Spreading of a density front in the K\"untz-Lavall\'ee model of porous media

We analyze spreading of a density front in the K\"untz-Lavall\'ee model of porous media. In contrast to previous studies, where unusual properties of the front were attributed to anomalous diffusion, we find that the front evolution is controlled by normal diffusion and hydrodynamic flow, the latter being responsible for apparent enhancement of the front propagation speed. Our finding suggests that results of several recent experiments on porous media, where anomalous diffusion was reported based on the density front propagation analysis, should be reconsidered to verify the role of a fluid flow

Molecular Dynamics Simulation of Solvent-Polymer Interdiffusion. I. Fickian diffusion

The interdiffusion of a solvent into a polymer melt has been studied using large scale molecular dynamics and Monte Carlo simulation techniques. The solvent concentration profile and weight gain by the polymer have been measured as a function of time. The weight gain is found to scale as t^{1/2}, which is expected for Fickian type of diffusion. The concentration profiles are fit very well assuming Fick's second law with a constant diffusivity. The diffusivity found from fitting Fick's second law is found to be independent of time and equal to the self diffusion constant in the dilute solvent limit. We separately calculated the diffusivity as a function of concentration using the Darken equation and found that the diffusivity is essentially constant for the concentration range relevant for interdiffusion.Comment: 17 pages and 7 figure

A model for reactive porous transport during re-wetting of hardened concrete

A mathematical model is developed that captures the transport of liquid water in hardened concrete, as well as the chemical reactions that occur between the imbibed water and the residual calcium silicate compounds residing in the porous concrete matrix. The main hypothesis in this model is that the reaction product -- calcium silicate hydrate gel -- clogs the pores within the concrete thereby hindering water transport. Numerical simulations are employed to determine the sensitivity of the model solution to changes in various physical parameters, and compare to experimental results available in the literature.Comment: 30 page

The Stress-Strain State and Potential Crack Trajectories in 2D Elastic Brittle Materials from Steady-State Flow Experiments

A steady-state flow method is used to examine micromechanisms of brittle failure in 2D elastic cracked media submitted to uniaxial compressive stress. The steady-state flow experiments were conducted with an incompressible Newtonian fluid in a Hele Shaw cell. Thin linear rubber inclusions were inserted in the cell to model preexisting cracks and flow was visualised by a continuous injection of methylen blue dye. Several experiments with different configurations of inclusions were conducted: 1) one single inclusion inclined at different angles b to the flow direction, 2) left lateral shear of right or left-stepping en échelon inclusions with various overlapping and 3) several randomly-distributed inclusions. The flow lines around the inclusions show very strong similarity with the trajectories of growth of similarly arranged cracks in uniaxially compressed brittle plates. Although the similarity between Hele shaw flow and elastic deformation is not fully understood yet, the method may be used to visualise both crackinduced perturbation of the stress field and crack interaction and allows accurate predictions of the potential crack's trajectories for one or several pre-existing cracks in two dimensionsCivil Engineering and Geoscience

Numerical Analysis of Moisture Flow and Concrete Cracking by means of Lattice Type Models

Modelling of fluid-flow and the resulting effects on shrinkage and microcracking by means of a combination of two lattice models are presented. For the moisture transport, a Lattice Gas Automaton (LGA) is adopted since it can effectively model moisture loss, whereas for cracking simulation a Lattice Fracture Model (LFM) is used. The resultant moisture gradients are translated into strains by means of a shrinkage coefficient. Strains yield shrinkage stresses, which are applied as pre-stresses on the sample in order to invoke cracking. The principle of the coupling of two lattice models is explained. Some examples of preliminary analyses on a homogenous lattice and a lattice containing a single large aggregate are presented. Problems and advantages to the approach are debated, and existing gaps in knowledge are indicated.Structural EngineeringCivil Engineering and Geoscience