Study On Microstructure-Damage Evolution Of C80 High Performance Concrete Subjected To High Temperature

High strength and high performance concrete has been widely used in modern engineering because of its high strength,high bearing capacity and good performance.However,the compact structure also makes high strength and high performance concrete structure easy to burst under the high temperature.Adding polypropylene fiber in high strength and high performance concrete is a common method to resist the burst.The study on rule of microstructure-damage of high strength and high performance concrete at high temperature is very important to find the mechanism of burst,and it is also expected to explain the bursting preventation of polypropylene fiber.This paper is supported by the National Natural Science Foundation of China?Grant No.51278325?.The paper mainly uses the X ray CT technology?CT?to research meso-structural damage of high performance concrete?HPC?and C80 polypropylene fiber high performance concrete?PPHPC?at high temperature.In this paper,mercury intrusion testing?MIP?has also been used to research the micro-damage of C80 HPC and C80 PPHPC after high temperature,as well as the scanning electron microscopy?SEM?is used to study the micromorphology of C80 PPHPC after high temperature.The paper has been written by following aspects:1.Qualitative analysis of CT imagesThe CT images of HPC and PPHPC at different temperatures were obtained by experiments,and the components in the CT images are identified and analyzed.The CT images show that the pore structure of HPC and PPHPC increases with the development of temperature.Generally,less temperature cracks in HPC began to appear at 400?.However,temperature cracks in PPHPC start to show up at 300? and a large number of cracks increased rapidly at 400?.2.Quantitative analysis of CT imagesFirstly,histogram of CT images and the number of pixels have been set up and accumulated by the Matlab.It is turned out that the histogram peak shifts to the left and the pixel of low gray level increases with the development of temperature.In addition,the the statistical results of pixel number indicate that internal defects show deterioration gradually with temperature increasing.Secondly,The cracks in CT image of HPC and PPHPC have been extracted to calculate the length,width,area and perimeter.After tracing the variation of cracks in different temperature,it has been found that the length,width,area and perimeter of single crack will increase with the higher temperature.Then calculating the total number,total length,width,area and perimeter of cracks at the one of CT layers.The calculation of has shown that the character of total cracks are increased with the rising temperature.And the character of total cracks of PPHPC is lager than HPC3.Pore structure analysisThe pore structure of HPC and PPHPC specimens were measured by MIP.The results showed that the porosity trend of HPC is similar as PPHPC except 300?.As well as the average pore size decreases and pore specific surface increases with elevated temperature.The pore distribution indicates that HPC and PPHPC both seriously deteriorate after 400 ? and a new round of deterioration start to happen after 500?.After 600? and 700?,the damage of HPC and PPHPC is very severe.PPHPC start to deteriorate as the HPC have deteriorated after 300?.The “steam curing condition” have formed in pore strcture of HPC after 300?,which result in the lower porosity than PPHPC.The proportion of much harm pore in HPC is lager than PPHPC after 400?.Those phenomenons indicate that adding polypropylene fiber can improve the high temperature performance of high strength and high performance concrete.According to the knowledge of fractal geometry and MIP test results,the fractal dimension of pore structure is calculated by using two kind of mathematical models.The results show that the fractal dimension calculated by different mathematical models is different.Fractal dimension of HPC and PPHPC differ greatly in different temperature range and different pore range,which show the different fractal characteristics with the change of temperature.4.SEM analysisSEM was used to observe the micromorphology of PPHPC after room temperature,300?and 500?.At room temperature,C-S-H,Ca?OH?₂ and Aft can be indentified clearly.However,Aft have completely decomposed after 300? with the C-S-H gradually becoming loose.After 500?,Ca?OH?₂ decompose and C-S-H become extremely loose.The change of phase at the micro level directly leads to the deterioration of high strength and high performance concrete after high temperature.