Prediction of Drying Shrinkage of Portland Cement Paste: Influence of Shrinkage Mechanisms

Ultimate drying shrinkage of paste was investigated in terms of the Gibbs-Bangham shrinkage and the capillary stress mechanisms. Shrinkage measurements and nitrogen sorption isotherms were obtained on cement pastes of water-to-cement (W/C) ratios of 0.4,0.6 and 0.75. The contribution to ultimate drying shrinkage from these two mechanisms was obtained from ultimate shrinkage versus increase in surface free energy curves in the relative humidity (RH) range of 95 to 0 percent. The Gibbs-Bangham stress mechanism is the major contributor to shrinkage stresses in this RH range. The capillary stress mechanism is active in the RH range of 95 to 50 percent.The maximum Gibbs-Bangham stress shrinkage, which occurs at 0 percent RH, is linearly proportional to total surface area as obtained from volume-thickness (V-t) sorption analysis. Maximum capillary stress shrinkage occurs at about 50 percent RH. This component was found to be linearly proportional to cumulative capillary surface area. V-t adsorption analysis was used to estimate the cumulative capillary surface area. The equations for predicting maximum Gibbs-Bangham and capillary stress shrinkage were extended to include effect of RH of drying. They were tested on different shrinkage results of well-hyd-rated pastes of 0.4 and 0.6 W/C ratios and found to be accurate