Modelling the effect of damage on transport processes in concrete

The calculation of corrosion current density, during the process of electrochemical steel corrosion in concrete, requires modelling of the following physical and electrochemical processes: transport of capillary water, oxygen and chloride through the concrete cover, immobilization of chloride in the concrete, transport of OH− ions through electrolyte in concrete pores and cathodic and anodic polarization. The paper deals with a 3D numerical model for transport of capillary water, oxygen and chloride through the concrete. The model is formulated in the framework of continuum mechanics following basic principles of irreversible thermodynamics. The mechanical part of the model is based on the hygro-thermo dependent microplane model of concrete. Damage and cracking phenomena are modelled within the concept of smeared cracks (weak discontinuity). The interaction between the non-mechanical processes (distribution of temperature, capillary water, oxygen and chloride) and mechanical properties of concrete (damage) is taken into account. The strong and weak formulations of the model and the implementation into a 3D finite element code are discussed. The formulation is restricted to the processes leading to depassivation of reinforcement. The application of the model is illustrated on a numerical example in which the transient 3D finite element analysis of RC slab is carried out to investigate the influence of damage of concrete on depassivation time of reinforcement. In the analysis, the undamaged and damaged parts of previously loaded RC slabs are exposed to the aggressive influence of seawater. Due to external load, the RC slab was partly cracked before the exposure to seawater. Consequently, the damaged part of the slab exhibits a much shorter depassivation time than the undamaged part. This is due to the cracking of concrete, which significantly accelerates processes that are relevant for depassivation of reinforcement. It is shown that depassivation in the cracked concrete is reached almost immediately after the attack of chlorides. The numerical results are in good agreement with the available experimental observations.