Study On Deterioration Mechanism Of Ultra-High Performance Concrete Based On CT Scan

With the rapid development of the construction industry,the requirements for concrete used in construction projects are increasing,and ultra-high performance concrete(UHPC)has come into being.As a representative of high-strength and high-durability materials,UHPC has been gradually applied in large-span bridges,super high-rise buildings and hydraulic structures.However,based on current research,it is found that UHPC has a high risk of deterioration under corrosive and high temperature environments.This is due to the high electrochemical corrosion risk of the large number of steel fibers in UHPC in the chloride salt attack environment;on the other hand,the dense microstructure of UHPC significantly hinders the release of water vapor inside the matrix at high temperatures,resulting in deterioration and even bursting due to excessive accumulation of vapor pressure.Therefore,it is important to investigate the deterioration mechanism of UHPC in corrosive and high-temperature environments and to take targeted protective measures to improve the structural safety of UHPC and promote the promotion and application of UHPC.In order to monitor the deterioration process of UHPC and visualize the evolution of its internal structure during the deterioration process,this study adopts X-CT nondestructive testing technology to continuously test the corrosion and high temperature deterioration process of UHPC,and visualize and quantitatively analyze the deterioration process of UHPC internal pores,steel fibers,steel reinforcement and cement matrix through high-resolution three-dimensional reconstruction of X-CT.And on this basis,the corrosion and high temperature deterioration mechanism of UHPC was investigated in depth,and the main findings are as follows.(1)The increase of steel fiber doping will significantly reduce the resistance of current transmission in the UHPC matrix.During the energization process,the UHPC specimens with 2%and 3%steel fiber doping showed obvious signs of corrosion deterioration,while the 1%doping specimens did not have this phenomenon.The study concluded that UHPC with 1%steel fiber doping has a strong resistance to electrochemical corrosion,and the grid structure formed by interconnecting steel fibers is a key factor in inducing corrosion deterioration of UHPC.(2)Steel fiber corrosion triggered the UHPC concrete protective layer from the outside to the inside of the cracking or spalling,the initial damage area becomes the most serious area of UHPC corrosion degradation.According to the test results,determine the UHPC corrosion degradation process:steel fiber corrosion,concrete protective layer damage,steel corrosion.(3)According to the connection between the initial distribution of steel fibers and the region of specimen deterioration,it is believed that the signs of corrosion deterioration of steel fibers,steel reinforcement and cement matrix are more likely to occur in the region with higher initial density of steel fibers.(4)The incorporation of steel fibers has a negative impact on the microstructure of UHPC.With the increase of steel fiber incorporation,the percentage of large pores in UHPC increases,the pore structure deteriorates,and the denseness of the cement matrix decreases,thus increasing the risk of corrosion deterioration of UHPC in the eroding environment.(5)After high temperature treatment at 600℃,the steel-polypropylene hybrid fiber UHPC has a high relative compressive strength,and the admixture of steel-polypropylene hybrid fiber can effectively inhibit the formation and development of larger pores in UHPC at high temperature and improve the pore structure of UHPC after high temperature,which is extremely beneficial to improve the mechanical properties of UHPC after high temperature.(6)Due to the formation of pores within the UHPC matrix by heat melting of polypropylene fibers,the porosity,fractal dimension and the percentage of shaped pores of steel-polypropylene blended fiber UHPC increased significantly during the heating stage from 25 to 200℃,but the growth was slower during the heating stage from 200 to 600℃.The doping of steel-polypropylene hybrid fibers can effectively limit the further development of additional pores and irregular pores in UHPC at high temperatures and delay the degradation damage to the UHPC matrix at high temperatures.(7)Scanning electron microscopy analysis revealed that after high temperature,the UHPC cement matrix was loose and porous,with numerous cracks and pores staggered,the steel fibers were heat deformed and oxidized,and large voids appeared at the junction with the cement matrix,and a large number of microcracks were generated around the pores of polypropylene fibers,and the reduction of fiber properties and the severe deterioration of the matrix led to a significant decrease in the mechanical properties of UHPC.