Study On The Evolution Rule Of Macroscopic Properties Of Concrete With Initial Defects In The Western Region

The natural environment in the western part of China is harsh and the service life of concrete structures is much shorter than the designed value.The reason is related well with the initial defects in concrete before being subjected to loading.In this thesis the influences of initial defects on the macroscopic properties of concrete under three typical harsh environmental conditions(extremely low temperature,severe temperature change and extremely dry)and the evolution of the macroscopic properties of concrete with initial defects are studied.(1)Concrete specimens were stored in harsh environment after being standardly cured for 3d,7d and 28 d,respectively,and the elastic wave velocity is employed to quantitatively characterize the levels of drying initial defects.The result show that when the standardly curing time is smaller than 28 days and exposed to harsh environment at early age,initial defects will be generated in concrete.The shorter the standardly curing time the greater the initial defect level.(2)The quantitative analysis technology of panoramic microcrack and mercury intrusion porosimeter are used to test the characteristics of the microcrack and pore structure of the concrete with initial defects under harsh environment.Initial defects in concrete in harsh environment can be quantitatively characterized by microcracks characteristics(density,distribution,etc.)and pore structural characteristics(harmful pore content,harmless pore content,etc.).The higher the initial defect level of concrete,the greater the microcrack density,the higher the content of harmful pores are higher and the greater the initial damage extent calculated from the elastic wave velocity.(3)Distributions of initial microcracks in concrete under different harsh environments are different.Under the extreme low temperature and severe temperature changing environmental conditions,the initial microcracks in concrete are randomly distributed,while under the extremely dry environmental condition,the initial microcracks in concrete are mainly distributed in the outer region(2cm below the specimen surface).95% of the initial microcracks in the matrix in concrete are ITZ(interfacial transition zone)microcracks.(4)When the concrete age is 28 days,the mechanical properties and durability of concrete(frost resistance,transmission performance)are tested.In harsh environment,initial defects have adverse effects on the mechanical properties and durability(frost resistance,transport properties)of concrete.The higher the initial defect level,the greater the adverse effects on the macro properties.In extremely dry environment initial defects have adverse effects on the long-term mechanical properties of concrete instead of short-term mechanical properties.(5)In order to study the evolution of macroscopic properties of concrete with initial defects under harsh environment,the mechanical properties and durability(frost resistance and transmission performance)are tested in different age.In harsh environment,mechanical properties and transport performance of concrete with initial defects increase gradually with the increase concrete age.However the macro properties of concrete with initial defects are lower than concrete without initial defects at the same age.The frost resistance of concrete with initial defects gradually increase with age under extremely low temperature and while decrease gradually under extremely dry environment.