MICRO-SCALE STUDY OF MULTI-COMPONENT IONIC TRANSPORT IN CONCRETE

Abstract

Corrosion of reinforcing steel in concrete due to chloride ingress is one of the main causes of the deterioration of reinforced concrete structures, particularly in marine environments. It is therefore important to develop a reliable prediction model of chloride ingress into concrete, which can be used to predict the chloride concentration profiles accurately to help to assess the service life for reinforced concrete structures. Cementitious materials are porous media with a highly complex and active chemical composition. Ionic transport in cementitious materials is a complicated process involving mechanisms such as diffusion, migration, ionic binding, adsorption and electrochemical interactions taking place in the pore solution of the materials. The process is dependent on not only the microstructural properties of the materials such as porosity, pore size distribution and connectivity but also the electrochemical properties of the pore solution including ionic adsorption and ion-ion interactions. This thesis presents a numerical study on the multi-component ionic transport in concrete with the main focus on the microscopic scale.

This study first investigated the impact of the Electric Double Layer (EDL) on the ionic transport in cement-based materials. The EDL is a well-known phenomenon found in porous materials, which caused by the surface charges at the interface between solid surfaces and pore solutions. The numerical investigation is performed by solving the multi-component ionic transport model with considering the surface charges for a cement paste subjected to an externally applied electric field. The surface charge in the present model is taken into account by modifying the Nernst-Planck equation in which the electrostatic potential is dependent not only on the externally applied electric field but also on the dissimilar diffusivity of different ionic species including the surface charges. Some important features about the impact of surface charge on the concentration distribution, migration speed and flux of individual ionic species are discussed.

Then a new one-dimensional numerical model for the multi-component ionic transport in concrete to simulate the rapid chloride migration test is proposed. Advantages and disadvantages of the traditional methods used to determine the local electrostatic potential, i.e. electro-neutrality condition and Poisson’s equation, are illustrated. Based on the discussion a new electro-neutrality condition is presented, which can avoid the numerical difficulties caused by the Poisson’s equation, and remain the non-linearity of the electric field distribution. This model with the new electro-neutrality condition is employed to simulate the RCM test to prove its applicability. The new model is promising in solving the multi-component ionic transport problems especially in microscopic scale.

Lastly, a one-dimensional numerical investigation on the chloride ingress in a surface-treated mortar with considering the penetration of sealer induced porosity gradient was performed. The numerical model was carefully treated to apply governing equations of ionic transport to this situation of two pore structures, with every parameter clearly defined on the microscopic scale.