MODELING DRYING SHRINKAGE IN RECONSTRUCTED POROUS MATERIALS - APPLICATION TO POROUS VYCOR GLASS

A three-dimensional representation of the microstructure of porous Vyc or glass was generated from a transmission electron micrograph, and wa s analysed to compute the locations of all capillary-condensed water a s a function of relative humidity. On solid surfaces where capillary-c ondensed water was not present, an adsorbed water layer, whose thickne ss is a function of relative humidity, was placed. As a function of re lative humidity, fixed pressures were specified in all capillary-conde nsed water, and the change in specific surface free energy with relati ve humidity was taken into account for the adsorbed water layers. New finite-element codes were developed to determine the drying shrinkage, in response to the changes in the specific surface free energy of the adsorbed water layers and to the fixed pressures in the capillary con densed water. Existing finite-element and finite-difference codes were used to evaluate the elastic moduli, the electrical and thermal condu ctivity, and the fluid permeability of the material. Bulk properties s uch as fluid permeability and electrical and thermal conductivity agre ed well with experiment. By adjusting the elastic moduli of the solid backbone, which are not experimentally determined quantities, the comp uted porous glass elastic moduli, and computed low and high relative h umidity shrinkage all agreed well (less than or equal to 10%) with exp erimental values. At intermediate relative humidities, the agreement f or shrinkage was worse, partly due to inaccuracies in the simulated wa ter desorption curve, and partly due to the fact that water-induced sw elling of the solid backbone, an effect that is probably present in th e real material, was not taken into account in the model computations.