HARDENING CONCRETE EFFECTS IN MASSIVE STRUCTURES
Journal Title: ELECTRONIC JOURNAL OF POLISH AGRICULTURAL UNIVERSITIES - Year 2008, Vol 11, Issue 2
Abstract
Although cement materials have had more than one hundred years history, they are still building material of great importance. Positive and negative experiences with their application were used in subsequent realizations. Significant development of material engineering, especially in the last few years, has created new possibilities also in the cement materials technology. This has contributed to the considerable increase the durability of cement materials and the firmness of their structure as well. Those effects were possible to attain by dint of the modifications of the cement materials` structure that rested mainly on usage of specific chemical admixtures (super plasticizers) and mineral supplements (silica fume). The modifications of the cement materials structure allowed to reduce remarkably water-cement ratio and porosity. As a result one can obtain a mixture with much more active particles that are able to make structural bonds and much less capillary pores, which create main structural defects. It may appear that modelling problems have already been solved. In literature one can find many different hardening models. In this research the structural model of cement materials hardening is presented. Formulas, shown on the basis of established model assumptions, make possible to observe the development of concrete’s strength under compression in wide range of changes of its structure, defined by porosity coefficient. This coefficient, giving the ratios between molecular (gel) pores capacity and total pores capacity in hardening concrete, is a proper description of structures – formed processes character, integrally connected with structure of porosity of hardening cement paste. It means that there is an influence of mineral additives and physical and chemical active admixtures on development of hardening concrete strength. In the established model the fundamental influence on concrete’s strength, in the period of its structural changes, exerts the cement gel with dissipated, molecular, capillary and air pores. The distinction of porosities and equivalent influence on the development of concrete’s strength under compression fixing makes the identifying of significant model difference in scope of concretes characterized by dissimilar composition of matrix possible. The setting and hardening of concrete are accompanied by no stationary, heterogeneous temperature and moisture distributions caused by the hydration heat development. In this paper theoretical and experimental studies of temperature distributions induced in large High Performance Concrete (HPC) and Ordinary Concrete (OC) blocks during the hardening process are discussed. Two concrete mixes with water to binder ratios 0.52 (OC) and 0.32 (HPC) are used in the experimental blocks to identify the temperature fields and thermal stresses during their hardening. Concrete structures treated by outer load are exposed to loads resultant from material properties and technology of structure. These loads are mainly caused by unsteady, coupled fields of temperature and humidity. They come into being during truss and hardening of concrete under the influence of action of inner heat sources and inner dry up as a result ofhydration process in binder and also as a result of mass (humidity) and energy (heat) exchange with environment. These processes, causing irregular changes of volume in hardening concrete, are often the reason of forming the first scratches and cracks in structure, even in stage of its realization. Thermal and humidity distortions are essential in massive structures. The increasing of massive structure temperature evokes distortion and stress in this structure. Maximal values of thermal stresses are being noticed in initial period of concrete truss and hardening. Cracks can be made in zones of tensioned concrete on the surface of solid. In the consequence of later, natural cooling of structure of the influence of plastic and rheological distortions and changes of material features in hardening concrete, there is a phenomenon of inversion in solid’s distortion, causing tension of inner zones.
Authors and Affiliations
Jan Ślusarek
Keywords
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