Study On Thermal Compatibility Of Concrete Components And Thermal Recycling System Of Concrete

This dissertation systematically investigated thermal compatible behavior of of concrete components when exposed to ambient temperature difference (20℃～85℃) and high temperature (up to 900℃). One mixture proportioning design method of concrete that can resist thermal incompatibility was put forward. This method was based on adjusting thermal deformation of concrete components. In addition, by means of thermal incompatibility and chemical change of concrete components at high temperature, a highly efficient separation method to reuse the concrete waste was reported in order to obtain the high-quality aggregates and prepare the blended cementitious materials with dehydrated cement paste.There are three main parts in this dissertation. The first one is to measure thermal deformation of different components in concrete and then to evaluate the internal nature of thermal deformation of hardened cement paste (HCP). The second part is to verify thermal induced degradation on the concrete structure exposed to ambient temperature difference and high temperature due to thermal incompatibility of concrete components. One mixture design method for getting thermal compatible concrete is introduced by adjusting thermal deformation behavior of HCP. The last one is to indicate the feasibility of recycling the demolished concrete and preparing new type of cementitious material by means of the change of thermal physico-chemical properties.Some findings in the first part may be summarized as follows:1) Exposed to ambient temperature difference and high temperature, the main phases in concrete, HCP and aggregate, take on thermal incompatibility. This behavior depends on the temperature range, the thermal deformation behavior of HCP and the aggregate type.2) Thermal expansion of HCP is determined by the confined pore solution, the pore system and hydrated solid products. For the initially saturated HCP, the temperature and curing time influence its thermal response. With the temperature increase, the pore solution drains out of the body due to thermal induced pore pressure and the coefficient of thermal expansion (CTE) linearly decreases. The linear slope of the CTE-temperature curve and the degree of hydration of HCP keep a well linear relationship. Evidence may be potentially given that thermal expansion response of HCP, which is produced by pure ordinary Portland cement, could present its hydration development.3) Mineral admixtures and fibers used in this study can improve thermal expansion of HCP in the process of heating, while the big thermal shrinkage exists after one thermal cycle. The polymer latex can not only decrease the thermal expansion, but also cause low shrinkage or no shrinkage. As a result, designing the modified HCP needs to consider the whole thermal deformation during the thermal cycle.Some main conclusions in the second part can be drawn as follows:1) Experienced to thermal cycles, thermal fatigue cracking gradually initiates and propagates in concrete. After 120 thermal cycles, the reduction of the compressive strength of concrete is about 30%.2) Thermal expansion measurements of mortar and limestone aggregate reveal that the divergence of both linear CTEs presents temperature dependence. Thermal induced tensile stress around coarse aggregate occurs in RT～170℃range, while the hydrostatic compression around coarse aggregate happens between 170℃and 800℃. This effect is available to the gradual initiation of tangential and radical meso-cracks around coarse aggregate in concrete.In the last part, some important findings may be given as follows:1) By means of thermal degradation of concrete at high temperature, the recycled concrete may be successfully separated to obtain high-quality aggregates and prepare new cementitious materials with dehydrated phases.2) The recycled mortar experienced to thermal treatment takes on the rehydration activity. By using some by-products, such as lime and fly ash, the dry-mixed mortar may be designed with different strengths.3) The rehydration ability of the dehydrated HCP is determined by the dehydration temperature and initial water/cement ratio. By means of this behavior, new type of cementitious materials may potentially be designed by using additional additives.