A Study on the Mechanism of Plastic Shrinkage of Cement-Based Materials

Doctoral thesis, 1992

The work presented in this thesis has been concentrated on describing the mechanism of plastic shrinkage and on developing a method of calculating the progress of plastic shrinkage. The origin of plastic shrinkage and plastic shrinkage induced cracks has been investigated by many researchers during the last four decades, but no generally accepted theory could be found in the literature. In order to estimate the risk of cracking of fresh concrete exposed to certain climatic conditions, a complete understanding of the process of plastic shrinkage is necessary. The proposed model of plastic shrinkage is based on the idea that the capillary pressure in a saturated mixture exposed to drying is a function of the geometry of the spaces between the solid particles at the surface and the difference between the amount of evaporated water and the amount of water coming from inside the mixture. In order to describe these quantities, as well as the geometry of the spaces between the particles at the surface, the relationship between the capillary pressure and the amount of evaporated water, represented by a PW-curve or a PW'-curve, has been introduced. A set of equations that expresses the total relative deformation of a sample exposed to drying and the capillary pressure as functions of time was developed. The geometry of the pores at the surface and the factors affecting the amount of water transferred to the surface are studied by investigating the development of capillary pressure in three groups of materials with different behavior when mixing with water: sand beds, mixtures of non-reactive particles (silica fume and fly ash) with water and cement pastes. The model is verified on cement paste and mixtures of non-reactive particles with water. A qualitative verification of the model is made by showing that the development of the capillary pressure in mixtures of non-reactive particles with water and cement pastes depends on the following factors: 1. the rate of evaporation 2. the geometry of the pores at the surface 3. the thickness of the sample 4. the modulus of plastic shrinkage. Finally, it is shown that the highest and the lowest limits of the total relative deformation of a sample can be determined by using the proposed equation of plastic shrinkage.