Uniaxial And Multiaxial Mechanical Properties Of Freeze-thawing Concrete

Concrete structures in service inevitably suffer from the long-term effects of various environmental factors,such as freeze-thaw cycle,long overload,wind load,which leads to the reduce of mechanical properties and the increase of maintenance cost.In particular,the damage caused by freeze-thaw cycle is more serious in hydraulic structures.Uniaxial mechanical properties of concrete after freeze-thaw cycle have been studied for now.However the uniaxial mechanical properties of actively confined in concrete after freeze-thaw cycle has not been studied.The mechanical properties of concrete after confinement are significantly different from unrestrain concrete.It is necessary to discuss mechanical properties of restrained concrete after freeze-thaw cycle.With the development of science and technology,mesoscale simulation has become an efficient tool to explore the mechanical properties of concrete.In this thesis,a mesoscale concrete model was developed.After the validation of the proposed model,the influences of freeze-thaw damage on uniaxial mechanical properties and the improvement effect of restraint levels on concrete mechanical properties were investigated.The work of this thesis mainly includes the following four sides:(1)The load-displacement curve of concrete was obtained through uniaxial mechanical test after freeze-thaw cycle.The influence of freeze-thaw damage on uniaxial mechanical properties(elastic modulus,peak stress,peak strain,etc.)has been studied by combining statical data and basic theories.The weakening effect of freezethaw damage on mechanical properties of concrete was determined.(2)The load-displacement curve of concrete was obtained through multiaxial mechanical test after freeze-thaw cycle.Combining with the basic theory of restrained concrete,the improvement effect of restraint levels on freeze-thaw damage of concrete was defined by comparing the differences of mechanical properties of specimens under different confinement levels and different freeze-thaw cycles.(3)Concrete can be assumed to be a three-phase material at the mesoscale level,including aggregate,mortar,and interface transition zones(ITZ).Based on the consideration of the three-phase composition of concrete,a concrete random aggregate model was established,and the rationality of the meso-model was verified.(4)The concrete random aggregate model was improved by considering the effect of cyclic temperature and frozen water.Based on the reliability verification of the model,the uniaxial and multiaxial mechanical properties of concrete after freeze-thaw cycle have been studied.The formula of the elastic modulus and peak stress of concrete after freeze-thaw cycle were proposed.