| With the increasing improvement of engineering infrastructure and dam construction technology,a large number of hydraulic projects begin to be constructed in northwest cold and arid regions with high altitude and complex environmental conditions.However,the severe cold and dry natural environment of the region poses a serious challenge to the construction and maintenance of concrete structures.Lower curing temperature will adversely affect the pore structure and mechanical properties of concrete when winter construction is required under certain conditions.During the long-term operation of concrete buildings,the performance will deteriorate due to freeze-thaw(F-T)cycles,which will greatly shorten the service life of the structure.In the long life cycle of concrete structures in cold and arid regions,it is inevitable to face the challenges of the above two problems throughout the process from construction to operation.Therefore,it is significant for the development of hydraulic concrete buildings in northwest China to carry out numerical simulation research on damage characteristics and research on concrete constitutive relations under the coupling effect of non-standard curing and F-T cycles.In this paper,laboratory tests,theoretical analysis and numerical simulation are combined to study the influence of non-standard curing temperature and F-T cycles on concrete performance in Northwest China.The main research contents and achievements are as follows:(1)Based on the laboratory macro-mechanical test,the quick freeze-thaw cycle test of concrete under different curing temperature was carried out.The initial defects caused by nonstandard curing temperatures and the evolution of F-T damage were quantitatively analyzed by using non-destructive test(dynamic elastic modulus test,acoustic emission(AE)test)and destructive test(uniaxial compressive test),respectively,to reveal the influence of curing temperature on the freeze-resistance performance of concrete and the damage law of concrete under load after F-T damage.(2)Nuclear magnetic resonance(NMR)tests were conducted on concrete after different FT cycles at different curing temperatures to reveal the effect of curing temperature and F-T cycles on the pore structure of concrete.Combined with the comparative analysis of the microscopic morphology by scanning electron microscopy(SEM),the microscopic mechanism of the influence of the curing temperature and F-T cycles on the macroscopic mechanical properties of concrete was revealed.(3)Considering the microscopic mechanism of the complex physical process of concrete FT damage,based on the multi-field coupling theory,a complete set of three-field control equations of thermal-hydro-mechanical control equations were established.A trans-scale finite element model of concrete containing the three phases of aggregate-interface transition zone-mortar was generated using the Monte Carlo method.By introducing the microscale research results obtained from experiments,numerical simulations were conducted on the freeze-thaw degradation process of concrete under different curing temperature.The damage deterioration regularity of concrete during F-T cycle and the effect of curing temperature on the damage process were revealed.(4)Based on damage mechanics theory,load damage variable was defined by AE characteristic parameters during uniaxial compression,and the constitutive model of concrete under uniaxial compression based on acoustic emission was established.The deterioration of concrete caused by non-standard curing temperature and F-T action was introduced into the model in the form of coupling damage.Finally,the uniaxial compression constitutive model of concrete considering F-T damage and the influence of curing temperature was established.The experimental verification shows that the constitutive model can better reflect the influence of curing temperature and F-T cycles on the uniaxial compressive stress-strain relationship of concrete. |
