Investigation on the plastic shrinkage of cementitious materials under drying conditions: mechanism and theoretical model

摘要

The plastic shrinkage behaviours of fresh paste under drying conditions were investigated for several typical cementitious materials, including cement, silica fume, fly ash and calcium carbonate powder. For all four kinds of slurries, pore water under pressure (PWP) at different depths, shrinkage both in the vertical and horizontal directions, and water evaporation were measured simultaneously. Preliminary experimental investigations show an obvious anisotropic character of the plastic shrinkage behaviour in the vertical and horizontal directions and strong dependence on the PWP development. At the beginning of exposure, no PWP is generated, the total shrinkage is fully displayed by vertical displacement, and the responsible mechanism for plastic shrinkage is water evaporation. Once PWP arises, the investigated pastes exhibit vertical expansion and horizontal shrinkage; the responsible mechanism is the capillary tension in pore fluid. Based on the experimental observations, a two-phase theoretical model is proposed, which builds a quantitative relationship between water evaporation and vertical shrinkage before PWP is generated, as well as between PWP and horizontal shrinkage after PWP arises.