| During hydration, thermal, autogeneous, and drying deformations are responsible for driving early age stress development, that often leads to concrete cracking at early ages. Early age stress development is also controlled by aging viscoelastic properties like creep and relaxation. Five major features of hydrating blended cements including hydration kinetics, evolution of microstructure, development of mechanical properties, autogeneous deformation, and early age stress development are investigated in this research. Hydration kinetics, considered the primary source of age-dependent behavior, is analyzed experimentally and analytically. The implications of hydration on the macroscopic behavior are also studied. Properties such as, compressive strength, tensile strength, Young's modulus, fracture energy, creep compliance, and autogeneous deformation are shown to be tied strongly to hydration level. Two methods are employed to determine the degree of hydration of blended cements and the degree of reaction of pozzolans: isothermal calorimetry and differential thermal analysis (DTA). The analysis indicates that heat of hydration can be higher in blended cement systems, and that pozzolan reactions are slower than the cement reaction. Then, microstructural characteristics of blended cements as influenced by hydration and the presence of pozzolans, are investigated using sorption techniques. The results provide an estimate on the surface area of different hydration products and information on the pore structure of hydration products/gel. These results together with a micromechanics-based model prove to be significant in describing the humidity-driven shrinkage in cement paste. Predictions based on this model are made and verified from drying shrinkage and autogeneous shrinkage results. A new method for predicting the degree or heat of hydration and property development under nonisothermal conditions is proposed. A comprehensive stress calculation procedure that uses the relations between the properties and hydration level, and the method for predicting hydration is developed to predict the early age stress development. The new procedure uses heat of hydration as a measure of aging. Incorporation of temperature effects on properties is taken care through the calculation of heat of hydration. The prediction results are accurately verified by experiments. A significant improvement from the classical procedure can be achieved by the new procedure. |
