Diffusion velocity lattice Boltzmann formulation applied to transport in macroscopic porous media

This paper describes the application of a single relaxation time (SRT) lattice Boltzmann scheme to the transport in porous media with large spatial variations of diffusion coefficients. Effective diffusion coefficients can vary substantially within porous media because of their dependence on porosity and tortuosity which can span over several orders of magnitude, depending on pore size and connectivity. Moreover, when mass is transported with pore-water in porous media, the hydrodynamic dispersion, which depends on Darcy's velocity, contributes additionally to the usually anisotropic variation of the dissipative term. In contrast to the traditional treatment of spatially variable diffusion coefficient by the variation of a SRT, here the variability is accommodated through the use of diffusion velocity formulation which allows for larger variabilities of diffusion coefficient. The volume averaged properties of mass transport in macroscopic porous media are resolved through the additional source term which is similar to the existing force adjusting methods. The applicability of both the proposed schemes is demonstrated on two examples. The first demonstrates that the method is accurate for the large variation of diffusion coefficients and porosities. The second example introduces mass diffusion in a real, geometrically complex system with spatially contrasting properties

DEM simulattions of soil-pile interface under static and cyclic loading

In this study, Discrete Element Method (DEM) was used to simulate interface direct shear tests for both constant normal load (CNL) and constant normal stiffness (CNS) conditions. The model was calibrated and validated using laboratory data. Simulations were made for both static and cyclic tests for different amplitudes at normal stress levels ranging from 100kPa to 400kPa on sand having relative density of 50%. No major difference was observed between static CNS and CNL tests for shear stress behaviour. However, a decrease in normal stress was observed in CNS tests which were evident from the imposed boundary conditions. In cyclic CNS tests, degradation of shear stress was observed for higher displacement amplitudes. Shear band thickness was measured from rotation diagrams and was observed to be 5 to 10 times D50 for all the tests

Can a reliable prediction of cement paste transport properties be made using microstructure models?

Physical and mechanical properties of cement-based materials are directly linked to their micros.tmcture. In recent years, different microstructure modelling platforms have emerged (e.g., HMYOSTRUC, CEMHYD3D and µic) which simulate the evolution of the microstructure of cement paste during hydration. These microstructures can be utilized to obtain physical and mechanical properties. However, due to underlying assumptions in these models, morphologically different microstructures (different pore connectivity, pore size and percolation thresholds) are obtained from these platforms for the same water-cement ratio and cement composition. The question then arises whether the estimations of properties of cement paste using these microstructures is reliable or not. In this paper we discuss issues related to achieving reliable predictions of the transport properties using microstructures generated from these platforms

Effective diffusivity of cement pastes from virtual microstructures : role of gel porosity and capillary pore percolation

The role of capillary pores percolation and gel pores are investigated to explain the underlying differences between relative diffusivity obtained from different experimental techniques using microstructures generated from two different types of hydration model viz., CEMHYD3D (a voxel based approach) and HYMOSTRUC (a vector based approach). These models provide microstructures with different capillary pore connectivity for the same degree of hydration and the same porosity due to the underlying assumptions. In order to account for a C-S-H diffusivity at the micro-scale, a continuum micro-mechanics based model has been proposed. These simulations show that deperolation of capillary pores at around 20% of capillary porosity is essential in order to correctly predict diffusivity of cement paste with water-cement ratio by mass (w/c) in between 0.4 and 0.5. Furthermore from our analysis we present a viable postulate that the higher diffusivity measured by electric resistivity compared to other methods is due to differences in contribution from gel pores. For electrical resistivity measurement it is proposed that all gel pores are diffusive whereas for ion and tracer transport it is proposed that only nitrogen accessible gel pores are diffusive. (C) 2018 Elsevier Ltd. All rights reserved

A three-dimensional lattice Boltzmann method based reactive transport model to simulate changes in cement paste microstructure due to calcium leaching

In this paper, a newly developed lattice Boltzmann method based reactive transport model to simulate changes in microstructure of ordinary Portland cement paste due to calcium leaching is presented. The model takes three-dimensional digitized cement paste microstructure as input and is capable to capture an evolution of microstructure due to leaching, accounting for the dissolution of portlandite and corresponding increase in capillary porosity and the decalcification of C-S-H resulting in increase in gel porosity. The developed model has been applied to microstructures generated using two cement hydration models, CEMHYD3D and HYMSOTRUC, for three water-to-cement ratios. It was observed that the rate of leaching is directly proportional to ability of microstructure to transport calcium ions and higher fraction of percolated capillary pores result in higher rate of leaching. The model qualitatively reproduces experimentally observed changes in cement paste porosity and pore size distribution due to leaching. The quantitative validation of model at this scale is not possible by comparison of leaching obtained experiments and simulations which can be attributed to several factors including the differences in the scales of experiment and modelling study presented in this paper. (C) 2018 Elsevier Ltd. All rights reserved