| Mass transport process in concrete exposed to marine or de-icing salt environment is the main factor for the degradation of building material performance and the effect on the durability of reinforced concrete structure. Chloride penetration into concrete is the significant cause which induce the corrosion of steel embedded in the concrete and lead to the failure of the bonding property between the concrete and steel bars. Furthermore, water is the important media which carries chloride into the interior of concrete. The mechanisms of mass transport in concrete mainly include capillary absorption (convection), diffusion, pressure permeation and electro-migration phenomenon. However, the transport mechanisms under different environmental conditions obviously differ a lot. Generally, dry-wet cycling has been considered to be one of the most unfavorable conditions for the mass transport process in the concrete. In order to exactly evaluate the durability and service life of reinforced concrete structure, therefore, it is the key point to determine the transport mechanism and distribution profiles of the water and chloride ions in concrete. On the basis of the mesoscale method, the lattice network model developed recently, which is regarded to be very suitable for mass transport, is firstly utilized to analyze the water penetration and chloride ingress into concrete when subjected to drying-wetting cycles. And then, in order to consider the effect of cracking caused by environment condition or mechanical loading, the mass transport speed model through a single crack is established and the numerical simulation on mass transport in cracked concrete is carried out. The main contents and conclusions in this paper can be briefly summarized as follows:(1) With the consideration of the non-homogeneous characteristic of concrete composition, the theoretical background of the mesoscopic lattice network model and mass transport models under some kinds of conditions are systematically summarized in the present paper. The water transport equation of capillary absorption and one dimensional nonlinear convection diffusion equation of chloride ion transport in unsaturated concrete are reasonably established and deduced the finite element equation by means of the Galerkin weighted residual method. Compared with experimental data, the accuracy and reliability of the model have been validated by the numerical simulation of water and chloride transport.(2) The study on water penetration and chloride ingress into concrete subjected to drying-wetting cycles are carried out by means of mesoscale lattice network model. For the drying and wetting period, the same transport equations and different transport coefficients are adopted to study the effect of the cycle times and cyclic mechanism while the value of water content and chloride concentration in the wetting period are treated as initial and boundary conditions during drying period. The numerical results suggest that the depth of water and chloride ion penetration into concrete is strongly dependent on the wetting time while the drying time and the cycles mainly determine the chloride content in the surface of concrete, and reflects the phenomenon that the chloride concentration has been gathered in the surface layer10mm of concrete.(3) Cracks or microcracks, which exist in the concrete, can offer additional channels for water and chloride penetration into concrete, speeding up the mass transport rate and the corrosion of steel bars. Combined with the unsaturated flow theory, the cubic law of water transport in a single crack is adopted to build up the model for predicting water transport speed in cracks of concrete, which has been validated by experimental results. Considering the actual situation of crack and the influence of saturation, the approach of sensitivity-based parameter analysis suggests that the crack width, crack length and the initial saturation are the primary parameters affecting the mass transport rate in the cracked concrete. |
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