Multi-scale Study On Defects Of High Strength And High Performance Concrete At High Temperature

At high temperature,the mechanical properties,thermal properties and internal microstructure of concrete will change,and its performance will be seriously degraded.As a result,the deformation of concrete components and structures is aggravated,which can cause serious collapse and destruction of the entire structure.The high-strength and high-performance concrete is prone to burst after fire because of its dense internal structure,which is more harmful to structure.Therefore,it is important to study the defects of high strength and high performance concrete at multiple scales and discuss the evolution of defect degradation under high temperature.It is of great significance to improve the fire resistance of high-performance concrete,prevent and mitigate disasters,and strengthen post-fire reinforcement.In this paper,defects in concrete include pores and cracks.The defect of high-temperature damage is studied from three aspects: macro scale,mesoscale and micro scale.Cracks on the surface of concrete were measured using a comprehensive crack detector at the macro scale,and X-ray CT was used to scan the C60 high-performance concrete at a measurable scale.the internal structure of concrete was observed by SEM at the microscopic scale,and the deterioration and evolution of defects of C60 and C80 high performance concrete mixed with 0% and 0.2% polypropylene fiber was studied and compared.The main research content of this article is as follows:1.Analysis of macro defects of high-Temperature damage of high performance concrete after high temperature(1)Apparent pPhenomenon of high-Temperature damage of high performance concreteThe apparent appearance of C60 and C80 concrete mixed with 0% and 0.2% polypropylene fiber were described,The results showed that the surface color of C60 and C80 concrete changed from deep grey at normal temperature to pale white after exposing to high temperature;The surface cracks initiated,developed,and penetrated with increasing temperature,the temperature at which HPC0 begins to crack is lower than PPHPC.(2)Analysis of cracks on concrete surfaceStudy the law of the number and width of cracks on concrete surface with temperature.The test results showed that the crack distribution of <0.1mm was the most at the concrete surface at each temperature.With the increase of temperature,the crack width continued to expand.At 800?,the crack width of concrete surface was>0.5mm,the max crack width was 0.705 mm.The number of surface cracks in PPHPC was less than that of HPC0.2.Analysis of meso-defective damage of high-strength and high-performance concrete at high temperature based on CT(1)X ray CT image feature analysis of high strength and high performance concrete at high temperatureCT scanning was done for C60 and C80 high-performance concrete mixed with 0% and 0.2% polypropylene fiber,CT images were obtained at each temperature,and the changing characteristics of concrete CT images were analyzed with the temperature increasing.The results showed that: with the increase of temperature,the internal microstructure of HPC0 and PPHPC were deteriorating,but their deterioration rules were inconsistent.The microstructure of HPC0 gradually deteriorated with the increase of temperature before 400?;The microstructure of PPHPC did not change significantly before 300?,and the number of pores increased significantly at 400?.(2)Statistical analysis of pore parameters of high strength and high performance concrete at high temperatureThe variation of the number,area,and average pore diameter with temperature of the pores was studied.The results show that: with the increase of temperature,the number of pores,pore area and average diameter of concrete increased;For the same grade of concrete,at any temperature,concrete with many original pores had less pore volume and area,but its growth rate was low;the threshold temperature of PPHPC was 500?.(3)Analysis of fractal dimension of high strength and high performance concrete porosity at high temperatureFractal theory was applied to study the fractal rule of microstructure change of C60 and C80 high performance concrete by differential box counting method.The test results showed that: with the increase of temperature,the fractal dimension of HPC was basically increasing;at any temperature,the fractal dimension of C60 PPHPC was greater than that of HPC0,and the fractal dimension of C80 PPHPC was less than that of HPC0 before 500?.(4)Analysis of crack evolution law of high strength and high performance concrete at high temperatureThe cracks were separated from CT images of C60 high performance concrete at different temperatures,quantitative observation and analysis of internal fractures in terms of shape,number,length,width,area and perimeter were made,and the characteristics of C80 concrete crack were analyzed and compared.The test results showed that: with the increase of temperature,the length,width,area and perimeter of cracks basically increased;the fracture parameters,morphology and development law of HPC0 and PPHPC were different.(5)Analysis of the evolution of high strength and high performance concrete holes at high temperatureFour holes were selected from concrete CT slices for analysis to study the evolution of pore diameter and area with temperature.The results showed that as the temperature increased,the diameter of a single hole expanded or shrank;multiple individual holes merged into a large hole,and the pore area changed.3,Micro analysis on damage of high strength and high performance concrete at high temperature(1)Analysis of pore structure based on SEMEvolution of internal pore structure of high strength and high performance concrete by SEM were studied.The test results showed that: with increase of temperature,the pore area ratio of C60 HPC increased,PPHPC had a larger pore area than HPC0,the proportion of the small holes(<0.05?m)and transition holes(0.05?m-0.10?m)tended to decrease,and the proportion of macropores(>0.1?m)increased.(2)Crack analysis based on scanning electron microscopyEvolution of internal cracks structure of high strength and high performance concrete by SEM were studied.The test results showed that the internal cracks of concrete developed as the temperature rised,the crack width increased,and they penetrated each other at 500?;the fracture development trend was consistent with that observed by the CT test.