Coupled multi-physics simulation of chloride diffusion in saturated and unsaturated concrete

Abstract

Chloride-induced corrosion of steel reinforcement is one of the major long-term deterioration mechanisms for reinforced concrete infrastructures. Chloride transport through cement-based materials is a complex chemo-physical process involving ionic diffusion in concentrated solution, pore structure, chemistry, membrane permeability of the matrix, cracking, and the variation of the internal and external environmental conditions. Although in the literature there are plenty of both simplistic phenomenological models and sophisticated models, in this study, a new model is developed taking aim at capturing the fundamental physics and, at the same time, having a formulation as simple as possible that it can be effectively calibrated and validated using available limited experimental data. The model couples the ionic diffusion process with the concrete micro-structure evolution due to continued hydration accounting for hygro-thermal variations and their effects on both the diffusion and hydration processes. The formulation is implemented in a semi-discrete conduit transport network that mimics the internal heterogeneity of the cementitious material by connecting the matrix space between coarse aggregate pieces. This allows the model to replicate naturally the meso-scale tortuosity effect which is an important feature towards representing realistically the heterogeneity-induced variations of chloride concentration within the concrete. The limited model parameters are carefully calibrated and the formulation is validated by simulating multiple experiments ranging from diffusion through pastes to large concrete cylinders. The results of numerical simulations show the ability of the model to describe spatial and temporal evolution of the chloride concentration within the samples under varying chloride concentrations and temperature boundary conditions within both saturated and 1 unsaturated concrete.

The “Skin Effect” Assessment of Chloride Ingress into Concrete Based on the Identification of Effective and Apparent Diffusivity

“…Due to the first assumption, the surface layers must not be removed from the samples before the chloride diffusion process was started, and the experiment should be conducted in the isothermal and saturated conditions on concrete samples of at least 90 days to avoid excessive hydration process impact on the process. At the same time, the experiment should last long enough to capture changes in the concentration distribution as much as possible, but also as short as possible to limit the impact of hydration of cement relics in concrete skeleton on changes in the diffusion coefficient (e.g., [1]). Hence, the authors decided to use the research of the diffusion process lasting 180 days from its start which was described in Section 2.…”

“…The penetration of chloride ions into concrete structures is one of the main causes of their steel reinforcement's corrosion, especially in facilities located in the coastal zone, as well as those treated with de-icing salts. The ingress of chlorides into concrete is primarily a diffusion process taking place in the pore solution, but in general, it is more complex, and its course is also significantly influenced by the pore space structure, concentration and type of constituents dissolved in the pore liquid, interactions between chloride and other ions, age of concrete, temperature, and process of chloride binding by concrete skeleton [1]. The values of chloride diffusivity are the same influenced by the water-cement ratio, volume of aggregate, amount of cement and supplementary cementitious materials.…”

Experimental Study and Simulation Calculation of the Chloride Resistance of Concrete under Multiple Factors

“…The external service environment of cement is complex and changeable, in which a variety of soluble salts interact to form a multi-salt concentration field, which has been proven to be able to change or even significantly change the diffusion coefficient of chloride ions and is in the seabed structure [17][18][19][20][21][22]. The cement of buildings is affected by the hydrostatic pressure field at the same time, thus forming a multi-field coupling environment, which makes the chloride ion transport mechanism in this environment worthy of further discussion [23][24][25][26][27].…”

On the Tortuosity-Connectivity of Cement-Based Porous Materials

“…Ožbolt et al [16] proposed a 3D chemo-hygro-thermo mechanical model for reinforced concrete to simulate physical processes related to corrosion of steel bars. In a more recent work, Zang et al [17] have proposed a model that couples the ionic diffusion process with the concrete microstructure evolution.…”

“…So, the detailed modelling of cement-based materials microstructure requires to consider different levels of approximation, which would greatly complicate the modelling. In addition, micro-models consider a large number of parameters which are usually difficult to calibrate through specific tests [17]. Seeking to more practical approaches, in previous works, a constant tortuosity factor has been adopted which assumes that all the effects arising from pore orientation, connectivity, size variation, etc., can be encompassed by a mean value valid for all pore sizes [19][20][21].…”