Study On Microstructure Of Hybrid Frber Reinforced High Performance Concrete After Exposure To Hifh Temperature

High-performance concrete has a dense internal structure and high strength,and is widely used in tunnels and underground engineering.However,high-performance concrete has shortcomings such as easy bursting at high temperature and poor tensile properties.To improve its deficiencies,polypropylene fiber and steel fiber are mixed into the concrete.Polypropylene fiber can effectively improve high-temperature concrete bursting at high temperature;steel fiber plays a significant role in improving the high-temperature residual compressive strength and splitting tensile strength of concrete.This article relies on the National Natural Science Foundation of China’s Youth Science Foundation project:hybrid fiber impact on high-performance concrete tunnel segment performance after fire and improvement mechanism research(51678100).The main research contents and research results are as follows:(1)In order to find out the influence of different kinds and amount of fiber on the macro performance of C50 high performance concrete after high temperature,this paper studies the high temperature test phenomenon,mass loss,appearance,resistance of steel fiber reinforced concrete,polypropylene fiber reinforced concrete,steel polypropylene fiber reinforced concrete and normal concrete without fiber under the action of 20℃,200℃,400℃,600℃ and 800℃The variation of pressure,splitting tensile strength and high temperature burst with temperature.The results show that the mass loss rate of concrete after high temperature is increased by adding fiber.Steel fiber can effectively improve the compressive and splitting tensile strength of concrete at room temperature and high temperature,and the increase of the strength is more obvious with the increase of the content of steel fiber.Polypropylene fiber effectively prevents the concrete from cracking at high temperature.The concrete specimens mixed with fiber have both advantages..(2)The change of microstructure greatly affects the macro performance of concrete.In this study,the changes of micro and micro morphology of fiber reinforced concrete after high temperature were qualitatively studied by SEM and super eyes digital microscope,and the changes of pore surface area,average pore diameter,porosity,pore size distribution and porosity ratio of fiber reinforced concrete after high temperature were quantitatively analyzed by mercury intrusion test.Through the observation of SEM and super eyes digital microscope,it is found that there are micro cracks in the hydration products after 400℃.Under the action of 600℃ and 800℃,the cracks gradually penetrate,and the strengthening effect of steel fiber on the mechanical strength is greatly weakened.Mercury injection test shows that the pore surface area and average pore diameter do not deteriorate before 200℃,and the pore surface area decreases and the average pore diameter and porosity increase with the increase of temperature after 200℃.After 600℃,the pore surface area,average pore diameter and porosity of fiber-reinforced concrete specimens are significantly worse than those of plain concrete specimens,and the most probable pore diameter is significantly increased..(3)It is very important to study the relationship between the macro mechanical properties and the micro structure of concrete.The traditional mathematical analysis methods can not accurately get the statistical laws of the two.In this study,fractal theory and grey correlation theory are used to study the internal pore characteristics of concrete,and the relationship between the pore structure parameters such as the pore surface area,average pore diameter,porosity and pore size distribution and the mechanical strength is discussed.The results show that in the pore structure parameters,the grey correlation between pore surface area and mechanical strength is the largest,which has the greatest influence on the strength;in the pore proportion,the grey correlation between pore and compressive strength is the highest,which is consistent with the fractal dimension with pore diameter greater than 50nm and the grey correlation between compressive strength and fractal dimension with pore integral dimension;and the grey correlation with splitting tensile strength is the highest The porosity with a specific aperture of 20nm-50nm is consistent with the rule that the fractal dimension with an aperture less than 50nm has the highest grey correlation with the splitting tensile strength.