Study On The Improved Creep Model And Calculation Theory Of Hydraulic Concrete

As the main material of hydraulic structures,it is very important to study the mechanical properties of hydraulic concrete.As a typical composite material with multi-scale random characteristics,concrete exhibits obvious creep characteristic under sustain load.The creep of concrete has an important influence on the distribution and development of stress-strain in structure.It is of great significance to systematically study the creep characteristic of hydraulic concrete for the better design and construction of engineering as well as the realistic safety assessment of hydraulic structures.There are many factors that affect the creep behavior of concrete and many scholars have done a lot of experimental research in laboratory.In the environment with standard temperature and humidity,the creep of sealed specimens under low stress level represents the basic creep of concrete.However,in engineering practice,the local stress of concrete structure may exceed the linear creep range(around 40% of the strength),and concrete may exhibit nonlinear creep characteristics.Besides,creep failure phenomenon may occur at high stress level(beyond 70% of the strength).In addition,the realistic service environment of concrete structure is far from the standard environment,and the influence of temperature and humidity on concrete creep should also be paid enough attention.The traditional creep experiment is time-consuming and laborious.Based on the experimental results,the development of efficient and reliable creep model and calculation theory can be a beneficial supplement to experimental research.The development of efficient and reliable creep model and calculation theory for investigating the complex creep mechanism of concrete has become a fronted issue in the fields of engineering and material science.This dissertation is devoted to develop and apply theoretical analysis and numerical calculation methods to study the complex creep behavior of concrete materials,including the basic creep,nonlinear creep and creep behavior under complex environments.The main achievements of this dissertation are as follows:(1)A state-of-the-art overview of mainstream creep model and calculation theory for nonlinear creep and creep under complex environment is given out.The dissertation systematically reviews and analyzes the key technical problems in the current research of concrete creep,and puts forward the main research content of this paper based on these.(2)With respect to the aging viscoelastic constitutive equations of concrete,an efficient and reliable numerical solution method is put forward based on Comsol Multiphysics by directly solving the viscoelastic constitutive equations in differential type.The homogenization theory in the calculation of effective viscoelasticity of composites is systematically reviewed,which lays a foundation for the establishment of multi-scale homogenization model for concrete creep,and provides a reference for the study of effective mechanical behavior of other composite materials.(3)A multi-scale homogenization model is established to estimate the basic creep of ordinary Portland cement concrete.The hydration mechanism of ordinary Portland cement is analyzed,the multi-scale microstructure characteristics of concrete materials are reasonably divided.Referring to experimental results in literatures,the calculation results of multi-scale homogenization model are comprehensively analyzed and verified with those of mainstream creep models.(4)A coupled creep and damage model considering rate effect is proposed to study the mechanisms of nonlinear creep and creep failure of concrete under medium and high stress levels.The simulation results are fully validated with the experimental results in literatures.It is shown that considering the rate effect has an important influence on the correct understanding of the coupling mechanism of creep and damage.In middle and high stress levels,the initial damage of the material is caused by the strain incompatibility between aggregate and mortar.The damage will increase the creep of the material,and the creep of the material will further affect the development of the damage.(5)The influence of ambient temperature and humidity on concrete creep is studied by using MPS theory.A systematical review of the development and shortcomings of MPS theory is given out.The shortcomings of the original MPS theory in characterizing the size effect of drying creep and the humidity dependence of transient thermal creep are improved.Experimental data in literatures are adopted to verify the improved MPS theory.Results prove the correctness of the modifications and the practicability of the improved MPS theory.