RESUMEN
Diffusion processes are crucial to the durability and confinement capacity of cement-based materials as well as property development. The liquid phase in cement-based materials is the pore solution, whose composition changes with age and is a function of the cement system composition. Water structure and dynamics are recognized to be affected by the presence of ions. Fundamental understanding of the physical processes underlying these changes can be critical in the elucidation of the physical origin of durability issues and in the development of new admixtures. Here, the structure and dynamics of water and ions present in pore solutions are studied using molecular dynamics simulations. Self-diffusion coefficients are computed for bulk solutions mimicking the complex composition of pore solutions. Specific ion effects on water dynamics are interpreted in terms of water reorientation time. The composition dependency of ion dynamics explains the evolution of the ionic conductivity of the pore solutions.
RESUMEN
The detection of porosity changes within a soil matrix caused by internal erosion is beneficial for a better understanding of the mechanisms that induce and maintain the erosion process. In this paper, an electromagnetic approach using Spatial Time Domain Reflectometry (STDR) and a transmission line model is proposed for this purpose. An original experimental setup consisting of a coaxial cell which acts as an electromagnetic waveguide was developed. It is connected to a transmitter/receiver device both measuring the transmitted and corresponding reflected electromagnetic pulses at the cell entrance. A gradient optimization method based on a computational model for simulating the wave propagation in a transmission line is applied in order to reconstruct the spatial distribution of the soil dielectric permittivity along the cell based on the measured signals and an inversion algorithm. The spatial distribution of the soil porosity is deduced from the dielectric permittivity profile by physically based mixing rules. Experiments were carried out with glass bead mixtures of known dielectric permittivity profiles and subsequently known spatial porosity distributions to validate and to optimize both, the proposed computational model and the inversion algorithm. Erosion experiments were carried out and porosity profiles determined with satisfying spatial resolution were obtained. The RMSE between measured and physically determined porosities varied among less than 3% to 6%. The measurement rate is sufficient to be able to capture the transient process of erosion in the experiments presented here.
RESUMEN
We report the characterization of the spectral response of a wavelength-self-tunable single-sideband filter operating at 16-GHz radio frequency. We demonstrate, for what we believe to be the first time, simultaneous generation of three dynamic Bragg gratings inside the InP:Fe photorefractive crystal of such a filter. In addition, two signal processing configurations are tested. Finally, measurements are confronted with simulations on the device, which is dedicated to chromatic dispersion compensation in fiber-radio systems.