| To develop the fire-resistant concrete materials is one of the main fire prevention measures of building structure. Lightweight aggregate concretes are well known for their advantages of being lightweight and fire resistant and for having high strength. They have wide use in high-rise buildings and large span bridges. Lightweight aggregates, such as pumice, expanded perlite and clay ceramsite, which are made by burning, naturally have excellent resistance to high temperature. Thus, it is expected that lightweight aggregate concretes also have high application potential in chimneys, high temperature furnaces, tunnel fireproof layers, and other fire-resistant constructions. However, on the other hand, owing to the high porosity and bibulous rate of lightweight aggregate, the moisture content of lightweight aggregate concretes will be increased greatly either comes from prewetting of the lightweight aggregates, thereby increasing the possibility of spalling expose to the high temperature. If the lightweight aggregate concretes undergo explosive spalling in fire, its high temperature resistance cannot be made fully effective in fire protection, thus limiting its application in high-temperature resistant concrete structure.In view of this situation, in this paper, two types of modified materals were used to modify lightweight aggregates (ceramsites), and two kinds of lightweight aggregate concretes were prepared by the modified lightweight aggregates. The study on mechanical properties and microstructure evolutions of the modified lightweight aggregate concretes after exposure to elevated temperatures were analyzed by universal material testing machine system for mechanical test, thermal gravimetric analysis, fourier transform spectrometer and X-ray diffraction techniques for modified materials analysis, scanning electron microscope and high-temperature electric resistance furnace with a visible window for meso/micro-structure observation. Meanwhile, the empirical formulas of the residual mechanical property of modified lightweight aggregate concretes after high temperature is given according to the regression statistics. The concrete uniaxial compression statistical damage constitutive model is fit well to describe stress-strain curves and damage evolution curve of after the high temperature modification lightweight aggregate concretes after exposure to high temperatures.The results showed that:(1) the water absorption of the ceramsite was reduced after the surface coating treatment. The modified lightweight aggregate concretes had much lower risk and extent of spalling than ordinary lightweight aggregate concretes and ordinary concretes. (2) After exposure to 1200℃, the residual mechanical properties of modified lightweight aggregate concretes still retain in a high level, exhibiting their superior resistance to high temperature. (3) The microstructure of concretes will produce a series of changes after heating, after being heated will produce a series of changes, at different temperatures, the bone paste interfacial zone of the ordinary concretes, ordinary lightweight aggregate concretes and the modified lightweight aggregate concretes have different thermal damage characteristics (thermal cracking and pore structure etc). Those characteristics are related to the spalling and the changes of mechanical properties of concretes at high temperature. (4) The theoretical curves of the constitutive relation are in good agreement with the experimental results.The new modified lightweight aggregate concretes have the advantages of high temperature resistance, which can be used in the construction of high temperature operation, such as smelting, oil exploitation ect, as well as sprayed on the surface of existing concrete structure as the refractory layer. The research results provided technical basis for the development and preparation of modified lightweight aggregate concretes in high-temperature environment, and it has a good reference value for guiding the engineering application. |
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