Study On Dynamic Damage Assessment Method Of Fiber Concrete For Urban Underground Engineering

China is in the rapid development stage of green building industrialization,concrete-based infrastructure construction scale is increasing day by day,high-performance fiber concrete because of its good anti-cracking performance and corresponding durability can be a green building material,with a wide range of application prospects,Play an important role in the development and utilization of urban underground engineering.As an important green building material,the mechanical performance and damage mechanism of concrete quasi-brittle materials are the basic scientific issues of high-performance concrete research,and the research of damage mechanism of high-performance fiber concrete and engineering applications improve material performance,improve the quality of the project,It is of great theoretical and practical significance to reduce the comprehensive cost of the whole life cycle of engineering.Relying on Qingdao subway project,based on the existing urban underground engineering cracking damage disease investigation,based on the experimental research,using the combination of theoretical analysis and engineering practice,this paper systematically carried out the evaluation method of dynamic damage characteristics of urban underground engineering fiber concrete.Firstly,the basic problems such as fiber concrete ratio optimization,macro-injury evaluation,digital image 3D-DIC technology look at the evolution of damage,and secondly,the study of dynamic mechanical characteristics of fiber concrete from static to dynamic load at different loading rates was carried out.With the help of the three-axis ultra-low permeability damage test system,according to the change of the measured value of permeability,the damage characteristics under various conditions,such as the change of the enveloped pressure value,the change of loading speed,the cycle and unloading,reveal the damage evolution mechanism of the dynamic addition and unloading process of fiber concrete,and finally,the fiber concrete is applied in the submarine tunnel tunnel of Qingdao subway.The applicability of fiber concrete and the evolution law of strength damage during the service period of offshore environment are discussed.The main research contents and results of the thesis are as follows:?1?Fiber concrete fit ratio optimization and macro-injury evaluationUsing the fiber mortar extensibility method,the optimal doping amount of fiber required to meet the fluidity and construction requirements of fiber concrete is determined to be 1.2 kg/m³,and the length of polypropylene fiber 0.7kg kg/m³ and PVA fiber 0.5 kg/m³ are added for mixing and using.The shrinkage rate of the optimized fiber concrete test block was only 65%of the base concrete,and the fracture coefficient reached 0.883.?2?Research on the fine viewing damage of fiber concrete based on digital image 3D-DIC technologyThe technique of digital image 3D-DIC was introduced,and the mechanism of damage evolution of fiber concrete test blocks under compressed loading conditions was carried out.First of all,under the condition of single-axis loading,according to the evolution of the whole-field strain cloud map of the test block,the damage process can be quantified and segmented into three stages of initial compression,stable expansion of cracks,and accelerated expansion of cracks to destruction;Secondly,the pre-peak cycle loading experiment was carried out,and the damage characteristics of the test block crack position from production to per-run destruction were analyzed.Finally,a similar material simulation test was carried out,and a strain comparison of load damage with the PMMA test block and the fiber concrete test block was produced,and under the same loading conditions,the PMMA test block showed obvious brittle damage characteristics,and the duration of the crack to the destruction was significantly shorter than the fiber concrete test block,which confirmed the fiber mixing,Concrete changed from brittle damage to obvious plastic damage,which improved the ability to resist damage cracking.?3?Study on the dynamic mechanical properties of fiber concrete at different loading ratesThe study of dynamic mechanical characteristics of fiber concrete test blocks from static to dynamic load different loading rates(10^-5/s¹0^-2/s)was carried out,and the mechanical properties of test block compression and stretching were analyzed.The research shows that the pressure strength and tensile strength of fiber concrete are increased with the increase of load rate,and the relative benchmark concrete in peak strain and toughness index is improved.A dynamic damage model of fiber concrete with strength dynamic growth factors is established,and the fitted curve is well matched.?4?Fiber concrete dynamic plus unloading damage evaluation 1Considering the perforation effect of engineering practice,the damage characteristics under the conditions of the circumference value change,load speed change,cycle and unloading of the test block are evaluated according to the change of the measured value of the penetration rate by means of the three-axis ultra-low permeability damage test system.First of all,by carrying out a single addition and unloading experiment with different circumference values,it is proved that the effect of the perforation effect on the infiltration of concrete is obvious,the higher the circumference value,the lower the degree of cracking of the test block damage.When the circumference is 1MPa,the damage penetration of the test block is reduced by 50%compared to the non-perimeter pressure effect,and when the perforation pressure is loaded to4MPa,the infiltration of the infiltration is reduced by 50%compared to the perforation pressure value of 1 to 2MPa,which reveals that the perforation effect effectively inhibits the expansion of the internal damage cracks in concrete.Secondly,by carrying out a single add-off test of different loading speeds?0.5,1.0,1.5,3.0 MPa/s?,it is proved that the loading speed has a great effect on the damage cracking of the test block,the faster the loading speed,the more obvious the damage characteristics.Finally,a three-axis cycle active load test with a circumference value of 1MPa and axial loading speed of 1.5MPa/s was carried out,which revealed that with the increase of loading times,the damage characteristics of the test block changed from elastic deformation to obvious plastic deformation damage.?5?In the evaluation of the applicability of fiber concrete,through the on-site application of the Qingdao Metro undersea tunnel,the evolution law of fiber concrete strength damage under the condition of doubling sea water concentration in the marine environment is studied,and the maximum strength damage factor of fiber concrete for 100 years is calculated to not exceed 0.1,which meets the design requirements of the 100-year service life of the lining service period.