Research On Freeze-thaw Damage And Performance Evolution Of Engineered Cementitious Composites With High Ductility

The brittleness and fragility of cement-based materials and insufficient durability are the two key problems that concrete structures have faced for a long time,which has seriously affected their safety and service life.The concrete structure in the cold area is in a freeze-thaw environment for a long time during its service period,and the freeze-thaw effect greatly accelerates its damage and deterioration process.In order to effectively solve the problem of brittleness and cracking of cement-based materials,and focus on assessing their frost resistance durability.In this paper,starting from the toughening and crack control of cement-based materials,an ultra-high ductility engineered cementitious composites(ECC)with the ultimate tensile deformation capacity of 3%was designed and prepared;then the freeze-thaw cycle test was adopted in the background of freeze-thaw environment.Utilizing nuclear magnetic resonance(NMR),electrochemical impedance spectroscopy(EIS)and digital image correlation(DIC)testing techniques,combined with theories of thermodynamics,damage mechanics,elasticity mechanics and porous medium mechanics,the ECC freeze-thaw damage and performance evolution was systematically studied.The main contents are as follows:(1)Evolution characteristics of freeze-thaw damage of ECC pore structure.Using the number of freeze-thaw cycles as the time parameter,a continuous freeze-thaw cycle test within the FTs0-FTs300 period was performed on the ECC,and the pore structure parameters such as T2 spectrum,pore size distribution curve and porosity were measured by NMR technology,and combined with thermodynamic theoretical analysis,revealing the evolution of freeze-thaw evolution of ECC pore structure.According to the existing conductive path form of the micro structure of concrete materials,the ECC micro structure conductive path evolution model and EIS equivalent circuit model based on pore structure damage were developed;with the connected pore path resistance RCCP and the non-connected pore path resistance RCP as parameters,a freeze-thaw damage model of ECC pore structure based on EIS was established,and the evolution mechanism of freeze-thaw damage of ECC pore structure was clarified.(2)ECC freeze-thaw damage analysis based on pore structure evolution.The ECC with discrete structure was simulated as a continuous medium,and the equivalent relationship between the pore water-ice phase transition frost heave stress and the surface tensile stress was given.The freeze-thaw damage was regarded as the cumulative fatigue damage caused by the repeated action of the frost heave stress.The ECC freeze-thaw damage evolution equation was derived;the damage degree of the pore structure was defined,and the freeze-thaw damage model with the porosity as the key parameter was further obtained.(3)The freeze-thaw evolution characteristics of ECC mechanical properties under freezing and thawing.The freeze-thaw damage ECC in the FTs0-FTs300 cycle was subjected to the mechanical loading test in the two states of freezing and thawing,and combined with the DIC test technology,the ECC mechanical performance freeze-thaw evolution characteristics were studied.Compared with the thawing state,innovative exploration and analysis the response characteristics of freeze-thaw damage ECC mechanical properties to low-temperature freezing environment.(4)The mechanism of the influence of pore structure evolution on the mechanical properties of ECC.Taking the evolution characteristics of pore structure as the core,the ECC mechanical strength prediction model of freeze-thaw damage in the thawing state was derived through the elastic mechanics theory and mesostructured central pore model.The pore structure in the freeze-thaw damage model was equivalent to the pore structure in the strength prediction model,and the effective strength-freeze-thaw damage degree relationship was obtained.Assuming that the effective frost heave pressure of saturated pores in the frozen state provides "prestress" in the opposite direction when the material was loaded,the concept of equivalent low-temperature freezing strength was given,and combined with the meso-structure hollow sphere model,a ECC strength calculation model for freezing and thawing damage under low temperature and freezing conditions was proposed.