Mechanism of plastic shrinkage cracking in portland cement and portland cement-silica fume paste and mortar

Abstract The objective of this paper is to discuss the mechanism of plastic shrinkage in portland cement and portland cement-silica fume paste and mortar. The effects of the three delivery forms of silica fume; asreceived powder, densified powder, and slurry, on plastic shrinkage are presented. In paste, plastic shrinkage is primarily related to development of tensile capillary pressure during drying. The higher the surface area of particles, the higher the tensile capillary pressure, and consequently, the more vulnerable the system would be to plastic shrinkage cracking. In mortar, plastic shrinkage is controlled by both capillary pressure in the paste and presence of fine aggregate particles. These particles serve to reduce cracking by (a) arresting cracks, and (b) refining the size and distribution of capillary pores. Data of rate of evaporation of water can only indicate degree of drying. Charts developed by Portland Cement Association to calculate the rate of evaporation can not predict whether or not plastic shrinkage cracks occur. A quantitative method could perhaps be developed to predict plastic shrinkage cracking.