Study of the isothermal drying characteristics of normal concrete subjected to low air velocity convection

Abstract

The drying of capillary moisture from structural concrete used in enclosed areas takes place by free convection characterized by low air velocity. The slow drying results in the persistence of an unsaturated state in concrete rendering it susceptible to deterioration. The study of critical process parameters, such as, mass loss flux, convective transfer coefficients, critical and equilibrium moisture contents and their dependence on the pertinent process variables is fundamental to design of concrete elements for durability. While these parameters have been widely researched for non-cementitious porous building materials, studies related to concrete are not comprehensive and far apart in literature. This study addresses the gap by investigating the drying characteristics of OPC-based normal concrete for varying levels of water-to-cement ratio, curing age and characteristic length of concrete specimens besides the dry bulb temperature and relative humidity of air. Interestingly, the constant rate mass flux is noted to depend on water-to-cement ratio thereby revealing the dependence of the convective transfer coefficients on surface characteristics. It is also observed that the Anomalous diffusion model satisfactorily describes the change in degree of saturation of a specimen with drying time and allows a reliable estimation of the critical degree of saturation. The latter, when plotted as a critical point curve, exhibits a non-uniform increase, as has been reported previously for a wide variety of other porous materials. The modified Oswin model is shown to provide a suitable description of the degree of saturation at equilibrium attained under steady-state temperature and relative humidity conditions.