Cement paste deformability

Volume deformations are largely a function of the nature and quantity of the cement paste in the concrete and it has been shown [113] that studies on... 

Neubauer, C.M. (1997) On the Chemistry, Micro structure, and Deformation Properties of Cement Pastes Towards a New Strategy for Controlling Drying Shrinkage. PhD. Thesis, Northwestern University. 

There are several hypotheses as the rheological properties of cement pastes are concerned. As it is commonly known the rheology deals with the flowing and deformation of materials imder stress. The Newtonian fluids show a simple relationship between the shear stress and shear rate. When a thin layer of fluid is placed between the two parallel plates, of which one is fixed and the second will be subjected to the shearing force F, then the shearing of this layer will occur. The dynamic equilibrium will be attained when the force F, in the condition of stationary flow, will be balanced by the viscosity of Newtonian fluid and the relation between the shear stress and shear rate gradient will be linear (Fig. 5.1). 

The adhesion of steel to cement paste consists primarily in the meclianical interpenetrating of both materials and hence the so-called elastic component of shear strength is produced. At low load the plastic deformation occurs and subsequently... 

Creep—slow deformation of a moist hardened cement paste under the influence of an external force. 

A system of microcracks appears during the initial period of hardening of the cement paste. It is later subjected to the action of external loads and imposed deformations, which vary over time. Local stress concentrations contribute considerably to crack propagation also under relatively low to average stresses. In certain conditions, the microcracks may exhibit healing (cf. Section 9.7). In most cases, however, they remain and may develop into major cracks leading to disintegration of elements. 

Shrinkage of the cement-based matrix is the change in its volume due to moisture variation or chemical reactions and thermal effects during and after the hydration process. The cement paste is the source of shrinkage while other components are inert and only may control the deformations due to shrinkage. 

Whilst the microhardness values of hydrated portland cement paste lie between those for Reg Set cement paste with 2 and 5% CaCl2, the modulus of elasticity values for hydrated portland cement paste are higher than for any of the Reg Set cement pastes. It is suggested that the processes of microstructural deformation occurring in microhardness and modulus of elasticity measurements are influenced in different ways by bond formation during the hydration of Reg Set cement in the presence of CaCl2. 

Cement, lime, limestone filler and filler from other rocks, such as a mixture of diabase and dolomite, as well as sulfur filler, were used in the past (Fiber and Pichler 1993). A substantial effect on the permanent deformation behaviour was seen only with the use of sulfur (Denning and Carswell 1981b Fromm and Kennepohl 1979). 

Everyday experience tells us that if we increase the concentration of the dispersed particles, the viscosity of a mixture increases, going from a free-flowing liquid through to a paste, and eventually to a soft solid, as more and more particles are added. A familiar example of this is when we add cement or flour to water. What we need to know is, what influences the increase in viscosity in terms of the character of the liquid phase and the added particles For instance, what is the effect of the shape of the particles their size distribution the interaction between them, and their deformability, etc. All these factors will be considered below. 

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