Study On The Mechanical Properties Of Carbon Fiber Concrete And Nano-SiO₂ Modified Carbon Fiber Concrete Afterroom Temperature And High Temperature

Concrete has become the most widely used building material in the world due to its high strength and durability,simple and convenient construction,etc.With the improvement of China’s urbanization requirements,the demand for infrastructure construction is huge.In the long-term use process,the disadvantages of traditional concrete also begin to emerge.High performance concrete arises at the historic moment in this process,and the core of which is to realize the high performance of cement-based materials through fiber.By giving full play to the reinforcing effect of fiber,the flexural strength,tensile strength,impact resistance,ductility and toughness of concrete are improved.Nano SiO₂is a good interface improvement material and filling material.The introduction of nano SiO₂can improve the grading effect of concrete,increase the interface strength between carbon fiber and concrete matrix,and increase the integrity and mechanical properties of concrete,thus showing obvious Performance stacking effect.Based on previous studies,this paper studies the key mechanical properties of carbon fiber concrete and nano-SiO₂modified carbon fiber concrete at room temperature and high temperature,and analyzes the microstructure of concrete by scanning electron microscope.The main contents can be summarized as follows:（1）In this experiment,12 kinds of concrete samples were prepared,including carbon fiber concrete with different carbon fiber content（0wt%,0.6wt%,1.0wt%and1.4wt%）and different carbon fiber length（5mm,7mm and 10mm）,nano-SiO₂modified carbon fiber concrete with different carbon fiber content（0.6wt%,1.0wt%and 1.4wt%）and nano-SiO₂content（0.5wt%,1.0wt%and 2.0wt%）.The results show that the compressive strength of concrete mixed with a proper amount of carbon fiber has been improved,but the effect of the improvement is limited,and the improvement of splitting tensile strength and bending strength is more obvious.Compared with single-mixed carbon fiber,the composite of carbon fiber and nano-SiO₂mixed into concrete shows better mechanical properties.（2）A total of 432 concrete specimens were prepared with the sizes of 150 mm×150 mm×150 mm and 100 mm×100 mm×400 mm.Among them,324 specimens were treated at 400℃,600℃and 800℃respectively.The experimental phenomena in the process of high temperature treatment and the appearance changes of specimens after high temperature treatment were recorded.The specimens after high temperature treatment were naturally cooled to room temperature,and then the specimens were subjected to mechanical performance experiments.（3）The key mechanical properties of concrete samples at room temperature were recorded,and the effects of carbon fiber content,carbon fiber length and nano-SiO₂content on the mechanical properties of concrete were analyzed,and the mechanism of these changes was explained.The results show that if too little or too much carbon fiber is added to concrete,the effect of enhancing the mechanical properties of concrete is limited,and even negative effects may occur.And the nano-SiO₂mixed into it also showed a similar situation,and did not show the rule that the larger the proportion of nano-Si O2 in the concrete,the greater the mechanical properties.（4）The residual mechanical properties of concrete samples after high temperature were recorded.Based on the mechanical properties of concrete at room temperature,compare the damage of concrete caused by high temperature and analyze the residual rate of mechanical properties of ordinary concrete,carbon fiber concrete and nano-SiO₂modified carbon fiber concrete.Explains the mechanism of high temperature causing the mechanical properties of concrete to decrease.The results show that the damage of high temperature to ordinary concrete is greater than that of carbon fiber concrete and nano-SiO₂modified carbon fiber concrete.Nano-SiO₂can reduce the internal porosity and improve the compactness of the matrix after high temperature.The synergistic effect of nano-SiO₂and carbon fiber is the main factor to improve the mechanical properties of nano-SiO₂modified carbon fiber concrete after high temperature.（5）The fracture surfaces of different types of concrete samples were scanned by electron microscope to observe the difference between carbon fiber reinforced concrete and nano-SiO₂modified carbon fiber reinforced concrete and ordinary concrete,as well as the failure mode of carbon fiber mixed in them.The changes of macro mechanical properties of concrete were explained by the internal microstructure.It was found that when the amount of carbon fiber is small,the carbon fiber can be evenly dispersed in the concrete matrix,and when the amount of carbon fiber is too much,clustering and agglomeration are likely to occur,resulting in loss of mechanical properties.The main failure modes of carbon fiber in the concrete matrix are pulling and breaking.