Herein, a three-dimensional numerical model based on computational fluid dynamics (CFD) for fresh concrete is developed to predict the slump and slump flow. Fresh concrete is considered as a non-Newtonian fluid, and its rheological behaviour is characterised by the Bingham and Herschel-Bulkley (H-B) models, respectively. Experiments are also conducted to validate the reliability and accuracy of this model. Through parametric investigations, the influence mechanisms of relevant factors on the flow characteristics of fresh concrete are analysed and discussed. The results show that the model predictions agree well with the experimental results. The predicted results obtained using the H-B rheological model are more accurate compared to the Bingham model, with average relative errors of 1.73 %, 2.03 % and 3.95 % for slump, slump flow and T₅₀₀, respectively. The flowability of fresh concrete is negatively correlated with power index, yield stress and consistency, while it is positively correlated with density. Grey relational analysis indicates that density has the greatest effect on the results of slump and slump flow, followed by yield stress and consistency, and finally the power index. The CFD-based numerical model presented in this study provides an important approach for better understanding the flow behaviour of fresh concrete from a rheological perspective.

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When serving in the marine environment, reinforced concrete structures are prone to be attacked by chloride ingress, which generally co-occurs with varying humidity and temperature changes. Therefore, considering the interaction between moisture transport and heat transfer, and their individual and coupling effects on chloride transport, this paper presents a novel numerical modelling framework for chloride penetration in concrete under different environmental conditions. In this framework, a novel thermal conductivity model and temperature-depended chloride binding isotherms are also developed, considering the heterogeneous characteristics of concrete. The proposed model is validated against a series of experimental data. By assuming the cyclic humidity and temperature boundary conditions as trigonometric type, this study further discusses the effect of average value, amplitude value and period length of cyclic environmental changes on the chloride transport in concrete. The results indicate that variation in humidity and temperature averages can alter the peak values of chloride content but have less effect on the chloride penetration depth. However, the increased humidity amplitude could significantly promote both the peaks and the penetration depths due to intensive chloride convection caused by moisture transport. This paper is supposed to provide a better understanding of chloride penetration in concrete under a realistic engineering environment.

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Pore structure characteristics of cementitious materials play a critical role in the transport properties of concrete structures. This paper develops a novel framework for modeling chloride penetration in concrete, considering the pore structure-dependent model parameters. In the framework, a multi-scale transport model was derived by linking the chloride diffusivities with pore size distributions (PSDs). Based on the three-dimensional (3D) microstructure generated by “porous growth” and “hard core-soft shell” methods, two sub-models were computationally developed for determining the multi-modal PSDs and pore size-related chloride diffusivities. The predicted results by these series of models were compared with corresponding experimental data. The results indicated that by adopting pore size-related diffusivities, even if the total porosities were the same, the proposed multi-scale chloride transport model could better capture the effect of different PSDs on chloride penetration profiles, while the model without pore structure-depended parameters would ignore the differences. Compared with the reference transport models, which adopt averaged chloride diffusivities, the chloride penetration depths predicted by the proposed multi-scale model are in better agreement with experimental data, with 10%–25% reduced prediction error. This multi-scale transport model is hoped to provide a novel computational approach on 3D microstructure generation and better reveal the underlying mechanism of the chloride penetration process in concrete from a microscopic perspective.@en

This paper develops a novel lattice diffusive model to quantitatively study the chloride diffusion coefficient in unsaturated cementitious materials, in which the pore voxels are redistributed to make a better representation of a real microstructure of hardened cement paste. Considering the hierarchical microstructure and different drying-wetting cycles, water distributions in multiscale pore structures are modelled and the structure characteristics of water-filled pores, including water connectivity, water tortuosity and effective porosity, are computationally extracted based on that. A lattice diffusion network is established to predict relative chloride diffusion coefficient by combining the effect of both water saturation degree and pore structure characteristics. The predicted results are validated against experimental data, and a concise analytical equation is proposed to predict the relative chloride diffusion coefficient. The equation indicated that the relative chloride diffusion coefficient is proportional to water connectivity but inversely proportional to the square of water tortuosity. Besides, the lattice model's quantitative results reveal that the water connectivity and water tortuosity are highly related to pre-water loading processes, and influenced by the gel pore fraction, which in turn will affect the relative chloride diffusion coefficient. Compared with existing equations and non-redistributed models, the present model could improve the prediction accuracy significantly.

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Understanding the transport mechanisms within unsaturated porous media is essential to the durability problems associated with cement-based materials. However, the involvement of multi-ions electrochemical coupling effect, especially under unsaturated condition makes the transport mechanisms even more complex. In this study, the moisture and multi-ionic transport in unsaturated concrete have been modeled in three-dimensional cases. The contribution from both water vapor and liquid has been considered in moisture transport. By adopting the constitutive electrochemical law, the electrostatic potential induced by inherent charge imbalance was calculated. With parameter calibration, the numerical results agreed well with the experimental data, proving the validity of the presented model. Results from a parametric analysis showed that neglecting multi-ions coupling effect will lead to an underestimated chloride concentration, and saturated degree has an obvious impact on the coupling strength among different ions. In addition, the existence of coarse aggregates will not only block mass transport but also make the discrepancies between two-dimensional model and three-dimensional model results more obvious. Other findings which have not been reported in existing studies are also highlighted.

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