chloride ingress in concrete. The results show that the chloride ion concentration increases quickly in concrete cover when a structure is exposed to deicing salts at a mountainous location; whereas permeability of concrete cover is an insignificant parameter when the concrete is in direct or splash water contact. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction The corrosion initiation period [6] includes the time during which substances such as water, chloride ions and carbon dioxide flow through the concrete cover (i.e. the clear concrete cover protecting the steel reinforcement) and reach the certain concentration necessary to trigger corrosion of the steel reinforcement. The initiation period is characterized by chemical reactions between the various substances and the movement of the various substances in the concrete. Two models are used, each taking into account a different scale: microscopic and macroscopic models. Microscopic models consider the flow of ions and their chemical balance in the concrete, such as the models Stadium [15], Ms Diff [29], Masi [16], by Shin [27] and Schmidt- Döhl [25,26]. Macroscopic models take into account the various thermal variation as well as hydrous and ionic movements. The chemical reactions are considered only through parameters simulating the chemical effects on transport, like the models by Roelfstra [21], TransChlor [6], ClincConc [28], by Meijers [17], Saetta [22] and Ishida [10,14]. While microscopic models simulate the phase changes more precisely and take into account the porosity reduction, they often require extensive testing to obtain valid data for the model parameters. The TransChlor model is an original model to address chloride ion movement with water in concrete and to consider microclimates [4,5] reconstituted from real climates. The data is taken from meteorolo- gical stations and is reconstituted by considering exposure level of a reinforced concrete structural element. The water movement is accelerated in situations when their adhesion forces between the water and the porous structure of concrete contribute to this movement. This phenomenon is known as capillary suction. The vapour movement is less sensitive to capillarity suction. Instead it follows a diffusion process. Transport of water and water vapour is differentiated in the model [2,21,23]. The water transport parameters are obtained from laboratory tests on water adsorption at low temperatures [6,7]. The TransChlor model basically uses the Fick's diffusion law for water vapour transport [3,18], thermal diffusion and chloride ion diffusion in water [6]. This law allows the simulation of the diffusion process due to a concentration gradient of these substances. Although the Fick's diffusion law represents vapour diffusion well, it does not describe capillary suction of water as observed in own laboratory tests [6] and by other researchers [11,21,24]. Accurate capillary suction modelling is thus obtained by considering the kinetics of the phenomenon. 2. Transport model The chloride ion transport in concrete cover is modelled with TransChlor. The relevant parameters considered are the microclimate, the presence of deicing salts and the concrete cover permeability of the structural element. The microclimate is a function of the structural element's exposure to solar radiation (zones in the shade are distinguished from those exposed to the sun), the annual average carbon dioxide concentration and the geographically linked weather conditions (air temperature, relative humidity and precipitation) [6]. The TransChlor model considers the thermal diffusion process and the hydrous transport by capillarity suction and vapour diffusion as a function of the carbonation state, while simulating chloride ion transport in concrete [4,6,8]. The various transport processes are modelled with Fick's Eqs. (1)–(3). The model uses the finite element method to solve for the ion propagation within the concrete and the finite differences method (an implicit method) to solve for the duration of progression of the propagation front. Cement and Concrete Research 38 (2008) 1401–1408 ⁎ Corresponding author. Laval University, Pavillon Adrien-Pouliot, local 2928G, Québec, Canada G1K 7P4. E-mail address: david.conciatori@gci.ulaval.ca (D. Conciatori). 0008-8846/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2008.06.006 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp