Effect Of High Temperatures On Micro-characteristics Of Concrete And Properties Of High Temperature Exposed Aggregates Containing Recycled Concretes

With the rapid development of economy and urbanization, China has made a very large number of infrastructure constructions. Concrete played an important role in this process. However, new problems appear in this process. For instance, the frequency and loss of fire were increasing with concentrated living of population; the tremendous demand of concrete also has made aggregate prices rise constantly. A method can be used to solve this problem. Firstly, we must evaluate the concrete structures of fire. Secondly, according to the results of the assessment, the structures to be used should be repaired. Finally, the rest can be used to do recycled aggregates. To achieve this aim, the high temperature on mechanical properties of concrete and the properties of high temperature exposed aggregates containing recycled concrete must be studied.Firstly, the high temperature on mechanical properties of concrete were studied in this paper, temperature and duration of high temperature were variables in this process. Secondly, the microstructures of concrete after high temperature were analyzed. Furthermore, the properties of recycled aggregates containing high temperature exposed concrete were studied. Finally, the mechanical properties of high temperature exposed aggregates containing recycled concrete were studied. The main research results are as follows:1. The results of mechanical properties of concrete after high temperature show that the cube compressive strength and prism compressive strength of concrete after high temperature decrease with the increasing of temperature and duration of high temperature; When the temperature is above 200℃, the stress-strain curve of concrete after high temperature become flat.2. The microstructures of concrete with exposure high temperature for 2h were analyzed. The results show that the surface and interior of concrete after high temperature crack gradually, furthermore, these cracks widen and extend with the increasing of temperature. The different components of concrete with exposure high temperature have various physical or chemical changes. For instance, the dehydration reaction of Ca（OH）₂ occur at 400℃. Ca（OH）₂ and CaCO₃ decomposes at 600℃. At 800℃, a lot of Ca₂SiO₄ appear in concrete, the content of SiO₂ and CaCO₃ decrease significantly, CaO disappear almost. The results also show that the element content of concrete exposure high temperature has changed, but the element composition has not changed.3. The experiments of aggregate show that the proportion of less than 2.36 mm particles, the water content ratio and water absorption ratio of the recycled aggregates containing high temperature exposed aggregate increase with the original concrete heating temperature and quantity of high temperature exposed aggregate raising, but the apparent density decreases. Besides, the bulk density and compact density of the recycled aggregates containing high temperature exposed aggregates increase with original concrete heating temperature higher. On crushing value indicators, compared to the crushing value indexes containing 400℃ or 600℃ recycled aggregates, the crushing value indexes containing 800℃ recycled aggregates are lower.4. The cube compressive strength and prism compressive strength of recycled concrete decrease with the increasing of heating temperature and proportion of recycled aggregates. If recycled concrete containing higher temperature and proportion of recycled aggregates, the ascending segment slope of stress-strain curves down faster.