SENSITIVITY OF BRITTLENESS OF THE CONCRETE TO THE PORE STRUCTURE OF THE CEMENT PASTE

Abstract

One of the essential factors which affects the performance of the concrete buildings under earthquake loads is the degree of brittleness of the concrete. It is possible to investigate the brittleness directly by dynamic experiments, whereas the static experiments which make it possible to get some numerical values. In this work, the second way is choosen and the brittleness of the concrete is expressed as the ratio between the compressive strength and the tension strength of the concrete (The brittleness index). The relationship between the brittleness, the grading of the aggregate mixture and the pore structure of the cement paste is investigated. The degree of sensitivity of the values determining the brittleness to the pore structure of the cement paste is also determined; then the factors determining the sensitivity correlation constants are investigated. The correlations between properties are strong because their sensitivities to the pore structure of the cement paste are close. All concrete properties are related to the inner structure of the cement paste because the properties of the concrete depend on the reactions of the hydration occurring in the cement paste. In the modern engineering, understanding the effect of the inner structure of the cement paste or how the materials of the concrete change the inner structure of the cement paste or the effects of the interface between the cement paste and the aggregates are important. From the past to the present, within the help of the developing technology, the inner structure of the cement paste is continuosly researched. The first researches of the inner structure of the cement paste are the crystallization theory of Le Chatelier and the Michealis' colladial theory (1). Powers and his friends proved that as a result of the cement hydration, the fibres like Labermant, form in the cement paste and these fibres are in colladial dimension and they have a great inner surface. Also, the structure of these fibres is gel. Besides this gel, in the cement paste, there are calcium hydroxide crystals, the secondary components, unhydrated cement particles and the pores. These pores are surrounded by the gel. Since the size of these pores is capillary, they are called as capillary pores (2). There are 4 main components of the cement; C2S, C3S, C3A and C4AF. After the hydration, these components turn into the hydrated components. The hydrated components are C-S-H (Calcium Silicate Hydrate) and CH (Calcium Silicate). C-S-H supply the binding property of the cement. The inner structure of the cement paste is porous. The pores form as a result of the hydration reactions because of the formed hydrated products. The capillary pores are formed after the hydration when used water evaporates. The capillary porosity depends on the water/cement ratio and the hydration degree. As the hydration degree increases, the more hydrated products are formed and they fill the pores so the numbers of the pores decrease. However, when the water/cement ratio increases, it is impossible that the hydrated products fill the pores because the amount of the water and the numbers of the pores increases, so the formed hydrated products are not enough to fill these pores. The gel pores are in the hydration products and their numbers and the total volume increase within the hydration process. The dimension of the gel pores is smaller than the dimension of the capillary pores. Besides these pores, there are air voids, the dimension of which is smaller. The air voids are formed because of the insufficient settlement of the concrete or some additives which drag air in the concrete. The interface between the cement paste and the aggregate is called as the weakest chain in the concrete. The inner structure of the interface between the cement paste and the aggregate differs from the inner structure of the cement paste. There are various models for the micro structure of the interface. According to Barnes;