Research On Microstructure And Compressive Performance Of High Strength Concrete With Polypropylene Fiber Subjected To High Temperature

High strength concrete has the advantages of high compressive strength, strong resistance to deformation, and low porosity. Whereas, HSC is prone to burst when exposed to high temperature because of its dense structure, low permeability, which probably results in sharp reduction of bearing capacity in the structural element, tremendously decreasing the fire resistance and safety of concrete structure. Many researches have shown that the addition of polypropylene fiber is one of the main measures to improve the spalling resistance property in HSC. Therefore, it is practically significant to explore the effect of polypropylene fiber on compressive performance of HSC after exposure to high temperature, and the fire resistance mechanism.By combining the micro-structural analysis with macro-mechanics testing, this paper discussed the variation of the internal pore structure and compressive strength in HSC with different temperature, attempting to provide experimental basis for the research on the performance of HSC after exposed to high temperature.The main researches as follows:1. The effect of length and diameter of polypropylene fiber on the compressive strength of HSC was studied, after testing the compressive strength of HSC with polypropylene fiber before and after exposed to high temperature, which demonstrated that, with the dosage of1.8kg/m3, the compressive strength of HSC with polypropylene fiber with the length of15mm was superior to that of8mm. At room temperature, there is little effect of fiber diameter on the compressive strength of HSC. However, the compressive strength of HSC with the25μm polypropylene fiber was superior to that of35μm. Meanwhile, the relationship between residual compressive strength, relative residual compressive strength of HSC, and temperature was established respectively.2. A set of sample preparation methods was decided for micro-structure analysis of HSC by several tests, one of which is cutting, grinding and polishing, before exposed to high temperature, as is called sample preparation method for quantitative analysis. The other, called sample preparation method for qualitative analysis, is directly taken from the test block. These samples above will be observed through scanning electron microscope, from various angles, according to different demands, to acquire distinct images for further research on micro-structure in HSC without disturbance of the concrete structure.3. The distinguish feature of micro-structure in HSC with polypropylene fiber was summarized before and after different high temperatures, by means of qualitative analysis of the micro-images in HSC, which confirmed that, the higher the temperature is, the more seriously the concrete microstructure is destroyed, the more gradually the cement paste dehydrates, the looser the structure becomes. The variation of micro-structure in HSC with polypropylene fiber was compared before and after different high temperatures, based on different analysis including non-interface zone of cement paste, aggregate and cement paste interface, polypropylene fiber in the cement and the pore structure of concrete.4. The pore area ratio of specimens and porosity distribution were analyzed in HSC with polypropylene fiber after high temperature. The pore in the high-strength concrete can be divided into macro pore (>0.1μm), transition pore (0.05～0.10μm), micro pore (<0.05μm), according to the different influence of pore on compressive strength and permeability of HSC, in which macro pore mainly influence on compressive strength and permeability of HSC, micro pore mainly influence on shrinkage and creep. It is shown that:the pore area ratio of high-strength concrete at300℃is lower than the pore area ratio at room temperature, the pore area ratio of high-strength concrete at500℃is higher than the pore area ratio at room temperature and300℃. According to the above,it can be concluded that the pore area ratio would be higher with the increasing of the temperature.5. The fitting relationship between the macro pore area ratio and compressive strength of different fiber series after high temperature was established, according to which large pore area ratio can be calculated through the measurement of compressive strength.6. The effect of pore size distribution on concrete compressive strength is larger than the pore area ratio. The fitting formula between the most probable aperture and the compressive strength was established.